Thông tin tài liệu
ENHANCED BROADBAND ANTI-REFLECTION IN
LASER PROCESSED SILICON NANO-STRUCTURE
ARRAYS FOR SOLAR CELLS
FU XIAOTIAN
(B. Eng. (2nd Upper.) NUS)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF ELECTRICAL AND COMPUTER
ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2014
DECLARATION
I hereby declare that the thesis is my original work and it has been written by me
in its entirety.
I have duly acknowledged all the sources of information which have been used in
this thesis.
This thesis has also not been submitted for any degree in any university
previously.
Name: FU XIAOTIAN
Signature: _________________
Date: _________________
Acknowledgements
ACKNOWLEDGEMENTS
I would like to express my heartfelt appreciation and gratitude to my supervisors,
Prof. Hong Minghui, Dr. Bram Hoex and Dr. Ian Marius Peter for their invaluable
guidance and great support throughout my Master project. Without their valuable
advice and encouragements, the progress of this project will not be as smooth as it
is. I am deeply grateful to Prof. Hong Minghui for the high standard he held on
me. Without his dedicate care, my research work would be slow down. His acute
sense and strict attitude in research field inspire and give me great help.
It is my pleasure to recognize all the members in Laser Microprocessing Lab for
sharing their experience in research and giving me kind help and useful discussion.
Special thanks would be expressed to Dr. Luo Fangfang, Dr. Du Zheren, Mr.
Yang Jing and Mr. Wang Dacheng for your help in both my study and life, and I
deeply appreciate the time shared with you. I wish you best luck in your career.
I am grateful to all the members from Monocrystalline Silicon Wafer Group and
Simulation Group in Solar Energy Research Institute of Singapore (SERIS) for
giving me their kind advice and experience in research. The scholarship provided
by SERIS for my Master degree is gratefully acknowledged.
Last but the most importantly, I would like to give my great thanks to my parents
for their great encouragement and constant support during my years of pursuing
degree in National University of Singapore.
i
Table of contents
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ..................................................................................... i
TABLE OF CONTENTS ........................................................................................ ii
SUMMARY .......................................................................................................... vii
LIST OF FIGURES ............................................................................................... ix
LIST OF SYMBOLS ........................................................................................... xiii
CHAPTER 1 INTRODUCTION ............................................................................ 1
1.1
Background .................................................................................................. 1
1.2
Motivation .................................................................................................... 2
1.3
Research objectives...................................................................................... 4
1.4
Organization of thesis .................................................................................. 5
CHAPTER 2 BACKGROUND AND LITERATURE REVIEW .......................... 7
2.1
Silicon solar cells ......................................................................................... 7
2.1.1
Introduction .......................................................................................... 7
2.1.2
Optical properties of silicon ................................................................. 8
2.1.3
Optical loss ......................................................................................... 11
ii
Table of contents
2.1.4
Surface texturing................................................................................. 13
2.1.5
Light trapping ..................................................................................... 15
2.2
Light scattering by nanoparticles (NPs) ..................................................... 18
2.2.1
Properties of surface plasmons ........................................................... 19
2.2.2
Surface plasmon excitation................................................................. 21
2.3
Numerical simulation ................................................................................. 23
CHAPTER 3 EXPERIMENTAL SETUP AND FABRICATION DETAILS ..... 27
3.1
Introduction ................................................................................................ 27
3.1.1
Laser micro-lens array (MLA) lithography ........................................ 27
3.1.2
Laser interference lithography (LIL) .................................................. 29
3.1.2.1 Principle of LIL............................................................................... 30
3.1.2.2 Lloyd’s mirror setup........................................................................ 31
3.1.3
3.2
Top-down fabrication of silicon nanowires ........................................ 33
Fabrication details ...................................................................................... 36
3.2.1
Substrate selection and wafer cleaning............................................... 36
3.2.1.1 RCA I cleaning................................................................................ 36
3.2.1.2 RCA II cleaning .............................................................................. 37
3.2.1.3 5% Hydrofluoric Acid Dip.............................................................. 37
3.2.2
Photoresist coating.............................................................................. 38
iii
Table of contents
3.2.3
Exposure and develop......................................................................... 40
3.2.3.1 Concentric rings nano-structure arrays ........................................... 40
3.2.3.2 Silicon nanowire arrays................................................................... 40
3.2.4
Metallic thin film deposition .............................................................. 41
3.2.5
Lift-off ................................................................................................ 43
3.2.6
Chemical etching and 3D nano-structures fabrication ....................... 46
CHAPTER 4 HYBRID 3D SILICON SURFACE NANO-STRUCTURE
ARRAYS FOR SOLAR CELLS BY LASER MICRO/NANO-PROCESSING.. 49
4.1
Introduction ................................................................................................ 49
4.2
Characterization methods........................................................................... 50
4.2.1
Optical microscope (OM) imaging ..................................................... 50
4.2.2
Scanning electron microscope (SEM) imaging .................................. 51
4.2.2.1 Silicon concentric nano-rings array ................................................ 51
4.2.2.2 Silicon nanowires array................................................................... 53
4.2.3
UV-Vis spectroscopy.......................................................................... 56
4.2.3.1 Silicon concentric nano-rings array ................................................ 57
4.2.3.2 Silicon nanowires array with different pillar heights ...................... 59
4.2.3.3 Silicon nanowires array with different pillar diameters .................. 62
4.3
“Mushroom”-shape silicon nanowires array.............................................. 64
iv
Table of contents
4.3.1
Scanning electron microscope (SEM) image ..................................... 66
4.3.2
UV-Vis spectroscopy.......................................................................... 67
4.4
Theoretical analyses of enhanced light trapping in hybrid 3D silicon nano-
structures array ...................................................................................................... 69
4.4.1
Optical performance ........................................................................... 69
4.4.1.1 Reflectance of silicon nano-structures array ................................... 69
4.4.1.2 Absorption of silicon nano-structures array.................................... 71
4.4.2
4.5
E-field distribution in single standing nano-structure ........................ 72
Summary .................................................................................................... 76
CHAPTER 5 BROADBAND ENHANCEMENT OF ANTI-REFLECTION IN
SILICON MICRO/NANO-STRUCTURES ......................................................... 78
5.1
Introduction ................................................................................................ 78
5.2
Experimental details................................................................................... 79
5.2.1
Metallic nanoparticles deposition ....................................................... 79
5.2.2
Pyramid micro-structures ................................................................... 79
5.2.3
Laser surface texturing ....................................................................... 81
5.2.4
Thermal annealing of metallic nanoparticles...................................... 82
5.3
Characterization ......................................................................................... 84
5.3.1
Scanning electron microscope imaging .............................................. 84
v
Table of contents
5.3.1.1 Flat silicon with metallic nanoparticles deposition ......................... 84
5.3.1.2 Pyramid micro-structure with metallic nanoparticles deposition ... 85
5.3.1.3 Laser surface texturing with metallic nanoparticles’ deposition .... 86
5.3.2
UV-Vis spectroscopy.......................................................................... 89
5.3.2.1 Silicon micro/nano-structures with metallic nanoparticles’
deposition....................................................................................................... 89
5.3.2.2 Silicon back surface texturing ......................................................... 97
5.4
Summary .................................................................................................... 99
CHAPTER 6 CONCLUSIONS AND FUTURE WORK ................................... 101
6.1
Research achievements ............................................................................ 101
6.2
Suggestion for the future work................................................................. 103
References: .......................................................................................................... 105
vi
Summary
SUMMARY
Light collection efficiency act as a key factor affecting the performance of many
optical and optical-electronic devices. For silicon wafer solar cells, the high
reflection at the front surface, due to its high refractive index, is a hindrance to the
devices’ light collection efficiency. To minimize unwanted reflection and increase
light trapping, anti-reflection surfaces by micro/nano-texturing are one of the
most promising candidates, which is featuring an improved anti-reflection as well
as enabling the probability of manufacturing high-efficiency solar cells on a large
scale. This thesis focuses on broadband enhancement of light absorption for solar
cells by silicon surface texturing using laser micro/nano-processing.
Laser technology has become one of the commonly applied techniques for highefficiency silicon wafer solar cell fabrication. In this thesis, laser interference
lithography (LIL) and micro-lens array (MLA) lithography are adopted as maskfree and efficient techniques, associated with metal catalyst assisted chemical
etching, to fabricate silicon nano-structures arrays. The surface anti-reflection
performance and light trapping are significantly improved by sub-wavelength
structures at ultra-high aspect ratios, which create gradient refractive index from
air ambient to wafer substrate. Furthermore by varying the surface geometry and
feature design of the nano-structures arrays, the light absorption in the nanostructures are boosted by extending the light travelling path and changing the
silicon volume ratio at the nano-structures layers. Meanwhile, with the decoration
vii
Summary
of metallic nanoparticles on silicon nano-structures, the surface plasmon
resonance can be excited to further enhance broadband anti-reflection, achieving
an ultra-low reflection across the broad spectrum from 300 to 1200 nm.
viii
List of figures
LIST OF FIGURES
Chapter Two
Figure 2.1 Evolution of silicon solar cell efficiency [36] ....................................... 7
Figure 2.2 In the wavelength regime 300 nm < λ < 1200 nm, which is relevant to
silicon solar cell operation, the absorption coefficient 𝛼0 and correspondingly the
absorption length 𝐿𝛼 of c-Si strongly depend on wavelength. [39]...................... 10
Figure 2.3 Schematic illustration of optical loss processes in a solar cell [43]. ... 12
Figure 2.4 Comparison of light travelling path on flat and textured silicon surfaces.
............................................................................................................................... 14
Figure 2.5 Silicon surface structures fabricated by (a) KOH etching, (b) catalystassisted chemical etching, (c) plasma etching and (d) femtosecond laser induced
texturing [25] [48]. ................................................................................................ 15
Figure 2.6 Schematic diagram illustrating reflection and transmission of light for a
pyramidal textured silicon solar cell [43]. ............................................................ 17
Figure 2.7 Schematic illustration of plasmon collective oscillation of a spherical
gold colloid, showing the displacement of the conduction electron charge cloud
relative to the nuclei [54]. ..................................................................................... 20
Figure 2.8 SPP excitation configurations: (a) Otto geometry (b) Kretschmann
geometry, (c) diffraction on a grating, and (d) diffraction on surface features [60].
............................................................................................................................... 21
Figure 2.9 Rayleigh expansion for the diffracted fields [70]. .............................. 25
Chapter Three
Figure 3.1 Schematic of the experimental setup used for laser micro-lens array
lithography. ........................................................................................................... 28
ix
List of figures
Figure 3.2 Schematic illlustration of a standing wave generated by the
interference of two coherent laser beams.............................................................. 31
Figure 3.3 Schematic illlustration of a Lloyd's mirror setup for laser interference
lithography of periodic structures on photoresist.................................................. 32
Figure 3.4 Scanning electron micrographs showing (a) the arrays of silicon
nanowires prepared by using inductively coupled plasma etching [81]; and (b) the
arrays of silicon nanopillars fabricated by optical lithography and reactive ion
etching [82]. .......................................................................................................... 33
Figure 3.5 Scanning electron micrographs of Ag–Si after treatment in an aqueous
solution containing (a) 5.3 M HF and 0.18 M H2O2 for 1 min; (b) 5.3M HF and
1.8M H2O2 for 1 min. Inset shows an enlarged image at the top surface region
[89]. ....................................................................................................................... 35
Figure 3.6 Measurement of the film thickness using a step profiler. .................... 39
Figure 3.7 Schematic drawing of an electron beam evaporator. ........................... 42
Figure 3.8 Process flow for the single-layer lift-off process................................. 45
Figure 3.9 Schematic illustration of catalyst assisted wet etching process........... 47
Figure 3.10 Schematic diagram depicting the experimental setup for catalyst
assisted wet etching............................................................................................... 48
Chapter Four
Figure 4.1 Optical microscope top view images of fabricated (a) silicon concentric
nano-rings array and (b) silicon nanowires array.................................................. 51
Figure 4.2 SEM micrographs of silicon concentric nano-rings arrays fabricated at
heights of (a) 750 nm, (b) 3 µm and (c) 12µm. .................................................... 53
Figure 4.3 SEM micrographs of silicon nanowires array fabricated with (a) 300
nm diameter and pillars’ height of (b) 300 nm, (c) 1 µm, (d) 6 µm, (e) 12 µm and
(f) 40 µm. .............................................................................................................. 54
Figure 4.4 SEM image of silicon nanowires array fabricated with (a) less than
half period, (b) half period and (c) larger than half period diameters of the pillars
at the same height.................................................................................................. 56
x
List of figures
Figure 4.5 Reflectance spectra of silicon concentric nano-rings arrays with (a)
750 nm, (b) 3 µm and (c) 12 µm ring heights. ...................................................... 58
Figure 4.6 Measured reflectance spectra of silicon nanowires arrays with (a) 1
µm, (b) 6 µm, (c) 12 µm and (d) 40 µm pillar heights. ........................................ 61
Figure 4.7 Measured reflectance spectra of silicon nanowires array with pillar
diameters of (a) less than, (b) equal to and (c) larger than half period. ................ 63
Figure 4.8 Schematically cross section view of LIL-based lift -up process. ........ 64
Figure 4.9 Schematic process flow for the fabrication of 3D silicon nanostructures array fabrication without using a lift off process.................................. 65
Figure 4.10 SEM micrograph of a "mushroom"-shape silicon nanowires array. . 67
Figure 4.11 Measured reflection spectra of silicon nanowires array with and
without the "mushroom"-shape crowns. ............................................................... 68
Figure 4.12 Measured spectrally resolved reflectance of planar silicon, normal
and “mushroom”-shape silicon nanowires. ......................................................... 70
Figure 4.13 Measured spectrally resolved absorption spectra of planar, normal
and “mushroom”-shape silicon nanowires. ......................................................... 72
Figure 4.14 Plot of the simulated absolute value of square of electric field in ydirection (abs(Ey)2) for (a) planar and (b) “mushroom”-shape nanowires structures
at wavelengths of 300 nm and (c) 1000 nm. ......................................................... 75
Chapter Five
Figure 5.1 (a) Top view and (b) cross section SEM images of the KOH etched
silicon surface. ...................................................................................................... 80
Figure 5.2 (a) Schematic diagram of the laser ablation for silicon surface texturing
and (b) a SEM image of the resulting c-Si surface. .............................................. 82
Figure 5.3 Schematic of silicon nanowires array with metallic nanoparticles'
decoration. ............................................................................................................. 83
Figure 5.4 SEM images of (a) Ag, (b) Au, (c)Ag/Au and (d) Cu/Ag/Au metallic
nanoparticles' decoration of flat silicon surfaces. ................................................. 84
xi
List of figures
Figure 5.5 SEM images of metallic nanoparticles' decoration of the KOH etched
silicon surface. ...................................................................................................... 85
Figure 5.6 SEM images of the laser-textured silicon surfaces being decorated with
(a) Ag/Au, and (b) Cu/Ag/Au alloy nanoparticles ................................................ 86
Figure 5.7 SEM images of metallic nanoparticles' decoration of the silicon
nanowires array at heights of (a) 500 nm, (b) 3 µm, (c) 12 µm and (d) 40 µm. ... 88
Figure 5.8 Measured reflection spectra of flat silicon surfaces with and without
the metallic nanoparticles' decoration. .................................................................. 89
Figure 5.9 Measured reflection spectra of the KOH etched silicon surface with
and without metallic nanoparticles' decoration. .................................................... 92
Figure 5.10 Measured reflection spectra of laser textured silicon surfaces with and
without metallic nanoparticles' decoration............................................................ 93
Figure 5.11 (a) SEM image and (b) Measured reflection spectra of silicon
nanowires arrays with and without metallic nanoparticles' decoration................. 95
Figure 5.12 Measured reflection spectra of 320 µm-thick laser-textured Si front
surfaces without and with the backside surface texturing. .................................... 98
Figure 5.13 Measured reflection spectra of 320, 530, 890 µm-thick front side
laser-textured Si sample with the backside surface texturing. .............................. 99
xii
List of symbols
LIST OF SYMBOLS
c-Si
Crystalline silicon
PV
Photovoltaic
ARC
Anti-reflection Coating
RI
Refractive Index
NPs
Nanoparticles
EM
Electromagnetic
FCA
Free Carrier Absorption
SPR
Surface Plasmon Resonance
SPPs
Surface Plasmon polaritons
TIR
Total Internal Reflection
UV
Ultraviolet
IR
Infrared
NIR
Near Infrared
°C
Degree Celcius
VLS
Vapor-Liquid-Solid
CE
Catalytic Etching
xiii
List of symbols
MLA
Micro-lens Array
ROC
Radius of Curvature
LIL
Laser Interference Lithography
D
Grating dimension
P
Periodicity
NA
Avogadro’s constant
n
Refractive index
λ
Wavelength
k
Boltzmann constant
θ
Two light waves intersect angle
h
Arrays thickness
OM
Optical Microscopy
SEM
Scanning Electron Microscopy
RCWA
Rigorous coupled-wave analysis
xiv
Chapter 1 Introduction
CHAPTER 1
INTRODUCTION
1.1
Background
Silicon surfaces covered by a layer of micro- or nano-structures, which effectively
suppresses reflection, while simultaneously enhance the scattering and absorption
of light, have attracted great interest for their fundamental properties and
potentially practical applications [1-4]. Ever-increasing demand for high
efficiency of various optical and opto-electronic devices has driven the design and
fabrication to achieve excellent anti-reflection performance [1-4]. Among the
various opto-electronic devices, solar cell has attracted much attention. Solar
electricity, as a source of clean and renewable energy free from contaminations
and carbon emissions, is regarded as one of the best solutions to replace fossil based electricity for future electricity supply [5, 6]. The direct conversion from
solar energy to electricity can be realized by solar cells based on the photovoltaic
(PV) effect [7]. When light, intrinsically electromagnetic (EM) wave, impinges on
a different media, it is partially reflected due to the refractive index (RI)
mismatching. In this case, high reflection at the interfaces affects the performance
of the devices [8-11]. Therefore, the surface anti-reflection becomes an important
factor to enhance its cell efficiency [12-14]. In order to minimize unwanted
reflection and improve the light absorption efficiency, different types of anti1
Chapter 1 Introduction
reflection surfaces have been widely studied for solar cells, like anti-reflection
thin film deposition and surface texturing. Among all of these, nano-structures
have received steadily growing research interest due to their unique optical
properties and potential applications superior to their bulk counterparts [15-17].
