Linear detection of sub-bandgap energy photons in silicon: A photo-assisted Shockley-Read mechanism

Two-photon absorption is a third-order process and, as such, displays a quadratic dependence relatively to the incident light intensity. This TPA process has been harnessed in a new generation of ultrafast light intensity correlator setups [1,2]. This quadratic dependence of the photocurrent to the sub-bandgap light intensity has been checked in various semiconductor-based devices, and verified over more than 8 orders of magnitude in GaAs or GaN light sensors. This is not the case in the silicon-based devices that have been investigated by various authors [1,3]: while a quadratic dependence is clearly observed at relatively high optical fluxes (typically above 10 μW in a diffraction-limited situation at a wavelength of 1.55 μm), a linear dependence is systematically observed in the low flux regime. Here we show that this linear absorption of sub-bandgap energy photons in silicon originates from a photo-assisted Shockley-Read (SR) process [4]. In this process, sub-bandgap energy photons promote electrons from deep level traps to the conduction band, freeing quantum states on these levels. These newly freed states contribute to the SR capture-recombination mechanism, and thus enhance the dark current of the silicon diode.