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

The central result of this work is the definite proof that the mechanisms o f the direct current induced step bunching in the middle and high temperatu re domains are different. We used the recently developed technique for refl ection electron microscopy (REM) observation of Si surfaces during equilibr ium and during crystal growth to document the impact of the growth on the p rocess of step bunching induced by direct current heating of an Si crystal. We found completely different effects of crystal growth on the stability o f the vicinal surfaces in the two temperature domains 1160-1240 degrees C a nd 1260-1320 degrees C. In the high temperature domain step bunching takes place at step-down direction of the electric current during sublimation, eq uilibrium and growth; whereas in the 1160-1240 degrees C domain bunching ta kes place at step-up current during sublimation and at step-down current du ring growth. These findings support the concept of local mass transport in the high temperature domain - the surface migration of adatoms is effective ly interrupted at each step by a high rate exchange between the adlayer and the crystal phase. At 1160-1240 degrees C the mass transport is global - a datoms easily cross the steps without taking part in the crystal-adlayer ex change. Since earlier studies of other researchers support the concept of l ocal mass transport in the low temperature domain, 900-1050 degrees C, a di fficult question arises - why do the properties of the steps, with respect to the mass transport over the crystal surface, have a temperature dependen ce which is not monotonous? To explain the transition from local mass trans port in the low temperature domain to global mass transport in the middle t emperature domain we advance a hypothesis for a transition from a low tempe rature state of adsorption (Takayanagi-like adatoms, existing above the (7 x 7)<->(1 x 1) transition) to a high temperature state of adsorption (adato m with three dangling bonds) with much lower activation energy for desorpti on. (C) 1999 Elsevier Science B.V. All rights reserved.