The ionic charges of zincblende type crystals are discussed. Since the static ionic charges are, in general, quite small in these crystals, the magnitudes of the dynamic ionic charge, e*T, are attributed largely to the effects of charge redistribution. The fairly sizeable values of e*T in Se and Te trigonal crystals must, in fact, be attributed entirely to charge redistribution effects since their static ionic charges are identically zero. Efforts to account for the e*T values in ZnS type crystals using either the shell model or the deformation dipole model have not been too successful. It is urged that theorists turn their attention to ab initio energy-band calculations of long wavelength dynamical properties of zincblende and diamond type crystals, namely: the first-order electric moment (infrared) tensor, ∂M/∂uT; the first-order electric susceptibility (Raman) tensor, ∂χ0/∂uT; and the change in the electric susceptibility with interatomic distance (piezo-optic) tensor, ∂χ0/∂∂.

We describe the investigations of cyclotron resonance, and its formulation in terms of intraband transitions between Landau levels, that were carried out at the Naval Research Laboratory and the University of Pennsylvania in the 1950’s and 1960’s. Measurements were carried out as a function of magnetic field at fixed wavelength in the infrared in both the Faraday and Voigt configurations on an intrinsic sample of InSb sample for which ωP << ωC, and on doped n‐type samples for which ωP is comparable to ωC. Azbel’‐Kaner cyclotron resonance, which is also observed in the Voigt configuration, was investigated at microwave frequencies in degenerate p‐type PbTe where the cyclotron orbit of the carriers is comparable to the skin depth. The results showed that AK‐CR is a particularly effective tool for determining the effective mass of carriers in semiconductors at microwave frequencies when ωP cannot be made smaller than ωC.

Photomodulation Raman spectroscopy is employed to affect and to measure the band bending at the interface of the heterostructure system ZnSeGaAs. Our samples are pseudomorphic layers of undoped ZnSe (001) grown by molecular beam epitaxy (MBE) on undoped GaAs (001) films terminated with a 2 × 4 surface reconstruction. The interdiffusion of Zn (Ga) into GaAs (ZnSe) during growth produces an intrinsic band bending at the interface. The ZnSe overlayer with energy band gap EZn-Se = 2.67 eV offers a transparent window for the photomodulating pumping beam of photon energy EPM to reach the heterojunction while the Raman measurements are in progress. We observed a decrease in the Raman scattering intensity of LOZnSe and an increase in the scattering intensity of LOGaAs for EPM > EZn-Se while almost the reverse behaviour occurred for EPM < EZn-Se. These observations are explained in terms of the presence of hole traps at this 2 × 4 reconstructed interface.

In this paper, which we dedicate to Professor Claude Benoit a la Guillaume on the occasion of his 65th birthday, we present an overview of resonant inelastic light scattering by excitations of two-dimensional electron plasmas in semiconductor quantum structures. We consider, in particular, intersubband excitations and highlight aspects of resonant inelastic light scattering that play a key role in the elucidation and understanding of the novel physics of electron plasmas in semiconductor systems of reduced dimensions.

Mechanisms of resonant inelastic light scattering by the electron gas are evaluated here for their impact in the current research of semiconductor nanostructures. Recent experiments in the fractonal quantum Hall regime highlight the power of the resonant inelastic light-scattering method in studies of electron electron interactions in semiconductors of reduced dimensions. These applications follow from its capabilities to measure spin-density and charge-density collective modes as well as excitations that are not predicted by conventional response functions of the electron gas. We consider measurements of intersubband excitations of two-dimensional systems in GaAs quantum wells and observations of gap excitations in the fractional quantum Hall effect.

Raman studies using the allowed and the forbidden longitudinal optical (LO) Fröhlich interactions are employed to investigate the heterostructure of ZnSe/GaAs as a counterpart to three-photon mixing or second harmonic generation studies. Our samples are an undoped ZnSe/GaAs (001) heterojunction with a ZnSe overlayer varying from 50 to 5000 Å. The scattering intensities for the allowed and the forbidden LO interactions at both resonant and nonresonant incident frequencies for ZnSe are investigated. The nonresonant forbidden LO phonon shows the effect previously observed for the pseudomorphic region of ZnSe thickness as well as for thicker films. The allowed LO intensities show a clear interference effect similar to recent interference effects observed in second harmonic generation for identical ZnSe/GaAs heterostructures. At resonant incident frequency in ZnSe, forbidden LO phonon scattering shows strong enhancement and is an order of magnitude larger than the allowed scattering due to wave-vector-dependent scattering in ZnSe.

