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Enhanced electrical conduction in anatase TaON via soft chemical lithium insertion towards electronics application Atsushi Suzuki, Yasushi Hirose, Takafumi Nakagawa, Satoshi Fujiwara, Shoichiro Nakao, Yutaka Matsuo, Isao Harayama, Daiichiro Sekiba, and Tetsuya Hasegawa ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00750 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018
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ACS Applied Nano Materials
Enhanced electrical conduction in anatase TaON via soft chemical lithium insertion towards electronics application Atsushi Suzuki†, Yasushi Hirose*†‡, Takafumi Nakagawa⊥, Satoshi Fujiwara†, Shoichiro Nakao‡, Yutaka Matsuo⊥§, Isao Harayama∆◊, Daiichiro Sekiba∆◊, and Tetsuya Hasegawa*†‡ †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡
Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan
⊥
Department of Mechanical Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
§
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China
∆
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ◊
Tandem Accelerator Complex, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan
oxynitride semiconductor, epitaxial thin film, soft chemical lithium insertion, electron mobility, transparent electrode, thin film transistor, anatase TiO2
ABSTRACT: Metal oxynitride semiconductors with the d0 or d10 electron configuration are promising materials for nontoxic pigments and photocatalysts, but their electrical properties have scarcely been studied. Anatase TaON (δ-TaON) is a metastable polymorph of TaON, and its epitaxial thin films show good semiconducting properties such as wide tunability of electrical conductivity and a rather high electron mobility comparable to anatase TiO2. However, the density of carrier electrons (ne) provided by anion vacancies is limited to ~1×1020 cm−3, so establishing a method for carrier doping of anatase TaON remains a critical issue for its use in electronics applications. In this report, we used soft chemical insertion of Li into interstitial sites of anatase TaON epitaxial thin films by using an n-butyllithium solution, and the resulting material showed higher ne (3.5×1020 cm−3) than anion-deficient anatase TaON films. Additionally, the Li-inserted anatase TaON showed an enhanced Hall mobility (μH) of over 30 cm2V−1s−1, and a lower resistivity of 6.7×10−4 Ωcm at room temperature. In contrast, direct vapor phase deposition of Li-doped TaON caused Li substitution for Ta, where large difference in charges between Li+ and Ta5+ was compensated by an increase in the O/N ratio. These results indicate that soft chemical insertion of Li after growth of the host crystal is an effective method for carrier doping of anatase TaON.
Introduction 0
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Metal oxynitride semiconductors with the d or d electron configuration are promising materials for nontoxic pigments1 and photocatalysts2–6 due to their suitable band gaps and band alignment. In contrast, few studies have explored their use in electronic devices, for example as a channel in thin-film transistors, an active layer in photovoltaic cells, and a transparent electrode, mainly because it is difficult to synthesize single-crystalline or dense ceramic samples suited for measurements of electrical transport properties. Recently, we synthesized anatase TaON (δ-TaON),7 a metastable polymorph of TaON,8,9 in an epitaxial thin film with bandgap of 2.37 eV. The anatase TaON films showed semiconducting properties promising for electronics applications. Its electrical conductivity can be widely tuned from insulator to degener-
ate semiconductor by introducing anion vacancies as a source of carrier electrons. Furthermore, this aniondeficient anatase TaON has a high electron mobility (~17 cm2 V−1 s−1 at 300 K)7 comparable to anatase TiO2 (~17 cm2 V−1 s−1 at 300 K)10, a functional oxide semiconductor used in various applications such as a transparent conductor, a thin film transistor, a ferromagnetic semiconductor, and a resistive random access memory.11–14 However, the carrier electron density of anion-deficient anatase TaON is limited to ~1×1020 cm−3, about one order of magnitude lower than that of conventional oxide semiconductors doped with an appropriate donor, such as Al-doped ZnO, Sbdoped SnO2, Sn-doped In2O3, and Nb-doped TiO2.15 As a result, lowest electrical resistivity of anatase TaON is ~10-2 Ω cm, which is rather high value for applications requiring good electrical conductivity like a transparent elec-
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trode. Thus it is important to develop a carrier doping method for anatase TaON before it can be widely used in electronics applications. In this study, we focused on soft chemical Li insertion into anatase TaON epitaxial thin films by using an nbutyllithium solution (Figure 1).16 The soft chemical Li insertion reaction of TaON is represented by following equations. xC4H9Li + Ta5+ON LixTa(5-x)+ON + xC4H9•
(1)
2C4H9• C8H18
(2)
This reaction is applicable to anatase crystals such as TiO2 (refs. 16,18) and Mg0.05Ta0.95O1.1N0.7,19 and the inserted Li occupying the interstitial sites works as a good electron donor in anatase TiO2.20 Thus, soft chemical Li insertion is expected as a feasible way for carrier doping for anatase TaON.
