Considerable attention has been devoted recently to the dependence of the widths of the Raman phonons of semiconductors on pressure and on isotopic mass. The dependence on pressure is usually small and monotonic unless the phonon happens to be close to a singularity of the two‐phonon density of states (DOS) which determines its width. In the latter case, strong nonmonotonic dependences of the phonon width on pressure and on isotopic mass can appear. We have investigated the E2high phonons of ZnO crystals with different isotopes and observed a wide range of FWHM depending on isotopic masses. Ab initio calculations of the two‐phonon DOS provide an explanation for this variation of the FWHM: the E2high frequency falls on a sharp ridge of the 2‐DOS corresponding to combinations of TA and LA phonons. Changes in isotopic mass result in a motion of the E2high frequency up and down that ridge which produces the changes in FWHM. These phenomena suggest a decrease of the FWHM with pressure which seems to be present in existing data obtained at 300 K. Similar phenomena are discussed for the E2low phonons. Applications of the isotope and pressure techniques to the elucidation of two‐phonon spectra will be presented.

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.

Self-energy effects of Raman phonons in isotopically disordered semiconductors are deduced by perturbation theory and compared to experimental data. In contrast to the acoustic frequency region, higher-order terms contribute significantly to the self-energy at optical phonon frequencies. The asymmetric dependence of the self-energy of a binary isotope system ${m}_{1\ensuremath{-}x}{M}_{x}$ on the concentration of the heavier isotope mass x can be explained by taking into account second- and third-order perturbation terms. For elemental semiconductors, the maximum of the self-energy occurs at concentrations with $0.5lxl0.7,$ depending on the strength of the third-order term. Reasonable approximations are imposed that allow us to derive explicit expressions for the ratio of successive perturbation terms of the real and the imaginary part of the self-energy. This basic theoretical approach is compatible with Raman spectroscopic results on diamond and silicon, with calculations based on the coherent potential approximation, and with theoretical results obtained using ab initio electronic theory. The extension of the formalism to binary compounds, by taking into account the eigenvectors at the individual sublattices, is straightforward. In this manner, we interpret recent experimental results on the disorder-induced broadening of the TO (folded) modes of SiC with a ${}^{13}\mathrm{C}$-enriched carbon sublattice [S. Rohmfeld, M. Hundhausen, L. Ley, N. Schulze, and G. Pensl, Mater. Sci. Forum 338-342, 579 (2000); Phys. Rev. Lett. 86, 826 (2001)].

We have measured disorder-induced effects of the phonon self-energy on the low-temperature Raman spectrum of isotopically tailored silicon crystals. The frequency and the linewidth of the Raman peak show isotope disorder-induced shifts and broadenings resulting in a nonlinear dependence on composition ("bowing"). This broadening exhibits an asymmetric variation with respect to the intermediate mass M = 29 amu. This dependence on composition is similar to, but much smaller than, the one seen in diamond and is attributed to higher-order perturbations in the mass disorder. Weak disorder-induced excitations axe observed from 30 cm(-1) to 60 cm(-1) below the main Raman peak. Their spectral positions for various isotopic compositions do not simply follow the harmonic scaling law omega proportional to (M) over bar (-1/2) expected for a virtual crystal with average mass TT In addition, their intensities axe not proportional to the second moment g(2) of the mass-fluctuation parameter.

Raman measurements are reported for isotopically tailored CuBr at low temperature. The transverse optic (TO) mode shows the expected similar to mu (-1/2) behavior with the reduced mass, with a small deviation that can be easily explained in terms of anharmonicity. However, for the longitudinal optic (LO) mode we observe a broad structure which can be resolved in three features of total width around 17 cm(-1). The nature of this structure is assessed by means of the Fermi resonance model, i.e., a two-phonon combination couples with the LO mode. The behavior of this structure with different Cu and Br isotopes is analyzed in detail.

We present a detailed investigation of the Raman spectra of isotopically tailored CuBr at low temperature. The transverse optic (TO) phonon of CuBr exhibits an almost perfect Lorentzian line shape, whereas the longitudinal optic (LO) phonon displays a complex broad structure. The change of the TO frequency with the variation of the isotope composition can be well described within the virtual crystal approximation (VCA), which corresponds to a $\ensuremath{\omega}\ensuremath{\propto}{\ensuremath{\mu}}^{\ensuremath{-}1/2}$ dependence on the reduced mass \ensuremath{\mu}. Slight deviations from this general trend are attributed to anharmonic renormalization and agree semiquantitatively with results extracted from previous measurements of the temperature dependence of the Raman spectra. In the LO case, the broad structure is resolved into three separate features, A, B, and C. While A and B are rather broad, C is a narrow peak located at the high-energy side of the LO structure. Two different trends are observed when analyzing the evolution of the LO structure with isotope substitution: peak B shows a $\ensuremath{\propto}{\ensuremath{\mu}}^{\ensuremath{-}1/2}$ behavior, analogous to that of the TO phonon, whereas peaks A and C shift almost only with the copper mass. The LO line shape is explained in terms of the Fermi resonance (FR) model, i.e., an interaction between the LO mode and a combination band of two acoustic phonons. We have performed a shell model calculation, with parameters taken from inelastic neutron scattering measurements, in order to obtain the one- and two-phonon densities of states (DOS). This calculation yields Raman line shapes in remarkable agreement with the experimental observations.

