Silicon surface phase transition studies using low-energy electron microscopy and diffraction.

摘要

Surface phenomena have long been studied by Low-energy Electron Diffraction (LEED), but the past decade has seen the rise of the Low-energy Electron Microscope as a powerful tool to investigate local properties of surfaces in real-time at high temperature, even during deposition. We have further extended the capabilities of the tool and used it to probe the effect of monolayer amounts of Ge on two Si surface phase transitions.; After a detailed description of the microscope, a novel dark-field technique is described for LEEM imaging of würtzite crystals (here, GaN) that leverages the structure's bulk properties for surface imaging. The interaction of the electrons with the different terraces of the material produces LEED intensity variations with respect to location and diffracted beam, giving rise to contrast in LEEM images. The method can achieve single bilayer vertical resolution on this class of materials.; Thermal roughening transitions have traditionally been treated as surface phenomena of a bulk material. We further present a chemisorbed “A-on-B” system (Ge on Si{lcub}001{rcub}) in which roughening only occurs in the film. We find that the roughening temperature of two monolayers of Ge on Si is 900 ± 25°C, lying between the roughening temperatures of Ge(001) and Si(001). Decreasing the film thickness raises the transition temperature. Although a roughening transition is usually explained accurately via a 2-D solid-on-solid model, we show that in thin-film roughening, additional surface energy terms related to surface reconstruction and film thickness must be considered to obtain acceptable results.; Finally, the effect of Ge on the celebrated Si(111) (7 x 7) disordering transition has been studied. We find that submonolayer quantities of Ge can increase the transition temperature from the clean Si(1 11) transition at 860°C to more than 950°C. The stabilization is explained through the use of a total-energy calculation which shows the lowering of the (7 x 7) surface energy with added Ge.