Recent advances in surface micro/nano-fabrication and thin film deposition
technologies provide versatile approaches to decorate silicon surfaces with
engineered structures to reduce optical reflection, especially for silicon solar cells
with an absorber layer of a few micrometers.
Following Feynman’s challenge that “there is plenty of room at the bottom” [18],
the capability in sculpting silicon with extraordinary precision and efficiency is
very much needed for the development of nanotechnology. Extensive effort has
been devoted by various research groups in the quest for greater structural control
at the nanometer level [19] to precisely generate such miniscule structures. The
resulting nano-structures show complex and interesting optical properties, which
lead to a large number of opportunities, including but not limited to the potential
applications in the areas of optical and opto-electrical devices.
1.2
Motivation
For monocrystalline silicon solar cells, the most commonly adopted light trapping
structures are pyramidal textures [20, 21], which have feature sizes of a few
micrometers (2~10 µm). However, with reducing the thickness of silicon solar
cells to no thicker than a few micrometers, they are no longer favored as absorber
2
Chapter 1 Introduction
layers. Meanwhile, the crystalline silicon (c-Si) solar cells exhibit a comparably
weaker absorption at near-bandgap spectrum due to the indirect bandgap of
silicon, which results in a narrow absorption spectrum. To bypass these
limitations, nano-scale textured silicon surface provides the better solution for
omnidirectional and broadband anti-reflection [2, 22-28]. As photon management
schemes for c-Si solar cells with thin absorber layers, such silicon nano-structures
usually possess unique photon management properties to compensate for light
absorption loss when arranged into random or regular arrays.
With the fabrication of surface nano-structure arrays on silicon substrates, high
optical absorption is indicated because of the strong light trapping by multiple
scattering of the incident light among silicon nano-structures and the optical
antenna effect [29-32]. Therefore, the surface anti-reflection properties can be
further improved by the effective graded refractive index (RI) from subwavelength structures. The effective RI depends on the volume fraction of silicon
and air to compensate the RI mismatching at the interface [33, 34]. This causes
low reflectance and eliminates antireflective coating step required in the
manufacturing process. With further adopting the metallic nanoparticles together
with the nano-structures, a broadband reflection spectrum can be promised. Hence,
the fabrication of c-Si solar cells decorated with three-dimensional (3D) nanostructures, which vary in features, has attracted a great deal of interest among
researchers as it renders the design of high-efficiency solar cells with possibly
reduced material cost.
3
Chapter 1 Introduction
1.3
Research objectives
The aim of this study was to explore the techniques for the fabrication of precisely
located and well-arranged 3D nano-structures arrays on silicon using micro-lens
array (MLA) and laser interference lithography (LIL). To further improve the
antireflection performance and light trapping efficiency of silicon based solar
cells.
The focus of this research can be divided into several parts. Firstly, this study
focuses on the large-area synthesis of silicon nanowire arrays with tunable size,
length and period. This approach makes use of interference lithography,
anisotropic etching of silicon, electron-beam (E-beam) evaporation of Ag and Au
layer, lift-off, and catalyst assisted etching.
Secondly, a study is carried out to investigate the optical performance of the
silicon nano-structured surfaces when various surface geometries were applied.
The methods of MLA, LIL and multiple etching are adopted to fabricate the
nanorings array and “mushroom”-shape silicon nanowires with varying feature
sizes and periods.
Finally, in order to reduce the reflection in a broadband region, metallic
nanoparticles are deposited to the aforementioned nano-structures. The tunability
in the size and distribution of the synthesized metallic nanoparticles are examined
by varying the deposited film thickness and annealing temperature. Meanwhile, a
back surface texturing is applied to further enhance the reflection reduction at a
broadband range.
4
Chapter 1 Introduction
1.4
Organization of thesis
This thesis is divided into six chapters and their contents are listed as follows:
Chapter One gives an introduction on the anti-reflection performance of optical
and opto-electrical devices, especially solar cells. An introduction of the c-Si
surface micro/nano-structures fabrication is indicated. The motivation, objective
and contribution of this study are also addressed.
Chapter Two describes the fundamental physics of antireflection for silicon based
solar cell. The optical properties of silicon solar cell are introduced. The current
antireflection and light trapping techniques are discussed with their advantages
and limitations. A theoretical introduction of light scattering and absorption
affected by metallic nanoparticles deposited on the substrate are also included.
Chapter Three describes the methods and techniques of the fabrication process. A
detailed procedure of the fabrication of silicon micro/nano-structures arrays by
laser processing is presented.
Chapter Four investigated the fabricated silicon surface structures for their antireflection performance. The reflection in the silicon surface nano-structures arrays
are shown according to different designs of their features in both experimental
measurement and numerical simulation.
Chapter Five investigates the anti-reflection enhancement of the silicon
micro/nano-structures by two means, which are metallic nanoparticles deposition
and back surface texturing. Their anti-reflection performance is discussed.
5
Chapter 1 Introduction
Chapter Six provides the conclusions of this study as well as recommendation for
future work.
6
Chapter 2 Background and literature review
CHAPTER 2
BACKGROUND AND LITERATURE REVIEW
2.1
Silicon solar cells
2.1.1
Introduction
Silicon wafer solar cells currently dominate the photovoltaic market with a market
share of ~ 90%. The first silicon wafer solar cell with a pn junction was fabricated
in the 1950s at Bell Labs by Chapin et al [35]. This solar cell only had an energy
conversion efficiency of 6% but as can be seen in Figure 2.1 the efficiency of the
c-Si wafer solar cell has been improved significantly in recent years up to 25.6%
[36].
Figure 2.1 Evolution of silicon solar cell efficiency [36]
7
Chapter 2 Background and literature review
Optimizing the solar cell efficiency is all about minimization of losses. Therefore,
the boost of silicon solar cells efficiency comes with its priority in our study. The
dominant losses of a silicon wafer solar cell are optical, resistive, and
recombination losses. In this thesis we will only focus on a reduction of optical
losses particularly for ultrathin c-Si solar cells.
2.1.2
Optical properties of silicon
When light impinges on the silicon surface, it is partially reflected due to the
optical contrast between c-Si and air. The reflection coefficient 𝑅𝑆𝑖 can be
calculated by the well know Fresnel equations. The transmitted light penetrates
into the material, where it is attenuated due to absorption. The rate of absorption
is determined by the law of Lambert-Beer,
𝐼(𝑥 ) = (1 − 𝑅𝑆𝑖 )𝑒 (−𝛼𝑒𝑓𝑓 𝑥) 𝐼0
（2.1）
which characterizes the intensity of a light beam that has traveled a distance 𝑥
inside a material with effective absorption coefficient 𝛼𝑒𝑓𝑓 , relative to an incident
intensity 𝐼0 . The absorbed photons either lose their energy by generating
electron/hole pairs at a rate 𝐺 = 1⁄𝑡 (optical generation), where 𝑡𝑒 is the
𝑒
excitation time constant, or by exciting free carriers (free carrier absorption, FCA).
The excited carriers eventually thermalize and transfer their energy as heat in the
lattice by carrier/lattice collisions. The optically generated electron/hole pairs
eventually recombine radiatively, or likewise produce heat by non-radiative
8
Chapter 2 Background and literature review
recombination, unless they are extracted out of the material as e.g. happens in a
solar cell. The time constant for recombination is denoted by the carrier lifetime 𝜏.
The most important quantities and dependencies of the energy transfer of light to
silicon will be described as following.
Absorption by optical generation of charge carriers
In opto-electric semiconductors, the generation of electron/hole pairs occurs for
incident photons with energies ℎ𝜈 > 𝐸𝑔 . The generation rate is 𝐺 =
𝐼𝜎𝑜𝑝𝑡
⁄ ,
ℎ𝜈
with 𝐼 being the absorbed intensity, 𝜎𝑜𝑝𝑡 the optical absorption cross section and
ℎ𝜈 the photon energy. For silicon, the photon energies required for optical
generation are ℎ𝜈 > 𝐸𝑔 = 1.12 eV (corresponding to 𝜆 = 1107 nm).
However, silicon is an indirect band gap semiconductor, as the energy maximum
of the valence band and the energy minimum of the conduction band are located
at different values of crystal momentum [37]. Therefore, the absorption of a
photon with an energy below the direct bandgap of c-Si requires an interaction
with the crystal lattice (phonon absorption or emission), which leads to a strong
dependence of the band-to-band absorption coefficient 𝛼0 on 𝜆. Only for photon
energies ℎ𝜈 > 3.4 eV ( 𝜆 = 365 nm), corresponding to the direct band gap of
silicon, the absorption coefficient saturates around 𝛼0 ≈ 106 cm−1. Figure 2.2
depicts 𝛼0 as well as the corresponding penetration depth 𝐿𝛼 in the range 250 nm
< 𝜆 < 1300 nm [38].
9
Chapter 2 Background and literature review
Figure 2.2 In the wavelength regime 300 nm < λ < 1200 nm, which is relevant to silicon
solar cell operation, the absorption coefficient 𝛼0 and correspondingly the absorption
length 𝐿 𝛼 of c-Si strongly depend on wavelength. [39]
Absorption by free charge carriers
The absorption coefficient 𝛼0 can be significantly increased by the presence of a
large number of free charge carriers 𝑁𝑓𝑐𝑐 due to FCA. In such a case, an effective
absorption coefficient 𝛼𝑒𝑓𝑓 = 𝛼0 + 𝑁𝑓𝑐𝑐 𝜎𝐹𝐶 holds, with 𝜎𝐹𝐶 as the FCA cross
section. As 𝜎𝐹𝐶 increases approximately with 𝜆2 , FCA predominantly plays a role
for infrared light [40] and is therefore relatively less important in normal solar cell
operation. Relevant data is also found e.g. in Refs. [41] or [42].
10
Chapter 2 Background and literature review
As mentioned above, free carriers are either created optically or, alternatively, by
doping, due to the dependence of the intrinsic carrier density 𝑛𝑖 on the
temperature according to
𝑛𝑖 (𝑇) = √𝑁𝐶 𝑁𝑉 𝑒 (−𝐸𝑔 ⁄2𝑘𝑇 )
（2.2）
with 𝑁𝐶 and 𝑁𝑉 as the effective densities of states for conduction and valence
bands, respectively.
2.1.3
Optical loss
When producing high efficiency solar cells, optical loss, which lowers the shortcircuit current, becomes one of the most significant restriction of improving the
power from a solar cell. Figure 2.3 schematically illustrates the main sources of
optical losses. It consists of light which could have generated an electron-hole
pair, but on the contrary, is reflected from the front surface or not absorbed inside
the solar cell, such as parasitic absorption losses. All these optical loss processes
should be minimized in order to optimize the performance of the solar cell.
11
Chapter 2 Background and literature review
Figure 2.3 Schematic illustration of optical loss processes in a solar cell [43].
However, in realistic manufacturing of solar cell, optical loss is yet to be
eliminated. Therefore, in order to achieve more promising performance of the
cells, a number of ways can be applied to reduce the optical loss:
Top contact coverage of the cell surface can be minimized for reducing
light reflection at the surface contacts. However, this potentially increases the
resistive losses of the solar cell by an increase in series resistance.
Anti-reflection coatings (ARC) can be used on the top surface of the cell.
However, the ARC film is limited at one specific wavelength due to its fixed RI.
By surface texturing, the reflection of the incoming light can be
significantly reduced.
12
Chapter 2 Background and literature review
The solar cell can be made thicker to increase absorption. The optical path
length in the solar cell may be increased by a combination of surface texturing at
the front and rear surface.
