Size tunable Ge quantum dot phototrasnsistors for optical interconnects with high figure of merits

We report a unique, CMOS approach for the inclusion of size-tunable (7-50-nm), spherical Ge quantum dots (QDs) into gate stacks of Si MOS transistors, through selective oxidation of SiGe pillars over the buffer layer of Si₃N₄ on top of the Si substrate. The Ge-QD MOS phototransistors feature extremely low dark current densities (I_off ~ 0.27 pA/μm²), high I_on/I_off (>10⁶), steep subthreshold swing (~175 mV/dec at 300 K), superior external quantum efficiency (~240%), and fast response time of 1.4ns under illumination of 850 nm, providing a core building block for high-performance Ge optical transducers for on-chip optical switches and transducers for Si-based optical interconnect applications. Most importantly, the detection wavelength of the Ge QD is tunable from near infrared to near ultraviolet by reducing the QD size from 50 to 7 nm, and the optimal photoresponsivity is tailored by the Ge QD size and the effective thickness of gate dielectrics.