Silver gallium telluride (AgGaTe2) is a ternary chalcogenide crystal that belongs to a family of materials with chalcopyrite structure. It is the member of I-III-VI2 group where I = Ag, Cu; III = Ga, Al, In; VI = S, Se, and Te. These materials are known because of their attractive physical properties which have potential for practical application in different types of devices. Particularly, it is known that the AgGaTe2 crystals are prospective for using as thermoelectrical materials. The high absorption coefficient makes these materials interesting for the absorbing layer for thin-film solar cells production. It is known, that AgGaTe2 crystalize in the chalcopyrite structure, which is described by the I-42d space group. Today, the information on some physical properties important for the construction of the devices for practical usage is still lacking. Particularly, there is no information reported on elastic properties, nonlinear properties, and parametric effects under external fields like stress, electromagnetic field, temperature, etc.
In this work, we present the results of complex theoretical investigations of the optical, electronic, parameters of AgGaTe2 crystals. The presented results are received by the ab initio calculations. The calculations were carried out using the plane-waves pseudopotential method based on the density functional theory (DFT). The effects of exchange and correlation of electrons were taken into account as the local density approximation LDA/CA-PZ parameterization and the generalized gradient approximation GGA/PBE were used. The valence electrons have the following configuration: Ag 4d10 5s1; Ga 3d10 4s2 4p1 and Te 5s2 5p4. The unit cell geometry was relaxed by BFGS algorithm. For self-consistent electronic minimization, the eigen energy convergence tolerance was chosen at 24×10-7 eV and the tolerance of the electronic total energy convergence was 10-5 eV/atom. The convergence parameters used during optimization were as follows: the maximum force 3×10–2 eV/Å; maximum pressure 5×10–3 GPa and maximum displacement 1×10‑4 Å.
It was established that the crystal has a direct bandgap located at Г-point of the Brillouin zone. The calculated band gap value is equal to Eg = 0.164 eV and Eg = 0.324 eV for the GGA and LDA functionals [1]. The underestimation of the band gap was fixed using the scissor operator of 1.156 eV and 0.996 eV values, respectively. The analysis of the calculated total and partial density of state diagrams shows that the valence band top levels are formed by 4d states of silver and 5p states of tellurium and the bottom levels of the conduction band are formed by the 5p states of tellurium and gallium 4s states.
Using the band structure diagrams the optical spectra of the dielectric function, refractive indices and extinction coefficients were obtained. The calculated optical spectra reveal excellent agreement with the published experimental spectra received from optical ellipsometry for the wide range of energies. The dielectric function, refractive indices and extinction coefficient reveal relatively insignificant anisotropy along with main crystallographic directions.
Fig. 1. The calculated refractive and extinction coefficients of the AgGaTe2 crystals for (a) EZ and (b) E||Z.
The elastic constants coefficients for the tetragonal symmetry of Cij matrix are calculated using DFT calculation [2]. The elastic properties have a significant anisotropy as can be seen from 3D spatial distribution of Young’s modulus E(φ, θ) [3]. From the elastic properties, the characteristics of the sound propagation along different directions in a solid were estimated. The sound velocities for the longitudinal νl and transversal νt modes of the AgGaTe2 crystal were calculated using the Сij values obtained in this work. Good agreement between all calculated and corresponding experimental data was achieved, which serves as firm proof of the applicability of the used calculation method to the studies of anisotropic crystalline materials.
M. Ya. Rudysh thanks the support by the PRELUDIUM 15 program of Polish National Science Center (Grant No. 2018/29/N/ST3/02901).
References:
1. M. Ya. Rudysh, M. Piasecki, G.L. Myronchuk, P.A. Shchepanskyi, V. Yo. Stadnyk, O.R. Onufriv, M.G. Brik, Infrared Phys. Technol. 111, 103476 (2020).
2. M. Ya. Rudysh, P.A. Shchepanskyi, A.O. Fedorchuk, M.G. Brik, C.-G. Ma, G.L. Myronchuk, M. Piasecki, J. Alloys Compd. 826, 154232 (2020).
3. A. Majchrowski, M. Chrunik, M. Rudysh, M. Piasecki, K. Ozga, G. Lakshminarayana, I.V. Kityk, J. Mater. Sci. 53 (2), 1217-1226 (2018).
