| 1. |
- Biznárová, Janka, 1995, et al.
(författare)
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Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits
- 2024
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Ingår i: npj Quantum Information. - 2056-6387. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270 μs, corresponding to Q = 5 million, and a highest observed value of 501 μs. Through materials analysis techniques and numerical simulations we investigate the dominant source of energy loss, and devise and demonstrate a strategy toward its mitigation. Growing aluminum films thicker than 300 nm reduces the presence of oxide, a known host of defects, near the substrate-metal interface, as confirmed by time-of-flight secondary ion mass spectrometry. A loss analysis of coplanar waveguide resonators shows that this results in a reduction of dielectric loss due to two-level system defects. The correlation between the enhanced performance of our devices and the film thickness is due to the aluminum growth in columnar structures of parallel grain boundaries: transmission electron microscopy shows larger grains in the thicker film, and consequently fewer grain boundaries containing oxide near the substrate-metal interface.
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| 2. |
- ANDERSSON, MICHAEL, 1988, et al.
(författare)
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Microwave characterization of Ti/Au-graphene contacts
- 2013
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:17, s. 173111-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we report on a microwave characterization of the interface between Ti/Au contacts and chemical vapor deposition graphene using structures of Corbino geometry, with primary focus on extracting and modeling the capacitance associated with the contact region. It is found that with the current contact resistivity, ρc∼10^−6 Ωcm2, the contact capacitance, on the order Cc∼1 μF/cm2, has a negligible effect on microwave transmission through the contact below ∼100 GHz. Finally, a parallel plate capacitance model for the contact is presented.
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| 3. |
- ANDERSSON, MICHAEL, 1988, et al.
(författare)
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Towards Graphene Electrodes for High Performance Acoustic Resonators
- 2013
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Ingår i: WOCSDICE. ; , s. 99-100
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Konferensbidrag (refereegranskat)abstract
- The tunable FBAR is a promising building block for versatile microwave systems. Utilizing graphene electrodes promises higher tunability and frequency. Increased parasitic resistance may hamper the Q-factor of the resonator. This paper reports the initiated study of graphene and contacts at DC and microwave frequencies for optimization of these parameters leading to graphene FBARs.
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| 4. |
- Bray, C., et al.
(författare)
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Temperature-dependent zero-field splittings in graphene
- 2022
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 106:24
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene is a quantum spin Hall insulator with a 45μeV-wide nontrivial topological gap induced by the intrinsic spin-orbit coupling. Even though this zero-field spin splitting is weak, it makes graphene an attractive candidate for applications in quantum technologies, given the resulting long spin-relaxation time. On the other side, the staggered sublattice potential, resulting from the coupling of graphene with its boron nitride substrate, compensates intrinsic spin-orbit coupling and decreases the nontrivial topological gap, which may lead to the phase transition into trivial band insulator state. In this work, we present extensive experimental studies of the zero-field splittings in monolayer and bilayer graphene in a temperature range 2-12 K by means of subterahertz photoconductivity-based electron spin-resonance technique. Surprisingly, we observe a decrease of the spin splittings with increasing temperature. We discuss the origin of this phenomenon by considering possible physical mechanisms likely to induce a temperature dependence of the spin-orbit coupling. These include the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electron-phonon interactions, and the presence of a magnetic order at low temperature. Our experimental observation expands knowledge about the nontrivial topological gap in graphene.
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| 5. |
- Buron, J.D., et al.
(författare)
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Correlation between THz AC and micro-four-point-probe DC conductivity mapping of graphene sheets
- 2012
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Ingår i: Laser and Tera-Hertz Science and Technology, LTST 2012. - Washington, D.C. : OSA.
-
Konferensbidrag (refereegranskat)abstract
- We present quantitative correlation mapping of the sheet conductance of large areas of graphene. Terahertz time-domain spectroscopy (THz-TDS) maps the nanoscale conductance averaged over the beam spot size whereas micro four-point probe (M4PP) maps the micro-scale conductance.
