Bonding and Optical Anisotropy of Vacancy-Ordered Ga2Se3

The bonding, electronic structures and dielectric functions of vacancy-ordered orthorhombic and monoclinic Ga 2 Se 3 are calculated by using the ab initio pseudopotential method and the linear response theory. We have found that in these compounds the bonds around vacancies relax such that the bond length fulfills the order as s p 2 -like bond ≥ s p -like bond, and the dangling-bond states of anion atoms located nearest to such vacant sites appear at the valence-band top. These states produce the bonding, i.e. one-dimensional band, along the vacancy-ordering direction and become the origin of large optical anisotropy at the absorption edge. The present results well explain the recent experiments.