Photoinduced features of energy bandgap in quaternary Cu sub(2)CdGeS sub(4) crystals

The quaternary chalcogenide crystal Cu sub(2) CdGeS sub(4) 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 sub(-2). 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 Cu sub(2) CdGeS sub(4) were performed by the general gradient approximation (GGA) and local density approximation (LDA) methods. Possible intrinsic defects are shown to affect the optical absorption spectra considerably. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu sub(2) CdGeS sub(4) are also reported.