Search

A theory of graphene-based magnetic field Hall sensors sensitivity dependence on temperature is summarized. The existence of low-temperature range with sensitivity, almost independent on temperature, is predicted; at higher temperatures, when thermally-induced carrier concentration in graphene prevails, the sensitivity decreases with temperature. The experimental studies of the temperature dependence of magnetic sensitivity of Hall sensors on single layer graphene base were carried in temperature range from 300 °K to 430 °K. The values of sensitivity, obtained for room temperatures ~ 230 V·А‑1·Т‑1 exceed essentially the maximum sensitivity of the traditional Hall sensors on silicon base ~ 100 V·А‑1·Т‑1.

Dominating electron-electron scattering enables viscous electron flow exhibiting hydrodynamic current density patterns such as Poiseuille profiles or vortices. The viscous regime has recently been observed in graphene by non-local transport experiments and mapping of the Poiseuille profile. Here, we probe the current-induced surface potential maps of graphene field effect transistors with moderate mobility using scanning probe microscopy at room temperature. We discover micron-sized large areas appearing close to charge neutrality that show current induced electric fields opposing the externally applied field. By estimating the local scattering lengths from the gate dependence of local in-plane electric fields, we find that electron-electron scattering dominates in these areas as expected for viscous flow. Moreover, we suppress the inverted fields by artificially decreasing the electron-disorder scattering length via mild ion bombardment. These results imply that viscous electron flow is omnipresent in graphene devices, even at moderate mobility., Comment: 89 pages, 24 figures, published in Nano Letters

PtSe

The design, fabrication, and characterization of wafer-scale, zero-bias power detectors based on 2D MoS2 field-effect transistors (FETs) are demonstrated. The MoS2 FETs are fabricated using a wafer-scale process on 8μm-thick polyimide film, which, in principle, serves as a flexible substrate. The performances of two chemical vapor deposition MoS2 sheets, grown with different processes and showing different thicknesses, are analyzed and compared from the single device fabrication and characterization steps to the circuit level. The power-detector prototypes exploit the nonlinearity of the transistors above the cut-off frequency of the devices. The proposed detectors are designed employing a transistor model based on measurement results. The fabricated circuits operate in the Ku-band between 12 and 18GHz, with a demonstrated voltage responsivity of 45V W−1 at 18GHz in the case of monolayer MoS2 and 104V W−1 at 16GHz in the case of multilayer MoS2, both achieved without applied DC bias. They are the best-performing power detectors fabricated on flexible substrate reported to date. The measured dynamic range exceeds 30dB, outperforming other semiconductor technologies like silicon complementary metal–oxide–semiconductor circuits and GaAs Schottky diodes.

Advanced science 2201272 (2022). doi:10.1002/advs.202201272, Published by Wiley-VCH, Weinheim

It has been shown that there can be a significant temperature increase in graphene field-effect transistors (GFETs) operating under high drain bias, which is required for power gain. However, the possible effects of self-heating on the high-frequency performance of GFETs have been weakly addressed so far. In this article, we report on an experimental and theoretical study of the effects of self-heating on dc and high-frequency performance of GFETs by introducing a method that allows accurate evaluation of the effective channel temperature of GFETs with a submicrometer gate length. In the method, theoretical expressions for the transit frequency ( ${f}_{\text {T}}$ ) and the maximum frequency of oscillation ( ${f}_{\text {max}}$ ) based on the small-signal equivalent circuit parameters are used in combination with the models of the field- and temperature-dependent charge carrier concentration, velocity, and saturation velocity of GFETs. The thermal resistances found by our method are in good agreement with those obtained by the solution of the Laplace equation and by the method of thermo-sensitive electrical parameters. Our experiments and modeling indicate that the self-heating can significantly degrade the ${f}_{\text {T}}$ and ${f}_{\text {max}}$ of GFETs at power densities above 1 mW/ $\mu \text{m}^{2}$ , from approximately 25 to 20 GHz. This article provides valuable insights for further development of GFETs, taking into account the self-heating effects on the high-frequency performance.

