L. K. Vodop’yanov

Raman scattering and optical normal vibrations of the Zn1-xCdxSe crystal lattice

The nonresonant spectra of the Raman scattering in Zn1−xCdxSe crystals (0≤ x≤1) are investigated. The mode doublet was found to gradually shift with increasing x from the longitudinal-transverse frequencies of ZnSe to the longitudinal-transverse frequencies of CdSe. Furthermore, an additional branch of weak modes was observed between the doublet components. The frequencies of this branch are shown to correspond to the frequencies of the cadmium impurity vibrations in the ZnSe lattice at x=0 and the zinc impurity vibrations in the CdSe lattice at x=1. The concentration dependences of the modes and atomic displacements are analyzed by using the isodisplacement model that includes the interaction between ZnSe-and CdSe-like vibrations. It is concluded that a one-mode behavior with the formation of an additional weak branch in the longitudinal-transverse splitting region is characteristic of this system.

Raman scattering in multilayer structures with CdTe quantum dots in ZnTe

Raman spectra in superlattices composed of layers of self-assembled CdTe quantum dots separated by ZnTe barriers are investigated. As the barrier thickness increases, a high-frequency shift of all peaks is observed, which is explained by a decrease in the lattice constant averaged over the volume of the entire structure. Peaks are found at a CdTe TO mode frequency of 140 cm−1 and also at 120 cm−1. The first peak is assigned to the symmetric Coulomb (interface) mode of the quantum dot material, and the low-frequency peak is assigned to the symmetric mode of the phonons captured in the quantum dot. This combination of modes in structures with quantum dots has not been observed previously.

Effect of elastic stresses on the IR spectra of lattice vibrations in epitaxial ZnSe films on (001)GaAs substrates

The lattice IR reflection spectra of epitaxial ZnSe films are studied for different thicknesses on a (001)GaAs substrate. The frequency of the TO mode is found to increase for films with thicknesses of 0.8 and 1.2 µm that exceed the critical value dcr≈0.1 µm for ZnSe/GaAs pairs. The effect is explained by the existence of regions with residual stress in the film.

Infrared lattice-reflection spectroscopy of Zn1−xCdxSe epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Results are presented of the first measurements of infrared reflection spectra of Zn1−xCdxSe films (x=0–0.55; 1) grown on a GaAs substrate by molecular-beam epitaxy. It is shown by a mathematical analysis of the experimental spectra that the investigated Zn1−xCdxSe alloy system manifests a unimodal rearrangement of its vibrational spectrum as the composition is varied.

Electronic states and vibration spectra of CdTe/ZnTe quantum dot superlattices

Electronic and vibrational states in CdTe/ZnTe quantum dot superlattices are studied using optical spectroscopy techniques (photoluminescence in a wide temperature range, IR reflection, and Raman scattering). The effect of the ZnTe barrier layer thickness on the luminescence spectra of the structures is discussed. The luminescence from electronically coupled islands is assumed to be due to spatially indirect excitons because of the specific features of the CdTe/ZnTe heterostructure band structure. A combination of quantum-dot vibrational modes, which has not been observed earlier, is detected in the Raman spectra. Analysis of the lattice IR reflection spectra shows that, in the case of large barrier thicknesses between the quantum-dot planes, elastic stresses are concentrated in the Zn1−xCdxTe layers, whereas in structures with lower barrier thicknesses the elastic-strain distribution exhibits a more complicated pattern.

Cathodoluminescence and Raman scattering in Ga1−xAlxP epitaxial films

Low-temperature cathodoluminescence and Raman scattering of Ga1−xAlxP epitaxial layers (0≤x≤0.8) grown by liquid phase epitaxy on the GaP(100) substrate are studied. The obtained cathodoluminescence spectra indicate that the dependence of the indirect energy gap on the composition parameter x is nonlinear. This nonlinearity can be described by the parabolic function with the inflection parameter b=0.13. Raman scattering studies show that the phonon spectrum of Ga1−xAlxP consists of one (Al-P)-like vibrational mode and three (Ga-P)-like modes.

