Plasmonic metal nanoantennas arrayed on silicon have attracted attention as silicon-based sub-bandgap infrared detectors. Localized surface plasmons induced on metal nanoblocks can provide a broadband responsivity, and the silicon-based configuration offers the potential for complementary metal oxide semiconductor-compatible infrared photodetectors. The responsivity of such devices, however, needs further improvement. In this report, a nanoantenna structure consisting of a nanoblock with electrical connection to it built in a narrow, vertical, and deep nanohole is presented, which generates a photocurrent in response to backside illumination. Inclined evaporative deposition of copper onto an array of nanoholes, 150 nm in diameter and 560 nm in depth, simultaneously forms nanoblocks and thin conductive films. The structure's effectiveness is demonstrated from the reflectance and responsivity to infrared laser illumination 1.1–1.8 µm in wavelength. When the periodicity of the nanoholes is 500 nm, a responsivity of 9.8 mA W−1 at 1.55 µm is obtained, which is sustained over a broad band. The responsivity–density relationship has a limitation between periodicities of 250 and 500 nm, possibly because of overlapped near fields around antennas and the resultant altered resonant mode. With its simple fabrication and efficient functionality, the nanoantenna structure contributes to the realization of silicon-based infrared imaging.
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