Spin orbit and tetragonal crystalline field interaction in the valence band of CuInSe₂‐related ordered vacancy compound CuIn₇Se₁₂

Abstract

Thin films of the off-tie-line ordered vacancy compound CuIn 7 Se 12 were deposited on optically flat glass substrates by multi-source co-evaporation method. The preliminary structural, compositional and morphological characterizations were done using X-ray diffraction, energy dispersive X-ray analysis and atomic force microscopy. The X-ray diffraction data were further analysed applying the Nelson -Riley method and CTB plus η = η experiment rule, respectively, for lattice constants (a = 5.746 Å and c = 11.78 Å) and bond length estimations (R Cu-Se = 2.465 Å and R In-Se = 2.554 Å). A detailed analysis of the optical absorption spectra of the compound, which exhibited a three-fold optical absorption structure in the fundamental gap region, yielded three characteristic direct energy gaps at 1.37, 1.48(7) and 1.72(8) eV indicative of valence band splitting, which were evaluated using Hopfield's quasi-cubic model. The 0.04 eV increase in spin -orbit splitting parameter of the compound (0.27 eV) compared to that of CuInSe 2 (0.23 eV) is found to be suggestive of the smaller contribution of Cu d orbitals to hybridization (determined by the linear hybridization model) in this Cu-deficient compound. Spectral response spectra exhibit, in addition to a maximum around 1.34 ± 0.03 eV, two other defect transition peaks near 1.07 and 0.85 eV. The binding energies of Cu, In and Se in the compound were determined using X-ray photoelectron spectroscopy.

Structural, optical, electrical and low temperature thermoelectric properties of degenerate polycrystalline Cu7Se4 thin films

“…At the optical absorption edge, the variation of α with photon energy hν is given by the Tauc relation for direct transitions, where n can take values 1/2 and 3/2 for direct allowed and direct forbidden transitions, respectively and also for indirect transitions, where n can take values 2 and 3 for indirect allowed and indirect forbidden transitions, respectively. Here n is an integer that determines the type of electronic transition causing the absorption, E g is the band gap energy, E p is the phonon energy, ν is the frequency of the incident beam and A and B are constants . The plot of ( αhν ) 2 versus hν is shown in Fig.…”