Vibrational spectroscopy of the Ga1−xAlxP epitaxial layers grown on GaP(111) substrate by the liquid-phase epitaxial technique

The paper reports the results of measurements of the lattice IR reflection and Raman scattering spectra for the Ga1−xAlxP (x=0–0.8) films grown on the GaP(111) substrate by the liquid-phase epitaxy technique. The dispersion analysis of the experimental spectra has demonstrated that, for the studied system of the Ga1−xAlxP alloy, the vibrational spectra of the alloys with different compositions exhibit three modes of the Ga-P vibrations and one mode of the Al-P vibrations. The frequencies of modes only slightly depend on the composition x of the Ga1−xAlxP alloy, but the composition considerably affects the oscillator strengths of these modes.

Long-wavelength optical phonons in the ZnTe/Zn0.8Cd0.2Te superlattice

The lattice IR reflection spectra of a ZnTe/Zn0.8Cd0.2Te superlattice measured at temperatures of 300 and 10 K are analyzed. The ZnTe/Zn0.8Cd0.2Te superlattice is grown by molecular-beam epitaxy on a GaAs substrate with a ZnTe buffer layer. It is found that the lattice IR reflection spectra of the studied structure exhibit only one reflection band. Dispersion analysis of the experimental spectrum has revealed the presence of one lattice TO mode close in frequency to the mode of pure ZnTe. This result is explained by a shift in the frequency of the lattice modes of the ZnTe and Zn0.8Cd0.2Te layers of the superlattice toward each other. In turn, this shift is caused by internal elastic stresses in the superlattice due to a mismatch between the lattice parameters of the materials of these layers.

Effect of band inversion on the phonon spectra of the Hg{sub 1-x}Zn{sub x}Te alloys

The narrow-gap II-VI and IV-VI alloys are convenient objects for studying the electron-phonon interaction. However, the concentration of free carriers in the IV-VI alloys is rather high (∼1018 cm−3), which complicates studying this effect, by the optical reflection method. The concentration of free carriers in the narrow-gap Hg1-xZnxTe alloys is considerably lower (∼1016 cm−3). Therefore, the plasma component exerts less effect on the lattice reflection spectrum. The reflection spectra of Hg1-xZnxTe crystals with x = 0.1–1 were studied in the far-IR region in the range of 30–700 cm−1 at 40–300 K. Using the dispersion analysis and the Kramers-Kronig method, the frequencies of the TO phonons of the HgTe-like and ZnTe-like modes were determined depending on the composition. It is shown that the reconstructed phonon spectrum involves two modes. The temperature dependences of the frequencies of the TO phonons and the damping parameter were measured for the narrow-gap alloy with x = 0.1 in the range of 80–200 K. A decrease in the frequency of the TO phonon of the soft mode in the vicinity of the inversion point of the bands at T = 110 K was for the first time found by optical methods. The damping parameter slightly increases in the vicinity of this temperature. The result obtained qualitatively agreed with the theoretical model of Kawamura et al., which makes allowance for the effect of the electron-phonon interaction on the frequency of the soft mode in the IV-VI compounds.

Manifestation of interaction effects in Raman spectra of CdTe quantum-dot layers with ZnTe narrow barriers

The Raman spectra of superlattices consisting of layers of CdTe self-assembled quantum dots separated by ZnTe narrow barriers with thicknesses of 10 and 5 monolayers are investigated. It is found that, apart from the bands previously observed at frequencies of ∼120 and ∼140 cm−1 for samples with thicker barriers (25 and 12 monolayers), the Raman spectra exhibit a band at ∼147 cm−1 in the frequency range of CdTe vibrational modes. This band is attributed to a symmetric vibrational mode of a pair of quantum dots with oppositely directed oscillations of the dipole moments. It is this type of vibrational mode in the material surrounding the ZnTe quantum dot that accounts for the shift of the band at ∼200 cm−1 near the LO mode of ZnTe vibrations toward lower frequencies.