Compositional and physico-optical characterization of 0–5% Al-doped zinc oxide films prepared by chemical spray pyrolysis

Abstract

Highly transparent polycrystalline Al-doped ZnO thin films were deposited in open atmosphere by chemical spray pyrolysis on fused silica and glass substrates at 623 K.The influence of Al doping, 0 to 5 %, was studied. XPS results revealed a linear relationship between Al content in the precursor solutions and Al content in the films.XPS depth profiling showed that any carbon contamination is restricted to the uppermost surface of the films. Optical transmission measurements revealed an increasing amount of dispersion centres as well as a band gap shift to higher values with increasing Al content in the films. At fixed Al concentration, the comparison of the absorption coefficient for increasing film thickness showed that the films are very homogeneous, not changing their materials properties like absorption coefficient and band gap. PACS: 81.15.Rs, 82.80.Pv, 68.53.Dv, 78.66.Fd Keywords: thin films; chemical synthesis; doping; optical properties. Confidential: not for distribution. Submitted to IOP Publishing for peer review 23 December 2009Compositional and optical characterization of Al doped ZnO films prepared by CSP 2 IntroductionAl-doped ZnO is a multifunctional material. In the form of thin layers, it has been studied for different applications as transparent conductive oxide, infrared mirror, antireflection layer, gas sensor, etc [1]. Al-doped ZnO films are commonly prepared by sputtering or CVD [2][3][4][5]. However, the preparation by chemical spray pyrolysis (CSP) is an interesting alternative due to the potentially very low installation and maintenance cost of the technique. Chemical spraying can be done in open air without the need of special chambers and pumps for a controlled atmosphere or vacuum. Furthermore, CSP can be considered as environmentally friendly since mainly aqueous precursor solutions and air as driving gas are used. Sometimes, depending on the substrate and the material to be deposited, it might be necessary to add small quantities of ethanol (typically 50 ml per liter) and/or acetic acid (1 -10 ml per liter) to the aqueous precursor solution in order to stabilize the precursor (∼10 -2 M) in the aqueous solution by forming stable acylates [6] and to enhance the wettability of the solution to the substrate [7]. Further advantages of spray pyrolysis are: compact or porous films can be obtained depending on the spray conditions; doping can easily be performed by introduction of the doping element into the precursor solution; CSP in open atmosphere can easily be up-scaled to large area deposition [8]. A review on the possibilities of CSP was given by Patil [9].For CSP, the selection criteria for a precursor in aqueous spray solution is determined by its chemical stability, at least over the period of deposition time, its ease of availability and its cost. This leads directly, in case of ZnO films, to the selection of zinc acetate and zinc chloride as precursors. Zinc alcoxides could also be used, but have to be stabilized in an aqueous spray solution and are more expensive. Previou...

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