Abstract

Metal-semiconductor-metal (MSM) photodetectors are very attractive for optical fiber communication systems and future high-speed chip-to-chip connections. MSM photodetectors not only have high-speed, but also their fabrication technology is compatible with the technology of large scale FET integrated circuits. Generally, the recombination time of carriers in a semiconductor is a key factor that limits the speed of a MSM photodetector. However, as the finger-spacing of a MSM detector is reduced into submicron range, the transit time of photon-generated carriers between two interdigitated metal fingers can be much smaller than the recombination time, and therefore the speed of the photodetector is not limited by the recombination time but the transit time. Thus, high speed MSM photodetectors can be fabricated on a thin layer of high-quality semiconductor which has very long recombination time and high mobility. Moreover, carrier velocity-overshoot effect due to the ultra-small finger­spacing can be utilized to further increase the speed of the detector. We have fabricated high­speed MSM photodetectors with sub-100 nm finger-width and finger-spacing in GaAs. The smallest finger-width and finger-spacing of these MSM photodetectors are 30nm respectively and are, we believe, the smallest ever reported in literature. A 400nm thick p-type GaAs layer was grown on a semi-insulating GaAs substrate. The thickness of the GaAs layer was kept thin for increasing operation speeds of the MSM detectors. Ti/Au Schottky interdigitated gate fingers of sub-100 nm width and spacing were defined using a high resolution electron beam lithography system and a lift-off process. The following figure shows a scanning electron micrograph of a GaAs MSM photodetector of a 40nm finger-width and a 160nm finger-spacing. The MSM photodetectors were characterized by a laser system of a pulse width of 70ps and a wavelength of 800nm. Preliminary results show that the FWHM of pulse response of the MSM photodetectors of sub-100nm finger-width and finger-spacing are well below 25ps, limited by our measuring system (25ps is the resolution of sampling oscilloscope). The actual response time of the detector should be a few picoseconds or less, since the electron transit time between the 30nm finger spacing is less than 0.2ps.

© 1991 Optical Society of America