A PHYSICAL MODEL FOR EXPERIMENTAL DIFFRACTION MEASUREMENTS OF TiO2 THIN FILMS GROWN ON SrTiO3 SUBSTRATE VIA PULSED LASER DEPOSITION PI PENG SUPERVISORS: PROF. ANDRIVO RUSYDI Dr. LUIS RODRIGUEZ Dr. ONG BIN LEONG DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2018
bandwidth and peak intensity in comparison with experimental values. Line identification and Phase identification From the Bragg law presented in Eq. (2.1), the d-spacing for a peak is calculated using its respective angular position. As shown in Appendix 1, the four peaks located at angle positions 2θ=22.82°, 46.53°, 72.61° and 104.26° correspond to (001), (002), (003) and (004) STO peaks. The first thin film peak at 2θ=36.7ᵒ is undoubtedly assigned to anatase (004). The unknown peak displayed in Fig. 12 at 2θ=79.76ᵒ in samples PTS0318 and PSS0218, although it might be confused with rutile (212), is assigned to anatase (008). The reason is, as shown in Fig. 12, from the adjustment of model to anatase (004), a high order Bragg peak appears in the same 2θ position as the unknown peak. Hence, it is anatase (008). This example highlights the capabilities of the model: whereas with the traditional line identification the unknown peak could have been assigned to rutile (212), with the utilization of present model, once anatase (004) has been modelled, a higher Bragg order appears and matches the peak at 79.76ᵒ. Fig. 12 shows only anatase (004) and (008). According to the simulation, the reason why the other anatase peaks are missing is that peaks such as (002) and (003) are very weak, and hence covered up by noise; anatase (001) is of medium intensity, however it locates at around 2θ=4.64° which is out of the experimental range. With the absence of any peaks of other allotropes such as rutile, the TiO2 in the three samples shows only anatase phase. The presence of anatase (004) and (008) confirms the assumption made when the model was built: due to the particular STO and anatase lattice parameters, the TiO2 film grows with a preferential orientation that minimizes the lattice mismatch with the
Grain size Fig. 12 shows a noticeable difference among anatase peaks, , in terms of intensity and bandwidth. This is due to the different grain size within the thin film. The grain size gives a rough measurement on the structural morphology and degree of order inside a crystal. With smaller grain size, there are more crystallites that may take various orientation, and result in broadening the angle of diffraction and decreasing the intensity in an XRD pattern. Similar effect can be seen with very thin TiO2 films, where the amount of crystalline planes satisfying Bragg condition is limited. For a single crystal thin film, the measurements for grain size reveals the thin film thickness. In this subsection, the grain size for TiO2 in each of the sample is calculated using two different ways and compared: first one is by Scherrer equation, and the second one is by simulation with the model described in section 3. The Scherrer equation takes the form: (4.1) stands for the grain size, is the Scherrer constant that can treated as equal to 0.9, λ is incident wavelength that equals to 0.15418 nm and (in rad) is the FWHM of the peak. In order to obtain the grain size from the simulation, the procedure is to find the value of repetition number N which gives the best matching to both peak intensity and FWHM. The results for simulation using different repetition number are plotted in Figs. For PTS0217 sample, N=3 gives the best matching. For the other two samples, there is one additional difficulty as the FWHM value obtained from the model can be