S. P. Kozyrev

IR-active vibrational modes of CdTe and CdSe colloidal quantum dots and CdTe/CdSe core/shell nanoparticles and coupling effects

The frequencies of the vibrational modes of CdTe and CdSe quantum dots and CdTe/CdSe core/shell nanoparticles prepared by the colloid chemistry method are determined using IR transmission and IR reflection spectroscopy. The experimental IR transmission spectrum of CdTe and CdSe nanocrystals exhibits a broad minimum located between the frequencies of the transverse optical (TO) and longitudinal optical (LO) phonons of bulk CdTe and CdSe crystals. The frequencies of the modes for ensembles of CdTe and CdSe quantum dots are considerably shifted toward lower frequencies as compared to those calculated for single quantum dots. This is explained by the dipole-dipole interaction between quantum dots. The frequencies of modes for the structures with core/shell nanoparticles differ little from the calculated frequencies. This suggests a weakening of the interaction in these structures due to the enhancement of dielectric screening.

Anomalous properties of optical lattice vibrations in HgTe: A double-well model of the lattice potential for a Hg atom

A double-well potential model has been proposed for a Hg atom in the HgTe crystal lattice in which the Hg atom either occupies the center of the anion tetrahedron or is located at the off-center position. The fundamental TO mode of the Hg-Te vibrations at a frequency of ∼120 cm−1 corresponds to the vibrations of the off-center Hg atom, and the additional mode is associated with the vibrations of the Hg atom located at the center of the anion tetrahedron. The model has made it possible to explain both the strong increase in the intensity of the additional mode with increasing temperature, which, at T = 300 K, is comparable to the intensity of the fundamental mode, and the anomalous change (decrease) in the damping parameter of the fundamental TO mode with increasing pressure, which corresponds to a more ordered crystal structure.

Manifestation of anomalous properties of HgTe lattice vibrations in Hg1 − xZxTe (Z = Cd, Mn, Zn) alloys

Alloys (solid solutions) Hg1 − xZxTe (Z = Cd, Mn, Zn) with a variation in their composition are characterized by the double-mode type of rearrangement of the vibrational spectrum of the crystal lattice, and these alloys retain the properties of lattice vibrations of the binary alloy components. The HgTe-enriched alloys under consideration also retain the anomalous properties of HgTe lattice vibrations with their additional low-frequency 105 cm−1 mode, which is forbidden according to the symmetry selection rules. An analysis has been made of the specific features of the manifestation of the 105 cm−1 mode in the alloys as compared to HgTe, in particular, its splitting into several modes ωn (n = 0, 1, 2) due to the presence of two cations Hg and Z (Cd, Mn, Zn) in the cation sublattice. The thermal activation of the low-frequency 105 cm−1 mode in the HgMnTe and HgZnTe alloys almost coincides with that observed in HgTe, but the activation energy of the mode for HgCdTe differs substantially from that for HgTe. Moreover, the conditions providing the activation of the 105 cm−1 mode in the n-HgCdTe and p-HgCdTe alloys are different.

Low-frequency optical lattice vibrations in Hg1 − xCdxTe alloys

The lattice reflection spectra of the Hg1 − xCdxTe (x = 0.06–0.70) alloys measured in the low-frequency range of optical vibrations (the region of the anomalous mode of Hg-Te vibrations in HgTe) at room temperature are interpreted. The low-frequency modes observed at frequencies of 98, 105, and 112 cm−1 for all compositions of the Hg1 − xCdx Te alloy are assigned to the modes of Hg-Te vibrations, as was previously done for modes of Cd-Te vibrations in the quasi-molecular approximation. According to the double-well potential model for the Hg atom in the crystal lattice of the alloy, the Hg atom either can occupy the center of the anion tetrahedron or can be located in the off-center position. The fundamental strong mode of Hg-Te vibrations at a frequency of about 120 cm−1 (at T = 300 K) corresponds to the vibrations of the off-center Hg atom, and the low-frequency vibration modes correspond to the vibrations of the Hg atom located at the center of the anion tetrahedron.

Low-frequency optical spectra of lattice vibrations of the Hg1−x ZnxTe alloy

The paper reports on assignment of lattice reflection spectra of the Hg1−xZnx Te alloys (x = 0.10, 0.35, and 0.55) measured in the 40–300-K temperature interval and on a comparative analysis of the lattice vibration parameters for two alloy systems, HgZnTe and HgCdTe, in the low-frequency optical vibration region (the region of the anomalous Hg-Te vibrational mode in HgTe). While the parameters of the fundamental lattice modes are closely similar, the temperature dependences of the low-frequency modes of the two alloy systems are radically different. Their thermal activation energies for the HgZnTe and HgCdTe alloys are 3.3 and 50.0 meV, respectively. The results of the experiment are treated in terms of the double-well potential model for the Hg atom in the alloy lattice.

Specific features of the properties of Hg-Te lattice vibrations in Cd1 − yHgyTe alloys enriched with CdTe

In alloys (solid solutions) Cd1 − yHgyTe enriched with CdTe, the properties of Hg-Te lattice vibrations are determined by a single-well lattice potential for the Hg atom located at the center of the anion tetrahedron. The HgTe-enriched alloys retain the anomalous properties of Hg-Te lattice vibrations, which are determined by a double-well lattice potential for the Hg atom located in the off-center position at low temperatures; a shallower well of the potential for the Hg atom located at the center of the anion tetrahedron is filled upon thermal activation. The polarizability of the ion pair in the TO mode of Hg-Te lattice vibrations with the Hg atom located at the center of the anion tetrahedron for the CdTe-enriched alloys considerably exceeds the polarizability of the pair with the off-center Hg atom for the HgTe-enriched alloys.

IR-active vibrational modes of CdTe, CdSe, and CdTe/CdSe colloidal quantum dot ensembles

The frequencies of the vibrational modes of CdTe and CdSe quantum dots and CdTe/CdSe core/shell nanoparticles prepared by the colloidal chemistry method are determined using IR transmission and IR reflection spectroscopy. The experimental IR transmission spectrum of CdTe and CdSe nanocrystals exhibits a broad minimum located between the frequencies of the transverse (TO) and longitudinal (LO) optical phonons of bulk CdTe and CdSe crystals. The frequencies of the modes for ensembles of CdTe and CdSe quantum dots are considerably shifted toward lower frequencies as compared to those calculated for single quantum dots. This is explained by the dipole-dipole interaction between quantum dots. The frequencies of modes for the structures with core/shell nanoparticles differ little from the calculated frequencies. This suggests a weakening of the interaction in these structures due to the enhancement of dielectric screening.

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.

Specific features of the properties of lattice vibrations in the Cd1–yHgyTe alloy formed by the CdTe semiconductor and the HgTe semimetal

The specific features of the properties of Hg-Te lattice vibrations have been considered within the percolation model of a mixed Cd1–yHgyTe crystal (alloy), in which the composite medium is formed by the alloy regions enriched in HgTe and CdTe with different Hg-Te vibrational states and, hence, with different characteristics of vibrational modes (frequency and damping parameter). In the HgTe-enriched alloy, the properties of Hg-Te lattice vibrations are determined by a double-well lattice potential for the Hg atom with the off-center localization at low temperatures. In the CdTe-enriched alloys, the properties of Hg-Te lattice vibrations are determined by a single-well lattice potential for the Hg atom in the center of the anion tetrahedron, and the Hg-Te vibrational states are localized. For the HgTe-enriched alloy, the Hg-Te vibrational states are extended. In the percolation scheme of the transformation of the vibrational spectrum of the Cd1 - yHgyTe alloy with the composition y, the “137 cm−1” mode is a split-off mode of Hg-Te vibrations, or one of the modes of the percolation doublet of HgTe-like vibrations.