Impact of the growth on the stability–instability transition at Si (111) during step bunching induced by electromigration

Abstract

The central result of this work is the definite proof that the mechanisms of the direct current induced step bunching in the middle and high temperature domains are different. We used the recently developed technique for reflection electron microscopy (REM) observation of Si surfaces during equilibrium and during crystal growth to document the impact of the growth on the process of step bunching induced by direct current heating of an Si crystal. We found completely different effects of crystal growth on the stability of the vicinal surfaces in the two temperature domains 1160–1240°C and 1260–1320°C. In the high temperature domain step bunching takes place at step-down direction of the electric current during sublimation, equilibrium and growth; whereas in the 1160–1240°C domain bunching takes place at step-up current during sublimation and at step-down current during growth. These findings support the concept of local mass transport in the high temperature domain — the surface migration of adatoms is effectively interrupted at each step by a high rate exchange between the adlayer and the crystal phase. At 1160–1240°C the mass transport is global — adatoms easily cross the steps without taking part in the crystal–adlayer exchange. Since earlier studies of other researchers support the concept of local mass transport in the low temperature domain, 900–1050°C, a difficult question arises — why do the properties of the steps, with respect to the mass transport over the crystal surface, have a temperature dependence which is not monotonous? To explain the transition from local mass transport in the low temperature domain to global mass transport in the middle temperature domain we advance a hypothesis for a transition from a low temperature state of adsorption (Takayanagi-like adatoms, existing above the (7×7)↔(1×1) transition) to a high temperature state of adsorption (adatom with three dangling bonds) with much lower activation energy for desorption.