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Photoinduced features of energy band gap in quaternary Cu2CdGeS4 crystals

Quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using the cw 532 nm green laser with energy density about 0.4 J/cm2. The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for polarized and un-polarized photoinducing laser light to separate contribution of the intrinsic defect states from the pure states of the valence and conduction bands. To understand the origin of the observed photoinduced absorption near the fundamental edge, the benchmark first-principles calculations of the structural, electronic, optical and elastic properties of Cu2CdGeS4 were performed in the general gradient approximation (GGA) and local density approximation (LDA) approaches. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior within the 0.15...0.20 eV energy range near the absorption edge; the optical anisotropy was also found in the deep inter-band transition spectral range. Peculiar features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron density distribution. Possible intrinsic defects are shown to affect considerably the optical absorption spectra. Pressure effects on the structural and electronic properties were modeled by optimizing the crystal structure and calculating all relevant properties at elevated hydrostatic pressure. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu2CdGeS4 are reported.