(463f) Chemical Vapor Deposition Growth And The Evolution Of The Microstructure Of Indium Arsenide On Gallium Arsenide

The development of devices based on the 6.1 Å class of zincblende compound semiconductors (InAs, GaSb and AlSb) has suffered from the lack of a commercially-produced lattice-matched semi-insulating substrate for epitaxial growth. Films grown on available substrates, such as GaAs at 7% lattice parameter mismatch, typically are riddled with mismatch related defects. A chemical vapor deposition process, Metal organic vapor phase epitaxy, can used to synthesize such large lattice mismatched materials. The gas phase stoichiometry used during the deposition affects the nucleation rate and surface transport. Both of these factors can influence the resulting defect structure and morphology. Under certain growth conditions, both InAs and GaSb films grown on GaAs are comprised of columnar grains in which the 6.1 Å material is tilted relative to the substrate by a few degrees. When InAs is grown on (100) GaAs under conditions that delay nucleation and promote growth, the InAs within each grain is tilted by a up to six degrees in one of just six different directions relative to the substrate. This study investigated this systematic tilt and its correlations with island morphology in the InAs/(100) GaAs epitaxial system from the early stages of island growth through coalescence to a continuous thick film. The island morphology, local crystal orientation, their dependence on island size, and their evolution as the film grows were studied using combinations of secondary electron microscopy, atomic force microscopy, x-ray diffraction, and back-scattered electron diffraction techniques, specifically orientation imaging microscopy. The results show that a characteristic pattern of tilt develops early in the growth process within individual islands when they grow to greater ~1 µm across before coalescing with neighbors. These results confirm that the tilt is established at the interface rather than within the island volume. HREM imaging of the interfacial dislocation network revealed a change in the make-up of the mismatch dislocation array from side to side across the island. Near the center of the island, where the InAs was aligned crystallographically with the GaAs, the 60° dislocations present occurred in pairs to produce a net Burgers vector in the [100] direction of zero. Near the island's outer edges, the 60° dislocations in the interface all had Burgers vector, rendering a non-zero burgers vector density in the [100] direction that was oriented as needed to produce the crystal tilt measured above the array. These results are consistent with a model proposed by Spencer and Tersoff (B.J. Spencer, and J. Tersoff, Phys. Rev. B63 (2001) 205424) and with the systematic pattern of tilt observed in films. Both the model and the experimental results suggest that film quality is improved when the grown conditions favor island coalescence at small island size. The systematic crystallographic tilt observed in larger islands is attributed to particulars of the misfit dislocation array that forms in the InAs/GaAs interface as the island grows laterally, unimpeded. The discovery of critical island sizes at which morphology, crystal orientation and defect generation processes all change indicates that that detailed characterization of such transitions is a key to defect control in heteroepitaxy that occurs by an island growth process. The tilt microstructure can be evaded by growing films under conditions that promote coalescence at small island size; for example, those that favor copious nucleation. Changes in the gas phase stoichiometry can alter the island formation rate and hence density of InAs islands, allowing for the coalescence of the islands during the early stages of lateral growth. The imposition of physical boundaries using of a masked substrates as in lateral epitaxial growth (LEO) can modify the generation of the tilt. Both of these strategies for minimizes tilt microstructure will be presented.