Thermal Desorption of Ultraviolet-Ozone Oxidized Ge(001) for Substrate Cleaning

X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and reflection high-energy electron diffraction (RHEED) have been used to show that 30 min exposures of a degreased and deionized-water-rinsed Ge(001) wafer to ultraviolet (UV)-ozone in laboratory air is sufficient to remove C contamination and form a nonpermeable passive amorphous Ge0₂ layer with a thickness of ~1.8 nm. Subsequent annealing in ultrahigh vacuum (UHV) at >390 °C for >30 min resulted in desorption of the oxide layer and the exposure of a clean well-ordered Ge(001)2Xl surface. No impurities, including C and O, were detected by either XPS or AES. EELS spectra from the clean surface showed well-defined peaks corresponding to transitions involving dangling bonds, surface states, and surface plasmons. Shorter UV-ozone exposures (i.e., < 30 min) often resulted in residual C contamination while incomplete oxide removal was obtained at lower oxide desorption temperatures. Ge overlayers deposited by molecular beam epitaxy at temperatures between 200 and 450 °C on UV-ozone processed substrates were found by a combination of plan-view and cross-sectional transmission electron microscopy to be highly perfect single crystals with abrupt film/substrate interfaces and no detectable dislocations or extended defects.