Abstract

Undoped single crystals ${\rm{ZnGe}}{{\rm{P}}_2}$ have been studied by optical absorption spectroscopy in the range of 0.65–3.0 µm, where the unwanted absorption by intrinsic point defects plays a decisive role in highly efficient nonlinear optical devices. An increased optical dichroism up to 70% is achieved due to rearrangement of deep levels after low-temperature annealing (LTA). Zinc vacancies among other defects (phosphorus vacancies, cationic antisites) make the main contribution to the local structure transformation. A surprising fact is that the amplitude of optical dichroism at the deep acceptor ${{\rm{V}}_{{\rm Zn}}}$ exceeds the maximum photopleochroism ${\sim}{{55}}\%$ for direct optical transitions in ${\rm{ZnGe}}{{\rm{P}}_2}$. Structural improvement based on the distorted tetrahedron model is discussed.

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