Studies on the structural, electrical and optical properties of thermally oxidized copper nickel oxide thin films

Abstract

Thin films of copper nickel oxide (CuNiO2) were prepared by thermal oxidation of metallic copper nickel film formed by DC magnetron sputtering of quimolar Cu50Ni50 target. The as-deposited copper nickel films were thermally oxidized in oxygen atmosphere at different temperatures in the range 250-350oC. The asdeposited and oxidized films were characterized for their chemical composition, structure and surface morphology, electrical and optical properties. The films oxidized at 350oC were of CuNiO2 with polycrystalline nature and crystallite size of 38 nm. The influence of oxidation temperature on the physical properties was systematically investigated. The films oxidized at temperature of 350oC were CuNiO2with electrical resistivity of 7 Ωcm and optical band gap of 2.50 eV. Introduction Cuprous oxide is a p-type semiconducting oxide material finds potential applications in the field of gas sensor, solar cells, electrochromic devices and ptype transparent conductive coatings. Doping of nickel into copper oxide modifies the electrical properties by introducing acceptor levels in copper oxide [1]. CuO-NiO mixed metal oxide nanoparticles are potential as a catalyst for various organic reactions such as methanol synthesis and humidity and non-enzymatic glucose sensors [2-4]. Copper nickel oxide in thin film form find applications as NO2 gas sensor, anode for fuel cells, solar selective coatings, solar cells, electrochromic devices, light emitting diodes, photodiodes, p-type gate in heterojunction field effect transistors, p-type transparent conducting coatings, antifungal coatings and super-capacitor coatings [5-19]. CuxNi1-xOthin films with different copper compositions were deposited by various methods such as spray pyrolysis, sol-gel process electro-deposition, pulsed laser deposition, pulsed plasma deposition, co-evaporation, DC magnetron sputtering and RF magnetron sputtering [6-12,14,16,17,19-35]. In the literature, Yang et al. [27] reported that the p-Cu0.1Ni0.9O thin films formed by pulsed plasma deposition showed the electrical resistivity of 0.19 Ωcm and optical band gap of 3.7 eV. Miyata et al. [31] noticed the electrical resistivity increased from 96 to 8x105Ωcm and optical bands gap increased from 2.6 to 3.9 eV with increase of nickel content in RF reactive magnetron sputtered CuNiO (Ni = 0 -100 at.%) films. Elsayed et al. [14] formed nanocrystalline CuNiO2 films by sol-gel process with optical band gap of 2.4 eV. The electrical resistivity of 27 Ωcm and optical band gap of 2.0 eV were obtained by RF magnetron sputtering by using equimolar Cu50Ni50 target [34]. Menaka et al. [22] deposited CuxNi1-xO (x = 0.02 – 0.08) films by spray pyrolysis and noticed that the electrical resistivity decreased from 250to 88 Ωcm and optical band gap decreased from 3.94 to 3.58 eV with increase of copper content from 2 to 8 at.% respectively. In the present investigation, an attempt is made in the preparation of copper nickel oxide (CuNiO2) thin films by thermal oxidation of metallic CuNi thin films formed by DC magnetron sputtering method. The thermally oxidized CuNiO2 thin films were characterized for their composition, structure, electrical and optical properties. Experimental Thin metallic CuNi films were deposited by employing DC magnetron sputtering technique using the composite target of Cu50Ni50 (99.95% purity) with 50 mm diameter. The sputter chamber was evacuated to achieve the ultimate pressure of 1x10-5mbarusing diffusion pump backed by rotary pump. Pressure in the sputter chamber was measured with Pirani and Penning gauges. Before introducing the glass substrates in sputter chamber, the substrates were cleaned with degreasing solution followed by washing in deionized water. Later, the substrates were immersed in potassium dichromate solution for an hour and finally cleaned ultrasonically in a stainless steel tank for 15 minutes and dried with flow of nitrogen gas then transferred into chamber for deposition of copper nickel thin films. Target to substrate distance maintained was 50 mm. After achieving the ultimate pressure, argon gas was admitted into the sputter chamber through fine controlled needle valve to achieve sputter pressure of 3x10-2mbar. The films were deposited on glass substrate held at room temperature and the DC power fed to the sputter target was 90 W. The duration of the deposition of the film was 10 min. As-deposited metallic CuNi films were thermally oxidized in oxygen atmosphere for three hours in the temperature (Ta) range 250oC to 350oC in order to transform in to copper nickel oxide films. The as-deposited and thermally oxidized films were characterized for their chemical composition, structure and surface morphology, electrical and optical properties. Thickness of the deposited CuNi film determined with Veeco Dektak (Model 150) was 220 nm. Chemical composition of the metallic and oxide films was determined with energy dispersive X-ray analysis (EDAX) (Oxford instruments Inca Penta FETX3). Ccrystallographic structure of the Correspondence to: S Uthanna, Department of Physics, Sri Venkateswara University, Tirupati – 517 502, India; Tel: +919440650393; E-mail: uthanna@ rediffmail.com