1. Introduction

Fluorites and sesquioxides have attracted attention as luminescent materials for scintillators and laser gain media owing to their high transparency and optical isotropy [1,2]. HfO₂ and Lu₂O₃ are potential host materials because of wide band gaps (5.8 and 5.5 eV) [3,4], high densities (10.1 and 9.5 Mg m⁻³), and high effective atomic numbers (67.3 and 67.4), respectively. However, these crystals are hard to grow from the melt due to the high melting points of HfO₂ (3031 K) and Lu₂O₃ (2763 K). Although advanced powder technologies can fabricate transparent polycrystalline ceramics [5,6], preparation and pretreatment of starting powders are still complicated procedures to obtain highly transparent ceramics. In addition, a reversible phase transformation of HfO₂ among the monoclinic, tetragonal, and cubic structures prevents the melt solidification and sintering of HfO₂ without additives.

Crystal growth from the vapor phase is an alternative for synthesizing transparent crystals and ceramics having a high-melting point and a reversible phase transformation. Generally, the conventional vapor deposition process mostly results in thin films ranging in thickness from the nanometer to the submicron scale due to the typically low deposition rates, which are usually less than one micron per hour. Metal–organic chemical vapor deposition (MOCVD) is advantageous for the rapid production of transparent thick films compared with physical vapor deposition techniques including sputtering and pulsed laser deposition. However, there are no reports of CVD deposition of HfO₂ transparent thick films, and thus, luminescent studies on HfO₂ have been limited to thin films [7,8] and powders [9,10].

We have previously demonstrated that laser-assisted MOCVD can prepare transparent single-crystal CeO₂ thick films at a high deposition rate of 15 µm h⁻¹ [11]. Although Feng et al. have also prepared Lu₂O₃ films [12], discussion on growth mode, morphological observation, and luminescence properties was insufficient.

In the present study, we synthesized epitaxially grown transparent thick films of Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ using the laser-assisted MOCVD and compared their growth mode, morphology, and luminescence properties. Although Eu³⁺ ion exhibits a rather slow response as scintillation imaging, an intense red emission from Eu³⁺ ion can be used as a spectroscopic probe for site symmetry determination among HfO₂ polymorphs [13,14].

2. Experimental procedure

2.1. Sample preparation

The laser-assisted MOCVD apparatus used in the present work has been described elsewhere [15]. Metal–organic precursors of hafnium acetylacetonate (Hf(acac)₄; Mitsuwa Chemicals, Japan), lutetium and europium tris-dipivaloylmethanates (Lu(dpm)₃ and Eu(dpm)₃; Toshima Manufacturing, Japan) were heated to 463, 453, and 443 K, respectively. The resultant vapors were transferred into the deposition chamber using Ar as a carrier gas, and O₂ gas was separately introduced into the chamber through a double-tube nozzle. The total chamber pressure was maintained at 0.2 kPa. The Eu³⁺ molar fraction in the precursor vapor was 3.8–4.0 mol%, which was calculated from the mass change of each precursor.

A commercially available (100) yttria-stabilized zirconia single-crystal plate (YSZ; 5 mm × 5 mm × 1 mm; both-side polished; lattice parameter: a = 0.514 nm) was used as a substrate and was preheated to 1000 K on a heating stage. During the deposition, a CO₂ tube laser (wavelength: 10.6 µm; maximum laser output: 60 W; SPT Laser Technology, China) was used to irradiate the substrate through a ZnSe chamber window, which caused the substrate temperature to increase to 1253 K. The deposition was conducted for 0.3 ks.

2.2 Sample characterization

The phase composition and out-of-plane orientation of the films were investigated by X-ray diffraction (XRD; Bruker D2 Phaser), and the in-plane orientation was studied using an X-ray pole figure measurement (Rigaku Ultima IV). The morphology of the films was observed using a scanning electron microscope (SEM; Jeol JCM-6000 and Hitachi SU8010). The crystal structures were visualized using the VESTA [16] software package.

