Development of Deposition Phase Diagrams for Thin Film Si:H and Si 1-x Ge x :H Using Real Time Spectroscopic Ellipsometry

The growth of hydrogenated silicon (Si:H) and silicon-germanium alloys (Si 1-x Ge x :H) by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) substrates has been studied by real time spectroscopic ellipsometry (RTSE). The motivation is to develop deposition phase diagrams that can provide greater insight into the optimization of amorphous Si 1-x Ge x :H (a-Si 1-x Ge x :H) for multijunction photovoltaics. In initial studies, the phase diagram for bottom cell a-Si 1-x Ge x :H (E g ~ 1.4 eV) is found to exhibit fundamental similarities to that for Si:H when both materials are prepared under standard PECVD conditions that optimize pure a-Si:H. These similarities suggest directions for optimizing a-Si 1-x Ge x :H by identifying conditions under which a smooth, stable surface is obtained to the largest possible bulk layer thickness. In phase diagram development for PECVD Si 1-x Ge x :H on c-Si, it has been found that the highest surface stability and smoothest surfaces are obtained using cathodic deposition (self bias: ~–20 V) with a H 2 -dilution level just below that of the amorphous-to-(mixed-phase microcrystalline) [a→(a+μc)] transition for a thick layer. Due to the promising nature of these results, full phase diagrams are compared for cathodic and anodic Si 1-x Ge x :H as well as for cathodic and anodic Si:H, all on c-Si substrates. The cathodic phase diagram for Si 1-x Ge x :H reveals a narrow range of significant improvement in surface structural evolution near the a→(a+μc) transition, and for a-Si:H reveals an extension of the ultrasmooth protocrystalline regime to a much wider range of thickness.