Heating by a direct electric current can produce step bunches on vicinal semiconductor surfaces. Under extreme conditions, steps crossing from one bunch to another bend sufficiently to create bands of steps of the opposite sign (antibands). Unusual large scale scanning tunneling microscopy images reveal a mechanism where field-induced concentration gradients produce a spatially variable step velocity that drives the antiband formation. A continuum step model allows quantitative analysis of crossing step shapes, yielding an effective charge for the diffusing adatoms of $q_{\mathrm{eff}}=0.13\mathrm{}$ electron units at 1270 °C.

• Received 16 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.5531