The in-line transmittance was measured using an ultraviolet–visible spectrophotometer (Jasco V-630) over the wavelength range of 190–1100 nm. The photoluminescence spectrum excited at a wavelength of 275 nm using a deep-ultraviolet light-emitting diode (DUV-LED) was measured using a spectrometer (Ocean Insights HR2000+) over the wavelength range of 400–800 nm at room temperature. The excitation spectrum and fluorescence decay curve were recorded using a fluorescence spectrophotometer (Jasco FP-8500).

3. Results and discussion

3.1. Epitaxial growth mode

Figure 1 shows out-of-plane and in-plane XRD patterns of Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films grown on (100) YSZ substrates. Both the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films were epitaxially grown with in-plane orientations.

 

Fig. 1. (a, b) Out-of-plane XRD patterns of (a) Eu³⁺:HfO₂ and (b) Eu³⁺:Lu₂O₃ films grown on (100) YSZ substrate. (c–e) In-plane XRD patterns of (c) {011} HfO₂ plane of the Eu³⁺:HfO₂ film, (d) {111} Lu₂O₃ plane of the Eu³⁺:Lu₂O₃ film, and (e) {111} YSZ plane of the YSZ substrate.

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The Eu³⁺:HfO₂ film was indexed with a monoclinic baddeleyite structure (ICSD No. 27313; Space group: P2₁/c; lattice parameters: a = 0.511 nm, b = 0.517 nm, c = 0.529 nm, and γ = 99.216°) with (001) orientation (Fig. 1(a)). The four-fold peak with two satellites was observed in the in-plane XRD pattern for the Eu³⁺:HfO₂ film (Fig. 1(c)). Because the {011} HfO₂ plane has a two-fold symmetry, the in-plane XRD pattern was associated with the formation of 90°-rotated and ±9.5°-rotated domains.

The plan-view and cross-sectional atomic arrangements of epitaxially grown (001) HfO₂ and (100) Lu₂O₃ domains on (100) YSZ are illustrated in Fig. 2. The primary and 90°-rotated domains adopted the in-plane orientation relationship of [100] HfO₂ || [010] YSZ and [010] HfO₂ || [001] YSZ, respectively. The ±9.5°-rotated domains could be feasibly formed like a coincident site lattice, as illustrated in Fig. 2(a), adopting the in-plane orientation relationship of [610] HfO₂ || [010] YSZ for −9.5° domain. An edge-sharing HfO₇ heptahedral chain is straight along the c axis in the monoclinic structure of HfO₂ resulted in the (001) orientation (Fig. 2(b)), not (100) and (010) orientations [15].

 

Fig. 2. (a, c) Plan-view and (b, d) cross-sectional atomic arrangements of (a, b) (001) HfO₂ domains and (c, d) (100) cubic Lu₂O₃ domains epitaxially grown on (100) YSZ substrates.

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The Eu³⁺:Lu₂O₃ film was indexed with a cubic bixbyite structure (ICSD No. 40471; Ia$\bar{3}$; a = 1.039 nm) with (100) orientation (Fig. 1(b)). According to the in-plane XRD patterns of the (100) Eu³⁺:Lu₂O₃ film and the (100) YSZ substrate (Figs. 1(d) and 1(e)), the (100) Eu³⁺:Lu₂O₃ film grew epitaxially with the in-plane orientation relationship of [001] Eu³⁺:Lu₂O₃ || [001] YSZ. Because Lu₂O₃ and YSZ have a small lattice mismatch (1.1%), the (100) Eu³⁺:Lu₂O₃ film grew on the (100) YSZ substrate via a cube-on-cube growth mode, as illustrated in Figs. 2(c) and 2(d).

