In this study, the chemical composition of the film Hf_0.24Si_0.20O_0.52Pr_0.05 was determined through the simulation of the corresponding RBS spectrum using the SIMNRA program (Figure 1). The RBS analysis shows that the as-deposited film cannot be considered as a matrix of SiO₂ and HfO₂ only, as this is usually assumed for hafnium silicates. In our case, we deal with a hafnium silicate matrix enriched with silicon as well as doped with Pr³⁺ ions.

The inset of Figure 1 displays the refractive index evolution upon annealing treatment between 800°C and 1,100°C. The uncertainty of the refractive index is 0.01. Nevertheless, it was notable that it decreased with T _A. In a previous study on as-deposited film, it was found that the refractive index was about 2.2 8 , exceeding the value corresponding to the stoichiometric HfSiO₄ matrix (1.7) due to Si enrichment 8 . However, upon annealing, the refractive index is found to be about 1.85 (T _A = 800°C) and 1.82 (T _A = 1,100°C). If we exclude the decrease of porosity, this evolution could be explained by the increasing contribution of some phases with lower refractive index upon annealing (like SiO₂ (1.46)) 8 .

Figure 2a represents the evolution of the FTIR spectra as a function of T _A. The FTIR spectrum of as-deposited film is represented by two broad vibration bands in the ranges of 500 to 750 and 800 to 1,200 cm⁻¹. An annealing treatment stimulates the appearance of several bands that peaked at about 827, 1,084, and 1,250 cm⁻¹ (dashed lines in Figure 2a) corresponding to the LO₂-TO₂, TO₃, and LO₃ vibration modes of the Si-O bond, respectively. Moreover, the increase of the LO₃ mode intensity is attributed to the increase in the number of Si-O-Si bonds. This is a signature of the formation of the SiO₂ phase due to a phase separation process, leading to the decrease of the refractive index for T _A ≥ 800°C.

This phase separation is confirmed also by an increase of the vibration mode intensity in the range of 600 to 780 cm⁻¹, corresponding to Hf-O bonds for the formation of the HfO₂ phase 7 14 . The appearance of well-defined peaks at 760 and 660 cm⁻¹ for T _A ≥ 1,050°C attests the presence of the monoclinic HfO₂ phase 16 . Besides, for T _A ≥ 1,050°C, two new absorption peaks that centered at 900 and 1,000 cm⁻¹ appeared (detailed in Figure 2b). As we showed earlier 8 , at such temperature, undoped HfSiO _x did not reveal the presence of Si-O-Hf bonds. Thus, the vibration band at 900 and 1,000 cm⁻¹ can be attributed to Si-O-Pr asymmetric mode. Similar incorporation of rare-earth ions into Si-O bonds and the formation of rare-earth silicate phase was observed earlier for SiO _x materials doped with Er³⁺, Nd³⁺, or Pr³⁺and annealed at 1,100°C 17 18 19 . Thus, based on this comparison, one can conclude about the formation of Pr silicate revealed by FTIR spectra.

To get more information about the evolution of film structure, we performed XRD analyses. For as-deposited and 900°C annealed films, XRD spectra show a broad peak in the range of 25.0° to 35.0° with a maximum intensity located at 2θ ≈ 31.0° (Figure 3a). The shape of the XRD peak demonstrates the amorphous nature of both layers. With T _A increase, several defined peaks appear, emphasizing the formation of a crystalline structure. Thus, for T _A = 950°C, intense XRD peaks at 2θ ≈ 30.3°, 35.0°, and 50.2° were detected. They correspond to the (111), (200), and (220) planes of the tetragonal HfO₂ phase, respectively, confirming the FTIR analysis 8 . The peak at 2θ ≈ 60.0° can be considered as an overlapping of the reflections from the (311) and (222) planes of the same HfO₂ phase. When T _A reaches 1,050°C, the appearance of peaks at almost 2θ ≈ 24.6° and 28.5° occurs. The first peak is attributed to the monoclinic HfO₂ phase (Joint Committee on Powder Diffraction Standards (JCPDS) no. 78–0050). The second one, at 28.5°, could be ascribed to several phases such as Pr₂O₃ (2θ _[222] ≈ 27.699°) (JCPDS no. 78–0309), Pr₆O₁₁ (2θ _[111] ≈ 28.26°) (JCPDS no. 42–1121), Si (2θ _[111] ≈ 28.44°) (JCPDS no. 89–5012), or Pr₂Si₂O₇ (2θ _[008] ≈ 29.0°) (JCPDS no. 73–1154), due to the overlapping of corresponding XRD peaks. This observation is in agreement with the FTIR spectra (Figure 2b) showing the Hf-O vibrations and formation of Pr clusters.

