Nanowire arrays with suitable periodicity, aspect ratio, and dielectric constant exhibit interesting optical properties compared to their bulk counterparts, including the ability to tune the bandgap and associated absorption wavelengths. For detectors and photovoltaic applications, vertical arrays of nanowires offer the potential to de-couple the two relevant length scales, i.e. the optical absorption length and the distance a photo-generated carrier must travel to be collected. Nanowire arrays also offer the possibility to change the overall electromagnetic environment, basically through tapering the dielectric transition between air and the semiconductor. In this presentation, we describe branched semiconductor nanowire arrays formed by electrochemical deposition into the partially branched pores of a porous anodic alumina (PAA) template. Over most of their length, the nanowires have diameters of 100 to 150 nm. Near the end where the electrochemical deposition starts, branched structures with diameters around 20 nm are formed. The PAA technique yields relatively dense arrays of nanowires. Following an in-situ annealing step, the main segments of the nanowires are found to be single crystal. The devices exhibited a very low reflectance (less than 5%) over a broad wavelength range. The branched structures provide a tapered dielectric transition between air and the body of the nanowires. A model has been developed in order to understand the length and geometry factors of the dielectric transition as well as the absorption characteristics of various regions within the branched structures.
Jawaharlal Nehru Centre for Advanced Scientific Research
Indo-US Science & Technology Forum
NSF - Office of International Science and Engineering
MGRN 121, Purdue University, West Lafayette, IN