3.2. Morphology and appearance

Figures 3(a) and 3(b) show cross-sectional and surface SEM images of the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films prepared on YSZ substrates. The Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films exhibited 1.2 µm-thick and 1.6 µm-thick dense cross-sections. The surfaces of the films were smooth, and no grain boundary was observed (insets in Figs. 3(a) and 3(b)). From the cross-sectional SEM images, deposition rates of 15 and 20 µm h⁻¹ were determined for Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃, respectively.

 

Fig. 3. (a, b) Cross-sectional SEM images of (a) Eu³⁺:HfO₂ and (b) Eu³⁺:Lu₂O₃ films grown on (100) YSZ substrates. The insets show surface SEM images of the films. (c) In-line transmittance spectra of Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films grown on (100) YSZ substrates. The inset show photographs of those films together with a blank YSZ substrate.

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Figure 3(c) shows the in-line transmittance spectra of the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films on YSZ substrates. The optical band gaps of the YSZ raw substrate and the films were estimated to be 4.5–4.6 eV by using the Tauc plot. This is because YSZ has a narrower band gap (5.0 eV) than HfO₂ (5.8 eV) and Lu₂O₃ (5.5 eV) [3,4]. The decline in the transmittance of Eu³⁺:HfO₂ film in the wavelength range of 250–350 nm might be associated with light scattering at domain boundaries and/or light absorption due to oxygen deficiency. The oxygen vacancy in HfO₂ might be formed to compensate charge valance when replacing Hf⁴⁺ with Eu³⁺ ion. At a wavelength of 650 nm, the transmittances of Eu³⁺HfO₂ and Eu³⁺:Lu₂O₃ films were 100% and 95%, respectively, relative to the YSZ substrate. Photographs of the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films were highly transparent, as were the blank YSZ single-crystal substrates (inset in Fig. 3(c)).

3.3. Luminescence properties

Figures 4(a) and 4(b) show the photoluminescence excitation and emission spectra of the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films. In the excitation spectra of the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films, broad absorption bands over the wavelength range of 200–300 nm were associated with O²⁻–Eu³⁺ charge transfer (CT) transitions and weak peaks in the wavelength range of 300–450 nm were due to 4f–4f transitions within Eu³⁺ ions. The photoluminescence emission lines of the Eu³⁺:HfO₂ and Eu³⁺:HfO₂ films corresponded to ⁵D₀ → ⁷F_J (J = 0–4) transitions of the Eu³⁺ ions, and the ⁵D₀ → ⁷F₂ transition observed at 614 and 615 nm was the strongest emissions for the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films, respectively.

 

Fig. 4. (a, b) Photoluminescence excitation and emission spectra of (a) Eu³⁺:HfO₂ and (b) Eu³⁺:Lu₂O₂ films excited at a wavelength of 275 nm. (c) VRBE diagram for Eu²⁺ in Lu₂O₃ and Eu^2+/3+ in HfO₂. CB: conduction band; VB: valence band; NR: non-radiative relaxation; HRBE: host referred binding energy; E^CT: charge-transfer energy; E_c, E_x, and E_v: energy levels of conduction band, exciton, and valence band, respectively. Arrow 1: the energy of the CT band of Eu³⁺: 4.5 eV; arrow 2: ⁵L₆ ← ⁷F₀ excitation within Eu³⁺ ion; arrow 3: the ⁵D₀ → ⁷F₂ transition with red emission.

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The probability of ⁵D₀ → ⁷F₂ electric-dipole transition is sensitive to the Eu³⁺ surrounding symmetry, while that of the ⁵D₀ → ⁷F₁ magnetic-dipole transition is independent of the surrounding symmetry. Therefore, the relation of these luminescence intensities is known as the asymmetry ratio [13,14], and the larger asymmetry ratio is for lower surrounding symmetry of Eu³⁺ ion. The asymmetry ratios for the Eu³⁺-doped ZrO₂ with monoclinic structure and Y₂O₃-stabilized tetragonal structure were 2.2–2.7 and 1.3, respectively [13]. The asymmetry ratio of the Eu³⁺:HfO₂ film was 2.1 (Fig. 4(a)), confirming the Eu³⁺:HfO₂ film had the monoclinic symmetry.

