Growth and characterization of germanium and silicon nanostructures

Growth and Characterization of Germanium and Silicon Nanostructures Huang Jinquan A Thesis Submitted for the Degree of Doctor of Philosophy Department of Electrical and Computer Engineering National University of Singapore 2010 i Abstract In this dissertation, the growth and characterization of five different types of germanium (Ge) and silicon (Si) nanostructures are presented The nanostructures include one-dimensional Ge nanowires (GeNWs), GeSi oxide nanotubes (GeSiOxNTs), heterostructures of GeNW-GeSiOxNT, Si nanowires (SiNWs) and near zerodimensional Ge nanodots (GeNDs) The first three were obtained using bottom-up approaches where the materials were self-assembled together with the aid of metal catalysts The formation of the SiNWs, on the other hand, was by a top-down process making use of metal nanodots formed using an anodized aluminium oxide (AAO) template AAO was also utilized as a thermal evaporation mask for the deposition of the regular arrays of GeNDs The formation mechanism of each type of nanostructure was investigated in detail GeNWs were obtained via the vapour-liquid-solid growth catalyzed by active gold (Au) droplets On the other hand, the formation of the GeSiOxNTs required passivation of the Au catalyst so that growth was limited to the rims of the Au dots Consequently, the GeNW-GeSiOxNT heterostructure was a result of timely control of the Au passivation such that formations of hollow tubes and solid wires took place at different time For the top-down fabrication of SiNWs, uniform and well-aligned SiNWs were produced by chemical wet etching using AAO-templated chromium/gold nanodots as a hard mask blocking material This dissertation also explored some unique properties of the as-synthesized nanostructures In particular, thermal conductance measurements have shown that the wire-tube heterostructure demonstrated a thermal rectification as high as 6% The different charge-trapping characteristics of the GeNDs were also studied using the scanning capacitance microscopy technique ii Acknowledgements First and foremost, I am particularly grateful to my thesis supervisors: Wai Kin for his support and guidance, especially his enigmatic encouragement in looking out for serendipity which indeed miraculously happened; Shijie for allowing me extreme freedom in pursuing any area of my interest in my Ph.D studies I am also extremely fortunate to have worked with Sing Yang, my “master” in all areas including the correct approach to research, the intelligent tricks, e.g how to be at least not-wrong when I cannot prove I am right, in manuscript preparation, the happy hours in WalaWala, my first Kilkenny beer (and the countless ones after), etc Special thanks also go out to: Nancy for the numerous TEM sessions she performed for me and I sincerely wish that her eyesight did not suffer as a result; Prof John Thong for his gracious accommodation in CICFAR, and Mrs Ho and Chee Keong for their help in preventing a logistical nightmare Their hearts must still be fluttering with fear after my two unintentional, and fortunately unsuccessful, attempts to “destroy” the lab using fire and flood Meng Lei and Chee Leong for the regular tea sessions during my thesis writing days though they seldom helped to wash the tea sets Rongguo and Cong-Tinh for the thermal rectification measurements Folks like Anna, Pi Can, Ren Yi, Wang Rui, Huijuan, Ziqian, Alfred, Heng Wah, Shi Fa, Jason and many others for their wonderful company; some must have profited a lot over the bets and the mahjong games that I lost during my stay in CICFAR Lastly, I am eternally grateful to my family My sisters and my brother for taking care of my mum, who has always treated me with unconditional love and care My dad, who now must have been blessing me in the other world His strict home teachings had trained me well and helped me tide over the difficult time in my Ph.D studies Table of Contents iii Table of Contents Abstract i Acknowledgements ii Table of Contents iii List of Figures vii List of Tables xiii Chapter Introduction and Motivation 1.1 Nanotechnology 1.2 Semiconductor Nanostructures 1.3 Challenges and Opportunities in Syntheses of Si and Ge Nanostructures 1.4 Organization of Thesis Chapter 2.1 Literature Review VLS Growth of Si and Ge Nanowires 2.1.1 VLS Mechanism and Its Variants 2.1.2 Factors affecting VLS Growth 12 2.2 SiNWs through Catalytic Etching 22 2.2.1 One-step Etching in Ionic Metal HF Solutions 23 2.2.2 Etching in HF/H2O2 with Patterned Metal Catalyst 29 2.3 VLS and Catalytic Etching as Complementary Methods 30 2.3.1 Material Types 30 2.3.2 Axial Orientation 31 Table of Contents iv 