Figure 1. Schematic illustration of soft chemical Li insertion into anatase TaON. Crystal structure was drawn by VESTA.17 Crystal structure of the thermodynamically most stable β-TaON are presented in Supporting Fig. S1. Experimental Preparation of Anatase TaON Epitaxial Thin Films. As precursors, (001)-oriented undoped anatase TaON epitaxial thin films were fabricated on the (100) plane of (LaAlO3)0.7-(SrAl0.5Ta0.5O3)0.3 (LSAT) substrates by using nitrogen-plasma-assisted pulsed laser deposition (NPAPLD). Details of the growth conditions are reported elsewhere.7 Briefly, a ceramic pellet of Ta2O5 was ablated by a KrF excimer laser. The deposition was conducted under a partial N2 gas pressure of 1×10−5 Torr, where the supplied N2 gas was activated by a radio-frequency (RF) wave plasma source with an input power of 200 W or 250 W. The substrate temperature was set at 750 °C unless otherwise noted. The oxygen and nitrogen contents of the TaON thin films grown under these conditions were almost stoichiometric within the experimental error of ∼10%, which was confirmed by a scanning electron microscope equipped with an energy dispersive X-ray spectroscope and nuclear reaction analysis using a 15N(p, αγ)12C resonance reaction at 898 keV.7 The film thicknesses were 40–60 nm with experimental error of ~10%, measured by a stylus profiler (Veeco, Dektak 6M).
Soft Chemical Li Insertion. The produced anatase TaON epitaxial thin films were soaked in an nbutyllithium/hexane solution (1.6 M, 1.6×10−2 M, or 1.6×10−4 M) at 60 °C for 6 h in a glove box filled with N2 gas. The solution temperature and the reaction time were determined based on the results of pilot experiments (Supporting Note). After the reaction, each film was washed with hexane. Because the obtained films were sensitive to air, they were covered with a laminate film or vacuum grease in a glove box, except for the samples whose surface morphology was investigated. Characterization of Li-inserted Anatase TaON. The crystal structure and surface morphology of the Li-doped anatase TaON films were examined by X-ray diffraction (XRD) using Cu-Kα radiation on a four-axis diffractometer (Bruker, discover d8) and by atomic force microscopy (AFM) (SII instruments, SPI4000 with SPA400), respectively. The Li content in the films was determined by secondary ion mass spectrometry (SIMS) or ΔE–E telescope elastic recoil detection analysis (ERDA). In the SIMS analysis, we used Li-ion-implanted anatase TaON thin films as standards for quantitation. The experimental error in Li concentration was ±20%. ERDA was performed with a 38.4-MeV 35Cl beam generated by a 5-MV tandem accelerator (Micro Analysis Laboratory, The University of Tokyo [MALT]).21 The electrical resistivity (ρ), carrier electron density (ne), and Hall mobility (μH) of the films were determined by the van der Pauw method using Au/Ti electrodes. ρ and ne include experimental error of ~10% mainly due to the error in film thickness. Results and Discussion The Li insertion into the anatase TaON epitaxial thin films through the reaction with n-butyllithium solution was confirmed by using SIMS (Fig. 2). Although the Li distribution was not completely homogeneous through the depth (Fig. 2, inset), we could tune the average Li concentration nLi from ~2×1016 cm−3 to ~3×1020 cm−3 by changing the concentration of the n-butyllithium solution. Figure 3a shows θ–2θ XRD patterns of an anatase TaON thin film before and after the reaction with a 1.6 M n-butyllithium solution. Both diffractograms show a 004 peak of anatase TaON without any impurity phases. Notably, the reaction caused the out-of-plane lattice parameter of the film to increase from 10.262±0.003 to 10.272±0.003 Å (+0.10% expansion), as calculated from the 004 diffraction peak (Fig. 3a, inset), where the experimental error was estimated from the standard deviation of the angle observed for 002 diffraction of the LSAT substrate (±0.011°). Similar volume expansion has been reported for Li-inserted anatase TiO2 (refs. 20, 22) and anatase Mg0.05Ta0.95O1.15N0.85,19 so we infer that the reaction topotactically inserted Li into the interstitial sites of anatase TaON. In our Li-doped anatase TaON thin film, the c/a ratio increased with Li insertion, while it decreases in anatase TiO2 and Mg0.05Ta0.95O1.15N0.85 in powders and polycrystalline thin films,19,20,22 probably due to the restriction of the in-plane lattice constant of the epitaxial
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ACS Applied Nano Materials thin film. AFM revealed no precipitation or etch pit in the anatase TaON after the reaction (Fig. 3b and 3c).