Lu₂O₃ bixbyite structure has two different cation sites of C₂ and S₆ (C_3i) symmetry. Because Lu₂O₃ has three times more C₂ site than S₆ site and ⁵D₀ → ⁷F_J transition of the Eu³⁺ ion at the S₆ site exhibits forbidden character due to its inversion symmetry, the emission from Eu³⁺ ion at the C₂ site was much stronger than that at the S₆ site, as shown in Fig. 4(b). The asymmetry ratio of 3.8 for the Eu³⁺:Lu₂O₃ film was comparable to the reported values for Eu³⁺:Lu₂O₃ bulk (3.8) and nanocrystals (4.0) [17].

Dorenbos studies vacuum referred binding energies (VRBE) of electrons in divalent and trivalent lanthanide impurity states and host band states comprehensively in various materials including rare-earth sesquioxides, and transition metal oxides [18,19]. Figure 4(c) shows VRBE diagram for Eu^2+/3+ states in HfO₂ and Lu₂O₃ constructed based on the literature [18,19]. Typical optical transitions in Eu³⁺:HfO₂ are also illustrated in the figure. The band gap energies of 6.5 eV for Lu₂O₃ and 6.3 eV for HfO₂ were used in the literature. Eu²⁺ levels were located in the band gaps 4.5 eV and 5.2 eV above the valence band levels of Lu₂O₃ and HfO₂, respectively (red bars in Fig. 4(c)). Under DUV irradiation centered at 275 nm (4.5 eV), an electron can be transferred from O²⁻ to Eu³⁺ via CT excitation (arrow 1 in Fig. 4(c)), and Eu³⁺ is formally reduced to Eu²⁺ [14]. Emissions from higher excited states (⁵D_J>0) may only be observed at a low concentration of Eu³⁺ ions or at a low temperature because of efficient non-radiative relaxation processes feeding the ⁵D₀ state [20]. Therefore, photoluminescence emissions of not only Eu³⁺:HfO₂ but also Eu³⁺:Lu₂O₃ were dominant by ⁵D₀ → ⁷F_J transitions of the Eu³⁺ ions (arrow 3 in Fig. 4(c)).

The Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films exhibited intense red emission under irradiation by ultraviolet light from a low-pressure mercury-vapor lamp, as shown in Figs. 5(a) and 5(b). The blank YSZ substrate weakly emitted blue light. Figure 5(c) shows the fluorescence decay curves monitored at 614 and 615 nm for the ⁵D₀ → ⁷F₂ transitions of Eu³⁺ ions in HfO₂ and Lu₂O₃ under the CT excitation at 218 and 256 nm, respectively. The decay curve of a Eu³⁺:Lu₂O₃ transparent polycrystalline ceramic prepared by spark plasma sintering [21] was also depicted for comparison. These curves were fitted with a first-order exponential equation, and the decay times were determined to be 1.17 ± 0.02 and 1.21 ± 0.08 ms for Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films, respectively, which are comparable with that of the Eu³⁺:Lu₂O₃ ceramic (1.05 ± 0.02 ms) and literature values (1–1.6 ms) [21].

 

Fig. 5. (a, b) Photographs of (a) Eu³⁺:HfO₂ and (b) Eu³⁺:Lu₂O₃ films grown on (100) YSZ substrate under UV irradiation together with blank YSZ substrates. (c) Fluorescence decay curves of the Eu³⁺:HfO₂ and Eu³⁺:Lu₂O₃ films excited at 218 and 256 nm, and monitored at 614 and 615 nm, respectively, together with the results of Eu³⁺:Lu₂O₃ transparent ceramic [21].

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4. Conclusions

The results of the present study can be summarized as follows: