Thin Solid Films 517 (2009) 3106–3109 Contents lists available at ScienceDirect Thin Solid Films j o u r n a l h o m e p a g e : w w w e l s ev i e r c o m / l o c a t e / t s f Fabrication of TiO2-based transparent conducting oxide on glass and polyimide substrates T Hitosugi a,b,⁎, N Yamada b, N.L.H Hoang c, J Kasai b, S Nakao b, T Shimada b,c, T Hasegawa b,c a b c Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Japan Kanagawa Academy of Science and Technology (KAST), Kanagawa, Japan Department of Chemistry, University of Tokyo, Tokyo, Japan a r t i c l e i n f o Available online 20 November 2008 Keywords: Titanium oxide Transparent conducting oxide Sputtering Titanium dioxide a b s t r a c t We report on preparation and properties of anatase Nb-doped TiO2 transparent conducting oxide films on glass and polyimide substrates Amorphous Ti0.96Nb0.04O2 films were deposited at room temperature by using sputtering, and were then crystallized through annealing under reducing atmosphere Use of a seed layer substantially improved the crystallinity and resistivity (ρ) of the films We attained ρ = 9.2 × 10− Ω cm and transmittance of ~ 70% in the visible region on glass by annealing at 300 °C in vacuum The minimum ρ of 7.0 × 10− Ω cm was obtained by 400 °C annealing in pure H2 © 2008 Elsevier B.V All rights reserved Introduction Transparent conducting oxides (TCOs) are materials realizing high optical transmittance and high electrical conductivity at the same time They are indispensable in devices that require electrical contact and optical access, such as flat panel displays (FPDs), light-emitting diodes (LEDs), and solar cells [1,2] Currently, Sn-doped indium oxide (ITO) is the most widely used TCO, because of its excellent transparent conducting properties [3] and the ease of film growth However, rapid progress in opto-electronic devices requires TCOs with additional characteristics For example, the emission intensity of GaN-based LEDs is expected to be raised by using a TCO with a high refractive index In solar cell applications, TCOs with higher infrared transparency are desired in order to elevate the energy conversion efficiency These situations have motivated us to develop alternative TCOs with unique properties unattainable from existing TCO materials, such as ITO, ZnO and SnO2 [4] Recently, we have reported on pulsed laser deposition (PLD) growth of anatase Ti1−xNbxO2 (TNO) transparent conductor [5,6] This material is characterized by a wide band gap (3.2 eV) [7] and relatively low effective mass ~ m0 (m0: free electron mass) [8], and shows electrical and optical properties comparable to those of ITO Moreover, TNO exhibits other remarkable features, i.e., high refractive index, high transmittance in the infrared region, and high chemical stability in a reducing atmosphere The report on the anatase TNO has ⁎ Corresponding author Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Japan E-mail address: hitosugi@wpi-aimr.tohoku.ac.jp (T Hitosugi) 0040-6090/$ – see front matter © 2008 Elsevier B.V All rights reserved doi:10.1016/j.tsf.2008.11.090 stimulated studies on growth, mechanism and application of this TCO, [9–13] In this paper, we present the fabrication of TNO polycrystalline films on glass and plastic (polyimide) substrates Amorphous thin films were deposited at first and then annealed to obtain transparent conductive TNO films In order to achieve low temperature processing not exceeding 300 °C and high electrical conductivity, we used a seed layer, from which nucleation was initiated during the annealing Experimental details Sputter-deposited amorphous films deposited on unheated nonalkali glass (Corning 1737) or polyimide plastic substrates were crystallized to obtain transparent conductive TNO films [14,15] The temperature of the unheated substrate was in a range of 70–80 °C during deposition A sintered Ti1 − xNbxO2 − δ (x = 0.037 or 0.06) disks (diameter: in.), annealed in reducing atmosphere in order to introduce oxygen vacancies, were used as a target The base pressure of deposition chamber was maintained at ~5 × 10− Pa Deposition was conducted in a mixture of Ar and O2 with various ratios