Impact of buffered layer growth conditions on grown-in vacancy concentrations in molecular beam epitaxy silicon Germanium

Silicon-Germanium (SiGe) has become increasingly attractive to semiconductor manufacturers over the last decade for use in high performance devices. In order to produce thin layers of device grade SiGe with low concentrations of point defects and well-controlled doping profiles, advanced growth and deposition techniques such as molecular beam epitaxy (MBE) are used. One of the key issues in modeling dopant diffusion during subsequent processing is the concentration of grown-in point defects. The incorporation of vacancy clusters and vacancy point defects in 200nm SiGe/Si layers grown by molecular beam epitaxy over different buffer layers has been observed using beam-based positron annihilation spectrpscopy. Variables included the type of buffer layer, the growth temperature and growth rate for the buffer, and the growth temperature and growth rate for the top layer. Different growth conditions resulted in different relaxation amounts in the top layer, but in all samples the dislocation density was below 10⁶ cm^-2. Preliminary results indicate a correlation between the size, type and concentration of vacancy defects and the buffer layer growth temperature. At high buffer layer growth temperature of 500°C the vacancy point defect concentration is below the PAS detectable limit of approximately 10¹⁵ cm^-3. As the buffer layer growth is decreased to a minimum value of 300°C, large vacancy clusters are observed in the buffered layer and vacancy point defects are observed in the SiGe film. These results are relevant to the role played by point defects grown-in at temperatures below ∼350°C in modeling dopant diffusion during processing.