Structural transitions on Si(111) surface during Sn adsorption, electromigration, and desorption studied by in situ UHV REM

Using in situ ultrahigh vacuum reflection electron microscopy, we have studied structural transitions on the Si (1 1 1) surface induced by Sn deposition up to 2 ML coverage (1 ML = 7.8 center dot 10(14) cm(-2)) and by Sn desorption at substrate temperatures T = 200-860 degrees C. We have shown that Si-Sn intermixing in the adsorption layer during Sn deposition onto the Si(1 1 1)-(7 x 7) surface at T > 650 degrees C leads to the shift of the monatomic steps in the step-up direction, and there is formed a mosaic (root 3x root 3)-Sn phase. The electromigration of Sn adatoms induced by DC resistive heating has been shown to redistribute adsorbed Sn layer and to cause local (root 3x root 3)-Sn double left right arrow "1 x 1"-Sn structural transitions on the surface. We have estimated the lower bound of the Sn adatom positive effective charge on the Si(1 1 1) surface q(eff)(Sn) >= 0.001 center dot e. During AC annealing, the rate of desorption-induced "1 x 1"-Sn domain area shrinkage has been measured as a function of substrate temperature, and the activation energy of Sn adatom desorption has been estimated to be 2.5 +/- 0.1 eV.