Si-Ge intermixing induced at mesa sidewalls of Si-capped Ge epitaxial layers on Si for operation wavelength tuning in Ge photonic devices

A spontaneous formation of SiGe is reported, which is induced at mesa sidewalls of Si-capped Ge epitaxial layers selectively grown on Si. Ultrahigh-vacuum chemical vapor deposition is used to grow Ge mesa stripes on (001) Si wafers partially covered with SiO2 masks, followed by a growth of Si capping layer. Micro-Raman spectra reveals that SiGe is formed selectively on the {113} sidewalls of Ge mesa structure running in the [110] direction, resulting from an intermixing between the Si capping and Ge layers, whereas no such SiGe is detected on the flat-top (001) mesa surface. An increased amount of SiGe is observed for stripe patterns misaligned from the [110] direction. This suggests that the atomic step/roughness on the sidewall contributes to the intermixing. The observed SiGe formation would be applied to the bandgap engineering to tune the operation wavelengths in Ge photonic devices on Si. In fact, a significant blue shift in the direct-gap light emission peak is observed for submicron-wide Ge mesa structures with no flat-top (001) surface, where the surface is totally surrounded by SiGe.