ZnO:Al nanostructures synthesized on pre-deposited aluminum (Al)/Si template: Formation, photoluminescence and electron field emission
Introduction
Nanostructures, such as nanowires, nanorods and nanobelts have attracted much attention as semiconductor nanostructured materials having many unique properties with potential applications, such as in gas sensors [1], dye-sensitized solar cells (DSSCs) [2], and optoelectronic devices [3]. Zinc oxide (ZnO) nanostructures have been considered as promising materials for electronic and photonics applications because of their large excitation binding energy of 60 meV with wide direct band gap at 3.37 eV [4]. The ZnO nanostructures are doped with approximate dopant (i.e., gallium, aluminum, indium) [5], which can increase their electrical properties [6], [7]. Therefore, a large amount of transparent conducting oxides (TCO), such as indium oxide, tin oxide, and zinc oxide, have been widely studied over the years. However, Al doped ZnO are considered as attractive alternative materials as transparent conducting electrode because they are nontoxic and inexpensive compared with indium tin oxide (ITO). Although many studies have been focused on ZnO:Al thin films by utilizing sputtering process [8], [9], [10], one dimensional nanostructured materials of ZnO:Al were rarely investigated.
Thermal evaporation is often applied to synthesize the highly crystalline nanostructures. The method is simple and straightforward, enabling its integrate with the doping process. However, Al is highly oxidized at high temperatures. Therefore, preparation of ZnO:Al nanostructures continue to present challenges. To address this challenge, Hsu et al. [11] reported that the ZnO:Al nanowires can be synthesized by thermal evaporation by introducing a high-voltage source to bombard the Al source material into vapor, which subsequently mixes with the Zn vapor and then condensed on the substrate to form the ZnO:Al nanorods. Wang et al. [12] also developed an alloy-evaporation deposition method of Zn–Al to synthesize the single-crystalline of ZnO:Al nanostructures. However, concerning the semiconductor process in microelectronics, the ZnO:Al nanostructures should be fabricated for large area deposition. In this work, we report a simple method whereby an Al film (~ 200 nm) was pre-deposited on Si substrate by DC sputtering with Zn powder acting as the vapor source. We believe that the Al layer with Zn vapor forms as a Zn–Al cluster which acts as the dopant source of Al, and subsequently precipitates as the ZnO:Al nanostructures during the crystal growth.
FESEM (field emission scanning electron microscopy), HRTEM (high-resolution transmission electron microscopy), XRD (X-ray diffractometer), and EDS (energy dispersive X-ray spectroscopy) techniques are used for the structural characterization. All of these techniques revealed that the ZnO:Al nanostructures were precipitated from the liquid droplets of Al–Zn. The field emission properties of ZnO:Al nanostructures exhibited a low turn-on field at 3.8 V/μm which is relatively lower than that of control sample of ZnO nanowires (5 V/μm); the current density was at 1 µA/cm2. The photoluminescence (PL) spectra revealed that the ZnO:Al nanostructures have a high intensity of green emission and a blue-shift emission at 496 and 382 nm, respectively as compared to those of pure ZnO nanostructures.
Section snippets
Experimental
Silicon substrates were pre-deposited with an Al thin layer of thickness 200 nm. The source material of Zn (0.5 g, 99%, 200mesh) was loaded into alumina boat and then placed at the center zone of quartz reactor. In the beginning, an argon flow rate of 40 sccm was introduced into the reaction system to maintain the vacuum pressure at 10 Torr at all times. After that, a 0.5 sccm of oxygen flow was introduced to the system when the temperature reached 650 °C, and then kept at this temperature for
Results and discussion
Comb-like ZnO:Al with a high aspect ratio and pure ZnO nanostructures were grown on the silicon (Si) substrate as shown in Fig. 1(a) and (b), respectively. The dimensions of the ZnO:Al and pure ZnO nanostructures are about 200–300 nm in diameter and up to several ten microns in length. Fig. 1(c) further shows the thin film XRD pattern of as-synthesized products. Obviously, all products belong to a hexagonal (wurzite) ZnO with lattice constants a = 0.325 nm and c = 0.52 nm, which are consistent with
Conclusions
A pre-deposited Al layer on Si substrate was employed to synthesize ZnO:Al nanostructures by thermal evaporation at 650 °C. The TEM EDS mapping showed that the Al element was highly scattered on the ZnO nanostructures. A blue-shift emission peak was found in ZnO:Al nanostructures (382 nm) in contrast to that of pure ZnO nanostructures (385 nm). The turn-on and threshold field of ZnO:Al nanostructures was estimated as ~ 3.8 and 25 V/µm, respectively. This result is superior to pure ZnO
Acknowledgements
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 95-2218-E-035-007 and NSC 96-2221-E-035-058.
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