Evolution of macrosteps on 6H-SiC(0001): Impurity-induced morphological instability of step trains

We report on the observation of the impurity-induced morphological instabil ity of step trains during sublimation growth on 6H-SiC(0001). 6H-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 repulsi on force between steps, which energetically establishes extremely regular e quidistant step trains on the 6H-SiC(0001) surface. Upon nitrogen doping, t hese regular step trains on 6H-SiC(0001) become unstable: the equidistant s tep trains are transformed into meandering macrosteps by nitrogen adsorptio n on the growing crystal surface. We discuss the effect of nitrogen adsorpt ion through the consideration of asymmetric step kinetics and shed more lig ht on the mechanism of step bunching on the 6H-SiC(0001) surface. The compe tition between the repulsive step-step interaction and the asymmetric step kinetics plays a vital role in the observed morphological transition.