We present a comprehensive Raman study of self-energy effects in isotopically tailored silicon crystals at low temperature. Changes in the measured Raman spectra induced by isotope mass disorder are analyzed, including the weak excitations on the low-energy tail that arise approximately 30–60 cm−1 below the main Raman peak. In order to obtain a detailed picture of the renormalization, we simulate all lineshape properties within the framework of the coherent potential approximation. Comparison with earlier studies on diamond, germanium and α-tin illuminates the common aspects of isotope disorder-induced effects in elemental semiconductors.

Raman spectra of 69GaP were measured at T = 10 K under hydrostatic pressures up to 8 GPa. In the pressure range from 0 to 4 GPa the overall width of the transverse optic (TO) peak is found to decrease from ≈4.2 to ≈0.2 cm—1 and then to remain nearly constant at higher pressures. The dramatic changes in the TO Raman lineshape are attributed to a strong pressure dependence of the third-order anharmonic decay of the TO mode into phonons of lower energy. Our experimental findings are compared to results of recent first-principles calculations.

We present the design of a device for the simultaneous application of uniaxial stress and hydrostatic pressure. This new apparatus will for the first time allow measurements at constant strain. Results of the simultaneous application of uniaxial stress and hydrostatic pressure to a semiconductor laser are presented and discussed.

We have measured the phonon dispersion of wurtzite CdSe along <(Gamma A)over bar> by inelastic neutron scattering at room temperature. The sample was grown with the very weakly neutron absorbing isotope Cd-116 and natural selenium. These experiments extend the picture so far obtained from earlier measurements on (CdS)-Cd-114 and deliver more information on the accuracy of theoretical models. The experimental data are compared with ab initio and shell model calculations. (C) 1999 Elsevier Science B.V. All rights reserved.

We have measured Raman spectra of several SIC polytypes (3C, 6H, 15R) made from natural silicon (approximate to(28)Si) and Si-30. The isotope shifts of the phonon frequencies show characteristic variations with their effective wave vector in the zinc blende (3C) modification which arises from Brillouin zone backfolding. This allows us to determine the phonon eigenvectors of 3C SiC for the dispersion branches along the [111] direction. The observed magnitudes of the Si and C ion displacements, as well as their relative phase, confirm bond charge model and ab initio calculations. [S0031-9007(99)08863-8].

The transverse optical (TO) phonon in GaP has been measured in crystals with varying Ga isotopic composition by means of Raman scattering. In high resolution experiments at low temperature, we find a striking double peak structure in the line shape of natural GaP which varies with the Ga isotope abundance in a systematic manner. We calculate the spectra assuming anharmonic interaction of the TO phonon with a band of acoustic modes given by a model two-phonon density of states. This approach explains the measured Raman profiles and the isotope effects.

We have measured by Raman scattering changes of the optic phonon energy and line width in Si and GaAs with isotopic composition. The phonon energies of isotopically pure samples show the expected dependence on the average atomic mass in Si and the reduced mass in GaAs, respectively, as well as small anharmonic contributions. In isotopically disordered samples we find frequency shifts of 1.15(20)cm−1 for 28Si0.530Si0.5 and 0.31(20)cm−1 for the TO phonon of natGaAs, induced by mass disorder which also contributes to the line broadening. We give theoretical estimates of these effects.

The Raman spectrum of the transverse optical (TO) phonon in GaP is known to exhibit an asymmetric line shape at room temperature, broadened to lower energies. High resolution Raman measurements on GaP crystals at low temperature reveal a characteristic fine structure in the line shape of the TO phonon that has been studied systematically by variation of the isotopic composition of the gallium atoms. Calculations of the Raman spectra based on third-order anharmonic phonon phonon coupling account for the experimental findings. In this paper we give an extended view into the nature of the line shape, The understanding of the low temperature line shape is necessary for the interpretation of changes in the spectra with rising temperature. The temperature dependence of the TO Raman line of natural GaP is presented as well as a qualitative explanation for the broadening in terms of anharmonic effects.

The design of a device for the simultaneous application of uniaxial stress and hydrostatic pressure is presented. Shown in detail is the design of the part that applies the uniaxial force to the sample and first results of the simultaneous application of uniaxial stress and hydrostatic pressure to a semiconductor laser.