HF-treated Si surfaces and the oxidation kinetics in pure water or in clean room air have systematically been studied by x-ray photoelectron spectroscopy (XPS). The oxidation of heavily-doped n-type Si appears to proceed parallel to the surface, resulting in the layer-by-layer oxidation. The oxide growth rate in pure water for heavily-doped n-type Si is significantly higher than that of heavily-doped ptype Si. This is explained by the electron tunneling from the Si conduction band to adsorbed O2 molecules to form the O2 state. O2 ions easily decompose and induce a surface electric field, enhancing the oxidation rate. The growth rate of native oxide on heavily-doped n-type Si is less sensitive to the crystallographic orientations than the case of lightly doped Si where the steric hindrance against oxygen molecules significantly lowers the oxidation rate of the (110) and (111) surfaces. We suggest that the decomposed oxygen can penetrate into Si without steric hindrance. It is also found that the oxidation of heavily-doped n-type Si in pure water is effectively suppressed by adding a small amount (10 ∼ 3600 ppm) of HCI.
The principal role of silicon-fluorlne bonds in the chemical nature of HF etched Si surfaces has been investigated by angle resolved x-ray photoelectron spectroscopy. The native oxide growth kinetics and the fluorine coverage have been systematically measured as functions of HF concentration, pure water rinse time, air or N2+02 gas exposure time and gas phase H20 coneentration. It is shown that the native oxide growth is strongly suppressed. by the existence of Si-F bonds of about 0.12 monolayers on the surface. This is explained by a nodel in which Si-F bonds chemically stabilize the surface reactive sites sueh as atomic steps as supported by the result of the layer by layer oxidation of Si.
Current transport through ultrathin gate oxides grown on chemically cleaned Si(100) surfaces has been systematically investigated. It is shown that current through oxides thinner than 4.2 nm is controlled by direct tunneling (DT), while Fowler-Nordheim tunneling (FNT) predominates in transport through SiO2 thicker than 5.1 nm. In the oxide thickness range between 4.2 and 5.1 nm, DT limits the current at low electric fields and FNT at high fields. The observed tunneling current is quantitatively explained by a theory based on the Wentzel-Kramers- Brillouin method (WKB approximation). Also, the influence of the Si surface microroughness on the tunneling current is discussed.
Hydrogen-terminated Si(111) and Si(100) surfaces obtained by aqueous HF or pH-modified (pH=5.3) buffered-HF (BHF) treatments have been characterized by a Fourier transform infrared (FT-IR) attenuated-total-reflection (ATR) technique. The BHF treatment provides better surface flatness than the HF treatment. Pure water rinse is effective for improving the Si(111) surface flatness, while this is not the case for Si(100) because the pure water acts as an alkaline etchant and promotes the formation of (111) microfacets or microdefects on the (100) surface.
Growth kinetics of native oxide on Si(111) surfaces treated in pH-modified buffered HF (BHF) solutions has been systematically studied by angle-resolved X-ray photoelectron spectroscopy. A BHF-etched (pH=5.3) Si(111) surface has no Si-F bonds and dose not oxidize for 300 min in clean room air. FT-IR-attenuated total reflection (ATR) measurements of Si-H bonds existing on the BHF-treated Si(111) surface have revealed that the surface is nearly step-free and atomically flat. This explains the chemical stability of the Si(111) surface.
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