A Systematic Study on the Growth of GaAs Nanowires by Metal−Organic Chemical Vapor Deposition

The epitaxial growth of GaAs nanowires (NWs) on GaAs(111)B substrates by metal-organic chemical vapor deposition has been systematically investigated as a function of relevant growth parameters, namely, temperature, arsine (AsH3) and trimethyl-gallium (TMGa) flow rates, growth time, and gold nanoparticle catalyst size. When growing in excess As conditions (V/III molar ratios greater than four), the NW growth rate is independent of AsH3 concentration, while it is linearly dependent on TMGa concentration, and it is thermally activated. The NW morphology is primarily affected by the growth temperature, with very uniform NWs growing at around 400 degrees C and severely tapered NWs growing above 500 degrees C. A simple phenomenological expression that allows prediction of the NW growth rate over a wide range of growth parameters has been derived. The growth rate dependence on the seed nanoparticle size has also been investigated, which reveals valuable information on the role of catalyst supersaturation and Ga surface diffusion in the growth mechanism. The NW growth rate is found to be almost independent of Au nanoparticle size down to diameters of approximately 20 nm over a wide range of temperatures and TMGa and AsH3 molar flows. For smaller NW radii, the growth rate becomes size-dependent and is strongly affected by the V/III molar ratio; at relatively low V/III ratios, smaller NWs grow more slowly due to the Gibbs-Thompson effect, while at higher V/III ratios (V/III >50), Ga adatom diffusion becomes the dominant mass-transport mechanism, and smaller NWs grow faster than larger ones. The growth-limiting mechanisms in the above growth regimes are finally discussed, and important quantities such as pyrolysis efficiency of the precursors, supersaturation, and surface diffusion length are deduced by comparing the experimental results with the NW growth rates predicted from first principles.