Roughness and Phase Evolution in Si1-xGex:H: Guidance for Multijunction Photovoltaics

In this study, the amorphous-phase roughening transition thickness has been determined as a function of process variables in plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon-germanium alloys (a-Si_1-xGe_x:H). The process variables include the H₂-dilution gas flow ratio, the alloying flow ratio, the electrode configuration, and the He-dilution ratio. One clear feature of this study is a maximum in the amorphous roughening transition thickness (and hence surface stability) at a H₂-dilution ratio just below the transition from amorphous to mixed-phase (amorphous +microcrystalline) growth. A second feature for high Ge content films is a significant increase in the roughening transition thickness for cathode PECVD (with a self-bias of ~-20 V). Additional features of interest involve suppression of the mixed-phase transition for (i) alloying with Ge, (ii) biasing the substrate cathodically, and (iii) diluting the gas with He