Browsing by Author "Holtz, M."

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    Effect of pressure on a defect-related band-resonant vibrational mode in implanation-disordered GaAs
    Sauncy, T.; Holtz, M.; Zallen, Richard H. (American Physical Society, 1994-10)
    We have used hydrostatic pressure as a means for studying a resonant Raman mode observed at 47 cm(-1) in highly disordered, ion implanted, unannealed GaAs. The mode shifts weakly (-0.07 +/- 0.15 cm(-1)/GPa), supporting an identification of this band-resonant vibration as stemming from the breathing mode of the gallium vacancies, which are expected to be in high concentration. We measure a pressure coefficient of the longitudinal-optic phonon in these (5.5 nm) nanocrystals of GaAs to be 3.6 +/- 0.1 cm(-1)/GPa. The good agreement between our value and the pressure shift of this phonon in bulk GaAs implies that the bulk modulus is independent of size at least down to this size crystallite.
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    Effect of pressure on defect-related emission in heavily silicon-doped GaAs
    Holtz, M.; Sauncy, T.; Dallas, T.; Massie, S. (American Physical Society, 1994-11)
    We report cryogenic high-pressure measurements of a defect-related emission at 1.25 eV in silicon-doped GaAs. The pressure measurements prove that the 1.25-eV photon energy is relative to the conduction band, implying a deep defect level 0.30 eV above the valence band and an electron-capture process from the conduction band into the defect. The defect level moves up in the band gap at a rate of 23±3 meV/GPa. These results are consistent with a vacancy-related defect level, possibly stemming from a gallium-vacancy–silicon-at-gallium (second-nearest-neighbor) defect complex.
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    Raman-scattering depth profile of the structure of ion-implanted GaAs
    Holtz, M.; Zallen, Richard H.; Brafman, O.; Matteson, S. (American Physical Society, 1988-03)
    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.
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    Resonant Raman-active acoustic phonons in ion-implanted GaAs
    Holtz, M.; Zallen, Richard H.; Brafman, O. (American Physical Society, 1988-09)
    We have observed a new, strong feature at 47 cm-1 in the first-order Raman spectrum of ion-implanted GaAs, prior to any anneal. It is not present in the Raman spectrum of either amorphous or single-crystal GaAs. The peak is strong between excitation photon energies ∼1.5 and 2.2 eV. Above 2.2 eV it is masked by the Raman spectrum of the amorphous GaAs component of the mixed microcrystalline-amorphous system. Its frequency and line shape are not dependent upon implant species or energy. The photon-energy dependence of the intensity of the amorphous GaAs component of the Raman spectrum is found to be completely accounted for by the photon-energy dependence of the optical penetration depth over the full range studied (1.55–2.71 eV). This then serves as an internal intensity standard for our measurements, permitting us to separate scattering efficiencies from mere scattering volume effects. The longitudinal-optical phonon of the microcrystalline remnant resonates near the E1 interband transition peak in the electronic density of states, consistent with a feature corresponding to a Raman-active crystal phonon. However, the new feature at 47 cm-1 is observed to resonate strongly at an energy near the E0 and E0+Δ0 electronic transition energies and not near E1. We propose that this feature is an acoustic vibration of microcrystalline GaAs, observed via defect-assisted scattering between an electron or hole and the crystalline-amorphous interface regions characteristic of ion-implanted GaAs. The additional scattering breaks the k=0 infinite-crystal selection rule, and double-resonance effects result in intense scattering for phonons in special regions of the Brillouin zone. Electronic wave functions with sufficiently large wavelengths (on the scale of the crystallite size) are strongly affected by the disorder. A phenomenological model accounts for the resonance behavior reasonably well.
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    Resonant Raman-active acoustic phonons in the mixed amorphous-microcrystalline phase of ion-implanted GaAs
    Holtz, M.; Zallen, Richard H.; Brafman, O. (American Physical Society, 1988-02)
    We have observed a new, strong, low-frequency peak (at 47 cm-1) in the Raman spectrum of ion-implanted GaAs having a mixed amorphous-microcrystalline microstructure. It is strongly resonant near 1.7 eV, just above the band gap, in contrast to the longitudinal-optic phonon line of the microcrystals (which resonates differently) and the bands of the amorphous component (which do not resonate). We tentatively interpret this peak in terms of acoustic phonons made Raman active by the presence of microcrystal-amorphous interface regions, and discuss several models.