Annealing and optical homogeneity of large ZnGeP2 single crystal

A high-quality ZnGeP2 (ZGP) single crystal with large size of Φ30 mm × 80 mm was grown by a modified vertical Bridgman method. ZGP wafers were annealed with ZGP polycrystalline powder for 300 h at 550, 600 and 650 °C, respectively. The as-grown and annealed crystals were characterized by X-ray diffraction (XRD) analysis, Fourier transform infrared spectroscopy (FTIR), IR microscope and energy-dispersive spectroscopy (EDS). Results show that the quality of all wafers is improved evidently after annealing and the optimum annealing temperature obtained is 600 °C. The IR transmittance of the wafer measured by FTIR is up to 56.78 % at wavelength of 2.0 μm nearby and exceeds 59.00 % in the wavelength range of 3.0–8.0 μm. The deviations from stoichiometry decrease, and the homogeneity of the crystal is also improved after annealing. In this paper, scanning infrared map was proposed as a new nondestructive method to evaluate optical quality and homogeneity of crystal through comparing the IR transmittance with the three-dimensional IR spectral contour map.