Preparation And Growth Mechanism Of Hydrogenated Nanocrystalline Silicon Films

In recent years, with the concept of sustainable development and environment protection deeply rooting among the people and gradual exhaustion of ordinary petrifaction energy source, tapping new energy source becomes more and more importent. In that field, PV solar energy is considered to be one of the most promising options. But in view of the limited reduction space upon cost of solar cells of silicon wafer, it is hard to compete with conventional energy. The cost of silicon wafer account for over95%of overall costs of solar cells raw material and energy consumption. Therefore, the main way to reduce the cost of solar cells is to manufacture thin films solar cells. Hydrogenated nanocrocrystalline silicon (nc-Si:H) has been confirmed as a promising PV absorber material, owing to its low cost, high conversion efficiency, good stability and good performance of photoluminescence and electroluminescence at room temperature.Impurities play a central role in semiconductor technology. In the case of Si nanocrystal-based solar cells, both phosphorus-doped n-type nc-Si:H thin films and boron-doped p-type nc-Si:H thin films are essential component. Since nc-Si:H thin films are mixed-phase material, where small and isolated Si nanocrytallites disperively distribute in an amorphous matrix, doping of Si nanocrystals is quite complicated and not fully understood yet. In this paper, Intrinsic, p-type and n-type nc-Si:H thin films were prepared by electron cyclotron resonance plasma-enhanced chemical vapar depositon (ECR-PECVD). Techniques such as TEM, XRD, Raman, XPS and FTIR were used to analyze the structure of the films. Hall system was carried out on the doped films to determine the electrical properties. The optical quality was measured by transmission spectrum. OES and Langmuir probe were used to investigate the plasma surrounding of the ECR-PECVD system. Then we made a deep research about the mechanism of nc-Si:H doping based on the experimental results. These studies provide abundant experimental and theoretical foundations to the further application of nc-Si:H. The results of this paper are showed below:1. Intrinsic nc-Si:H thin filmsIn this work we analyze the effect of substrate temperature, depostition time, working pressure and gas-flow rate on the structure of as-deposited films. Intrinsic nc-Si:H thin films were successfully prepared at low substrate temperature (250℃) by ECR-PECVD. Some useful rules and knowledge about the preparation of nc-Si:H thin films by ECR-PECVD have been grasped for next doping experiment, e.g. crystallinity of the films increased with the increase of substrate temperature. When the deposition time extend, the crystallinity of as-deposited films increased, at the meantime the grain orientation would changed to (220) preferential.2. n-type nc-Si:H thin filmsThe crystallinity of the as-grown phosphorus-doped n-type nc-Si:H thin films had a strong fall with the increase of PH3flow when the substrate temperature was250℃. And this effect reduced when the substrate tempereature rose to350℃. The influence of Ar/H2ratio on the characteristics of n-type nc-Si:H thin films prepared by ECR-PECVD was investigated. It was suggested that introducing Ar into the H2plasma strengthened the dissociation of PH3, and thus enhanced the doping efficiency. The content of H bonded as Si-H2and (Si-H2)n modes was getting smaller with the Ar/H2ratio increasing, which meant that both the asymmetry and stability of the films were improved. Thus it was a feasible method for improving the properties of phosphorus-doped nc-Si:H thin films that a moderate amount of Ar was introducted into the H2plasma.3. p-type nc-Si:H thin filmsB doped p-type nc-Si:H thin films could be prepared at low substrate temperature (250℃) by ECR-PECVD. The films showed excellent electrical properties and good optical properties, according with applicaton performance requirements of the window layer of solar cells. The influence of Ar/H2ratio on B-doped characteristics of nc-Si:H thin films prepared by ECR-PECVD has been also investigated. Diluting process gas H2with Ar did enhance the doping efficiency of B2H6. But unlike the n-type situation, Ar diluting did not increase the density of carrier, which means the increased amount of B in the films must be inactivated (B30).Then the plasma surrounding has been diagnosised by OES and Langmuir probe testing. The microcosmic mechanism of the influence of Ar/H2on the characteristics of P and B doped nc-Si:H films have been discussed deeply.