The reflection of a polished silicon surface in air is over 30% due to its high
refractive index. The reflectivity R between two materials of different refractive
indices for normal incidence is determined by:
𝑅=(
𝑛0 − 𝑛𝑆𝑖 2
)
𝑛0 + 𝑛𝑆𝑖
（2.3）
where 𝑛0 is the refractive index of the surroundings and 𝑛𝑆𝑖 the complex
refractive index of silicon [43]. For an unencapsulated cell, 𝑛0 = 1. For an
encapsulated cell, 𝑛0 = 1.5 (RI of glass).
2.1.4
Surface texturing
For optical and photovoltaic applications, surface texturing, either in combination
with an ARC layer or by itself, is commonly applied to reduce surface reflection
and enhance the light travelling path as well. The textured surface of a solar cell,
by means of “roughening”, reduces reflection by i ncreasing the chance that
reflected light has another interaction with the sample, instead of being lost to the
surrounding [44]. Figure 2.4 shows this principle schematically for a standard
pyramid textured surface where most of the light undergoes.
13
Chapter 2 Background and literature review
Figure 2.4 Comparison of light travelling path on flat and textured silicon surfaces.
Researchers have demonstrated several processing methods to modify the surface
morphology of silicon surfaces [1, 8, 10, 25, 45, 46]. The etching methods,
including chemical, electrochemical and dry etching, have been widely employed
[47, 48] as shown in Figure 2.5.
Dynamic etching of porous silicon surfaces at the thickness of 100 nm was
demonstrated by Striemer et al. to achieve an average reflection of 3.7% across
the terrestrial solar spectrum [48]. Laser ablation also provides another dry
processing to fabricate Si surfaces for anti-reflection [23-25, 27, 28]. Black Si
surfaces fabricated by short pulse lasers were studied by Mazur et al. [25] Conical
shape surface structures can be created by the laser processing, resulting in a
reflection below 5% in the visible range of the spectrum.
14
Chapter 2 Background and literature review
Figure 2.5 Silicon surface structures fabricated by (a) KOH etching, (b) catalyst-assisted
chemical etching, (c) plasma etching and (d) femtosecond laser induced texturing [25]
[48].
2.1.5
Light trapping
The need to absorb all the light is not the only parameter that needs to be
considered when deciding your solar cell device thickness. Also the bulk minority
carrier diffusion length should be taken into account. When the bulk minority
carrier diffusion length is much shorter than the thickness of the solar cell
increasing the thickness of the solar cell will not increase the short circuit current
density. In addition, a thinner solar cell may have a higher voltage by reducing the
voltage losses due to the recombination. Consequently, an optimized solar cell
structure will typically have effective "light trapping" in which the optical path
15
Chapter 2 Background and literature review
length is several times of the actual device thickness. The optical path length of a
device, which is usually defined in terms of device thickness, refers to the total
distance a photon could travel within the structure before it exits the device at the
front or rear surface. A solar cell with no light trapping features may have an
optical path length that is only slight higher than the device thickness, while on
the other hand, one with good light trapping may have an optical thickness of
more than 50 times of device thickness.
Effective light trapping is usually achieved by varying the angle at which light
travels in the solar cell. Applying an angled surface is one way. A textured surface
will not only reduce reflection as previously described but also couple light
obliquely into the silicon, thus giving a longer optical path length than the
physical device thickness. The angle at which light is refracted into the
semiconductor material is as follows, according to Snell's Law,
𝑛1 sin 𝜃1 = 𝑛2 sin𝜃2
(2.4)
where 𝜃1 and 𝜃2 are the angles for the light incident on the interface relative to the
normal plane of the interface within the mediums with refractive indices 𝑛1 and
𝑛2 , respectively.
In a textured single crystalline solar cell with upward pyramids, the presence of
crystallographic planes make the angle 𝜃1 equal to 36 degree as shown in Figure
2.6. The light travelling path inside the device is thus increased by the texture.
16
Chapter 2 Background and literature review
Figure 2.6 Schematic diagram illustrating reflection and transmission of light for a
pyramidal textured silicon solar cell [43].
The amount of light reflected at an interface is calculated from the Fresnel
reflection formula. For light polarized in parallel with the surface, the amount of
reflected light is:
tan2 (𝜃1 − 𝜃2 )
𝑅∥ =
tan2 (𝜃1 + 𝜃2 )
(2.5)
For light polarised perpendicular to the surface, the amount reflected is:
𝑅⊥ =
sin2 (𝜃1 − 𝜃2 )
sin2 (𝜃1 + 𝜃2 )
(2.6)
For unpolarised light, the reflected amount is the average of the two:
𝑅𝑇 =
𝑅∥ + 𝑅⊥
2
17
(2.7)
Chapter 2 Background and literature review
If light passes from a high refractive index medium to a low refractive index
medium, there is the possibility of total internal reflection (TIR). The angle at
which occurs is the critical angle and is found by setting 𝜃2 to 0. Using the TIR,
light can be trapped inside the cell with multiple passes through the cell, thus
allowing a high optical path length at the reduced thickness of the device.
2.2
Light scattering by nanoparticles (NPs)
By the fabrication of the sub-wavelength structures, higher light absorption is
achieved in visible and NIR range, which is significant for solar cells [47, 49, 50].
Sub-wavelength structures, e.g. hybrid moth-eye structures [34] and ZnO coated
Si nano-cone [51], have been shown to exhibit ultra-low reflection by numerical
simulations. Another point worth mentioning for anti-reflection performance of Si
solar cell is metallic nanoparticles (NPs) induced surface plasmon resonance
(SPR). With EM wave excitation, metallic NPs can be used in the anti-reflection
surfaces to excite localized SPR to increase optical absorption by light trapping
[2-4, 8, 22, 52]. The localized EM field around metallic NPs can be significantly
enhanced at the resonance [53]. The light scattering properties become dominant
for NPs at a large size. Both effects contribute to the anti-reflection performance.
Generally, Au and Ag NPs are mostly used for their SPR excitations at visible
range. To reduce the material cost for anti-reflection surface fabrication, other
metallic NPs, such as Al and Cu, were also investigated. They are more suitable
for the applications in UV and IR ranges at their SPR resonances [4, 52]. To
18
Chapter 2 Background and literature review
reduce optical reflection in a wide wavelength range, alloyed NPs, like Ag-Au
NPs, were also applied [8, 22]. Nano-structured metallic particles show complex
and interesting properties, which contribute to anti-reflection performance for
photovoltaic applications by adopting surface plasmon resonance (SPR).With
electromagnetic (EM) wave excitation, the deposited metallic NPs excite
localized SPR, thereby enhance optical absorption by light trapping.
2.2.1
Properties of surface plasmons
Surface plasmon polaritons (SPPs), often referred to as surface plasmons (SPs),
are resonant electromagnetic field which are strongly confined to metallic
surfaces that enable them to sustain coherent electron oscillations. These
electromagnetic surface waves arise via the coupling of the electromagnetic fields
to the electron plasma oscillations of the conductor [54, 55].
Localized surface plasmons (LSPs), a type of SPs, are charged density oscillations
confined to metallic nanoparticles (sometimes referred to as metal clusters) and
metallic nano-structures [54]. LSPs are non-propagating excitations of the
conduction
electrons
of
the
metallic
nano-structures
coupled
to
the
electromagnetic field. We will see that these modes arise naturally from the
scattering problem of a small and sub-wavelength conductive nanoparticle in an
oscillating electromagnetic field. The curved surface of the particle exerts an
effective restoring force on the driven electrons, so that a resonance can arise,
leading to a field amplification both inside and in the near-field zone outside the
19
Chapter 2 Background and literature review
particle. This resonance is called the localized surface plasmon or short localized
plasmon resonance [56]. Another consequence of the curved surface is that
plasmon resonances can be excited by direct light illumination of appropriate
frequency irrespective of the wave vector of the exciting light. In contrast, an SPP
mode can only be excited when both the frequency and wave vector of the
exciting light match the frequency and wave vector of the SPP [57, 58].
Figure 2.7 Schematic illustration of plasmon collective oscillation of a spherical gold
colloid, showing the displacement of the conduction electron charge cloud relative to the
nuclei [54].
A typical example is shown in Figure 2.7, where the conduction electrons of a
spherical gold colloid oscillate coherently in response to the electric field of the
incident light [54]. Excitation of LSPs by an electric field (light) at an incident
wavelength, where resonance occurs, results in strong light scattering, in the
appearance of intense SP absorption bands, and an enhancement of the local
electromagnetic field. The frequency and intensity of the SP absorption bands are
characteristic of the type of materials (typically, gold, silver, or platinum), and are
highly sensitive to the size, its distribution, and shape of the nano-structures, as
well as to its surrounding environment [59].
20
Chapter 2 Background and literature review
2.2.2
Surface plasmon excitation
Figure 2.8 SPP excitation configurations: (a) Otto geometry (b) Kretschmann geometry,
(c) diffraction on a grating, and (d) diffraction on surface features [60].
As seen from the SPP dispersion relations, the SPP wavevector is larger than the
photon wavevector in the adjacent dielectric medium. Thus, light illumination a
smooth surface cannot be directly coupled to surface polaritons. Special
experimental arrangements have been designed to provide the conservation of the
wave vector. The photon and SPP wavevector can be matched using either photon
tunneling in the total internal reflection geometry (e.g. by a prism) or diffraction
effects (e.g. via a grating or on surface defects) as shown in Figure 2.8.
21
Chapter 2 Background and literature review
There are three main techniques by which the missing momentum can be
provided. The first makes use of prism coupling to enhance the momentum of the
incident light [61, 62]. The second makes use of a periodic corrugation in the
surface of the metal [63]. The third involves scattering from a topological defect
on the surface, such as a sub-wavelength protrusion or hole, which provides a
convenient way to generate SPs locally [59, 64].
In the prism coupling technique, incident light passes thro ugh an optically dense
medium. In this case, a prism, to increase its wave vector momentum. Under
suitable wavelength and angles, total internal reflection (TIR) can be achieved
where the incident beam reflects off at an interface between the optically dense
glass and less dense dielectric [Otto configuration as shown Figure 2.8 (a)] or
metallic layer [Kretchmann configuration as shown in Figure 2.8 (b)]. Although
no light comes out of the prism in TIR, the electrical field of the photons extends
about a quarter of a wavelength beyond the reflecting surface. The coupling gap
provides the evanescent tunnel barrier across which the radiation couples,
allowing the surface plasmons to be excited at the dielectric metal interface [60].
The second method to overcome momentum mismatch is periodic corrugation,
such as grating coupling, as shown in Figure 2.8 (c). This involves incident light
being directed towards a grating with spatial periodicity similar to the incident
irradiation. The incident beam is either diffracted away from the grating or
produces an evanescent mode that travels along the interface. This evanescent
mode has wave vectors parallel to the interface with reciprocal lattice vectors
22
Chapter 2 Background and literature review
added or subtracted from it. Numerically, these vectors are represented by
2𝑛𝜋
𝐷
,
where n is an integer and D gratings dimensions [65].
On a rough surface, the SPP excitation conditions can be achieved without any
special arrangements. Diffraction of light on surface features can provide
coupling to the SPP modes on both the air-metal and glass-metal interfaces
[Figure 2.8 (d)]. This is possible since in the near field region, all wave vectors of
diffractive components of light are present [65-67]. Thus, SPPs can be excited in
conventionally illuminated rough surfaces. The problem with random roughness
is the irregular SPP excitation conditions, resulting in the low efficiency of lightto-SPP coupling. This is a non-resonant excitation and there is a strong presence
of the reflected excitation light close to the surface. Depending on the metal film
thickness and depth of the defect, SPPs can be excited on both interfaces of the
film. Such non-resonant SPP excitation processes result in a complex field
distribution over the surface due to interference of SPPs excited on different
interfaces of the film and the illumination light [67].
2.3
Numerical simulation
In this study, the Rigorous Coupled Wave Analysis (RCWA) [68, 69] method was
applied to investigate the optical properties of the silicon nano-structures. This
numerical simulation provides an insight of reflection and absorption conditions
in the nano-structure as well as its electric-field distribution.
23
Chapter 2 Background and literature review
RCWA is a rigorous method to solve Maxwell’s equations by performing a
Fourier transform of both the electromagnetic field and the structure. In general
terms, RCWA solves the diffraction problem by a grating defined by a stack of
layers which have all identical periods in the x- and y-directions. When defining
the structures, a single period is approximated as a stack of uniform layers in the
z-direction and repeated infinitely in x- and y-directions to form the whole
structure, which describes a 3D spatial variation of the complex refractive index.
Reflectance R, transmittance T and absorptance A of the structures can then be
retrieved. The calculation returns the diffraction efficiencies of the transmitted
and reflected orders for an incident plane wave from the top and for an incident
plane wave from the bottom, both for TM and TE polarizations, by the grating of
the following incident plane wave:
i.
If incident from the top layer,
𝑖𝑛𝑐 exp [𝑖 (𝑘 𝑖𝑛𝑐 𝑥 + 𝑘 𝑖𝑛𝑐 𝑦 + 𝑘 𝑖𝑛𝑐 (𝑧 − ℎ) )]
𝐸𝑡𝑜𝑝
𝑥
𝑦
𝑧 𝑡𝑜𝑝
(2.8)
𝑖𝑛𝑐
𝐻𝑡𝑜𝑝
exp [𝑖 (𝑘𝑥𝑖𝑛𝑐 𝑥 + 𝑘𝑦𝑖𝑛𝑐 𝑦 + 𝑘𝑧𝑖𝑛𝑐
𝑡𝑜𝑝 (𝑧 − ℎ) )]
(2.9)
2
2
2
𝑖𝑛𝑐 − 𝑘 𝑖𝑛𝑐 ;
where 𝑘𝑧𝑖𝑛𝑐
𝑦
𝑡𝑜𝑝 = −√(2𝜋 ∗ 𝑛 𝑡𝑜𝑝 ⁄𝜆 ) − 𝑘𝑥
ii.
If incident from the bottom layer,
𝑖𝑛𝑐
𝐸𝑏𝑜𝑡𝑡𝑜𝑚
exp[𝑖(𝑘𝑥𝑖𝑛𝑐 𝑥 + 𝑘𝑦𝑖𝑛𝑐 𝑦 + 𝑘𝑧𝑖𝑛𝑐
𝑏𝑜𝑡𝑡𝑜𝑚 𝑧)]
(2.10)
𝑖𝑛𝑐
𝐻𝑏𝑜𝑡𝑡𝑜𝑚
exp[𝑖(𝑘𝑥𝑖𝑛𝑐 𝑥 + 𝑘𝑦𝑖𝑛𝑐 𝑦 + 𝑘𝑧𝑖𝑛𝑐
𝑏𝑜𝑡𝑡𝑜𝑚 𝑧)]
(2.11)
2
2
𝑖𝑛𝑐 − 𝑘 𝑖𝑛𝑐 .
2
where 𝑘𝑧𝑖𝑛𝑐
𝑦
𝑏𝑜𝑡𝑡𝑜𝑚 = −√(2𝜋 ∗ 𝑛 𝑏𝑜𝑡𝑡𝑜𝑚 ⁄𝜆 ) − 𝑘𝑥
24
Chapter 2 Background and literature review
The z-component of the Poynting vector of the incident plane wave is applied
with ±0.5 in both condition for simulation.
