Optical Characteristics of Silicon-Based Asymmetric Vertical Nanowire Photodetectors

Wavelength-selective absorption phenomena of silicon-based vertical nanowire (NW) photodetectors (PDs) are investigated using 3-D numerical simulations. The difference in the refractive indexes between silicon NW and its surrounding material induces diameter-dependent waveguide effects at several specific wavelengths for asymmetric as well as symmetric structures. But the asymmetric NW PDs with a large difference in diameter between the top side and the bottom side have loss of waveguide effects. All the NW PDs achieve high external quantum efficiency (EQE) peaks with an increase in NW height. Decreasing the NW pitch or increasing the NW density also increases the EQE peak values for awide range of wavelengths, but too densely packed NWs induce high coupling between the nearest NWs, thus losing wavelength-selective properties. Vertical NW PDs having slightly bottom-wide asymmetric structure decrease reflectance and transmittance of the incident light because of the narrow top-side and wide bottom-side silicon NW cross sections, respectively, thus achieving higher EQEs. Bottom-wide asymmetric NW PDs are promising to enhance the optical characteristics as well as to maintain waveguide properties for optical sensor applications.