Interface-layer formation mechanism ina−Si:Hthin-film growth studied by real-time spectroscopic ellipsometry and infrared spectroscopy

Real-time spectroscopic ellipsometry (SE) and infrared attenuated total reflection spectroscopy (ATR) have been applied to investigate interface layer formation mechanisms in an $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ film on a c-Si substrate covered with native oxide (∼30 Å) in a conventional rf plasma-enhanced chemical vapor deposition. These real-time monitoring techniques allow us to determine a depth profile of hydrogen content for the SiH and ${\mathrm{SiH}}_2$ bonding states, together with the microscopic structural evolution during the $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ deposition. The analyses of these real-time measurements show the formation of a 35-Å-thick H-rich layer having an average hydrogen content of ∼17 at. % at the $a-\mathrm{S}\mathrm{i}:\mathrm{H}$/substrate interface. A deuterium diffusion experiment performed after a 130-Å-thick $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ deposition supports the H-rich layer formation at the interface. This interface layer formation is primarily caused by the H-rich three-dimensional island growth on the substrate in the early $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ deposition stage. In a following coalescence process, we found a significant reduction in the hydrogen content in $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ bulk layer, accompanied by a clear surface smoothening of the $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ layer. The above results indicate that surface diffusion of precursors promotes a dense $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ network formation during the coalescence and confines H-rich $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ islands at the film/substrate interface region.