MICROSCOPIC, ETCHING AND X-RAY STUDIES ON DOPED CdI₂ DENDRITIC SINGLE CRYSTALS. G,C. Trigunayat and Binay Kumar, Department of Physics & Astrophysics, University of Delhi, Delhi-110007

Dendritic single crystals of cadmium iodide are grown by vapour method, both without doping and with PbI₂ and CdBr₂ doping. They are examined for their polytypism by x-ray diffraction. The undoped crystals exclusively exhibit the basic polytype 4H, with small degree of arcing, but with no streaking existing on their oscillation photographs. In contrast, the oscillation photographs of doped crystals reveal frequent presence of higher polytypes, particularly the 12-layered rhombohedral one. The arcing and streaking of the reflections on the oscillation photographs undergo a marked change with the change of nature and amount of doping.

The basal surfaces of the as-grown doped and undoped crystals are examined by optical and scanning electron microscopy. A wide variety of growth features are observed, viz. growth steps inclined at 120deg., bunching of steps, overgrowths, along with typical dendritic features.

The basal surfaces of the crystals are also subjected to etching, which is carried out by controlled condensation of water vapour on the surface. Hexagonal etch pits are observed in the case of undoped crystals while for the doped crystals both hexagonal and triangular pits are observed. The relative number of the hexagonal and triangular pits are found to depend on the extent of doping. The results have been analysed to provide useful information regarding the mode of crystal growth and defect content in the crystal and regarding the formation of polytypes.