Phosphorus Doped Nanocrystalline Silicon Thin Films And Related Characteristics

Plasma enhanced chemical vapor deposition (PECVD) is one of the mature and simple thin film deposition methods. It suits to be used to deposit large area high-uniformity film. But now this method faces two problems: one is the micro-structural characterization for thin films; the other is the transport property of thin films. Micro-structural characterization and the transport properties decide electric characteristic. So preparing high quality thin film rapidly is a key element for solar cell. Aiming at this target, scientists try to optimize deposition parameters and use recrystallization method.PECVD method is used to deposit polycrystalline silicon thin films at low temperature. The influence of deposition parameters including substrate temperature and ratio of SiH₄ on crystalline volume fraction and average crystalline grain size was studied systematically. Based on the experimental results above, phosphorus-doped nc-Si:H thin films were deposited and the influences of phosphorus doping on the crystalline volume fraction, electrical conductivity, average grain size, lattic strain and optical energy gap of nc-Si:H films studied. The results can be accounted for by film growth theories and HQD model. It shows that:(1)Decreased silane concentration helps to the increase in crystalline volume fraction X_c and decrease in inflection point temperature for intrinsic nc-Si:H films. And average grain size d, X_c change similarly with the substrate temperature increasing.(2)The mobility of free carriers is a key factor to affect the electrical conductivity of intrinsic nc-Si:H films and free carrier concentration is a limited and second one. While free carrier concentration and mobility are both the main factor to affect electrical conductivity for phosphorus-doped nc-Si:H films. Phosphorus doping can help to crystallization, but extra doping can lower the crystalline volume fraction and electrical conductivity for the enhanced lattic strain. The results above can be accounted for by HQD model.(3) The grain size of nc-Si:H thin films have no preferable direction derived; The smaller grain size is, the larger lattic strain is.The reason is that lattic strain increases with the increase in surface pressure from smaller grain size.(4)The micro-structural characterization for nc-Si:H thin films has a close relationship with its optical energy gap (E_opt). E_opt is strongly affected by p-doping concentration.(5)Silver oxide films were prepared by magnetron-controlled sputtering method. Increased ratio of O₂ to Ar can help the oxidation of silver. When the ratio of O₂ to Ar arrives at 2:1, silver is completely oxidized and the corresponding new optical disc including covered silver oxide layer prepared at 2:1 ratio of O₂ to Ar has a better signal-to-noise.