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| 6. |
- Buron, J. D., et al.
(författare)
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Graphene Conductance Uniformity Mapping
- 2012
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 12:10, s. 5074-5081
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a combination of micro four-point probe (M4PP) and non-contact terahertz time-domain spectroscopy (THz-TDS) measurements for centimeter scale quantitative mapping of the sheet conductance of large area chemical vapor deposited graphene films. Dual configuration M4PP measurements, demonstrated on graphene for the first time, provide valuable statistical insight into the influence of microscale defects on the conductance, while THz-TDS has potential as a fast, non-contact metrology method for mapping of the spatially averaged nanoscopic conductance on wafer-scale graphene with scan times of less than a minute for a 4-in. wafer. The combination of M4PP and THz-TDS conductance measurements, supported by micro Raman spectroscopy and optical imaging, reveals that the film is electrically continuous on the nanoscopic scale with microscopic defects likely originating from the transfer process, dominating the microscale conductance of the investigated graphene film.
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| 7. |
- Buron, J. D., et al.
(författare)
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Quantitative mapping of large area graphene conductance
- 2012
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Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035. - 9781467315975
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Konferensbidrag (refereegranskat)abstract
- We present quantitative mapping of large area graphene conductance by terahertz time-domain spectroscopy and micro four point probe. We observe a clear correlation between the techniques and identify the observed systematic differences to be directly related to imperfections of the graphene sheet on the length scale of several micrometers.
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| 8. |
- Evelt, M., et al.
(författare)
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Chiral charge pumping in graphene deposited on a magnetic insulator
- 2017
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 95:2
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate experimentally that a sizable chiral charge pumping can be achieved at room temperature in graphene/yttrium iron garnet (YIG) bilayer systems. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. The strength of effect shows a nonmonotonous dependence on the spin-wave frequency, in agreement with the proposed theoretical model.
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| 9. |
- Gaska, Karolina, 1986, et al.
(författare)
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Influence of manufacturing process on electrical properties of LDPE-GnP nanocomposites
- 2017
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Ingår i: 25th Nordic Insulation Symposium on Materials, Components and Diagnostics.
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Konferensbidrag (refereegranskat)abstract
- In this report electrical properties of the nanocomposite samples, prepared from graphene nanoplatelet (GnP) loaded low density polyethylene (LDPE) by extrusion and compression molding, were examined in order to elucidate the impacts of the nanoplatelets size and material’s manufacturing process. It is shown that the extrusion forces a strong anisotropy in material’s morphology. The graphene nanoplatelets become aligned along the flow direction. As compared to pure LDPE, a significant reductions of the through-plane low field electric conductivity is found in such samples. On the other hand, the samples produced by press molding exhibit slightly higher level of electric conductivity, which is connected to their less aligned microstructure and filler dispersion. For comparison results of measurements on LDPE-graphene monolayer sandwiches are also presented.
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| 10. |
- Guo, W., et al.
(författare)
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Rapid chemical vapor deposition of graphene on liquid copper
- 2016
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Ingår i: Synthetic Metals. - : Elsevier BV. - 0379-6779. ; 216, s. 93-97
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Tidskriftsartikel (refereegranskat)abstract
- Molten copper is used to catalyze the graphene synthesis by chemical vapor deposition. The Cu has no grains above melting temperature, which is favorable for graphene growth. Using a vertical cold wall system, the deposition rate is drastically increased as compared with common hot-wall tube furnaces, pushing the method one step forward towards applications. A molybdenum-graphite Joule heater is used to avoid mechanical deformation of the carrier foil for the catalyst to ease the subsequent processes. The rapid deposition makes it possible to observe graphene growth on liquid Cu even at low pressure, where severe Cu evaporation simultaneously occurs.
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| 11. |
- Indykiewicz, Kornelia, 1986, et al.