In this work, we present the fabrication and characterization of bulk-heterojunction solar cells on monolayer graphene (MLG) with nickel-grids (Ni-grid) as semitransparent conductive electrode. The electrodes showed a maximum transmittance of 90% (calculated in 300–800 nm range) and a sheet resistance down to 35 Ω/□. On these new anodes, we fabricated TCO free BHJ-SCs using PTB7 blended with PC70BM fullerene derivative as active layer. The best device exhibited a power conversion efficiency (PCE) of 4.2% in direct configuration and 3.6% in inverted configuration. The reference solar cell, realized on the ITO glass substrate, achieved a PCE of 6.1% and 6.7% in direct and inverted configuration respectively; for comparison we also tested OSCs only with simple Ni-grid as semitransparent and conductive electrode, obtaining a low PCE of 0.7%. The proposed approach to realize graphene-based electrodes could be a possible route to reduce the overall impact of the sheet resistance of this type of electrodes allowing their use in several optoelectronic devices.

Advanced materials 2108473 (2022). doi:10.1002/adma.202108473, Published by Wiley-VCH, Weinheim

Terahertz (THz) inspection is among the most versatile tools to perform non-destructive, quantitative characterization of conductive thin-films. In this work we apply high-resolution THz near-field measurements for the inspection of graphene-based thin-films for flexible applications. We show and discuss sheet resistance results for wafer-scale CVD-grown graphene as well as for conductive thin-films produced from dispersions of exfoliated graphene-flakes on various substrates.

Molybdenum disulfide (MoS 2 ) is a widely studied two-dimensional (2D) semiconductor with high potential as channel material in field effect transistors (FET) [1] . A major challenge for MoS 2 -based FETs is low hysteresis operation [2] . Al 2 O 3 is a common encapsulation layer or gate dielectric, even though it causes an n-doping effect on MoS 2 -FETs after their encapsulation [3] , [4] . Here, we present a method for scalable MoS 2 FET gate dielectric deposition and encapsulation using a stack of monolayer hexagonal boron nitride (h-BN) and Al 2 O 3 . This heterostructure prevents the n-doping effect of Al 2 O 3 and leads to reduced hysteresis even under ambient conditions.

Advanced functional materials 2102929 (2021). doi:10.1002/adfm.202102929, Published by Wiley-VCH, Weinheim

In this work, a novel one-dimensional geometry for metal-insulator-graphene (1D-MIG) diode with low capacitance is demonstrated. The junction of the 1D-MIG diode is formed at the 1D edge of Al2O3-encapsulated graphene with TiO2 that acts as barrier material. The diodes demonstrate ultra-high current density since the transport in the graphene and through the barrier is in plane. The geometry delivers very low capacitive coupling between the cathode and anode of the diode, which shows frequency response up to 100 GHz and ensures potential high frequency performance up to 2.4 THz. The 1D-MIG diodes are demonstrated to function uniformly and stable under bending conditions down to 6.4 mm bending radius on flexible substrate.

In this letter, we report on the performance of graphene field-effect transistors (GFETs) in which the extrinsic transit frequency ( ${f}_{T}$ ) and maximum frequency of oscillation ( ${f}_{\text {max}}$ ) showed improved scaling behavior with respect to the gate length ( ${L}_{g}$ ). This improvement was achieved by the use of high-quality graphene in combination with successful optimization of the GFET technology, where extreme low source/drain contact resistances were obtained together with reduced parasitic pad capacitances. GFETs with gate lengths ranging from $0.5~\mu \text{m}$ to $\text {2}~\mu \text{m}$ have been characterized, and extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ frequencies of up to 34 and 37 GHz, respectively, were obtained for GFETs with the shortest gate lengths. Simulations based on a small-signal equivalent circuit model are in good agreement with the measured data. Extrapolation predicts extrinsic ${f}_{T}$ and ${f}_{\text {max}}$ values of approximately 100 GHz at ${L}_{g}=\text {50}$ nm. Further optimization of the GFET technology enables ${f}_{\text {max}}$ values above 100 GHz, which is suitable for many millimeter wave applications.