The Rayleigh-expansion of the diffracted electric fields is shown in the following
figure.
Figure 2.9 Rayleigh expansion for the diffracted fields [70].
𝑑𝑖𝑓
𝑚,𝑛
𝐸𝑡𝑜𝑝
= ∑ 𝐸𝑡𝑜𝑝
exp [𝑖 ((𝑘𝑥𝑖𝑛𝑐 + 𝑚𝐾𝑥 )𝑥 + (𝑘𝑦𝑖𝑛𝑐 + 𝑛𝐾𝑦 )𝑦 + 𝑘𝑧𝑖𝑛𝑐
𝑡𝑜𝑝 (𝑧 − ℎ))]
(2.12)
𝑑𝑖𝑓
𝑚,𝑛
𝐻𝑡𝑜𝑝
= ∑ 𝐻𝑡𝑜𝑝
exp [𝑖 ((𝑘𝑥𝑖𝑛𝑐 + 𝑚𝐾𝑥 )𝑥 + (𝑘𝑦𝑖𝑛𝑐 + 𝑛𝐾𝑦 )𝑦 + 𝑘𝑧𝑖𝑛𝑐
𝑡𝑜𝑝 (𝑧 − ℎ) )]
(2.13)
25
Chapter 2 Background and literature review
2
𝑖𝑛𝑐
𝑖𝑛𝑐
2
2 ,
√
⁄
where 𝑘𝑧𝑚,𝑛
𝑡𝑜𝑝 = − (2𝜋 ∗ 𝑛 𝑡𝑜𝑝 𝜆 ) − (𝑘𝑥 + 𝑚𝐾𝑥 ) − (𝑘𝑦 + 𝑛𝐾𝑦 )
𝐾𝑥 =
(2𝜋) /𝑝𝑒𝑟𝑖𝑜𝑑_𝑥 and 𝐾𝑦 = (2𝜋) /𝑝𝑒𝑟𝑖𝑜𝑑_𝑦, and they are the unit vectors along x
and y axis. The (m,n)th order has a parallel momentum equal to (𝑘𝑥𝑖𝑛𝑐 + 𝑚𝐾𝑥 )
→ +(𝑘𝑦𝑖𝑛𝑐 + 𝑛𝐾𝑦 ) →. We define two points Otop= (0,0,h) at the top of the grating,
𝑥
𝑦
and Obottom= (0,0,0) at the bottom of the grating [70].
26
Chapter 3 Experimental setup and fabrication details
CHAPTER 3
EXPERIMENTAL SETUP AND FABRICATION
DETAILS
3.1
Introduction
In this chapter, the fabrication methods of laser processed silicon nano-structures,
including laser micro-lens array (MLA) and laser interference lithography, are
discussed. Meanwhile, detail experimental setup and procedures employed for the
fabrication and characterization are presented. The fabrication processes include
spin-coating of photoresist, lithography, electron beam evaporation, lift-off and
catalytic etching. The characterization processes were carried out by microscope,
scanning electron microscope (SEM) and UV-Vis spectroscopy.
3.1.1
Laser micro-lens array (MLA) lithography
Laser micro-lens array (MLA) lithography is used to define the patterns on the
photoresist layer [71, 72]. MLA is a parallel exposure technique which can realize
more than 10,000 patterns simultaneously in a large area with a very short
exposure time. It is carried out by combining a micro-lens array and a UV
femtosecond laser. No mask is required as the patterns were realized by
27
Chapter 3 Experimental setup and fabrication details
controlling the stage movement and shutter switching, which could be used to
fabricate arbitrary patterns flexibly.
Figure 3.1 shows the experimental setup of the laser micro-lens array lithography.
The laser system used for the exposure was a Ti:Sapphire second harmonic
generation femtosecond laser (Spectra Physics Tsunami, Mode 3960, λ = 400 nm,
τ = 100 fs, repetition rate=82 MHz).
Figure 3.1 Schematic of the experimental setup used for laser micro-lens array
lithography.
The laser beam emitted from the laser head and passed through an attenuator
before it was directed to the reflective mirrors and irradiated on the sample
through the MLA. The beam splitter/attenuator was used to reduce the laser
28
Chapter 3 Experimental setup and fabrication details
energy going into the system. A mechanical shutter was used to control the
exposure time. The beam expander was used to enlarge the laser beam size,
making it cover the whole MLA with a uniform laser beam. The MLA is the key
part of this lithography system. It is used to split the single incident laser beam
into more than 10,000 tiny light beam arrays, which irradiate on the sample
surface simultaneously. In the exposure process, the sample surface and the MLA
must be kept exactly parallel, which was controlled by the tip and tilt anises of the
stage. This is a parallel fabrication technique which can shorten the fabrication
time significantly. The sample with photoresist coating was mounted on a 7-axis
translation nanopositioning stage with the numerical computer control.
Controlling the stage movement and the shutter switching by computer
programming, more than 10,000 designed micro-patterns can be fabricated on
photoresist simultaneously by laser exposure and photoresist develop.
Three pieces of MLA were used in the laser MLA lithography, which have the
periods of 30 µm, 75 µm and 100 µm. The MLA with a smaller radius of
curvature (ROC) has a smaller focusing point, which can produce the smaller
feature size of the structures. In this study, the MLA can fabricate the surface
structures with the feature size of less than 800nm. The difference of these three
pieces of MLA was the line width of the fabricated structures, which comes from
the ROC and numerical aperture [73].
3.1.2
Laser interference lithography (LIL)
29
Chapter 3 Experimental setup and fabrication details
3.1.2.1 Principle of LIL
When two monochromatic waves are superposed, the resulting intensity is the
sum of two individual intensities of these two waves. It is well known as the wave
interference [74]. Practically, the interference of two coherent light waves
generates sinusoidal varying standing wave patterns. LIL is a lithographic
technique using a laser as the light source to record the periodic patterns onto a
layer of light sensitive polymer materials, such as photoresist [75, 76]. It is a
maskless and simple patterning method to fabricate periodic, quasi-periodic and
spatially coherent structures over a large area [77-80]. Figure 3.2 shows a
schematic illustration of the resulting fringes generated by the interference of two
light beams that forms a standing wave. The resulting patterns are a series of
gratings with a certain periodic length. The factors that affect the period of the
structures are the light wavelength 𝜆 and the angle 𝜃 at which the two light waves
intersect, as given by Eq. (3.1).
𝜆
Standing wave period 𝑃 = 2sin(𝜃)
30
(3.1)
Chapter 3 Experimental setup and fabrication details
Figure 3.2 Schematic illlustration of a standing wave generated by the interference of
two coherent laser beams.
3.1.2.2 Lloyd’s mirror setup
There are several different setups for laser interference lithography and each setup
has its own advantages. Figure 3.3 shows the schematic drawing of the setup for
laser interference lithography by Lloyd’s mirror configuration. A laser beam from
a laser source is directed to a spatial filter by a set of mirrors. The spatial filter
consists of an objective lens and a pinhole with its diameter approximately equal
to or smaller than the focused laser beam size. The function of the spatial filter is
to filter out the high frequency noise so that a uniform Gaussian beam profile can
be obtained. Meanwhile, it also acts as a beam expander where the beam diameter
after the pinhole increases thus giving uniform beam intensity. The expansion of
31
Chapter 3 Experimental setup and fabrication details
the laser beam diameter by the spatial filter gives a closely approximated plane
wave over the exposure area [76].
Figure 3.3 Schematic illlustration of a Lloyd's mirror setup for laser interference
lithography of periodic structures on photoresist.
In front of the spatial filter, there is a rotating stage with a mirror mounted
perpendicularly to the samples holder. A part of the expended beam is reflected
by the mirror onto the sample held at 90° to the mirror surface and interferes with
the other portion of the beam that falls directly on the samples to create a gratinglike standing wave, which is recorded on the photoresist layer of the sample. Due
to the fact that the mirror is at a right angle with respect to the sample surface,
light beams always reflect at the same angles as the original incident beam.
Therefore, altering the angle of the incident beam by rotating the rotation stage
varies the period of the pattern as mentioned and thus periodic structures with
different dimensions can be created.
The period of the fringes can be as low as a half of the light wavelength of the
incident laser beam. Therefore, depending on the types of laser used, the
structures of submicron dimensions can be produced. The use of LIL technique
enables the size of the periodic structures created on the photoresist to be as small
32
Chapter 3 Experimental setup and fabrication details
as half of the incident beam wavelength. The applications of LIL include the
patterning of magnetic media materials and magnetic random access memory
(MRAM) devices, the structuring of polymers and creating distributed structures
for quantum dots. However, the utilization of the LIL technique for efficient
surface antireflection performance for solar devices still requires intensive study.
3.1.3
Top-down fabrication of silicon nanowires
Silicon nanowire arrays can be fabricated by a combination of top-down
lithography and dry/wet etching. While conventional lithographic technique and
reactive ion etching have been successfully employed to fabricate the ordered
arrays of silicon nanowires [81-83] (see Figure 3.4), the extensive effort has also
been devoted by various research groups on the wet etching of silicon nanowires.
a
b
Figure 3.4 Scanning electron micrographs showing (a) the arrays of silicon nanowires
prepared by using inductively coupled plasma etching [81]; and (b) the arrays of silicon
nanopillars fabricated by optical lithography and reactive ion etching [82].
33
Chapter 3 Experimental setup and fabrication details
In recent years, a simple catalytic etching technique that makes use of metal (Au,
Ag or Pt) particles to prepare large-area aligned silicon nanowire arrays on singlecrystal silicon wafers has been reported [84-88]. In this technique, metal particles
were deposited on the silicon wafer, and then the silicon substrates covered with
metal clusters were immersed into the etching solution, where the silicon
nanowire arrays were formed via a wet chemical etching. This method utilizes the
catalytic actions of metals for the dissolution of silicon in HF-based solutions.
Since this method does not require electrochemical equipment with an external
electrical supply, it is well-suited for the mass production of silicon nanostructures.
It was found that cylindrical holes with diameters of tens of nanometers were
formed in silicon by wet chemical etching in an aqueous solution containing HF
and hydrogen peroxide (H2O2) when silver nanoparticles were loaded on the
silicon surface before the etching process [89]. The pore formation was found to
be initiated by the reduction of H2O2, as represented by the equation below:
𝐻2 𝑂2 + 2𝐻 + + 2𝑒 − → 2𝐻2 𝑂
(3.2)
Due to the low catalytic ability of the silicon surface for the reaction, the etching
of silicon is slow in a HF–H2O 2 solution. However, the etch rate of silicon
becomes much higher when the reaction is catalyzed by Au, Pt or Ag particles.
Holes are generated as the oxidants (H2O2) are reduced at the metal particles. As a
result of the removal of electrons from Ag particles, the potential of the silver
particles shifts toward a positive value to a level enabling injection of positive
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Chapter 3 Experimental setup and fabrication details
holes into silicon and oxidative dissolution of silicon in fluoride-containing
solutions by
𝑆𝑖 + 6𝐹 − + 4ℎ+ → 𝑆𝑖𝐹62−
(3.3)
In this process, the holes are injected into silicon through the metal/silicon
interface and the holes are attracted near the silver particles due to the image force
induced by them. Hence, the reaction above takes place near the metal/silicon
interface. However, when holes are injected, some of them escape from the image
force. This leads to the generation of the nanoporous silicon layer at the silicon
surface when samples are treated in solutions containing H2O2 at high
concentrations, as shown in Figure 3.5 (b). This porous layer at the top surface
layer consists of a microporous region and an underlying region containing pores
of about 10 nm in size.
Figure 3.5 Scanning electron micrographs of Ag–Si after treatment in an aqueous
solution containing (a) 5.3 M HF and 0.18 M H2O2 for 1 min; (b) 5.3M HF and 1.8M
H2O2 for 1 min. Inset shows an enlarged image at the top surface region [89].
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Chapter 3 Experimental setup and fabrication details
It has been reported that the oxidative dissolution of silicon in HF solution, either
caused by chemical oxidation or electrochemical oxidation, is accompanied by the
formation of a nanoporous silicon layer on the surface [90]. The formation of this
nanoporous silicon layer is enhanced under the condition in which a large number
of positive holes are injected into silicon. This also leads to the strong
luminescence signal detected from the porous silicon layer produced adjacent to
the metal-coated silicon surfaces.
3.2
Fabrication details
3.2.1
Substrate selection and wafer cleaning
In this experiment, p-type silicon wafers were used as substrates since it is the
most widely used material for solar cells manufacturing. In addition, the silicon
substrate is able to present a preferred crystal orientation, which provide clean and
smooth cross section profile from edge cutting.
The wafers were of orientation and had a resistivity of 5 - 9 Ω·cm. In order
to eliminate the samples from possible contaminations, the wafers were cleaned
by immersing cassettes of 4 - 6 wafers into RCA (Radio Corporation of America)
solutions, and heating them at 80 - 90 °C for 15 min. The exact procedure will be
outlined below.
3.2.1.1 RCA I cleaning
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Chapter 3 Experimental setup and fabrication details
The wafers were firstly cleaned using RCA I solution. This oxidizes organic films
and complexes Group I and II metals as well as other metals, such as Au, Ag, Cu,
Ni, Zn, Cd, Co and Cr.[91] The RCA I solution is a high pH solution consisting of
hydrogen peroxide (H2O2), ammonium hydroxide (NH4OH) and de-ionized (DI)
water in the proportion of 1:1:5 by volume. It was then heated to 80 - 90 °C
before the wafers were immersed in the solution for 15 min. The wafers were then
rinsed in DI water with nitrogen (N 2) bubbler for another 10 - 15 min. The RCA I
solution slowly dissolves the thin native oxide layer on silicon and continuously
grows a new oxide layer by re-oxidation. This combination of etching and reoxidation helps to dislodge particles from the wafer surface.[91]
3.2.1.2 RCA II cleaning
RCA II is a low pH solution consisting of hydrochloric acid (HCl), H 2O2 and DI
water in the proportion of 1:1:6 by volume. This solution removes alkali ions and
cations (ionic and heavy metal atomic contaminants) like Fe 3+, Al3+, and Mg2+
which form insoluble hydroxides in basic solutions of RCA I.[91] It serves to
remove the ionic and heavy metal contaminants on the wafer. The RCA II
solution was then heated to 80 - 90°C before the wafers were immersed in the
solution for 15 min. The wafers were then rinsed with DI water with nitrogen (N 2)
bubbler for another 10 - 15 min.
3.2.1.3 5% Hydrofluoric Acid Dip
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Chapter 3 Experimental setup and fabrication details
The hydrogen peroxide in the RCA I and RCA II cleaning procedures caused the
surfaces of the wafers to be coated with a layer of oxide of 1 ~ 2 nm due to its
high oxidizing nature. Furthermore, it was well established that silicon exposed to
oxygen or air ambient forms a thin native oxide layer on its surface even at room
temperature. Such native oxides can grow up to a thickness of around 0.5 ~ 1 nm.
