Diffraction Analysis of the Size Distribution of Noncrystalline Regions on Single-Crystal Surfaces.

Low-energy electron diffraction (LEED) is used to investigate the sizes and separations of three-dimensional overlayer islands. Angular intensity profiles of the substrate diffraction beams are monitored as a function of the overlayer dose. The profiles are fit with a model calculation for assumed island size and separation distributions to determine the average size and separation of the islands. The fitting procedure compensates for the instrumental contribution to the angular profile, and the influence of thermal diffuse scattering is considered. Cleaved GaAs(110) surfaces serve as the substrates for the experiments. The overlayer material is MBE deposited In, which grows in a Volmer-Weber mode as three-dimensional, prism-shaped islands oriented in the (110) direction. Depositions are made on room temperature substrates in monolayer increments to four monolayers. Profiles are measured in the (110) direction after each deposition. The average island size increases monotonically from 60 +/- 15 angstroms at a dose of one nominal monolayer to 140 +/- 15 angstroms at four monolayers. The average island separation remains essentially constant at 70 +/- 15 angstroms for these doses. We invoke coalescence of the islands to explain the behavior of the island separation. The effective coverage, or the amount of surface actually covered by the indium islands, increases to 70% +/- 2% at four monolayers. The technique can be extended to other overlayer systems in which the overlayer diffraction beams, if any are present, do not coincide with those of the substrate. These include liquid or amorphous two or three-dimensional overlayers, and angular structures such as faceted islands and etch pits.