On defects in tetragonal ZnGeP₂ crystals

High quality ZnGeP₂ (ZGP) crystals were grown by means of the horizontal gradient freezing technique and characterized by X-ray diffraction (XRD) and optical measurements. This compound solidifies congruently and on further cooling undergoes a β→α phase transition. At room temperature, the tetragonal α-ZGP crystals contained a small amount of coherently oriented cubic β-ZGP clusters, indicating an incomplete phase transition. The β-ZGP phase was also partially retained by the rapid quenching from the melt. Optical absorption measurements have shown that thermal annealing in vacuum of the melt grown ZGP reduces the absorption coefficient in the near IR region. A parallel reduction in the relative amount of the β-ZGP phase was also observed. These result suggests that the point defects considered to be responsible for the large absorption coefficient in the near-IR, are related to the presence of the coherent β-ZGP regions within the α-ZGP single crystals.