Chemical Vapor Deposition Methods

In order to realize device applications in optoelectronics, the practical configuration will necessitate the growth of ZnO nanorod on a substrate, with an electrically active nanojunction. There are two main methods reported for the synthesis of aligned ZnO nanorods on substrates: either by chemical vapor deposition methods or hydrothermal synthesis. In both fabrication methods, the anisotropic growth of the ZnO crystal is due to a faster rate of growth in the c-axis, resulting in the morphology of a rod shape with the [0001] direction normal to the substrate. If the nucleation density on the substrate is high, as promoted by precoating the surface with a catalyst or a buffer layer of ZnO, the very dense vertical growth is a result of the crowding effect. Hence most of the vertically aligned growth reported in literature is not due to true epitaxial growth with respect to the substrate, but arises from the overcrowding effect.

It is now well established that vapor phase methods can produce high-quality aligned ZnO nanorod arrays, but gas phase reactions generally require prohibitively high growth temperatures of between 800-900°C. Yang and co-workers at the University of Berkeley demonstrated the fabrication of ZnO nanowire arrays on a-plane sapphire in a furnace [12]. The starting materials and the substrates were heated to 900° C in an argon flow and the Zn vapor was generated by carbothermal reduction of ZnO, and transported to the substrates where ZnO nanowires grew. A thin layer of Au catalyst was predeposited on the sapphire. The growth mechanism follows the well-known VLS mechanism. Selective growth can be readily achieved by patterning the Au thin film before growth. Wang and Summers [13] demonstrated large-scale hexagonal-patterned growth of aligned ZnO nanorods by using a catalyst template produced by a self-assembled monolayer of submicron polystyrene spheres.

Aligned growth of ZnO nanorods without catalyst has also been achieved using low-temperature chemical vapor deposition and metal organic chemical vapor deposition (MOCVD). Although no metal catalyst was coated on the sapphire, a very thin layer of ZnO buffer layer was grown at a low temperature prior to the ZnO nanorod growth. G. C. Yi et al. utilized diethylzinc (DEZn) and oxygen as reactants and ZnO nanorods can be grown on sapphire at 400-500° C [14].

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