Elsevier

Superlattices and Microstructures

Volume 75, November 2014, Pages 127-135
Superlattices and Microstructures

Synthesis, structural and electrical properties of annealed ZnO thin films deposited by pulsed laser deposition (PLD)

https://doi.org/10.1016/j.spmi.2014.07.043Get rights and content

Highlights

  • ZnO thin films were prepared by using PLD.

  • The XRD reveal the presence of hexagonal wurtzite structure of ZnO films.

  • These annealed films are promising materials for electrical component applications.

Abstract

The effects of annealing times on the structural and electrical properties of ZnO thin films by XRD, FTIR, and dc conductivity were investigated. The XRD reveal the presence of hexagonal wurtzite structure of ZnO with preferred orientation (0 0 2). The grain sizes were found to be in the range 7.765–15.905 nm. The values of optical phonon frequency (νo) were obtained to be 1.26–1.44 × 1013 Hz. The electrical conductivity shows that all samples are semiconductor and it decreases with increasing annealing times. The calculated activation energy for the films was found to be 0.0322–0.126 eV. The increase in electric conductivity in the films annealed at 400 °C for 1 h compared to that of the film annealed at 400 °C for 4 h can be attributed to the decrease in grain boundary scattering due to the reduction in grain size.

Introduction

ZnO thin films have attracted a lot of research attention due to their high electrical response, low cost, simple manufacturing process, absence of toxicity and good process compatibility [1], [2], [3]. It is a extensively used material in different applications such as short wavelength light emitting devices and day light UV detector and for fabrication of the next generation optoelectronic devices in the UV region, gas sensor, and optical or display devices [4], [5]. ZnO thin films attract considerable attention due to its typical properties such as excellent substrate adherence, piezoelectric properties in visible and near infrared region [6], [7]. ZnO films can be prepared using different techniques such as molecular beam epitaxy, chemical spray pyrolysis, vacuum evaporation, sputtering and pulse laser deposition(PLD) [7]. PLD has become an extensive technique for fabricating transparent conducting oxides among numerous deposition techniques owing mainly to the reason that high quality thin films can be fabricated at lower growth temperature even room temperature [8].

Some researchers have study the effects of substrate temperature and post-annealing times on structural and el electrical and optical properties of ZnO films grown by PLD. In these papers, most ZnO films were deposited on crystal substrates at substrate temperature from room temperature to above 500 °C and the target was ZnO in microstructure [9], [10], and post-annealing temperature is usually higher than 400 °C [11], [12]. The effect of low annealing temperature on the structural and electrical properties of ZnO films such that the target is ZnO nanoparticles is rarely investigated. In this paper ZnO thin films were fabricated using PLD at room temperature under vacuum of 8 × 10−4 Pa such that the target is ZnO nanoparticles and then were annealed in air at fixed temperature (400 °C) at different times.

In this work, here we report the fabrication of ZnO thin films by PLD. Employing different annealing times at fixed temperature it was possible to produce nanostructured films. The properties of these annealed thin films at different times, such as structural and electrical properties, were investigated.

Section snippets

Preparation of ZnO nanoparticles using microwave irradiation

All chemicals were purchased from Sigma Aldrich. In a typical procedure, 5 gm zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (99.99%) was dissolved into 30 mL distillated water. After stirring for several minutes, aqueous solution of 5 mol sodium hydroxide was slowly added to the reaction mixture. It was then stirred for 20 min. Finally, the mixture was placed under microwave irradiation for 3 min. The white solid product was washed with distilled water and dried in air at 80 °C. The average grain size of

XRD

Fig. 1 Shows the XRD pattern of deposited ZnO thin films annealed at 400 °C for different times 0, 1, 2 and 4 h. From these figures the XRD peaks which are observed and identified are (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) indicate a preferred orientation. These results are in good agreement with the JCPDS card no. 04-015-0825, where the lattice constant for pure ZnO were evaluated using Rietveld refinement program [9]. The XRD patterns shows that annealed ZnO thin films have (0 0 2)

Conclusion

Structural and electrical properties of annealed ZnO thin films at different times were investigated. X-ray diffraction measurements of the thin film showed hexagonal close-packed structure and preferential orientation along the c-axis. The XRD reveal the presence of hexagonal wurtzite structure of ZnO with preferred orientation (0 0 2). The particle size is calculated using Debye–Scherer equation and the average grain size were found to be in the range 7.77–15.71 nm. The density values of

Acknowledgments

The authors would like to acknowledge the support of the Ministry of Higher Education, Kingdom of Saudi Arabia for supporting this research through a grant (PCSED-003-14) under the Promising Centre for Sensors and Electronic Devices (PCSED) at Najran University, Kingdom of Saudi Arabia.

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