Raman-scattering depth profile of the structure of ion-implanted GaAs
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We have carried out an extensive Raman-scattering investigation of the structure of beryllium-implanted gallium arsenide. Single-crystal GaAs was bombarded with 45-keV Be+ ions, and backscattering Raman measurements were made, prior to any anneal, as a function of ion fluence, laser photon energy, and depth (via chemical-etch removal of surface layers). Line-shape and intensity analyses of the observed first-order Raman spectra, especially of the longitudinal-optical- (LO) phonon line (which is superimposed on the broad spectral signature of amorphous GaAs), support a structural model of the implantation-induced damage layer as a fine-scale mixture of amorphous and crystalline GaAs. The etch studies yield a structural depth profile in terms of the depth dependence of the amorphous volume fraction (derived from measured scattering intensities) and of the characteristic crystallite size. The first 1500 Å is a high-damage layer having nearly constant structure; this is followed by a structurally graded transition region in which the crystalline volume fraction and the crystallite size smoothly increase until the bulk crystal is reached at about 4000 Å. For a fluence of 5×1014 ions/cm2, the near-surface high-damage plateau is characterized by an amorphous volume fraction of 0.25 and a crystallite size of 60 Å. This plateau begins at the surface; there is no evidence of the near-surface decrease in disorder which appears in some commonly used theoretical simulations. Varying the laser photon energy from 1.55 to 2.71 eV reveals that the LO intensity (arising from the crystalline component) increases at both ends of this spectral range. The intensity increase at low photon energies reflects the increasing optical penetration depth (i.e., effective scattering volume), but the increase at high photon energies signifies a real rise in the scattering efficiency. We interpret this as a resonance-Raman effect associated with the approach toward the E1 interband transition. This resonance is partially quenched as the crystallite size is decreased for heavily implanted samples.