Silicon Nanowire Photodiodes with Whispering Gallery Modes

Abstract

Silicon (Si) photodiodes have been widely used in many industries, including communications, self-driving vehicles, security and healthcare with the advantages of high crystal quality, cost-effective, CMOS-compatible mature fabrication technology, and strong optical confinement with high index contrast between Si and SiO2. However, optical properties of Si are very poor, because of its indirect bandgap, which is unfavorable for efficient light emission and absorption near the band edge. Whispering gallery modes (WGMs), a type of modes that can travel around a closed concave surface with repeated total internal reflection, can be a candidate to overcome the limitation of Si as a photonic and photovoltaic material. In this presentation, we introduce Si nanostructures with a size comparable to the optical wavelength that have great potential to be an effective resonator platform, forming multiple WGMs. We observed spatially well-separated and non-degenerate modes in the wide near-infrared spectral regime in a weakly tapered Si nanopillar, suggesting that the diameter and slope of the Si nanopillar are crucial determinants for the formation and control of WGMs. In addition, we present a novel design, hourglass-shaped Si nanowire to significantly increase the light-trapping path with low-Q WGMs and Si photodetectors based on the nanostructure. The devices enhanced their photoresponse over broadband wavelength region, exhibited a higher responsivity and external quantum efficiency than commercial Si photodetectors at the wavelength range of 700-1100 nm. We also analyzed Si solar cells with hourglass-shaped Si nanowire. WGM identified by optical simulations accounted for the greatly improved absorption at the broadband solar spectrum. As such, this presentation will discuss our strategies for designing wavelength-scale nanostructured resonators to improve the performance of photodiodes.