We investigate an influence of the crystal structure imperfections on the electronic properties and dielectric functions of the In 0.5 Tl 0.5 I semiconductor in the frame of the density functional theory calculations. The tensor of electron effective mass m à ij for the InI, In 0.5 Tl 0.5 I and TlI crystals has been calculated for the valence and conduction bands at different K-points of the Brillouin zone. The dielectric functions e(hm) of the imperfect crystals based on In 0.5 Tl 0.5 I solid state solution with iodine vacancy and a thallium interstitial atom were calculated taking into consideration the inter-band and intra-band electron transitions. The studies of the imperfect crystals reveal increased low-frequency and stationary electron conductivity with anisotropy resulted from the anisotropy of the electron effective mass tensor. Our findings explain the origin of crucial changes in the band structure by formation of the donor half-occupied levels close to the unoccupied conduction bands due to the crystal structure defects, i.e., iodine vacancy or a thallium interstitial atom. It has been shown that in the case of real crystals, in particular metal-halides, the proper consideration of defects in quantum-chemical calculations results in a better matching with experimental data and, opposite to the perfect structure calculations, gives opportunities to explain the observed phenomena.
Complex dielectric functions ε(E) = ε1(E) + ε2(E) were experimentally evaluated within the spectral range E = 2–25 eV and E = 2–20 eV for SrTiO3 and NdGaO3 single crystals, respectively, using synchrotron-based spectroscopic ellipsometry measurements. The ellipsometric spectra were evaluated within a framework of optical layer model taking into account sample surface roughness and anisotropy of NdGaO3. The parameters of Herzinger-Johs oscillator model were fitted to reproduce sufficiently all features of the optical spectra within the spectral range 2–10 eV. Only slight differences were revealed for spectra polarized along b and c crystallographic axes of the NdGaO3, which can confirm weak optical anisotropy.
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