R. Lauck

Lattice properties of PbX (X=S, Se, Te): Experimental studies and ab initio calculations including spin-orbit effects

During the past five years the low-temperature heat capacity of simple semiconductors and insulators has received renewed attention. Of particular interest is its dependence on isotopic masses and the effect of spin-orbit coupling in ab initio calculations. Here we concentrate on the lead chalcogenides PbS, PbSe, and PbTe. These materials, with rock salt structure, have different natural isotopes for both cations and anions; a fact that allows a systematic experimental and theoretical study of isotopic effects, e.g., on the specific heat. Also, the large spin-orbit splitting of the 6p electrons of Pb and the 5p of Te, using a computer code which includes spin-orbit interaction, allows an investigation of the effect of this interaction on the phonon-dispersion relations and the temperature dependence of the specific heat, and on the lattice parameter. It is shown that agreement between measurements and calculations significantly improves when spin-orbit interaction is included.

Electronic and phononic properties of the chalcopyrite CuGaS2

The availability of ab initio electronic calculations and the concomitant techniques for deriving the corresponding lattice dynamics have been profusely used for calculating thermodynamic and vibrational properties of semiconductors, as well as their dependence on isotopic masses. The latter have been compared with experimental data for elemental and binary semiconductors with different isotopic compositions. Here we present theoretical and experimental data for several vibronic and thermodynamic properties of CuGa2, a canonical ternary semiconductor of the chalcopyrite family. Among these properties are the lattice parameters, the phonon dispersion relations and densities of states (projected on the Cu, Ga, and S constituents), the specific heat and the volume thermal expansion coefficient. The calculations were performed with the ABINIT and VASP codes within the LDA approximation for exchange and correlation and the results are compared with data obtained on samples with the natural isotope composition for Cu, Ga and S, as well as for isotope enriched samples.

Vibrational and Thermal Properties of ZnX (X=Se, Te): Density Functional Theory (LDA and GGA) versus Experiment

We calculated the phonon dispersion relations of ZnX (X=Se, Te) employing ab initio techniques. These relations have been used to evaluate the temperature dependence of the respective specific heats of crystals with varied isotopic compositions. These results have been compared with mea- surements performed on crystals down to 2 K. The calculated and measured data are generally in excellent agreement with each other. Trends in the phonon dispersion relations and the correspond- ing densities of states for the zinc chalcogenide series of zincblende-type materials are discussed.

Temperature-dependent Raman scattering of natural and isotopically substituted PbS

Lead sulfide is an important semiconductor that has found technological applications for over a century. Raman spectroscopy, a standard tool for the investigation and characterization of semiconductors, has limited application to this material because of the forbidden nature of its first order scattering and its opacity to visible lasers. Nevertheless, useful vibrational spectra from two-phonon processes are obtained with red lasers, probably because of a resonance in the concomitant electronic transitions. Herewith, we report temperature dependent Raman spectra covering the 10–300 K range, for two samples with different sulfur isotopic compositions. The results are analyzed by comparison with ab initio cal- culations of the lattice dynamics of PbS and the corresponding densities of one and two-phonon states. Emphasis is placed on the analysis of the two phonon band centered at ∼430 cm–1. The combined use of ab initio calculations and an extended temperature range (compared to previous reports) allows an unequivocal identification of some of the main features in the second-order Raman spectra. Our study also provides a much firmer background for the understanding of anomalous temperature dependences in the anharmonic shifts of second-order Raman spectra in PbS. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Anomalous pressure behavior of ZnSe Raman spectrum

The effects of pressure on the 1- and 2-phonon Raman spectrum of 68Zn76Se are investigated at 295, 125, and 11 K. At 295 K, the TO(Γ) line-width exhibits an anomalous 10-fold broadening in the pressure range 10.5–13 GPa, prior to the B1 phase transition. Other spectral features remain reasonably sharp. At 125 and 11 K the TO(Γ) anomaly is not seen. After the phase transition, a broadband is also seen in the TO(Γ) region for a wide range of downstroke pressures. Two explanations are considered: strong anharmonic coupling between TO(Γ) and (TO2–TA1)(X,K) difference modes, and structure or local bonding changes within the ZnSe lattice. First principles calculations of the 1- and 2-phonon density of states as a function of pressure are carried out and used to test the effect of the TO(Γ)→(TO 2–TA 1)(X,K) decay process. The results cannot rule out contributions from additional anharmonic channels and/or structural disorder effects.

Phonons and electrons in chalcopyrite semiconductors

In recent years the phonons and the electron phonon interaction of binary tetrahedral semiconductors have been profusely investigated by ab initio techniques and compared with experimental results. Of particular interest have been binary compounds in which the cations contain semi-core d-electrons (CuCl, CuI, AgI) which display anomalies related to the semi-core d-states (3dCuCl, 4dAgI). Here we present the corresponding data and anomalies which have been observed in ternary compounds of chalcopyrite structure (e.g. CuGaS2, AgGaX2 (X = S, Se, Te)). We present new ab initio calculations of the phonon dispersion relations of AgGaS2 and compare them with available Raman and IR data. Anomalies in the temperature dependence of the electronic gaps, which have been found in the binary chalcogenides, are also hinted at by the results for the ternary compounds with chalcopyrite structure. In view of the large number of atomic combinations possible for these materials (AgGaS2, AgGaSe2, CuGaTe2, ...) we believe that...

Phonons and Phonon‐Mixing in ZnSe Isotopic Crystals, Pressure‐cycled Domains, and Nanorods

Pressure‐Raman spectra are compared for bulk crystalline 68Zn76Se and ZnSe nanorods. The broadening of TO(Γ) and LO(Γ) is explained by multiphonon mixing and the position of a gap in the 2‐phonon DOS. Nucleation of the phase transition in 68Zn76Se leads to striking similarities in the Raman spectra of retrieved bulk and nanorod samples.