Numerical Simulation of the Growth of ZnO Nanostructures in a Tube Furnace by Physical Vapour Deposition
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1074-I05-06
Numerical Simulation of the Growth of ZnO Nanostructures in a Tube Furnace by Physical Vapour Deposition Sharvari Dalal1, Federico Gallo2, Andrew J Flewitt1, and William I Milne1 1 Electrical Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, United Kingdom 2 Ravensworth Gardens, Cambridge, CB1 2XL, United Kingdom ABSTRACT Zinc oxide is a versatile II-VI naturally n-type semiconductor that exhibits piezoelectric properties. By controlling the growth kinetics during a simple carbothermal reduction process a wide range of 1D nanostructures such as nanowires, nanobelts, and nanotetrapods have been synthesized. The driving force for the nanostructure growth is the Zn vapour supersaturation and supply rate which, if known, can be used to predict and explain the type of crystal structure that results. A model which attempts to determine the Zn vapour concentration as a function of position in the growth furnace is described. A numerical simulation package, COMSOL, was used to simultaneously model the effects of fluid flow, diffusion and heat transfer in a tube furnace made specifically for ZnO nanostructure growth. Parameters such as the temperature, pressure, and flow rate are used as inputs to the model to show the effect that each one has on the Zn concentration profile. An experimental parametric study of ZnO nanostructure growth was also conducted and compared to the model predictions for the Zn concentration in the tube.
INTRODUCTION Zinc oxide (ZnO) is a direct gap semiconductor with a wide band gap that affords optical transparency (~3.3eV). Furthermore, it has a large exciton binding energy (~60meV) which is more than two times higher than thermal energy at room temperature. Therefore, there has been great interest in the production of ZnO nanostructures, as their small size and large surface to volume ratio could enable a number of nanowire devices, including UV nanolasers, light emitting diodes and chemical sensors. A variety of complex ZnO nanostructures, such as nanowires, nanobelts, and tetrapods, have been deposited using a vapour deposition method involving carbothermal reduction. In this method, a mixture of zinc oxide powder and carbon powder is heated in an atmosphere of nitrogen and oxygen. Carbothermal reduction leads to the formation of zinc vapour, which then reacts with oxygen in the presence of a catalyst to produce nanostructures. This work addresses the difficulty in predicting the nature of the nanostructures that will be produced as a function of furnace geometry and processing conditions by proposing a numerical model for deposition.
Zn vapor supersaturation and supply rate help determine the type of crystal growth that results. Control over the level of supersaturation would allow more precise manipulation of the morphology, density and diameter of nanostructures that are produced. The Zn vapour concentration depends on several macroscopic variables such as the pressure, temperature and flow rate. Therefore, a model has been developed which attempt
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