2.3.3 Nanowire Morphology 34 Summary 36 Chapter 3.1 Theory 37 Anodic Aluminium Oxide 37 3.1.1 Anodization Process 38 3.1.2 Mechanism for Formation of Regular Hexagonal Pore Arrays 39 3.1.3 Anodization of Al with Pre-textured Surface 44 3.1.4 Ultra-Thin AAO as an Evaporation Mask 47 3.2 Scanning Capacitance Microscopy (SCM) 49 3.2.1 SCM Operation Principle 50 3.2.2 SCM Operation Modes 56 Summary 62 Chapter GeNWs and GeSiOxNTs 63 4.1 Introduction 63 4.2 Experiment and Results 64 4.2.1 Sample preparation 64 4.2.2 GeNWs 66 4.2.3 GeSiOxNTs 69 4.3 Growth Mechanism 76 Summary 82 Chapter Heterostructures of GeNW and GeSiOxNT 83 5.1 Introduction 83 5.2 Experimental Details 85 Table of Contents v 5.3 Results and Analysis 86 5.3.1 Structural Characterization 86 5.3.2 Chemical Composition 90 5.4 Growth Mechanism 93 5.5 Application in Thermal Rectification 98 Summary 104 Chapter Well-aligned and Uniform SiNWs by Catalytic Etching 105 6.1 Introduction 105 6.2 Experimental Setup and Procedures 108 6.2.1 Control Experiment 108 6.2.2 Fabrication Procedure 110 6.3 Results and Discussion 112 6.3.1 General Morphology 112 6.3.2 Crystallinity 116 6.3.3 Precise Diameter Control 117 6.3.4 Other Masking Metals 121 Summary 127 Chapter GeNDs and Their Charge Trapping Characteristics 129 7.1 Introduction 129 7.2 GeND Fabrication 130 7.3 Results and Discussion 131 7.3.1 Surface Morphology of GeNDs 131 7.3.2 SCM Characterization 133 7.3.3 Group II GeNDs 142 Table of Contents vi Summary 144 Chapter Conclusion 145 8.1 Summary of Findings and Conclusion 145 8.2 Future Works 148 References 149 Appendix A: List of Publications 170 A1 Thesis-related Publications 170 A2 Other Publications 170 List of Figures vii List of Figures Figure 1-1 Intel central processing unit (CPU) transistor count trend Figure 2-1 (a) Au-Si binary phase diagram showing the compositional and phase evolution during the nanowire VLS growth process (b) Schematic depiction of the nanowire VLS growth Figure 2-2 Plot of the optimum growth temperature as a function of the diameter of the gold particle seeds for CVD growth of GeNWs 16 Figure 2-3 Variation in the shapes of GeNWs at different temperatures 20 Figure 2-4 Variation in the diameter and the aspect ratio of the GeNWs as (a) a function of pressure of GeH4 at 290 °C, and (b) a function of the growth temperature at 40 Torr of GeH4 21 Figure 2-5 (a) Scanning electron microscopy (SEM) micrographs of large-area SiNWs obtained in this project by catalytic etching in HF/AgNO3 (b) SEM image of the SiNWs at a higher magnification 24 Figure 2-6 Schematic depiction of the formation of vertically aligned SiNWs on a Si surface in ionic AgNO3/HF solution 27 Figure 2-7 HRTEM images of (a) an alloy-wire interface of a SiNW with a growth axis, (b) an alloy-wire interface of SiNW with a growth axis, (c) HRTEM cross-sectional image, and (d) the equilibrium shape for the wire cross sections predicted by Wulff construction 32 Figure 2-8 SEM micrographs of regular arrays of (a) Si nanowires of oval crosssections, (b) Si nanofins and (c) cylindrical nanowires obtained through laser interference lithography with different conditions combined with catalytic etching 35 Figure 3-1 Scanning electron microscopy (SEM) micrographs taken at (a) a 0o-tilt view and (b) a 45o-tilt view of an AAO template (with barrier layer removed) used in this project (c) SEM images of regular metal nanodots and (d) carbon nanotubes synthesized through the use of AAO templates 38 Figure 3-2 Simplified schematic of an electrolytic cell for aluminium anodization 39 Figure 3-3 Schematic diagrams for the electric-field strength distribution in some typical oxide barrier layers with the electrolyte-oxide interface marked by A, B, C and the oxide-metal interface marked by A’, B’, C’ 41 List of Figures viii Figure 3-4 (a) Two neighbouring pores having a separation larger than 2dE (b) The pores move towards each other to achieve a wall thickness of 2dE (c) The pores move closer with 2dW < 2dE (not drawn to scale) and a balanced curvature of 2θ < 180o (d) Two neighbouring pores that are too close to each other and (e) their self-adjustment to increase the wall thickness 42 Figure 3-5 SEM micrographs of (a) a barrier layer with hexagonally packed structure, viewed at a 0o-tilt, and (b) an oblique angle view of the cross-section of a typical AAO used in this project 43 Figure 3-6 SEM micrographs of AAO templates obtained from different acid electrolytes 44 Figure 3-7 Schematic depiction of formation of self-ordered porous AAO through a two-step anodization 46 Figure 3-8 Effect of surface pretexturing on anodization 47 Figure 3-9 SEM micrographs of ordered AAOs with inter-pore distances of (a) 100 nm, (b) 