GaAs hetero-epitaxial layers grown by MOVPE on exactly-oriented and off-cut (1 1 1)Si: Lattice tilt, mosaicity and defects content

Integration of III-V devices with Si-photonics and fabrication of monolithic III-V/Si tandem solar cells require the heteroepitaxy of III-V compounds on Si. We report on the lattice tilt, mosaicity and defects content of relaxed GaAs grown by MOVPE on exactly-oriented and 4°-offcut (111)Si. Thin GaAs single-layers grown at 400°C and annealed at 700 °C show ∼3×108 cm-2 density of surface pinholes. Double-layer samples were obtained by GaAs overgrowth at 700 °C. GaAs epilayers are tilted by (0.05-0.14)° with respect to Si. Rotational twins were observed in X-ray diffraction (XRD) pole figures: the most abundant originate from 60°-rotation of GaAs around the 1¯1¯1¯ growth direction and are identified as micro-twins along the GaAs/Si hetero-interface. Twins obtained by rotations around the 1¯1¯1, 11¯1¯, and 1¯11¯ directions or by combined rotations around the growth direction and one the former, were also observed. The GaAs mosaicity and block size were studied through high-resolution XRD intensity mapping: for single-layer samples crystal blocks are ascribed to 3-5 nm thin micro-twins, whose size does not change upon annealing. In double-layer samples thicker (32-35 nm) micro-twins occur. GaAs samples grown on offcut (111)Si show less rotational twins but a reduced mosaic block size with respect to exactly-oriented Si.