The scanning tunneling microscope is used to study arsenic-related point defects in low-temperaturegrown GaAs. Tunneling spectroscopy reveals a band of donor states located near E v + 0.5 eV arising from the defects. Images of this state reveal a central defect core, with two satellites located about 15 A from the core. The structure of the defect is found to be consistent with that of an isolated arsenic antisite defect (As on a Ga site) in a tetrahedral environment.

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To investigate the effect of growth area on interface dislocation density in strained-layer epitaxy, we have fabricated 2-.um-high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5 X 10\ 10 4 , and 10 2 cm--2 • 3500-, 7000-, and 8250-A-thick In o (J5 Gao 95 As layers, corresponding to 5, to, and 11 times the experimental critical layer thickness as measured for large-area samples, were then deposited by molecularbeam epitaxy. For the 3500-A layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 em -I for mesas as large as 100 pm. A pronounced difference in the linear interface dislocation densities along the two interface (110) directions indicates that a dislocations nucleate about twice as much as /3 dislocations. For samples grown on the highest dislocation density substrates, the linear interface-dislocation density was found to vary linearly with mesa width and to extrapolate to a zero linear interface-dislocation density for a mesa width of zero. This behavior excludes dislocation multiplication or the nucleation of surface half"loops as operative nucleation sources for misfit dislocations in these layers. Only nucleation sources that scale with area (termed fixed sources) are active. In specimens with lower substrate dislocation densities, the density of interface dislocations still varies linearly with mesa size, but the slope becomes independent of substrate dislocation density, indicating that surface inhomogeneities now act as the dominant source for misfit dislocations. Thus, in 3500-A-thick overlayers, substrate dislocations and substrate inhomogeneities are the active fixed nucleation sources. Since only fixed nucleation sources are active, a single strained layer wiH dramatically reduce the threading dislocation density in the epilayer. For the 7ooo-A layers, we observe a superlinear increase in linear interface-dislocation density with mesa size for mesas greater than 200,um, indicating that dislocation mUltiplication occurs in large mesas. For mesas less than 200 pm in width, linear interface-dislocation density decreases linearly with mesa size, but extrapolates to a nonzero linear interface-dislocation density for a mesa size of zero. This nonzero extrapolation suggests an additional active source which generates a dislocation density that cannot be decreased to zero by decreasing the mesa size. Cathodoluminescence eeL) images using radiative recombination indicate that the additional source is nucleation from the mesa edges. Despite a doubling in epilayer thickness from 3500 to 7000 A, the linear interface-dislocation density for mesas 100 [tm in width is stm very low, approximately 1500 cm --l. The 8250"A layers possess interface-dislocation densities too high to be accurately determined with CL. However, increases in CL intensity as mesa width is reduced indicate that the interface-dislocation density is decreasing and that growth...

Optical transmission data covering the l15Y-llC absorption edge are presented, together with photoluminescence (PL) results, for AlxGal_xAs crystals of high purity [n(293 K) < 10 17 cm-3 ] for 0< x < 0.9. The results indicate that the fundamental absorption edge is dominated by electron-hole interaction even in an alloy. In the direct-gap region the bound-exciton ground-state peak is clearly resolved. In the indirect-gap region the relaxation of crystal-momentum conservation in optical transitions for an alloy apparently has significant effects on the ll5y-llC absorption edge, and the discrete peak in absorption spectra disappears. From the variation of the absorption coefficient nth at the ll5Y -llC edge with x. we can deduce the corresponding variation of the excitonic binding energy Eex, excitonic effective mass m*, and electron mass m, for the llC minimum. Extrapolated values for these quantities in AlAs are Ecx = 5 meV, m* = 0.060 m o , and me = 0.11 mo for a value ath = 2.3 X 10 4 em-I. Further, there is a shift of the r 15V -llC absorption edge towards lower energies only in the indirect-gap region. Thus the usually employed quadratic expression for the bowing of band gaps in alloys is inadequate for the ll5y-llC gap in AlxOal_xAs. We observe no bowing at all «0.01 eV) in the direct-gap region, in sharp disagreement with earlier experiments employing other methods. Our PL data for the indirect-gap region indicate no significant bowing in the ll5Y -X IC gap either «0.01 eV). The PL results for the direct-gap region establish significant deviations in near-band-gap luminescence peak positions from the band gap established in absorption, in agreement with detailed balance predictions.

We present a comprehensive study of epitaxially grown and As-coated GaAs(100) surfaces as a function of As desorption temperature and background pressure. We have used low-energy electron diffraction to determine surface reconstruction, and core-level and valence-band soft-x-ray photoemission spectroscopy to perform chemical and electronic characterization of these surfaces. We find gradual changes in surface geometry and composition, and a limited (-120 meV) Fermi-level movement over numerous reconstructions in the 250-650'C annealing temperature range. The surface ionization potential and work function exhibit large changes between different surface reconstructions. In conjunction with other techniques, work-function measurements present evidence of surface inhomogeneity for many of the desorption temperatures and surface reconstructions. This inhomogeneity appears related to the existence of differently reconstructed patches on the surface. Our results emphasize the complexity of reconstructed GaAs(100) surfaces and the advantages of a multiple-technique approach for their characterization.