Photovoltaic and Photoconductive Action Due to PbS Quantum Dots on Graphene/SiC Schottky Diodes from NIR to UV

We demonstrate photovoltaic and photoconductive responses to near-infrared light in devices formed by depositing a film of gel permeation chromatography purified PbS quantum dots (QDs) on top of n-SiC epitaxial layers with natively grown, low-leakage 10-15 monolayer thick epitaxial graphene (EG) Schottky contacts. The QD film layer was removable by selective chemical etching, resetting the EG/SiC Schottky diode: the sub-bandgap response could be restored in subsequent PbS-QD depositions. The EG in these devices simultaneously forms Schottky contacts to SiC and ohmic contacts to PbS-QD, enabling electrical screening and isolation of these interfaces from each other. After PbS-QD deposition, the diodes exhibit photovoltaic and photoconductive responses at photon energies far below the SiC bandgap, extending to the NIR gap of the QD film. Scanning photocurrent microscopy illustrates that this is due to charge transfer from the QD film to the n-type 4H-SiC through a trap-limited, rectifying PbS-QD/SiC heterojunction with ideality n = 2 in parallel with the EG/SiC Schottky diode. The photoconductive gain at this QD/SiC interface could be useful for IR detection in wide-bandgap platforms. Response times as fast as 40 ms are suitable for imaging applications, although careful contact design is required to optimize work-function matching and spreading resistance.