150 nm, and (c) 200 nm 47 Figure 3-10 Procedures of formation of metal dot arrays by evaporation through an AAO template 48 Figure 3-11 (a) Typical setup for atomic force microscopy (AFM) (b) Force-distance diagram showing the different regimes of tip deflection 51 Figure 3-12 Basic SCM detection system 53 Figure 3-13 The capacitance measured by the SCM sensor varies as the carriers move towards and away from the conductive cantilever tip 54 Figure 3-14 (a) High-frequency CV curves for a heavily and a lowly doped n-type semiconductor The CV curves in (b) shows the δC/δV for both n- and p-type materials 55 Figure 3-15 (a) 2D Topography image by AFM of the SRAM test sample used in this project, and (b) its reconstruction in 3D 56 Figure 3-16 SCM contrast images of the SRAM sample taken in (a) amplitude mode, and (b) hybrid-data mode with a 90o lock-in phase 58 Figure 3-17 Section analysis along the white line indicated in Figure 3-16(b) 58 Figure 3-18 High frequency CV curves and the corresponding differential capacitance δC/δV dependence on the dc bias for (a) n-type, and (b) p-type semiconductors 60 List of Figures ix Figure 3-19 Effect of different charges on (a) the high frequency CV curve, and (b) the δC/δV curve 61 Figure 4-1 Block diagram of a thermal evaporation system 65 Figure 4-2 (a) SEM micrograph of individual Au-dots obtained by annealing a nm Au film (b) Size distribution of 100 typical Au-dots randomly selected across the sample 66 Figure 4-3 (a) Setup, and (b) temperature setting for GeNW growth 67 Figure 4-4 (a) and (b) SEM images showing GeNWs with smooth surface morphology (c) TEM image of several Ge nanowires, which have a uniform diameter of about 80 nm.(d) High resolution TEM (HRTEM) image of a single Ge nanowire showing the growth direction and its SAED image (inset) 69 Figure 4-5 Block diagram of the experimental setup for the growth of GeSiOxNTs 70 Figure 4-6 (a) SEM image of the as-synthesized GeSiOxNTs (b) Close examination of the nanotubes reveals that each nanotube is a long, tubular structure with uniform diameter (c) and (d) SEM images showing the open-ended GeSiOxNTs and the wavy surface of the walls of the tubular structure 71 Figure 4-7 (a) TEM image of a single GeSiOxNT and (b) its HRTEM image 72 Figure 4-8 (a) Ge3d core level XPS spectra and (b) Si2p XPS spectra of GeSiOxNTs 74 Figure 4-9 STEM-EDX mapping of (b) Ge, (c) O and (d) Si of a typical GeSiOxNT in (a) 74 Figure 4-10 TEM spot EDX spectrum of a typical GeSiOxNT 75 Figure 4-11 (a) to (c): TEM images of a single GeSiOxNT showing gradual shape transformation under 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germanium nanowires and germanium-silicon oxide nanotubes and growth mechanisms”, Nanotechnology, vol 20, no 42, article no 425604, pp 425604-1 to 425604-8, 2009 Huang, J Q.; Chim, W K.; Wang, S J.; Chiam, S Y.; Wong, L M., “From germanium nanowires to germanium-silicon oxide nanotubes: Influence of germanium tetraiodide precursor”, Nano Letters, vol 9, no 2, pp 583-589, 2009 Huang, J Q.; Chim, W K.; Wang, S J.; Chiam, S Y.; Wong, L M., “Germanium nanostructures: Under control”, Nature Publishing Group, Asia Materials research highlight, 15 April 2009 Wong, K M.; Chim, W K.; Huang, J Q.; Zhu, L., “Scanning capacitance microscopy detection of charge trapping in free-standing germanium nanodots and the passivation of hole trap sites”, Journal of Applied Physics, vol 103, no 5, article no 054505, pp 054505-1 to 054505-5, 2008 A2 Other Publications Ren, Y.; Chim, W K.; Chiam, S Y.; Huang, J Q.; Pi, C.; Pan, J S., “Formation of nickel oxide nanotubes with uniform wall thickness by low-temperature thermal oxidation through understanding the limiting effect of vacancy diffusion and the Appendix A: List of Publications 171 Kirkendall phenomenon”, Advanced Functional Materials, vol 20, pp 3336-3342, 2010 Wong, L M.; Chiam, S Y.; Huang, J Q.; Wang, S J.; Pan, J S.; Chim, W K., “Energy band alignment of Cu2O and ZnO thin film heterojunctions”, Accepted for publication in Journal of Applied Physics, vol 108, no 3, article no 033702, pp 033702-1 to 033702-6, 2010 ... exploration of self-assembled synthesis and the possible applications of new Si and/ or Ge nanostructures and, (b) achieving a controlled growth of Si and Ge nanostructures 1.4 Organization of Thesis... Abstract In this dissertation, the growth and characterization of five different types of germanium (Ge) and silicon (Si) nanostructures are presented The nanostructures include one-dimensional... Temperature profiles of Ge and GeI4 sources and Au-dotted Si substrate for the growth of the wire-tube heterostructures 93 Figure 5-8 Schematic depiction of the growth mechanism of type-1