An n-Type Transparent Conducting Oxide: Nb12

Jul 21, 2011 - Internet at http://pubs.acs.org. 'AUTHOR INFORMATION .... Law, D. S. L.; Vittadini, A.; Casarin, M.Phys. Rev. B 2000,61, 13445–13457...
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An n-Type Transparent Conducting Oxide: Nb12O29 Takeo Ohsawa,*,† Junpei Okubo,† Tohru Suzuki,† Hiroshi Kumigashira,‡,§,|| Masaharu Oshima,‡,||,^ and Taro Hitosugi†,§ †

WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Department of Applied Chemistry, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan Synchrotron Radiation Research Organization, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan ^ Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, Bunkyo, Tokyo 113-8656, Japan

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bS Supporting Information ABSTRACT: A Nb12O29 film is found to show transparent conducting properties with lowest resistivity of 3.3  10 3 Ω 3 cm at room temperature. High transmittance (∼70% at wavelength of 435 nm and ∼50% at wavelength of 730 nm, 120 nm in thickness) in the visible is maintained with an optical band gap of ∼4.0 eV. The optical dielectric constant and effective mass were both larger than those for conventional transparent conducting oxides, leading Nb12O29 to exhibit high transparency. The n-type carrier density and Hall mobility at room temperature were 1.8  1021 cm 3 and 0.8 cm2 V 1 s 1, respectively. The origin of the charge carriers lies in the mixed-valent character of Nb, as revealed by photoemission measurements. This intrinsically doped d-electron-based transparent conducting oxide shows unique characteristics such as high refractive index and high-temperature stability, that is, maintaining high conductivity after annealing at 1000 C in vacuum.

’ INTRODUCTION The past decade has witnessed a strong industrial demand for transparent conducting oxides (TCOs), which are used as core materials in a wide range of opto-electronic devices, such as flat panel displays, light-emitting diodes (LEDs), touch panels, and solar cells.1 To date, the TCOs used for these devices have been limited to Sn-doped In2O3 (ITO),2 5 F-doped SnO2 (FTO), and Al-doped ZnO (AZO)6,7 systems (conventional TCOs) that show both high electrical conductivity and high transmittance in the visible. To enhance device performance, development of new TCOs is of crucial importance; for example, a TCO with a suitable work function and refractive index would improve the efficiency of solar cells and GaN-based LEDs, respectively.8,9 While materials exploration for TCOs has long been focused on compounds with s-electron conduction, we have reported a new type of d-electron-based TCO, a Nb-doped anatase TiO2 film (TNO), exhibiting low resistivity (2 3  10 4 Ωcm) and high transparency (97% in the visible region).10 This material has different properties compared to conventional TCOs whose conduction mechanisms originate from their s-electron character. Such d-electron systems are promising as alternative TCOs, and further exhibit unique character, such as high refractive index, stability against chemicals, and transparency in infrared, which would lead to an increase of efficiency for opto-electronic r 2011 American Chemical Society

materials. All of the TCOs reported to date, regardless of their s- or d-electron conduction characteristics, are wide-bandgap semiconductors either extrinsically doped or having a nonstoichiometric composition to introduce carriers into the crystals. Further, the development of indium-free TCOs has become crucial, aiming for a sustainable society.11 In this article, we report on the transparent conducting properties of a Nb12O29 film on a glass substrate for use as a new TCO. Remarkably, it was found that this material shows transparent conducting properties without any chemical doping, which is quite different from conventional transparent conductors, and thus establishes a new class of TCOs: intrinsically doped d-electron-based TCOs. Cava et al. first reported on the magnetism and metallic conductivity of Nb12O29,12,13 which is a rare example of a material that simultaneously exhibits long-range magnetic order, antiferromagnetism, and metallic conductivity. From the viewpoint of charge localization, the formula Nb12O29 can be written as Nb24+ Nb105+O29, where the electrons from two Nb4+(4d1) orbitals per block of 12 NbO6 octahedra are responsible for the Received: April 1, 2011 Revised: July 21, 2011 Published: July 21, 2011 16625