f(O2) = [O2/(Ar + O2)] under a total pressure of 1.0 Pa The RF power (13.56 MHz) applied to the target was kept constant at 120 W during sputtering Before the film deposition, the target surface was sputter-cleaned by pure Ar for 10 in order to remove surface oxide layers and contamination, and was subsequently pre-sputtered for under the same conditions as for film growth The as-deposited amorphous films were annealed in a rapid thermal annealing furnace, where the annealing temperature was raised at a rate of 100 °C/min Deposition condition and annealing conditions are summarized in Table Carrier transport properties were measured using the standard Hall T Hitosugi et al / Thin Solid Films 517 (2009) 3106–3109 3107 Table Summary of deposition and annealing conditions and resistivity of the samples Results and discussions Sample Composition Substrate number Ti1− xNbxO2 Fig 1(a) schematically shows the structure of the presently fabricated TNO films A seed layer with a thickness of 30 nm was first deposited on the unheated substrate at f(O2) = 5%, and, subsequently, a main film layer was grown at f(O2) = 0.05% Fig 1(b) is a plot of the resistivity (ρ) of the annealed TNO films against annealing temperature These films were subjected to 60 annealing in pure H2 (1 × 105 Pa) or vacuum (3 × 10− Pa) and were confirmed to be in the anatase polycrystalline phase, except for the film prepared at 250 °C, from X-ray diffraction measurements The film annealed at 250 °C was still amorphous, and thus, we determined the optimal annealing temperature to be 300–400 °C The single–layer film without seed layer does not crystallize when annealed at 300 °C, f (O2) Annealing conditions (Target) (Main film) (Temperature) (Film) (Seed layer) (Atmosphere) x = 0.037 x = 0.04 x = 0.037 x = 0.04 x = 0.037 x = 0.04 x = 0.06 – x = 0.06 – x = 0.037 – non-alkaline glass non-alkaline glass non-alkaline glass non-alkaline glass non-alkaline glass polyimide 0.05% 300 °C 5% Vacuum 0.05% 400 °C 5% H2 0.05% 400 °C no seed layer H2 0.05% 400 °C 5% Vacuum 0.05% 400 °C no seed layer Vacuum 0.05% 300 °C 5% H2 Resistivity 9.2 × 10− Ω cm 7.0 × 10− Ω cm 9.8 × 10− Ω cm 6.4 × 10− Ω cm 7.6 × 10− Ω cm 1.9 × 10− Ω cm bar geometry The Nb content x of the film sputtered using x = 0.037 target was determined to be x = 0.040 ± 0.001 by Rutherford backscattering spectrometry (RBS) The value of x tends to show slightly larger than the composition of the target, and composition difference between seed and top layers was not detected Structural properties were characterized by X-ray diffraction and cross-sectional transmission electron microscopy (TEM) Fig (a) Schematic structure of double-layer film (b) Resistivity as a function of annealing temperature Fig Temperature dependence of (a) resistivity, (b) carrier density, (c) Hall mobility of anatase Ti0.96Nb0.04O2 polycrystalline film on glass substrate (sample #2) 3108 T Hitosugi et al / Thin Solid Films 517 (2009) 3106–3109 dopants are substituted for Ti sites in anatase TiO2 without segregation High carrier activation efficiency is a unique characteristic of TNO in both epitaxial and polycrystalline films, in sharp contrast to those of ITO, typically b50% [16] The μH increases with decreasing temperature, implying that the room temperature ρ is dominated by phonon scattering In other words, grain boundary scattering is not a dominant factor in determining ρ Transmittance (Tr) and reflectance (R) spectra of a TNO film after annealing (thickness ~ 200 nm) are shown in Fig 3(a) The Tr values in a wavelength region of 400–800 nm are 60–80%, while R ranges from 10 to 40% The large R is due to the relatively high refractive index of anatase TNO, approximately 2.4 at 500 nm The absorbance (A) in the visible region, evaluated from the formula A = 1−(Tr + R), is as low as 10%, indicating excellent transparency of the present TNO films (Fig 3(b)) By using the above-mentioned low temperature process, we fabricated TNO films on plastic film substrate Fig 4(a) is an X-ray diffraction pattern of a TNO film deposited on polyimide (sample #6), clearly indicating an anatase (101) peak Fig 4(b) compares transport properties between