Calculation of the vibrational spectra of AgGaS2 crystal with chalcopyrite structure

Abstract

Ternary semiconductor materials with a chalcopyrite structure belonging to the I(cid:21)III(cid:21)VI 2 group (where I = Ag, Cu; III = Al, Ga, In; VI = S, Se, Te) attract considerable attention due to interesting physical properties. The structural, electronic and optical properties of these crystals make them promising materials for a number of applications in nonlinear optics devices, detectors, photodiodes, solar cells, etc. In this work, the theoretical (cid:28)rst-principle calculations of the structural, vibrational and dielectric properties of an AgGaS 2 crystal with a chalcopyrite structure are performed. We report the results of the calculations of the phonon band structure ω ( q ) , total and partial density of phonon states N ( ω ) , infrared spectra and Raman spectra, the Born e(cid:27)ective charge tensor for each atom, and dielectric constants (for ω = 0 and ω → 0 ) using the theory of perturbation of the density functional. Theoretical-group analysis of vibrational modes is carried out and the classi(cid:28)cation of vibrations in the phonon spectra of the investigated compound is performed. The results of the phonons calculations showed the stability of the crystal structure of the studied compound. It follows from the calculations of the phonon spectrum that there is a signi(cid:28)cant anisotropy of low-frequency acoustic branches of vibrations. There is also a presence of low-frequency optical branches that interact with acoustic ones. It was found that the density of states can be divided into three parts (cid:22) low-frequency, medium-frequency and high-frequency regions, which are formed by the oscillations of Ag, Ga and S atoms, respectively. The calculated and identi(cid:28)ed frequencies of vibrations modes in the center of the Brillouin zone ( Γ point) showed good agreement with the available experimental data on IR and Raman spectra and theoretical data obtained using the Phonon program. A symmetric classi(cid:28)cation of vibrational modes for a crystal using group theory is performed. Estimation of the Born e(cid:27)ective charge tensor and dielectric constants in an AgGaS 2 crystal indicates their signi(cid:28)cant anisotropy.