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5ALT'22
LASER SYSTEMS AND MATERIALS
LS-P-9
Promising materials for optoelectronics - CdGa2Se4: Theoretical calculations and experimental studies of electronic properties
I.A. Mamedova1, Z.A. Jahangirli12, E.H. Alizade1, T.G. Kerimova1, T.G. Mammadov1, N.A. Abdullayev12*
CdGa2Se4 single crystals crystallize in a tetragonal structure in space symmetry group I 4. These compounds are characterized by birefringence, significant values of the coefficient of nonlinear susceptibility, and bright photoluminescence, which puts these compounds forward among the promising materials for optoelectronics.
The electronic band structure and projected onto atoms partial densities of states (PDOS) are theoretically calculated from first principles using density functional theory (DFT). An analysis of the partial density of states shows that the group of valence bands in the energy range from approximately -6.5 eV to 0 eV originates from the s states of Gal and Ga2, as well as the p states of the chalcogen Se atoms. It also follows from the partial density of states that the bottom of the conduction band is formed mainly from the s states of the Gal and Ga2 atoms with some addition of the p states of the Se atom. According to the calculated electronic band structure, the top of the valence band and the absolute minimum of the conduction band are at the r point of the Brillouin zone and, accordingly, according to our calculations, CdGa2Se4 is a direct-gap semiconductor with a band gap of ~ 2.5 eV.
Since spectral ellipsometry is one of the accurate methods for determining the optical characteristics of crystals, we carried out spectral ellipsometric studies of CdGa2Se4 single crystals. These measurements are based on determining the change of polarization state of light as a result of its interaction with the surface of crystals upon reflection. The measurements were carried out on an optical range ellipsometer M-2000 DI (J.A. Woollam Co, Inc.). The spectral dependence of the ellipsometric parameters A and Y was measured in the photon energy range of 0.7-6.5 eV with a step of 50 meV at radiation incidence angles in the range of 60°-75° with a step of 5°.
A comparison is made between the ab initio calculations in the 0-14 eV spectral range and the experimentally measured the real and imaginary parts of the dielectric function, coefficients of refraction, extinction, and absorption of CdGa2Se4 for different polarizations of the incident light (along and perpendicular to the tetragonal c axis). A good agreement is observed between the characteristic critical points at energies of approximately 2.4 and 4 eV.
It was found that the spectral dependence of the absorption coefficient a is well approximated by the dependence ahv~(Eg-hv)1/2, for direct allowed optical transitions. This is also evidenced by the high values of the absorption coefficient a (104-105 cm-1), its sharp increase in the energy range of 2-3.5 eV.
The work was supported financially by the Science Development Foundation under the President of the Republic of Azerbaijan (grant no. EIF-BGM-3-BRFTF-2+/2017-15/02/1).
1-Institute of Physics of NAS of Azerbaijan, ave.H.Javid, 131, AZ1143, Baku, Azerbaijan 2-Baku State University, st.Z. Khalilov, 33, AZ-1148, Baku, Azerbaijan e-mail address: abnadir@mail.ru
