Comparison of compressive and tensile relaxed composition-graded GaAsP and (Al)InGaP substrates

The authors present a comparison of metal organic chemical vapor deposition grown compositionally graded metamorphic buffers, which enable virtual substrates with very high quality crystal lattices with lattice constants from 5.45 to 5.65 A (threading dislocation density, ρt, around 104 cm−2). The structures, grown on GaP or GaAs, consist of graded In-fraction InGaP and AlInGaP or graded P-fraction GaAsP. They show that surface roughness and locally strained regions of phase separation (branch defects) limit misfit dislocation glide velocity and escalate threading dislocation density. High surface roughness and branch defects in (Al)InGaP lead to the lowest quality virtual substrates we observed, with ρt of around 3×106 cm−2. In contrast, graded mixed-anion films of GaAsP avoid branch defects and minimize surface roughness, giving superior defect densities, as low as 104 cm−2 at useful lattice constants halfway between that of Si and Ge. Tensile graded GaAs1−zPz layers yield the smoothest films (0.78 nm r...