The finite difference time-domain (FDTD) method is used to simulate the light absorption enhancement in a plasmonic metal-semiconductor-metal photodetector (MSM-PD) structure employing a metal nanograting with phase shifts. The metal fingers of the MSM-PDs are etched at appropriate depths to maximize light absorption through plasmonic effects into a subwavelength aperture. We also analyse the nano-grating phase shift and groove profiles obtained typically in our experiments using focused ion beam milling and atomic force microscopy and discuss the dependency of light absorption enhancement on the nano-gratings phase shift and groove profiles inscribed into MSM-PDs. Our simulation results show that the nano-grating phase shift blue-shifts the wavelength at which the light absorption enhancement is maximum, and that the combined effects of the nano-grating groove shape and phase shift degrade the light absorption enhancement by up to 50%. 1. Introduction In recent years, sub-wavelength nanostructured metal nanogratings have been identified as promising candidates for realising high-speed improved sensitivity metal-semiconductor-metal photodetectors (MSM-PDs) [1]. The strong interaction of a nanostructured metal grating with an incident light enables trapping the light through the metal finger slit into the semiconductor substrate, leading to substantial improvement in light absorption, and opening the way for a wide range of potential applications, such as optical fiber communications, high-speed chip-to-chip interconnects, and high-speed sampling [2, 3]. An MSM-PD is simply composed of two back-to-back Schottky diodes and has interdigitated metal fingers deposited onto a semiconductor substrate as an active light absorption layer. Upon detection of photons, it collects the electric currents generated by photo-excited charge carriers in the semiconductor region, which drift under the electric field applied between the metal fingers. The speed of MSM-PDs can be limited intrinsically by the carrier transit time between the electrodes. The interdigitated electrodes in MSM-PDs result in a huge increase in bandwidth and reduction in dark current, in comparison to conventional PIN photodiodes with active areas of similar size [2–4]. Recently, the use of surface plasmon-assisted effects has been reported for the development of MSM-PDs with a high-responsivity bandwidth product [5, 6]. Several theoretical and experimental results have been reported on the extraordinary optical transmission through the sub-wavelength metallic apertures, and metal gratings [7–20].
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