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The Journal of Physical Chemistry C properties mentioned above. A model in which one electron per formula unit is localized at a specific Nb4+ site, and the other electron is delocalized, has been proposed to account for the metallic conductivity and antiferromagnetic ordering with a Neel temperature of 12 K.12 16 The NbOx crystallographic shear structures contain intersecting octahedral planes whose shear edges lead to a relatively short Nb Nb separation, suitable for direct metal orbital overlap,17 19 leading to a Nb d orbital

Figure 1. (a) Deposition rate (rdepo) and (b) resistivity (F) as a function of partial oxygen ratio, P(O2), during NbOx deposition by dc magnetron sputtering. (c) Temperature dependence of F for the metallic film.

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dominated conduction band.15 Further, R€uscher et al. reported on the optical properties on Nb12O29, but they did not point out the transparency in the visible.20 We here demonstrate that this material is a transparent conductor in thin film form, and discuss the origin of its transparent conducting properties.

’ EXPERIMENTAL METHODS All NbOx thin films were deposited on silica glass substrates at room temperature (RT) by the dc magnetron off-axis sputtering method. We used a Nb metal disk (2 in. in diameter) as the sputtering target, with a background pressure that was typically lower than ∼5  10 5 Pa. Sputtering was performed in a mixed Ar/O2 atmosphere at various partial oxygen ratios, P(O2), defined as F(O2)/(F(Ar) + F(O2)), where F(O2) and F(Ar) are the flow rates of oxygen and Ar gas, respectively, under a constant total pressure of 1.0 Pa. A dc power of 50 W was applied to the target during all sputtering process, and substrates were rotated during deposition to produce films with uniform thicknesses (∼100 nm in thickness). After deposition, the films were annealed at 1000 C for 10 min in vacuum (∼1  10 4 Pa) to crystallize the Nb12O29. The electrical resistivity (F) and Hall effects of the films were measured with a standard van der Pauw geometry at temperatures from 10 to 300 K. Optical properties were determined by UV visible IR spectrophotometry over the wavelength range of 300 2500 nm. Structural properties were characterized by high-resolution X-ray diffraction (XRD, Rigaku) and cross-sectional high-resolution transmission electron microscopy (HRTEM, Hitachi). We used the grazing incidence XRD (GIXRD) method that is typically used for the surface-sensitive characterization of solids. The composition of the metallic Nb12O29 film was analyzed by Rutherford backscattering spectroscopy (RBS). Synchrotron-radiation photoemission spectroscopy (SR-PES) was conducted at BL-2C, Photon Factory, Japan, at a photon energy of 800 eV, which has a probing depth of approximately 1 2 nm. The PES spectra were measured using a Scienta R-2002 analyzer, and the Fermi energy (EF) was calibrated using a gold spectrum. ’ RESULTS AND DISCUSSION As-deposited films, except for the film prepared at P(O2) = 0%, were highly insulating, with typical sheet resistances of

Figure 2. (a) Optical properties of as-deposited (insulating) and annealed (metallic) films deposited under the same conditions. Transmittance data for glass substrate is also shown. Sample photograph is shown in the inset. (b) Refractive index (left axis) and extinction coefficient (right axis) plotted as a function of wavelength. (Rhυ)1/2 against photon energy is shown in the inset. 16626

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The Journal of Physical Chemistry C several TΩ/0. These films were then subjected to vacuum annealing at 1000 C. Figure 1a,b respectively shows the deposition rate (rdepo) and resistivity (F) of the annealed films as a function of P(O2). Note that F for an as-deposited film at P(O2) = 0% is shown as a reference. In Figure 1a, the value of rdepo remained relatively constant at about 15 nm/min below a P(O2) of 6.0%, indicating a metallic mode of reactive sputtering method. With increasing P(O2), the Nb metal target became oxidized to form Nb oxide (NbOx), and rdepo decreased abruptly at approximately P(O2) = 6.5%, representing a rapid transition from the metallic to reactive mode.21 This decrease was accompanied by a sudden increase of F. This led to the surprising result that the lowest F of ∼3.3  10 3 Ω 3 cm was obtained at a critical P(O2) of 6.5%. As P(O2) was further increased, F generally increased rapidly, indicating a very narrow deposition window for fabricating a conducting NbOx film. These results indicate that the P(O2) value during deposition is one of the most important parameters for obtaining conducting NbOx films. Figure 1c shows the temperature (T) dependence of F for the lowest resistivity NbOx sample prepared at P(O2) = 6.5%. The F decreased with reducing temperature, indicating metallic conduction (dF/dT > 0) down to 30 K. In addition, the carrier density (ne) and Hall mobility (μH), determined by standard van der Pauw measurements, were 1.8  1021 cm 3 and 0.8 cm2 V 1 s 1 at RT, respectively, suggesting that many free electrons were present in the film, compared to the case for other TCOs reported to date.2 5 However, the F values for the conducting NbOx film were still 1 order of magnitude higher than those for conventional TCOs due to the low Hall mobility, throughout the entire temperature range. The resistivity ratio (F10K/F300K) plotted against T was maintained at ∼0.92 even at low temperatures, being several times larger than those of standard metals such as Au and Ag. This small temperature dependence suggests that the conduction electrons in NbO x are predominantly scattered by ionic impurities or disorders rather than by phonons. We show in Figure 2a the optical properties of as-deposited and annealed NbOx films deposited at P(O2) = 6.5%. Both films, with uniform thicknesses of 120 nm, have sufficiently high transmittance in the visible: approximately 50% for red light, and above 70% for blue light. The transmittance in the red/ infrared region decreased only for the annealed film, indicating the generation of free carriers in the film. Figure 2b shows the refractive index (n, left axis, red) and extinction coefficient (k, right axis, blue) of the conducting NbOx thin film as a function of wavelength, as measured by optical ellipsometry. At a wavelength of 400 nm, the value n = 2.2 is larger than that of ITO but smaller than that of TiO2 (approximately 2.5 at the same wavelength). The k value increased in the near-infrared region from 1000 to 1800 nm, clearly indicating free-carrier absorption resulting from a plasma wavelength, λp = 2πc(ε0εopt m*/nee2)1/2, which approaches the visible region; here c, ε0, εopt, m*, ne, and e represent the velocity of light, vacuum dielectric constant, optical dielectric constant, effective mass, carrier density, and elementary electric charge, respectively. In addition to broad free-carrier absorption, a weak absorption band is apparent around a wavelength of 900 nm. This absorption likely results from a d d transition and/or a charge-transfer transition originating from localized Nb4+ with 4d1 configuration.22 The inset of Figure 2b shows a linear extrapolation at the absorbance edge for the annealed film in the (Rhυ)1/2 versus photon energy plot, where R and hυ represent the absorption coefficient and photon energy, respectively. This

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Figure 3. (a) GIXRD scan of a metallic film with JCPDS data for Nb2O5 and Nb12O29 plotted below. (b) Cross-sectional HRTEM image of a Nb12O29 film on a glass substrate, taken along the [010] zone axis. Contrast patterns due to [3  2] and [2  3] bright columns can be seen. A number of defects in the form of minor [2  2] and [3  3] blocks are seen in the inset image.

plot yields an optical bandgap of ∼4.0 eV, indicating that the sample is a wide gap material. Next, we characterized the structural aspects of these NbOx thin films by means of XRD. Figure 3a shows GIXRD data for the metallic film (same sample as red curve in Figure 1c), taken at an incidence angle of 0.5. GIXRD is a surface-sensitive technique for characterizing the structure of solids, and has been widely employed for thin films.23 As shown in Figure 3a, several peaks appeared in addition to the broad background intensity from the glass substrate. This result suggests that this film is polycrystalline with a structure of Nb2O5,24 Nb12O2925 or a mixture of both compounds, but it is difficult to determine the crystal structure using GIXRD. Considering that Nb12O29 shows metallic conductivity with F = 4  10 3 Ωcm,12,13 in contrast to insulating Nb2O5,12,13,26,27 we speculated that this metallic film is primarily composed of Nb12O29. The composition analysis by RBS showed that the oxygen-to-niobium 16627

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Figure 4. Photoemission spectra of (a) Nb 3d core level, (b) valence band, and (c) near EF for as-deposited and annealed Nb12O29 films. A peak due to Nb4+ in addition to Nb5+ is clearly seen in the inset of panel a. Remarkably, the metallic Fermi edge is clearly observed only for the annealed film in panels b and c, indicating the metallic nature of the film, in accordance with the transport properties.

ratio was ∼2.4 ( 0.1 (not shown in figure), supporting that the material is Nb12O29. To gain more insight into the structure of this transparent conducting film, we characterized the sample using HRTEM. Figure 3b shows HRTEM images taken along the [010] zone axis (b-axis), for the conducting Nb12O29 film. Nb12O29 has two crystal structures: a monoclinic and an orthorhombic phase.17,19,26 28 Both polymorphs have a basic unit comprising a block of [4  3] corner-shared NbO6 octahedra. According to previous reports, atoms appear dark, and the channels between them in the ReO3type columns are seen as bright dots.16 18 As can be seen in the HRTEM image, a pattern of [3  2] bright columns appears in the ac-plane. The square arrangement of NbO6 octahedra in the [4  3] blocks of the Nb12O29 structure gives rise to a [3  2] square pattern of channels along the b-axis.17 19 Additionally, there are some slabs of an orthorhombic arrangement, as well as monoclinic Nb12O29.17 19,26 Thus, we concluded that this metallic Nb12O29 film consists of both monoclinic and orthorhombic forms. However, as shown in the inset of Figure 3b, a large number of defects were observed in the form of ReO3-type blocks with different orientations and block sizes, such as the [2  2] or [3  3] arrangements typical of Nb oxides.29 These defects may act as scattering centers, suggesting further reduction of F when higher-quality crystals are grown. It should be stressed here that this sample exhibits transparent conducting properties in the absence of chemical doping, which is strikingly different from the case for conventional TCOs whose carriers are mainly produced by extrinsic carrier doping into wide-bandgap semiconductors. What discriminates this material from normal metals or conventional TCOs is the presence of a moderate carrier density without loss of transparency. Nb12O29 contains two and four unpaired electrons per unit cell for the monoclinic and orthorhombic forms, respectively. One out of two for these electrons is delocalized and yields metallic conduction.14 16 From these results, assuming monoclinic Nb12O29, the estimated value of ne is 1.7  1021 cm 3, which is in good agreement with the measured value of 1.8  1021 cm 3. This appropriate ne value, along with the band gap close to 4.0 eV, is the key to obtaining transparent conducting properties. The largest advantage of this material over the conventional TCO is the thermal properties. This Nb12O29 does not lose high conductivity even after annealing at 1000 C in vacuum. This is

