| 1. |
- He, Hans, 1989, et al.
(author)
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The performance limits of epigraphene Hall sensors doped across the Dirac point
- 2020
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116:22
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Journal article (peer-reviewed)abstract
- Epitaxial graphene on silicon carbide, or epigraphene, provides an excellent platform for Hall sensing devices in terms of both high electrical quality and scalability. However, the challenge in controlling its carrier density has thus far prevented systematic studies of epigraphene Hall sensor performance. In this work, we investigate epigraphene Hall sensors where epigraphene is doped across the Dirac point using molecular doping. Depending on the carrier density, molecular-doped epigraphene Hall sensors reach room temperature sensitivities of S-V=0.23V/(VT) and S-I=1440V/(AT), with magnetic field detection limits down to B-MIN=27 nT/root Hz at 20kHz. Thermally stabilized devices demonstrate operation up to 150 degrees C with S-V=0.12V/(VT), S-I=300V/(AT), and B-MIN similar to 100 nT/root Hz at 20kHz. Our work demonstrates that epigraphene doped close to the Dirac point could potentially outperform III-V Hall elements in the extended and military temperature ranges.
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| 2. |
- Kim, Kyung Ho, 1984, et al.
(author)
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Chemical Sensing with Atomically Thin Platinum Templated by a 2D Insulator
- 2020
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In: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 7:12
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Journal article (peer-reviewed)abstract
- Boosting the sensitivity of solid‐state gas sensors by incorporating nanostructured materials as the active sensing element can be complicated by interfacial effects. Interfaces at nanoparticles, grains, or contacts may result in nonlinear current–voltage response, high electrical resistance, and ultimately, electric noise that limits the sensor read‐out. This work reports the possibility to prepare nominally one atom thin, electrically continuous platinum layers by physical vapor deposition on the carbon zero layer (also known as the buffer layer) grown epitaxially on silicon carbide. With a 3–4 Å thin Pt layer, the electrical conductivity of the metal is strongly modulated when interacting with chemical analytes, due to charges being transferred to/from Pt. The strong interaction with chemical species, together with the scalability of the material, enables the fabrication of chemiresistor devices for electrical read‐out of chemical species with sub part‐per‐billion (ppb) detection limits. The 2D system formed by atomically thin Pt on the carbon zero layer on SiC opens up a route for resilient and high sensitivity chemical detection, and can be the path for designing new heterogenous catalysts with superior activity and selectivity.
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| 3. |
- Makarovsky, O., et al.
(author)
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Enhancing optoelectronic properties of SiC-grown graphene by a surface layer of colloidal quantum dots
- 2017
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In: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:3
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Journal article (peer-reviewed)abstract
- We report a simultaneous increase of carrier concentration, mobility and photoresponsivity when SiC-grown graphene is decorated with a surface layer of colloidal PbS quantum dots, which act as electron donors. The charge on the ionised dots is spatially correlated with defect charges on the SiC-graphene interface, thus enhancing both electron carrier density and mobility. This charge-correlation model is supported by Monte Carlo simulations of electron transport and used to explain the unexpected 3-fold increase of mobility with increasing electron density. The enhanced carrier concentration and mobility give rise to Shubnikov-de Haas oscillations in the magnetoresistance, which provide an estimate of the electron cyclotron mass in graphene at high densities and Fermi energies up to 1.2 x 10(13) cm(-2) and 400 meV, respectively.
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| 4. |
- He, Hans, et al.
(author)
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Highly efficient UV detection in a metal-semiconductor-metal detector with epigraphene
- 2022
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In: Applied Physics Letters. - : American Institute of Physics Inc.. - 0003-6951 .- 1077-3118. ; 120:19
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Journal article (peer-reviewed)abstract
- We show that epitaxial graphene on silicon carbide (epigraphene) grown at high temperatures (T >1850 °C) readily acts as material for implementing solar-blind ultraviolet (UV) detectors with outstanding performance. We present centimeter-sized epigraphene metal-semiconductor-metal (MSM) detectors with a peak external quantum efficiency of η ∼85% for wavelengths λ = 250-280 nm, corresponding to nearly 100% internal quantum efficiency when accounting for reflection losses. Zero bias operation is possible in asymmetric devices, with the responsivity to UV remaining as high as R = 134 mA/W, making this a self-powered detector. The low dark currents Io ∼50 fA translate into an estimated record high specific detectivity D = 3.5 × 1015 Jones. The performance that we demonstrate, together with material reproducibility, renders epigraphene technologically attractive to implement high-performance planar MSM devices with a low processing effort, including multi-pixel UV sensor arrays, suitable for a number of practical applications. © 2022 Author(s).
