High-speed metal-semiconductor-metal photodetectors fabricatd on SOI-substrates

Interdigitated metal-semiconductor-metal (MSM) Schottky diode photodetectors based on silicon-on-insulator-substrates (SOI) have attracted research interest in the past years due to their high-speed compatibility at longer wavelengths with GaAs-based MSM PD's. This paper presents DC and transient characteristics of SOI MSM PD's with different top Si layer dopings and thicknesses (0.2 µm, 0.5 µm, 1 µm, 2 µm and 4 µm). The detectors with metal finger width and spacing of 2–3 µm and active areas of 100 × 100 µm² were fabricated by using conventional optical lithography and lift-off technique. For the p-type detectors the Schottky electrode metallization consisted of Ti/Au (60/130 nm), whereas the n-type detectors were metallized with aluminium (100 nm). I–V characteristics have been measured showing a low dark current. C–V measurements exhibit a small saturation capacitance. The speed of the detectors was investigated with a double heterostructure (DH) laser at a wavelength of 850 nm, a single heterostructure (SH) laser at 910 nm and a Ti : Sapphire laser at 800 nm. At the wavelength of 850 nm the risetime for the 0.5 µm top Si layer detector was 70 ps (bias-voltage 10 V), corresponding to 3 dB bandwidth of 5 GHz. For the detectors with thicker active top layers the bandwidths were slightly smaller, but also in the GHz-region. The results indicate that the photocarriers generated beneath the thin photoactive top Si layer deep in the bulk are isolated through the buried SiO₂-layer. High speed for Si-based detectors may thus be achieved even at a long wavelength. The photoresponsivity has also been measured showing that reasonable efficiency is achievable for SOI detectors.