In some oxygen-deficient oxide films 20 21 , the phase separation is observed with the crystallization of the stoichiometric oxide matrix in the initial step and then in metallic nanoclustering. The aforesaid results are also coherent with our previous study of nonstoichiometric Hf-silicate materials in which we have evidenced the formation of HfO₂ and SiO₂ phases as well as Si nanoclusters (Si-ncs) upon annealing treatment 14 22 . To underline this point, we performed a TEM observation of 1,100°C annealed sample and observed a formation of crystallized Si clusters. Figure 3b exhibits the corresponding selected area electron-diffraction (SAED) pattern. The analysis of dotted diffraction rings indicates the presence of several phases. Among them, one can see the signature of monoclinic and tetragonal HfO₂ phases, Pr₂O₃ phase, and crystallized Si phase (the Table one found in Figure 3b). This latter confirms the presence of Si-ncs but in a small amount (a few spots on the corresponding ring).

Figure 4a shows the PL spectra of Pr³⁺-doped hafnium silicate films, which were excited by a 285-nm wavelength for Pr³⁺ ions. Remarkable emission is observed with peaks centered at about 475, 487, 503, 533, 595, 612, 623, 640, 667, 717, and 753 nm. They are associated to the Pr³⁺ energy level transitions ³P₁→³H₄, ³P₀→³H₄, ³P₀→³H₅, ¹D₂→³H₄, ³P₀→³H₆, ³P₀→³F₂, ³P₀→³F₃, and ³P₀→³F₄, respectively, as shown in Figure 4b 23 . The maximum emission intensity corresponds to the peak centered at 487 nm due to the ³P₀→³H₄ transition.

On the first step, the effect of annealing on Pr³⁺ PL properties was investigated (Figure 4c). The PL intensity evolution is shown in Figure 4d for the representative peak at 487 nm. The PL intensity increases with T _A rising from 800°C up to 1,000°C and then decreases with further T _A increase. At the initial stage, the annealing process is supposed to decrease the non-radiative recombination rates 24 . Thereafter, the quenching of the Pr³⁺ emission that occurred for T _A > 1,000°C can be due to the formation of the Pr³⁺ silicate or Pr oxide clusters (Figure 2) similar to the case observed in 17 18 . Moreover, it is interesting to note that the position of peak (Pr³⁺: ³P₀→³H₄) redshifts from 487 nm (T _A ≤ 1,000°C) to 492 nm (T _A = 1,100°C) as shown in Figure 4c. At the same time, two split peaks contributed to the ¹D₂→³H₄ transition that joined as one sharp peak which centered at 617 nm. All these results can be explained by the dependence of Pr³⁺ PL parameters on the crystal field associated with the type of Pr³⁺ environment 25 . Furthermore, the Pr³⁺ surrounding has been influenced by the crystallization of the HfO₂ phase for films annealed at T _A > 1,000°C.

Taking into account the formation of Si-ncs in Pr-doped HfSiO _x samples annealed at 1,100°C for 1 h, one can expect the appearance of a PL emission due to exciton recombination inside Si-ncs, which is usually observed in the 700- to 950-nm spectral range 17 18 . However, our study of these samples did not reveal the Si-nc PL emission. Two reasons can be mentioned. The first one is the low density of Si-ncs, confirmed by the SAED pattern (Figure 3b). The second one is the efficient energy transfer from the Si-ncs to Pr³⁺ ions. However, based on the comparison of energetic diagrams of Pr³⁺ ions and Si-ncs, we observed that the energy levels of Si-ncs and Pr³⁺ ions have no overlapping. Thus, the energy transfer from Si-ncs toward Pr³⁺ ions should be very weak, contrary to an efficient sensitizing of other rare-earth ions such as Er³⁺ or Nd³⁺ in SiO _x or HfSiO _x matrices 23 24 . Thus, in the case of Pr-doped HfSiO _x samples, Si-ncs do not seem to be a major actor for the energy transfer. Nevertheless, due to the low amount of Si-ncs, their PL signal is not detectable.

