Room temperature measurements of the far-infrared (FIR) reflectance spectra are reported for the polar optical phonons in a series of bulk CdxZn1$-${}xTe (0 $\le${} x $\le${} 1) and CdSexTe1$-${}x (0 < x $\le${} 0.35) crystals grown by Bridgman technique. The composition-dependent spectra exhibited many fundamental aspects of lattice vibrations, while the results of long-wavelength optical modes for the end member binary compounds displayed phonon values in good agreement with the existing inelastic neutron scattering and/or Raman spectroscopy data. Using a standard methodology of multilayer optics with effective-medium dielectric tensors, we simulated the FIR transmission and reflectivity spectra at oblique incidence (Berreman's effect) in many II-VI free-standing thin films, epilayers and superlattices. Berreman's approach provided us with a strong basis for analyzing the infrared experimental data and offered an effective means of estimating the zone-center transverse and longitudinal optical ($\omega${}TO, $\omega${}LO) phonon frequencies in polar-semiconductor thin films and heterostructure materials of increasing technological importance.

Lattice dynamical theory of thermal expansion and mode Grüneisen parameters in cubic boron monophosphide is reported in the quasiharmonic approximation within the framework of a second-neighbor rigid-ion model. In this scheme, we optimized the involved force constants by using nonlinear least-squares procedures with constrained parameters and weighting of the available data on critical-point phonons and elastic and lattice constants. Theoretical results of the phonon dispersion curves along high-symmetry directions (both at ambient and 89 kbar pressures), mode Grüneisen parameters, and thermal expansion coefficient {α(T)} are compared and discussed with the existing experimental and ab initio calculations. Consistent with x-ray data, our calcualtion for the variation of α(T) with temperature in BP is found to be remarkably similar to that of Β-SiC, and unlike most other III-V-compounds, it does not attain negative values at lower temperatures.

Measurements of the Raman scattering and extended x-ray-absorption fine-structure (EXAFS) spectroscopy are reported on a series of Bridgman-grown zinc-blende CdTe${}_{1$-${}x}$Se${}_{x}$ (0.35 $\ge${} $x$ > 0.05) ternary alloys to empathize their lattice dynamical and structural properties. Low-temperature Raman spectra have revealed the classic CdTe-like (TO${}_{1}$, LO${}_{1}$) and CdSe-like (TO${}_{2}$, LO${}_{2}$) pairs of optical phonons. The composition-dependent peak positions of the LO${}_{2}$ modes exhibited shifts towards the higher-energy side, while those of the LO${}_{1}$ phonon frequencies have unveiled the slight redshifts. Detailed analyses of EXAFS data by using the first-principles bond orbital model have enabled us to estimate both the lattice relaxations and nearest-neighbor radial force constants around the Se/Te atoms in the CdTe/CdSe matrix. These results are methodically integrated in the ``average $t$-matrix'' formalism within the Green's-function theory for defining the impurity perturbations to comprehend the composition-dependent optical phonons in CdTe${}_{1$-${}x}$Se${}_{x}$ alloys. Based on our comprehensive calculations of impurity modes in the low-composition regime $x$\rightarrow${}$ 0, we have assigned the weak phonon feature observed near $\sim${}175 cm${}^{$-${}1}$ in the low-temperature infrared reflectivity spectroscopy study to a Se${}_{\mathrm{Te}}$ localized vibrational mode.

In II-VI compound semiconductors, we have used a comprehensive Green's function theory to study the vibrational properties of isotopic defects and to ascertain the microstructure of complex centers involving dopants and intrinsic impurities. The phonons generated by a realistic lattice-dynamical model for the host materials are integrated in simulating the Green's functions to help explicate the observed localized vibrational modes (LVMs) for various defect centers. Contrary to the distinct force constants required for isolated defects, the isotopic shift of LVMs has offered strong revelations for inflexible ‘impurity-host’ interactions in each isotopic defect. In compound semiconductors a unique force variation correlation with bond covalency is proposed providing corrections to the nearest-neighbor (NN) force constants for the closest mass isoelectronic and impurities carrying static charges. The articulation is extremely useful for defining perturbations and for analyzing the infrared absorption data on LVMs of complex defect centers. In corroboration with experiments, the Green's functions theory of impurity modes in Li-doped CdTe:Al (ZnSe:Al) has established second NN Li${}_{\mathrm{Cd}\left(\mathrm{Zn}\right)}$-Al${}_{\mathrm{Cd}\left(\mathrm{Zn}\right)}$ pairs indicating the passivation of group-I acceptors via interaction with group-III elements as donors. The proposal of an antisite complex model Al${}_{\mathrm{Zn}}$-Zn${}_{\mathrm{Se}}$-Al${}_{\mathrm{Zn}}$ for the $X$ center is consistent with the existing absorption results on impurity modes and is equally justified by theoretical considerations—making it the more likely identity for the native defect compensating neighboring Al${}_{\mathrm{Zn}}$ donors in ZnSe. DOI: http://dx.doi.org/10.1103/PhysRevB.85.195203 ©2012 American Physical Society