Advanced electronic materials 2001210 (2021). doi:10.1002/aelm.202001210 special issue: "Hot Topic: Carbon, Graphite, and Graphene", Published by Wiley-VCH Verlag GmbH & Co. KG, Weinheim

Graphene and bilayer graphene quantum dots are promising hosts for spin qubits with long coherence times. Although recent technological improvements make it possible to confine single electrons electrostatically in bilayer graphene quantum dots, and their spin and valley texture of the single particle spectrum has been studied in detail, their relaxation dynamics remains still unexplored. Here, we report on transport through a high-frequency gate controlled single-electron bilayer graphene quantum dot. By transient current spectroscopy of single-electron spin states, we extract a lower bound of the spin relaxation time of 0.5~$\mu$s. This result represents an important step towards the investigation of spin coherence times in graphene-based quantum dots and the implementation of spin-qubits., Comment: 6 Pages, 4 Figures

Flexible energy harvesting devices fabricated in scalable thin-film processes are important components in the field of wearable electronics and the Internet of Things. We present a flexible rectenna based on a one-dimensional junction metal-insulator-graphene diode, which offers low-noise power detection at terahertz (THz) frequencies. The rectennas are fabricated on a flexible polyimide film in a scalable process by photolithography using graphene grown by chemical vapor deposition. A one-dimensional junction area reduces the junction capacitance and enables operation in the D-band (110 - 170 GHz). The rectenna on polyimide shows a maximum voltage responsivity of 80 V/W at 167 GHz in free space measurements and minimum noise equivalent power of 80 pW/$\sqrt{\text{Hz}}$., Comment: 20 pages, 4 figures, 1 table

We demonstrate dispersive readout of individual charge states in a gate-defined few-electron quantum dot in bilayer graphene. We employ a radio frequency reflectometry circuit, where an LC resonator with a resonance frequency close to 280 MHz is directly coupled to an ohmic contact of the quantum dot device. The detection scheme based on changes in the quantum capacitance operates over a wide gate-voltage range and allows to probe excited states down to the single-electron regime. Crucially, the presented sensing technique avoids the use of an additional, capacitively coupled quantum device such as a quantum point contact or single electron transistor, making dispersive sensing particularly interesting for gate-defined graphene quantum dots., Comment: 5 Pages, 4 Figures

This article presents a fully integrated radio frequency (RF) rectifier on a flexible polyimide (PI) substrate. This circuit is designed and implemented using a chemical-vapor-deposition (CVD) graphene monolithic microwave integrated circuit (MMIC) process, where a metal–insulator–graphene (MIG) diode is employed to obtain the necessary nonlinearity for the rectifier. The smooth surface enables good titanium dioxide (Tio2) barrier interface formation and a higher carrier transport performance of the graphene. Passive components are also developed on the same substrate. To carry out the circuit design, a large-signal diode model is created based on measured results on a test key. A rectifier is deliberately designed for eliminating the input matching network by properly sizing the MIG diode and the load impedance. Removing the matching network results in a compact size and reduces the loss of the input of the rectifier. Nevertheless, the measured input reflection coefficient at 2.4 GHz is −17.5 dB, and measured, peak output dc voltage is 93 mV at an input power of 15.7 dBm. To the best of the authors’ knowledge, this design achieves the highest output voltage of all graphene-based rectifiers on a flexible substrate at 2.4 GHz. Thanks to the CVD graphene MMIC process, a rectifier array could be realized and thus, low-energy devices such as wireless-sensor-network (WSN) and medical implants are potential applications.

Integrated logic circuits require transistors that have a sufficiently high mobility, but also a high current on/off ratio. In this respect, two-dimensional (2D) molybdenum disulfide (MoS 2 ) has been demonstrated as a suitable channel material for n-type field-effect transistors (FETs) with high performance. Single-layer graphene (SLG) has been shown to reduce contact resistance in such MoS 2 devices [2] , [3] . Here, we experimentally demonstrate a scalable method for low resistivity contacts to MoS 2 using only chemical vapor deposited (CVD)-grown SLG, corroborated through simulations that explore the scalability potential of contact resistance optimization based on this approach.

The deposition of dielectric materials on graphene is one of the bottlenecks for unlocking the potential of graphene in electronic applications. In this paper we demonstrate the plasma enhanced atomic layer deposition of 10 nm thin high quality Al$_2$O$_3$ on graphene using a monolayer of hBN as protection layer. Raman spectroscopy was performed to analyze possible structural changes of the graphene lattice caused by the plasma deposition. The results show that a monolayer of hBN in combination with an optimized deposition process can effectively protect graphene from damage, while significant damage was observed without an hBN layer. Electrical characterization of double gated graphene field effect devices confirms that the graphene did not degrade during the plasma deposition of Al$_2$O$_3$. The leakage current densities were consistently below 1 nA/mm for electric fields across the insulators of up to 8 MV/cm, with irreversible breakdown happening above. Such breakdown electric fields are typical for Al$_2$O$_3$ and can be seen as an indicator for high quality dielectric films.

