Control of Nanowire Growth by Droplet Dynamics with Optical Applications

Abstract

Self-catalyzed GaAs nanowires (NWs) are grown epitaxially on Si(111) substrates using molecular beam epitaxy (MBE). The dynamics of the droplet are examined to improve NW yield and to control NW morphology. Control and understanding of the NW diameter via droplet dynamics is applied to NW photovoltaics and to novel corrugated NW distributed Bragg reflectors (DBRs). At the beginning of the MBE growth, a Ga pre-deposition step, between 0 s and 500 s in duration, is introduced to improve the yield of the NW arrays. The effect of the pre-deposition time was examined for five different hole diameters and yield was increased to nearly 100% for the appropriate combination of hole diameter and pre-deposition time. Two models were used to model the NW growth progression under different atomic flux ratios. The first model considers the contributions from direct and diffusion fluxes to the droplet and solves coupled equations for the droplet contact angle and the NW radius. The second model treats the contact angle as constant. Both models explained the accompanying experimental observations. Both models could be used to model future NW growths and the choice between the two would depend on the availability of contact angle data and whether the crystal phase must be considered. Absorption in NWs is determined by the diameter and the HE1n modes. The effectiveness of a linearly tapered inverted conical NW is demonstrated using finite element simulations. The photocurrent of an optimized inverted conical NW array is found and shown to be similar to that achieved by optical nanocones and nanowires. Diameter modulations can also be introduced into NW structures periodically to produce corrugated NW distributed Bragg reflectors (DBRs). The tunability of the reflectance peaks is demonstrated and explained by changes to the effective refractive index of the structure.