Abstract

Previously, the response time for the fastest transit-time-limited metal-semiconductor- metal (MSM) photodetectors, that have a metal finger spacing of 0.5 μm and a finger width of 0.75 μm on a thin layer of GaAs, was 4,8 ps;¹ and that for the fastest recombinationtime-limited MSM photodetectors, that have a finger spacing and width of 200 nm on a layer of low-temperature-grown (LT) GaAs, was 1.2 ps² We fabricated the transit-time- limited MSM photodetectors on crystalline Si substrates and semi-insulating (SI) bulk GaAs, and the recombination-time-limited MSM photodetectors on a LT GaAs layer. The detectors have a finger spacing and finger width ranging from 50 to 300 nm, and typical detection area of 10 × 10 μm.² Fig. 1 shows an MSM detector of a 50-nm finger spacing and width, which are we believe the smallest ever reported. Interdigitated finger electrodes consisting of 150-nm Ti and 150-nm Au, which form Schottky contacts on the GaAs and Si substrates, were defined using high- resolution electron beam lithography and a lift-off technique. The SI GaAs substrate has a resistivity of ~5×l0⁷ Ω-cm, mobility of ~6500 cm²/V sec, and a carrier concentration of ~1.5 × 10⁷ cm^-3. The Si substrate is a lightly p-type doped to a concentration of 8 × 10¹⁴ cm^-3 . The LT GaAs layer was 1 μm thick and was grown on a SI GaAs substrate at 210 °C and then annealed at 600 °C for 1 h. For high speed measurements, coplanar transmission lines with a lmewidth of 16 μm and a spacing of 9 μm, and a characteristic impedance of 60 Ω were fabricated on the substrates.

© 1992 Optical Society of America