Absorption and Photoluminescence Spectra of Heavily Zn-Doped CuGaS2 Crystals

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Abstract
In heavily Zn-doped (up to 5%) CuGaS2 crystals, reductions of excitonic absorption and above-band-gap absorption were observed for optical transitions of the polarization parallel to the optic axis, probably related to a change in the exciton oscillator strength ratio of the transitions of polarizations parallel and perpendicular to the optic axis. An impurity absorption whose concentration dependence is consistent with electron transitions from ZnGa acceptors to the conduction band is reported for the first time. An emission appearing at 2.1∼2.3 eV is considered to be due to the electron transitions from a donor level of sulfur vacancies or their complexes with ZnGa to the ZnGa acceptor level, involving the effect of broadening of the latter level. The existence of isoelectronic-trap ZnCu-ZnGa pairs is suspected from time-resolved emission spectra.