Article pubs.acs.org/IC
Structural and Thermal Properties of Ternary Narrow-Gap Oxide Semiconductor; Wurtzite-Derived β‑CuGaO2 Hiraku Nagatani,† Issei Suzuki,† Masao Kita,‡ Masahiko Tanaka,§ Yoshio Katsuya,§ Osami Sakata,§ Shogo Miyoshi,¶ Shu Yamaguchi,¶ and Takahisa Omata*,† †
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ‡ Department of Mechanical Engineering, Toyama National College of Technology, 13 Hongo-machi, Toyama 939-8630, Japan § Synchrotron X-ray Station at SPring-8, National Institute for Materials Science, Kouto, Sayo, Hyogo 679-5148, Japan ¶ Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *
ABSTRACT: The crystal structure of the wurtzite-derived β-CuGaO2 was refined by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Its structural characteristics are discussed in comparison with the other I−III−VI2 and II−VI oxide semiconductors. The cation and oxygen tetrahedral distortions of the β-CuGaO2 from an ideal wurtzite structure are small. The direct band-gap nature of the β-CuGaO2, unlike β-Ag(Ga,Al)O2, was explained by small cation and oxygen tetrahedral distortions. In terms of the thermal stability, the β-CuGaO2 irreversibly transforms into delafossite α-CuGaO2 at >460 °C in an Ar atmosphere. The transformation enthalpy was approximately −32 kJ mol−1, from differential scanning calorimetry. This value is close to the transformation enthalpy of CoO from the metastable zincblende form to the stable rock-salt form. The monovalent copper in β-CuGaO2 was oxidized to divalent copper in an oxygen atmosphere and transformed into a mixture of CuGa2O4 spinel and CuO at temperatures >350 °C. These thermal properties indicate that β-CuGaO2 is stable at ≤300 °C in both reducing and oxidizing atmospheres while in its metastable form. Consequently, this material could be of use in optoelectronic devices that do not exceed 300 °C.
1. INTRODUCTION Ternary I−III−VI2 oxide semiconductors with a wurtzitederived β-NaFeO2 structure1 are very attractive as materials that expand the range of available energy band gaps in oxide semiconductors. The energy band gaps of oxides possessing this structure are 5.6 eV for β-LiGaO2, 3.0 eV for β-AgAlO2, and 2.2 eV for β-AgGaO2.2−7 They form alloys with ZnO over a wide composition range because of their structural similarity to wurtzite ZnO. This has allowed band gap engineering of ZnO from near-ultraviolet to ultraviolet using ZnO−LiGaO2 alloys and from near-ultraviolet to visible using ZnO−AgGaO2 alloys.8−12 The latter alloys give oxide semiconductors with a narrow band gap, while those with wide band gaps have played a key role in the performance of previous oxide semiconductors. Recently, we reported β-CuGaO2 as a new material possessing the β-NaFeO2 structure.13 The energy band gap of this material is direct and 1.47 eV. This means that the ternary I−III−VI2 oxide semiconductors with a wurtzite-derived βNaFeO2 structure cover a wide region of energies, as shown in Figure 1. An energy band gap of 1.47 eV matches the required © XXXX American Chemical Society
Figure 1. Band gap vs pseudowurtzite lattice parameter a0 for II−VI and I−III−VI2 oxide semiconductors.
energy to achieve the theoretical maximum conversion efficiency for a single-junction solar cell.14 The p-type conduction of the β-CuGaO2 and the small lattice mismatch with ZnO are promising for the fabrication of a p/n heterojunction of β-CuGaO2 with n-type ZnO. These features Received: November 4, 2014
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DOI: 10.1021/ic502659e Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
atomic displacement parameters were refined in the present analysis together with the profile and background parameters. The following agreement indices were calculated: profile, Rp = ∑|yio − yic|/∑yio; weighted profile, Rwp = [∑wi(yio − yic)2/∑wi(yio)2]1/2; Bragg, RB = ∑| I0(hK) − I(hK)|/∑I0(hK), and the goodness-of-fit S = Rwp/Re, where Re = [(N − P)/ ∑wi(yio)2]1/2, yio and yic are the observed and calculated intensities, respectively, wi is the weighting factor, I0(hK) is estimated integrated intensities from observed diffraction data, I(hK) is the calculated integrated intensities, N is the total number of yio data, and P is the number of adjusted parameters. Thermal Analysis and High-Temperature XRD Measurement. The thermal stability of the samples was studied by TG-DTA (TG/DTA 6300, Seiko Instruments Inc., Japan) in an oxygen or argon gas atmosphere. In the measurements, the heating rate was 5 K min−1, and gas flow rate was 200 mL min−1. The structural transformation from β-CuGaO2 to α-CuGaO2 was characterized by DSC (XDSC7000, Seiko Instruments, Inc.). The high-temperature XRD patterns under N2 or air were recorded in the 2θ range of 20−60° in the temperature range from room temperature to 631 °C on an X-ray diffractometer in a θ−2θ geometry (RINT2500, Cu Kα radiation, Rigaku, Japan) equipped with Anton Paar HTK attachment.
