Real time spectroscopic ellipsometry of thin film Si1-xGex:H: phase diagrams for optimization in photovoltaics applications

The growth of silicon-germanium alloys (Si_1-xGe_x:H) by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon substrates has been studied by real time spectroscopic ellipsometry (RTSE). The motivation is to develop deposition phase diagrams that can be used in the optimization of amorphous Si_1-xGe_x:H (a-Si_1-xGe_x:H) for multijunction photovoltaics. In initial studies, the phase diagram for bottom cell a-Si_1-xGe_x:H (E_g ∼ 1.4 eV) is found to exhibit basic similarities to that for Si:H prepared under the same PECVD conditions. This similarity suggests directions for optimizing a-Si_1-xGe_x:H by identifying conditions under which smooth, stable surfaces are obtained to the largest possible bulk layer thicknesses. For low temperature (200 °C), low power PECVD of a-Si_1-xGe_x:H, high surface stability and smooth surfaces are obtained by cathodic deposition (self bias: ∼ -20 V), using a H₂-dilution level just below that of the amorphous-to-(mixed-phase microcrystalline) transition [a→(a+μc)].