School of Materials Engineering

Molecular ABSTRACT Molecular beam epitaxy has been used to grow both GaAsQ cSbQ ^ films lattice-matched to InP and GaAs. Sb /GaAs strained layer superlattices on GaAs*substrates. Films grown on InP substrates are of a composition inside the well-known solid-phase miscibility gap for this alloy. Because these films are metastable, they exhibit an unusual microstructure which includes both ordering and clustering effects. Nevertheless, we have obtained low- temperature phot ol umi nes cence linewidths of under 8 meV. This represents the best linewidth for this material reported to date. Correlations between film microstructure and the optical quality of these alloys have been observed. Strained layer GaAs. Sb /GaAs superlattices grown on GaAs substrates have been characterized by x-rayxdi^fraction, photoluminescence, optical absorption, and photoreflectance. Structural parameters as determined by x-ray diffraction have been used in an envelope function superlattice band structure model to estimate the band offsets and indicate that the superlattices are Type II, with a large valence band discontinuity. GaAs. Sb has been considered as a potential alloy for applications primarily in optical devices because its bandgap spans the technologically important 1.3 - 1.5 um range. Early investigation of this alloy grown by liquid phase epitaxy revealed the presence of a large solid-phase miscibility gap in this system, which severely limited the compositional range over which this material could be grown. However, with the advent of highly nonequi librium growth techniques such as molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE), growth of GaAs. Sb over the entire compositional range has been reported. ' Characterization of material grown inside the miscibility gap, generally nearly lattice-matched to InP (x ~ 0.5), indicates that this material suffers from a microscopic nonuniformity which may directly affect the optical quality of the alloy. ' In this paper, results of optical characterization of MBE-grown GaAs, Sb on InP and correlations with the film microstructure will be presented. In addition, a extermination of the GaAs. Sb /GaAs heterojunction band discontinuties is made through the characterization 01 strained GaAs. Sb /GaAs superlattice structures. 2. EXPERIMENTAL TECHNIQUES All structures were grown by MBE on (100) InP (for bulk layers) or GaAs (for superlattices) substrates. Growth temperatures were typically 500-540°C and growth rates 0.5-1.2 um/hr. Both ASp and Sbp were supplied by large capacity graphite cracking sources. The cell fluxes were such that the surface was group V stabilized under all conditions studied. Undoped GaAsn r-Sbn c grown in this manner typically had room temperature hole U. D /- U_. -O concentrations of 1-2 x 10 cm as measured by Hall effect. Epitaxial layers of GaAs. Sb on InP were characterized by a variety of techniques including transmission electron microscopy (TEM), low-temperature photoluminescence (PL) and optical absorption. The GaAs, Sb /GaAs superlattices were additionally characterized by 77K photoreflectance and x-ray diffraction.