Abstract-Towards quantum-limited coherent detection of terahertz waves in charge-neutral graphene

S. Lara-Avila, A. Danilov, D. Golubev, H. He, K. H. Kim, R. Yakimova, F. Lombardi, T. Bauch, S. Cherednichenko,  S. Kubatkin,

https://www.nature.com/articles/s41550-019-0843-7

Spectacular advances in heterodyne astronomy have been largely due to breakthroughs in detector technology. To exploit the full capacity of future terahertz (∼300 GHz–5 THz) telescope space missions, new concepts of terahertz coherent receivers are needed, providing larger bandwidths and imaging capabilities with multipixel focal plane heterodyne arrays. Here we show that graphene uniformly doped to the Dirac point, with material resistance dominated by quantum localization and thermal relaxation governed by electron diffusion, enables highly sensitive and wideband coherent detection of signals from 90 to 700 GHz and, prospectively, across the entire terahertz range. We measure on proof-of-concept graphene bolometric mixers an electron diffusion-limited gain bandwidth of 8 GHz (corresponding to a Doppler shift of 480 km s−1 at 5 THz) and intrinsic mixer noise temperature of 475 K (which would be equivalent to ~2 hf/kB at f = 5 THz, where h is Planck’s constant, f is the frequency and kB is the Boltzmann constant), limited by the residual thermal background in our setup. An optimized device will result in a mixer noise temperature as low as 36 K, with the gain bandwidth exceeding 20 GHz, and a local oscillator power of <100 pW. In conjunction with the emerging quantum-limited amplifiers at the intermediate frequency, our approach promises quantum-limited sensing in the terahertz domain, potentially surpassing superconducting technologies, particularly for large heterodyne arrays.

Abstract-Multi-scale investigations of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC

• C. Bouhafs^a,, 
• A.A. Zakharov^b, 
• I.G. Ivanov^c, 
• F. Giannazzo^d, 
• J. Eriksson^c, 
• V. Stanishev^a, 
• P. Kühne^a,
• T. Iakimov^c, 
• T. Hofmann^e, 
• M. Schubert^e, 
• F. Roccaforte^d, 
• R. Yakimova^c, 
• V. Darakcheiva^a

• ^a Terahertz Materials Analysis Center, Department of Physics, Chemistry and Biology, IFM, Linköping University, Linköping, S-58183 SE, Sweden
• ^b MaxLab, Lund University, S-22100 Lund, Sweden
• ^c Department of Physics, Chemistry and Biology, IFM, Linköping University, Linköping, S-58183 SE, Sweden
• ^d CNR-IMM, Strada VIII, 5, 95121 Catania, Italy
• ^e Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
• 
  

http://www.sciencedirect.com/science/article/pii/S0008622317301549

In this work, we report a muti-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 × 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiO_xlayer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO₂ and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation.