On defects in tetragonal ZnGeP2 crystals

High quality ZnGeP2 (ZGP) crystals were grown by means of the horizontal gr adient freezing technique and characterized by X-ray diffraction (XRD) and optical measurements. This compound solidifies congruently and on further c ooling undergoes a beta-->alpha phase transition. At room temperature, the tetragonal alpha-ZGP crystals contained a small amount of coherently orient ed cubic beta-ZGP clusters, indicating an incomplete phase transition. The beta-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 thtr absorption coefficient in the nea r IR region. A parallel reduction in the relative amount of the beta-ZGP ph ase was also observed. These result suggests that the point defects conside red to be responsible for the large absorption coefficient in the near-IR, are related to the presence off the coherent beta-ZGP regions within the al pha-ZGP single crystals. (C) 1999 Elsevier Science B.V. All rights reserved .