Detection of Interfaces States Correlated with Layer-By-Layer Oxidation on Si(100)
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in interface state densities near the midgap was observed periodically in accordance with the periodic changes in interface structures with the progress of oxidation [7]. Here, Si" and Si3 ' denote a Si atom bonded to one oxygen atom and three Si atoms and a Si atom bonded to three oxygen atoms and one Si atom, respectively. Recently, we reported the periodic changes in atomic-scale surface roughness with the progress of oxidation formed on Si(l00) [3]. Therefore, the key idea in the present paper is to detect the interface state density near the midgap which must change periodically in accordance with periodic changes in surface roughness. In the present paper the surface structures of ultrathin silicon oxides, whose thickness is close to the thickness of structural transition layer, formed on Si(100) surface and their correlation with interface structures and states will be described. EXPERIMENTAL DETAILS The extremely uniform oxide films were formed as follows. First, the 5 Pim thick n-Si layer epitaxially grown on Si(100) at 1100I C was terminated with hydrogen by the treatment in 0.5 % hydrofluoric acid solution. Second, on this surface 1.2-nm-thick preoxide was formed in H.SO 4-HO. solution (HSO4 :H20 2=4:l ) at 85-90'C for the study of interface state density, while an oxide layer with a thickness of about 0.6 nm was formed by heating this hydrogen-terminated Si surface in 4 Torr dry oxygen at 300'C for the study of surface roughness. Through this preoxide the oxidation was performed at 600-880°C in the oxygen gas, the dew point of which is 33 Mat. Res. Soc. Symp. Proc. Vol. 592 © 2000 Materials Research Society
below -95 'C, with the pressure of I Torr in order to keep purity of oxygen gas. The SiO/Si interface structures and interface state distribution in Si bandgap were measured by high resolution XPS. The interface state densities were measured nondestructively using the method developed by Lau and Wu [8]. First, an organic molecule of 2-propanol was adsorbed on the oxide film. Second, the oxide was charged by the irradiation of electron beam with the electron energy of less than 2 eV, which is necessary for the suppression of electron-bombardmentinduced damage in silicon oxide, [9] to produce a voltage drop across the oxide film. Third, the charge-induced changes in the potentials of surface and interface, which are necessary for the determination of interface state densities, were determined by measuring the charge-induced shifts in the C Is core levels of organic molecules and those in the Si 2p core level of the Si substrate. Because the thickness of depletion layer developed by the charge-up of oxide film is more than one order of magnitude larger than electron escape depth of 2.7 nm for Si 2p photoelectrons, the charge-induced shift in the Si 2p core level of the Si substrate is almost equal to that at SiO./Si interface. Because the shapes and values of full width half maximum of C Is and Si 2p photoelectron spectra hardly change by charge-up, the organic molecule and the oxide film are almost u
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