Hydrogenated silicon (Si:H) and silicon-germanium alloy (Si 1-x-Ge x:H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH 4 and GeH 4 and measured during growth using real time spectroscopic ellipsometry (RTSE). A two-layer virtual interface analysis has been applied to study the structural evolution of Si:H films prepared in multistep processes utilizing alternating layers of high and low H 2-dilution materials, which have been designed to produce predominantly amorphous silicon (a-Si:H) films with a controlled distribution of microcrystallites. The compositional evolution of alloy-graded a-Si 1-x-iGe xH has been studied as well using similar methods. In each study, the depth profile of the microcrystalline silicon (μc-Si:H) content,f μc, or the Ge content, x, has been extracted. Additionally, RTSE has been used to monitor post-deposition exposure of a-Si:H, a-Si 1-xGe x:H, and a-Ge:H films to hydrogen plasmas in situ in order to study the sub-surface modification and etching that would be anticipated when a highly H 2-diluted layer is deposited on a layer prepared with lower dilution. These analyses provide guidance for enhancing the performance of Si:H based solar cells through controlled fractions of microcrystallites in bulk amorphous i-layer materials using modulated H 2 dilution, through controlled bandgap profiling using compositionally graded a-Si 1-xGe x:H, and through a better understanding of the modification of underlying layers during the deposition of subsequent layers in multilayer stacks.