The transparent conductor Nb12O29 was found to exhibit high-temperature stability superior to that of con-ventional transparent conducting oxides (TCOs). The Nb12O29 thin film was fabricated on a glass substrate,followed by annealing at 1000 °C in vacuum, and a high conductivity of 3.0×102 Ω−1 cm−1 at room temper-ature was maintained even after vacuum annealing at 800 °C. Although the conductivity of the Nb12O29 filmwas decreased by annealing at 300 °C in air, the initial high conductivity was recovered upon subsequentvacuum annealing. These properties indicate that Nb12O29 is a promising material applicable in caseswhere conventional TCOs cannot be used.
The increasing demand for functional oxides with superior con-ductivity, semiconductivity, magnetic and optical properties has trig-gered the rapid expansion of research on such materials in recentyears [1,2]. Among the functional materials, transparent conductingoxides (TCOs), which are key materials in flat-panel displays, solarcells and light-emitting diodes, have attracted considerable attentionin industrial and academic fields [3,4]. A crucial factor in applicationsrequiring TCOs is high-temperature chemical stability. In general,conventional TCOs, such as Sn-doped In2O3 (ITO), F-doped SnO2,Al-doped ZnO and Nb-doped TiO2 [5,6], are wide bandgap semicon-ductors which are either extrinsically doped or prepared with anoxygen-deficient composition in order to introduce carriers intotheir crystal lattices. Consequently, dopants are easily deactivated atelevated temperature due to carrier compensation or the formationof defect complexes, eventually resulting in lower conductivity. Inthe case of thin-film Si solar cells, the ITO electrodes do not withstandthe high-temperature process in reductive conditions, and thus, SnO2
is used as a transparent conductor. Furthermore, chemically stableTCOs at high temperature are required in fabrication processes, suchas for charge-coupled devices and charge-injection devices [7,8].
We have recently reported the d-electron-based transparent con-ductor Nb12O29 [9], which exhibits a unique transparent conductionmechanism. This material shows a moderate carrier density due tothe mixed-valence nature of Nb, along with large optical dielectric con-stant and effective mass, resulting in a larger plasma wavelength thatmaintains transparency in the visible part of the spectrum [10]. Thisma-terial is different from conventional TCOs in that its transparency and
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conductance are intrinsic, and thus an extrinsic chemical dopant isunnecessary. As a result, Nb12O29 is expected to show higher chemicalstability in comparison to doped TCOs.
Here, we describe the high-temperature stability of the trans-parent conductor Nb12O29, which is found to resist changes in con-ductivity at temperatures up to 1000 °C in vacuum annealing, whichis the highest operating temperature among all reported TCOs. Fur-thermore, we investigate the effect of annealing in air and show thatalthough the conductivity decreases upon heating, it can be recoveredby annealing in vacuum.
2. Experiments
A thin film of NbOx was deposited on a silica glass substrate by dcmagnetron off-axis sputtering at room temperature, after which thefilm was crystallized into Nb12O29 by post-deposition annealing for10 min in vacuum (~1×10−4 Pa). A 2-in. Nb metal target was usedfor the deposition, which was conducted at a power of 50 W. The de-tails of the preparation of the samples have been reported previously[9]. To investigate the high-temperature stability of this material,polycrystalline Nb12O29 films were annealed at 100–1100 °C eitherin vacuum or in ambient air. The electrical resistivity (ρ) of theNb12O29 films was measured with a standard vander Pauw geometryat room temperature, and their structural properties were character-ized by grazing incidence X-ray diffraction (XRD: Rigaku, SmartLab).
3. Results and discussion
First, we discuss the post-deposition annealing temperature de-pendence of ρ at room temperature. The as-deposited amorphousfilms showed pronouncedly insulating properties (ρ~106 Ω cm),and ρ after annealing at 600 and 800 °C in vacuum was still as high
Fig. 2. (a) Resistivity as a function of the annealing temperature in vacuum. Note that
219T. Ohsawa et al. / Thin Solid Films 526 (2012) 218–220
as 1.5 Ω cm, as shown in Fig. 1(a). However, annealing at 1000 °Cresulted in conducting films with ρ reaching 3.3 mΩ cm, which wasa decrease by eight orders of magnitude as compared with ρ of theas-deposited films. This high conductivity can be attributed todelocalized electrons in Nb12O29 [11–13]. Fig. 1(b) shows ρ plottedas a function of annealing time at 1000 °C. It was found that annealingfor 5 min is sufficient to obtain conducting Nb12O29 films, with thelowest ρ of 3.3 mΩ cm reached after annealing for 10 min.
With the aim to investigate their stability in terms of conduc-tivity against high-temperature annealing, the conducting crystallineNb12O29 films were annealed again in vacuum. Fig. 2 shows theannealing temperature dependence of ρ at room temperature. It is re-markable that Nb12O29 maintained high conductivity even afterannealing at temperatures up to 800 °C, with only a slight increasein ρ at 1000 °C. This is the strongest advantage of Nb12O29 as a trans-parent conductor since conventional TCOs cannot sustain high conduc-tivity after annealing at such high temperatures, which is discussedbelow.
