Elsevier

Ceramics International

Volume 42, Issue 9, July 2016, Pages 10847-10853
Ceramics International

Microstructural and optical properties of Ta-doped ZnO films prepared by radio frequency magnetron sputtering

https://doi.org/10.1016/j.ceramint.2016.03.214Get rights and content

Abstract

Ta-doped ZnO films with different doping levels (0–5.02 at%) were prepared by radio frequency magnetron sputtering. The effects of the doping amount on the microstructure and the optical properties of the films were investigated. The grain size and surface roughness first significantly decrease and then slowly increase with the increase of Ta doping concentration. Both the grain size and the root mean square (RMS) roughness reach their minimum values at the doping content of 3.32 at%. X-ray Diffraction (XRD) patterns confirmed that the prepared Ta-doped ZnO films are polycrystalline with hexagonal wurtzite structure and a preferred orientation along the (002) plane. X-ray photoelectron spectroscopy (XPS) analysis reveals that Ta exists in the ZnO film in the Ta5+ and Ta4+ states. The average optical transmission values of the Ta-doped ZnO films are higher than those of the un-doped ZnO film in the visible region. The band gap energy extracted from the absorption edge of transmission spectra becomes large and the near band edge (NBE) emission energy obtained from PL spectra blueshifts to high energy when the Ta doping content grows from 0 at% to 5.02 at%, which can be explained by the Burstein–Moss shift.

Introduction

Zinc oxide (ZnO) has drawn many attention due to its excellent physical properties such as large exciton binding energy (60 meV), excellent transparency in the visible range, wide direct band gap (∼3.37 eV), and high piezoelectric constant. It has potential applications in gas sensors, UV lasers, surface acoustic wave devices, and film bulk acoustic resonators. The properties of ZnO can be improved and the range of its applications can be expanded by doping different foreign elements into ZnO. ZnO doped with group III elements such as Al, Ga, and In shows enhanced conductivity and can be used as a transparent conducting oxide (TCO) [1], [2], [3], [4], [5]. ZnO doped by transition elements such as Co, Mn and Fe exhibits ferromagnetic (FM) behavior and can be used as a dilute magnetic semiconductor (DMS) in spintronics devices [6], [7], [8], [9], [10]. Furthermore, p-type ZnO is produced by doping ZnO with N, P or Sb elements [11], [12], [13] for use in the blue and ultraviolet solid-state light emitters and detectors. Additionally, the use of doped ZnO can improve the gas sensor selectivity due to the specific interactions between the molecules to be detected and the ions in the sensing materials [14], [15], [16].

The efficiency of the dopant element is related to its electronegativity as well as to the difference between its ionic radius and the ionic radius of Zn [17]. Ta is an efficient doping element for ZnO due to the high valence difference between Ta+5 (or Ta+4, Ta3+) ions and substituted Zn+2 ions. Therefore, a very small amount of Ta dopant can provide enough free carriers and reduce the ion scattering effect [18]. Moreover, the ionic radii of Ta5+ (0.064 nm), Ta4+ (0.068 nm), and Ta3+ (0.072 nm) are close to the radius of Zn2+ (0.074 nm); thus, it is theoretically possible for Ta to substitute for Zn in the ZnO lattice. However, there are only a few reports in the literature on Ta-doped ZnO films [17], [18], [19], and to the best of our knowledge, many properties of Ta-doped ZnO film, for example luminescence, have not yet been studied systematically.

In this study, we prepared Ta-doped ZnO films with various Ta contents by RF magnetron sputtering. The composition, morphology, microstructure, optical and photoluminescence properties of Ta-doped ZnO films were investigated.

Section snippets

Preparation of Ta doped ZnO films

Ta-doped ZnO films have been deposited on Si and glass substrates by radio frequency magnetron sputtering. ZnO ceramic (purity: 99.99%, diameter: 6 cm, thickness: 3 mm) was used as the sputtering target. High purity (99.99%) Ta metallic wires, with a diameter of 1.5 mm and a length of 6 cm, were hung above the ZnO target and served as the dopant source. The doping amount was controlled by changing the number of Ta wires. The number of Ta wires varied from 1 to 5 and the corresponding samples were

Results and discussions

The prepared films were analyzed by EDS to identify their compositions with the results shown in Fig. 1(a). Only three elements (Zn, O and Ta) are found in the films. Fig. 1(b) shows the atomic percent contents of the three elements in Ta-doped ZnO films. Ta atomic percent is 0 at% for S0, 0.34 at% for S1, 0.97 at% for S2, 2.02 at% for S3, 3.32 at% for S4, and 5.02 at% for S5. Zn fraction decreases from 46.20 at% to 37.14 at% and the O atom fraction slightly increases from 53.80 at% to 57.84 at% as the

Conclusion

Un-doped and Ta-doped ZnO films have been prepared by radio frequency magnetron sputtering. Ta content in the ZnO films measured by EDS is in the range of 0–5.02 at%. The surface morphology, microstructure, optical and photoluminescence properties of the prepared films were studied. The results show that the grain size, film thickness and surface roughness of the prepared films decrease with increasing Ta doping concentration from 0 at% to 3.32 at%. The smallest RMS roughness is 0.56 nm for the ZnO

Acknowledgments

This study was supported by the National Natural Science Foundation of China (Nos. 61504096, 61401306, 61301045 and 61306010), the Natural Science Foundation of Tianjin City (Nos. 13JCZDJC36000 and 15JCYBJC24000), the Excellent Young Teachers Program of Tianjin, and the Youth Top-notch Talents Program of Tianjin.

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