Elsevier

Materials Letters

Volume 109, 15 October 2013, Pages 265-268
Materials Letters

Structural and optical properties of post annealed Mg doped ZnO thin films deposited by the sol–gel method

https://doi.org/10.1016/j.matlet.2013.07.104Get rights and content

Highlights

  • Mg doped ZnO (MZO) thin films were grown on quartz substrate using sol–gel method.

  • Crystallinity and RMS roughness of MZO films were enhanced upon annealing.

  • Transmittance of all the MZO films and their optical band gap were decreased with the rise in annealing temperature.

Abstract

Thin films of magnesium doped zinc oxide (MZO) were synthesized using the sol–gel method and annealed at different temperatures under ambient condition. The morphological properties of the post annealed MZO films were investigated using X-ray diffraction (XRD) while atomic force microscopy (AFM) was employed to probe the topographical alteration of the films. The optical properties of the post annealed MZO films were examined by UV-visible spectroscopy and the Tauc method was used to estimate the optical band gap. The studies revealed that, with the rise of annealing temperature the crystallinity and the surface roughness of the MZO films were increased whereas the optical transmittance and the energy bandgap were decreased.

Introduction

A wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature have established ZnO as a promising material for optoelectronic devices such as fast ultraviolet detector, light emitting diode, laser diode etc. [1]. Nevertheless, there are few limitations in the application of pure ZnO for the integrated optical devices as they often require a much wider band gap [2]. Hence, the band gap tuning of ZnO becomes significant. The band gap of ZnO can be tailored by alloying ZnO with group II elements e.g. Be, Mg, Ca, Cd, and Sr [3]. Moreover, the radius of the Mg2+ ion (0.57 Å) closely matches with the radius of Zn2+ ion (0.60 Å), so the incorporation of Mg2+ ion into ZnO lattice is quite feasible, thus forming MZO. The enhanced optical band gap of MZO has been successfully employed to sense the mid and deep UV light [4]. The MZO thin films can be synthesized using various techniques such as RF sputtering, molecular-beam epitaxy, sol–gel, pulse laser deposition and chemical vapor deposition [2], [3], [4]. However, the sol–gel route offers many advantages like simple, inexpensive preparation of a large-area homogeneous thin film along with excellent compositional control, lower crystallization temperature and uniform film thickness. Furthermore, at different steps of fabrication, optoelectronic devices often require thermal treatments. So the study of the effect of thermal treatment on the structural and optical properties of MZO films requires serious attention for optoelectronic applications of these films.

Section snippets

Experimental procedure

Zinc acetate dihydrate, methanol and diethanolamine (DEA) were used as starting material, solvent and stabilizer, respectively. First, 6.585 g of zinc acetate dihydrate was dissolved in 40 ml methanol and then DEA was slowly added under magnetic stirring to prepare a solution of 0.75 M. Magnesium doping of ZnO was performed by adding magnesium acetate tetrahydrate to the methanol along with the zinc acetate dihydrate in two different Mg/Zn ratios. For the sample MZO1, Mg/Zn ratio was 1.0 at% and

Results and discussion

The XRD spectra (Fig. 1) revealed the influence of annealing treatment in the temperature range of 400–700 °C on the structure of the MZO thin films. The X-ray pattern (Fig. 1) depicted that all the films were polycrystalline in nature with hexagonal wurtzite structure. Furthermore, for all the MZO films, neither MgO nor Mg phase was detected in the X-ray pattern. It was observed (Table 1) that the XRD peak related to (002) plane shifted systematically towards higher angle with the increase in

Conclusions

In this paper the structural, topographical and optical properties of sol–gel derived MZO thin films deposited on quartz substrate were studied after annealing the films at different temperatures. The investigation showed that the lower boundary of the grain size of all the films was increased with annealing temperature thus the crystallinity. The analysis also revealed that surface roughness of MZO films was increased upon annealing and this was responsible for the reduction in transmittance

Acknowledgments

The authors are grateful to Dr. S. Chattertjee from the department of Electronics and Communications Engineering, SMIT for his help with the spin coating. The authors would also like to thank Mr. N. Paitya and Mr. H. Pal from the department of Applied Electronics and Instrumentation, SMIT for their help with UV–vis measurements.

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The authors A. Ahmed and R. Labar had contributed equally to this work.

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