make this new material suitable as a light absorber in thin-film solar cells. The ternary I−III−VI2 oxides with a wurtzite-derived βNaFeO2 structure are the oxide analogue of the ternary chalcogenides with a chalcopyrite structure, such as CuInSe2.15 While the ternary chalcopyrite chalcogenides have been extensively studied because of their promising application in solar cells and phosphors, we have a limited understanding of the oxides with a β-NaFeO2 structure. Further studies on their properties are necessary to understand their potential applications in optoelectronic devices. For the present paper, we studied the structure of β-CuGaO2 to discuss its structural features with the other β-NaFeO2- and wurtzite-type oxide semiconductors. For this purpose, we refined the crystal structure of β-CuGaO2 by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Thermal stability and phase transformation into delafossite α-CuGaO2 are discussed based on thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), and hightemperature X-ray diffraction (XRD).
3. RESULTS AND DISCUSSION 3-1. Structure of β-CuGaO2. Figure 2 shows the observed and calculated X-ray diffraction profiles and the difference
2. EXPERIMENTAL SECTION Chemicals. For the precursor preparation, Na2CO3 (99.8%; Wako Pure Chemical Industries, Japan) and Ga2O3 (99.99%; Kojundo Chemical Laboratory, Japan) were used as starting materials. To exchange the Na+ ions in the precursor β-NaGaO2 to Cu+ ions, CuCl (99.9%; Wako Pure Chemical Industries) was used. All the chemicals were used as purchased. Sample Preparation. β-CuGaO2 was synthesized via ionexchange of Na+ ions in the β-NaGaO2 precursor with Cu+ ions from CuCl, similar to the synthesis of β-AgGaO2.16,17 The β-NaGaO2 precursor was prepared by the solid-state reaction of Na2CO3 and Ga2O3. Na2CO3 and Ga2O3 were weighed, mixed, and then pressed into disks (17.2 mm diameter) at 256 MPa. The disks were fired at 900 °C for 20 h and then crushed and mixed with CuCl to achieve a βNaGaO2/CuCl molar ratio of 1:1. The mixed powder was then heated at 250 °C for 48 h under vacuum (460 °C in an inactive gas atmosphere, and the β-CuGaO2 is a metastable phase because the transformation from the βCuGaO2 to α-CuGaO2 was irreversible. Figure 8 shows the schematic illustration of α-CuGaO2 with a delafossite structure. While the Cu and Ga atoms alternately occupy the cation site,
Figure 8. Schematic illustrations of the crystal structures of β-CuGaO2 with a delafossite structure.28
which is 4-fold and tetrahedrally coordinated to oxygen atoms, in wurtzite-derived β-CuGaO2 (Figure 3), the Cu atoms are 2fold and in linear coordination to oxygen atoms, and the Ga atoms are 6-fold and in octahedral coordination to oxygen atoms in the α-CuGaO2 with delafossite structure. The αCuGaO2 can be described as the layered structure consisting of Cu2O and Ga2O3 layers. Because not only coordination states but also ordering manner of cations is different between these E
DOI: 10.1021/ic502659e Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry two forms, the structural transformation from β-CuGaO2 to αCuGaO2 is understood as reconstructive transformation involving the rearrangement of both cations and anions. Figure 9 shows the DSC curve for the phase transformation from the
from the metastable zincblende or wurtzite form to the stable rock-salt form or from metastable rock-salt form to stable wurtzite form. Under oxygen-containing atmospheres, the monovalent copper in the β-CuGaO2 is oxidized into divalent copper, and the β-CuGaO2 transformed into a mixture of CuGa2O4 spinel and CuO >300 °C. These results indicate that the β-CuGaO2 is a metastable phase, but practically stable at
