Evolution of microstructure and dislocation dynamics in InxGa1−xP graded buffers grown on GaP by metalorganic vapor phase epitaxy: Engineering device-quality substrate materials

This study explores the dislocation dynamics of strain relaxation in graded composition buffers of ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P}$ grown on GaP $({\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P/GaP})$ by metalorganic vapor phase epitaxy. Transmission electron microscopy, cathodoluminescence imaging, atomic force microscopy, and triple-axis x-ray diffraction are applied to the characterization of ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P/GaP}$ with final compositions ranging from $\text{x=0.09}$ to $\text{x=0.39}$ and growth temperatures ranging from 650 to $\text{810\hspace{0.05em}°}\mathrm{C}.$ The previously reported escalation of defect density with continued grading of ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P/GaP}$ beyond $\text{x∼0.3}$ is discovered to be due to the formation of dislocation pileups. A new defect microstructure with a branching morphology and featuring sharp local strain fields, hereafter referred to as branch defects, is observed to pin dislocations and cause the dislocation pileups. Branch defect morphology varies strongly with growth temperature, becoming significantly stronger with increasing growth temperature and causing severe material degradation above $\text{700\hspace{0.05em}°}\mathrm{C}.$ Further experiments show that branch defects evolve during growth and that the onset of branch defect formation is delayed by increasing growth temperature. Comparison with the literature suggests that the evolution of branch defects may control the microstructure of indium-bearing phosphides and arsenides over a very wide range of conditions. In the absence of branch defects at high growth temperatures and low indium compositions near $\text{x∼0.1,}$ nearly ideal dislocation dynamics dominated by dislocation glide kinetics are recovered, providing the first experimental proof of a kinetic model for graded buffer relaxation. This new understanding of the evolution of microstructure and dislocation dynamics in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P/GaP}$ suggests that growth temperature must be optimized as a function of composition for optimal material quality. A simple process optimization in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P/GaP}$ graded to $\text{x=0.39}$ results in an overall threading dislocation density of ${\text{4.7×10}}^6 {\mathrm{cm}}^{\mathrm{-2}},$ which is the lowest reported value to date for $\text{x>0.3.}$ Combining the new observations with earlier findings, we present three basic design rules for producing practical, device-quality graded buffers: branch defects must be avoided or suppressed, growth temperature must be maximized, and surface roughness must be minimized. Using these design rules, we also present optimization strategies for achieving device-quality substrate materials. Applying these design rules and optimization strategies, we hope to achieve threading dislocation densities of ${\text{<10}}^6 {\mathrm{cm}}^{\mathrm{-2}}$ in ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{P/GaP}$ over the full range of useful compositions.