Resonant Raman-active acoustic phonons in ion-implanted GaAs
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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.