Graphene-based photodetectors have shown responsivities up to 10$^8$A/W and photoconductive gains up to 10$^{8}$ electrons per photon. These photodetectors rely on a highly absorbing layer in close proximity of graphene, which induces a shift of the graphene chemical potential upon absorption, hence modifying its channel resistance. However, due to the semi-metallic nature of graphene, the readout requires dark currents of hundreds of $\mu$A up to mA, leading to high power consumption needed for the device operation. Here we propose a novel approach for highly responsive graphene-based photodetectors with orders of magnitude lower dark current levels. A shift of the graphene chemical potential caused by light absorption in a layer of colloidal quantum dots, induces a variation of the current flowing across a metal-insulator-graphene diode structure. Owing to the low density of states of graphene near the neutrality point, the light-induced shift in chemical potential can be relatively large, dramatically changing the amount of current flowing across the insulating barrier, and giving rise to a novel type of gain mechanism. This readout requires dark currents of hundreds of nA up to few $\mu$A, orders of magnitude lower than other graphene-based photodetectors, while keeping responsivities of $\sim$70A/W in the infrared, almost two orders of magnitude higher compared to established germanium on silicon and indium gallium arsenide infrared photodetectors. This makes the device appealing for applications where high responsivity and low power consumption are required., Comment: 14 pages, 7 figures

Platinum diselenide (PtSe 2 ) is a promising two-dimensional (2D) material of the noble-metal dichalcogenides (NMDCs), a subgroup of the transition-metal dichalcogenides (TMDCs). It has been shown to exhibit a high negative piezoresistive gauge factor (GF) [1] and a charge carrier mobility of up to 210 cm 2 /Vs while being air-stable for many months [2] . It can be grown at CMOS-compatible temperatures by thermally assisted conversion (TAC) [3] . PtSe 2 can be tuned from a semiconductor to a semimetal by varying the number of layers [4] – [7] . Experimental data of electronic devices based on PtSe 2 show large variations in the electronic properties that cannot be explained by the material thickness alone. Here, we show that the nanocrystalline structure of TAC-grown PtSe 2 films greatly influences the electronic properties of PtSe 2 -based devices.

The prospects of using nanofilms of indium-containing III-V semiconductors, gold and single-layer graphene in magnetic field sensors, intended for application in radiation environment were evaluated on the results of testing in neutron fluxes. Semiconductor sensors are capable of withstanding radiation levels typical for the ITER-type fusion reactors, while gold sensors are stable even under environment expected in the first fusion power plant DEMO. Graphene is promising for creating sensors that combine high magnetic field sensitivity and high irradiation resistance.

IEEE journal of the Electron Devices Society 8, 457-464 (2020). doi:10.1109/JEDS.2020.2988630, Published by IEEE, [New York, NY]

An in-situ study of Hall sensors based on single-layer graphene in neutron fluxes of a nuclear reactor up to the fluence of 1.5×20 n·m−2 was conducted. The sensitivity of the sensors to the magnetic field remained stable throughout the experiment, while the resistance changes correlated with the increase in sample temperature due to radiation heating. The experiment confirmed the theoretical expectations regarding the high resistance of graphene sensors to neutron irradiation. Necessary further improvement of sensor technology to optimize their characteristics, as well as irradiation testing to determine the maximum permissible neutron fluence.

A gate-defined quantum dot in bilayer graphene is utilized as a sensitive electrometer for probing the charge density of its environment. Under the influence of a perpendicular magnetic field, the charge carrier density of the channel region next to the quantum dot oscillates due to the formation of Landau levels. This is experimentally observed as oscillations in the gate-voltage positions of the Coulomb resonances of the nearby quantum dot. From the frequency of the oscillations, we extract the charge carrier density in the channel and from the amplitude the shift of the quantum dot potential. We compare these experimental results with an electrostatic simulation of the device and find good agreement., Comment: 5 Pages, 4 Figures

