We designed and fabricated an epitaxial-graphene-channel field-effect transistor (EG-FET) featuring an asymmetric dual-grating-gate (ADGG) structure working as a current-driven terahertz detector and experimentally demonstrated a 10 ps-order fast response time and a high responsivity of 0.3 mA/W to 0.95 Terahertz (THz) radiation incidence at room temperature. The ADGG and drain–source bias dependencies of the measured photoresponse showed a clear transition between plasmonic detection under periodic electron density modulation conditions with depleted regions and photothermoelectric (PTE) detection under entirely highly doped conditions without depleted regions. We identified the PTE detection that we observed as a new type of unipolar mechanism in which only electrons or holes contribute to rectifying THz radiation under current-driven conditions. These two detection mechanisms coexisted over a certain wide transcendent range of the applied bias voltages. The temporal photoresponses of the plasmonic and PTE detections were clearly shown to be comparably fast on the order of 10 ps, whereas the maximal photoresponsivity of the PTE detection was almost twice as high as that of the plasmonic detection under applied bias conditions. These results suggest that the ADGG-EG-FET THz detector will be promising for use in 6G- and 7G-class high-speed wireless communication systems.

Graphene is a promising material in next-generation devices. Large-scale epitaxial graphene should be grown on Si substrates to transfer the accumulated technologies to integrated devices. We have for this reason developed epitaxy of graphene on Si (GOS) and device operation of the backgate field-effect transistors (FETs) using GOS has been confirmed. It is demonstrated in this paper that the GOS method enables us to tune the structural and electronic properties of graphene in terms of the crystallographic orientation of the Si substrate. Furthermore, it is shown that the uniformity of the GOS process within a sizable area enables us to reliably fabricate topgate FETs using conventional lithography techniques. GOS can be thus the key material in next-generation devices owing to the tunability of the electronic structure by the crystallographic orientation of the Si substrate.

This paper reviews recent advances in the research and development of graphene‐based plasmonic metamaterials for terahertz (THz) laser transistors. The authors’ theoretical discovery on THz laser transistors in 2007 was realized as a distributed‐feedback dual‐gate graphene‐channel field‐effect transistor (DFB‐DG‐GFET) in 2018, demonstrating ∼0.1 µW single‐mode emission at 5.2 THz and ∼80 µW amplified spontaneous 1–7.6 THz emission at 100 K. To realize room‐temperature, dry‐cell‐battery operating intense THz lasing with fast direct modulation, various approaches based on graphene plasmonic metamaterials are investigated and introduced as real device implementations, including (i) replacement of the laser photonic cavity with plasmonic cavity enormously improving the THz photon field confinement with larger gain overlapping, (ii) introduction of THz amplification of stimulated emission via current‐driven graphene Dirac plasmons (GDPs), and (iii) controlling the parity and time‐reversal symmetry of GDPs enabling ultrafast direct gain‐switch modulation. Possible real device structures and design constraints are discussed and addressed toward coherent light sources applicable to future 6G‐ and 7G‐class THz wireless communication systems.

Surface chemistry involved in the epitaxy of graphene by sublimating Si atoms from the surface of epitaxial 3C-SiC(111) thin films on Si(111) has been studied. The change in the surface composition during graphene epitaxy is monitored by in situ temperature-programmed desorption spectroscopy using deuterium as a probe (D2-TPD) and complementarily by ex situ Raman and C1s core-level spectroscopies. The surface of the 3C-SiC(111)/Si(111) is Si-terminated before the graphitization, and it becomes C-terminated via the formation of C-rich (6√3 × 6√3)R30° reconstruction as the graphitization proceeds, in a similar manner as the epitaxy of graphene on Si-terminated 6H-SiC(0001) proceeds.

Enhancement of CO₂adsorption on functionalized graphene with epoxy oxygen was found by ambient pressure XPS, and supported by DFT calculations that include vdW interactions.