(författare)
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Current-induced enhancement of photo-response in graphene THz radiation detectors
- 2022
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 12:11
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Tidskriftsartikel (refereegranskat)abstract
- Thermoelectric readout in a graphene terahertz (THz) radiation detector requires a p-n junction across the graphene channel. Even without an intentional p-n junction, two latent junctions can exist in the vicinity of the electrodes/antennas through the proximity to the metal. In a symmetrical structure, these junctions are connected back-to-back and therefore counterbalance each other with regard to rectification of the ac signal. Because of the Peltier effect, a small dc current results in additional heating in one and cooling in another p-n junction, thereby breaking the symmetry. The p-n junctions then no longer cancel, resulting in a greatly enhanced rectified signal. This allows simplifying the design and controlling the sensitivity of THz radiation detectors.
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| 12. |
- Khan, Munis, 1991, et al.
(författare)
-
High mobility graphene field effect transistors on flexible EVA/PET foils
- 2024
-
Ingår i: 2D Materials. - 2053-1583. ; 11:3
-
Tidskriftsartikel (refereegranskat)abstract
- Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1, unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.
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| 13. |
- Khan, Munis, 1991, et al.
(författare)
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High mobility graphene on EVA/PET
- 2022
-
Ingår i: Nanomaterials. - : MDPI AG. - 2079-4991. ; 12:3
-
Tidskriftsartikel (refereegranskat)abstract
- Transparent conductive film on a plastic substrate is a critical component in low cost, flexible and lightweight optoelectronics. CVD graphene transferred from copper- to ethylene vinyl acetate (EVA)/polyethylene terephthalate (PET) foil by hot press lamination has been reported as a robust and affordable alternative to manufacture highly flexible and conductive films. Here, we demonstrate that annealing the samples at 60 ∘C under a flow of nitrogen, after wet etching of copper foil by nitric acid, significantly enhances the Hall mobility of such graphene films. Raman, Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to evaluate the morphology and chemical composition of the graphene.
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| 14. |
- Li, Mengyue, 1985, et al.
(författare)
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Single-crystalline Bi2Sr2CaCu2O8+x detectors for direct detection of microwave radiation
- 2015
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 106:15
-
Tidskriftsartikel (refereegranskat)abstract
- We test radiation detectors made from single-crystalline Bi2Sr2CaCu2O8+x flakes put on oxidized Si substrates. The 100-nm-thick flakes are lithographically patterned into 4 x 12 mu m(2) large rectangles embedded in thin-film log-spiral antennas. The SiO2 layer weakens the thermal link between the flakes and the bath. Two modes of radiation detection have been observed. For a bolometric type of sensors a responsivity of similar to 300 V/W and a noise equivalent power of 30 nW/root Hz has been deduced at 70 K. Much more sensitive is the non-bolometric device showing characteristics similar to a Golay-type detector while being at least a thousand times faster. Making smaller (sub-mu m) structures is expected to significantly improve the performance of these devices and makes them very competitive among other microwave and terahertz detectors.
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| 15. |
- Lindvall, Niclas, 1985, et al.
(författare)
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Cleaning graphene using atomic force microscope
- 2012
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:6, s. Article Number: 064904-
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Tidskriftsartikel (refereegranskat)abstract
- We mechanically clean graphene devices using an atomic force microscope (AFM). By scanning an AFM tip in contact mode in a broom-like way over the sample, resist residues are pushed away from the desired area. We obtain atomically flat graphene with a root mean square (rms) roughness as low as 0.12 nm after this procedure. The cleaning also results in a shift of the charge-neutrality point toward zero gate voltage, as well as an increase in charge carrier mobility.
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| 16. |
- Lindvall, Niclas, 1985, et al.
(författare)
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Measurements of weak localization of graphene in inhomogeneous magnetic fields
- 2015
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Ingår i: JETP Letters. - : Pleiades Publishing Ltd. - 1090-6487 .- 0021-3640. ; 102:6, s. 367-371
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Tidskriftsartikel (refereegranskat)abstract
- Weak localization in graphene is studied in inhomogeneous magnetic fields. To generate the inhomogeneous field, a thin film of type-II superconducting niobium is put in close proximity to graphene. A deviation from the ordinary quadratic weak localization behavior is observed at low fields. We attribute this to the inhomogeneous field caused by vortices in the superconductor. The deviation, which depends on the carrier concentration in graphene, can be tuned by the gate voltage. In addition, collective vortex motion, known as vortex avalanches, is observed through magnetoresistance measurements of graphene.
