Metal-semiconductor-metal photodiodes on textured silicon membranes

Abstract

High-speed and high-responsivity silicon photodetectors, which can be readily integrated with electronics, would make silicon-based optoelectronic receivers the preferred technology for short distance fiber-optic and free-space optical communication.. However, the long absorption length in silicon (-10 pm at 830 nm) results in detectors with a poor high-speed response. Previous work [l-31 focused on reducing the absorption length by reducing the wavelength (1 pm at 630 nm and 0.1 pm at 400 nm) even though fiber attenuation is more favorable at longer wavelength, whilc light sources are more readily available at 830 nm. In this paper, we present a novel silicon Metal-Semiconductor-Metal (MSM) photodetector structure with a 3.0 GHz bandwidth and 0.17 A/W DC responsivity at 830 nm. The fabrication process is simple and relies on conventional silicon fabrication processes. The structure is an interdigitated MSM detector fabricated on a silicon membrane. The back surface of the membrane is textured to trap the light within the membrane. This detector provides good absorption at longer wavelengths without sacrificing bandwidth. The membrane is created by reactive ion etching using CF4. The back surface is RIE-textured in an Ar/CF4 mixture. Transmission through a 5 pm membrane was measured to be 7.8%, compared to 30% for an untextured membrane, demonstrating the increased absorption. The MSM detector has a finger width of 2.5 km and a finger spacing of 3.75 pm. The bulk detector prior to membrane creation had a responsivity of 0.24 A/W and an internal quantum efficiency of 80% at 5 V. After membrane fabrication, front illumination of the detectors show a responsivity of 0.17 A/W and an internal quantum efficiency of 60%, compared to 0.21 A/W and 45% for back illumination. The transient response of the detectors was obtained by applying 830 nm optical pulses from a current spiked GaAs laser diode. The transient response of the novel detector at 10 V shows a full-width-half-maximum (FWHM) of 74 ps and a fall time of 128 ps. The -3 dB bandwidth is 3.0 GHz (2.7 GHz at 5 V bias) as determined from the fourier transform of the pulse response. For comparison we measured the detector prior to membrane formation. The pulse response showed a FWHM of 267 ps and a bandwidth of 326 MHz at 10 V bias, which clearly demonstrates the effect of the membrane. In summary we have fabricated a novel high-speed silicon detector which can bc integrated with silicon circuits. The detector can be illuminated from either side of the membrane, It was demonstrated to have a superior bandwidth and similar responsivity at 830 nm compared to previously published silicon MSM detectors [l] measured at 630 nm, despite the much larger absorption length.