Role of ZnO thin film in the vertically aligned growth of ZnO nanorods by chemical bath deposition
Graphical abstract
Size-controlled ZnO nanorod arrays were vertically grown on ZnO thin films by a chemical bath deposition method at 90 °C using hexamethylenetetramine as a reducing agent.
Introduction
Zinc oxide (ZnO) nanostructures have long been gaining attention due to their intriguing properties such as wide and direct band gap (3.37 eV) at room temperature and simultaneous revelation of high electrical, optical, and piezoelectric performance. ZnO nanostructures have found applications in the fields of piezoelectric transducers, photovoltaic devices, gas sensors, biosensors, transistors, and optoelectronic devices [1], [2], [3], [4], [5], [6], [7]. In particular, highly-oriented ZnO nanowire or nanorod arrays are required for high-performance optoelectronic devices, and their growth method are of critical importance.
Several groups have grown vertically aligned ZnO nanowire/nanorod arrays on silicon or glass substrates without the use of any textured thin film as a seed layer. These arrays were synthesized at a temperature range of 400–600 °C, using metal-organic chemical vapor deposition (MOCVD) [8], [9], [10], pulsed laser deposition (PLD) [11], or chemical vapor transport (CVT) [12]. However, all of these methods employ complex and expensive processes at high temperatures. As an alternative of these physical vapor methods, solution-based ZnO nanorod growth methods were emerged, which do not need high temperature or vacuum [13], [14], [15]. Chemical bath deposition (CBD) has particularly received a great interest because of its simple experimental setup, low cost, and good potential for scaling up. As far as the CBD growth of ZnO nanorods is concerned, using hexamethylenetetramine (HMTA) ensures high crystallinity and good morphological property of ZnO nanorods, as compared with other reducing agents [16], [17].
Vertically aligned ZnO nanorod arrays have also been grown on various substrates with the support of textured ZnO seed layers such as ZnO colloids, ZnO nanocrystal layers, and ZnO thin films [18], [19], [20]. For instance, Greene et al. prepared ZnO colloids and nanocrystals in aqueous solution by the hydrolysis of zinc salts on Si substrate at 200–350 °C [19]. Solis-Pomar et al. deposited ZnO thin films with different thicknesses by atomic layer deposition [20]. However, these methods also require high temperature or costly processes. As previously demonstrated for various applications [21], [22], [23], [24], [25], [26], [27], sputtering ZnO thin films on a substrate may be a much simpler way to prepare a ZnO seed layer for the subsequent growth of vertically aligned ZnO nanorods. Till now, no systematic study has been reported on a correlation between the property of sputtered ZnO seed film and the features of ZnO nanorods grown from that.
In this work, ZnO seed layers are deposited on Si substrate by RF magnetron sputtering at room temperature, with varying the layer thickness. Thickness-dependent surface morphology of the seed layer and its effect on the vertical growth of ZnO nanorod arrays are investigated. The CBD technique is employed to grow the ZnO nanorod arrays using HMTA as a reducing agent.
Section snippets
Experimental
The fabrication process of vertically aligned ZnO nanorod arrays includes two steps: preparation of textured ZnO thin films using RF sputtering and ZnO nanorod growth using CBD method.
Results and discussion
Fig. 1 shows the AFM images of ZnO seed layers prepared by RF sputtering. The growth rate was kept constant at 8 nm/min for all samples, and the sputtering time was altered to obtain ZnO seed layers of 80, 120, 160, and 200 nm, respectively, in thickness. It is found from the AFM images that ZnO thin films are composed of small grains and the grain size depends on the sputtering time. For the 10 min-sputtered ZnO thin film, small ZnO grains are observed on the surface (Fig. 1(a) and (b)). The
Conclusions
ZnO thin films were deposited on Si substrate at room temperature by RF sputtering. The film thickness was adjusted in the range of 80–200 nm by controlling the sputtering time. Both the surface roughness and the average grain size of ZnO film turned out to increase with an increase in the film thickness. ZnO nanorods were grown on these ZnO thin films on Si substrate, using a CBD technique. It was disclosed that dense, hexagonal-shaped ZnO nanorods were almost vertically aligned, and their
Acknowledgements
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013R1A1A2010630).
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