MOLECULAR-BEAM-EPITAXIAL GROWTH AND CHARACTERIZATION OF HIGH-QUALITY ALLOYS AND MULTIPLE-QUANTUM WELLS ON INP SUBSTRATES USING A POST-EVAPORATION-HEATED ARSENIC SOURCE

We report the growth of high-quality As-based ternary and quaternary alloys lattice matched to InP using a valved arsenic source that can post-heat the As beam after evaporation. We find that the optimum group-V-to-group-III beam-equivalent pressure ratio for growth of (In,Ga)As alloys using this source is considerably lower than values reported previously for growth using conventional As-4 sources. Consequently, high-quality (In,Ga)As, (In,Al)As, and (In,Al,Ga)As alloys (and quantum wells made from these alloys) can be grown under the same growth conditions, i.e., substrate temperatures between about 525 degrees C and 540 degrees C and V/III pressure ratios between 10:1 and 15:1. Thick-film alloys and multiple-quantum-well structures grown under these conditions show superior structural and optical quality. Strong excitonic features are observed in the room-temperature absorption spectra of a number of multiple-quantum-well structures with well widths ranging from 30 Angstrom to 170 degrees. Calculations of the exciton transition energies using a simple empirical two-band model are in excellent agreement with experiment, even for a structure containing quantum wells in tensile strain in which the ordering of ground-state light- and heavy-hole excitons is reversed. The optical absorption spectrum of a 50-Angstrom -period (In,Ga)As/ (In,Al)As superlattice shows room-temperature excitons involving electronic states at both the bottom and top of the minibands. Exciton line widths for these quantum-well structures, measured using low-temperature photoluminescence, are consistent with the limits imposed by random alloy fluctuations. We tentatively explain the lower optimum V/III pressure ratio for growth of (In,Ga)As in terms of the increase in kinetic energy of Asq molecules (compared with the kinetic energy of molecules from a conventional AS(4) source) and the consequent enhancement in the efficiency of dissociation of AS(4) molecules into As-2 molecules at the growing surface.