Influence of cation vacancies and Bi impurity on the electronic structure and photoelectric properties of orthorhombic GeS

Quantum-chemical studies of GeS electronic structure containing intrinsic point defects (vacancies in the cation (V-Ge) and anion (V-S) sublattices), isolated substitutional impurities Bi-Ge and {V-Ge-Bi-Ge}-type complexes were performed using density functional theory in the LDA+U-approximation as well as their role in the formation of photoelectric characteristics of the crystals was discussed. It was established that the localization of Bi impurity predominantly in the germanium positions induces the appearance of donor-type levels in the bandgap compensating the acceptor levels formed by cation vacancies which lead to the increase of dark resistivity as well as the sharp increase of photosensitivity of GeS crystals. The features of chemical bonding in the defect-free and defective GeS crystals were analyzed on the basis of electronic density distribution maps. Electronic density maps clearly show the covalent-ionic bond nature within the corrugated double-layer packets with the predominant charge concentration on Ge-S (Bi-S) bonds as well as the weak van der Waals bond components between double-layer packets with the participation of germanium electronic lone pair.