From Terahertz to X-ray: Developing New Graphene-Based Photodetector Technologies

The latest technological developments have resulted in a push for faster, cheaper and simpler photodetector technologies across a wide range of temperatures, wavelengths and sensitivities for use in industrial and research applications. Graphene, a 2D allotrope of carbon, is seen as an interesting route for future photodetectors. Recent research into graphene has focussed on fundamental physics, fabrication processes and future commercial applications. Fundamental research has demonstrated many interesting properties, including the potential for high carrier mobility, high conductivity, approximately constant photon absorption and extreme tensile strength. These properties led to promising developments for graphene-based photodetectors, such as the demonstration of ultrafast photodetection on a femtosecond timescale for pulsed lasers. This document discusses novel graphene-based photodetector technologies from concept to theory, design, fabrication and experimental demonstration. Three detectors, from terahertz to X-ray, were fabricated in essentially the same graphene field effect transistor (GFET) structure, with photons coupling to different components of the detector to provide a measurable photosignal. A simulated cryogenic, colour sensitive, bilayer graphene single photon counting photodetector exploited the tuneable band gap of bilayer graphene to trade-off resolution against temperature to enable higher temperature operation, requiring less costly and complex cryogenics, with photons coupling directly to the bilayer graphene. The passive terahertz detector utilised photons from a broadband terahertz source coupling to antennae to generate a photoresponse via the Dyakonov-Shur effect with an 𝑁𝐸𝑃=0.85±0.15μWHz-0.5, with further work ongoing to demonstrate narrowband terahertz detection. The X-ray GFET was developed to investigate the energy sensitivity to X-ray photons coupling to the absorber based on work in the literature, where charge carrier modulation generates a field that changed the conductivity of the graphene channel. Using pulsed optical lasers to probe the behaviour and sensitivity of the detector gave 𝛥𝐸~480keV (for 𝐸=30MeV) with a photoresponse dependent on the gate voltage. No X-ray sensitivity was observed for Fe-55 sources, but it was observed for an X-ray generator; this inconsistency possibly suggests a different mechanism, such as bolometry, to that proposed previously in the literature.