Observation of Chemisorbed O₂ Molecule at SiO₂/Si(001) Interface During Si Dry Oxidation

抄録

We recently proposed a unified Si oxidation reaction model mediated by point defect generation [1-3]. Vacancies produced at the SiO₂/Si interface (V⁰) become chemically active sites (V⁺ and V⁻) after carrier trapping, and O₂ dissociation at V^+/− subsequently occurs, which induces the strain and the following V⁰ generation at the SiO₂/Si interface, thus forming a reaction loop. This reaction model demonstrated two reaction loops: a rapid loop that proceeds with a single step (loop A) and a slow loop with a double step (loop B). However, it is still unclear what causes the branching of the two loops. In this study, we investigated the mechanism behind the branching of these loops, focusing on chemisorbed O₂ at the SiO₂/Si interface.

XPS observations were performed with the surface reaction analysis apparatus (SUREAC2000) at BL23SU, SPring-8. Sb-doped n-type Si(001) with 2 Ωcm resistivity wafer was prepared. O 1s and Si 2p spectra were alternately obtained by a real-time XPS measurement for the Si sample during the sample was irradiated with a 0.06 eV O₂ supersonic molecular beam at room temperature (RT).

Fig. 1 (a) shows the time evolution of the intensity of O 1s spectra (I_O-1s). This uptake curve can be fitted with a Langmuir-type adsorption model, I_O-1s = I_sat[1-exp(-kt)], where I_sat is a saturation level. From the fitting, the surface (before point G) and interface (after point G) oxidation region, as shown in Fig.1(a), were determined. For each point of A−M in Fig. 1(a), we performed curve fitting as shown in Fig. 1(b, c). The O 1s spectra can be separated into five components of a~e. The components a, b, and c correspond to ins (Si-O-Si), tri (Interstitial O), and ad (Si-O), respectively. d and e are assigned to paul (chemisorbed O₂) state. Because the paul with ins O on the backbond has a comparatively long lifetime, the d and e components can be observed in the XPS spectra. The paul is still observed even at point M in the interface oxidation region. Since the surface is entirely covered with SiO₂ at the interface oxidation region, the paul can be assigned to the chemisorbed O₂ at the P_b1 center (P_b1-paul); a dangling bond with backbond O at the SiO₂/Si interface. Based on the P_b1-paul and our unified oxidation model, we constructed a branching model of the loops A and B (Fig. 1(d)). In our model, the oxidation process requires minority carrier (hole) trapping. Loop A proceeds via hole trapping, while spontaneous dissociation of paul without hole causes loop B. Here, the branching ratio of loop A can be nearly 0 at RT because of small k_mCT (reaction coefficient of minority carrier trapping). This can be confirmed by fitting the time evolution of SiO₂ thickness (X_O). Therefore, the oxidation at RT dominantly proceeds via loop B. Because loop B involves P_b1-paul in the process and the dissociation of P_b1-paul can be a rate-limiting step due to small k_mCT at RT, the growth rate of SiO₂ thickness (dX_O/dt) can be expressed as the equation in Fig.1 (e). a and N_Pb1-paul are a constant and the amount of P_b1-paul, respectively.

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