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| 17. |
- Lindvall, Niclas, 1985, et al.
(författare)
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Towards transfer-free fabrication of graphene NEMS grown by chemical vapour deposition
- 2012
-
Ingår i: Micro and Nano Letters. - : Institution of Engineering and Technology (IET). - 1750-0443. ; 7:8, s. 749-752
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Tidskriftsartikel (refereegranskat)abstract
- Graphene, an atomic monolayer of sp(2)-hybridised carbon atoms, is a promising material for future NEMS based on its remarkable electronic and mechanical properties. Through the rapid progress of chemical vapour deposition of large-scale, high-quality graphene, these applications seem to be close to reality. However, issues related to the graphene transfer process limit the reproducibility of such devices. In this Letter, the authors present two different approaches for fabricating suspended graphene devices without any transfer step, using both catalytically and non-catalytically grown graphene. The authors achieve high reproducibility in manufacturing flat and uniform suspended graphene beams. While good mechanical properties are observed, the electrical performance is still poor, requiring improvements.
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| 18. |
- Lindvall, Niclas, 1985, et al.
(författare)
-
Transfer-free fabrication of suspended graphene grown by chemical vapor deposition
- 2012
-
Ingår i: 7th IEEE International Conference on Nano/Micro Engineered and Molecular Systems. NEMS 2012, Kyoto, 5 - 8 March 2012. - 9781467311243 ; , s. 19-22
-
Konferensbidrag (refereegranskat)abstract
- Graphene, a true two-dimensional material with extraordinary mechanical- and electronic properties, is thought to be ideal for nanoelectromechanical systems (NEMS), like mass- and force sensors. Here, we present two different ways to fabricate suspended graphene for the intended use in future NEMS applications. The fabrication schemes do not require transfer of graphene from a catalyst where the graphene is grown on to another supporting substrate. The transfer is a source of several issues causing irreproducibility in large-scale production of graphene devices. We obtain suspended graphene membranes by locally removing the copper thin film on top of which the graphene is catalytically grown. The membranes are uniform and exhibit mechanical properties similar to those of exfoliated graphene. Also, suspended graphene beams with electrical interconnects are fabricated from non-catalytically grown graphene on SiO 2. Both approaches represent the first steps towards transfer-free fabrication of suspended graphene for NEMS applications.
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| 19. |
- Liu, Lihui, 1985, et al.
(författare)
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A Mechanism for Highly Efficient Electrochemical Bubbling Delamination of CVD-Grown Graphene from Metal Substrates
- 2016
-
Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 3:8
-
Tidskriftsartikel (refereegranskat)abstract
- In most cases, transfer of chemical-vapor-deposited 2D materials from metallic foil catalysts onto a target substrate is the most necessary step for their promising fundamental studies and applications. Recently, a highly efficient and nondestructive electrochemical delamination method has been proposed as an alternative to the conventional etching transfer method, which alleviates the problem of cost and environment pollution because it eliminates the need to etch away the metals. Here, the mechanism of the electrochemical bubbling delamination process is elucidated by studying the effect of the various electrolytes on the delamination rate. A capacitor-based circuit model is proposed and confirmed by the electrochemical impedance spectroscopy results. A factor of 27 decrease in the time required for complete graphene delamination from the platinum cathodes is found when increasing the NaOH ratio in the electrolyte solution. The opposite trend is observed for delamination at the anode. The surface screening effect induced by nonreactive ions in the vicinity of the electrodes plays a key role in the delamination efficiency. The analysis is generic and can be used as a guideline to describe and design the electrochemical delamination of other 2D materials from their metal catalysts as well.
