Germanium doping of wider-band-gap CuGaSe2 chalcopyrites: Local and electronic structure

We present here a complementary study on germanium doping of the wider-band-gap CuGaSe2 (CGS) chalcopyrite. In photoluminescence studies, the occurrence of a new emission line was identified as Ge related and explained as a donor-acceptor-pair recombination. The precise role the Ge is playing in this doping of CGS is revealed by x-ray absorption spectroscopy and ab initio calculations based on the density-functional theory. Extended x-ray absorption fine-structure spectroscopy (EXAFS) as well as x-ray absorption near-edge spectroscopy performed at the Ge K-, Cu K-, and Ga K-edge show that the Ge dopants occupy the cationic sites of Ge-Cu or Ge-Ga of the host lattice. The complementary ab initio calculations support the EXAFS results. They further indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band-structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe2 originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted antibonding Ge-Se 4sp(3) defect band appearing well above the Fermi level for the Ge-Ga(1+) point-defect system can be directly linked to a Ge-dopant-related donor-acceptor-pair transition as observed in our photoluminescence spectra