To remove this layer of oxide prior to subsequent process, the wafers were
immersed in a 5% hydrofluoric acid (HF) for around 60 sec. Then, the wafers
were immersed in DI water with a nitrogen bubbler for 20 min to remove the
remaining HF acid. The wafers were then blown dry using nitrogen gas from a
nitrogen gun and ready for the next processing step. Wafers that are scratched or
with defects after the entire wafer cleaning process were not used for the good of
reproducibility and comparability.
3.2.2
Photoresist coating
The resist used in this study is S1805 (positive resist) and ma-N1407 (negative
resist). It was coated onto the substrate via a spin coater. Prior to the spinning
process, a few droplets of photoresist were applied to the surface of the silicon
until it was thick enough to cover the entire surface area of a 2 cm by 2 cm silicon
substrate.
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Chapter 3 Experimental setup and fabrication details
Figure 3.6 Measurement of the film thickness using a step profiler.
The spin coating was basically a two-step process. The first step consisted of a 5
sec spinning interval at 1000 rpm to ensure the deposited photoresist spreaded out
uniformly over the entire surface of the sample. The second step consisted of a 45
sec spin coating at 4000 rpm, and this step determined the final thickness of the
spin-coated photoresist. In this study, the resist thickness was calibrated by a step
profiler (see Figure 3.6) to ensure the consistency in the thickness of the resist
layer and a thickness of 300 nm was obtained.
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Chapter 3 Experimental setup and fabrication details
This was then followed by a soft-bake process. The hot plate’s temperature was
set at 100°C and the duration of the soft bake is 1 min and 30 sec. The purpose of
soft bake was to evaporate away excess solvents within the photoresist, as well as
improve the adhesion of photoresist to the sample.
3.2.3
Exposure and develop
3.2.3.1 Concentric rings nano-structure arrays
In this study, the MLA with the period of 30 μm was used to fabricate the
concentric rings nano-structures arrays with line width smaller than 300 nm. The
spin-coated silicon wafers are fixed onto the precisely controlled nano-stage. By
moving the stage in nano scale, the photoresist is exposed focused laser beam in
pre-design route (concentric rings are adopted in this research). Develop of
photoresist ma-N1407 was done by immersing the sample into the ma-D533
developer, followed by rinsing with DI water and drying by nitrogen (N 2) gas.
The developer dissolved the negative photoresist that was not exposed by the laser.
3.2.3.2 Silicon nanowire arrays
The sample was exposed by the interference of two laser beams, which resulted in
the generation of periodic grating structures on the photoresist. The exposure was
carried out with the Llyod’s mirror setup with a helium cadmium (He-Cd)
continuous wave (CW) laser (Kimmon, Japan) as the laser source. The laser
40
Chapter 3 Experimental setup and fabrication details
wavelength λ was 325 nm with a long coherence length of 10 cm, which made it
suitable for interference lithography purpose.
After a single exposure, an array of periodic line structure was formed. The
subsequent cross exposure of the photoresist by rotating the sample by 90 degrees
enabled the forming of holes arrays on the silicon surface. The period of the
structure is set by changing the angle of the rotating stage. In this experiment, the
angle was set from 12 to 21 degrees with an increment of 3 degrees. The periods
obtained were approximately ranging from 800 to 450 nm according to Eq. 3.1
(𝑃 =
𝜆
2 sin 𝜃
) and gave the pillars with diameters of approximately as a half of the
periods. The exposure time required to fully expose the photoresist mainly
depended on the laser fluence (mJ/cm2) and develop time. Develop of the exposed
photoresist S1805 was performed using the MF319 developer. The exposed
sample is immersed into the developer for ~25 seconds, after which the sample is
removed and cleaned with DI water. The final step involved the dry blowing with
N2 gas to drive off any remaining solutions and clean the sample surface. [91]
3.2.4
Metallic thin film deposition
Prior to the thin film deposition, particularly for sputtering or evaporation process,
a post-develop bake is recommended. This can drive off excess solvent so that
there is less out gassing during the film deposition. However, the bake should not
be too long time or at a very high temperature, otherwise the resist will reflow
41
Chapter 3 Experimental setup and fabrication details
slightly. In this experiment, the samples were baked at 100 °C for 1 min to dry out
the liquid solvent that remained before metal deposition takes place.
Figure 3.7 Schematic drawing of an electron beam evaporator.
Metallic thin film deposition was carried out by an electron-beam evaporator. As
shown in Figure 3.2, the deposition chamber is evacuated to a pressure of 10 -6
Torr. The material to be evaporated is in the form of ingots. E-beams can be
generated by thermionic emission, field electron emission or the anodic arc
method. The generated electron beam is accelerated to a high kinetic energy and
focused towards the ingots. When the accelerating voltage is between 20 ~ 25 kV
and the beam current is a few Amperes, 85% of the kinetic energy of the electrons
is converted into thermal energy as the beam bombards the surfaces of the ingots.
The surface temperature of the ingots increases, resulting in the formation of a
42
Chapter 3 Experimental setup and fabrication details
liquid melt. Although some of incident electron energy is lost to excite X-rays and
secondary electron emission, the material vapor evaporates from the melted ingot
material in a vacuum.[92]
During the metal deposition process, 2 nm of Cr thin film was firstly deposited to
increase the adhesion between different thin film layers, i.e., between the silicon
substrate and Au (or Ag) thin films. 30 nm of Au (or Ag) film was subsequently
deposited on top of the Cr layer.
3.2.5
Lift-off
The common method to produce
metal or oxide nano-structures for
semiconductors, namely lift-off, which is known as an additive process as
opposed to the subtractive etching process. In the lift-off process, a sacrificial
photoresist layer is printed using an inverse mask pattern. The metallic pattern is
created by blanket coating the photoresist pattern with metal and washing away
the sacrificial layer. Any material which was deposited on the sacrificial layer is
removed, any material which was in direct contact with the substrate remains on
the surface. In the etching process, a metallic pattern is fabricated by the first
blanket coating the substrate with a metal layer, then patterning photoresist with
the desired mask pattern and etching away the metal which is not covered by the
photoresist. Wet chemical etching, however, results in isotropic behavior and
undercutting of the photoresist. Dry etching is another method but usually
43
Chapter 3 Experimental setup and fabrication details
requires reactive ion plasma tools or direct laser processing. Moreover, toxic
gases, like Cl2, are needed for plasma etching and many defects are induced.
Figure 3.8 outlines the basic steps involved in single layer photoresist lift-off in
this experiment. The obvious advantage of a standard photoresist lift-off is its
process simplicity, because it involves only one masking step.
The main disadvantage is the sidewall effect. Arising from the fact that the
metallic film is also deposited on the sidewall of the photoresist. This part of the
metal film remains adhered to the substrate even after the resist is removed. This
sidewall makes the lift-off step to be difficult, since it is harder for acetone to
come into contact with the photoresist “behind” the sidewall. Depending on how
robust the film and substrate are, sidewalls from the deposited film can be
removed using a gentle swipe of a clean-room swab or a directed stream of
acetone from a squeeze bottle. The substrate should be immersed in acetone until
the whole film has been lifted off and there are no traces of film particulates –
once the particles dry on the substrate, they are very difficult to be removed.
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Chapter 3 Experimental setup and fabrication details
Figure 3.8 Process flow for the single-layer lift-off process.
In this experiment, after the patterning was made on the photoresist using laser
MLA lithography and LIL, a layer of thin metal film was evaporated on the top of
the patterned photoresist. Lift-off was performed by using acetone at room
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Chapter 3 Experimental setup and fabrication details
temperature with the assistance of ultrasonic agitation to transfer the patterns. A
direct transfer of patterns to the metal film was obtained for negative photoresist;
meanwhile, a negative image was transferred to the metal film for positive
photoresist.
3.2.6
Chemical etching and 3D nano-structures fabrication
Catalytic assisted etching is applied to selectively etch the silicon substrate
covered by a patterned metal (Au or Ag) layer resulted from lift-off process. The
etch rate of silicon becomes much higher when the reaction is catalyzed by the
patterned Au or Ag layer in a HF–H2O2 solution (refers to chapter 3.1.3). The
catalyst assisted etching provide the top-down etching process to achieve the 3D
nano-rings and nanowires structures.
The silicon in direct contact with the metal layer will be etched at a much faster
rate compared to the silicon surface that is exposed directly to the etching solution
[93], allowing the fabrication of silicon nano-structures with high aspect ratios as
shown in Figure 3.9.
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Chapter 3 Experimental setup and fabrication details
Figure 3.9 Schematic illustration of catalyst assisted wet etching process.
During catalytic etching, holes are generated as the oxidants, such as H 2O 2, are
reduced at the metal layer. As a result of the removal of electrons from the metal,
the potential of the metal shifts towards a positive value to a level enabling
injection of holes into silicon and oxidative dissolution of silicon in the fluoridecontaining solution.[94]
47
Chapter 3 Experimental setup and fabrication details
Figure 3.10 Schematic diagram depicting the experimental setup for catalyst
assisted wet etching.
Etching was carried out at a wet bench, with the experimental setup as shown in
Figure 3.10. Several parameters have to be controlled precisely: (i) etching
duration, (ii) concentration of HF, and (iii) concentration of H 2O2. The etching
process was carried out in a chemical solution containing 4.6 Molar (M) of HF
and 0.44 M of H 2O2 for 5 to 30 min. The sample was then rinsed for 10 min in DI
water to remove the remaining HF and H 2O2. After the etching process, the
sample was blown dry by using a N2 gun.
After the catalytic etching, the samples were process with the standard RCA I and
RCA II cleaning to remove the residue metallic layer as well as the remaining
photoresist. Therefore, it is expect that all the sacrificial metal was removed from
the samples prior to characterisation.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
CHAPTER 4
HYBRID 3D SILICON SURFACE NANO-STRUCTURE
ARRAYS FOR SOLAR CELLS BY LASER
MICRO/NANO-PROCESSING
4.1
Introduction
Silicon substrates with a rough surface exhibit optical enhancement properties due
to increased surface area and sub-wavelength surface structures. Various
researchers have studied the development of novel silicon surface structures for
their interesting optical properties, such as nanoparticles, nanodots and nanorods.
To further push the boundary, the 3D silicon nano-structures, which demostrate
improved optical performance, have gained interest.
In this chapter, we focus on the optical performance of silicon concentric nanorings array and nanowires array, which attribute to 3D surface geometry. This
study offers a novel way for the fast fabrication of surface textures in a large scale
with feature sizes comparable to light wavelength. Therefore, the achieved surface
micro/nano-structures provides a layer with gradient refractive index in between
the air ambient and substrate, which is normally required as anti-reflection coating
(ARC). Meanwhile, the silicon nano-structures allow an enhanced surface anti-
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
reflection performance even at a reduced substrate thickness. Furthermore, the
nanowires are modified in features to achieve more complex surface textures.
Both experimental
measurements and numerical simulations shows an
improvement of the optical properties resulting from the change in the design of
the silicon nano-structures.
4.2
Characterization methods
4.2.1
Optical microscope (OM) imaging
To investigate the uniformity of the fabricated nano-structures array over a large
scale, OM images are taken. Figure 4.1 (a) and (b) are the optical microscope top
view of the silicon concentric nano-rings structure and silicon nanowires array
respectively. Both structures exhibit an excellent uniformity over a large area of
about 1.8 cm ×1.8 cm.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.1 Optical microscope top view images of fabricated (a) silicon concentric nanorings array and (b) silicon nanowires array.
4.2.2
Scanning electron microscope (SEM) imaging
4.2.2.1 Silicon concentric nano-rings array
To further investigate the fabricated concentric silicon nano-rings structure,
individual pattern of the arrays are presented as following. Figure 4.2 shows the
SEM images of the fabricated concentric nano-rings arrays. By controlling the
time of catalyst assisted wet etching at 5 min, 15 min and 35 min, the height of
the nano-rings are 750 nm, 3 µm and 12µm as shown in Figure 4.2 (a), (b) and (c)
respectively. Meanwhile, the line width of the concentric nano-rings are 800 nm,
600 nm and 450 nm as observed on the surface, which result on an aspect ratio up
to around 27:1. In Figure 4.2 (a), additional small nano-rings with same height are
fabricated in between the concentric nano-rings to complex the structure for
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
further reflection measurement since the exposed bulk silicon surface area add up
to the reflection by a significant amount.
The period of the fabricated nano-rings array could be flexibly tuned by changing
the MLA period as well as moving the nano-stage for multiple exposure within
one MLA period. In this study, the fabricated concentric nano-rings array has a
period of 15 µm by 30 µm MLA and four-time exposure.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.2 SEM micrographs of silicon concentric nano-rings arrays fabricated at
heights of (a) 750 nm, (b) 3 µm and (c) 12µm.
4.2.2.2 Silicon nanowires array
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.3 SEM micrographs of silicon nanowires array fabricated with (a) 300 nm
diameter and pillars’ height of (b) 300 nm, (c) 1 µm, (d) 6 µm, (e) 12 µm and (f) 40 µm.
In this study, the period of the fabricated silicon nanowires array is resulted to be
around 600 nm by setting the angle of the rotating stage of the LIL system at the
angle of 16 degree. Using the 325 nm wavelength laser with a power of 10 mW
and fixing the exposure time to 2 min before and after rotating the samples by 90
degrees, the silicon nanowires array obtains the diameter of pillars at a half of the
period, which is around 300 nm as shown in Figure 4.3 (a). By controlling the
time of catalyst assisted wet etching to be 1 min, 5 min 10 min, 15 min and 30
min, the heights of the nanowires are around 300 nm, 1 µm, 6 µm, 12 µm and 40
µm as shown in Figure 4.3 (b) - (f).
From Figure 4.3, it can be seen that the obtained silicon nanowires is free from
metallic impurities from the fabrication process. Therefore, their optical
performance is not affected by the presence of a metallic layer or metal particles.
With increasing the etching time, the silicon nanowires array could reach a height
up to more than 40 µm. Therefore, an ultra-high aspect ratio up to 120:1 is
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
achieved. In addition, the silicon nanowires array maintain a good periodicity at
heights from 1 to 12 µm. When the height of silicon nanowires array increases to
~ 40 µm, the mechanical strength of the nano-structure fails to support the
nanowires. Several adjacent silicon nanowires array lean together to form silicon
nanowires array clusters, which compromises the silicon nanowires arrays’
periodicity.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.4 SEM image of silicon nanowires array fabricated with (a) less than half
period, (b) half period and (c) larger than half period diameters of the pillars at the same
height.
Figure 4.4 shows the silicon nanowires array fabricated with different diameters
of the pillars by varying the angle of rotation stage, exposure fluence and develop
time. Meanwhile, the samples with different feature sizes are immersed into the
catalytic etching solution at same time, which will provide the same height of
nanowires.
4.2.3
UV-Vis spectroscopy
All the reflection spectra of the silicon surface micro/nano-structures array are
measured with an UV-Vis scanning spectrophotometer (Shimadzu Corporation).
Normal incident light is used in the experiment. For periodic nanostructures, the
1st order Floquet reflection wave is comparable to the normal reflected wave.
Therefore, the measurement of oblique reflection is necessary. Integrating sphere
method is adopted for reflection measurement.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
4.2.3.1 Silicon concentric nano-rings array
The reflection spectra of the silicon concentric nano-rings array at heights of 750
nm, 3 µm and 12 µm are shown in Figure 4.5, and the reflection spectra
correspond to the concentric nano-rings array on their left accordingly.
From the measured spectra, the average reflection of the silicon surface reduces
from 12.4% to 3.7% at the wavelength range (300 ~ 1300 nm) by adopting the
concentric nano-rings array. The maximum reflection reduces from 31.7% to 6.8%
at 1300 nm. However, it can be observed from all three spectra that the reflection
of the textured surfaces reach a peak at the wavelength smaller than 400 nm
comparing to the overall reflection.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.5 Reflectance spectra of silicon concentric nano-rings arrays with (a) 750 nm,
(b) 3 µm and (c) 12 µm ring heights.
Different from the flat silicon surface, the textured silicon surface has plenty of
micro-rings at different feature sizes, which enlarge the surface area and re-routes
the incident light inside silicon substrate. The incident light bounces from one
structure to another. This multi-internal reflection process can largely increase the
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
optical length of the incident light. Therefore, the opportunity for light absorption
by the silicon substrate is enhanced greatly.
Though the silicon surfaces with the concentric nano-rings array can reduce
reflection at the interface, the resulting surface structures leave the large area of
flat silicon in between the features even at high aspect ratios, as it is shown in
Figure 4.5, and therefore, limit the anti-reflection performance.
4.2.3.2 Silicon nanowires array with different pillar heights
Figure 4.6 illustrates the measured spectra (300 -1300 nm) of silicon nanowires
with different heights. It is obvious that the reflection of the silicon nanowires at
the height of 1 µm is around 10% at the wavelength range (300 ~ 1000 nm). A
sharp increase in reflection up to 24.3% in the 1000 -1300 nm wavelength range
is clearly visible in Figure 4.6 (a). Compared with the concentric nano-rings at a
similar height, the lower reflection is shown in nanowires at 1 µm high, which
gives an average reflection of 11.8% and broadband reflection below 24.3%.
However, the anti-reflection performance is not improved when the aspect ratio of
the nano-ring is further increased. This can be attributed to several factors. Firstly,
the low aspect ratio (~ 3:1) of structures limits the surface roughness and area,
which subsequently limits the light trapping. Furthermore, the light absorption by
the Si surface is weak, especially at the wavelength range (1000 ~ 1300 nm),
which is close to the single crystalline Si band gap at 1100 nm. A certain fraction
of the light enters the Si substrate and is reflected back from the backside surface.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Moreover, the limited anti-reflection performance reasons from the total area of
the flat surface at the sidewalls, top and bottom surfaces of silicon nanowires,
which give rise to an increase to their surface reflection. Figure 4.6 (a) shows that
the surface profile at the top side of silicon nanowires at 1 µm high is nearly
identical to the original flat Si. It is much smoother than that of nanowires at
increased heights due to the short etching time of 5 min. Though the top area of
single pillar is small, the overall top area is significant considering the high array
density. The total flat top area contributes to the high reflection of the surface.
When the height of silicon nanowires increases to 6 and 12 µm, the surface
reflection decreases throughout the 300 to 1300 nm wavelength range. The
nanowires, which are sub-wavelength structures, approximately act as an
intermediate layer with an effective RI. The effective RI can be calculated by the
volume fraction of silicon and air inside this layer. Consequently, it compensates
the RI mismatching at Si-air interface and reduces the reflection. Meanwhile, the
ultra-high aspect ratio provides much higher opportunities for light trapping
among silicon structures. The enlarged vertical dimension significantly increases
the surface area available for light trapping. Meanwhile, compared to the pillars at
1 µm high, it can be observed that the top surface areas become rougher due to the
longer etching time required to fabricate 6 and 12 µm pillars. Such rough top
surfaces help to reduce reflection as well. Therefore, the broadband reflection of
nanowires at the height of 6 µm can be reduced to below 16.6% and an average
reflection at 5.0% can be achieved. Furthermore, at a height of 12 µm the average
reflection of silicon nanowires is 1.2% and broadband reflection is below 2.8%.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
The maximum reflection at 2.8% is observed at a wavelength of 1300 nm. The
relatively high reflection in the infrared-range is due to the backside surface
reflection of Si substrate.
Figure 4.6 Measured reflectance spectra of silicon nanowires arrays with (a) 1 µm, (b)
6 µm, (c) 12 µm and (d) 40 µm pillar heights.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
When the height of silicon nanowires is further increased to ~ 40 µm, the silicon
nanowire clusters are generated. Thus the light trapping effect is weakened due to
the less effective surface aspect ratio, leading to the reduced light absorption. The
overall anti-reflection performance (average reflection at 4.3% and broadband
reflection below 6.4%) is weakened compared to the silicon nanowires at the
height of 12 µm.
4.2.3.3 Silicon nanowires array with different pillar diameters
To investigate the effect of pillar densities on the surface reflection, the silicon
nanowires are fabricated into different diameters. By varying laser fluence during
the LIL exposer process, the photoresist patterns are resulted in different feature
sizes after lift-off. Therefore, the silicon nanowires are fabricated to be at
diameters of smaller than, larger than and equal to a half of the periods.
Meanwhile, the silicon nanowires are fixed at a height of 6 µm and a period of
600 nm. Figure 4.7 illustrates the measured spectra of silicon nanowires array at
different densities.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.7 Measured reflectance spectra of silicon nanowires array with pillar
diameters of (a) less than, (b) equal to and (c) larger than half period.
It can be observed in Figure 4.7 (b) that when the pillar diameters equal to a half
of the period, the average reflection over the whole spectrum reaches the lowest
value which is 3.2%. And the highest refection is 6.2% at the wavelength of 1200
nm. However, when either reducing or increasing the diameter of the nanowires
relative to a half of the period, the reflection increases over the whole band. In
Figure 4.7 (a), the diameter of the nanowire is reduced to around 90 nm, and the
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
flat silicon substrate is mostly exposed as shown in the SEM image. The average
reflection shows to be 8.7% and a maximum of 17.9% achieved at 1200 nm. This
increased reflection is mainly caused by the reflection at the flat silicon substrate
surface in-between the silicon nanowires, which directly reflect the incident light.
On the other hand, the diameter of the nanowire is increased to 450 nm as shown
in Figure 4.7 (c). The average reflection is 7.2% and a maximum of 9.3% is
achieved at 1200 nm. In this case, the total area of the nanowires top surface
appears to be similar as flat silicon surface, and therefore, light reflection is
boosted.
4.3
“Mushroom”-shape silicon nanowires array
During the fabrication process of the silicon nanowires array, a problem
associated with the lift-off method identified. The sidewall of the photoresist
patterns resulting from LIL were found to be not perfectly perpendicular to the
substrate, as shown schematically in Figure 4.8.
Figure 4.8 Schematically cross section view of LIL-based lift -up process.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
This sidewall of the patterns turns out to be in a bump shape, which makes it
easier for the deposited metal to form a continuous film layer. This metal film
layer covers all the photoresist patterns and thus makes it difficult for acetone to
permeate into the structure to remove the photoresist.
In order to solve the problem, a new fabrication process was developed with the
multiple step of etching and no sigle layer lift-off process (see Figure 4.9) is
proposed in the fabrication of “mushroom”-shape silicon nanowire arrays. In this
process, the sample directly proceeds with catalyst assisted etching after the
deposition of metal film, with both metal film and remaining photoresist on the Si
substrate. Subsequently, an additional isotropic KOH etching is applied to the
sample before removal of the residue films.
Figure 4.9 Schematic process flow for the fabrication of 3D silicon nano-structures array
fabrication without using a lift off process.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Since the photoresist mask and the metal film remain on the substrate d uring the
process, the residue dots of photoresist keep on covering the heads of the silicon
nanowires and keep them from reacting with the etching solution. The thicker the
residue layer is, the slower the etching process is. Therefore, the etching speed of
the heads of the silicon nanowires is much slower than it is at the sidewalls of the
nanowires. Eventually, an enlarged head part is formed at each single standing
silicon nanowire which strongly resembles a “mushroom”-shape.
In this experiment, the same samples specification and equipment are applied for
the fabrication of “mushroom”-shape silicon nanowires array. The enlarged
diameter ratio of the nanowire’s head relative to the bottom of the structure can be
varied by etching time and thickness of photoresist (~300 nm in this case). Then
the samples undergo a 15 min catalyst assisted etching, and subsequently dipped
into 17% KOH solution for 5 min. Eventually, the metal and photoresist residue
layers are removed by cleaning steps as in Section 3.2.1.
4.3.1
Scanning electron microscope (SEM) image
Figure 4.10 illustrates a SEM micrograph of the “mushroom”-shape silicon
nanowires array. The fabricated nanowires have a height of 6 µm at an area of 1.8
cm2 × 1.8 cm2. The “mushroom”-shape silicon nanowires array maintain a good
periodicity and uniformity, and the residue layer is able to be fully removed from
the top of the nanowires, therefore giving a clear view of the “mushroom”-shape
head parts. The diameters of the head and the nanowires’ sidewalls are 230 nm
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micro/nano-processing.
and 200 nm, respectively, which shows an increase of the diameter from
nanowires’ sidewall to the top by around 15%.
Figure 4.10 SEM micrograph of a "mushroom"-shape silicon nanowires array.
4.3.2
UV-Vis spectroscopy
Other than metallic nanoparticles decoration on silicon micro/nano-structures, the
hybrid 3D silicon nano-structures are proposed to further enhance the antireflection performance for solar cells. Figure 4.17 shows the experimental
reflection spectra of the silicon nanowires array and fabricated “mushroom”shape silicon nanowires array, as shown in Figure 4.10, at the same height, period
and sidewall diameter.
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micro/nano-processing.
Figure 4.11 Measured reflection spectra of silicon nanowires array with and
without the "mushroom"-shape crowns.
It can be observed that an overall reduction of the reflectance is achieved, thus the
spectrum is flattened in broadband by fabricating the silicon nanowires into the
proposed “mushroom” shape. Especially, the maximum reflection is greatly
suppressed form 2.7% to 1.3% at a wavelength of 1200 nm. Meanwhile, the
average reflection is reduced from 1.9% to 1.0%, and a reflection lower than 1.0%
was achieved for the for a 300 – 1000 nm wavelength range.
In this case, the enlarged head of the silicon nanowires with the sub-wavelength
feature sizes is able to further enhance the scattering of incident light as well as
the back reflected light, and trap them in the silicon substrate. Meanwhile, this
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
structure provides an enhanced electron field inside the “mushroom”-shape head
part comparing to the normal silicon nanowires (detailed in chapter 4.4).
4.4
Theoretical analyses of enhanced light trapping in hybrid 3D silicon
nano-structures array
4.4.1
Optical performance
In order to investigate the enhancement of antireflective properties of
“mushroom”-shape nanowires surface textures, the reflectance and absorption of a
5 µm-thick planar silicon absorber layer and one decorated with the surface
textures of both normal and “mushroom”-shape silicon nanowires at 5 µm are
calculated. During the simulation process, the surrounding environment and all
the numerical factors of the silicon nano-structures array are set to the same
condition.
4.4.1.1 Reflectance of silicon nano-structures array
Figure 4.12 shows the spectrally resolved reflectance spectra for the two nanoscale surface textures and the planar surface as reference.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
50
"Mushroom"
nanowires
Silicon nanowires
Planar
Reflection (%)
40
30
20
10
0
400
600
800
1000
1200
Wavelength (nm)
Figure 4.12 Measured spectrally resolved reflectance of planar silicon, normal and
“mushroom”-shape silicon nanowires.
Compared to the planar surface, nano-scale surface textures reduce the surface
reflectance over the spectrum. This is because in the layer of the silicon nanostructures, the silicon volume ratio corresponds to a gradient effective refractive
index profile bridging between the low-index air and high-index silicon.
Furthermore, the “mushroom”-shape silicon nanowires surface texture has a more
significant antireflection effect than the normal silicon nanowires surface text ure.
The fabricated “mushroom”-shape silicon nanowires display a changing silicon
volume ratio from top to bottom of the layer of nanowires. The anti-reflection
effect results from an effective index profile gradually change from low to high
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
from air to bulk silicon interface. Since the “mushroom”-shape silicon nanowire
without the top part shows to be a needle-like texture, the reflection is reduced
over the whole spectrum. On the other hand, the “mushroom”-shape top of the
nanowire provides an enlarged and roughen the surface area of the nano-structure,
thereby, an enhance light trapping path is achieved, which results in a lower
overall reflection.
4.4.1.2 Absorption of silicon nano-structures array
Figure 4.13 shows the spectrally resolved absorption spectra of the nano-scale
structures at normal incidence of normal and “mushroom”-shape silicon
nanowires as well as the planar reference for absorber layers of 5 µm thickness,
respectively. Both nano-scale structures show significant light absorption
enhancement compared to the planar reference. To achieve similar absorption,
planar structure of about 10-fold thickness is required. Multiple absorption peaks
are observed for both surface nano-structures, where the peak positions vary.
Specifically, in the short wavelength range, “mushroom”-shape nanowires surface
absorbs light more efficiently, which could come from the path length
enhancement when light is trapped in the cavities between adjacent periods as
dimension of the cavities is comparable to the wavelength of blue light. In the
long wavelength range, the “mushroom”-shape nanowires structure contributes to
higher absorption as well. This could be attributed to the strong antireflective
effect of the “mushroom”-shape nanowires surface structures due to the gradient
effective refractive index, as discussed above.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
100
Reflection (%)
80
60
40
Planar
Silicon nanowires
"Mushroom"
nanowires
20
0
400
600
800
1000
1200
Wavelength (nm)
Figure 4.13 Measured spectrally resolved absorption spectra of planar, normal and
“mushroom”-shape silicon nanowires.
4.4.2
E-field distribution in single standing nano-structure
To investigate the local variation of absorption within the solar cells, Poynting’s
theorem is used to calculate the local absorptance from the electric field
distribution and plotted the spatially resolved absorption in one plane,
𝐴=
𝜋
2
∫ Σ𝑚(𝜀 ) (|𝐸𝑥 | 2 + |𝐸𝑦 | + |𝐸𝑧 |2 ) 𝑑𝜐
𝜆 𝜐
72
(4.1)
Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
In this equation, A is the absorptance, ε is the location dependent permittivity, λ is
the wavelength, E x, Ey, E z are the electric fields in x, y and z directions.
RETICOLO-2D returns the diffraction efficiencies of the transmitted and
absorbed orders for an incident plane wave from the top and for an incident plane
wave from the bottom, both for TM and TE polarizations (refers to chapter 2.3).
The period of the “mushroom” shape silicon nanowire is set to be 600 nm and
diameter is half period with height of 1 µm, which are the same as the
experimental results. For simulation process, the boundary is set to infinitely
repeat the designed structure.
