The change in the chemical surface state of polished Si wafers [p-type, (100) oriented] during storage in air at room temperature was investigated for storage times up to half a year. Measurements were performed by x-ray Photoelectron Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS). Immediately after the HF treatment (1 min 5% HF, 2 min water rinse) vibrational spectroscopy (HREELS) shows a predominant coverage of the surface with hydride groups (80%–90% of a ML), which can be inferred from the presence of the stretching (2100 cm−1), scissor (900 cm−1) and bending (640 cm−1) vibrations in the spectra. A slight additional coverage with oxygen is proved by XPS and originates from Si-OH groups (3670 cm−1) and oxygen-related hydrocarbon groups (XPS). These Si-OH groups result from an exchange reaction of Si-F with water during the two-minute water rinse. The development of an oxygen coverage during subsequent storage in air occurs extremely slowly and shows a logarithmic behavior. A monolayer coverage of oxygen (7×1014/cm2) is reached after approximately 7 days of storage in air. HREELS spectra exhibit the concurrent development of the asymmetric Si-O-Si vibration, which indicates that oxygen penetrates the lattice and breaks Si—Si bonds. During this period the Si-O-Si frequency shifts from about 1060 to 1100 cm−1. The penetration of backbonds of Si—H gets evident by broadening of the Si-H stretching vibration and finally by a shift to higher wavenumbers. Chemically shifted components of the Si 2p line (partially oxidized Si) are present with the SiO2−x component (chemical shift ≳3.4 eV) becoming dominant after roughly a week. Further oxidation proceeds essentially by an increase of the SiO2 peak in combination with a steeper slope of the logarithmic growth curve. The SiO2 thickness after half a year is about 8 Å. The frequency of the Si-O-Si vibration shifts up to 1120 cm−1, which can be related to a growing angle of the Si-O-Si bridge. Si—H groups are still present, the final peak position is about 2220 cm−1. The measurements show an extended induction period until the monolayer range of oxide coverage is attained. We ascribe this to the passivation of the surface by hydrogen and to a HF treatment according to Very-Large-Scale-Integration standards.
Si wafers with (100) or (111) oriented surfaces were treated in hydrofluoric acid (40% HF, 1 min) and then water rinsed for different times from 10 s to more than 50 h. Oxygen uptake and oxide formation were investigated by x-ray photoelectron spectroscopy and high-resolution electron energy-loss spectroscopy. The initial state after the HF dip is characterized by a coverage with Si–hydride and small amounts of oxygen and fluorine. The interaction with the liquid phase of water was investigated up to the monolayer range. It shows distinct features: The first step is a rapid exchange of Si–F with H2 O to form Si–OH groups followed by a slow nucleophilic attack of OH− on surface Si–H to produce Si–OH. Growth law is logarithmic and extends to 3–5 h of water contact. The surface Si–OH act as nuclei for the attack of water on the polarized Si–SiOH backbonds. Interior Si–H and Si–OH groups develop. Further attack of OH− on interior Si–H yields Si–OH. Condensation of Si–OH forms Si–O–Si bridges and SiO2−x nuclei appear. Strain and altered surface topography lead to a changed rate of the logarithmic oxide growth. The oxide formation is accompanied by a slight corrosive attack of H2 O, leading to roughening of the surface.
Origin of asymmetric growth during solid-state amorphization studied with molecular-dynamics simulation Niobium surfaces show features that have been blamed on the unavoidable oxidation that occurs during the preparation of the Nb. To quantitatively measure the oxidation, XPS and AES profile measurements have been carried out for the typical procedures used in the preparation of Nb surfaces, such as ultrahigh vacuum annealing (UHV), oxipolishing (OP), electropolishing (EP), or handling in air, H 2 0, and HP2' For the different surface preparations the oxidation proceeds as follows: At the Nb surface an inhomogeneous NbP-NbO layer 1 nm or less thick exists which does not seem to depend much on oxidation technique. Then a dielectric oxide Nb 2 0 s grows homogeneously in UHV (s 2 nm) or in Ope s 3 nm), whereas EP(:~3 nm) yields rough oxide layers that pick up impurities and charges easily. With time, all oxides grow more or less rapidly to a stable thickness of 6 nm. These results explain the observed behavior of Nb surfaces, especially their responses to the most sensitive measurements, namely, in superconducting rf cavities and tunnel junctions.
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