Photovoltaics, Controlled nucleation and growth process for large grained polycrystalline silicon.

Research has been conducted to develop a new means of producing large grained polycrystlline thin films for photovoltaic applications. The process is one of growth of controlled crystalline nuclei in an a-Si film. For this process to be practicle, we must develop a-Si films with crystallization behavior such that the rate of spontaneous crystallization (nucleation rate) is essentially zero at temperatures where the growth rates are > 104 A/min. Amorphous Si films, deposited on oxidized single crystal substrates by RF sputtering, E-beam evaporation, and CVD, have been examined. The spontaneous crystallization behavior and the rates of nucleation and growth of crystals in the Si films have been evaluated using X-ray diffraction and TEM. The results show that microstructural features and impurities in the RF sputtered films result in very high nucleation rates and extremely low growth rates. The CVD films have high nucleation rates associated with the high deposition temperatures. The growth rates for Si crystallites into a-Si in the CVD films are only a factor of 20 below desired levels.For process demonstration one requires a deposition process that will produce extremely high purity (- 1018/cm3) a-Si films at low deposition temperatures. It remains to be seen if this can be done economically by E-beam or conventional CVD processes.Solar Energy Research Institute, U.S. Dept. of Energy