## 3 Results  
### Characterization of ALD TiO\({}_{2}\) Thin Films  
The cross-section TEM images and selective-area diffraction (SAD) patterns of TiO\({}_{2}\) films are shown in Figure 1. From the TEM images, we can measure the thickness of 1000-cycle TiO\({}_{2}\) film as 66 nm, and estimate the thicknesses of 125-cycle, 250-cycle and 530-cycle TiO\({}_{2}\) films as 8 nm, 15 nm and 35 nm, respectively. Amorphous TiO\({}_{2}\) exists in 8-nm-, 15-nm- and 35-nm-thick TiO\({}_{2}\) films as shown in Figure 1a-c, respectively. Anatase and amorphous TiO\({}_{2}\) phases coexist in the 66-nm-thick TiO\({}_{2}\) film as shown in Figure 1d. Anatase TiO\({}_{2}\) can be found near the interface of the Si and TiO\({}_{2}\) film. These observations can be confirmed by the SAD pattern. In Figure 1e, the SAD pattern of 35-nm-thick film is constituted by the Debye rings of a-TiO\({}_{2}\) and the pattern of the Si crystal. From the SAD pattern of 66-nm-thick TiO\({}_{2}\) shown in Figure 1f, we can find the mixture of the diffraction patterns of anatase TiO\({}_{2}\) and the Si crystal. According to the TEM results, we observe the phase transformation of the TiO\({}_{2}\) thin film as the cycle number increases during the ALD process. Heterogeneous nucleation and growth of anatase TiO\({}_{2}\) took place from the surface of the Si [31]. The phase transformation of the TiO\({}_{2}\) films from amorphous to anatase might reduce the filed-effect passivation of TiO\({}_{2}\) film (discuss in Section 3.2) [26; 32]. Figure 2 shows the depth profile analysis of oxygen, silicon and hydrogen elements for 66-nm-thick TiO\({}_{2}\) films on silicon. At the interface of Si and TiO\({}_{2}\), we can find the large amount of hydrogen atoms which serve in the chemical surface passivation for Si. The concentration of hydrogen decreases from the interface to the surface of the TiO\({}_{2}\) film. To further check its crystallinity, the 66-nm-thick TiO\({}_{2}\) film was analyzed by Raman spectroscopy. Figure 3 shows the Raman spectra of the 66-nm-thick TiO\({}_{2}\) film and a Si wafer. Both spectra were normalized to their maximum values of intensity (at -520 cm\({}^{-1}\) which is the phonon peak of crystalline Si). The TiO\({}_{2}\) film shows three relatively stronger phonon peaks located at around 141 cm\({}^{-1}\), 395 cm\({}^{-1}\) and 636 cm\({}^{-1}\), and one relatively weaker phonon peak at around 195 cm\({}^{-1}\) (see the inset of Figure 2). According to the report [33], these peaks can be assigned to the active phonon modes of anatase TiO\({}_{2}\), namely 3E\({}_{\rm g}\) (144, 196, and 638 cm\({}^{-1}\)), 2B\({}_{\rm lg}\) (398 and 519 cm\({}^{-1}\)), and 1A\({}_{\rm lg}\) (513 cm\({}^{-1}\)). Two phonon modes at 513 and 519 cm\({}^{-1}\) are not seen in the measured Raman spectrum of TiO\({}_{2}\), probably because of the much stronger signal of the Si phonon located at -520 cm\({}^{-1}\). Based on the Raman results, we again confirm that the 66-nm-thick TiO\({}_{2}\) film contains the anatase phase. Figure 1: TEM (transmission electron microscopy) bright field images and selective-area diffraction patterns: (**a**) Bright field image of 125-cycle TiO\({}_{2}\) film (8 nm); (**b**) Bright field image of 250-cycle TiO\({}_{2}\) film (15 nm); (**c**) Bright field image of 530-cycle TiO\({}_{2}\) film (35 nm); (**d**) Bright field image of 1000-cycle TiO\({}_{2}\) (66 nm); (**e**) SAD pattern of 530-cycle TiO\({}_{2}\) and Si; and (**f**) SAD pattern of 1000-cycle TiO\({}_{2}\) and Si. Figure 2: SIMS (secondary ion mass spectrometry) depth profiles of oxygen, silicon and hydrogen. When the crystalline size is smaller than the critical size, the full width at half maximum (FWHM) values of the Raman peaks become broader due to the quantum confinement effect. Therefore, we can deduce the crystalline size from the Raman results. The FWHM of the 144 cm\({}^{-1}\) peak is about 10 cm\({}^{-1}\).