TNO films deposited on polyimide and glass The Fig (a) Transmittance, reflectance, and (b) absorbance, of double-layer Ti0.94Nb0.04O2 polycrystalline film on a glass substrate (sample #2) showing the importance of seed layer As the f(O2) increases, crystallization temperature is reduced (not shown in figure), that is, crystallization temperature of seed layer is lower than that of main layer We speculate that when the seed layer crystallizes at 300 °C, the main film is crystallized due to the initiation from seed layer However, it is still unclear about the origin of the link between crystallization temperature and oxygen stoichiometry Notably, the values of vacuum-annealed films are almost identical to those of the H2-annealed ones The TNO film annealed at 300 °C in vacuum showed ρ as low as 9.2 × 10− Ωcm (sample #1, Table 1) The lowest resistivity ρ = 7.0 × 10− Ωcm (carrier density ne = 1.2 × 1021 cm− and Hall mobility μH = cm2 V− s− 1) was obtained by annealing at 400 °C in pure H2 (sample #2), while as-grown films showed × 101 Ωcm at room temperature Without seed layer, an annealed single-layer film (f(O2) = 0.05%) exhibited 9.8 × 10− Ωcm (sample #3), which is larger than the value of the film with seed layer Reason of this improvement in ρ is still in debate By applying a similar double-layer structure to Ti0.94Nb0.06O2, we attained ρ = 6.4 × 10− Ωcm (sample #4), while the film without seed layer exhibited 7.6 × 10− Ωcm (sample #5) These films show metallic temperature dependence of electron transport properties at low temperature Fig 2(a), (b) and (c) show ρ, ne, and μH, respectively, as functions of temperature (T) for the doublelayer Ti0.96Nb0.04O2 film prepared by 400 °C annealing in H2 atmosphere (sample #2) The ρ–T curve shows a metallic temperature dependence, dρ/dT N 0, and ne is almost independent of temperature, clearly indicating that the present polycrystalline TNO film can be regarded as a degenerated semiconductor From the ne value of ~ 1.2 × 1021 cm− 3, it is estimated that doped Nb atoms are activated with an efficiency of N90% This result strongly suggests that the Nb Fig (a) X-ray diffraction pattern of anatase Ti0.96Nb0.04O2 thin film on polyimide film “A(101)” denotes anatase (101) peak Amorphous film on polyimide was annealed in H2 atmosphere (1 × 105 Pa) at 300 °C (sample #6) (b) Comparison of transport properties between films on non-alkaline glass (sample #2) and on polyimide (sample #6) T Hitosugi et al / Thin Solid Films 517 (2009) 3106–3109 former exhibits slightly higher ρ of 1.9 × 10− Ωcm, reflecting lower ne and lower μH This might be due to surface roughness of the polyimide substrate and contamination from polyimide Conclusion We have established a low temperature preparation procedure (~300 °C) using seed layers for transparent-conducting anatase Nbdoped TiO2 (TNO) polycrystalline films on glass and polyimide substrates We achieved the lowest resistivity of ρ = 6.4 × 10− Ωcm and excellent optical transparency (transmittance ~70%, absorption b 10%) in the visible region on glass The optimal TNO film showed high carrier activation efficiency of N90% and metallic temperature dependence of ρ These results highlight anatase TNO as a promising candidate for nextgeneration TCOs Acknowledgment This work was supported by the Global COE Program for Chemistry Innovation, NEDO, MEXT Elements Science and Technology Project, and Grain-in-Aid for Young Scientists (B) 19760475, 2007 3109 References [1] D.S Ginley, C Bright, Mater Res Bull 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Films 517 (20 09) 3106–3109 3107 Table Summary of deposition and annealing conditions and resistivity of the samples Results and discussions Sample Composition Substrate number Ti1− xNbxO2 Fig 1(a)... polyimide Conclusion We have established a low temperature preparation procedure (~300 °C) using seed layers for transparent- conducting anatase Nbdoped TiO2 (TNO) polycrystalline films on glass and polyimide. .. – non-alkaline glass non-alkaline glass non-alkaline glass non-alkaline glass non-alkaline glass polyimide 0.05% 300 °C 5% Vacuum 0.05% 400 °C 5% H2 0.05% 400 °C no seed layer H2 0.05% 400 °C