## 3 Results and Discussion  
Figure 1 shows the FTIR spectra for the HfO\({}_{2}\) films with different post-annealing temperatures ranging from 400 to 650 \({}^{\circ}\)C. The absorbance peaks at 415, 512, 600, 623, and 750 cm\({}^{-1}\) are assigned to Hf-O bonding configuration [8; 9; 10; 11]. The peaks at 820, 1000, and 1108 cm\({}^{-1}\) correspond to the Si-O bonds in the films [8; 9; 10]. Considering the as-deposited HfO\({}_{2}\) sample, the Si-O peak at 1108 cm\({}^{-1}\) suggests that the as-deposited film already consists of an SiO\({}_{x}\)-like interfacial layer between the silicon wafer and HfO\({}_{2}\) layer. This is similar to the Al\({}_{2}\)O\({}_{3}\)/Si system, which attributes the presence of the interfacial SiO\({}_{x}\) layer to the first few ALD cycles [12]. The SiO\({}_{x}\) peak intensity enhances at the annealing temperature of 400 \({}^{\circ}\)C, but it then drops when the annealing temperature further increases. For the annealing temperatures of 550-650 \({}^{\circ}\)C, the peak is almost unobservable, which in turn infers that these samples have nearly no SiO\({}_{x}\) interfacial layer. On the other hand, the intensity of the Hf-O peak at 623 cm\({}^{-1}\) increases when the annealing temperature increases. The enhancement of the signal intensity may be a result of densification of the HfO\({}_{2}\) films during the annealing process. It seems that there is a competitive process between the formation of Si-O and Hf-O bonds. Oxygen atoms bond to silicon atoms at low temperatures, but they are consumed by bonding to Hf under high-temperature annealing conditions. Varying the annealing temperature can therefore control the interfacial layer. Figure 2a shows n-type PERC solar cell structure consisting of an Al\({}_{2}\)O\({}_{3}\) emitter passivation layer and an HfO\({}_{2}\) rear passivation layer. The TEM cross-sectional image of the HfO\({}_{2}\)/n-type Si interface annealed at 550 \({}^{\circ}\)C is shown in Figure 2b. The extent of the atom arrangement order can be used for evaluation of crystallization of films. It can be seen that the atoms in the HfO\({}_{2}\) region are orderly arranged, indicating the high crystallinity achieved at the annealing temperature of 550 \({}^{\circ}\)C. The interfacial SiO\({}_{x}\) layer with a thickness of approximately 2.5 nm can be seen, but both sides of this interfacial layer are partially crystallized. It has been reported that HfO\({}_{2}\) would be possibly crystallized at temperatures greater than 400 \({}^{\circ}\)C [13]. It should be noted that the interfacial layer in the as-deposited sample is approximately 3 nm of amorphous SiO\({}_{x}\).The crystallization near the HfO\({}_{2}\)/SiO\({}_{2}\) interface causes a thickness reduction in the actual amorphous SiO\({}_{x}\) layer. This phenomenon is more pronounced when the annealing temperature is elevated. In Figure 2b, the amorphous SiO\({}_{x}\) layer is significantly inhomogeneous, and the SiO\({}_{x}\) layer can hardly be distinguished in some regions. This is in agreement with the decreased signal intensity of SiO\({}_{x}\) found via FTIR, as it almost disappears at annealing temperatures greater than 550 \({}^{\circ}\)C. As the SiO\({}_{x}\) interfacial layer is usually considered to play  
Figure 1: Fourier transformation infrared spectra of HfO\({}_{2}\)/Si samples with different annealing temperatures. an important role in decreasing the interface defect density in the dielectric/Si system [12], annealing temperatures equal to or higher than 550 \({}^{\circ}\)C are not preferable. Figure 3 shows the \(Q_{f}\) and \(D_{it}\) values obtained from C-V measurements as a function of annealing temperature. The as-deposited film has a high \(Q_{f}\) value of 1.4 \(\times\) 10\({}^{13}\) cm\({}^{-3}\). This might be due to the formation of the Si-O interfacial layer, which causes the Hf atoms near the interface region to bond with oxygen atoms in oxygen-poor conditions. The oxygen vacancies are known to be the main cause of positive charges in HfO\({}_{2}\) films [14; 15].