Perfectly tetragonal, tensile-strained Ge on Ge_1-ySn_y buffered Si(100)

High-quality, tensile-strained Ge layers with variable thickness (>30 nm) have been deposited at low temperature (350-380 °C) on Si(100) via fully relaxed Ge1-y Sny buffers. The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable tensile strains up to 0.25% for y=0.025. Combined Raman analysis and high resolution x-ray diffraction using multiple off-axis reflections reveal unequivocally that the symmetry of tensile Ge is perfectly tetragonal, while the strain state of the buffer (∼200 nm thick) remains essentially unchanged. A downshift of the direct gap consistent with tensile strain has been observed.