Preparation and characterization of sol–gel Al-doped ZnO thin films and ZnO nanowire arrays grown on Al-doped ZnO seed layer by hydrothermal method

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Abstract

Al-doped ZnO (AZO) nanocrystalline thin films are prepared by a sol–gel technique. Effects of the Al-doped concentration on microstructural, electrical and optical properties of the AZO thin films are studied and discussed. Results indicate that the AZO crystalline thin film with a highly preferred c-axis orientation perpendicular to the substrate is grown, and the AZO thin film with a small crystal grain size of 30–40 nm, high transmittance of above 90% in visible region, and low resistivity of 1.9×10−2 Ω cm can be obtained when the Al-doped concentration is up to 1 at%. Furthermore, ZnO nanowire (ZnO NW) arrays with a large surface area are grown on the sol–gel derived AZO thin film, which acts as a seed layer, by using a hydrothermal method. Optical properties of the grown ZnO NW arrays reveal that a high transmittance in visible region can be obtained, and only a strong UV emission at about 380 nm is observed in the room-temperature photoluminescence spectra, which implies that few crystal defects exist inside the as-assembled ZnO NW arrays.

Introduction

One-dimensional ZnO semiconductor nanostructures, such as nanowires (NWs), nanospheres and nanoribbons, have attracted increasing attention in recent years due to various applications arising from their unique properties [1]. In particular, vertical ZnO nanowire (ZnO NW) arrays are regarded as a promising candidate for nanodevice assembly and applications in blue–UV light emitters [2] and photodetectors [3], field emission devices [4] and dye-sensitized solar cells [5]. Therefore, various methods including thermal evaporation, solution method and epitaxy technique have been employed to prepare the ZnO NW arrays [6], [7], [8]. Although sapphire is the most frequently used substrate for the growth of the ZnO NW arrays, it is not the best option to prepare the ZnO NW devices because it is insulating and expensive. The use of a lattice-matched and conducting buffer layer may circumvent these problems [9], [10], [11]. Therefore, the ZnO NW arrays grown on the doped ZnO seed layer are extensively studied. For example, the Al-, Ga- and In-doped ZnO thin films with high c-axis orientated crystalline structure along (0 0 2) plane are extensively reported for practical applications including transparent conducting electrode materials for various electronic devices such as solar cells, electroluminescence displays, etc. [12], [13], [14], [15], [16]. Among these doped ZnO thin films, Al-doped ZnO (AZO) thin films are attractive owing to their good conductivity, high transparency and relatively low cost [17], [18]. Moreover, they can be prepared by a variety of thin film deposition techniques including radio frequency magnetron sputtering [19], chemical vapor deposition [20], pulsed-laser deposition [21], sol–gel process, etc. [18], [22], [23], [24], [25], [26]. Among them, the sol–gel process is simple and easy, and can prepare the possibility of a small or large-area coating at relatively low temperature and cost [17], [27].

In recent years, the luminescent and electron field emission properties of the ZnO NW arrays grown on AZO seed layers are studied by many researchers [20], [28], but they reported the preparation of the AZO seed layers and the growth of the ZnO NW arrays by a radio frequency magnetron sputtering and a vapor–liquid–solid (VLS) method, respectively, which require expensive equipments and high process temperature. In this paper, the AZO thin films were prepared by the sol–gel process, and effects of the Al-doped concentration on microstructural, electrical and optical properties of the as-prepared AZO thin films were investigated. In addition, the ZnO NW arrays grown on the as-prepared AZO seed layer were grown using a hydrothermal method and the optical and morphological properties of the grown ZnO NW arrays were also characterized and evaluated.

Section snippets

Experimental study

The AZO thin films and the ZnO NW arrays grown on the AZO seed layer were prepared and grown by a sol–gel technique and a hydrothermal method, respectively. Zn(CH3COO)2·2H2O was first dissolved in a 2-methoxyethanol-monoethanolamine (MEA)-deionized water solution at room temperature. The molar ratio of MEA and deionized water to zinc acetate was kept at 1 and 0.5, respectively, and the concentration of zinc acetate was 0.75 mol/L. Then appropriate amounts of aluminum doping were achieved by

Microstructural and electrical properties of the AZO thin films

Fig. 1 shows the SEM images of the AZO thin film samples. It can be seen that the AZO thin films with a uniform crystal grain and flat surface morphology can be obtained by the sol–gel technique under present process conditions, but the crystal grain size of the sample changes with the Al-doped concentration, which starts to decrease with the doping of the Al and then increase with a further increases in the Al-doped concentration. For example, the crystal grain size of Sample B is smaller than

Conclusions

The AZO thin films have been successfully prepared by the sol–gel method. Effects of the Al-doped concentration on microstructural, electrical and optical properties of the AZO thin films have been studied. Results indicate that the as-prepared AZO thin films have a wurtzite c-axis preferred orientation perpendicular to the glass substrate, and the AZO thin film with small crystal grain size, high transmittance and low resistivity can be obtained when the Al-doped concentration is 1 at%. The ZnO

Acknowledgments

This work was supported by the Ministry of Science and Technology of China through 863-project under Grant 2009AA03Z218, the Major Program of the National Natural Science Foundation of China under Grant no. 90923012, and Xi’an Applied Materials Innovation Fund (XA-AM-200805).

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