Photoluminescence and EPR of Phosphorus Vacancies in ZnGep2

Zinc germanium diphosphide (ZnGeP2) is a nonlinear optical material used in mid-infrared optical parametric oscillators. The near-infrared photoluminescence (PL) from single crystals of bulk ZnGeP2 has been studied as a function of excitation power, wavelength, temperature, and polarization. At 5 K, a broad PL band extending from 0.7 µm to beyond 1 µm is typically observed. Two distinct emissions with different polarization, power, and temperature behaviors have been resolved. These bands have peaks in intensity near 1.6 eV and 1.4 eV. The relative intensities of these two bands were found to correlate with the presence of phosphorus vacancies, as determined by electron paramagnetic resonance (EPR). A resonance in the intensity of the 1.6-eV band occurs when pumping into a level ∼90 meV below the minimum conduction band. This level is tentatively assigned to the shallow state.