Optical dichroism in ZnGeP2 crystals at deep levels

Undoped single crystals Z n G e 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 V Z n exceeds the maximum photopleochroism ∼ 55 % for direct optical transitions in Z n G e P 2 . Structural improvement based on the distorted tetrahedron model is discussed.