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 Å (threading dislocation density, $ρt$⁠, around $104cm−2$⁠). 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 $3×106cm−2$⁠. In contrast, graded mixed-anion films of GaAsP avoid branch defects and minimize surface roughness, giving superior defect densities, as low as $104cm−2$ at useful lattice constants halfway between that of Si and Ge. Tensile graded $GaAs1−zPz$ layers yield the smoothest films (0.78 nm rms in a $5μm$ scan) with the lowest defect densities but are subject to cracking when graded beyond $z=0.5$ with a graded layer thickness of $<10μm$⁠. Compressive graded GaAsP yields excellent $ρt$ values $(3.3×105cm−2)$ with very thin buffers $(1.3μm)$⁠. The accelerated grade rate of the compressive buffers increases crosshatch roughness, which along with the higher defect density of GaP substrates, accounts for the higher defect density compared to tensile GaAsP on GaAs substrates.