Elsevier

Thin Solid Films

Volume 351, Issues 1–2, 30 August 1999, Pages 125-131
Thin Solid Films

Optical, electrical and structural properties of sol gel ZnO:Al coatings

https://doi.org/10.1016/S0040-6090(99)00211-4Get rights and content

Abstract

Single and multilayer transparent conducting aluminium doped zinc oxide films have been prepared on DESAG AF45 substrates by the sol gel dip coating process. Zinc acetate solutions of 0.1–0.5 M in isopropanol stabilised by diethanolamine and doped with a concentrated solution of aluminium nitrate in ethanol were used. Each layer was fired at 600°C in a conventional furnace for 15 min. The final coatings were then tempered under a flux of forming gas (N2/H2) at 400°C for 2 h. The coatings have been characterised by surface stylus profiling, optical spectroscopy (UV-NIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and combined Hall and van der Pauw techniques. Single layers with thickness, d, ranging from 25 to 186 nm are polycrystalline with the zincite structure and exhibit a small preferential orientation with the (002) direction perpendicular to the surface (texture coefficient, TC, ranging from 1.8 to 2.8). They consist of almost spherical particles with size ranging from 15 to 25 nm (thin sample) to 40 nm (thick sample) with crystallite sizes ranging from 23 to 40 nm (002), respectively. The film resistivity decreases from 5×10−1 Ω cm (d=25 nm) to 4×10−2 Ω cm (d=186 nm). Multilayer coatings built with about 20 nm thick layers are also slightly textured along the (002) direction (TC ranging from 1.9 to 2.8). A structural evolution in morphology from spherical to columnar growth was observed. The crystallite size calculated from the (002) reflex increases with the number of layers from 23 nm for a single layer to over 100 nm for ten layers. The resistivity decreases from 5×10−1 Ω cm for a single layer to 5×10−3 Ω cm for ten layers (d=174 nm). A model for the growth of the crystallites in sol gel multilayer coatings is presented.

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).

References (31)

  • K.-S. Weiβenrieder et al.

    Thin Solid Films

    (1997)
  • B. Szyszka et al.

    J. Non-Cryst. Solids

    (1997)
  • M. Ohyama et al.

    Thin Solid Films

    (1997)
  • W. Tang et al.

    Thin Solid Films

    (1994)
  • K.H. Kim et al.

    Thin Solid Films

    (1986)
  • M.A. Aegerter et al.

    J. Non-Cryst. Solids

    (1997)
  • Y. Takahashi et al.

    J. Non-Cryst. Solids

    (1997)
  • J.A. Anna Selvan et al.

    Mater. Res. Soc. Symp. Proc.

    (1996)
  • M. Ritala et al.

    Mater. Res. Soc. Symp. Proc.

    (1996)
  • V. Gupta et al.

    J. Appl. Phys.

    (1996)
  • M. Bertolotti et al.

    J. Mater. Res.

    (1990)
  • J.L. Deschanvres et al.

    J. Phys.

    (1993)
  • S. Hayamizu et al.

    J. Appl. Phys.

    (1996)
  • T. Nakada et al.

    Mater. Res. Soc. Symp. Proc.

    (1996)
  • J. Aranovich et al.

    J. Vac. Sci. Technol.

    (1979)
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