Figure 4.20 (a) shows the electric field of a planar silicon substrate. The region [-5,
0] is the silicon layer and the region [0, 5] is air layer, and the air-silicon interface
is set at 0. Figure 4.20 (b) and (c) shows the plot for absolute value of square of
electric field in the y-direction (abs (Ey2)) at 300 nm and 1000 nm as y-direction
shows the feature of the single stand nanowire. The white dashed line shows the
interface of air and silicon layer.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
Figure 4.14 Plot of the simulated absolute value of square of electric field in y-direction
(abs(Ey)2) for (a) planar and (b) “mushroom”-shape nanowires structures at wavelengths
of 300 nm and (c) 1000 nm.
In the planar reference, the electric field varies in z-direction while distribution
along x- and y-plane is uniform [Figure 4.20 (a)]. In structures with surface
textures, multiple concentration spots with strong absorption occurs are observed
[Figures 4.20 (b) and 4.20 (c)]. Particularly, the “mushroom”-shape nanowires
surface texture demonstrates a micro-lensing effect by which an array of spots
with a very high light intensity is found at the enlarged top parts of the
“mushrooms”. At these spots, the concentration factor is much more significant
than that of the silicon nanowires. This explains the higher absorption at larger
wavelengths for the “mushroom”-shape silicon nanowires surface textures. This
unique property could also potentially enable thinner absorber layers by applying
“mushroom”-shape silicon nanowires surface textures.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
4.5
Summary
In this chapter, silicon concentric nano-rings array and silicon nanowires array at
different feature sizes are fabricated by the MLA and LIL laser processing. Their
nano-structures as well as the anti-reflection performance are investigated
accordingly. Due to the complex surface geometry, the efficient light trapping
path is significantly improved by constructing multiple reflection inside the
structures. With increasing heights, the concentric nano-rings are able to achieve
an average reflection as low as 3% over wavelength range of 400 nm to 1000 nm.
For the silicon nanowire, the anti-reflection performance could be further
improved to around 1.0% when the height of the nanowire is around 12 µm.
Meanwhile, by investigating the reflection of silicon nanowires array with
different diameters relative to its period, we could optimize the structures with a
diameter approximately equals to half period.
On the other hand, by altering the configuration of the nano-structures, the hybrid
“mushroom”-shape nanowires in this study, provide an increase of the overall
absorption at broadband wavelength. Numerical simulation of the optical
properties and the E-field distribution of the hybrid silicon nanowires are
presented. An intensified absorption is observed for the “mushroom”-shape
nanowires. Therefore, absorber layer of only a few micrometers could accomplish
absorption comparable to a planar reference of about 10-fold thickness.
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Chapter 4 Hybrid 3D silicon surface nano-structure arrays for solar cells by laser
micro/nano-processing.
However, the resulting novel 3D silicon nano-structures complex the surface of
the substrate due to its ultra-high aspect ratio, which subsequently challenges the
following solar cells fabrication procedures. Therefore, it is critical that an
optimized feature is achieved to improve its anti-reflection as well as avoid a
complex solar cell fabrication process flow.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
CHAPTER 5
BROADBAND ENHANCEMENT OF ANTIREFLECTION IN SILICON MICRO/NANOSTRUCTURES
5.1
Introduction
Rough metal surface exhibits optical enhancement properties due to the strong
light scattering of the nano-structures and electric fields that result from light
induced surface plasmons (SPs) [95]. Noble metallic particles of sub-wavelength
sizes can sustain resonances of collective electron oscillations known as localized
surface plasmons (LSP) [96]. With their enhanced optical properties, a wide range
of applications have been investigated, such as imaging [97, 98] and photonic
devices [99, 100]. It is therefore a highly interesting scope to decorate the metallic
nanoparticles to the achieved silicon nano-structures in order to induce desired
light scattering and further intensify their anti-reflection performance.
In this chapter, the research focus on metallic nanoparticles which are deposited
on hybrid 3D nano-structures to ensure a better anti-reflection performance over a
broader wavelength range. Furthermore, facing the reflection raised from
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
backside of silicon substrate, back surface texturing will be adopted in this study,
which improves the overall anti-reflection performance.
5.2
Experimental details
5.2.1
Metallic nanoparticles deposition
Roughened silicon surfaces can reduce the optical reflection by light trapping. In
this section, the conventional anti-reflection surfaces for solar cells application
fabricated by KOH wet etching and surface texturing by laser-based ablation are
firstly introduced as reference for comparison with the 3D nano-structures. The
metallic nanoparticles are deposited to the KOH etched surfaces, laser processed
surface as well as the fabricated silicon nanowires array to investigate their
enhancement on anti-reflection performance. Furthermore, the backside surface
texturing is studied for its enhancement in light trapping.
5.2.2
Pyramid micro-structures
To fabricate micro-pyramid structures on silicon surfaces, an n-type silicon
substrate is firstly cleaned to remove the oxide layer as indicated in Section 3.2.1.
Then the wafer was immersed in a mixture of KOH and isopropyl alcohol in
water, which has the composition of 6 wt% and 20%, respectively. The etching
process is performed at 70 ◦C for a duration of 30 min.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.1 (a) Top view and (b) cross section SEM images of the KOH etched silicon
surface.
Figure 5.1 shows the SEM images of the KOH etched silicon surface. Micropyramids at base width from 5 to 15 µm and height from 2 to 8 µm can be
observed over the whole surface. The aspect ratio (height: width) of the
microstructures is approximately 2:1. The KOH wet etching cannot provide
surface structures with high enough aspect ratios, which limits the anti-reflection
performance.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
5.2.3
Laser surface texturing
Direct laser writing provides another method to texture silicon surfaces with
micro-structures at a higher aspect ratio to enhance light trapping. A fiber laser
ablation system was used to fabricate black silicons as shown in Figure 5.2 (a).
The laser wavelength used is 1064 nm, the pulse duration (full width at half
maximum, FWHM) is 1 ns and the laser spot size is ~ 20 µm. It can create microstructures on silicon substrates over a large area of 10 × 10 cm2 by scanning the
laser beam scanning over the silicon surface, which is shown schematically in
Figure 5.2 (b).
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.2 (a) Schematic diagram of the laser ablation for silicon surface texturing and
(b) a SEM image of the resulting c-Si surface.
Compared to the KOH wet etching, the laser ablation can create surface structures
at a higher aspect ratio, which is a key factor for better light trapping. The height
of the surface structures can reach more than 15 µm, at a width of about 10 µm
and aspect ratio of 3:2. Hybrid laser patterning, including different scanning
patterns along different directions, is adopted for the surface texturing. The
patterns are generated by the laser scanning along horizontal, vertical, diagonal
forward 45 degree and diagonal backward 45 degree directions, respectively.
5.2.4
Thermal annealing of metallic nanoparticles
Localized surface plasmon resonance (LSPR) can improve the anti-reflection
provided by nano-pillars (see Figure 5.3). With electromagnetic (EM) wave
excitation, metallic NPs can be used to excite localized SPR to increase optical
absorption by the light trapping. Meanwhile, light scattering properties become
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
dominant for nanoparticles at a large size. Both effects contribute to the antireflection performance.
Au and Ag nanoparticles are mostly used for their SPR excitations at visible range.
To reduce the material cost for anti-reflection surface fabrication, other metallic
nanoparticles are also investigated, such as Cu, which allow the SPR resonances
in IR range. Therefore, the emergence of the metallic nanoparticles in addition to
conventional anti-reflection surfaces can further improve the overall antireflection performance.
Figure 5.3 Schematic of silicon nanowires array with metallic nanoparticles' decoration.
To investigate the anti-reflection effect by metallic nanoparticles, Ag, Ag/Au bimetallic and Cu/Ag/Au tri-metallic nanoparticles were decorated on silicon
surfaces by thermal annealing. Metallic thin films were firstly deposited on silicon
surfaces by an e-beam evaporator. Thermal annealing at 200 ◦C (for Ag thin film)
for 30 min or 500◦C (for Ag/Au and Cu/Ag/Au films) for 30 min were then
carried out to create metallic nanoparticles decorated on silicon surfaces.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
5.3
Characterization
5.3.1
Scanning electron microscope imaging
5.3.1.1 Flat silicon with metallic nanoparticles deposition
Figure 5.4 SEM images of (a) Ag, (b) Au, (c)Ag/Au and (d) Cu/Ag/Au metallic
nanoparticles' decoration of flat silicon surfaces.
SEM images of Ag and Au nanoparticles deposited on the flat silicon surfaces are
shown in Figures 5.4 (a) and (b). It is observed that larger particle sizes of around
100 ~ 250 nm are achieved for Ag nanoparticles and much smaller sizes of around
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
10 ~ 30 nm are achieved for Au nanoparticles. However, the surface area
coverage is larger for Au nanoparticles decorated surfaces.
SEM images of Ag/Au and Cu/Ag/Au alloy nanoparticles formed on the flat
silicon surfaces are shown in Figures 5.4 (c) and (d), respectively. The Ag and Au
nanoparticles are clearly shown in Figure 5.4 (c) with different nanoparticles’
sizes. The surface area coverage is obviously increased in tri-metallic
nanoparticles of Cu/Ag/Au in Figure 5.4 (d).
5.3.1.2 Pyramid micro-structure with metallic nanoparticles deposition
Figure 5.5 SEM images of metallic nanoparticles' decoration of the KOH etched silicon
surface.
Figure 5.5 illustrates the SEM image of KOH etched silicon surfaces with the Ag
nanoparticles’ decoration. It is observed that larger particle sizes of around 100 ~
250 nm are achieved for Ag nanoparticles. And the surface coverage of the Ag
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
nanoparticles on the pyramid textures is about the same with Ag nanoparticles
decorated on the planar silicon surface.
5.3.1.3 Laser surface texturing with metallic nanoparticles’ deposition
Figure 5.6 SEM images of the laser-textured silicon surfaces being decorated with (a)
Ag/Au, and (b) Cu/Ag/Au alloy nanoparticles
Figure 5.6 shows the SEM images of the laser-textured silicon surfaces decorated
with Ag/Au and Cu/Ag/Au nanoparticles. The metallic nanoparticles are
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
significantly increased in size and surface coverage when Cu/Ag/Au nanoparticles
are decorated.
5.3.1.4 Silicon nanowires array with metallic nanoparticles’ deposition
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.7 SEM images of metallic nanoparticles' decoration of the silicon nanowires
array at heights of (a) 500 nm, (b) 3 µm, (c) 12 µm and (d) 40 µm.
Figure 5.7 shows the SEM images of the metallic nanoparticles' decoration of
silicon nanowires array at 500 nm, 3 µm, 12 µm and 40 µm. Ag nanoparticles are
deposited on the fabricated silicon nanowires array. The Ag nanoparticles
maintain a lower surface coverage at the sidewalls of the silicon nanowires,
however, the more Ag nanoparticles are grouped together and contribute to a
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
larger size at the top surface of the silicon nanowires. In Figure 5.7 (a), the Ag
nanoparticles coat are decorated on both the nanowires as well as the substrate.
When the nanowires get larger, the Ag nanoparticles are reduced in coverage at
the bottom of the nanowires as it is shown in Figures 5.7 (b), (c) and (d). When
the nanowires reach around 40 µm, the silicon nanowires clusters prevent Ag
atoms from reaching the substrate.
5.3.2
UV-Vis spectroscopy
5.3.2.1 Silicon micro/nano-structures with metallic nanoparticles’ deposition
Figure 5.8 Measured reflection spectra of flat silicon surfaces with and without
the metallic nanoparticles' decoration.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
The reflection spectra of the metallic nanoparticles deposited flat silicon substrate
(in Figure 5.4) were measured using UV-Vis spectroscopy.
The black, blue and red curves in Figure 5.8 show the measured reflection spectra
of a flat silicon surface and the flat silicon surfaces decorated with Ag and Au
nanoparticles. Comparing with the reflection of flat silicon, the reflection of the Si
surface decorated with Ag or Au nanoparticles is dramatically decreased,
especially at wavelengths shorter than 800 nm. The suppressed reflection over this
wavelength regime is mainly attributed to two reasons. First of all, the interaction
between metallic nanoparticles and the incident light contributes to the intrinsic
absorption of Ag or Au nanoparticles at LSPR wavelengths. Secondly, Ag or Au
nanoparticles at differently sizes are able to forward scatter light into the silicon
substrates [101], and thereby trap the propagating light at the interfaces between
silicon and air. The reflection of the Ag nanoparticles decorated flat silicon can be
reduced to 17.7% at the wavelength of 550 nm, while the reflection of the Au
nanoparticles decorated flat silicon decreases to 24.7% at the wavelength of 572
nm. In Figure 5.8, the resonance intensity and optical enhancement of the LSPR
in the reflection spectra highly depend on materials, size, and surface coverage
density [102]. Furthermore, the reflectance intensity of the Au nanoparticles
decorated flat silicon increases to 30% at the wavelength of 503 nm, which may
be due to more backward scattering from Ag nanoparticles with a larger diameter
(10 - 300 nm) after the annealing. In Figure 5.4 (b), it is observed that Au
nanoparticles are densely and randomly distributed so that interactions between
nanoparticles occur, which leads to the coupling by the neighboring Au
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
nanoparticles [103]. Therefore, the metallic nanoparticles can act as an
antireflection layer to effectively reduce the surface reflection of the flat silicon
surface at the wavelength range from 300 to 800 nm.
As illustrated in Figure 5.8, the pink and green curves show measured reflection
spectra of a flat silicon surface and the flat silicon surfaces being decorated with
Ag/Au and Cu/Ag/Au alloy nanoparticles. As compared to the flat silicon surface,
they exhibit more promising anti-reflection performance with average reflection
as low as 16.5% and 12% at the entire wavelength range, respectively. In addition,
compared to the flat single metallic nanoparticles decorated silicon surfaces,
alloyed nanoparticles achieve further reflectance reduction. This attributes to the
increased surface coverage density and size of the nanoparticles under the
optimized annealing conditions. The forward scattering from alloy nanoparticles
dominates, and strongly scatters the propagating wave into the silicon surface
[101]. Moreover, Cu/Ag/Au alloy nanoparticles help effectively reduce the
reflection from around 30% to 10% at the wavelength range from 800 to 1000 nm
as in Figure 5.8. This is mainly because of the LSPR induced by alloy
nanoparticles, and their frequencies are tuned to the longer wavelength range for
Cu nanoparticles. From the previous studies, the LSPRs of Ag and Au
nanoparticles appear to be around 400 and 530 nm, while Cu nanoparticles have a
LSPR wavelength around 570 nm [104], which provides more absorption for
alloy nanoparticles at the longer wavelengths range. Therefore, by decorating
Cu/Ag/Au alloy nanoparticles, the reflection of the silicon surfaces can be further
reduced over the entire wavelength range.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.9 Measured reflection spectra of the KOH etched silicon surface with and
without metallic nanoparticles' decoration.
Figure 5.9 illustrates the experimental reflection spectra of KOH etched silicon
surfaces with and without Ag nanoparticles’ decoration. A significant suppression
in reflection is achieved at the wavelength range from 300 nm to 400 nm. Ag
nanoparticles created by thermal annealing with Ag thin films thickness of 10 nm
(Ag10) and 20 nm (Ag20) have similar suppression behaviors on the reflection
spectra. The average reflection of the KOH etched silicon surface with Ag
nanoparticles’ (Ag20) decoration is reduced to as low as 9% at the wavelength
range of 300 to 1200 nm, and the maximum reflection of 17% appears at 321 nm.
