Characterization of metamorphic In_xAl_1-xAs/GaAs buffer layers using reciprocal space mapping

The depth profiles of metamorphic In_xAl_1-xAs (0.05≪x≪1) buffer layers grown on GaAs substrates were characterized using the x-ray reciprocal space mapping. Three types of metamorphic samples were investigated and compared: step grade, single-slope linear grade, and dual-slope linear grade. The lattice mismatch, residual strain, crystallographic tilt, tilt azimuth, and the full width at half maximum were obtained from the reciprocal space maps. The tilt angle of linearly graded buffer layers stayed low at low In compositions until In≈60%, at which composition the tilt angle increased abruptly. All linear-grade samples had an untilted relaxed structure in the low In region (below 60% In) and a tilted structure in the upper, high In region (above 60% In). The average lattice mismatch between the untilted relaxed structure and the tilted structure determines the tilt angle. The tilt angle of the step-graded layers increased at a near-linear rate as the In composition was increased. The tilt azimuth was intermediate between the <100> and <110> in-plane directions. The x-ray full width at half maximum generally increased with the In composition, but tended lower toward surface. We suggest a possible design strategy for the linear-grade metamorphic buffer layer based on our result.