Surface diffusion of in on Ge(111) studied by optical second harmonic microscopy

Surface diffusion of In on Ge(111) has been measured by optical second harmonic microscopy. This technique employs surface second harmonic generation to directly image submonolayer surface concentration profiles. The coverage dependence of the diffusivity D can then be obtained from a Boltzmann-Matano analysis. In the coverage range 0.1<θ<0.48, the activation energy E_diff decreased with increasing coverage, ranging from 31 kcal/mol at θ=0.1 to 23 kcal/mol at θ=0.48. Over the same coverage range, the pre-exponential factor D₀ decreased from 5×10² to 1×10^-1 cm²/s. This gradual change reflects a change in diffusion mechanism arising from the disordered nature of the Ge(111) surface. At low coverages, In adatoms sink into the top layer of Ge, and diffusion is dominated by thermal formation of adatom-vacancy pairs. At high coverages, diffusion occurs by normal site-to-site hopping. The gradual change in diffusion parameters with coverage was interrupted by an apparent phase transition at θ=0.16. At this point, both E_diff and D ₀ peaked sharply at 41 kcal/mol and 6×10₅ cm ²/s, respectively. The desorption energy E_des was measured by temperature programmed desorption. E_des decreased from 60 kcal/mol at submonolayer coverages to 55 kcal/mol at multilayer coverages.