Tantalum (V) oxide (Ta₂O₅) is a higher refractive index (n > 2) and lower phonon energy (100 - 450 cm⁻¹) material than other popular oxides (e.g., SiO₂). It can be widely applicable to various passive/active optoelectronics elements such as anti-reflection coatings for silicon solar cells [1] , photonic crystals fabricated using the autocloning method [2] [3] , and novel phosphors doped with rare-earths [4] . We have so far prepared various rare- earth (Er, Eu, Yb, Tm, Y, and Ce) doped Ta₂O₅ thin films using radio-frequency (RF) magnetron co-sputtering of rare-earth oxide (Er₂O₃, Eu₂O₃, Yb₂O₃, Tm₂O₃, Y₂O₃, and CeO₂) pellets and a Ta₂O₅ disc [5] - [18] , and we have obtained various photoluminescence (PL) properties from the films.

Furthermore, we have also prepared copper (II) oxide (CuO) and Ta₂O₅ co-sputtered (CuO-Ta₂O₅) films using the same co-sputtering method, and we have evaluated X-ray diffraction (XRD) and PL properties of the films after annealing [19] . We find that our CuO-Ta₂O₅ composite films annealed above 700˚C can be tetragonal CuTa₂O₆ phases, and good-quality CuTa₂O₆ films with almost no defect can be obtained using our co-sputtering method and subsequent annealing above 900˚C. CuTa₂O₆ films are applicable to chemisorption-type conductometric gas sensors [20] .

Chromium (Cr) is also one of transition metals, and Cr-doped garnets are well known as tunable solid-state laser materials in the red or near infrared regions [21] [22] . Novel Ta₂O₅-based functional materials are expected to be realized by doping with Cr into host Ta₂O₅. In this short report, we will demonstrate the first preparation of a Cr (III) oxide (Cr₂O₃) and Ta₂O₅ co-sputtered (Cr₂O₃-Ta₂O₅) composite film using simply co-sputtering of Cr₂O₃ and Ta₂O₅.

A Cr₂O₃-Ta₂O₅ film was deposited using our RF magnetron sputtering system (ULVAC, SH-350-SE). A schematic figure of the system was presented in our previous report [6] . A Ta₂O₅ disc (Furuuchi Chemical Corporation, 99.99% purity, diameter 100 mm) was used as a sputtering target in the system. We placed three Cr₂O₃ pellets (Furuuchi Chemical Corporation, 99.9% purity, diameter 20 mm) on the erosion area of the Ta₂O₅ disc as presented in Figure 1. The Cr₂O₃ pellets and the Ta₂O₅ disc were co-sputtered by supplying RF power to them. The flow rate of argon gas introduced into the processing vacuum chamber was 15 sccm, and the pressure in the chamber during deposition was kept at ~5.4 × 10⁻⁴ Torr. The RF power supplied to the target was 200 W. A fused-silica plate was used as a substrate, and it was not heated during sputtering. We prepared four specimens from the as-deposited Cr₂O₃-Ta₂O₅ sample by cutting it using a diamond-wire saw, and we subsequently annealed the four specimens in ambient air at 700˚C, 800˚C, 900˚C, or 1000˚C for 20 min using an electric furnace (Denken, KDF S-70).

The XRD patterns of the specimens were recorded using an X-ray diffractometer (RIGAKU, RINT2200VF+/ PC system). The PL spectra of the specimens were measured using a dual-grating monochromator (Roper Scientific, SpectraPro 2150i) and a CCD detector (Roper Scientific, Pixis: 100B, electrically cooled to −80˚C) under excitation using a He-Cd laser (Kimmon, IK3251R-F, wavelength λ = 325 nm).

Figure 2 presents XRD patterns of the four specimens annealed at 700˚C, 800˚C, 900˚C, and 1000˚C. The Cr₂O₃-Ta₂O₅ film annealed at 700˚C seemed to be almost amorphous because no significant diffraction peak was observed from the film. The Cr₂O₃-Ta₂O₅ film annealed at 800˚C seemed to be hexagonal Ta₂O₅ doped with Cr because a major peak corresponding to the (2 0 0); δ-Ta₂O₅ phase (JCPDS No.00-018-1304) was observed from the film. Furthermore, four significant peaks were additionally observed from the specimens annealed at 900˚C and 1000˚C in addition to the peaks corresponding to the above-mentioned (2 0 0) (hexagonal Ta₂O₅) phase. These peaks correspond to tetragonal CrTaO₄ ((0 0 3), (1 1 0), (1 0 1), and (2 0 3)) phases (JCPDS No. 00-039-1428). We found that our Cr₂O₃-Ta₂O₅ composite films annealed above 900˚C include both hexagonal Ta₂O₅ and tetragonal CrTaO₄ phases.

Figure 3 presents PL spectra of the specimens annealed at 700˚C, 800˚C, 900˚C, and 1000˚C. No significant

PL peak was observed from all the specimens. In our previous report, we found that the CuO-Ta₂O₅ composite films annealed at 700˚C - 900˚C were tetragonal CuTa₂O₆ phases, and we considered that our CuTa₂O₆ film annealed at 900˚C had almost no defect because broad PL peaks due to oxygen-vacancy trap levels were not observed [19] . Therefore, it seems that our Cr₂O₃-Ta₂O₅ composite films also have almost no defect because no significant PL peak was observed from the films as presented in Figure 3. As mentioned above, we can obtain tetragonal CrTaO₄ from our Cr₂O₃-Ta₂O₅ films after annealing above 900˚C. CrTaO₄ has also been prepared using other methods such as anodic spark deposition [23] . However, good-quality CrTaO₄ films without defects are expected to be obtained using our very simple co-sputtering method and subsequent annealing. We will try to calculate lattice parameters of the Cr₂O₃-Ta₂O₅ films annealed at the different temperatures, and characterize morphologies of the films using a scanning electron microscope.

We prepared Cr₂O₃-Ta₂O₅ composite films using our RF magnetron co-sputtering method for the first time. From the XRD patterns, the Cr₂O₃-Ta₂O₅ film annealed at 700˚C seemed to be almost amorphous, and the one annealed at 800˚C seemed to be hexagonal Ta₂O₅ doped with Cr. In addition, the Cr₂O₃-Ta₂O₅ films annealed at 900˚C and 1000˚C seemed to include tetragonal CrTaO₄ phases. Furthermore, it seems that almost no defect exists in our Cr₂O₃-Ta₂O₅ composite films annealed at 700˚C - 1000˚C because their PL spectra have no defect- related peak. It is expected that good-quality Cr₂O₃-Ta₂O₅ composite films including CrTaO₄ can be obtained using our very simple co-sputtering method and subsequent annealing above 900˚C.

Part of this work was supported by JSPS KAKENHI Grant Number 26390073. Part of this work was conducted at the Human Resources Cultivation Center (HRCC), Gunma University, Japan.

Kenta Miura,Takumi Osawa,Yuya Yokota,Osamu Hanaizumi, (2015) Preparation of Cr₂O₃-Ta₂O₅ Composites Using RF Magnetron Sputtering. Open Access Library Journal,02,1-5. doi: 10.4236/oalib.1102094