A. M. Yaremko

Fermi resonance in the phonon spectra of quaternary chalcogenides of the type Cu2ZnGeS4.

The experimental resonant and non-resonant Raman scattering spectra of the kesterite structural modification of Cu2ZnGeS4 single crystals are reported. The results are compared with those calculated theoretically within the density functional perturbation theory. For the majority of lines a good agreement (within 2-5 cm(-1)) is established between experimental and calculated mode frequencies. However, several dominant spectral lines, in particular the two intense fully symmetric modes, are found to deviate from the calculated values by as much as 20 cm(-1). A possible reason for this discrepancy is found to be associated with the Fermi resonant interaction between one and two-phonon vibrational excitations. The modelling of spectra, which takes into account the symmetry of interacting states, allows a qualitative description of the observed experimental findings. Due to the similarity of the vibrational spectra of Cu2A (II) B (IV) S4 (A  =  Zn, Mn, Cd; B  =  Sn, Ge, Si) chalcogenides, Fermi resonance is argued to be a general phenomenon for this class of compounds.

ANHARMONIC EFFECTS AND TEMPERATURE DEPENDENCE OF THE LATTICE SPECTRA OF FERROELECTRIC CRYSTALS A3B2C9

Abstract In the present work the Raman spectra of the family of ferroelectric crystals A 3 B 2 C 9 (where A=Cs, Rb; B=Sb, Bi; C=Br) are given and the assignment of some high frequency bands is fulfilled. The conclusion about the possible differences of the structures of the crystals Rb 3 Bi 2 Br 9 and Rb 3 Sb 2 Br 9 from the well-known and thoroughly investigated structure of the Cs 3 Bi 2 Br 9 crystal is made. It is shown that anharmonic interaction of vibrations can give rise to anomalous T -dependence of spectra in the low frequency region. For a crystal having methyl-ammonium (MA) groups the strong anharmonic effects can generate a very complicated structure around internal fundamental frequencies of the MA group which may be considered as a result of the phase transition effects.

THE RAMAN SPECTRA OF THE FAMILY CRYSTALS CS3BI2BR9, RB3BI2BR9 AND RB3SB2BR9

Abstract In the present work the Raman spectra of the family of ferroelectric crystals A3B2C9 (where A = Cs, Rb; B = Sb, Bi; C = Br) are given and the assignment of some high-frequency bands is fulfilled. Conclusions about possible differences between the structure of Rb3Bi2Br9 and Rb3Sb2Br9 crystals from the well-known and detailed structure of Cs3Bi2Br9 crystals are made. We also suggest that the significant variation between the spectra in the temperature range 77–300 K can be not only due to phase transition effects but also to anharmonic Fermi resonance between one- and two-phonon excitations.

Study of acoustical phonon modes in superlattices with SiGe QDs

Multilayers with SiGe nanoislands (QDs) grown in a broad temperature range are studied using Raman spectroscopy, HRXRD and compared with similar multilayers without islands. As the growth temperature increases, Si content in the islands increases, partially relieving strain. These structural transformations manifest themselves in both the intensity and frequency of the low‐frequency Raman peaks. Due to composition‐ and strain‐induced changes in the island band structure, excitation conditions come out of resonance, reducing Raman peak intensity. We have shown that at the interpretation of the Raman scattering by folded acoustic phonons for structures with nanoislands the real morphology of the island layer should be considered. The observed series of the low‐frequency Raman peaks, for the multilayered structures with the number of QD layers below ten, is due to the acoustic phonon modes within the islands. The enhancement of the scattering intensity due to resonance of the excitation light with the elect...

Exciton-phonon interaction in crystals and quantum size structures

In this report, the problem of electron-phonon interaction (EPI) in bulk semiconductors and quantum dots (QDs) is considered. It is shown that the model of strong EPI developed for organic molecular crystals can be successfully applied to bulk and nano-sized semiconductors. The idea of the approach proposed is to describe theoretically the experimental Raman (IR) spectra, containing the phonon replicas, by varying the EPI constant. The main parameter of the theoretical expression (βS) is the ratio of EPI constant (χS) to the frequency of the corresponding phonon mode (ΩS). The theoretical results show that variation of the QD size can change the value of χS.

The Correlation Between the Surface-Energy Minima and the Shape of Self-Induced SiGe Nanoislands

The effects of the composition, size, and thermal expansion coefficient of self-induced Ge and SiGe nanoislands formed on Si on the value of the islands’ total energy are examined. A correlation between the discrete minima in the surface energy of the islands and their shape is considered. The interdiffusion processes that are important at high temperatures of epitaxy are taken into account. The results of calculations are compared with experimental data obtained using atomic-force microscopy.

Correlation between the energy of SiGe nanoislands and their shape and size

The total energy of self-assembled SiGe nanoislands on a silicon substrate is investigated theoretically as a function of their geometric and physical parameters. It is demonstrated that the growth temperature and the silicon content in nanoislands affect the minimum of their energy. The results of numerical calculations for nanoislands are compared with experimental data obtained by atomic-force microscopy.

Exciton peculiarities in the As2S3 layer crystal

Abstract The effect of a weak van der Waals interaction between the layers on the edge reflection spectrum of As 2 S 3 is discussed. We point out specific features of the excitons in layer crystals.