Transition in the growth kinetics of vacancy islands on NbSe₂

The evolution of etch pits of NbSe₂ in air has been followed by AFM and STM. By minimizing the tip-surface interaction, the intrinsic evolution of monolayer deep depressions has been followed thoroughly. The time variation t^β of the lateral dimension of the triangular, monolayer deep depressions undergoes a transition from an exponent of β = 0.4 ± 0.1 to β = 1 ± 0.1 several tens of minutes after sample cleavage. The depression formation process is adequately described by the Burton-Cabrera-Frank theory (BCF) for the diffusion of etching molecules, most probably water molecules that are responsible for the chemical attack at defects. The transition in the time dependence to smaller sizes (< 28 nm) reveals a finite diffusion length of the diffusing entities.