Gallium zinc oxide coated polymeric substrates for optoelectronic applications

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Gallium zinc oxide coated polymeric substrates for optoelectronic applications

E. Fortunato, A. Gonçalves, A. Marques, V. Assunção, I. Ferreira, H. Águas, L. Pereira, R. Martins Materials Science Department/CENIMAT, Faculty of Sciences and Technology of New University of Lisbon, Campus da Caparica, 2829-516 Caparica, Portugal. ABSTRACT Highly transparent and conductive ZnO:Ga thin films were produced by rf magnetron sputtering at room temperature on polyethylene naphthalate substrates. The films present a good electrical and optical stability, surface uniformity and a very good adhesion to the polymeric substrates. The lowest resistivity obtained was 5×10-4 Ωcm with a sheet resistance of 15 Ω/sqr and an average optical transmittance in the visible part of the spectra of 80 %. It was also shown that by passivating the polymeric surface with a thin SiO2 layer, the electrical and structural properties of the films are improved nearly by a factor of 2.

INTRODUCTION Zinc oxide doped thin films have generated much attention in recent years, because of their material low cost, relatively low deposition temperature, non-toxicity and stability in hydrogen plasma, compared with indium tin oxide (ITO) and tin oxide (SnO2). It has been recently obtained by the present authors the best optoelectronic results on gallium zinc oxide (GZO) doped thin films at room temperature by rf magnetron sputtering on glass substrates [1]. However, when flexible devices are required, a polymeric substrate must be used. Nevertheless these substrates present certain challenges such as considerably lower working temperature and rougher surfaces as compared to glass substrates. In order to overcome these limitations we have optimised the r.f. magnetron sputtering process to be able to produce highly transparent and conductive GZO films exhibiting a low film stress and with a smooth surface [2, 3]. The films were grown by rf magnetron sputtering at room temperature on commercially available polyethylene naphthalate (PEN) substrates. PEN is a biaxially oriented film with high stiffness and mechanical strength, low thermal shrinkage, and high chemical resistance. In this paper a detailed description of the preparation conditions as well as on the electrical (Hall effect and resistivity as a function of temperature), optical (transmittance), structural (X-ray diffraction) and morphological (FE-SEM) properties will be presented.

EXPEIMENTAL DETAILS The GZO thin films with different thickness were deposited on PEN substrates with a thickness of 75 µm, supplied by DuPont (Kaladex®1020). Prior to the deposition of the GZO films the polymeric substrates were ultrasonically cleaned in isopropyl alcohol. The GZO films were deposited by rf (13.56 MHz) magnetron sputtering using a ceramic oxide target ZnO/Ga2O3 (95:5 wt%; 5 cm diameter from SCM, Tallman, NY) with a purity of 99.99 %. The sputtering

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