Structural, optical and electrical properties of undoped and co-doped Al/Ga ZnO thin films by spray pyrolysis

Abstract

Zinc oxide (ZnO) is a naturally occurring n-type semiconductor which is viewed as a promising candidate to replace indium tin oxide (ITO) in opto-electric applications, owing to its natural abundance and a wide bandgap (3.37). Synthesis of ZnO thin films via chemical spray pyrolysis technique is more convenient and cost effective compared to the mostly used deposition techniques such as magnetron sputtering which are highly expensive. Chemical spray pyrolysis provide easier way of doping the film by simply introducing dopant precursor ions into the starting solution. The doping of semiconductors with appropriate metals is generally one of the most effective ways of in research for developing sensitivity applications. In order to reduce resistivity of ZnO, more work have been done by doping ZnO with either group III or group VII elements using spray pyrolysis. Aluminum doped ZnO (AZO) and gallium doped ZnO (GZO) thin films have been extensively studied since they exhibit good optical transparency and a good electrical conductivity. Henceforth, there is a continuous search to explore other ways of further improving opto- electrical properties of ZnO through various doping techniques like co-doping. In this thesis, aluminum and gallium co-doped ZnO (AGZO) thin films were grown by simple, flexible and cost-effective spray pyrolysis method on glass substrates, at a temperature of 230 °C. Effects of equal co-doping with aluminum (Al) and gallium (Ga) on structural, optical and electrical properties were investigated by X-ray diffraction (XRD), UV-Vis-NIR spectrophotometry and Current – Voltage (I-V) measurements, respectively. XRD patterns showed a successful growth with high quality polycrystalline films on glass substrates. The predominant orientation of the films is (002) at dopant concentrations ≤ 2 at% and (101) at higher dopant concentrations. Incorporation of Al and Ga to the ZnO crystal structure decreased the crystallite size and increased residual stress of the thin films, as well as decrease in surface roughness of the films with increasing doping concentration. All films were highly transparent in the visible region with average transmittance of 80 %. Increasing doping concentrations increased the optical band gap from 3.12 to 3.30 eV. A shift of the optical band gap was observed from 400 nm to 380 nm with increase in equal co-doping. Co-doping improved the electrical conductivity of ZnO thin films. It has been found from the electrical measurements that films with dopant concentration of 2 at% have lowest resistivity of 1.621 x10-4 Ω.cm.