Optical, electrical and structural properties of sol gel ZnO:Al coatings
Introduction
Aluminium doped zinc oxide (AZO) coatings exhibit high transparency and low resistivity and this material is suitable for the fabrication of solar cells [1] and flat panel display electrodes [2]. They also find applications as surface acoustic devices [3], optical waveguides [4], gas sensors [5] and micro-machined actuators [6].
Doped and undoped ZnO thin films have been prepared by physical deposition methods such as laser deposition [7], different sputtering methods [1], [3], [8], [9], atomic layer deposition [2] and chemical deposition methods such as chemical vapour deposition [6], spray pyrolysis [10], chemical bath deposition [11] and the sol gel process [12], [13], [14], [15], [16], [17], [18], [19]. A recent compilation of the properties of sol-gel derived AZO films is given in [15]. The reported resistivities vary from 7×10−4[16] to 10 Ω cm whereas the reported resistivities of sputtered films are as low as 1×10−4 Ω cm [20]. These large discrepancies are currently not well understood.
The aim of this work is to investigate the influence of the multilayer deposition process on the structural, electrical and optical properties of sol gel ZnO:Al coatings as it is established that the fabrication process governs the microstructure [21] and influences the intrinsic properties of this material.
Section snippets
Preparation
The preparation of the coating solutions is shown in Fig. 1. Diethanolamine (DEA, Fluka puriss.) was first dissolved in isopropanol (iPrOH). Then zinc acetate dihydrate (ZnAc, Fluka puriss) was added under stirring. Doping of the solution was obtained by adding a 0.2 M solution of aluminium nitrate nonahydrate (Fluka puriss.) in ethanol in order to obtain a ratio Al/Zn=0.6 at.%. Finally iPrOH was added to achieve the desired concentration (0.1–0.5 mol/l). The solution became clear and
Experimental
Thickness measurements were performed with a Stylus Profiler (Tencor P10). The electrical properties have been measured with the van der Pauw and Hall method (MMR Technologies) with an applied magnetic field of 1.3 T. For X-ray diffraction (XRD) measurements a Siemens D500 equipment with a thin film attachment was used. 2θ-scans from 20 to 65° were performed at grazing incidence (θ=2°). The crystallite size was calculated from the FWHM of the (002) peak corrected for the instrumental line width
Structural Properties
The variation of single layer thickness with the concentration of the coating solution is plotted in Fig. 2. A non-linear dependence of the film thickness on the concentration is observed. Fig. 3shows the thickness of the films as a function of the number of coatings obtained with the 0.1 M solution. After the first layer deposition the thickness of the coating increases linearly (16.4 nm/layer) with the number of layers.
All samples are polycrystalline and exhibit the zincite structure (JCPDS
Discussion
Different morphologies have been observed in ZO and AZO coatings made by the sol gel process. A columnar growth was reported for undoped zinc oxide multilayer coatings with an individual layer thickness of ca. 20 nm [14], [18]. However, for AZO coatings only a layered structure with spherical grains [17] or a homogeneous distribution of spherical grains [15] have been observed. Usually the coatings present no texture or are slightly textured [16], [17], [19] but Ohyama et al. [13], [14], [15]
Conclusion
AZO sol gel multilayer coatings have been prepared with electrical resistivity as low as 5×10−3 Ω cm and good optical properties. The electrical properties of the coatings have been related to the structural and morphological properties of the coatings. As long as homogeneous nucleation takes place predominantly the particles are spherically shaped and randomly distributed. Therefore the film density is low and the resistivity is high. Heterogeneous nucleation is favoured by depositing very
Acknowledgements
The authors are grateful to Dr. T. Krajewski for the TEM pictures. This work was financed in part by BMBF (Förderzeichen 2A67/03 N 9040) and the State of Saarland (Germany).
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