MORPHOLOGICAL INSTABILITY OF A VAPOR-GROWN CRYSTAL - INSITU STUDY OF (0001) CADMIUM FACE

From in-situ observation of a vapour grown cadmium single crystal, the morphological evolution of the (0001) face is described in a large domain of supersaturation and crystal temperature. At low supersaturation, a great morphological instability results from the competition between composite pyramids which leads to a single dominant-growth pyramid as supersaturation is increased. The mechanism of growth is explained through the characteristic starlike shape morphology of the pyramid (twenty four vicinal facets) of elementary step height (5.6 angstrom) Beyond sigma almost-equal-to 30%, bunching occurs in some sectors, as a result of the fluctuating interstep distance and the pyramid slope increases very slowly with the supersaturation. The spiral-growth mechanism is predominant because of the too small interstep distance which inhibits the two dimensionnal nucleation. The morphological evolution is reproductible and reversible with the supersaturation. The mean free surface diffusion path and the edge free energy are estimated.