We will present our research on the development of a sensitive hybrid photodetector that utilizes a graphene-based charge sensor capacitively coupled to a thick semiconductor absorber in depletion. Due to its high-mobility and broadband optical conductivity, graphene is emerging as an interesting material for photosensing applications capable of spanning the electromagnetic spectrum (ultraviolet to terahertz). Achieving high sensitivities in practical applications remains elusive, however, due to the difficulty associated with coupling light into an atomically thin layer that is at least an order of magnitude thinner than the wavelength which it is sensing. To circumvent issues with direct absorption into an atomically thin layer, we have developed a photodetector concept based on a deeply depleted graphene/oxide/semiconductor (D2GOS) junction. This photodetecting junction consists of a graphene field effect transistor (GFET) that is capacitively coupled, using a thin oxide layer, to a thick semiconducting absorber. Unlike other graphene-based detecting concepts, absorption of photons takes place in the semiconducting substrate and not the ultra-thin graphene layer. In this concept, the GFET acts as a high-sensitivity local charge detector that is capable of sensing photo-induced charge collecting in the potential well that forms near the absorber/oxide interface when the junction is biased into deep depletion. As photo-induced charge collects within the potential well, the GFET’s conductivity is altered due to the presence of capacitively-coupled charge that forms within the graphene channel. The behavior of this hybrid device is analogous to a self-sensing metal/oxide/semiconductor (MOS) capacitor, which is the fundamental building block of modern CCD imaging technology.
Under this paradigm, we have successfully fabricated D2GOS device arrays using commercial CVD graphene that has been transferred atop a thin layer of HfO2 resting on a low-doped n-type silicon. We have measured responsivities in excess of 2500 A/W (25,000 S/W), for visible wavelengths, using moderate integration times. Consistent behavior for multiple devices fabricated on different chips was also observed, with signal-to-noise ratios comparable to commercial CCD technology. In addition to devices using Si absorbers, we have implemented the D2GOS detection concept in InGaAs which is an infrared absorber. Although similar devices have been fabricated in the past, we have developed an improved understanding of the working principles for this hybrid device and constructed a model to adequately describe its behavior.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.