Fabrication of TiO 2 -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 Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai, Japan b Kanagawa Academy of Science and Technology (KAST), Kanagawa, Japan c Department of Chemistry, University of Tokyo, Tokyo, Japan abstract article info Available online 20 November 2008 Keywords: Titanium oxide Transparent conducting oxide Sputtering Titanium dioxide We report on preparation and properties of anatase Nb-doped TiO 2 transparent conducting oxide films on glass and polyimide substrates. Amorphous Ti 0.96 Nb 0.04 O 2 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 - 4 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 - 4 Ω cm was obtained by 400 °C annealing in pure H 2 . © 2008 Elsevier B.V. All rights reserved. 1. 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 SnO 2 [4]. Recently, we have reported on pulsed laser deposition (PLD) growth of anatase Ti 1-x Nb x O 2 (TNO) transparent conductor [5,6]. This material is characterized by a wide band gap (3.2 eV) [7] and relatively low effective mass ~ m 0 (m 0 : 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 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. 2. Experimental details Sputter-deposited amorphous films deposited on unheated non- alkali 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 Ti 1 - x Nb x O 2 - δ (x =0.037 or 0.06) disks (diameter: 2 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 - 5 Pa. Deposition was conducted in a mixture of Ar and O 2 with various ratios f(O 2 )=[O 2 /(Ar + O 2 )] 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 min in order to remove surface oxide layers and contamination, and was subsequently pre-sputtered for 5 min 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 1 . Carrier transport properties were measured using the standard Hall Thin Solid Films 517 (2009) 3106–3109 ⁎ 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 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf