Characterization of residual strain in epitaxial Ge layers grown in sub-100 nm width SiO₂ trench arrays

We examined the selective epitaxial growth of Ge on Si(001) substrates with 40, 65, and 90 nm width trench arrays by ultra-high vacuum chemical vapor deposition. Because SiO₂ trench walls were present, no surface undulation in the direction parallel to the trenches was observed and compressive strain developed in the same direction. Based on reciprocal space mapping (RSM) measurements, this strain along the parallel direction increased from - 0.28 to - 0.72% as the width of the exposed Si substrate between the SiO₂ walls decreased from 90 to 40 nm, which was due to a decrease in strain relaxation. We calculated the effect of the Si trench width on changes in strain after removing the SiO₂ walls and compared the calculated values with the RSM results. No significant change in the strain relaxation was detected along the direction perpendicular to the trenches, and the strain changes were < 0.1%.