Phase diagrams have been studied to describe the RF PECVD process for intrinsic-hydrogenated silicon Si:H and silicon–low germanium alloy a-Si 1− x Ge x :H thin films using textured Al substrates that have been overdeposited with n-type amorphous Si:H (n + a-Si:H). UV, vis, IR, atomic force microscopy (AFM), Raman spectroscopy, small angle X-ray and cross-section transmission electron microscopy (TEM) are used to establish the phase diagram. The a-Si:H, a-Si 1− x Ge x and μc-Si:H processes are applied for optimization of triple-junction thin silicon-based n–i–p solar cells.