| Photovoltaic devices consisting of highly periodic, ultradense, silicon nanowire arrays and nanohole arrays have been fabricated with nominal nanowire widths of 20 nm, nanohole sizes of 12 nm, and lattice pitches of 32 nm, deep in the subwavelength regime for visible light. We have developed a set of surface passivation protocols that provide the extremely low surface recombination velocities typical of thick, high-quality, furnace-grown thermal silicon dioxide, but within an ultrathin layer on the order of 5 -- 10 nm thick. With this high quality oxide passivation, these devices exhibit good photovoltaic performance that rivals or exceeds all comparable devices reported in the literature. Using a collection of characterization techniques, including optical microscopy, scanning electron microscopy, cross-sectional transmission electron microscopy, and spectroscopic ellipsometry, we characterize the structure and morphology of these nanostructure arrays. The high perfection of the arrays enables absorptance calculations to be performed using rigorous coupled-wave analysis, which solves Maxwell's equations for periodic structures. The calculations show that these deep subwavelength nanostructures behave as homogeneous optical materials with effective refractive indices determined by the structural parameters. We solve approximate models to estimate their refractive indices. When the spectral responses of these devices were measured, their external quantum efficiencies track the calculated absorptances, except for a small multiplicative offset at shorter wavelengths due to a greater than unity internal quantum efficiency, which we estimate by dividing the absorptance into the external quantum efficiency. |
