The Influence of Hydrogen on CVD-Growth on Si(111) Surfaces

Abstract

Time-resolved scanning tunneling microscopy has been used to study the influence of atomic hydrogen on the growth behavior in silicon homoepitaxy and the etch attack of atomic hydrogen onto the silicon surface above 500 °C. In CVD of silicon on Si(111) using disilane (Si2H6) as gas phase precursor the presence of hydrogen on the surface plays an essential role in all stages of growth. At temperatures above 400 °C, for each Si atom deposited on the surface one H atom is adsorbed additionally. Under the influence of hydrogen the density of islands increases during nucleation on the bare substrate. The nucleation behavior in CVD, when quantitatively analyzed, differs distinctly from the one during MBE-growth. As opposed to MBE, the density of nuclei as a function of temperature and growth rate cannot be understood in terms of classical rate equation based nucleation theories. Under the influence of a closed hydrogen layer the topmost silicon layer is largely immobilized and lateral island coarsening can only take place after hydrogen desorption. At a high flux of disilane the growth rate is limited by this hydrogen desorption. The surface, which then has a hydrogen-coverage close to saturation, roughens linearly with coverage and completely decays into facets of the 〈110〉-zone. When the flux of the precursor gas is interrupted the surface flattens with a time constant determined by the hydrogen desorption. Atomic hydrogen is able to remove silicon from the surface visible in STM as step retraction. Above 500 °C we found a linear increase of the removal rate with temperature.