- Using the density functional theory and the non-equilibrium Green's function to investigate electronic characteristics (band structure, density of state, ...) and electronic transport [r]
(1)MINISTRY OF EDUCATION AND TRAINING
VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY
GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY
…… ….***…………
PHAM THI BICH THAO
ELECTRONIC TRANSPORT
IN SEMICONDUCTOR NANOSTRUCTURE BASED ON POLAR MATERIAL AlGaN/GaN
AND PENTA-GRAPHENE NANORIBBON
Speciality: Theoretical and mathematical physics Code: 44 01 03
SUMMARY OF THE PHD THESIS
(2)Supervisors: Assoc Prof Dr Nguyen Thanh Tien Prof Dr Doan Nhat Quang
Referee 1:Assoc Prof Dr Dinh Van Trung Referee 2:Prof Dr Dao Tien Khoa
Referee 3:Dr Pham Ngoc Dong
This dissertation will be defended in front of the evaluating assembly at academy level, place of defending: meeting room, Graduate University of Science and Technology, Vietnam Academy of Science and
Technology
This thesis can be studied at:
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Introduction
Nowadays, semiconductor technology is one of the most important fields in the development of science and technology Semiconductor technology is a foundation of the information society that has been motivating human so-ciety forward by changing in production, living, communication and even in human In semiconductor technology, semiconductor materials play a cru-cial role The first transistor was invented in 1947 based on germanium (Ge) semiconductor with a band gap at room temperature of 0.66 eV The first integrated circuit was born in 1958 and the bulk integrated circuit appeared in 1961 using germanium and silicon (Si) with a band gap of 1.12 eV Since 1965, silicon has become the main material for semiconductor integrated circuits Today, most semiconductor, integrated circuit or photovoltaic in-dustries are still based on silicon
Silicon and germanium are often referred as the first generation of semi-conductors The second generation including gallium arsenide (GaAs, band gap at room temperature is 1.42 eV) and indium phosphide (InP, band gap at room temperature 1.35 eV) was introduced in the 1970s The second gen-eration is primarily used in high-speed devices, microwave devices and inte-grated circuits Besides the larger band gap, the electron mobility of GaAs is more than six times larger than that of silicon In addition, the saturation velocity of GaAs is higher, i.e two times larger than that of silicon There-fore, devices based on GaAs are suitable for high-frequency operations In addition, field-effect transistors based on GaAs also have advantages such as low noise, high performance, ect However, GaAs has lower thermal con-ductivity and disruptive potential than semiconductors like GaN and SiC have, resulting in capacity limitations
(4)Currently, semiconductor heterostructure is widely used in many fields due to its great advantages Specifically, in the telecommunication field with semiconductor transistors, satellite television, warning systems, ect; Energy field with solar cell, light-emitting diode, information storage device, ect; Medical field with water filtration system, data processing system, ect Many studies show that the electric and optical properties of heterostructure semiconductor vary significantly compared to those of the bulk semiconduc-tor and by the external field Moreover, the heterostructure semiconducsemiconduc-tor also has superior intrinsic properties One of the properties is that the po-larization depends on the direction of the material and the structure of the material, especially the low-dimensional structures Therefore, the het-erostructure semiconductor has been an attractive topic in modern material research in recent decades
Strong polarization effect exists in many materials such as GaN, ZnO, MgO, InN, ect Although there have been a number of studies on the ef-fects of polarization on the electric properties of these structures, these constructions have not been fully and systematically studied In particular, the heterostructure semiconductor with polarizing confinement effect needs to be studied extensively In addition, the relationship between confinement effects and electronic transport characteristics should be studied in more detail
Along with the heterostructures, the allotropes of carbon are currently at-tracting much attention Until the mid-twentieth century, the two most com-mon allotropes of carbon in nature were diacom-monds and graphite Graphite is a material consisting of two layers of carbon (2D) atoms arranged in a hexagonal lattice Because only of the orbitals p make bonds, there is a unpaired orbit, pz, which can be used in electron transport
There-fore, graphite is a good conductor The next carbon allotrope, Buckminster Fullerenes, was discovered in 1984 by Richard Smalley et al Next, the single-walled carbon nanotubes were discovered by Iijima in 1990
Intensive study of graphene, a two-dimensional (2D) material consist-ing of carbon atoms in the hexagonal network, began in 2004 The most notable characteristic of graphene is the thinnest crystal with extremely hardness, outstanding elasticity and thermal conductivity With a two-dimensional structure and a large surface area of about 2675 m2/g, graphene
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ous chemical reactions and interactions This creates great prospects for changing the properties of graphene Over the years, many theoretical and empirical studies for graphene have been carried out Specifically, the syn-thesis of single-layer, multi-layer graphene or graphene nanoribbon on the metal substrate were performed The electronic, chemical, magnetic and electrochemical properties of graphene have also been considered
Although graphene has excellent physical and chemical properties, graphene is a gapless material, which makes it difficult to apply graphene in field effect transistors and other electronic devices
In 2015, a new allotrope of carbon, penta-graphene is predicted by Zhang et al Penta-graphene (PG) exhibits mechanical and kinetic stability even when the temperature reaches 1000 K In addition, penta-graphene has a direct band gap about 3.25 eV, which is higher than that of other allotropes of carbon Penta-graphene exhibits many unique electric, thermal and opti-cal properties Studies on penta-graphene also show that hydrogenation can increase the thermal conductivity of PG PG doping Si, B, N Ge and Sn can reduce the band gap of PG, while doping transition metal can increase or decrease the band gap and simultaneously greatly enhances the absorption of hydrogen With a large surface area ratio and band gap, PG is beneficial for adsorption of gas molecules Recently, due to the superior physical and chemical properties, PG has been studied through theoretical calculations and showed that they have great potential for application in the field of nanoelectronics, nano mechanics and catalysts
From penta-graphene, one can form four types of penta-graphene nanorib-bon (PGNR) with different boundary forms Investigation of the electronic properties of four types of PGNRs shows that structures exhibit semicon-ductor or metal properties A number of works have focused on the elec-tronic properties of pure PGNR structures, passivation by different atoms or magnetic properties of PGNR forms
From the above analysis, the effect of polarization on electronic trans-port in low-dimensional systems, in particular 2D systems, need to be stud-ied in detail In addition, the electronic transport of novel material like graphene, namely penta-graphene nanoribbon, should be investigated more fully Therefore, problems such as polar heterostructure, electronic proper-ties in low-dimensional systems have to consider the influence of polarized charge, structural properties and electronic transport properties in penta-graphene nanoribbon will be presented in this thesis
The purposes of research
Studying on electronic transport phenomena in semiconductor nano struc-tures such as AlGaN/GaN and penta-graphene nanoribbon
The objects of research
(6)GaN/GaN and penta-graphene nanoribbon The methods of research
- Using the variational method to determine the electronic properties and deriving analytical expressions related to the mobility specific to the electronic transport phenomenon in the AlGaN/GaN system
- Using numerical methods, programming by Mathematica to identify the variational parameters and illustrate the physical quantities graphically
- Using the density functional theory and the non-equilibrium Green's function to investigate electronic characteristics (band structure, density of state, ) and electronic transport properties (I(V), T(E), ) in penta-graphene nanoribbon material system
- Using Origin software to process data - Comparing with several experimental results The structure of thesis
The thesis is presented as follows:
Introduction: An overview of the thesis Content
Chapter 1: Overview of research materials
Chapter 2: Electronic distribution in AlGaN / GaN structure
Chapter 3: Electronic transport phenomenon in AlGaN / GaN structure Chapter 4: Phenomenon of electronic transport in doped penta - graphene nanoribbon
Conclusion: Summarizing the contributions of the thesis and stating prospects for further research
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Chapter 1
Overview of research materials
1.1 Heterostructure AlGaN/GaN
1.1.1 Heterostructure