The charge state of a C60 molecule is strongly dependent on its environment. We discuss the factors that determine the charge state of C60 molecules adsorbed on metal substrates. An estimate of the charge state of a C60 molecule adsorbed without covalent bonding on a smooth simple metal surface is calculated taking into account the Coulomb interaction of the C60−n ion with its image charge and the polarization of the molecule, assuming the metal to be a perfect conductor, and the van der Waals interaction, using a Lennard-Jones pairwise potential based on graphite. We find that at most two electrons can be transferred to the molecule regardless of the work function of the metal. The broadening of the LUMO level of the adsorbed molecule due to resonance with the substrate energy levels will allow for a partial charge transfer even for metal substrates with work functions as high as 4.5 eV.

We treat for the first time the contributions to the linear and nonlinear optical response of noble metal surfaces from surface electronic transitions involving intrinsic surface states, Rydberg states and surface modified continuum states, and in particular formulate SHG, and 3WM in general, in terms of three-step processes involving three virtual surface electronic transitions. The surface electronic transitions lead to differences in reflection by (111) and (110) surfaces and to an anisotropic reflection by the (110) surfaces. They also lead to differences in the energy and dispersion of the surface plasmons on Ag(111) and Ag(110) and to anisotropic dispersion of the surface plasmons on Ag(110). 3WM at the (110) and (111) surfaces of Cu and Ag is discussed in terms of specific three-step processes at high symmetry points in the surface Brillouin zones. Processes involving transitions between intrinsic surface states at the -Y point of the (110) surface Brillouin zone are shown to lead to resonances in the dependence of χ2y′y′z′ and χ2y′y′z′ the frequency of the input, or output EM waves.

As mentioned in the Preface of these Proceedings, one important aspect of the Workshop was to review recent advances in the field of inelastic fight scattering from elementary excitations in semiconductors. To that end, the workshop organizers requested an introductory lecture from A. Pinczuk that would cover developments in what could be called the artificially structured materials era, which has occupied the last decade or so. But rather than just review recent work for this introductory chapter, the organizers felt a more comprehensive history of research developments would be appropriate, particularly as this had not been attempted before. E. Burstein’s lecture on early developments in light scattering spectroscopy of semiconductors, together with M. Cardona’s encyclopaedic knowledge of the field, naturally lead to their co-option in such a task. What follows is a general account of key and other important developments in the subject to date, biased to some extent by the knowledge and preferences of the contributors. We apologize in advance for any inadvertent omission of other major relevant research work.

Resonant optical second-harmonic and sum-frequency generation are applied to probe electronic transitions at the Ca-terminated epi- taxial ${\mathrm{CaF}}_{2}$/Si(111) interface. A band gap of 2.4 eV is established for the interface states, a value twice as large as that in bulk Si, but only (1/5 of the band gap in ${\mathrm{CaF}}_{2}$. The experimental three-wave-mixing spectra can be modeled by a two-dimensional band gap and a narrow resonance 150 meV below the band edge, the latter being tentatively assigned to a transition to a bound two-dimensional exciton.

We have chosen CaF2/Si(111) as a prototype to study the two-dimensional band structure of an ordered interface using angle-resolved photoemission and optical second harmonic generation. A pair of interface state bands is found, one occupied, the other empty. The former disperses from EF−0.8 eV at Γ to EF−1.4 eV (−1.6 eV) at M (K). An interface band gap of 2.4 eV is determined via resonant second harmonic generation using a truly buried interface with 500 Å of CaF2 on top of Si. Therefore, the interfacial gap is twice as large as the gap in Si and five times smaller than the gap in CaF2. The pair of interface bands can be understood as bonding/antibonding combinations of the Si dangling bond orbital and the Ca 4s orbital, with Ca in the 1+oxidation state.

When 15 MeV ions bombard single crystal GaAs (111), (100), and (110) faces, they leave a strained, crystalline, surface layer with many defects (as well as a buried amorphous layer). Using Raman spectroscopy, we measure the shits and line widths of the optic phonons of these strained crystalline layers. Using simple models, the possible sources of the phonon shifts are quantitatively considered. We conclude that the strains, and a change in the ionic plasma frequency (LO-TO splitting) due to a ratio of interstitials, or antisites, to atoms in the crystals of ≈ 2% − 3% account for the major portion of the phonon shifts. These effects have been ascribed previously to phonon confinement.

The surface layer of single crystal GaAs that has been bombarded with high energy (2-15 MeV) ions is strained and contains many defects, but it remains crystalline. We have studied the isochronal annealing behavior of this layer using phonon shifts and widths, which were detected by Raman measurements. We do not find any sharp annealing stages to 500 C, at which temperature the phonons have returned to their positions in the unbombarded crystals. These results are consistent with the idea that the phonon shifts are primarily due to antisites and their complexes with other primary defects. However, other extended defects may still be present, and the Raman data show some evidence for this.