At even higher temperatures, ρ abruptly increased after annealingat 1100 °C, indicating the upper limit of the annealing temperature formaintaining high conductivity. The sudden increase in ρ resulted froma structural transition from a Nb12O29 to a NbO2 phase. As shown inFig. 2(b) (top), a strong peak around 33.1° appeared in the XRD pat-tern of the insulating film with ρ of 1.1 Ωcm, which closely matchedthat of the insulating NbO2 phase (plotted as bars) [14].
Looking at the stability of Nb12O29 films against annealing in air, ρincreased only slightlywhen the temperaturewas increased to 200 °C,afterwhich it increased rapidly, reaching values eight orders ofmagni-tude higher after annealing at 300 °C (Fig. 3), which is close to the ini-tial value of ρ of the as-deposited amorphous NbOx film. There are nonotable differences between the XRD pattern of the insulating film(Fig. 2(b), middle) and that of highly conducting film (Fig. 2(b), bot-tom), and a most intriguing feature of the insulating film is that itshigh conductivity can be restored by annealing at 1000 °C in vacuum.This result implies that excess electrons delocalized in Nb12O29 arecompensated during annealing in an oxidizing atmosphere. Fig. 4shows the transmission spectra of as-prepared (conducting) andair-annealed (insulating) Nb12O29 films. The transmittance at a wave-length of 800 nm for the film annealed in air was indeed higher than
(a)
(b)
Fig. 1. Resistivity of as-deposited insulating films after annealing in vacuum as a func-tion of (a) annealing temperature and (b) annealing time.
the conductive Nb12O29 film is annealed. (b) X-ray diffraction patterns of Nb12O29 filmssubjected to annealing at 1000 °C (red), 300 °C in air (green) and 1100 °C (blue). Barsindicate diffraction peaks from JCPDS data for Nb12O29 (red) and NbO2 (blue), respec-tively. Annealing time was set to 10 min.
1000
~ ~~ ~
Fig. 3. Resistivity as a function of annealing temperature in air. Annealing time was setto 10 min.
Fig. 4. Transmittance spectra of conducting Nb12O29 film (blue) and the insulating filmannealed at 300 °C in air (red). Annealing time was set to 10 min.
220 T. Ohsawa et al. / Thin Solid Films 526 (2012) 218–220
that of the as-prepared film. This increase in transmittance indicatedthat the free carrier density had decreased, supporting the hypothesisthat carriers are compensated by annealing.
Finally, we compare the high-temperature stability of Nb12O29
with that of other TCOs and discuss the origin of the high-temperature stability of conductive Nb12O29 thin films. ITO generallyloses its transparency and conductivity at 500 °C in vacuum [15,16],as does doped SnO2 at almost the same temperature [17]. The con-ductivity of Nb-doped TiO2 TCO on glass has been reported to persistup to 600 °C [10]. Conventional TCOs exhibit low chemical stability athigh temperatures, owing to the presence of dopants; for example, Snatoms in ITO form defect complexes known as neutral tin–oxygencomplexes [18,19], The case of Nb-doped TiO2 TCO is similar, inwhich the dopant forms defect complexes due to binding betweenNb at Ti sites (NbTi) with adjacent oxygen vacancies or interstitials[20–22]. Thus, semiconductor TCOs with an intrinsically wide bandgapare advantageous over doped TCOs since the former do not containextrinsic dopants.
Furthermore, we conjecture that the thin film form is crucial forhigh-temperature stability. The conditions of vacuum annealing inour setup included a partial oxygen pressure (PO2) of 1×10−5 Pa, asevaluated with quadrupole mass spectrometer. According to the phasediagram for bulk Nb–O systems [23,24], bulk Nb12O29 (NbO2.417) trans-forms into Nb2O5 in a vacuum with PO2 of 1×10−5 Pa. In contrast, thestructure of thin-film Nb12O29 persists up to 1000 °C, which might bethe result of stress from the substrate or the disorder of a number ofdefects in the film [9].
4. Conclusions
Nb12O29 in the form of a thin film was found to show high-temperature stability in vacuum. Its conductivity of 3.0×102 Ω−1 cm−1
was maintained even after annealing at 800 °C in vacuum, with aslight decrease after annealing up to 1000 °C, which is exceedinglyhigh in comparison with the limits of high-temperature stability ofconventional TCOs. This stability is unique to thin-film Nb12O29,which is likely because it is an intrinsic wide bandgap semiconduc-tor. The high-temperature stability of thin-film Nb12O29 will furtherexpand the application of TCOs in emerging optoelectronic devices.
Acknowledgments
This study was supported by theWorld Premier Research InstituteInitiative, promoted by the Ministry of Education, Culture, Sports, Sci-ence, and Technology (MEXT), Japan, for the Advanced Institute forMaterials Research, Tohoku University, Japan. We thank T. Miyazakiand S. Mizukami for electrical measurements. T. H. acknowledges fi-nancial support from NEDO, Japan.
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