Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS

Abnormal large grains influencing highly the optical and electrical properties are commonly observed in the bottom part of CVD-ZnS layers. We have analyzed the films using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared transmittance tests. Compared to normal grains, more homogeneous and simpler structures have been observed in abnormal large grains in CVD-ZnS. In addition to the obvious difference in sizes, the crystalline outline and interior structure of normal and abnormal large grains are found to be markedly different. Distorted structures and stacking faults, such as wurtzite phase and polytypes of ZnS, are not detected in the large grain areas. Preferred orientation of abnormal large grains is found to be sphalerite (1 1 1). These results could be explained as due to the recrystallization mechanism of abnormal large grains. Samples with large grains show lower infrared absorption band (at 6.2 mu m wavelength) compared to samples with normal size grains. However, the infrared transmittance is found to be lower for the samples with large grains. Microstructure patterns show the presence of plenty of microdefects in boundaries between large grains, which indicates that the recrystallization in the volume with large grains is not as perfect as the recrystallization in hot isostatic pressure process (HIP).