Elsevier

Ceramics International

Volume 38, Issue 5, July 2012, Pages 3875-3883
Ceramics International

The role of substrate temperature on the properties of nanocrystalline Mo doped ZnO thin films by spray pyrolysis

https://doi.org/10.1016/j.ceramint.2012.01.039Get rights and content

Abstract

Transparent conducting molybdenum (2 at.%) doped zinc oxide (MZO) films were prepared with various substrate temperatures by spray pyrolysis technique on glass substrates. The effect of substrate temperature on the structural, surface morphological, electrical, optical and photoluminescence properties of these films were studied. The X-ray diffraction analysis revealed that the films are polycrystalline in nature having a wurtzite structure with a preferred grain orientation in the (0 0 2) direction. The average crystallite size of the films increases from 17 nm to 28 nm with the increase of substrate temperature from 573 K to 623 K, thereafter it slightly decreases with further increase of substrate temperature to 723 K. Analysis of structural parameters indicates minimum strain and stress values for films deposited at a substrate temperature of 673 K. From atomic force microscopy (AFM) analysis, it is found that rms roughness of the films deposited at 623 K is a minimum, indicating better optical quality. The scanning electron microscopy (SEM) measurements showed that the surface morphology of the films changes with substrate temperature. Optical parameters such as optical transmittance, reflectance, refractive index, extinction coefficient, dielectric constant and optical band gap have been studied and discussed with respect to substrate temperature. Room temperature photoluminescence (PL) spectra show the deep-level emission in the MZO thin films. The films exhibit a low electrical resistivity of 6.22 × 10−2 Ω cm with an optical transmittance of 75% in the visible region at a substrate temperature of 623 K.

Introduction

Transparent conducting zinc oxide films have been extensively studied in recent years, because of their low cost precursor materials, relatively low deposition temperature and high stability in hydrogen plasma compared to ITO and SnO2 films [1]. These advantages are of considerable interest for solar energy conversion appilications. ZnO have high chemical and thermal stability and high abundance make it an attractive material for a wide variety of applications, such as UV emitters and detectors, gas sensors, light emitting devices and transparent conducting electrodes [2]. Zinc oxide (ZnO) is a II–VI n-type semiconductor with a wide band gap, large free exciton binding energy (60 meV), high transparency in the visible region and a wide range resistivity values (10−4 to 1012 Ω cm) [3]. Due to their optical and electrical properties metal oxide semiconductor films have been widely studied and received considerable attention in recent years. Some of them are good candidates for the application in transparent conductive films, if they are prepared off-stoichiometry or doped with suitable impurities. ZnO is one of the metal oxide semiconductors suitable for use in optoelectronics and an alternative material to ITO [4]. The electrical conductivity of zinc oxide depends on the carrier concentration contributed by oxygen vacancies or interstitial metal atoms in it [5]. Molybdenum (Mo) is one of the potential dopant materials for improving conductivity and transparency of the zinc oxide thin films. The substitution of Mo is possible due to the smaller radius of Mo (0.062 nm) compared to Zn (0.083 nm). Moreover, Mo ([Kr]: 4d55s1) is the more beneficial impurity to be doped into the ZnO matrix as it can donate 4 electrons to the free carriers due to the high valence difference between Mo6+ ions and substituted Zn2+ ions. Therefore, very small amount of Mo doping can give enough free carriers and reduce the ion scattering effect [6]. The investigations on MZO films are valuable in exploring substitute materials for ITO and may have potential application prospects in transparent optoelectronic devices.

Different deposition techniques are used to prepare MZO thin films such as RF/DC sputtering [7], [8] and ion beam sputtering deposition (IBSD) [9]. In comparison to other chemical deposition techniques, spray pyrolysis [10] has several advantages such as high purity and excellent control of chemical uniformity in multi-component system. Spray pyrolysis has been developed as a powerful tool to prepare various kinds of thin films such as metal oxides and nanophase materials. Another advantage of the spray pyrolysis technique is that it can be adapted easily for production of large-area films. Investigations on the preparation and characterization of MZO films are less in the literature. Xiu et al. [7] used RF magnetron sputtering to deposit MZO films onto a glass substrate was one of such reports. In the present study, transparent and conductive Mo doped ZnO thin films (MZO) are prepared at different substrate temperatures (Ts) using a spray pyrolysis method and the effect of substrate temperature (Ts) on the structural, optical, electrical and photoluminescence properties of MZO films was investigated.

Section snippets

Experimental

Mo doped ZnO thin films were prepared by spray pyrolysis technique at substrate temperatures of 573 K, 623 K, 673 K and 723 K. The precursor solution for spray pyrolysis was prepared by dissolving an appropriate amount of zinc acetate dehydrate (Sigma–Aldrich, 99.5%, Germany) and molybdenum chloride (Sigma–Aldrich, 99%, USA) in the 100 ml mixture of deionized water and ethanol (Merck, 99.9%, Germany) at room temperature. A small amount of acetic acid (Merck, 99.9%, Germany) was added into the

Structural studies

Fig. 1 shows the XRD patterns of 2 at.% MZO thin films deposited by spray pyrolysis at different substrate temperatures. Spray pyrolysis is a chemical deposition technique where the endothermic thermal decomposition takes place at the hot surface of the substrate to give the final product. The substrate temperature plays an important role in the film formation. When the substrate temperature is below 553 K, the spray falling on the substrate undergoes incomplete thermal decomposition (oxidation)

Conclusions

In this study, the influence of the substrate temperature on the structural, surface morphology, optical and electrical properties of MZO thin films grown on glass substrates by spray pyrolysis was investigated. From the X-ray diffraction (XRD) pattern, it was observed that the MZO thin films were polycrystalline with wurtzite structure. From AFM studies, it was found that the surface roughness of the thin films increases with the increase of substrate temperature. SEM analysis revealed the

Acknowledgement

Author M. C. S. Kumar is thankful to the Department of Science and Technology (DST), Govt. of India for the financial support through SERC-Fast Track project for young Scientists.

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