TRANSITION IN THE GROWTH-KINETICS OF VACANCY ISLANDS ON NBSE2

The evolution of etch pits of NbSe2 in air has been followed by AFM an d STM. By minimizing the rip-surface interaction, the intrinsic evolut ion of monolayer deep depressions has been followed thoroughly. The ti me variation t(beta) of the lateral dimension of the triangular, monol ayer deep depressions undergoes a transition from an exponent of beta= 0.4+/-0.1 to beta=1+/-0.1 several tens of minutes after sample cleavag e. The depression formation process is adequately described by the Bur ton-Cabrera-Frank theory (BCF) for the diffusion of etching molecules, most probably water molecules that are responsible for the chemical a ttack at defects. The transition in the time dependence to smaller siz es (<28 nm) reveals a finite diffusion length of the diffusing entitie s.