When the lattice mismatch between between the substrate and the layers or between the layer and the layer is suitable, an ideal crystal was made by two different material These structures are called heterostructures The discontinuity of the material arising in the heterojunction leads to a change in some important electrical and optical properties, such as carrier confine-ment (due to the discontinuity of the conduction band or the valence band) or radiation confinement (due to band gab discontinuity) The heterostruc-ture is made from a thin semiconductor layer (about 100 nm), sandwiched between two other semiconductor layers creating a potential well in the con-duction or valence band and is often referred to as a quantum well (QW) (eg AlGaAs/GaAs/AlGaAs) Heterostructures are largely based on semi-conductor alloys Semisemi-conductor alloys can consist of two elements, three elements or four elements (for example, GaAs, InAs, InP, GaAs, GaP, Al-GaAs, InGaAsP, )
1.1.2 Polar heterostructure
Polarization is an important characteristic of group III nitride semicon-ductors Wurtzite group III nitride structures not have inversion symme-try along thecaxis The strong ionicity of the metal - nitrogen bond results
(8)Figure 1.1: Atomic arrangement on Ga and N surface of GaN crystal Arrows indicate spontaneous polarization
The additional polarization in group III nitride structures due to strain is called piezoelectric polarizationPz The calculations for interface bound
charge and two-dimensional electronic gas (2DEG) depend on both piezo-electric polarizationPzand spontaneous polarityPsp The value ofPspin a
material is constant, whilePz is a function of strain and can be determined
from the following expression:
Pz=
a−a0
a0
e31−e33
C13
C33
(1.1) witha0 is the lattice constant when the system in not affected by stress or
compression,ais the lattice constant when the system is affected by stress
or compression, e31 and e33 as piezoelectric coefficients C13 and C33 as
elastic constants
1.1.3 The effect of polarized charge on electronic trans-port in AlGaN/GaN polar heterojunction
In AlGaN/GaN polar heterostructure, 2DEG density is very high, can reach n2d ≈ 2×1013 cm−2 and moblity at room temperature µ ≈ 1500
cm2/V.s The 2DEG sheet carrier density can be modulated by varying the
AlGaN barrier thickness as well as the Al content When a strong polariza-tion occurs (∼ MVcm−1), the electrons are electrostaticly repelled close
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and surface roughness scattering These scattering processes predominate at low temperatures, and even at room temperature when 2DEG is high density
In polar heterostructures, a new scattering mechanism does not exist in non-polar semiconductors and weak polarized semiconductors called dipole scattering This is due to the alloy disorder, resulting in the dipole moments in each unit cell being non-periodic with the lattice
The charge distribution in the direction of[0001]of the polar
heterostruc-turs can be determined by solving the Schrodinger equation and the Poisson equation in the approximate effective mass
1.2 Graphene and penta-graphene 1.2.1 Graphene
Graphene is a single graphite layer with high carrier mobility, in the range of 20005000 cm2/Vs Therefore, graphene is applicable to field-effect
transistors (FET) operating at high frequencies Graphene also exhibits high optical transparency of up to 97.7%which should be a potential candidate
for solar cell, storing three-dimensional data In addition, graphene also has high thermal conductivity, high Young's modulus and large surface area However, graphene is a gapless structure, so it is limited for applications in the field of optoelectronics As a result, different methods have been implemented to open the band gap of graphene such as doping, changing edge, applied field,
1.2.2 Graphene nanoribbon
Graphene nanoribbon (GNR) is a one-dimensional structure formed by cutting graphene in different crystal directions Based on the edge of graphene nanoribbon, there are two types of graphene nanoribbon: zigzag graphene nanoribbons (ZGNR) and armchair graphene nanoribbons (AGNR) In gen-eral, the properties of GNR are sensitive to many factors, such as doping, defects, edge changes, adsorption and external electric fields This offers many opportunities to tune and expand GNR applications Among the pro-posed methods, doping is one of the most frequently ways to adjust the properties of GNR
1.2.3 Penta-graphene
(10)dis-and the bond angles of penta-graphene structure is also affected by doping, passivation,
Figure 1.2: Penta-graphene nanoribbons
1.2.4 Penta-graphene nanoribbon
So far, types of penta-graphene nanoribbon (PGNR) have been created: zigzag PGNR (ZZPGNR), armchair PGNR (AAPGNR), zigzag-armchair PGNR (ZAPGNR) and sawtooth PGNR (SSPGNR) (Figure 1.2)