Mean Free Path of Photoelectronic Excitations in Hydrogenated Amorphous Silicon and Silicon Germanium Alloy Semiconductors

Hydrogenated amorphous silicon germanium alloy (a-Si1-xGex: H) films are prepared by plasma enhanced chemical vapor deposition (PECVD) and characterized by in situ real time spectroscopic ellipsometry (RTSE). From complex dielectric function spectra extracted, the broadening width energy (G) of the primary absorption feature centered near 3.7 eV is quantified using the Cody-Lorentz oscillator model. Mean free path length of photoelectronic excitations is calculated from Gamma based on reasonable estimates for speed of electron-hole photoexcitations. A model is applied with excited state lifetime assumed to be limited by scattering from network disorder and provides a relative measure of short-range order. The simple model applied is an extension of that widely used to characterize broadening of optical transitions in polycrystalline semiconductors. Decreases in mean free path of up to similar to 30% occur with germanium. Relative increases in mean free path with increased hydrogen during PECVD a-Si:H, a-Si0.73Ge0.27:H, and a-Si0.60Ge0.40:H occur from similar to 5% to similar to 8% and with hydrogen plasma treatment of a-Si:H by similar to 6%. Mean free path changes are tracked with thickness to provide short range order evolution and the effect of the underlying material. Mean free path of photoexcitations in electronic quality a-Si1-xGex: H is estimated at similar to 3.5 angstrom, on the same order as interatomic spacing.