Скачать 153.23 Kb.
|CITY UNIVERSITY OF HONG KONG|
PHYSICS AND MATERIALS SCIENCE
BACHELOR OF ENGINEERING (HONS) IN MATERIALS ENGINEERING
Submitted in partial fulfilment of the
requirements for the degree of
BACHELOR OF ENGINEERING (HONS)
City University of Hong Kong
Table of Contents i
2.1 Introduction of GaAs 1
2.2 GaAs Nanowires (GaAs NWs) 2
2.3 The growth mechanism of the fabrication of GaAs NWs 3
2.3.1 Vapor-solid-solid (VSS) mechanism 3
2.3.2 Vapor-liquid-solid (VLS) growth 4
2.4 The process parameters 5
2.4.1 The use of catalyst for GaAs NWs 5
2.4.2 The substrate temperature 6
2.4.3 The source temperature 7
2.4.4 The precursor ratio, Growth time and carriers gas 7
2.5 Characterization of the GaAs NWs 8
2.5.1 The diameter distribution of GaAs NWs 9
2.4.1 The electrical performance of NWs field effect transistors (NW FET) 10
2.5.2 The relation between the IDS – VGS and IDS – VGS of FET for GaAs NWs 11
2.6. Nanowires application: the NWs solar cell 12
4.1 The sample preparation: 17
4.3 The solar cell device fabrication: 19
4.4.1 Electrical and Photovoltaic (PV) measurement 21
5. Results and Discussion 21
5.1 Structural characterization 22
5.3 The electrical measurement of the GaAs NWs solar cell 27
6. Conclusion 32
7. References 33
 Müller, C. M.; Spolenak, R. Microstructure evolution during dewetting in thin Au films, Acta Mater. 2010, 58, 6035–6045. 35
I would like to express my heartfelt thanks to those who have guided and assisted me to complete this dissertation.
Firstly, I would like to thank my supervisor Dr. Johnny Ho who offered me a chance to join his team and to investigate this professional field. I would like to thanks for the professional and useful advices suggested by him so that I can have a clear mind to conduct the experiment and write the report.
Secondly, I would like to thank Dr. Xian Feng Chen for his further supervision. With his help, I can make sure my project is on the right track and I can follow my schedule to do the project
Thirdly, I would like to thank my project tutor Dr. Han Ning. He gave me a clear guideline and helps for the entire project. He shared his research experience, taught me the entire process of the experiment and gave me advices on my report.
Finally, I would like to thanks all of them for their times and I have learnt a lot from this project including the synthesis and characterization of the nanowires and the fabrication of nanowires solar cell.
List of figure
Figure 1. (a) is the appearance of GaAs, (b) is the cubic Zinc blende crystal structure of the GaAs  2
Figure 2.The growth mechanism of GaAs using Ni as a catalyst  3
Figure 3. the growth mechanism of the VLS  4
Figure 4. SEM image of the GaAs NW (scale bar = 2µm)  6
Figure 5.The general working principle of Philips X’Pert 9
Figure 6. (a) is the diameter statistics of the NWs obtained from the TEM images, (b) is the XRD pattern of the GaAs NWs, (c) is the SEM image and the schematics view of the back-gated GaAs NWFET with optimal condition but 200sccm H2 gas flow.  10
Figure 7.(a) the IDS – VGS and (b) IDS – VDS curves of GaAs NWFET  11
Figure 8. the nanostructure with improved carrier collection  13
Figure 9. the general process of photolithography  20
Figure 10, The XRD patterns of the GaAs NWs with different growth parameters (different thicknesses of Au catalysts) 22
Figure 11, (a) the morphology of NWs (2.5 nm Au catalyst) and (b) the corresponding diameter statistic 24
Figure 12, (a) the morphology of NWs (4 nm Au catalyst) and (b) the corresponding diameter statistic 24
Figure 13, (a) the morphology of NWs (6 nm Au catalyst) and (b) the corresponding diameter statistic 25
Figure 14, (a) the morphology of NWs (12 nm Au catalyst) and (b) the corresponding diameter statistic 25
Figure 15. (a) is the electrical measurement of sample Au catalyst thickness 2.5 nm, (b) is the electrical measurement of sample Au catalyst thickness 4 nm, (c) is the electrical measurement of sample Au catalyst thickness 6 nm, (d) is the electrical measurement of sample Au catalyst thickness 12 nm 27
