A COMPARATIVE-STUDY OF SI AND GAAS METAL-SEMICONDUCTOR-METAL PHOTODETECTORS

In this paper we present a comparative study of the optical DC and tra nsient properties of the Si- and GaAs-metal-semiconductor-metal photod etectors (MSM-PD's). The interdigitated metal fingers for the detector s were fabricated on n-type Si and semi-insulating GaAs substrates by using optical lithography and lift-off technique. The width and spacin g of the interdigitated metal fingers varied between 1 and 2 mu m, and the detector area varied between 100 x 100 mu m(2) and 500 x 500 mu m (2) For Si-MSM-PD's we used 150 nm thick aluminum and for GaAs-MSM-PD' s 280 nm thick Ti/Pt/Au multilayer metallization for the Schottky barr ier formation. The Schottky diode capacitance vs. bias voltage as well as dark and illuminated I-V characteristics as a function of optical power have been measured. Also the photoresponsivity vs, wavelength ha s been measured for various bias voltages. The speed of the detectors was studied by using a colliding pulse mode-locked laser. The experime ntal results show that the leakage currents in GaAs-MSM detectors are much smaller than in Si detectors, but still a reasonable photorespons ivity can be achieved also in Si detectors. The speed of the Si detect ors could be improved by fabricating them on an epitaxial n(+) - n(-)- structure, which results in a fast impulse response 625 ps (FWHM), 292 ps (fall time).