Effects of doping concentration and annealing temperature on properties of highly-oriented Al-doped ZnO films

doi:10.1016/j.jcrysgro.2005.10.047

Journal of Crystal Growth

Volume 287, Issue 1, 18 January 2006, Pages 78-84

https://doi.org/10.1016/j.jcrysgro.2005.10.047 Get rights and content

Abstract

Transparent and conductive high-preferential c-axis-oriented Al-doped zinc oxide (ZnO:Al, AZO) thin films have been prepared by the sol–gel route. Film deposition was performed by spin-coating technique on Si(1 0 0) and glass substrate. Structural, electrical and optical properties were performed by XRD, SEM, four-point probe, photoluminescence (PL) and UV-VIS spectrum measurements. The effects of annealing temperature and dopant concentration on the structural and optical properties are well discussed. It was found that both annealing temperature and doping concentration alter the microstructures of AZO films. Also, PL spectra show two main peaks centered at about 380 nm (UV) and 520 nm (green). The variation of UV-to-green band emission was greatly influenced by annealing temperatures and doping concentration. Reduction in intensity ratio of UV-to-green might possibly originate from the formation of Al–O bonds and localized Al-impurity states. The minimum sheet resistance of 10⁴ Ω/□ was obtained for the film doped with 1.6 mol% Al, annealed at 750 °C. Meanwhile, all AZO films deposited on glass are very transparent, between 80% and 95% transmittance, within the visible wavelength region. These results imply that the doping concentration did not have significant influence on transparent properties, but improve the electrical conductivity and diversify emission features.

Introduction

Zinc oxide (ZnO) is a wide-band gap II–VI semiconductor with very attractive properties such as high transparency in the 0.4–2 μm optical wavelength range, high piezoelectric constant, large electro-optic coefficient, and large exciton binding energy of ∼60 meV at room temperature [1], [2], [3], [4]. In recent years, the fabrication of ZnO-based films has attracted a considerable amount of interest due to their potential application in solar cells, gas sensors, optical waveguides, surface acoustic devices, piezoelectric transducers and varistors [5], [6], [7], [8], [9]. In addition, doping of ZnO with various elements has been reported to improve their electrical conductivity for use in optoelectric devices. Therefore, doped ZnO films are alternative materials to tin oxide and indium tin oxide, which have been widely used as transparent conducting oxide (TCO) because of their superior electrical and optical properties, abundance in nature, nontoxicity, and the excellent stability in hydrogen plasma, an unavoidable processing ambient in silicon-related fields [10].

It is well known that the structural properties and dopants may determine the electronic and photoluminescence (PL) properties of the materials. The typical dopants that have been used to enhance the conductivities of ZnO are the group III (B, Al, In, Ga) of the periodic table. Among them, Al-doped ZnO films have been widely studied and are considered as candidate materials for organic electroluminescence displays [11]. Several techniques have been used for the preparation of ZnO thin films such as spray pyrolysis [12], chemical vapor deposition [13], sputtering [14], reactive evaporation [15], pulsed laser ablation, filtered cathodic vacuum arc technique [16], [17], and hydrothermal method [18], [19]. A number of reports have also been published on sol–gel-derived undoped and impurity-doped zinc oxide films [20], [21], [22]. Despite the crystalline quality being inferior to other vacuum deposition techniques, the sol–gel technique still has distinct advantages such as cost effectiveness, simplicity, excellent compositional control, homogeneity and lower crystallization temperature. Moreover, incorporation of dopants is easier in this technique. Hence, this technique is suitable for frontier researches.

In the present study, Al-doped ZnO (AZO) thin films with 0–5% molar concentrations were prepared by sol–gel method. Previous works mostly focused on the electrical and optical properties influenced by either dopants or thermal treatments. Different from their investigation, the influences of dopant concentration and thermal treatment on the structural, electrical and optical properties were investigated.

Section snippets

Experimental procedure

Zinc acetate-2-hydrate [Zn(CH₃COO)₂·H₂O] and aluminum nitrate nonahydrate [Al(NO₃)₃·9H₂O] were chosen as the starting materials, and alcoholic solution were used as solvent. The details of the preparation method are similar to those described in earlier literatures. The concentration of zinc acetate was chosen to be 1 mol l⁻¹, and the precursor solution has been mixed thoroughly by a magnetic stirrer. Appropriate amounts of aluminum doping were achieved by adding aluminum nitrate to the precursor

Results and discussions

Typical XRD patterns of the 3 mol% AZO thin films annealed at various temperatures are shown in Fig. 1(a). XRD spectra in Fig. 1(a) show that the sol–gel-derived ZnO:Al films developed without the formation of secondary phases and clusters such as Al₂O₃ and amorphous ZnO. All films exhibit only the (0 0 2) peak, indicating that they have c-axis preferred orientation due to self-texturing phenomenon. From Fig. 1(a), it is readily observed that the intensity of the (0 0 2) diffraction peak increases

Conclusion

In conclusion, highly c-axis-oriented Al-doped ZnO thin films have been prepared by the well-established sol–gel technique under suitable thermal treatment. Structural, electrical, and optical properties were investigated to explore a possibility of producing TCO films through low-cost process. The impacts of the doping concentration and annealing temperatures on the structural and optical properties of the films were studied in detail. XRD results indicate that the crystallinity was enhanced

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Effects of doping concentration and annealing temperature on properties of highly-oriented Al-doped ZnO films

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussions

Conclusion

Ceram. Int.

Mater. Lett.

Synth. Met.

Mater. Sci. Eng. B

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

J. Crystal Growth

J. Eur. Ceram. Soc.

Mater. Sci. Eng. B

Thin solid films

Thin Solid Films

Mater. Res. Bull.

IEEE Trans. Ultrson. Ferroelectrics Freq. Control