Diffusion effects on thermal desorption spectra of hydrogen from (111) surfaces of diamond-type crystals

Abstract

We study the thermal desorption of atoms located in subsurface interstitial sites at the (111) surface of crystals with a diamond-type structure. From these sites they may either desorb or diffuse deeper into the crystal. A set of master equations is set up to describe the resulting random walk problem, and its solution yields both the desorption rate and total desorbed amount, as a function of time for constant temperature thermal desorption, or of temperature for flash desorption with a constant heating rate. Unequal values of the desorption and diffusion probabilities are allowed and the effect of initial occupation of deeper interstitial sites as well, is considered. In the latter case the desorption rate curve shows a double peak when the probabilities favour desorption, and in all cases a distinct long time or high temperature tail of the peak is found to result from diffusion. Based on this study, a model of H adsorption on Si(111) is presented and shown to explain several features of the experimental results of Schulze and Henzler.