Elsevier

Journal of Alloys and Compounds

Volume 589, 15 March 2014, Pages 346-352
Journal of Alloys and Compounds

Influence of Al doping on structural and optical properties of Mg–Al co-doped ZnO thin films prepared by sol–gel method

https://doi.org/10.1016/j.jallcom.2013.11.061Get rights and content

Highlights

  • Mg–Al co-doped ZnO thin films were prepared by sol–gel spin coating method.

  • The effects of Al doping on structural and optical properties of AMZO thin films were investigated.

  • The EDS spectra confirmed presence of Mg and Al elements in AMZO thin films.

  • The optical band gap of AMZO thin films increased with Al doping concentration increased.

  • The origin of the photoluminescence emissions was discussed.

Abstract

Mg–Al co-doped ZnO (AMZO) thin films were successfully deposited onto quartz glass substrates by sol–gel spin coating method. The structure, surface morphology, composition, optical transmittance, and photoluminescence properties of AMZO thin films were characterized through X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy, UV–VIS–NIR spectrophotometry, and fluorescence spectrophotometry. The results indicated that AMZO thin films exhibited preferred orientation growth along the c-axis, and the full width at half maximum of the (0 0 2) diffraction peak decreased first and subsequently increased, reaching a minimum of approximately 0.275° at 3% Al content. The calculated crystallite size increased from 30.21 nm to 40.73 nm. Al doping content increased from 1% to 3% and subsequently reached 19.33 nm for Al doping content at 5%. The change in lattice parameters was demonstrated by the c/a ratio, residual stress, bond length, and volume per unit cell. EDS analysis confirmed the presence of Mg and Al elements in ZnO thin films. The atomic percentage of Mg and Al elements was nearly equal to their nominal stoichiometry within the experimental error. In addition, the optical transmittance of AMZO thin films was over 85% in the visible region, and the optical band gap increased with increasing Al doping concentration. Room temperature photoluminescence showed ultraviolet emission peak and defect emission peak. The defect emission peak of AMZO thin films blue-shifted from 612 nm to 586 nm, with Al doping content increasing from 1% to 5%.

Introduction

Wide-band gap materials have been receiving considerable attention because of their applications in optoelectronic devices. Zinc oxide (ZnO) thin films are one of the most commonly used wide band gap materials. Considering their band gap of 3.37 eV and exciton binding energy of 60 meV at room temperature, ZnO thin films have elicited much interest because of their potential applications in laser-emitting diodes, laser diodes, surface acoustic devices, diluted magnetic semiconductors, solar cells, and organic electroluminescence devices, among others [1], [2], [3]. ZnO thin films have been prepared using various growth techniques, such as pulsed laser deposition [4], magnetron sputtering [5], molecular beam epitaxy [6], spray pyrolysis [7] and sol–gel method [8]. Among them, the sol–gel method has drawn much attention because of its advantages, such as homogeneity at the molecular level, accurate compositional control, low cost, lower crystallization temperature and easy reproducibility [9], [10].

Normally, pure ZnO thin films are n-type semiconductors, and their optical and electrical properties are lower and more unstable [11]. Therefore, doping is usually preferred to improve the optical and electrical properties of ZnO thin films. To date, many groups have investigated the optical and electrical properties of ZnO thin films by doping them with Mg, Cd, Al, Ga, Sn, and In elements [12], [13], [14], [15]. In single-doped ZnO thin films, the optical and electrical properties could not be improved simultaneously. Thus, co-doping is an effective method to simultaneously enhance the optical and electrical properties of ZnO thin films. The band gap and electrical property of ZnO thin films can be improved by Mg and Al co-doping. Most studies have investigated the optical and electrical properties of Al-doped Zn1−xMgxO thin films with different Mg doping concentrations. For example, Matsubara et al. [16] reported that the maximum band gap of Al-doped Zn1−xMgxO thin films with resistivity ρ  1 × 10−3 Ω cm was 3.97 eV, and the average transmittance of all the films was over 90% in the wavelength region. Yang et al. [17] demonstrated that the wide band gap and resistivity of optimized ZnMgAlO thin films were 4.5 eV and 1.6 × 10−3 Ω cm, respectively. Prathap et al. [7] reported that the energy band gap of Zn0.76Mg0.24O:Al thin films varied from 3.79 eV to 3.68 eV when Al doping concentration varied from 0% to 6%, which could be attributed to the formation of localized states in the band gap. Duan et al. [18] reported that the optical transmittance of Al-doped Zn1−xMgxO thin films with Al doping content at 1 at.% annealed in nitrogen increased to 70–80% compared with that of films annealed in vacuum (50–60%). The optical band gap of Al-doped Zn1−xMgxO thin films annealed in nitrogen increased with increased Mg doping concentration from 0% to 8%. However, the effect of Al doping on the structural and optical properties of Mg–Al co-doped ZnO thin films has not been reported.

The thermodynamic solid solubility of MgO in ZnO is less than 4 mol%, according to the phase diagram of ZnO–MgO binary system. Therefore, in the present study, Mg doping content of Mg–Al co-doped ZnO (AMZO) thin films was 3 at.%. AMZO thin films with different Al doping contents were deposited onto quartz glass substrates using sol–gel spin coating method. The main goal of this work was to investigate the influence of different Al doping contents on structural, surface morphology, optical band gap, and photoluminescence properties of AMZO thin films.

Section snippets

Experimental details

Mg–Al co-doped ZnO thin films doped with different Al contents were deposited onto quartz glass substrates by sol–gel spin coating method. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] was used as starting material. 2-Methoxyethanol (C3H8O2), monoethanolamine (MEA), aluminum nitrate nonahydrate [Al(NO3)3·9H2O], and magnesium acetate [Mg(CH3COO)2·4H2O] were used as solvent, stabilizer, and dopant source, respectively. The concentration of the solutions was 0.75 mol/L, and the molar ratio of MEA to

Structural characterization of AMZO thin films

Fig. 1 shows the XRD patterns of pure ZnO and AMZO thin films doped with varying Al contents. All the films have three diffractive peaks corresponding to the (1 0 0), (0 0 2), and (1 0 1) diffraction peaks of ZnO. All diffractive peaks can be indexed into the ZnO hexagonal wurtzite structure, indicating that Mg and Al doping did not change the wurtzite ZnO structure. The intensity of the (0 0 2) diffraction peak increased with increasing Al doping content up to 3%, and then decreased with further

Conclusions

Mg–Al co-doped ZnO thin films with different Al doping concentrations were deposited on quartz glass substrates using sol–gel spin coating method. Experimental results showed that AMZO thin films possessed a preferential orientation along the (0 0 2) plane, and the intensity of (0 0 2) diffraction peak increased initially with increase in Al doping concentration up to 3% and subsequently decreased with further increase in Al doping concentration. Al doping concentration was observed to have great

Acknowledgements

The authors are grateful for the financial support from the Natural Science Foundation of Tianjin, PR China (Grant Nos. 07JCZDJC00600 and 07JCYBJC06000).

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