Growth of ZnO:Ga thin films at room temperature on polymeric substrates: thickness dependence
Introduction
Plastic materials are replacing glass substrates in a variety of applications due to its lower cost associated with its lighter weight. Everywhere we look there are touch-input devices ranging from personal digital assistants (PDAs) to automatic telling machines (ATMs) and point of sale (POS) cash registers [1]. The common feature of all these devices is the utilisation of a transparent conductive oxide (TCO) used as electrode. Indium tin oxide (ITO) has been the predominantly TCO used for a number of years mainly for the flat panel display industry [2]. Nevertheless, recently the utilisation of alternatively TCOs like zinc oxide doped thin films have generated much attention, because of their material low cost, relatively low deposition temperature, non-toxicity and its stability in hydrogen plasma. The authors present the recently best-known optoelectronic results concerning gallium-doped zinc oxide deposited on glass substrates at room temperature by r.f. magnetron sputtering [3]. 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 highly conductive GZO films exhibiting a low film stress and with a smooth surface [4], [5]. The films were grown by r.f. magnetron sputtering at room temperature on commercially available polymeric substrates (Kaladex®1020, 75-μm thick, from DuPont). 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.
Section snippets
Experimental procedures
The GZO thin films with different thickness were deposited on PEN (polyethylene naphthalate) 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 r.f. (13.56 MHz) magnetron sputtering using a ceramic oxide target ZnO/Ga2O3 (95.5 wt.%; 5 cm diameter) with a purity of 99.99%. The sputtering was carried out under room temperature
Results and discussion
The deposited films were physically stable and present very good adherence to the polymer substrates. No crack or peel off of the films was observed after deposition.
Fig. 2 shows the electrical resistivity as a function of the GZO film thickness. With increasing thickness up to a value approximately 300 nm, the resistivity decreases to a minimum value of 6×10−4 Ω cm. After that point ρ is kept almost constant, independently of the thickness of the film used. This means that up to approximately
Conclusions
In this paper we present results concerning the thickness dependence (from 70 to 890 nm) of electrical, structural, morphological and optical properties presented by GZO deposited on polyethylene naphthalate (PEN) substrates by r.f. magnetron sputtering at room temperature. For thicknesses higher than 300 nm an independent correlation between the electrical, morphological, structural and optical properties are observed, meaning that the properties obtained are not dependent on the thickness of
Acknowledgements
The authors would like to acknowledge Augusto Lopes for the SEM analysis and Rui Martins for the XRD measurements. This work was supported by the ‘Fundação para a Ciência e a Tecnologia’ through Pluriannual Contracts with CENIMAT and by the projects: POCTI/1999/ESE/35578, POCTI/1999/CTM/35440 and POCTI/2001/CTM/38924.
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