Fabricaion Of Silicon Nanowires And The Optical Properties

Silicon nanowires (SiNWs) have received a great deal of attention recently, due to their unique optical, electronic, and thermal properties. Especially, the SiNW arrays have been proposed as a promising candidate for high-efficiency photovoltaic applications. In this thesis, we focus on the controllable growth of SiNW arrays with smooth surface via the metal-assisted chemical etching (MACE) technique, and also on both experimental and theoretical studies on the optical properties of the as-grown SiNW arrays by implementing the rigorous coupled-wave analysis (RCWA) approach. In addition, we have further numerically studied the optical properties of the crystalline Si/hydrogenated amorphous Si (a-Si:H) core/shell nanowire (CSNW) arrays via the RCWA simulation.To the best of our knowledge, in the SiNW arrays fabricated by MACE technique, it still remains to resolve a rather rough surface problem which can cause a high surface recombination velocity and thereby extremely limit the performance of NW photovoltaic devices. In the present work, we firstly show the realization of well-ordered SiNW arrays of smooth surface, fabricated by the MACE. It is found that the key success in the synthesis of high quality SiNW arrays lies in a better understanding of the role of H2O2 in MACE. From our growth investigation at various H2O2 concentrations with aids of TEM and photoluminescence measurements, we have observed that H2O2 not only serves as an oxidant, but also dissolves metal particles (Ag in our case) to maintain a certain concentration of free Ag+ ions that plays a key role in the formation of SiNW arrays having smooth surface. Thus it is indicated that surface smoothness in SiNWs can be tailored by simply adjusting the H2O2 concentration, and that the length of SiNW arrays can be controlled by the etching time. Also, we have proposed a reliable mechanism for the etching behavior of Si in MACE. Our growth results have clearly demonstrated that large-scale SiNW arrays with smooth surface can be successfully achieved by the MACE approach, which may open up a great opportunity for fabricating low-cost SiNW-based photovoltaic devices.We have also experimentally and numerically investigated the optical properties of the SiNW arrays and evaluated the performance of the SiNW arrays for the photovoltaic applications. It has been experimentally demonstrated that the reflection in the as-grown SiNW arrays can be significantly suppressed (<1%) over a wide solar spectrum (300-1000nm) in the as-grown SiNW arrays. Also, based on our established bundled model, we have used the RCWA method to simulate the reflection of the SiNW arrays, and found that the calculated results are in good agreement with the experimental data. From a further simulation studies on the light absorption and photocurrent in SiNW arrays, we have achieved the photocurrent enhancement of up to 425% per unit volume of material as compared to Si film, implying that effective light trapping and enhanced absorption can be realized in the as-grown samples. Also, we have demonstrated experimentally and theoretically that our SiNW arrays exhibit a wide angle- and polarization-independent antireflection behavior. The strong light trapping and omnidirectional antireflection together with surface smoothness realized in the as-grown SiNW arrays can serve as powerful tools to develop high-efficiency NW-based solar cells.Additionally, we have further numerically analyzed the optical properties of the crystalline Si/a-Si:H core/shell nanowire (CSNW) heterostructure arrays via the RCWA simulation. It is found that the CSNW arrays can further enhance the optical absorption and photocurrent, compared to the pure SiNW arrays and the Si film. We have also found out that this absorption enhancement lies mainly in the amplified leaky-mode resonances occurred in CSNWs, due to the highly-absorptive a-Si:H shell. Finally, we have obtained the optimal parameters of the filling ratio (f=0.5) and the pitch (p~600nm) of the CSNW arrays for photovoltaic applications, which can be used as guidelines to design CSNW-based photovoltaic applications.This work is supported by the National Major Basic Research Project of 2010CB933702, and Natural Science Foundation of China under contracts 10734020 and 11074169.