Highly c-axis oriented ZnO film on silica glass using zinc naphthenate precursor

Abstract

High transmittance and good electrical conductivity thin conducting oxides (TCOs) have potential use in a variety of technological applications including plasma display panels (PDPs), electroluminescent devices, flat cathode ray tube (CRTs) and solar cells [1–4]. Recently, zinc oxide (ZnO) thin films have emerged as one of the most promising oxide materials owing to their optical and electrical properties, together with their high chemical and mechanical stability. ZnO is a wide band gap (3.3 eV) semiconducting, piezoelectric, and photoconducting material. In particular ZnO films with preferential orientation along the c-axis have been shown to work as SAW devices because of large piezoelectric constant. Ohyama et al. [5] prepared ZnO films on silica glass substrates from zinc acetate sol containing 2-methoxyethanol and monoethanolamine (MEA) with extremely preferred orientation along (002) plane. Ohya et al. [6] reported transparent, electrically conductive ZnO thin films with polycrystalline structure from zinc acetate and alkoxide. Recently Wessler et al. [7] reported textured ZnO thin films on (0001) sapphire by spin-coating a sol of acetate and ethanolamine. In this letter, the preparing of highly c-axis oriented ZnO thin film on amorphous silica substrates from a zinc naphthenate precursor is reported. Crystallinity, surface morphology, surface roughness, and optical behavior were studied. A coating solution was prepared using zinc naphthenate (Nihon Kagaku Sangyo Co., Ltd., Japan) and by diluting the solution with toluene (concentration: 4 wt% metal/100 mL sol). Metal naphthenates are more advantageous than metal alkoxides as starting materials, in terms of cost, stability in air, and ease of handling. Preparation of the coating solution using zinc naphthenate was easily performed by the addition of toluene, while complicated procedures were needed for the preparation of sol from zinc alkoxide [6]. The starting solution was spin-coated onto the cleaned substrate at 1500 rpm for 10 s in air. The asdeposited film was prefired at 500 ◦C for 10 min in air. The coating process was repeated five times to prepare