K. Sinha

Evidence for solid-state effects in the electronic structure of C60 films: a resonance-Raman study

Abstract The Raman excitation profiles for C 60 films grown on silicon reveal the existence of optical transitions near 2.4 eV, well below the lowest dipole-allowed absorption line predicted for isolated icosahedral molecules. These transitions are not observed in C 60 dissolved in CS 2 . We propose an explanation in terms of a breakdown of the icosahedral inversion symmetry in C 60 crystals.

Raman scattering and the π-orbitals in amorphous carbon films

We report a Raman study of amorphous carbon films grown by different techniques. We compare Raman line shapes, depolarization ratios, and excitation profiles obtained from samples grown by rf‐plasma deposition or sputtering as opposed to arc‐evaporated carbon films. We find that all features of the Raman spectra are closer to results from crystalline graphite for samples deposited with an electron cyclotron resonance plasma using either a hydrocarbon gas source or a graphite sputtering target. This is surprising in view of the much larger optical band gap of these samples and also in view of recent e‐2e scattering results which suggest that the π orbitals are more graphitelike in arc‐evaporated carbon films. We offer a possible explanation of this paradox based on a reinterpretation of the optical properties of amorphous carbon which takes into account the special characteristics of the electronic states derived from π orbitals.

Phonons in epitaxially grown α‐Sn1−xGex alloys

We present a Raman scattering study of optical phonons in α‐Sn1−xGex alloys (x≤0.08) grown by molecular beam epitaxy on CdTe (100) substrates. The Raman spectra provide strong evidence for the growth of high quality films in the diamond‐structure phase. The composition dependence of the Raman modes shows some qualitative differences with results from the isomorphic Ge1−xSix system. We show that these differences can be understood in terms of a simple model that considers confinement and strain effects.

Raman study of photoexcited C60

Abstract The high-energy pentagonal pinch mode of C 60 is known to soften under high incident laser irradiance in an oxygen-free environment. We have studied this phenomenon in C 60 films, single crystals, and solutions in CS 2 . Our Raman, optical absorption, and EPR data indicate that the softened Raman peak is related to very long lived states in C 60 . The experiments are consistent with the recently proposed photoinduced polymerization of C 60 and also suggest the presence of C 60 anions in the photoexcited material.

Nonuniform strain profiles in cubic CdS/GaAs films measured by reflection high energy electron diffraction and Raman spectroscopy

We report a study of the strain relaxation in zinc‐blende CdS films grown on GaAs by molecular beam epitaxy. We use Raman spectroscopy to determine the depth dependence of the strain profile. This dependence is compared with the instantaneous surface strain relaxation measured by reflection high energy electron diffraction during growth. The Raman results show that the strain profile determined by RHEED remains largely ‘‘frozen’’ in the final sample. It is only near the heterostructure interface, where the strain is large, that evidence of additional relaxation is found in the Raman spectrum.

Extrinsic Nature of the 2.5 eV Raman Resonance in C60

Abstract We present experimental evidence that the resonance near 2.5 eV observed in the Raman excitation profile of C60 is induced by the interactions of the C60 molecule with the surrounding medium and is not to be expected in isolated molecules.

Nonuniform strain gradients in CdS/GaAs films measured by reflection high-energy electron diffraction and Raman spectroscopy : a microscopic approach

Strain relaxation at the lattice mismatched CdS/GaAs (100) interface has been investigated by reflection high‐energy electron diffraction (RHEED) and Raman scattering. In‐situ RHEED measurements indicate a gradual relief of the mismatch strain once the critical film thickness is exceeded. After‐growth Raman measurements reveal that the strain profile measured by RHEED remains ‘‘frozen’’ in the grown layer except very close to the CdS/GaAs interface, where further relaxation takes place. A microscopic calculation shows that the presence of nonuniform strain leads to phonon confinement effects, as localized modes appear in regions of similar strains.

Raman study of icosahedral C60

We report a high-resolution resonance Raman study of C60 films. We identify several peaks whose frequencies are in good agreement with the predictions of a first-principles quantum molecular dynamics calculation for an isolated C60 molecule having icosahedral symmetry. On the other hand, the depolarization ratios for most Raman peaks, some fine- structure features observed with high resolution, and the resonance photon energies suggest solid-state effects or the existence of C60 isomers of lower symmetry

Raman Scattering in α-Sn1_xGex Alloys

The Si-Ge system provides a prototypical example of a substitutional semiconductor alloy. Because of its potential technological applications, it has been intensively studied during the past thirty years. In particular, light scattering techniques have been used to investigate the compositional dependence of phonon modes. Most of this work has concentrated on the three main Raman active optical vibrations, assigned to “Si-Si”, “Si-Ge”, and “Ge-Ge” modes.1,2,3,4,5,6,7,8 In spite of this effort, however, the compositional dependence of Raman modes is not well understood. A powerful experimental approach to this problem would be the comparison of Raman spectra for different isovalent alloys such as Si1-xCx, Ge1-xSi, and α-Sn1-xGex. Unfortunately, only α-Sn1-xSix shows solid solubility for the entire compositional range 0 ≤ x ≤ 1. Non-equilibrium growth techniques such as Molecular Beam Epitaxy, however, open up a new alternative for the fabrication of these alloys. Recently, the successful MBE growth of α-Sn1-xGex has been reported.9 In this paper, we summarize recent Raman results on this system and discuss the implications of the results for the understanding of phonons in IV-IV alloys.

Resonance Raman Scattering In Amorphous Carbon Films And Graphite

We report resonance Raman results from amorphous carbon films and graphite. Our experiments confirm the presence of π-states in the amorphous samples, as suggested by theoretical models. The different resonance behavior of the high-energy and low-energy sides of the broad amorphous Raman peak is shown to be related to similar differences we observe in cristalline graphite for zone-edge and zone-center phonons.