Sub-bandgap polysilicon photodetector in zero-change CMOS process for telecommunication wavelength

Abstract

We report a defect state based guided-wave photoconductive detector at 1360-1630 nm telecommunication wavelength directly in standard microelectronics CMOS processes, with zero in-foundry process modification. The defect states in the polysilicon used to define a transistor gate assists light absorption. The body crystalline silicon helps form an inverse ridge waveguide to confine optical mode. The measured responsivity and dark current at 25 V forward bias are 0.34 A/W and 1.4 μA, respectively. The 3 dB bandwidth of the device is 1 GHz.

State‐of‐the‐art all‐silicon sub‐bandgap photodetectors at telecom and datacom wavelengths

“…The defect states that exist in devices formed of deposited silicon are attributed primarily to trap states associated with dangling bonds at the grain boundaries . This is particularly attractive means of incorporating deep levels for photodetection, as it can be realized with a ‘zero‐change’ bulk CMOS process . In both surface states arising from waveguide sidewall and grain boundaries, high speed (>2.5 Gb/s) operation has been observed .…”

High-speed polysilicon CMOS photodetector for telecom and datacom

“…19 We demonstrated a resonant photodetector and were able to achieve 0.34 A/W responsivity at 25 V forward bias and 1 GHz bandwidth at bias voltages above 5 V. Because of predoping in the polysilicon gate associated with all advanced CMOS processes for avoiding gate depletion, 21 we were not able to achieve a true lateral pn diode, and as a result, that device behaved as a photoconductor. Because of the weak electric field in the absorption region, the device had poor photocarrier collection and a slow response.…”

“…On the other hand, polysilicon with grain boundary midgap states has been used for building photodetectors and have achieved comparable or better performance compared to ion-implanted silicon resonant photodetectors without these reliability concerns. [16][17][18][19] Polysilicon detectors at 1280 nm and 1550 nm with a responsivity of 0.2 A/W (at 2.5 V) with 8 GHz bandwidth (at 10 V bias) are already demonstrated in a modified bulk CMOS 17 and have been used to demonstrate the first optical link using monolithic optical transmitters and receivers in a bulk CMOS process. 20 Also, polysilicon exists in the majority of CMOS processes as the gate for FETs, and as is shown in this work, can be used to implement high performance photodetectors without changes to the foundry CMOS processes.…”

“…The confinement factor of this mode in polysilicon is roughly 0.3. 19 The absorbing material is the polysilicon gate layer that is predoped either n or p for the NFETs and PFETs in this process. 21 Using a combination of doping mask layers, we were able to counter-dope one type of predoping and implement a lateral pþpnþ junction in the polysilicon layer.…”

“…[12][13][14][15][16][17][18][19] Defects can generate energy states inside the bandgap of silicon and assist absorption of infrared photons. Optical absorption is observed in moderately doped guided-wave silicon devices with pn junctions through the defects generated during ion implantation; 12 and silicon has been intentionally implanted at high dosages (10 13 to 10 14 cm…”