ZnGeP2 is a candidate material for tunable mid-infrared optical parametric oscillators pumped by a 2-micron laser. Performance, thus far, has been limited by appreciable optical absorption extending from the band edge near 0.7 micrometers to beyond 2 micrometers . In the present investigation, electron paramagenetic resonance (EPR) and electron- nuclear double resonance (ENDOR) is used to establish the specific identities of the defects responsible for the optical absorption in crystals grown by the horizontal gradient freeze technique. The dominant acceptor, observed by EPR in as-grown crystals, is a singly ionized zinc vacancy (VZn-). Photo-induced EPR provides evidence that the dominant compensating donor is a phosphorus vacancy (VP+). Photoluminescence data and optical absorption spectra are interpreted in terms of transitions between these donor-acceptor pairs.
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