Funder: H2020 Marie Skłodowska-Curie Actions; doi: https://doi.org/10.13039/100010665, Hexagonal boron nitride (h-BN) is well-established as a requisite support, encapsulant and barrier for 2D material technologies, but also recently as an active material for applications ranging from hyperbolic metasurfaces to room temperature single-photon sources. Cost-effective, scalable and high quality growth techniques for h-BN layers are critically required. We utilise widely-available iron foils for the catalytic chemical vapour deposition (CVD) of h BN and report on the significant role of bulk dissolved species in h-BN CVD, and specifically, the balance between dissolved oxygen and carbon. A simple pre-growth conditioning step of the iron foils enables us to tailor an error-tolerant scalable CVD process to give exceptionally large h-BN monolayer domains. We also develop a facile method for the improved transfer of as-grown h-BN away from the iron surface by means of the controlled humidity oxidation and subsequent rapid etching of a thin interfacial iron oxide; thus, avoiding the impurities from the bulk of the foil. We demonstrate wafer-scale (2 inch) production and utilise this h-BN as a protective layer for graphene towards integrated (opto) electronic device fabrication., European Union's Horizon 2020 research and innovation program under Grant Agreement No number 785219. European Union's Horizon 2020 research and innovation program under Grant Agreement No number 796388. the Royal Commission for the Exhibition of 1851. EU Marie Skłodowska-Curie Individual Fellowship (Global) under grant ARTIST (No. 656870). EPSRC (EP/P005152/1, and Doctoral Training Award EP/M508007/1). U.K. Department of Business, Energy and Industrial Strategy (NPL Project Number 121452).

Electronic and photonic devices based on the two-dimensional material graphene have unique properties, leading to outstanding performance figures-of-merit. Mastering the integration of this new and unconventional material into an established semiconductor fabrication line represents a critical step for pushing it forward towards commercialization.

Scientific reports 9(1), 18059 (2019). doi:10.1038/s41598-019-54489-0, Published by Macmillan Publishers Limited, part of Springer Nature, [London]

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get optimal maximum oscillation frequency (

This paper presents a 2.4 GHz transmitter on a flexible substrate using a graphene process. The four-way mixerless transmitter architecture is used to obtain high quality complex modulated signal. Key components such as Metal-insulator-graphene (MIG) diodes have been fabricated. Measured impedances of the flexible MIG diodes are used to simulate the transmitter performance and to evaluate the quality of the transmitted output signals. Simulated EVM rms of 4.16% is obtained for the 16-QAM modulation at the centre frequency.

In this letter, we present for the first time a mixer circuit based on Metal–Insulator–Graphene (MIG) diodes fabricated with large-scale monolayer graphene grown by chemical vapor deposition. A small-signal model extracted from the diode physical structure is used together with a large-signal model extracted from the dc characteristics of the MIG diode to build a down-conversion mixer. The measured conversion loss at a local oscillator power ( ${P}_{\text {LO}}$ ) of 5 dBm is lower than 15 dB, while RF-to-IF isolation is 36 dB with an input return loss and RF-to-LO isolation better than 10 dB over the frequency band from 1.7–6 GHz. Promising mixer results in combination with the CVD-based process promote the MIG diode-based mixer to be used in low-power, low-cost, microwave, and millimeter-wave circuit applications.

It has been argued that current saturation in graphene field-effect transistors (GFETs) is needed to get the highest possible maximum oscillation frequency (fmax). This paper numerically investigates whether velocity saturation can help to get better current saturation and if that correlates with enhanced fmax. For such a purpose, we used a drift-diffusion simulator that includes several factors that influence output conductance, especially at short channel lengths and-or large drain bias: short-channel electrostatics, saturation velocity, graphene-dielectric interface traps, and self-heating effects. As a testbed for our investigation, we analyzed fabricated GFETs with high extrinsin cutoff frequency fT,x (34 GHz) and fmax (37 GHz). Our simulations allow for a microscopic (local) analysis of the channel parameteres such as carrier concentration, drift and saturation velocities. For biases far away from the Dirac voltage, where the channel behaves as unipolar, we confirmed that the higher is the drift velocity, as close as possible to the saturation velocity, the greater fmax is. However, the largest fmax is recorded at biases near the crossover between unipolar and bipolar behavior, where it does not hold that the highest drift velocity maximizes fmax. In fact, the position and magnitude of the largest fmax depend on the complex interplay between the carrier concentration and total velocity which, in turn, are impacted by the self-heating. Importantly, this effect was found to severely limit radio-frequency performance, reducing the maximum fmax from around 60 to 40 GHz., 14 pages, 11 figures, supplementary material with 13 pages and 5 figures