Assignment of infrared absorption bands in ZnGeP2

Abstract

Zinc germanium diphosphide (ZnGeP2) is a nonlinear optical material useful for frequency conversion applications in the midinfrared. A broad absorption band peaking near 1.2 microns and extending past 2 microns is often observed. To identify the defects responsible for these absorption losses, we have performed an optical absorption investigation from 10 to 296 K on bulk crystals of ZnGeP2 grown by the horizontal gradient-freeze method. Three broad absorption bands in the spectral range from 1 to 4 microns are observed that are due to native defects. Comparison of photoinduced changes in absorption with photoinduced changes in EPR spectra allowed specific defects to be associated with each of the three absorption bands. A band peaking near 1.2 microns and another band peaking near 2.2 microns involve transitions associated with singly ionized zinc vacancies. A third absorption band, peaking near 2.3 microns and extending from 1.5 microns to beyond 4 microns, involves neutral phosphorus vacancies. Absorption bands due to anion-site donor impurities Se and S have also been studied.