quite surprising since other TCOs, such as ITO, FTO, AZO, and TNO cannot resist up to this high temperature. A TiO2 system maintains its conductivity after annealing at 700 C, at most, and ITO is far weaker to vacuum annealing. In the cation-doped system, dopants or oxygen vacancies form defect complexes, and lead to the reduction of conductivity.30 Since Nb12O29 TCO is intrinsically doped, chemical stability may be higher than conventional TCOs. Other important factors governing the optical transparency of TCOs are their εopt and m*opt values. In general, λp should be located in the infrared, far from the visible region, to avoid free carrier absorption in the visible. A large ne, as is observed in Nb12O29, leads to a decrease in λp that will induce absorption in the visible. In order to compensate for this and recover transparency, εopt and m*opt need to have large values to push λp into the infrared. To confirm this, we estimated εopt and m*opt from the optical properties shown in Figure 2b. According to Drude k2 = εopt [(nee2/ theory, n and k are described as n2 2 εom*opt) 3 (1/ω )], where εopt and m*opt represent the optical dielectric constant and effective mass, respectively. Using the n and k values from Figure 2b, εopt and m*opt can be respectively estimated to be 4.5 and 2.33m0, where m0 indicates the electron mass (see Supporting Information). The value of m*opt for the Nb12O29 film is, indeed, larger than those for ITO5 or Nb-doped TiO2,31 thus resulting in a larger λp and a recovery of transparency in the visible. The λp estimated using εopt and m*opt was 2.6 μm for the conducting Nb12O29 film. We used SR-PES32,33 to investigate the electronic structure before and after annealing. Figure 4a shows Nb 3d core level spectra for as-deposited and annealed films prepared at P(O2) = 6.5%. Both spectra basically correspond to a simple spin orbit doublet with a Nb 3d5/2 binding energy of 207.8 eV. For the annealed film, a clear shoulder peak at the lower binding energy side of the Nb 3d5/2 line shape is visible. As shown in the inset of Figure 4a, an additional line shape at 206.4 eV can be assigned to Nb4+, which is 1.4 eV lower than that of the peak of Nb5+.34,35 This result supports the idea that delocalized electrons play a role in the metallic conduction for the annealed film. The peak area ratio, Nb4+/Nb5+, estimated from the fitted line shapes shown in the inset of Figure 4a was ∼0.092. This value is fairly consistent with the value of 0.083 calculated by assuming that one electron in a unit cell is delocalized to give rise to metallic conduction.12 14 16628

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The Journal of Physical Chemistry C Figure 4b,c shows the valence band spectra for the two films. In Figure 4b, the broad strong feature from 10 to 3 eV is due to energetically favorable hybridized bands with primarily Nb 4d and O 2p characters.36 Linear extrapolation of the leading edge of the valence band spectra to the energy axis reveals that both films exhibit a valence band maximum (VBM) of 3.4 3.6 eV relative to the Fermi energy (EF), consistent with the optical band gap estimated from Figure 2a. In addition, weak emissions were apparent within the wide gaps for both samples. Figure 4c shows the spectra near the EF, along with a gold spectrum as a reference. Although the density of states at the EF was not seen for the asdeposited film, the spectrum for the annealed conducting Nb12O29 film clearly exhibits a Fermi edge, which thus provides direct evidence for the conducting nature of the Nb12O29 film, quite consistent with the transport properties.

’ CONCLUSIONS In summary, a Nb12O29 thin film was found to be a d-electronbased transparent conductor in the absence of extrinsic chemical doping, indicating that this is a new class of TCO: an intrinsically doped TCO. This film maintains high conductivity even after annealing at 1000 C in vacuum, which is a quite considerable advantage over conventional transparent conductors. The origin of the TCO properties lies in the mixed-valent character of Nb, which acts as a source for carrier generation, analogous to a chemically doped material. An appropriate carrier density in Nb12O29, along with a suitable energy gap, induces metallic conduction together with transparency in the visible. The mechanism presented in this paper can be used as a guiding principle for discovering additional transparent conductors for use in highperformance, low-cost opto-electronic devices. ’ ASSOCIATED CONTENT

bS

Supporting Information. Optical properties of the Nb12O29 film. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address: WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Phone: +81-22-217-5948. FAX: +81-22-217-5943. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was supported by the World Premier Research Institute Initiative, promoted by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, for the Advanced Institute for Materials Research, Tohoku University, Japan. We acknowledge Prof. M. Miyazaki and Dr. S. Mizukami for electric measurements, Prof. M. Kawasaki for optical measurements, and MST, Japan, for TEM measurements. T.H. acknowledges financial support from NEDO, Japan. ’ REFERENCES (1) Ginley, D. S.; Bright, C. MRS Bull. 2000, 25, 15–18. (2) Hamberg, I.; Granqvist, C. G. J. Appl. Phys. 1986, 60, R123–R159. (3) Tahar, R. B. H.; Ban, T.; Ohya, Y.; Takahashi, Y. J. Appl. Phys. 1998, 83, 2631–2645.

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