Figure 16. (a) SEM image of one typical GaAs NW PV cell prepared by 4 nm Au, and (b) is the corresponding IV curves under dark and 1 Sun illumination. 28
Figure 17. The principle of solar cell device (a: p-type, b: ambipolar, c: n-type) 30
Figure 18. (a) SEM image of one typical GaAs NW PV cell prepared by 2.5 nm Au, and (b) is the corresponding IV curves under dark and 1 Sun illumination. i
Figure 19. (a) SEM image of one typical GaAs NW PV cell prepared by 4 nm Au, and (b) is the corresponding IV curves under dark and 1 Sun illumination. ii
Figure 20. (a) SEM image of one typical GaAs NW PV cell prepared by 6 nm Au, and (b) is the corresponding IV curves under dark and 1 Sun illumination. ii
Figure 21. (a) SEM image of one typical GaAs NW PV cell prepared by 12 nm Au, and (b) is the corresponding IV curves under dark and 1 Sun illumination. iii
List of table
Table1. The 2θ values taken from XRD and the d value calculated by Bragg’s law...22
Table2. The standard powder diffraction pattern (Reference code: 00-014-0450 (Powder XRD)) ……………………………………………………………………...23
There are wide ranges of applications of III/V semiconductor in today’s world. In recent years, the synthesis of nano-scale materials is of paramount importance as it helps to reduce cost for material extraction and it may have unique favorable properties when compared to the bulk and thin film devices.
Particularly, the use of the III/V semiconductor - gallium arsenide (GaAs) is the well-known material for making NW solar cell devices due to its superior properties including the excellent carrier mobility, direct band gap (Eg = 1.45eV) and insensitivity to heat.
In this project, the GaAs nanowires were synthesized by the method of the low cost bottom up approach via the growth of NWs with the vapor-liquid-solid, VLS mechanism with the source temperature (890 oC), growth temperature (800 oC for annealing and 590 oC for growing), growth time for 1hour and 100 sccm H2 gas flow (pressure ~ 0.5 Torr). The mean diameter of the nanowires varied directly with the thickness of the gold catalyst. The solar cell devices were fabricated by the method of photolithography with two electrodes Aluminum and Nickel. Moreover, the efficiency and electrical properties of nanowire based solar cell devices with different sizes of NWs were studied. The optimal diameter of nanowires (50.8 ± 10nm) was synthesized by coating the substrate with 4nm gold catalyst thickness.
The greatest fill factor (~ 34.7%) and efficiency (~ 0.76%) were found for the nanowires solar cells with optimal nanowies diameter among 4 samples (thickness of catalyst: 2.5 nm, 4 nm, 6 nm, 12 nm) due to the ‘ambipolar’ property of this optimal size of nanowires. The proposed band model (qualitative model) explaining the high performance ambipolar structured solar cell structure was discussed. Further investigation of the ambipolar property of nanowires should be conducted in order to set a more accurate quantitative model in the future.
Nanotechnology has been widely used in varied industries. In past decades, the uses of nanowires were applied in a broad range including: electronics, optoelectronics and sensors [1-4].
One of reasons of using nanowires is due to its surface - dependent property. The entire nanostructure properties (electrical, optical, thermal, chemical, mechanical, biological etc.) can be effected by simply engineering the surface properties of the nanowires, which can provide flexibility on fabricating desired devices.
In today’s world, the new technology of the NWs can be used in conversion devices which convert the thermal and solar energy into electricity. The advantages of using nanowires solar cells over wafer-based or thin-film devices related to electrical, optical and strain relaxation effects; low cost and new charge separation mechanism. The radial junction design of solar cells helps to reduce reflection and it has an extreme light trapping. The single-crystalline structure of the nanowires can also be synthesized on the non-epitaxial substrate.
In the following sections, the literature search of the synthesis of GaAs NWs and the photovoltaic (PV) application (solar cell) are discussed.