Photoinduced features of energy bandgap in quaternary Cu₂CdGeS₄crystals

Abstract

The 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 a cw 532 nm green laser with energy density about 0.4 J cm(-2). 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 unpolarized photoinducing laser light to separate the contribution of the intrinsic defect states from that of 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 by the general gradient approximation (GGA) and local density approximation (LDA) methods. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior (shift of the absorption maxima in different polarizations) within the 0.15-0.20 eV energy range not only near the absorption edge; optical anisotropy was also found for the deeper 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 the optical absorption spectra considerably. 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 also reported.

Electronic structure and optical properties of Cu2CdGeS4: DFT calculations and X-ray spectroscopy measurements

“…The above DFT-calculations have established that Cu 2 CdGeS 4 is a direct gap semiconductor because its valence band maximum (VBM) and conduction band minimum (CBM) occurs at the C point (the Brillouin zone (BZ) centre). Additionally, the VBM of Cu 2 CdGeS 4 is formed by the relatively localized Cu 3d states, while the unoccupied Ge 4p, 4s states are the dominant contributors to the CBM, therefore the d-p charge transfer plays the most important role in this compound [18]. The Ge-S and Cd-S bonds are mainly of covalent type, while the Cu-S bonds are more ionic in Cu 2 CdGeS 4 [18].…”

“…1 compound is considered to be a very prospective material for application in nonlinear optics [12,16] and it is of increased attention due to its piezo-and pyroelectric properties [17]. In particular, the Cu 2 CdGeS 4 compound is expected to be a very suitable semiconductor for second harmonic generation in the infrared spectral range as well as a prospective photogalvanic material [18]. Further, Cu 2 CdGeS 4 single crystals are found to be p-type semiconductors and the studies of the characteristic absorption spectra have revealed [15] that the energy gap of the compound is equal to 2.05 eV at 290 K. This value is close to those (E g = 1.9-2.0 eV) determined by Filonenko et al [17] using measurements of spectra of the diffuse reflection of light in Cu 2 CdGeS 4 single crystals.…”

“…Therefore, electronic properties of the Cu 2 CdGeS 4 compound were studied recently [18] based on first-principles density functional theory (DFT) calculations as incorporated in the CASTEP (CAmbridge Serial Total Energy) module of the Materials Studio package. The above DFT-calculations have established that Cu 2 CdGeS 4 is a direct gap semiconductor because its valence band maximum (VBM) and conduction band minimum (CBM) occurs at the C point (the Brillouin zone (BZ) centre).…”

“…Additionally, the VBM of Cu 2 CdGeS 4 is formed by the relatively localized Cu 3d states, while the unoccupied Ge 4p, 4s states are the dominant contributors to the CBM, therefore the d-p charge transfer plays the most important role in this compound [18]. The Ge-S and Cd-S bonds are mainly of covalent type, while the Cu-S bonds are more ionic in Cu 2 CdGeS 4 [18]. Furthermore, the DFT-calculations of dependencies of the lattice parameters upon pressure indicate [18] that all three lattice parameter a, b and c decrease linearly with pressure and the largest compressibility of the Cu 2 CdGeS 4 compound is realized along the b crystallographic axis.…”

Photoinduced Operation by Absorption of the Chalcogenide Nanocrystallite Containing Solar Cells

“…3) using a method described in the ref. 18. Such covalence causes more quick reaction on the external polarized field.…”