Band-gap tuning of SiGe/Si multi quantum wells for waveguides and photodetectors

Silicon-germanium (SiGe) alloys are attractive for the monolithic integration of Si photonics with mainstream VLSI technology. The addition of Ge extends the wavelength range of silicon and SiGe/Si multi quantum wells (MQW's) can be epitaxially grown coherent with the Si substrate which allows an additional degree of freedom in bandgap engineering. In this work, SiGe quantum wells were grown at 525 degrees C using a commercially available, ultra-high vacuum chemical vapor deposition (UHV-CVD) system. A strong blue shift is observed in photoluminescence on annealing as-grown MQW's. Shifts in photoluminescence line energies, which are directly related to the changes in SiGe QW shape during annealing, are monitored. SiGe MQW's annealed using a two-step process in which strain and Ge peak concentration remain unchanged after the first (low temperature) step, show a much lower rate of interdiffusion during the second step. It is argued that strain and Ge incorporation alone cannot explain the enhanced initial interdiffusion, which is attributed to grown-in, nonequilibrium point defects. Further confirmation was obtained using Si ion implantation, which only increased the interdiffusion during the first seconds of the anneal, without changing the ''steady state'' diffusion coefficient. This opens up the possibility of point defect mediated local tailoring of the bandgap, since enhanced interdiffusion and its associated blue shift can be laterally controlled by lithographic masking of implanted regions. The intriguing prospect of low loss (band-gap shifted) waveguides coupled with wavelength tunable SiGe/Si MQW photodetectors, fully compatible with CMOS, is discussed.