Electric-current-induced step bunching on Si(111)

We experimentally investigated step bunching induced by direct current on v icinal Si(111)"1x1" surfaces using scanning electron microscopy and atomic force microscopy. The scaling relation between the average step spacing l(b ) and the number of steps N in a bunch, l(b)similar to N-alpha, was determi ned for four step-bunching temperature regimes above the 7x7-"1x1" transiti on temperature. The step-bunching rate and scaling exponent differ between neighboring step-bunching regimes. The exponent alpha is 0.7 for the two re gimes where the step-down current induces step bunching (860-960 and 1210-1 300 degrees C), and 0.6 for the two regimes where the step-up current induc es step bunching (1060-1190 and >1320 degrees C). The number of single step s on terraces also differs in each of the four temperature regimes. For tem peratures higher than 1280 degrees C, the prefactor of the scaling relation increases, indicating an increase in step-step repulsion. The scaling expo nents obtained agree reasonably well with those predicted by theoretical mo dels. However, they give unrealistic values for the effective charges of ad atoms for step-up-current-induced step bunching when the ''transparent'' st ep model is used.