M. Adamcyk

Molecular beam epitaxy growth of GaAs1−xBix

GaAs1−xBix epilayers with bismuth concentrations up to x=3.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.33±0.06 A. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ∼84 meV per percent Bi is observed.

Band gap of GaAs1−xBix, 0<x<3.6%

The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.

Surfactant enhanced growth of GaNAs and InGaNAs using bismuth

Dilute nitride GaNAs thin films and InGaNAs single quantum wells (QWs) have been grown by molecular beam epitaxy with a concurrent bismuth flux. Bi does not incorporate into the films and acts as a surfactant. Atomic force microscopy images reveal that, at sufficiently high bismuth flux, step flow growth occurs in GaN0.004As0.996 even at substrate temperatures as low as 4601C. This results in an order of magnitude decrease in the surface roughness. A similar smoothing effect is obtained when growing GaAs and AlGaAs thin films with bismuth. Furthermore, Bi is found to enhance the incorporation of nitrogen into GaNAs. The peak photoluminescence intensity from an In0.26Ga0.74N0.011As0.989 QW is increased by more than a factor of two with the surfactant. We conclude that Bi reduces the incorporation of defects and/or impurities in the dilute GaNAs based alloys. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 68.35.Ct

Comparison of strain relaxation in InGaAsN and InGaAs thin films

We compare the strain relaxation of In0.08Ga0.92As and In0.12Ga0.88As0.99N0.01 epitaxial thin films grown on GaAs (001) by elemental-source molecular-beam epitaxy. The epilayers we studied were essentially identical in their compressive lattice mismatch (0.62±0.02%), and thickness (600 nm). The strain state of the samples was determined by in situ substrate curvature monitoring, and by ex situ x-ray diffraction and plan-view transmission electron microscopy. We observe a slower rate of strain relaxation, and a 25% higher residual strain in the nitride. This is attributed to the presence of nitrogen interstitials in the InGaAsN epilayers and/or to the higher nitrogen bond strengths.

Faceting transition in epitaxial growth of dilute GaNAs films on GaAs

An abrupt transition to a {111} faceted growth mode is observed in molecular-beam-epitaxy growth of dilute GaNxAs1−x (x<0.05) films on (100) GaAs substrates. The faceted growth mode is favored by high growth temperatures, high nitrogen content, and high arsenic flux. The best electronic quality material, as measured by low-temperature photoluminescence, was obtained at high growth temperatures and high arsenic flux without exceeding the threshold for facet formation. The nitrogen content was found to be insensitive to the arsenic flux.

Film thickness and composition monitoring during growth by molecular beam epitaxy using alpha particle energy loss

The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates recoil implanted with α-particle emitters, we have been able to measure thickness and composition of deposited GaAs, AlGaAs and InGaAs in real time. The AEL method yields in situ real time results comparable in accuracy to those obtained by ex situ scanning electron microscope and high-resolution x-ray diffraction measurements.The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates recoil implanted with α-particle emitters, we have been able to measure thickness and composition of deposited GaAs, AlGaAs and InGaAs in real time. The AEL method yields in situ real time results comparable in accuracy to those obtained by ex situ scanning electron microscope and high-resolution x-ray diffraction measurements.

Effect of the starting surface on the morphology of MBE-grown GaAs

Abstract In this paper, we study the homoepitaxial growth of GaAs by molecular beam epitaxy on substrates that have different pre-growth roughness due to the method of removing the native oxide. The evolution of the surface roughness of 1 μm thick GaAs films grown at 553°C was monitored in real time using ultraviolet light scattering, and compared with ex situ atomic force microscopy measurements of the power spectral density (PSD) of the surface morphology. The PSD at a spatial frequency of 2 μm −1 , is approximately three orders of magnitude larger for films grown on thermally cleaned substrates than for films grown on substrates cleaned with atomic hydrogen. No mounding indicative of unstable growth was observed in the films cleaned with atomic hydrogen.

Smoothing of textured GaAs surfaces during molecular beam epitaxy growth

The surface morphology of homoepitaxial GaAs layers grown by molecular beam epitaxy on random and periodically textured substrates has been measured by atomic force microscopy and elastic light scattering. The random texture was obtained by thermal evaporation of the surface oxide and the periodic texture consisted of one-dimensional grating patterns fabricated by holographic lithography. The time evolution of the surface morphology was simulated numerically with a nonlinear growth equation that includes deposition noise and anisotropy in the surface diffusion. The surface of the random substrate develops shallow mounds as the large amplitude initial texture smooths out, an effect that has previously been attributed to unstable growth.

SURFACE ROUGHNESS AND RESONANT SCATTERING EFFECTS IN SOFT X-RAY SPECKLE FROM RANDOM SEMICONDUCTOR INTERFACES

We report the use of coherent soft X-ray scattering, or speckle, to study the morphology of random surfaces on single crystal semiconductor substrates. The experiments were carried out with photon energies in the 266–290 eV range. The effect of the magnitude of the roughness on the scattering was observed by measuring speckle patterns at various angles of incidence from two different surfaces with InP islands in the nanometer size range and in the micron range. The effect of element-specific resonant scattering was explored with a PMMA-coated textured silicon sample by tuning the wavelength to the carbon K-edge. Two dimensional numerical simulations of the coherent scattering have been carried out in the Fraunhofer approximation, using AFM data for the sample surface morphology. Good agreement with the observed speckle patterns was obtained, taking into account the 20 μm lateral coherence length of the source.

Coherent soft x-ray scattering from InP islands on a semiconductor substrate

Coherent soft x-ray scattering experiments from a semiconductor sample consisting of InP islands on a smooth semiconductor substrate are described. The soft x-ray scattering was performed with 266 eV photons produced by an undulator source. Using a position sensitive detector, we are able to detect diffusely scattered x rays in the vicinity of the specular reflection, with an in-plane momentum transfer of up to 6 μm−1. Using Huygens–Fresnel theory and atomic force microscope images of the surface structure, we simulated the scattering assuming a finite lateral coherence length for the incident radiation. The lateral coherence length of the incident beam was found to be 20 μm from a fit to the observed diffraction pattern from a pinhole. The effect of changes in the surface morphology on the speckle pattern was simulated to explore the potential of coherent soft x-ray scattering for the study of surface structure dynamics.