Evolution of macrosteps on6H−SiC(0001): Impurity-induced morphological instability of step trains

We report on the observation of the impurity-induced morphological instability of step trains during sublimation growth on $6H\ensuremath{-}\mathrm{SiC}(0001).$ $6H\ensuremath{-}\mathrm{SiC}(0001)$ provides a step system suitable for studying the effects of repulsive step interaction. Its large step height and stiffness result in a strong repulsion force between steps, which energetically establishes extremely regular equidistant step trains on the $6H\ensuremath{-}\mathrm{SiC}(0001)$ surface. Upon nitrogen doping, these regular step trains on $6H\ensuremath{-}\mathrm{SiC}(0001)$ become unstable: the equidistant step trains are transformed into meandering macrosteps by nitrogen adsorption on the growing crystal surface. We discuss the effect of nitrogen adsorption through the consideration of asymmetric step kinetics and shed more light on the mechanism of step bunching on the $6H\ensuremath{-}\mathrm{SiC}(0001)$ surface. The competition between the repulsive step-step interaction and the asymmetric step kinetics plays a vital role in the observed morphological transition.