Origin of the time dependence of wet oxidation of AlGaAs

The time-dependence of the wet oxidation of high-Al-content AlGaAs can be either linear, indicating reaction-rate limitation, or parabolic, indicating diffusion-limited rates. The transition from linear to parabolic time dependence can be explained by the increased rate of the formation of intermediate As{sub 2}O{sub 3} vs. its reduction to elemental As. A steadily increasing thickness of the As{sub 2}O{sub 3}-containing region at the oxidation front will shift the process from the linear to the parabolic regime. This shift from reaction-rate-limited (linear) to diffusion-limited (parabolic) time dependence is favored by increasing temperature or increasing Al mole fraction

Refractive index and hygroscopic stability of AlxGa1−xAs native oxides

The authors present prism coupling measurements on Al{sub x}Ga{sub 1{minus}x}As native oxides showing the dependence of refractive index on composition (0.3 {le} x {le} 0.97), oxidation temperature (400 {le} T {le} 500), and carrier gas purity. Index values range from n = 1.490 (x = 0.9, 400) to 1.707 (x = 0.3, 500 C). The oxides are shown to adsorb moisture, increasing their index by up to 0.10 (7%). Native oxides of Al{sub x}Ga{sub 1{minus}x}As (x {le} 0.5) have index values up to 0.27 higher and are less hygroscopic when prepared with a small amount of O{sub 2} in the N{sub 2} + H{sub 2}O process gas. The higher index values are attributed to a greater degree of oxidation of the Ga in the film

Contributions of inelastically scattered electrons to defect images.

We have made measurements of the contribution of inelastically scattered electrons to images of dislocations in Ni{sub 3}Ga and nanometer-sized defects in ion-irradiated Au under weak-beam dark-field diffraction conditions [1]. The purpose is to determine the conditions for data acquisition required to eventually make detailed and quantitative comparisons to simulations of images for various defect models, thus determining defect structure, composition, and local strain field. Such image simulations usually consider only elastically scattered electrons, and thus it is important to understand and possibly eliminate the contribution of inelastically scattered electrons to the experimental images for quantitative comparisons with image simulations. Experimental data have been acquired with either JEOL 2010F or 3000F microscopes, both equipped with Gatan Imaging Filter electron spectrometers. Samples examined in the 2010F were Au, ion-irradiated to low dose (10{sup 11}Kr ions at 1 MeV energy) to form individual defects (1-10nm sized Frank dislocation loops and partial stacking fault tetrahedra). Samples examined in the 3000F were Ni{sub 3}Ga with long dislocation defects. Imaging conditions included weak-beam dark-field with deviation parameter generally > 0.2 nm{sup -1}. Energy filter slit width was set to 10 eV and centered on the zero loss peak in both instruments to obtain images produced by elastically scattered electrons. In the Au experiment an energy window was also set to image the 10-40 eV loss range to investigate the contribution to the defect images of plasmon-loss electrons. Electron intensity in defect images and backgrounds was measured by summing pixel values in appropriately sized rectangles using Digital Micrograph (Gatan) software

Microstructure of compositionally modulated InAlAs

The authors have observed spontaneous, lateral composition modulation in tensile InAlAs alloy films grown as short-period superlattices on InP (001). They have analyzed these films using transmission electron microscopy, x-ray reciprocal space mapping, and polarized photoluminescence spectroscopy. They find the growth front is nonplanar, exhibiting {approximately} 2 nm deep cusps aligned with the In-rich regions of the compositionally modulated films. In addition to the measured 15 nm wavelength modulation in the [110] direction, a modulation of 30 nm wavelength is seen in the orthogonal [1{bar 1}0] direction. The photoluminescence from the modulated layer is strongly polarized and red shifted by 0.22 eV

