Thermal relaxation of silicon islands and craters on silicon surfaces

The thermal relaxation of isolated silicon islands and craters on the Si(111)-(7 x 7) and the Si(001)-(2 x 1) surfaces are observed by temperature variable scanning tunnelling microscopy. Thermal decomposition rates of silicon islands and filling-up rates of craters are measured at various substrate temperatures of 300-600 degrees C. Effects of the probe tip on the rates are measured and the reduced rates are determined without the tip effects. The sizes of islands and craters depend on time t with a Functional form of (t(0)-t)(2) and alpha congruent to 1 without the tip effects and alpha congruent to 2/3 with the tip effects. For the Si(111) surface activation energies for the island decomposition and crater filling-up are determined as 1.5+/-0.1 eV and 1.3+/-0.2eV, respectively. Pre-exponential factors are 2x10(11+/-1) s(-1) for islands and 3 x 10(9+/-2) s(-1) for craters. We have found that characteristic (5 x 5) islands with long lifetimes are formed during relaxation, but the (7 x 7) islands mostly have a short lifetime, The rotation of small islands is also observed during relaxation. For the Si(001) islands, an activation energy is determined to be 2.1+/-0.5 eV. The pre-exponential factor is 10(18+/-4) s(-1). The rates of the relaxation of Si(001) islands are ca, 100 times larger than those of the Si(111) islands. The characteristic islands on the Si(001) surface have rectangular shapes with aspect ratios near 3:1. We discussed the results in terms of two-dimensional vapor phase processes and the Schwoebel effect. (C) 1997 Elsevier Science B.V.