Selective growth of ZnO nanorods on pre-coated ZnO buffer layer
Introduction
One-dimensional (1D) semiconductor nanostructures, such as nanorods and nanowires, have become important fundamental building blocks for nanophotonic devices and offer substantial promise for integrated nanosystems [1], [2]. Nanorods of various compound semiconductors including InP, GaAs, and GaP have recently synthesized in several research groups [3], [4], [5]. Much attention recently has been paid to the nano-structured materials such as ZnO and GaN which radiate ultraviolet (UV) emission. Especially, since ZnO has a wide bandgap of 3.37 eV at room temperature, high mechanical and thermal stabilities, and much larger free exciton binding energy (60 meV) than that of GaN (25 meV), it ensures an efficient excitonic emission up to room temperature. Recently, UV lasing for ZnO nanowires has been demonstrated by Huang et al. [6]. It is expected that a lower threshold optical pumping density for lasing is due to the carrier confinement effect in 1D nanowires. Therefore, it has great potential applications for short wavelength photonic devices.
Various methods have been developed in synthesizing ZnO nanorods [7], [8], [9], [10], [11], [12], [13]. Previous effort in synthesis of high-quality ZnO rods had employed high-temperature process such as vapor–liquid–solid (VLS) mechanism, in which a metal liquid droplet acts as an active catalyst [14]. In this method, the growth temperature were maintained beyond 800°C and the nanorods were randomly grown on substrates. However, to establish an applicable process for the integrated photonic devices, one may have to develop a relatively low temperature and selective growth method for growing desired structures on the patterned templates. In this work, we demonstrated the possibility of the selective growth of ZnO nanorods on patterned ZnO buffer layer. The selectivity of ZnO naorods grown on low-temperature pulsed laser deposition (PLD)-film was significantly enhanced as compared with those directly grown on c-plane sapphire. The growth mechanism is discussed and the origin of the emission at low-temperature luminescence is also analyzed.
Section snippets
Experimental procedure
A high yield of ZnO nanorods were fabricated by the following procedure. The ZnO buffer layer was grown on c-plane sapphire by PLD technique at deposition temperature of 500°C in different oxygen pressure of 10−4, 10−3 and 10−2 Torr. A ceramic ZnO target (99.99%) was ablated in a vacuum chamber using a KrF excimer laser with wavelength of 248 nm and pulse duration of 25 ns. A metal grid as a mask covered on part of the substrate was used to pattern the ZnO film. The preparation of ZnO nanorods was
Results and discussion
Fig. 1 displays the atomic force microscope topography of a ZnO buffer layer with the oxygen pressure of 10−2 Torr. It is seen that the film surface was non-uniform and with an average roughness of 1.3 nm. Presented in Fig. 2 is the SEM photograph of the nanorods grown on ZnO buffer layer with the oxygen pressure of 10−2 Torr. High yield of the nanorods were observed on ZnO buffer layer (left-hand side of the figure) but rare nanorods were observed on the other part of the sapphire substrate,
Conclusions
We have demonstrated the possibility of selective growth of ZnO nanorods on a low temperature grown ZnO buffer layer. The vapor–solid mechanism is responsible for this selective growth of the ZnO nanorods. Although no apparent quantum confinement was observed in these samples from the PL spectra, both of strong edge-emission and phonon-assisted exciton emission in nanorods from low temperature PL spectrum shows good crystalline quality of the grown samples. This low temperature growth technique
Acknowledgements
The authors would like to thank the National Science Council (NSC) and the Ministry of Education of the Republic of China for financially supporting this research under Contract No. NSC 91-2112-M009-016 and 89-E-FA06-AB. And Mr. Hsu gratefully thanks NSC for providing a fellowship.
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