Structural Imperfections in Ulrathin Oxides Grown on Hydrogen Terminated Silicon Surfaces
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Figure 1 shows the oxidation induced changes in Si 2 p photoelectron spectra measured at photoelectron take off2 angle of 15 degrees[161. Here, Si', Si ÷,Sj3+ are defined and have almost the same binding energies with those determined in the literature[17]. Si4 denotes Si atom bonded with four oxygen atoms. Figure 2 shows the amounts of intermediate oxidation states as a function. of number of bridging oxygen atoms obtained from the spectral analysis of Fig. 1. The number of bridging oxygen atoms on the horizontal scale are calculated by considering the bonding configurations of intermediate oxidation states of Si atoms. According to this figure at the early stage of oxidation the amount of Si'1 first increases and then the amount of Si"- increases steeply. The latter implies the non-uniform oxide formation. The oxidation processes thus measured are simulated as follows: The three dimensional silicon lattice consisting of 40 atoms X 40 atoms in each layer on (111) plane is used for the simulation of oxidation. The bonding probability of silicon atom with oxygen atom is adjusted until the almost the same figure with that shown in Fig. 2 is obtained. Because Fig. 2 implies that the oxidation proceeds non-uniformly, Si atom bonded with oxygen atom is assumed to have higher bonding probability with oxygen atom as compared with other Si atoms. Figure 3 shows the simulated result. The distribution of bridging oxygens on each Si layer is shown in Fig. 4. From the top to the bottom of this figure the topmost layer, the second layer from the top, the third layer and so on are shown in this order. From the left to the right the amount of bridging oxygen atoms increase. This figure indicates that once the isolated bridging oxygen atoms are produced the oxidation proceeds in lateral direction around these oxygen atoms. With oxidizing further the oxidation
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Fig. 1 Oxidation induced changes in Si 2p photoelectron spectrum for rt-type Si(1 11) surface
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Fig. 2 Amounts of intermediate oxidation states as a function of number of bridging oxygen atoms.
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Fig. 3 Number of intermediate oxidation states as a function of number of bridging oxygen atoms, which is obtained from the optimized simulation of experimental data. 62
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Fig. 4 Distribution of oxygen atoms at and near the hydrogen-terminated silicon surface corresponding to the simulated results in Fig. 3.
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