Elsevier

Applied Surface Science

Volume 377, 30 July 2016, Pages 355-360
Applied Surface Science

Electrical stability of Al-doped ZnO transparent electrode prepared by sol-gel method

https://doi.org/10.1016/j.apsusc.2016.03.133Get rights and content

Highlights

  • Al-doped ZnO thin film was deposited by sol-gel method in different annealing temperature and duration.

  • We examined the environmental stability in ambient and damp heat condition.

  • We investigated chemical state of thin film.

  • Better stability was observed in the film annealed at high temperature (600 °C) along with longer duration (120 min).

  • An ultrathin aluminum oxide layer formation was predicted by XPS measurement which protects further oxidation and improves stability.

Abstract

Al-doped zinc oxide (AZO) thin films have been considered as a promising alternative to tin doped indium oxide (ITO), which is currently used in various optoelectronic applications. However, the environmental stability of AZO film is not satisfactory, in that the resistivity is significantly increases in air. Here, we investigate the resistivity stability of AZO thin films prepared by sol-gel method using various annealing temperatures and durations. The degradation of resistivity property was observed for AZO films stored in ambient or damp heat environment, where the degradation rate was influenced by annealing temperature. A significant improvement of electrical stability was attained in AZO films that were prepared at high annealing temperature. The films, which showed the highest and the lowest increasing rate of resistivity, were further characterized in detail to shed light on the possible mechanisms explaining the improved stability through crystallinity, surface morphology and elemental state of the thin film.

Introduction

Transparent conducting oxides (TCOs) are widely used in recent modern technologies, especially for optoelectronic applications. Among many candidates of TCO materials, the Sn-doped indium oxide (ITO) is commonly used as transparent electrode for its excellent conductivity and transparency [1]. At a time, however, the limited existence and high price of indium enforced researchers to search and develop alternative TCOs other than ITO. One of the most promising candidates is ZnO, since it exhibits excellent optical and electrical properties that are suitable for transparent electrode. The ZnO is an n-type semiconductor material, which exhibits a direct wide band gap of 3.36 eV and a relatively high exciton binding energy of 60 meV at room temperature [2]. Doped ZnO can be used for various applications in optoelectronics such as PV (photovoltaic) devices [3], LED (light emitting diode) [4], gas sensors [5], piezoelectric devices [6], and photo detectors [7]. Generally, the group III (group 13) elements (e.g. Al, Ga, B) are used as doping material since they assist to increase the number of carriers and therefore the conductivity. Among them, Al is one of the most preferred dopant because of its low cost and availability. Many researchers have reported different heat treatment conditions, doping effects and film development procedure, shedding light on various characteristics of the Al-doped ZnO (AZO) [8], [9], [10], [11], [12], [13]. The optoelectronic properties are generally dependent on the deposition and post deposition conditions [14], where the deposition techniques such as sputtering [2], [3], [15], [16], [17], [18], [19], spray pyrolysis [9], [20], sol-gel [10], [11], [12], pulsed layer deposition (PLD) [21], chemical vapor deposition (CVD) [22] are popular, where sol-gel is one of the most promising method because of its simplicity, easy control of doping or growth parameters and low cost [23].

Meanwhile, in terms of the electrical stability, the AZO thin films have a critical problem as the resistance of the film largely increases in air, unlike the ITO film that shows high stability in moisture environment [24]. There have been some reports concerning influences of the film thickness [25], the deposition method [25], [26], the capping layer [27], and the co-doping on the stability of AZO film [28], [29]. However, the stability of sol-gel prepared AZO film has not been extensively studied. The purpose of this work is thus to investigate the environmental stability of AZO film prepared by sol-gel method.

Section snippets

Experimental

The deposition of Al-doped ZnO thin film on soda lime glass substrate (48 × 28 mm) was performed by sol-gel method [11]. As a starting material, zinc acetate dehydrate (Zn(CH3COO)2·2H2O) was dissolved into 2-methoxyethanol that acts as solvent. Monoethanolamine (MEA) was added to the solution as a stabilizer. The molar ratio of MEA to zinc acetate was 1.0 and the concentration of zinc acetate was 0.5 mol/L. To prepare Al-doped ZnO thin films, aluminum nitrate nonahydrate (Al(NO3)3·9H2O) was added

Results

In general, the n type conductivity of an intrinsic ZnO derives from Zn interstitial and oxygen vacancy [31]. The electrical properties of doped ZnO films strongly depend on the carrier concentration, which is related to contributions from Al3+ on substitutional sites of Zn2+ ions, Zn interstitial atoms, and oxygen vacancies. In this experiment, the intrinsic ZnO film (after vacuum annealing at 500 °C for 30 min) showed the resistivity of 8.0 × 10−2 Ωcm. When ZnO film was doped with 1.18 at% Al, the

Discussion

The reduction of the resistivity of AZO thin film after doping and heat treatment can be ascribed to creation of extra free electrons through both Zn substitution by Al atoms (doping effect) and formation of oxygen vacancies during annealing in vacuum atmosphere. The degradation of resistivity in air was observed which is mainly due to the adsorption of oxygen and/or water molecule to the film [27]. In this experiment, the resistivity increased in air and oxygen atmosphere, but resistivity

Conclusion

AZO thin film was prepared by sol-gel method followed by vacuum annealing in different temperature and duration. The electrical stability of these films was investigated in air over time. The films annealed at low temperature showed large increase of the resistance, whereas those at high temperature remained stable. The damp heat test also assured the same increasing tendency of resistivity, though the order of magnitude is quite different. Improved crystallinity and surface morphology with

Acknowledgements

This work was partly supported by the Kyoto University Global COE Program, “Energy Science in the Age of Global Warming”. The authors also gratefully acknowledge the Osaka Municipal Technical Research Institute for the support in film thickness measurements.

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