Figure 2. Average Li concentrations nLi in anatase TaON thin films reacted with n-butyllithium solutions. The circles and square represent the Li-inserted films produced from a pristine film grown at 750 °C and 800 °C, respectively. The nLi −2 values of the films reacted with 1.6×10 M n-butyllithium solution were evaluated two times, and averaged values were plotted. Result of each measurement was provided in Supporting Table S1. Inset is a SIMS depth profile of the anatase TaON film reacted with the 1.6 M n-butyllithium solution. The large signal at the surface is an artifact. nLi was calculated by averaging the depth profile except for the regions within 5 nm of the surface or interface.
Insertion of Li drastically enhanced the electrical conductivity of the anatase TaON thin films. Figure 4 summarizes the ρ, ne, and μH of the Li-inserted anatase TaON thin films reacted with n-butyllithium solutions of various concentrations. The pristine TaON film showed a ρ of 4.0×102 Ω cm, and its ne could not be measured because of its high resistivity. The film reacted with an nbutyllithium solution of 1.6×10−4 M showed a ne on the order of 1018 cm−3, which is higher than the nLi of the film, ~1016 cm−3, suggesting that some anion vacancies were introduced during the reaction with n-butyllithium. As the concentration of n-butyllithium increased, ne monotonically increased to 3.5×1020 cm−3, which is more than three times larger than that achieved in anion-deficient TaON. This ne is comparable to nLi, indicating that interstitial Li worked as a good electron donor. The maximum μH, 18.3 cm2 V−1 s−1 appeared in the film prepared with the 1.6×10−2 M n-butyllithium solution (nLi = 5.9×1019 cm−3) and decreased at higher nLi, probably due to carrier scattering from the high concentration of the Li dopant.
Figure 3. (a) θ–2θ XRD patterns of an anatase TaON thin film before and after the reaction with the 1.6 M nbutyllithium solution. Inset shows XRD patterns magnified near the 004 diffraction peak of anatase TaON. (b-e) AFM images of anatase TaON thin films grown at (b, c) 750 °C and (d, e) 800 °C, respectively: (b, d) the pristine TaON film and −2 (c, e) the film reacted with the 1.6×10 M n-butyllithium solution.
Notably, the maximum μH of the Li-inserted anatase TaON thin film (μH ~18 cm2 V−1 s−1) is double that of the anion-deficient thin film prepared by vacuum annealing of the TaON thin film (μH ~9 cm2 V−1 s−1). This result implies that interstitial Li disturbs electrical conduction in anatase TaON lattice less than anion vacancies do, possibly because the singly ionized interstitial Li induces weaker ionized impurity scattering23 than the doubly or triply charged anion vacancies. Because of its increased μH, the Li-inserted anatase TaON thin film showed lower ρ than the reduced anatase TaON.7 The electrical conductivity of the Li-inserted anatase TaON thin film was further increased by using a precursor film fabricated at 800 °C, whose grain size is larger than that fabricated at 750 °C (Fig. 3d and 3e), which should reduce grain boundary scattering. The ρ and μH of the TaON film fabricated at 800 °C reached 6.7×10−4 Ω cm and 34 cm2 V−1 s−1, respectively, at room temperature after the reaction with a 1.6×10−2 M n-butyllithium solution (Fig. 3). This film showed degenerate semiconducting behavior (dρ/dT > 0) and a μH as high as 63 cm2 V−1 s−1 at 10 K.
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using an n-butyllithium solution. The Li-inserted films exhibited higher ne and higher μH, and thus lower ρ, than anion-deficient anatase TaON films. These properties demonstrated that the inserted Li worked as a good electron donor with less effect on carrier conduction, although stability under air has to be improved for future applications such as transparent electrode, for example, by using an appropriate surface protection layer. The ρ and μH of the Li-inserted anatase TaON films reached 6.7×10−4 Ω cm and 34 cm2 V−1 s−1, respectively, at room temperature, comparable to conventional oxide semiconductors such as Nb-doped TiO2. Furthermore, we found that using topotactic soft chemical Li insertion could avoid unintended Li substitution for Ta accompanied with carrier compensation due to anion off-stoichiometry, which appeared in films deposited directly by physical vapor deposition. Although the doping chemistry for mixed-anion semiconductors, including oxynitrides, has not been established yet, our results suggest that soft chemical insertion of a dopant (e.g. Li+, H+, F−, etc.) might be used for carrier doping of mixed-anion semiconductors showing strong carrier compensation due to flexible anion off-stoichiometry. Figure 4. (a) Resistivity ρ, (b) carrier density ne, and (c) Hall mobility μH of the Li-inserted (solid symbols) and vacuum−7 annealed (
