Dependence of structural and optical properties of sol–gel derived ZnO thin films on sol concentration

Abstract

In this work, ZnO thin films were prepared by sol–gel method and the dependence of structural and optical properties of these films on sol concentrations was deeply investigated. Unlike the previous studies, the ZnO thin films deposited in this study have approximately equal thickness, which excludes the influence of film thickness on the physical properties. The results show that low sol concentration is favorable for obtaining high c-axis oriented ZnO thin films with good crystalline quality. When the sol concentration is above 0.7 mol/L, the degree of c-axis orientation of ZnO thin films decreases and the optical quality is also degraded. Photoluminescence spectra indicate that the defect-related blue emission is increased with the enhancement of sol concentration. The mechanism of the blue emission is analyzed. The reason why high sol concentration is unfavorable for formation of high c-axis oriented ZnO thin films and obtaining high optical quality is also discussed.

Introduction

In recent years, semiconducting materials with a wide band gap such as GaN, ZnO, ZnSe, ZnS, etc. have attracted more and more attention due to their potential applications in optoelectronic devices. Compared with other materials, ZnO possesses some striking advantages: (1) at room temperature, it has a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV which is much larger than those of GaN (25 meV), ZnSe (22 meV) and ZnS (40 meV); (2) ZnO is non-toxic and is rich on earth; (3) ZnO has abundant forms such as single crystal bulk, powder, thin film, nanowire, nanotube, etc.; (4) ZnO thin films can be epitaxially grown on substrates at low temperature. So far, ZnO has been widely studied in the form of thin film. ZnO thin films are usually highly transparent in the visible region. If ZnO thin films are doped with Al, Ga or In, they will show excellent n-type conductivity. Therefore, these films can be used as transparent electrode materials or window layer materials in solar cells [1]. Highly c-axis oriented ZnO thin films possess good piezoelectric effect which allows them for application in surface acoustic wave devices [2]. The most concern is the luminescence properties of ZnO thin films. Many studies showed that high quality ZnO thin films possess excellent ultraviolet emission performance, so they have been the ideal candidate for the fabrication of ultraviolet light-emitting devices such as ultraviolet lasers [3], ultraviolet light-emitting diodes [4].

Many results showed that the structural, optical and electrical properties of ZnO thin films have a strong dependence on the deposition method as well as deposition parameters. So far, ZnO thin films can be prepared by lots of techniques such as metal-organic chemical vapor deposition (MOCVD) [5], spray pyrolysis [6], sol–gel method [7], molecular beam epitaxy [8], atomic layer epitaxy [9], electron beam evaporation [10], magnetron sputtering [11], pulsed laser deposition [12], etc. It is a wide concern that which technique is the best one for fabricating ZnO thin films for large-scale commercial use. Many studies showed that MOCVD, molecular beam epitaxy and atomic layer epitaxy all can deposit high quality ZnO thin films, but these methods need complex and expensive equipments and make them unsuitable for commercial use. Among other methods, the sol–gel method is an attractive technique for depositing ZnO thin films and has been paid much attention to by researchers. Its striking advantages include low cost, simple equipments, no need of vacuum deposition environments, easy control of composition, etc. Furthermore, the sol–gel method is especially suitable for fabricating doped ZnO thin films, for the doped atoms have homogeneity on molecular level due to the mixing of liquid precursors.

Although the sol–gel method is a relatively simple technique for depositing ZnO thin films, there are still some factors which will affect the properties of the prepared films. These factors include sol aging time [7], sol concentration [13], preheating temperature [14], post-annealing temperature [15], annealing atmosphere [16], etc. As for the influence of annealing conditions on the properties of ZnO thin films derived from sol–gel method, many results have been reported. However, there are few studies on the influence of sol concentration on the physical properties of ZnO thin films. Even though some results have been reported, there are still many differences between them. For example, O’Brien et al. [17] found that the ZnO thin film prepared by 0.3 mol/L sol showed the best c-axis orientation, and the film gradually became randomly oriented with the further increase of the sol concentration. Gritskova et al. [18] found that the ZnO thin film deposited by 0.1 mol/L sol had the best c-axis orientation; when the sol concentration was increased from 0.1 to 0.7 mol/L, the degree of c-axis orientation was gradually decreased and the intensity of (0 0 2) peak was largely reduced. Kim et al. [19] found that the ZnO thin film prepared by 0.7 mol/L sol presented the best c-axis orientation when the sol concentration was within the range of 0.3–1.3 mol/L. On the contrary, for the transmittance of ZnO thin films prepared by sol–gel process, O’Brien et al. [17] found that the transmittance of ZnO thin films in the visible range was gradually reduced when the sol concentration was increased from 0.3 to 1.3 mol/L. Gritskova et al. [18] found that the transmittance of ZnO thin films in the visible region was almost not changed when the sol concentration was increased from 0.1 to 0.7 mol/L. However, Kamaruddin et al. [13] found that the transmittance of ZnO thin films in the visible range was gradually enhanced when the sol concentration was increased from 0.3 to 0.7 mol/L. From the above-mentioned results, it can be seen that the studies about the influence of sol concentration on the physical properties of ZnO thin films are insufficient for understanding the relationship between sol concentration and properties of ZnO thin films. Considering the ZnO thin films prepared by sol–gel method have many potential applications in lots of fields [20], [21], it is necessary to deeply investigate the influence of deposition conditions especially sol concentration on the physical properties of ZnO thin films. Furthermore, in the previous studies [17], [18], [19], the ZnO thin films were prepared by spin-coating the sols with different concentration on substrates with the same times. As a result, the resulting films have totally different thickness. However, it is known that the film thickness has a great effect on the properties of ZnO thin films [22]. So, it is difficult to determine whether the differences of the physical properties of ZnO thin films prepared by various sol concentrations were resulted from sol concentration or from film thickness on earth. In this study, we prepared ZnO thin films with approximately equal thickness using sol different concentrations, excluding the influence of film thickness on the physical properties of these films. The dependence of structural and optical properties of ZnO thin films deposited by sol–gel process on sol concentration is clearly disclosed.