Detection of lateral composition modulation by magnetoexciton spectroscopy

An experimental signature for detecting spontaneous lateral composition modulation in a (InAs){sub n}/(GaAs){sub m} short period superlattice on an InP substrate based on magnetoexciton spectroscopy described. The authors find by aligning the magnetic field in three crystallographic directions, one parallel to and the other two perpendicular to the composition modulation direction, that the magnetoexciton shifts are anisotropic and are a good indicator for the presence of composition modulation

Photoluminescence Studies of Lateral Composition Modulated Short-Period AlAs/InAs Superlattices

We present low temperature photoluminescence data for a series of spontaneous lateral composition modulation in (AlAs){sub m}/(InAs){sub n} short period superlattices on InP with differing average lattice constants, i.e., varying global strain. The low temperature photoluminescence peak energies were found to be much lower than the corresponding energy expected for the equivalent In{sub x}Al{sub 1{minus}x}As alloy. The bandgap energy reductions are found to approach 500 meV and this reduction is found to correlated with the strength of the composition modulation wave amplitude

Stress-driven instability in growing multilayer films

We investigate the stress-driven morphological instability of epitaxially growing multilayer films, which are coherent and dislocation-free. We construct a direct elastic analysis, from which we determine the elastic state of the system recursively in terms of that of the old states of the buried layers. In turn, we use the result for the elastic state to derive the morphological evolution equation of surface profile to first order of perturbations, with the solution explicitly expressed by the growth conditions and material parameters of all the deposited layers. We apply these results to two kinds of multilayer structures. One is the alternating tensile/compressive multilayer structure, for which we determine the effective stability properties, including the effect of varying surface mobility in different layers, its interplay with the global misfit of the multilayer film, and the influence of asymmetric structure of compressive and tensile layers on the system stability. The nature of the asymmetry properties found in stability diagrams is in agreement with experimental observations. The other multilayer structure that we study is one composed of stacked strained/spacer layers. We also calculate the kinetic critical thickness for the onset of morphological instability and obtain its reduction and saturation as number of deposited layers increases, which is consistent with recent experimental results. Compared to the single-layer film growth, the behavior of kinetic critical thickness shows deviations for upper strained layers.Comment: 27 pages, 11 figures; Phys. Rev. B, in pres

Microstructure and interface properties of laterally oxidized Al{sub x}Ga{sub 1{minus}x}As

The selective and passivating nature of the oxidation of high Al content AlGaAs has been used to create high-performance vertical-cavity surface emitting lasers (VCSELs). This is accomplished by using the fact the Ga content of a AlGaAs film will drastically affect its oxidation rate, allowing the strategic placement of high Al content layers in the structure; these are then oxidized to form current-confining and optical-mode-defining apertures. Here, the microstructure and interface properties of Al{sub x}Ga{sub 1{minus}x}As materials that have been laterally oxidized in wet N{sub 2} for several compositions (x = 0.80, 0.82...1.00) and temperatures (360 C to 450 C) have been studied. The microstructure is found to be relatively insensitive to composition and oxidation temperature. The oxidation forms an amorphous solid solution (Al{sub x}Ga{sub 1{minus}x}){sub 2}O{sub 3} that transforms to polycrystalline, {gamma}-(Al{sub x}Ga{sub 1{minus}x}){sub 2}O{sub 3} is formed via post oxidation annealing of the oxide. The level of hydrogen present in the oxidized layers is 1.1 {times} 10{sup 21} cm{sup {minus}3}, which is too low for the amorphous phase observed to be a hydroxide rather than an oxide. The amount of As in the layer is reduced to <2 atm%, and no As precipitates are observed. The (Al{sub x}Ga{sub 1{minus}x}){sub 2}O{sub 3}/GaAs interface is abrupt, but prolonged oxidation will cause the GaAs to oxidize at the internal interfaces. The reaction front between the oxidized and the unoxidized Al{sub x}Ga{sub 1{minus}x}As has a 10 to 20nm-wide amorphous zone that shows a different contrast than the remainder of the amorphous oxide and is stable under electron irradiation