Abnormal diffusion profile of adatoms on extremely wide terraces of the Si(111) surface

Abstract

The authors experimentally studied the distribution of the adatom concentration on extremely wide terraces of the Si(111) surface whose dimensions are comparable with the diffusion length of adatoms. The extremely wide terraces were created during in situ experiments carried out by ultrahigh vacuum reflection electron microscopy by high-temperature annealing of Si(111) samples (more than 1000 °C) followed by rapid cooling to 750 °C to form 7 × 7 superstructure domains. A detailed analysis of the surface morphology of the terraces was carried out by ex situ atomic force microscopy under ambient conditions. Based on high-resolution (1.2 nm/pixel) atomic force microscopy images, panoramic topographic images of the terraces were formed. Digital processing of the panoramic images visualized the distribution of the adatom concentration n. For a terrace cooled from 1070 °C, central terrace regions show minimum n values around 0.13 BL (1 bilayer (BL) = 1.56×1015 cm−2); close to the monatomic step bordering the terrace, n increases to about 0.14 BL. The authors determined that this radial distribution n(r) at 1070 °C corresponds to the adatom diffusion coefficient D = 59 ± 12 μm2/s. It was found that, for a terrace cooled from 1090 °C, the approach assuming the same adatom diffusion length over the entire terrace does not describe the experimental n(r) distribution. For its analysis, the authors used the solution of the stationary diffusion equation under the assumption that D is not constant. Based on a numerical solution, the dependence of D on the experimentally measured n values was obtained. Under the assumption that adatom lifetime does not depend on n at 1090 °C, the adatom diffusion coefficient was found to decrease from 140 μm2/s at n = 0.093 BL (in the central terrace regions) to 5 μm2/s at n = 0.118 BL (near the step). The results of this work experimentally demonstrated that the control over adatom concentration can be used to significantly vary the diffusion properties of the adsorption layer on the crystal surface.