Ultrafast silicon based photodetectors

Abstract

We have fabricated different metal-silicon-metal photodetectors and characterized these devices for visible and near-infrared (IR) response. For wavelengths shorter than 1.1 μm, electron-hole pairs are generated in the Si. They are accelerated to saturation velocity and move towards the metal electrodes. For longer wavelengths, Si itself becomes transparent, but carriers are emitted from the internal semiconductor–metal interfaces. This so-called “internal photoeffect” is governed by different carrier dynamics, because “hot” electrons or holes are injected into the Si. Their significant excess energy leads to extremely fast electrical pulses. The resulting temporal response was measured with a new setup, using a Ti:sapphire laser and an optical parametric oscillator, which generates ultrafast optical pulses (170 fs) at IR wavelengths. Schottky-barrier emission from CoSi2, Cr, Ti, and Pt was investigated. A photodetector from Ti–Si–CoSi2 was excited with optical pulses of 1250 nm and showed an electrical pulse response of 3.2 ps full width at half maximum at 4 V bias. This is a record value. We can demonstrate, that under certain conditions an even faster response becomes possible.