Thus, the second step of our investigation was to study the mechanism of Pr³⁺ energy transfer under the 285-nm excitation wavelength. The energy diagram of Pr³⁺ ions does not present such an absorption band wavelength at 285 nm (Figure 4b). In addition, the 4f to 5d transition is witted in upper energy level between 250 and 220 nm 26 . This evidences the indirect excitation of Pr³⁺ ions by the 285-nm wavelength and confirms an energy transfer behavior. To investigate this behavior in detail, we take interest in the strong background PL from 350 to 550 nm for the layers annealed at 800°C to 900°C in Figure 4c. This broad band may be ascribed to more than one kind of defect 5 6 27 . For the layers annealed at higher T _A such as 1,000°C, the intensity of this PL band drops deeply while the Pr³⁺ PL intensity increases notably. This suggests that the energy transfers from host defects to Pr³⁺ ions.

To understand this point, PLE spectra were recorded for the ‘optimized’ sample (annealed at 1,000°C) at different detection wavelengths (400, 487, and 640 nm, corresponding almost to the background emission for the former and to Pr³⁺ PL for the two latter), and they are presented in Figure 5. All the PLE spectra show a remarkable peak at about 280 nm (4.43 eV), and this peak position is in good agreement with that observed for oxygen vacancies 28 . According to some references 6 29 , the O vacancies in the host matrix introduce a series of defect states (at about 1.85 to 4.45 eV) in the bandgap of HfO₂, which might provide recombination centers for excited e and h pairs. These excitons can effectively transfer energy to the nearby Pr³⁺ ions due to the overlapping with absorption levels of Pr³⁺ and, thus, to enhance the Pr³⁺ PL emission. Therefore, the Hf-related O vacancies in the host matrix serve as effective sensitizers to the adjacent Pr ions. An additional argument for this interaction is the increasing of Pr³⁺ PL intensity with T _A (from 900°C to 1,000°C) which caused the formation of HfO₂ grains, providing more Hf-related O vacancies. However, due to a decomposition process, formation of the Si-rich phase (Pr-doped SiO _x and/or Pr silicate) occurs too. The decrease of the intensity of the PL band that peaked at 400 nm and the increase of corresponding Pr³⁺ emission are a signature of the contribution of these Si-rich phase to the Pr³⁺ ion excitation (Figure 4c).

The excitation mechanism of Pr³⁺ ions was further explored by comparing two matrices. We carried out the PL experiments for three kinds of samples annealed at 1,000°C: undoped HfO₂, undoped HfSiO _x films, and Pr-doped HfSiO _x films excited by a 285-nm source (Figure 6). According to 6 , in HfSiO _x films, two types of O vacancies coexist: one is an O vacancy surrounded by Si atoms (Si-related O vacancy), while the other is an O vacancy surrounded by Hf atoms (Hf-related). Since the HfO₂ phase is ionic, it is obvious that it forms easier in the HfSiO _x film upon annealing, and thus, Hf-related O vacancy formation is most preferable than Si-related O vacancy 6 . Herein, a particular interest is focused on the emissions from the defects: the Pr-doped film shows a broad band peaked at 420 nm, while the peak positions redshift to about 450 and 490 nm for HfSiO _x and HfO₂ films, respectively. The 450-nm band can be fitted in energy into four Gaussian bands centered at 3.1, 2.84, 2.66, and 2.11 eV (table inset of Figure 6). The former two peaks are related to defects of the SiO _x phase, for instance, Si-related oxygen deficient centers 13 28 . The peak at 2.66 eV is ascribed to O vacancies related to the HfO₂ phase. The disappearance of the 2.66-eV PL component is accompanied with the appearance of the strong 487-nm emission and series of other Pr³⁺ transitions in Pr-doped HfSiO _x film, which implies the energy transfer from O vacancies to the Pr sites.

As a result, the Si-rich HfO₂ host not only serves as a suitable matrix to achieve efficient Pr³⁺ emission, but also provides a sufficient amount of O vacancies acting as effective sensitizers of rare-earth ions.