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| 5. |
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| 6. |
- Karimi, Bayan, et al.
(author)
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Electron-phonon coupling of epigraphene at millikelvin temperatures measured by quantum transport thermometry
- 2021
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:10
-
Journal article (peer-reviewed)abstract
- We investigate the basic charge and heat transport properties of charge neutral epigraphene at sub-kelvin temperatures, demonstrating a nearly logarithmic dependence of electrical conductivity over more than two decades in temperature. Using graphene's sheet conductance as an in situ thermometer, we present a measurement of electron-phonon heat transport at mK temperatures and show that it obeys the T4 dependence characteristic for a clean two-dimensional conductor. Based on our measurement, we predict the noise-equivalent power of ∼ 10 - 22 W / Hz of the epigraphene bolometer at the low end of achievable temperatures.
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| 7. |
- Alexander-Webber, J. A., et al.
(author)
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Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene
- 2016
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In: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 6
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Journal article (peer-reviewed)abstract
- Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.
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| 8. |
- He, Hans, et al.
(author)
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Polymer-encapsulated molecular doped epigraphene for quantum resistance metrology
- 2019
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In: Metrologia. - : Institute of Physics Publishing. - 0026-1394 .- 1681-7575. ; 56:4
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Journal article (peer-reviewed)abstract
- One of the aspirations of quantum metrology is to deliver primary standards directly to end-users thereby significantly shortening the traceability chains and enabling more accurate products. Epitaxial graphene grown on silicon carbide (epigraphene) is known to be a viable candidate for a primary realisation of a quantum Hall resistance standard, surpassing conventional semiconductor two-dimensional electron gases, such as those based on GaAs, in terms of performance at higher temperatures and lower magnetic fields. The bottleneck in the realisation of a turn-key quantum resistance standard requiring minimum user intervention has so far been the need to fine-tune the carrier density in this material to fit the constraints imposed by a simple cryo-magnetic system. Previously demonstrated methods, such as via photo-chemistry or corona discharge, require application prior to every cool-down as well as specialist knowledge and equipment. To this end we perform metrological evaluation of epigraphene with carrier density tuned by a recently reported permanent molecular doping technique. Measurements at two National Metrology Institutes confirm accurate resistance quantisation below 5n-1. Furthermore, samples show no significant drift in carrier concentration and performance on multiple thermal cycles over three years. This development paves the way for dissemination of primary resistance standards based on epigraphene
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| 9. |
- He, Hans, et al.
(author)
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Stable and Tunable Charge Carrier Control of Graphene for Quantum Resistance Metrology
- 2018
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In: 2018 Conference on Precision Electromagnetic Measurements (CPEM 2018). - : IEEE. - 9781538609736 - 9781538609743
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Conference paper (peer-reviewed)abstract
- Here we demonstrate a stable and tunable method to alter the carrier concentration of epitaxial graphene grown on silicon carbide. This technique relies on chemical doping by an acceptor molecule. Through careful tuning one can produce chemically doped graphene quantum resistance devices which show long-term stability in ambient conditions and have performance comparable to that of GaAs quantum resistance standards. This development paves the way for controlled device fabrication of graphene quantum hall resistance standards, which can be reliably tailored to operate below 5 T and above 4 K out-of-the-box, without further adjustments from the end-user.
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| 10. |
- He, Hans, 1989, et al.
(author)
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Uniform doping of graphene close to the Dirac point by polymer-assisted assembly of molecular dopants
- 2018
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
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Journal article (peer-reviewed)abstract
- Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC—achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly(methyl-methacrylate)—proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral, large-area graphene with carrier mobilities ~70 000 cm2V−1s−1at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin-coating wafer-scale substrates in ambient conditions, opens up a scalable technological route toward expanding the functionality of 2D materials.
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