Abstract
GaAs nanowire (NW) arrays were grown on Si substrates by selective-area molecular beam epitaxy (MBE) using the self-assisted vapor-liquid-solid (VLS) method. In the self-assisted VLS method, the GaAs NW diameter is determined by the size of a Ga droplet at the top of the NW. Absorption of the solar spectrum for photovoltaic devices is optimum with a GaAs NW diameter of ∼170 nm, requiring a Ga droplet during the growth of a similar size. Control of the Ga droplet size, and therefore the NW diameter, is demonstrated by controlling the V/III flux ratio during the MBE growth. A V/III flux ratio near unity rapidly increased the Ga droplet volume during the NW growth, thereby creating an inverse-tapered (funnel-shaped) NW morphology. Numerical simulations indicated that the inverse-tapered NW showed strong broadband absorption of the solar spectrum due to the absorption of consecutively longer wavelengths toward the base of the NW.
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