Studies of the growth kinetics of CaF2(1 1 1) by molecular beam methods and atomic force microscopy

The growth kinetics of the (1 1 1) surface of CaF2 single crystals is studied by molecular beam methods and atomic force microscopy for temperatures between 1015 and 1099 K and saturation ratios up to 354. After passing a transition stage with repeated two-dimensional nucleation, island growth and coalescence of the growing islands the surface develops growth hillocks with closed loop steps of a height of one F−–Ca2+–F− triple layer. The step distance in the hillocks decreases with increasing saturation ratio. The shape of the islands and terraces varies with increasing size from triangular over rounded hexagonal to circular, reflecting the changing importance of step and surface diffusion. Spiral growth is not observed. Condensation coefficients during the intermediate stage are of the order of 0.9 and increase to values close to 1 when the surface becomes covered by growth hills. The comparison of condensation coefficients and step distances yield a value of the mean diffusion length of 156 ± 40 nm at the experimental temperatures. For the step energy of 〈1 1 0〉 steps of type I values between 0.24 and 0.69 mJ/m are obtained. Experiments with molecular beam pulses show that the effective residence times on the terrace are smaller than 0.02 ms, the shortest time measurable with the experimental set-up.