Structural, electrical and optical properties of sol–gel AZO thin films

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Abstract

Transparent conductive Al-doped zinc oxide (AZO) thin films were prepared by a sol–gel method and their structural, electrical and optical properties were systematically investigated. A minimum resistivity of 4.2 × 10−3 Ω cm was obtained for the 650 °C-annealed films doped with 1.0 at.% Al. All films had the preferential c-axis oriented texture according to the X-ray diffraction (XRD) results. Optical transmittance spectra of the films showed a high transmittance of over 85% in the visible region and the optical band gap of the AZO films broadened with increasing doping concentration.

Introduction

ZnO is a wide band gap (3.37 eV) semiconductor which has been extensively investigated for various applications such as transparent conducting films for solar cells, surface acoustic wave (SAW) devices and opto-electronic devices [1], [2]. Its large exciton binding energy of 60 meV allows a lower threshold for optical pumping at room temperature [3]. Band structure and optical properties of ZnO are very similar to those of GaN, which is known to be a good material for the fabrication of optical devices such as light-emitting diodes (LED) or laser diodes (LD) [4]. ZnO normally has the hexagonal structure with a = 3.25 Å and c = 5.12 Å; each Zn atom is tetrahedrally coordinated to four O atoms, where the Zn d-electrons hybridize with the O p-electrons; layers occupied by zinc atoms alternate with layers occupied by oxygen atoms [5].

It is well known that chemical doping greatly influences the electronic and optical properties of ZnO. Al, In and Ga-doped ZnO thin films are one of the most promising transparent conductive oxides for advanced applications such as displays, electrochromic devices, etc. Due to the non-toxic, low cost, high thermal/chemical stability and resource availability, Al-doped ZnO (AZO) thin films are intensively studied to replace indium tin oxide (ITO) thin films in the above mentioned applications [6], [7], [8], [9], [10], [11]. Different types of techniques such as sputtering [6], spray pyrolysis [7], chemical vapor deposition [8] and sol–gel method [9] have been reported for the deposition of AZO thin films. Among them, the sol–gel technique offers the possibility of small-area as well as large-area coating of films at a low cost for technological applications. To achieve a minimum resistivity, it is necessary to optimize the dopant concentration in the films. However, there exists a big discrepancy in the optimum Al doping concentration of the AZO films reported in the literature. Alam et al. [9] reported a minimum resistivity was obtained for 0.8 at.% AZO films prepared by the sol–gel spin method, and it turned out to be 0.3 at.% for AZO films prepared via the spray pyrolysis method [10]. In contrast, an optimum doping concentration of 5.0 at.% and 4.0 wt.% was obtained for AZO films prepared by filtered cathodic vacuum arc technique [4] and radio frequency (RF) magnetron sputtering [11], respectively. It is therefore crucial to better understand the role of Al in improving the film conductivity in an effort to achieve the minimum resistivity. In this work, we report the preparation of sol–gel AZO thin films annealed at 650 °C with the dopant concentration in the range of 0–4.0 at.%. The structural, electrical and optical properties of the AZO films have been systematically investigated.

Section snippets

Experimental procedure

AZO thin films were prepared by the sol–gel method. Zinc acetate dehydrate (Zn(CH3COO)2 × 2H2O) was used as a starting material. Absolute ethanol and diethanolamine (HN(CH2CH2OH)2, DEA) were used as a solvent and stabilizer, respectively. Aluminum as a dopant was added in the form of Al(NO3)3 × 9H2O (extra purity) with five different Al/Zn ratios, i.e., 0, 0.5, 1.0, 2.0 and 4.0 at.%. A 0.5 M ethanolic solution of zinc acetate was prepared by dissolving 0.05 mol of zinc acetate 2-hydrate in 0.1 L of

Structural properties

XRD patterns of AZO films annealed at 650 °C in air for 1 h are shown in Fig. 1. It shows that all films are crystalline and exhibit the hexagonal structure (JCPDS 36-1451) with the c-axis oriented preferentially normal to the substrate surface. No diffraction peaks of Al2O3 were observed in the Al doping concentration range considered in this work. The crystallite size of the AZO films can be determined from Scherrer‘s equation [12]d=0.89λBcosθBwhere λ is the X-ray wavelength of 1.54 Å, θB is the

Conclusions

The structural, electrical and optical properties of sol–gel prepared AZO thin films were systematically investigated. The optimum Al doping concentration and the optimum annealing temperature were 1.0 at.% and 650 °C, respectively, where the film resistivity was 4.2 × 10−3 Ω cm. The resistivity of the AZO films was initially decreased with increasing Al content up to 1.0 at.%, and then increased at a higher doping concentration. All AZO films had a c-axis oriented texture. The grain size of the films

Acknowledgement

This work was supported by the 2007 Research Fund of the University of Ulsan.

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