Aluminium-Doped ZnO Films and Nanowires by a Modified Chemical Vapor Deposition Technique
S.Y. Pung, K.L. Choy
University of Nottingham, UK
Keywords: ZnO, nanowires, thin films, doping
Abstract:ZnO has unique properties such as wide-bandgap (3.37eV) and large exciton binding energy (60meV) at room temperature. These properties made ZnO a promising material for short wavelength photonic devices for examples, light emitting diode (LED) , solar cell  and room temperature UV laser. It is essential to produce n-type and p-type ZnO films or nanowires (NWs) with controlled conductivity in order to realize the potential applications of ZnO. Many techniques, for instances, Metal Organic Chemical Vapor Deposition (MOCVD) , Atomic Layer Deposition (ALD) , Magnetron sputtering , Pulsed Laser Deposition (PLD) , etc. have been employed to produce n-type and p-type ZnO films and NWs. Nevertheless, the inherent limitations of these techniques such as high equipment and processing cost, high synthesis temperature, difficult to control doping concentration and/or lack of process flexibility, might be the main drawback for producing high quality and cost effectively doped ZnO films and NWs. A modified CVD technique has been used to produce Al-doped ZnO films and nanowires. In this technique, the precursor solution (dopant) is located outside the reactor (horizontal furnace), has became the key strength of this technique. This unique setup allows a better control of the precursor supply rate and supply period, uniform doping attributes to the uniform precursor flux in the reactor, flexibility of changing different types of precursor and composition during synthesis process, possibility of refilling the precursor solution during the synthesis process, low synthesis temperature as the precursor solution normally decomposes at the temperature range <600oC, a wider choice of precursor, and a low setting up cost [7-8]. Al-doped (n-type) ZnO films and NWs are successfully produced by this modified CVD technique. In this study, aluminium related solution and Zn powders have been used as Al and Zn sources, respectively. The aluminium related aerosol is then generated from the vibration of transducers and is delivered into the quartz tube by Argon carrier gas. Oxygen gas, with flow rate between 0 to 10% of the total gas flow, is supplied to facilitate the growth of ZnO films and NWs. A highly c-oriented Al-doped ZnO film is deposited as shown in Fig. 1. The electrical resistivity of these films can be adjusted in the range of 100 – 102 .cm by varying the concentration of precursor solution. Fig. 2 shows the vertically aligned Al-doped ZnO NWs synthesized on a ZnO film pre-coated Si substrate simply by changing the synthesis parameters. These results, although are yet to optimized, have shown the great potential of modified CVD as a low cost and flexible technique to produce doped ZnO NWs. It is worth to point out that different type of dopant precursors can be used in this modified CVD technique to produce p-type, n-type or a p-n junction with controlled conductivity.