Research On Amorphous Silicon Thin Films And The Optimization Of A-Si/c-Si Interface

With the continuous development of semiconductor material and its processing technology, semiconductor device has been widely applied in military and civilian fields. People nowadays have developed various kinds of silicon devices with many functional performance based on purified monocrystalline silicon through different technologies and/or different geometrical structures. However, how to improve the efficiency of semiconductor devices, fabricate them with high qualities, is always the efforts of researchers and engineers. And the in-depth exploration and research on the interface optimization of silicon semiconductor devices, is of great significance. Hydrogenated amorphous silicon(a-Si:H) thin films have many characteristics like high temperature coefficient of resistance, high light absorption rate, easily controlled forbidden band width, etc. And due to its advantages of no limit to substrate type, large area deposition compatibility at low temperature, simple production process, compatible with traditional silicon semiconductor technology, etc., a-Si:H thin film has been applied in many fields as thin film solar cells, thin film transistor and infrared detectors. However, amorphous silicon thin film itself has many defects, and when it is used to build a variety of heterojunction structures with crystalline silicon or other substrates, the interface quality will directly affect the performance of silicon-based devices. Thus the quality control and the optimization of amorphous silicon thin film and the interface of a-Si/c-Si structure become an very important basic research subject.The hydrogenated amorphous silicon(a-Si:H) thin films were fabricated by radio frequency magnetron sputtering deposition technology with different hydrogen flow rates and afterwards annealing heat treatment. The microstructure of the thin film and its optical performance were studied by use of several modern methods like FT-IR, Raman, UV-Vis, etc. A new way to monitor the minority carrier lifetime of crystal silicon was used to indirectly represent the defects in amorphous thin films by building an a-Si/c-Si heterojunction. The results indicated that with the increase of hydrogen flow rate in the process of PVD technology, Si-H bondings in a-Si:H thin films have changed significantly, resulting in apparent microstructural evolution. The in-situ annealing heat treatment has very strong effects on the optimization of a-Si/c-Si interface. When the hydrogen flow rate is taken at 15 sccm and the annealing heat treatment time is set as 1h, the passivation effect on c-Si by depositing a-Si:H thin film onto its surface reaches its best, and the minority carrier life of the specimen can be improved by almost 4 times. The change of hydrogen flow rates, and the process with or without annealing, will finally have its influence on the surface roughness, the hydrogen content, the order of amorphous network, and the film defects itself, etc.It is noteworthy that the etching effect by hydrogen ions in the process of film growing and the hydrogen escaping during the annealing, to a certain extent, reduce the defects in a-Si:H thin films and improve the interface optimization. The results also indicate that when the hydrogen flow rate is increased over a certain threshold, on the contrary, the hydrogen content in a-Si:H thin films will be reduced, and so will be increased the minority carrier lifetime of crystal silicon. In addition, after the in-situ annealing, the minority carrier life of crystal silicon is increased with a maximum ratio by nearly 80 %. It can be concluded that the improvement of film passivation to the surface of crystal silicon is caused by the evolution of internal Si-H bondings and the reduction of semiconductor surface activity. These effects are conducive to the exciton dissociation and the carrier transport in the a-Si/c-Si interface.