Quantum carrier reinvestment-induced ultrahigh and broadband photocurrent responses in graphene-silicon junctions

In an earlier work, we had reported a method that enables graphene-silicon junctions to display exceptionally high photovoltaic responses, exceeding 10⁷ V/W. Using a completely different method that has recently been reported to result in ultrahigh gain, we now show that these junctions can also demonstrate giant photocurrent responsivities that can approach ∼10⁷ A/W. Together, these mechanisms enable graphene-silicon junctions to be a dual-mode, broad-band, scalable, CMOS-compatible, and tunable photodetector that can operate either in photovoltage or photocurrent modes with ultrahigh responsivity values. We present detailed validation of the underlying mechanism (which we call Quantum Carrier Reinvestment, or QCR) in graphene-silicon junctions. In addition to ultrasensitive photodetection, we present QCR photocurrent spectroscopy as a tool for investigating spectral recombination dynamics at extremely low incident powers, a topic of significant importance for optoelectronic applications. We show that such spectroscopic studies can also provide a direct measure of photon energy values associated with various allowed optical transitions in silicon, again an extremely useful technique that can in principle be extended to characterize electronic levels in arbitrary semiconductors or nanomaterials. We further show the significant impact that underlying substrates can have on photocurrents, using QCR-photocurrent mapping. Contrary to expectations, QCR-photocurrents in graphene on insulating SiO₂ substrates can be much higher than its intrinsic photocurrents, and even larger than QCR-photocurrents obtained in graphene overlaying semiconducting or metallic substrates. These results showcase the vital role of substrates in photocurrent measurements in graphene or potentially in other similar materials which have relatively high carrier mobility values.