Roles of Surface Termination in Photoluminescence Mechanisms of Porous Si
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analyzer, and differentiated AES spectra were measured with a modulation voltage of 10 V . PL spectra were measured using 325 nm He-Cd laser light for oxygen dosing experiments and 333.6 - 363.8 nm Ar+ ion laser light for H/O exchange experiments. Oxygen exposure was carried out separately in the SR-PES or the AES chamber using the same ionization gun. In this oxygen dosing experiment, PS samples were exposed to oxygen excited by electron impact at room temperature. The flux density at the surface was determined by calculating the angular distribution of the effused gas using Clausing's relation [11]. To avoid unexpected oxidation, PS samples were kept in pure ethanol before the SR-PES, AES and PL measurements. They were placed in the chambers while wet and then the chambers were evacuated [1]. FTIR measurements were carried out in a nitrogen atmosphere. RESULTS AND DISCUSSION Effects of Oxygen Dosing on Electronic States and PL Properties of PS Measured Si 2p core spectra were decomposed into the Si 2p and Si 2p spin-orbit partner lines [12,13]. Figure 1 shows Si 2p3 2 core spectra (by dots) oitained from as-anodized porous Si and after oxygen exposure. To investigate the oxidation states, the Si 2P3/ 2 core spectra were further decomposed into several components. Since oxygen is not apparent on the surface of as-anodized porous Si which is mainly covered with mono- and di-hydride as reported in our previous papers [1-3], the Si 2 p3 /2 spectrum of as-anodized PS was decomposed into bulk Si denoted by Si0 , -- Si-H denoted by Si-H, and =Si-H components [14]. Upon oxygen exposure up to a dose of 3 x 1017 cm- 2, the signal for the dlydride component almost disappears. On the basis of this result, the Si 2p spectra of the oxygen-exposed PS samples were decomposed into bulk Si (Si0 ) and Si-H components, and five components in intermediate oxidation states, i.e., Si 0 (Sill), SiO (Si2 +), Si 20 3 (Sill), SiO * and SiO (Si4") [12,13]. The SiO2 * component has been reported by Niwano at al. for thin Si(001) Oxides [12]. In the case of PS, better curve fitting has also been obtained with the Sit 2* component than without it, particularly, in the initial stage of oxidation up to an oxygen dose of 7 x 1017 cm2. Each of the distribution functions of these components were a Lorentzian convoluted by a Gaussian. Precise decomposition procedures have been described in our previous paper [4]. Fitting curves using these models are also shown in Fig. 1 by solid lines. Good curve fitting has been achieved, as indicated in Fig. 1. Peak intensities of the decomposed components as a function of the oxygen dose are shown in Fig.2. As the oxygen dose increases, Si suboxides develop and Sit 2 peak intensity gradually increases. After an excited-oxygen dose of 71 x 1017 cm 2 , the most intense surface oxide is Sit 2 ; the surface oxides include oxides in the intermediate oxidation states. PL peak energies and PL peak intensities corresponding to the results of the SR-PES experiments are shown in Fig.3. As the oxygen dose increases, the P
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