UV/ozone induced oxidation of silicon(1-x) germanium(x) alloys and thermal desorption of the oxide film

This research project examines the fundamental issues associated with UV/ozone induced oxidation of Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} alloys and thermal desorption of the oxide film. A low temperature technique has been demonstrated for cleaning Ge wafer surfaces for the growth of high quality epitaxial films. Using x-ray photoelectron spectroscopy (XPS) and other techniques, it is shown that 30 min exposures of a degreased and deionized-water-rinsed Ge wafer to ultraviolet (UV) emission from a Hg lamp in laboratory air is sufficient to remove C contamination and form a non-permeable passive amorphous GeO{dollar}sb2{dollar} layer with a thickness of {dollar}sim{dollar}3 nm. Subsequent annealing in ultrahigh vacuum (UHV) at {dollar}>{dollar}400{dollar}spcirc{dollar}C resulted in desorption of the oxide layer leaving behind a clean well-ordered Ge surface with no C or O impurities. Si{dollar}sb{lcub}rm 1-x{rcub}{dollar}Ge{dollar}sb{lcub}rm x{rcub}{dollar} alloy films with different Ge fractions, grown epitaxially by molecular beam epitaxy (MBE) on Si (111) substrates, were oxidized by exposure to UV-ozone for different times. This resulted in the growth of a uniform oxide film containing both Si and Ge in the +4 state. The oxide thickness initially increased rapidly but reached a saturation value. This varied from 1.3 to 3 nm for Ge fractions of 0 to 100% in the semiconductor alloy. This behavior is consistent with the existence of a space charge region resulting from photo-excitation of electrons from the SiGe valence band into the oxide. The photoelectrons assist in ionizing surface oxygen, a critical step in the oxidation reaction.; The desorption kinetics of the GeO{dollar}sb2{dollar} film was studied using temperature programmed desorption (TPD) and isothermal annealing in the XPS instrument. The desorption took place over a narrow temperature span {dollar}<{dollar}10{dollar}spcirc{dollar}C at {dollar}sim{dollar}400{dollar}spcirc{dollar}C. The XPS spectra of the Ge 3d peak during desorption showed no increase in the GeO signal even though GeO is the desorbing species. Isothermal annealing experiments confirmed that the oxide film desorbs abruptly at around 400{dollar}spcirc{dollar}C. The oxide desorption does not take place by a layer-by-layer process. It is proposed that desorption is accomplished by the formation of holes in the film, and behaves as a nucleation-limited process.