Metal-germanium-metal ultrafast infrared detectors

We demonstrate silicon-based ultrafast metal-semiconductor-metal (MSM) photodetectors for near infrared optocommunication wavelengths. They show a response time of 12.5 ps full width at half maximum (FWHM) at both 1300 and 1550 nm wavelengths. The overall external quantum efficiencies are 13% at 1320 nm and 7.5% at 1550 nm. The sensitive volumes are 270 nm thick Ge films, grown on Si(111) by molecular beam epitaxy. Interdigitated Cr metal top electrodes with 1.5-5 mum spacing and identical finger width form Schottky contacts to the Ge film. A Ti-sapphire femtosecond laser with an optical parametric oscillator and an electro-optic sampling system are used to evaluate the temporal response, which is limited by the transit time of the carriers between electrodes. In addition, results on Si-Ge MSM heterostructure detectors with plate capacitor geometry are presented. At 1550 nm an ultrafast response of 9.4 ps FWHM and an overall quantum efficiency of 0.9% are measured. (C) 2002 American Institute of Physics