# Defect Tailoring in HfO2/Si Films upon Post-Deposition Annealing and Ultraviolet Irradiation
## 4 Discussion  
Figure 11 summarizes the results from fluorescence and UV-visible spectroscopy with two electronic band diagrams. Figure 11a presents the electronic band diagrams from the fluorescence data for samples S1-S2 (left) and samples S3-S4 (right). The positions of the conduction band and valence band edges were calculated from [PERSON] et al. 's data [8]. The dashed, red arrows indicate the radiative transitions. The level below the conduction band corresponds to empty levels where electrons arrive at 270 nm excitation to then produce radiative decay. This level has been observed by several authors [18; 23; 24; 25; 26] and it has been attributed either to defect states related to the observed Urbach tail below the band gap in polycrystalline HfO\({}_{2}\)[20] or to resonant states originating from the presence of localized Hf 5 d polaron states at the Fermi level, which, in turn, push the unoccupied states at 2 eV above the Fermi level [22; 23]. These polaron states arise from the oxygen vacancies indicated by the fluorescence band at 2.8 eV [18; 21; 22; 23]. The levels within the mobility gap were assigned considering the observed fluorescence bands. As described in Section 3.4.1, in S1 and S2 samples, the observed fluorescence peaks are associated with H\({}^{+}\)/Cl\({}^{-}\) states [17], oxygen vacancies, and OH-states [16], respectively. The thermal treatment at 350 \({}^{\circ}\)C increases the number of oxygen vacancies, while upon UV treatment, the peak associated with oxygen vacancies at 2.8 eV is reduced, but novel transitions at 2 eV, 3 eV, and 3.5 eV are observed. Also, the H\({}^{+}\) states are not observed in samples S2 and S4, and there is an increase in the fluorescence intensity of OH states, in contradiction with the results from [PERSON] et al. [14], who reported that UV + rapid thermal annealing at 450 \({}^{\circ}\)C led to a reduction in OH states, which confirms the possibility of tailoring the defect states with a careful selection of UV and thermal treatment conditions. From this diagram, it is  
Figure 10: (**a**) Indirect transitions allowed from the (F(R)hv)\({}^{1/2}\) vs. hv plot, and (**b**) direct transitions allowed from the (F(R)hv)\({}^{2}\) vs. hv plot for samples S1–S4. possible to state that the normalized areas of the fluorescence peaks are, then, a measure of the density of the electronic states within the mobility gap, as shown on the right side of Figure 11a. Figure 11b shows the electronic band diagram for the S1 (left) and S2-S4 samples (right) from the point of view of direct electronic transitions within the mobility gap calculated from the F(R)hv\({}^{2}\) = 0 plot [21]. In this diagram, the depicted transitions were assigned considering the fluorescence bands and the discussed assignation to different electronic states. For the case of sample S1, the direct band gap was directly obtained from the F(R)hv\({}^{2}\) = 0 plot as well as the transition at 5 eV, which was already assigned to the resonant states below the band gap [22; 23]. The transitions at 3.6 eV and 2.3 eV arose from the occupied oxygen vacancy levels to the conduction band and from the valence band to unoccupied H\({}^{+}\) states, respectively. The oxygen vacancy levels and H+ levels were very close (ca. 0.3 eV), which may lead to thermalization, thus explaining the high DOS for the H\({}^{+}\) level. For samples S2-S4, the fundamental VB-CB transition was not observed due to an increase in Eg in these samples, but as the samples presented fluorescence upon excitation at 270 nm, the resonant states were kept in the diagram, and Eg was set at 6 eV. Thus, the transition from the oxygen vacancy level to the CB was still observed at 3.6 eV, and a new 4 eV transition arose from the novel states, which may be a result of the varying levels of Hf 5d and O 2p both up and down [23]. This new state is 2 eV above the valence band maximum, which is of interest for photocatalytic applications [8].