Atmospheric plasma deposition of transparent semiconducting ZnO films on plastics in ambient air Makoto Watanabe a,b , Linying Cui a , Reinhold H. Dauskardt a,⇑ a Department Materials Science and Engineering, Stanford University, Stanford, CA 94305-4034, USA b High Temperature Materials Unit, National Institute for Materials Science, Ibaraki 305-0047, Japan article info Article history: Received 20 August 2013 Received in revised form 6 January 2014 Accepted 6 January 2014 Available online 18 January 2014 Keywords: Semiconducting oxides Transparent films Low temperature deposition Plastic substrate Atmospheric plasma deposition abstract Transparent zinc oxide (ZnO) thin films have been successfully synthesized on poly (methyl methacrylate) (PMMA), polycarbonate (PC), and polyethylene terephthalate (PET) substrates by atmospheric plasma deposition in ambient air at room temperature. The structural, optical and electrical properties of the ZnO films as well as their adhesion to the polymer substrates were investigated for various deposition conditions. The film surface exhibited a dome-shaped topography comprised of nanometer-sized grains. The size of both the domes and the grains became larger as the plasma power increased. The visible transmittance increased above 95% with decreasing plasma power. The resistivity exhibited a wide variation in the range of 10 2 –10 8 ohm cm. The adhesion energies to PMMA varied from 0.2 to 1.5 J/m 2 with increasing plasma power. While a finer grain struc- ture achieved with lower plasma power was preferable for higher transmittance, it resulted in lower adhesion to the plastic substrates. The study demonstrated the feasibility of depositing semiconducting transparent ZnO films on polymer substrates at low tempera- ture in ambient air using atmospheric plasma deposition. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO) has attracted considerable attention as a transparent conducting oxide and one of the candidate materials for indium tin oxide (ITO) replacement due to its high visible transmittance, low electrical resistivity with metal-dopants, and abundant Zn-based mineral sources [1,2]. While ITO is the most widely used conduct- ing oxide, low temperature deposition severely deterio- rates the quality of the films [3]. Low temperature deposition is particularly desirable for deposition on plas- tic substrates which is required for the fabrication of flex- ible and roll-to-roll polymer electronics. While many studies of ZnO deposition on polymer sub- strates have been, all of the procedures require vacuum/in- ert gas process environments [3–7] or wet-chemical approaches [8,9]. For example, Al-doped ZnO films were deposited on polyisocyanate (PI) or poly carbonate (PC) substrates using radio frequency (RF) magnetron sputter- ing in argon [4,5]. The films exhibited an optical transmit- tance higher than 80% and a resistivity in the range of 4.1  10 3 to 9.7  10 4 ohm cm. ZnO films have also been deposited onto polyethylene terephthalate (PET) sub- strates using a wet-chemical process and conditions for preparation of ZnO crystalline films in aqueous solutions were investigated at near ambient conditions [8]. Direct- current (DC) magnetron sputtering in argon was also ap- plied to deposit ZnO or Al-doped ZnO onto PET substrates, and the optical and electrical properties have been re- ported [3,6]. Finally, ZnO films have been deposited on polyimide (PI) and polytetrafluoroethylene (PTFE) sub- strates by pulsed laser deposition in vacuum at room tem- perature with an optical transmittance above 80% and an 1566-1199/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2014.01.007 ⇑ Corresponding author. Address: Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Durand Bldg., Rm. 121, Stanford, CA 94305-4034, USA. Tel.: +1 650 725 0679; fax: +1 650 725 4034. E-mail address: dauskardt@stanford.edu (R.H. Dauskardt). Organic Electronics 15 (2014) 775–784 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel