Microstructure and optical properties of Pr3+-doped hafnium silicate films
© An et al.; licensee Springer. 2013
Received: 9 October 2012
Accepted: 23 December 2012
Published: 21 January 2013
In this study, we report on the evolution of the microstructure and photoluminescence properties of Pr3+-doped hafnium silicate thin films as a function of annealing temperature (TA). The composition and microstructure of the films were characterized by means of Rutherford backscattering spectrometry, spectroscopic ellipsometry, Fourier transform infrared absorption, and X-ray diffraction, while the emission properties have been studied by means of photoluminescence (PL) and PL excitation (PLE) spectroscopies. It was observed that a post-annealing treatment favors the phase separation in hafnium silicate matrix being more evident at 950°C. The HfO2 phase demonstrates a pronounced crystallization in tetragonal phase upon 950°C annealing. Pr3+ emission appeared at TA = 950°C, and the highest efficiency of Pr3+ ion emission was detected upon a thermal treatment at 1,000°C. Analysis of the PLE spectra reveals an efficient energy transfer from matrix defects towards Pr3+ ions. It is considered that oxygen vacancies act as effective Pr3+ sensitizer. Finally, a PL study of undoped HfO2 and HfSiO x matrices is performed to evidence the energy transfer.
Rare-earth elements are important optical activators for luminescent devices. Among various rare-earth luminescent centers, trivalent praseodymium (Pr3+) offers simultaneously a strong emission in the blue, green, orange, and red spectral range, satisfying the complementary color relationship [1, 2]. Consequently, Pr3+-doped glass/crystals are often used as phosphor materials [2, 3]. SiO2-(Ca, Zn)TiO3:Pr3+ phosphors prepared with nanosized silica particles exhibit an intense red photoluminescence (PL) . The Pr3+ emission was achieved for Si-rich SiO2 (SRSO) implanted with Pr3+ ions, but its intensity was lower .
Hafnium dioxide (HfO2) and hafnium silicates (HfSiO x ) are currently considered as the predominant high-k dielectric candidates to replace the conventional SiO2 due to the rapid downscaling of the complementary metal-oxide semiconductor (CMOS) transistors [5, 6]. It is ascribable to their good thermal stability in contact with Si, large electronic bandgaps, reasonable conduction band offset in regard to Si, and their compatibility with the current CMOS technology. Our group has first explored the structural and thermal stability of HfO2-based layers fabricated by radio frequency (RF) magnetron sputtering [7, 8] and their nonvolatile memory application [9, 10].
It is worth to note that both HfO2 and HfSiO x matrices have lower phonon frequencies compared to those of SiO2, and as a consequence, both are expected to be suitable hosts for rare-earth activators. Thus, PL properties have been investigated for the HfO2 matrix doped with Tb3+, Eu3+[11, 12], or Er3+[12, 13] and have been explained by the interaction of rare-earth ions with host defects. Recently, our group has demonstrated that an enhancement of Er3+ PL emission can be achieved for the Er-doped HfSiO x matrix in comparison with that of the Er-doped HfO2. It was also observed that an energy transfer from the HfO2 host defects towards Er3+ ions, whereas the existence of Si clusters allowed an enhancement of the Er3+ ion emission under longer-wavelength excitation. Consequently, the mechanism of the excitation process, when Si clusters and oxygen-deficient centers act as Er3+ sensitizers, has been proposed to explain an efficient rare-earth emission from Er-doped HfSiO x hosts  similar to that observed for the Er-doped SRSO materials .
In this paper, we study the microstructure and optical properties of Pr-doped hafnium silicate films fabricated by magnetron sputtering versus annealing temperature. We demonstrate that an efficient Pr3+ light emission is achievable by tuning the annealing conditions. The excitation mechanism of Pr3+ ions is also discussed.
The films were deposited onto p-type (100) 250-μm-thick Si wafers by RF magnetron sputtering of a pure HfO2 target topped by calibrated Si and Pr6O11 chips. The growth was performed in pure argon plasma with an RF power density of 0.98 W∙cm−2; the Si substrate temperature was kept at 25°C. After deposition, a post-annealing treatment was carried out under a nitrogen flow, at temperatures (TA) varying from 800°C up to 1,100°C for 1 h.
The refractive index (n) (given always at 1.95 eV) and the film thicknesses were deduced from spectroscopic ellipsometry data. The chemical composition of the films was determined by Rutherford backscattering spectrometry (RBS) using a 1.5-MeV 4He+ ion beam with a normal incidence and a scattering angle of 165°. The infrared absorption properties were investigated by means of a Nicolet Nexus (Thermo Fisher Scientific, Waltham, MA, USA) Fourier transform infrared (FTIR) spectroscopy at Brewster’s incidence (65°) in the range of 500 to 4,000 cm−1. X-ray diffraction (XRD) experiments were performed using a Philips Xpert MPD Pro device (PANalytical B.V., Almelo, The Netherlands) with CuKα radiation (λ = 1.5418 Å) at a fixed grazing angle incidence of 0.5°. Cross-sectional specimens were prepared by standard procedure involving grinding, dimpling, and Ar+ ion beam thinning until electron transparency for their observation by transmission electron microscopy (TEM). The samples were observed using a FEG 2010 JEOL instrument, operated at 200 kV. The PL emission and PL excitation (PLE) measurements were carried out using a 450-W Xenon arc lamp as excitation source at room temperature corrected on spectral response with the help of a Jobin-Yvon Fluorolog spectrometer (HORIBA Jobin Yvon Inc., Edison, NJ, USA).
Results and discussion
Composition and structural characterizations
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 TA. In a previous study on as-deposited film, it was found that the refractive index was about 2.2 , exceeding the value corresponding to the stoichiometric HfSiO4 matrix (1.7) due to Si enrichment . However, upon annealing, the refractive index is found to be about 1.85 (TA = 800°C) and 1.82 (TA = 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)) .
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 TA ≥ 1,050°C attests the presence of the monoclinic HfO2 phase . Besides, for TA ≥ 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 , 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 [17–19]. Thus, based on this comparison, one can conclude about the formation of Pr silicate revealed by FTIR spectra.
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).
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 TA rising from 800°C up to 1,000°C and then decreases with further TA increase. At the initial stage, the annealing process is supposed to decrease the non-radiative recombination rates . Thereafter, the quenching of the Pr3+ emission that occurred for TA > 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 (TA ≤ 1,000°C) to 492 nm (TA = 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 . Furthermore, the Pr3+ surrounding has been influenced by the crystallization of the HfO2 phase for films annealed at TA > 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 . 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 TA 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.
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.
In summary, we have fabricated the Pr3+-doped hafnium silicate layers by RF magnetron sputtering. The effect of the annealing temperature on the film properties has been investigated by means of ellipsometry, XRD, and FTIR spectroscopies. We showed that the highest Pr3+ PL intensity is obtained for 1,000°C annealing. The PL and PLE measurements demonstrate that the Pr3+ ions were efficiently excited by oxygen vacancies in the films, and thus, remarkable Pr3+ PL can be obtained by a non-resonant excitation process. The present results show the promising application of Pr-doped films for future optoelectronic devices.
The authors would like to thank Dr. Ian Vickridge from SAFIR, Institut des NanoSciences de Paris for the RBS data as well as Dr. Sophie Boudin from CRISMAT Lab for the measurement of PL and PLE spectra. This work is supported by the CEA/DSM/ENERGY contract (Project HOFELI) and the Chinese Scholarship Council (CSC) program.
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