In this case, the coupling of Ag nanoparticles and the incident light is strongly
enhanced at their LSPR wavelength at around 400 nm. Meanwhile, the textured
silicon surfaces of micro-pyramids, in Figure 5.5, are able to extend the light
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
propagation length and trapping path, which eventually leads to an improved
chance for light absorption by Ag nanoparticles and silicon surfaces.
Figure 5.10 Measured reflection spectra of laser textured silicon surfaces with
and without metallic nanoparticles' decoration.
Compared to the KOH etched silicon surface with Ag nanoparticles’ decoration,
laser textured black Si with alloyed nanoparticles’ decoration can exhibit better
broadband anti-reflection performance. The experimental reflection spectra of the
fiber laser textured Si surfaces with different alloy nanoparticles’ decoration
(Figure 5.6) are shown in Figure 5.10. The average reflection of the laser-textured
Si surfaces is 8.3% for the 300 – 1200 nm wavelength range, while the silicon
surfaces decorated with Ag/Au and Cu/Ag/Au nanoparticles are reduced to 6.9%
and 5.5%, respectively. It is observed that there is a significant suppression of
reflection in the 300 to 500 nm wavelength range for Ag/Au alloy nanoparticles
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
decoration, which is caused by the excitation of the LSPR. Meanwhile, the light
scattered from the nanoparticles contributes to anti-reflection performance as well.
In addition, the Cu/Ag/Au tri-metallic nanoparticles assist in bringing down the
reflection even further in the infrared range. The maximum reflection at 9.2%
locates at 1200 nm. Similar to the flat silicon with Cu/Ag/Au nanoparticles, this is
due to the fact that the LSPR wavelength of Cu NPs being located at the near
infrared wavelength. Thus, the tri-metallic NPs’ decoration results in a better
broadband anti-reflection performance.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.11 (a) SEM image and (b) Measured reflection spectra of silicon
nanowires arrays with and without metallic nanoparticles' decoration.
To further improve the anti-reflection performance as presented in Chapter 4, a
hybrid structure by integrating silicon nanowires array and metallic nanoparticles
were designed and fabricated (see Figure 5.7) similar as was done in Chapter 4.
Since the silicon nanowires array at a height of 12 µm exhibits better antireflection performance, experimental reflection spectra of the silicon surface with
and without the Ag nanoparticles’ decoration are illustrated in Figure 5.11. The
silicon nanowires with the Ag nanoparticles decoration flatten the reflection
spectrum compared to the surface without nanoparticles’ decoration, and a
reflection lower than 1.0% was achieved in the whole broadband spectrum from
300 to 1200 nm with an average reflection at 0.8%. The maximum reflection at
1.0% appeares at 572 nm.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
The silicon nanowires exhibit a high aspect ratio of 40:1, which therefore
provides a large enough surface area to deposit Ag nanoparticles with the desired
surface coverage. When increasing the Ag nanoparticles density, their coupling
and the incident light is intensified, which contributes to minimize the surface
reflection. A reflection reduction is observed at the wavelength range from 300
nm to 500 nm. This reduced reflection is mainly caused by the enhanced light
absorption from Ag nanoparticles at the resonance since the LSPR of Ag
nanoparticles is around 400 nm. Furthermore, in the near infrared range,
specifically from 1000 nm to 1200 nm, silicon nanowires array with Ag
nanoparticles decoration exhibit better anti-reflection performance. It is due to the
reduction of reflection from the backside Si surface. Meanwhile, more Ag
nanoparticles are decorated at the sidewalls, top and bottom of silicon nanowires.
Therefore, with more light scattering by Ag nanoparticles with varying sizes, the
effective light trapping path is enhanced for the silicon nanostructures. Therefore,
the opportunities for light scattering within the nano-structures are largely
increased. By the combination of complex silicon surface structures with Ag
nanoparticles, the incident as well as the back reflected light are more effectively
trapped inside the nano-structures and reduce the overall reflection. Furthermore,
the metallic NPs of varying sizes contribute to absorption over certain wave range,
which result in reducing the overall reflectance. In this experiment, the Ag NPs
exhibit significant improved absorption of light at 900 nm to 1200 nm and
contribute to overall reduction of reflection. Moreover, the Ag NPs attached on
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
the tops and sidewalls of nanowires lead to a rougher surface of silicon nanowires,
and further reduce the surface reflection.
5.3.2.2 Silicon back surface texturing
In our previous analyses, it is found that the surface reflectance of the silicon
micro/nano-structures is quite high in the 1000 to 1300 nm wavelength range. It is
because the bandgap energy of crystal silicon corresponds to the wavelength,
which is close to 1100 nm, it may be also because the light at this long
wavelength penetrates deeper into the Si and reflects back from the backside of
the Si surfaces.
To solve this problem, the rear of the sample is laser-textured to reduce the
reflection from the back surface of the substrate and boost the light trapping
performance. Laser direct writing with the fiber laser ablation is used to fabricate
black Si on both surfaces of silicon substrate.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.12 Measured reflection spectra of 320 µm-thick laser-textured Si front surfaces
without and with the backside surface texturing.
Figure 5.12 shows that laser texturing on 320 µm-thick Si backside surface can
reduce the front surface reflectance in the 1000-2500 nm wavelength. This is
because when the sample is thinner, the reflected light from the backside surface
is stronger. Laser texturing on the backside surface can decrease the reflected
light from the backside surface, resulting in the reduction of the measured
reflection.
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Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Figure 5.13 Measured reflection spectra of 320, 530, 890 µm-thick front side lasertextured Si sample with the backside surface texturing.
Figure 5.13 shows that the increase of Si thickness can reduce the measured front
surface reflection over the wavelength from 1000 to 2500 nm. It is because with
the increase of sample thickness, the reflected light from the backside surface
decreases gently due to the light absorption inside Si.
5.4
Summary
In this chapter, the enhancement of anti-reflection performance for a broad
wavelength range is investigated. The silicon nano-structures fabricated are
decorated with metallic nanoparticles for the excitement of their LSPR and light
scattering, which contribute to the reduction of reflectance over a wide spectrum
from 300 nm to 1200 nm.
99
Chapter 5 Broadband enhancement of absorption in silicon micro/nano-structures.
Furthermore, by adopting back surface texturing, the anti-reflective property of
the substrate is significantly improved at the wavelength from 1000 nm to 2500
nm.
Nevertheless, the metallic nanoparticles absorb and scatter the incident light
simultaneously. Therefore, when applying the metallic nanoparticles deposited 3D
nano-structures to silicon solar cells, the optimum conditions of metal material,
features size and surface coverage are required.
100
Chapter 6 Conclusions and future work.
CHAPTER 6
CONCLUSIONS AND FUTURE WORK
6.1
Research achievements
In this thesis, two major experimental works have been presented. Firstly, the fabrication
of the large-area 3D silicon micro/nano-structures and the investigation of their optical
properties. Both micro-lens array (MLA) and laser interference lithography (LIL) are
applied to fabricate the 3D micro/nano-structures array. Secondly, the broadband
enhancement of the anti-reflection performance for the silicon nano-structures is studied.
Various approaches are demonstrated to achieve broadband absorption, such as metallic
nanoparticles decoration, hybrid design of 3D nano-structures and back surface texturing.
The main contributions and results are summarized as follow:
1.
Period silicon nano-rings and nanowires array have been fabricated by the MLA
and LIL on silicon substrates over a large areas (1.8 cm × 1.8 cm) in a short time (a few
minutes). This patterning technique increases the throughput of the nano-structures
fabrication, while minimizes the defect during the process.
2.
With increasing its height, the fabricated micro/nano-structures array is able to
effectively reduce the reflectance of the silicon substrate compared to conventional
texturing over the spectrum from 400 nm to 1000 nm. Well-ordered silicon nanowires
array at ultra-high aspect ratios are successfully fabricated and they demonstrate
101
Chapter 6 Conclusions and future work.
improved anti-reflection in UV-Visible-NIR range. By varying parameters such as the
nanowires height and the density of the array, this study provides methods to optimize the
silicon nanowires array for optimising their antireflection performance.
3.
A novel approach is adopted to achieve the hybrid “mushroom”-shape silicon
nanowires. This method consisted of a two-step chemical etching without a lift-off
process for the fabrication of silicon nanowires. The diameter of the fabricated nanowires
is found to be larger at the top compared to the bottom of the nanowires. The ratio
between the top and bottom diameter can be controlled by the thickness of photoresist
patterns and the etching time used in the experiment. The reflection of the “mushroom”shape silicon nanowires has been demonstrated in comparison to the original silicon
nanowires array with similar dimensions. The hybrid 3D “mushroom”-shape silicon
nanowires effectively reduce the reflection over a broad range from 250 nm to 1200 nm.
4.
Optical simulation of the “mushroom”-shape silicon nanowires array has been
conducted for its enhancement in broadband anti-reflection. The E-field distribution in
the single standing “mushroom”-shape silicon nanowires has been calculated and an
enhanced field at the enlarged top part is observed at either small or large wavelengths,
which contributes to their absorption.
5.
Decoration of various silicon surfaces by metallic NPs is investigated to reduce
surface reflection. Surfaces such as planar, pyramid and direct laser-ablated silicon
surfaces as well as silicon nanowires are studied. Textured silicon and metallic NPs have
different mechanisms for reducing the optical reflection. By combining these two
methods together to fabricate hybrid surfaces, broadband anti-reflection is achieved
compared to the textured silicon surfaces. The Ag NPs decorated silicon nanowires array
102
Chapter 6 Conclusions and future work.
exhibits ultra-low reflection at the broadband wavelength. Experimentally, this study
demonstrated that silicon surface with a reflection lower than 1.0% over the 300-1200 nm
wavelength range can be fabricated.
6.
Finally the rear of the silicon sample was textured to further improve the light
trapping in the sample. The experimental results show that the back surface texturing
obviously reduces the reflection at the wavelength larger than 1000 nm.
6.2
Suggestion for the future work
There are some suggestions for the future studies and applications of the research
presented in this thesis.
1.
To increase the power of the laser employed in LIL system to enlarge the
fabrication area, the adjustment of the Lloyd’s interferometer are necessary to reduce the
intensity non-equality between the original beam and the reflected beam. By enhancing
the beam interference contrast, the resolution of the lithography can be improved down to
sub-150 nm dimensions.
2.
The shape of the nano-structures fabricated can be altered by changing the angle
of the two exposures used in in the LIL system. Angles of 30 and 45 degree, instead of a
90 degree tuning angle, lead to the oblique array nano-structures, such as nano-rod arrays.
So far the structures fabricated by LIL are in the circular shape and only square arrays are
attempted. Meanwhile, the shape of micro/nano-structures fabricated by the MLA
lithography can be modified by the laser exposure process as well, which will result in
more complex nano-structures, for instance “C”-shape or split nano-rings array.
103
Chapter 6 Conclusions and future work.
Therefore, the multiple resonance effect could be excited by the complex nano-feature
array structures.
3.
The structures developed in this work should be tested at the solar cell device
level. This will require a careful optimization of the solar cell fabrication process flow to
ensure that the positive optical effects that can be achieved by the nano-structures is not
negated by an increase in surface recombination losses or resistive losses.
104
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[...]... sub-wavelength structures, higher light absorption is achieved in visible and NIR range, which is significant for solar cells [47, 49, 50] Sub-wavelength structures, e.g hybrid moth-eye structures [34] and ZnO coated Si nano-cone [51], have been shown to exhibit ultra-low reflection by numerical simulations Another point worth mentioning for anti-reflection performance of Si solar cell is metallic nanoparticles... fabrication of precisely located and well-arranged 3D nano -structures arrays on silicon using micro- lens array (MLA) and laser interference lithography (LIL) To further improve the antireflection performance and light trapping efficiency of silicon based solar cells The focus of this research can be divided into several parts Firstly, this study focuses on the large-area synthesis of silicon nanowire... methods and techniques of the fabrication process A detailed procedure of the fabrication of silicon micro/ nano -structures arrays by laser processing is presented Chapter Four investigated the fabricated silicon surface structures for their antireflection performance The reflection in the silicon surface nano -structures arrays are shown according to different designs of their features in both experimental... Figure 2.1 Evolution of silicon solar cell efficiency [36] 7 Chapter 2 Background and literature review Optimizing the solar cell efficiency is all about minimization of losses Therefore, the boost of silicon solar cells efficiency comes with its priority in our study The dominant losses of a silicon wafer solar cell are optical, resistive, and recombination losses In this thesis we will only focus on... decorate silicon surfaces with engineered structures to reduce optical reflection, especially for silicon solar cells with an absorber layer of a few micrometers Following Feynman’s challenge that “there is plenty of room at the bottom” [18], the capability in sculpting silicon with extraordinary precision and efficiency is very much needed for the development of nanotechnology Extensive effort has... comparably weaker absorption at near-bandgap spectrum due to the indirect bandgap of silicon, which results in a narrow absorption spectrum To bypass these limitations, nano-scale textured silicon surface provides the better solution for omnidirectional and broadband anti-reflection [2, 22-28] As photon management schemes for c-Si solar cells with thin absorber layers, such silicon nano -structures usually... of the silicon micro/ nano -structures by two means, which are metallic nanoparticles deposition and back surface texturing Their anti-reflection performance is discussed 5 Chapter 1 Introduction Chapter Six provides the conclusions of this study as well as recommendation for future work 6 Chapter 2 Background and literature review CHAPTER 2 BACKGROUND AND LITERATURE REVIEW 2.1 Silicon solar cells 2.1.1... monocrystalline silicon solar cells, the most commonly adopted light trapping structures are pyramidal textures [20, 21], which have feature sizes of a few micrometers (2~10 µm) However, with reducing the thickness of silicon solar cells to no thicker than a few micrometers, they are no longer favored as absorber 2 Chapter 1 Introduction layers Meanwhile, the crystalline silicon (c-Si) solar cells exhibit... for ultrathin c-Si solar cells 2.1.2 Optical properties of silicon When light impinges on the silicon surface, it is partially reflected due to the optical contrast between c-Si and air The reflection coefficient 𝑅𝑆𝑖 can be calculated by the well know Fresnel equations The transmitted light penetrates into the material, where it is attenuated due to absorption The rate of absorption is determined by. .. reflection reduction at a broadband range 4 Chapter 1 Introduction 1.4 Organization of thesis This thesis is divided into six chapters and their contents are listed as follows: Chapter One gives an introduction on the anti-reflection performance of optical and opto-electrical devices, especially solar cells An introduction of the c-Si surface micro/ nano -structures fabrication is indicated The motivation, ... cells on a large scale This thesis focuses on broadband enhancement of light absorption for solar cells by silicon surface texturing using laser micro/nano -processing Laser technology has become... scattering by nanoparticles (NPs) By the fabrication of the sub-wavelength structures, higher light absorption is achieved in visible and NIR range, which is significant for solar cells [47,... fabrication of silicon micro/nano-structures arrays by laser processing is presented Chapter Four investigated the fabricated silicon surface structures for their antireflection performance The
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