Energy harvesting is a key enabling technology for autonomous low-power electronics with enormous potential. Distributed, communicating sensors already play an important role in environmental monitoring, logistics, biomedical diagnostics, and consumer electronics, but powering these in a sustainable, carbon-neutral, and autonomous way is a largely unsolved problem. A promising solution is to recycle excess radiation from wireless communications networks using rectennas (antenna-coupled rectifiers). However, existing rectennas are limited to the microwave range due to their insufficient speed and efficiency. As modern communication patterns demand higher bandwidth, terahertz (THz) rectennas become key enablers for future wireless sensor networks. Critical figures of merit are the zero-bias current responsivity, resistance, and cutoff frequency of the rectifier, as well as the voltage responsivity of the rectenna system in free-space illumination. Here, three different graphene-based rectifier concepts suitable for energy harvesting in the THz range have been investigated. All rectifiers used graphene grown by scalable chemical vapor deposition (CVD) as the channel material. The first scientific contribution is based on a metal-insulator-graphene (MIG) diode with a one-dimensional lateral Ti-TiO2-graphene junction. The diode is integrated into a metallic bowtie antenna and fabricated on a flexible polyimide substrate using a scalable, thin-film compatible fabrication process. The rectenna demonstrates rectification in free-space measurements between 110 and 170 GHz with a responsivity of up to 80 V/W. As a power detector, the rectenna shows a competitive noise equivalent power of 80 pW/√Hz. This work has been published in a peer-reviewed journal and presented both orally and in poster form at international conferences. The 1D-MIG concept was expanded with a second insulating layer (Ta2O5) to form 1D- MIIG diodes. Temperature-dependent measurements and numerical device modelling suggest that the diode’s large zero-bias responsivity of 9.7 A/W originates from an interplay of in-plane thermionic emission, a modulation of the graphene-insulator barrier height, and trap-mediated transport through the Ta2O5 layer. This work is the first demonstration of an edge-contacted double-insulator diode. It represents a novel high-responsivity, low-capacitance diode type with large asymmetry up to 100 000. This work has been published in a peer-reviewed journal. Finally, the 1D-MIG diode concept has been applied to form a one-dimensional contact between graphene and sputtered silicon. The resulting lateral graphene-Si diode is thin-film compatible and highly nonlinear with a zero-bias current responsivity of 2.9 A/W. The second scientific contribution explores ballistic rectennas based on CVD-grown graphene. So far, ballistic rectification in graphene has only been shown in rectennas made from mechanically exfoliated graphene. In contrast, this work demonstrates the fabrication of multiple rectennas using scalable, commercially available CVD-grown graphene on a 2-inch wafer. The inherent zero-bias performance of ballistic rectifiers was exploited to form a low-impedance parallel connection of rectifiers to one antenna, thereby increasing the antenna/rectifier coupling efficiency. The rectennas showed an optical voltage responsivity of up to 0.77 V/W in free-space measurements and a noise equivalent power of 89 nW/√Hz between 490 and 680 GHz. The fabrication process also allows for the co-integration with an on-chip energy storage device. A publication in a scientific journal is in preparation. The third scientific contribution explores intrinsic rectification in graphene field-effect transistors with dissimilar metal top contacts at source and drain. The work function of graphene near the contacts is tuned by the metals, resulting in a gate-tuneable zero-bias current responsivity of up to 0.4 A/W while maintaining a low source-drain resistance of 500 Ω. In summary, this work explored various scalable concepts of using graphene for rectification up to the THz range. This was achieved by exploiting graphene’s two-dimensional structure, tuneable work function, and long charge carrier mean free path. The results enable energy harvesting from ambient THz radiation, contributing to the state of the art in energy harvesting for sustainably powered autonomous electronics devices and systems.
Andreas Hemmetter (Thu,) studied this question.