Low temperature growth of microcrystalline silicon and its application to solar cells

Low temperature growth of microcrystalline silicon using plasma enhanced chemical vapor deposition (PECVD) with monosilane diluted by hydrogen has been studied. It is demonstrated that lower ion energy and sufficient amount of atomic hydrogen are crucial for crystal formation at low temperatures. We have clarified the ambivalent roles of hydrogen: one is surface termination during the growth, and the other is surface reaction including abstraction and exchange between surface Si-H bonds and atomic H. We present experimental evidence for the importance of surface coverage, i.e. the optimum surface Si-H coverage in low temperature epitaxy on Si(001) by PECVD. The surface reaction between Si-H and impinging atomic hydrogen has been studied using deuterium dilution. We found that an exchange reaction between Si-H and weakly adsorbed D on the surface has an intimate relation to crystal formation. High-rate growth of microcrystalline silicon is a key issue from an industrial point of view because of its indirect band gap. On the basis of guidelines of low ion bombardment energy and sufficient amount of atomic hydrogen, we have developed a novel deposition method combining higher deposition pressure and depletion of silane source gas (High-Pressure-Depletion method). A growth rate as high as 50 A/s is achieved at a growth temperature of 250 degreesC. It is also demonstrated that our novel deposition method is successfully applied to solar cell fabrication and that a cell efficiency of 8.6% is achieved at a deposition temperature of 140 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.