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| 20. |
- Lockhart de la Rosa, César Javier, 1987, et al.
(författare)
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Frame assisted H2O electrolysis induced H2 bubbling transfer of large area graphene grown by chemical vapor deposition on Cu
- 2013
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 102:2
-
Tidskriftsartikel (refereegranskat)abstract
- An improved technique for transferring large area graphene grown by chemical vapor deposition on copper is presented. It is based on mechanical separation of the graphene/copper by H-2 bubbles during H2O electrolysis, which only takes a few tens of seconds while leaving the copper cathode intact. A semi-rigid plastic frame in combination with thin polymer layer span on graphene gives a convenient way of handling-and avoiding wrinkles and holes in graphene. Optical and electrical characterizations prove the graphene quality is better than that obtained by traditional wet etching transfer. This technique appears to be highly reproducible and cost efficient.
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| 21. |
- Maussang, K., et al.
(författare)
-
Temperature dependance of Intrinsic Spin Orbit Coupling Gap in Graphene probed by Terahertz photoconductivity
- 2023
-
Ingår i: International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz. - 2162-2027 .- 2162-2035.
-
Konferensbidrag (refereegranskat)abstract
- Graphene is a quantum spin Hall insulator, with a nontrivial topological gap induced by the spin-orbit coupling. Such splitting is weak (∼ 45 μ eV) in the absence of external magnetic field. However, due to rather long spin-relaxation time, graphene is an attractive candidate for applications in quantum technologies. When it is encapsulated in hexagonal boron nitride, the coupling between graphene and the substrate compensates intrinsic spin-orbit coupling and decreases the nontrivial topological gap, which may lead to phase transition into a trivial band insulator state. In this work, we have measured experimentally the zero-field splittings in monolayer and bilayer graphene by the means of subterahertz photoconductivity-based electron spin resonance technique. The dependance in temperature of such splittings have been also studied in the 2-12K range. We observed a decrease of the spin splittings with increasing temperature. Such behavior might be understood from several physical mechanisms that could induce a temperature dependence of the spin-orbit coupling. These includes the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electronphonon interactions, and the presence of a magnetic order at low temperature.
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| 22. |
- Nam, Youngwoo, 1983, et al.
(författare)
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Graphene p-n-p junctions controlled by local gates made of naturally oxidized thin aluminium films
- 2012
-
Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:5, s. 1987-1992
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Tidskriftsartikel (refereegranskat)abstract
- Graphene structures with both top- and bottom-electrostatic gates are studied. The top gate is made of thin aluminium (Al) film deposited directly onto graphene, with no prior dielectric layer in between. Natural oxidation of Al at the interface with graphene results in an insulating barrier proving useful in making top gates to graphene. For electrically disconnected top gate, graphene resistance as a function of the slowly-varying back-gate voltage shows hysteresis which reveals dielectric properties of the barrier. The estimated barrier thickness is only 2 nm allowing for very sharp profiles of the electric field in graphene devices. By applying voltages to both back- and top gates, effective p–n–p junctions with sharp interfaces can be created.
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| 23. |
- Nam, Youngwoo, 1983, et al.
(författare)
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Graphene thermocouple fabricated on a flexible and transparent substrate
- 2024
-
Ingår i: AIP Advances. - 2158-3226 .- 2158-3226. ; 14:6
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the realization of reliable, high-quality, micro-sized graphene-based field-effect devices on a flexible and transparent substrate, ethylene vinyl acetate (EVA)/polyethylene terephthalate (PET), using a convenient hot-press lamination transfer and employing parylene-N as a dielectric material for gating. Using this technique, we fabricate a graphene thermocouple on the EVA/PET substrate. Specifically, the graphene is patterned in a U-shape, and its legs are equipped with two independent top gates. Full control of the carrier density and type by electrostatic gating in the two graphene regions allow the formation of a thermocouple layout, exhibiting an enlarged thermovoltage signal when the two regions are doped with opposite types of carriers and leading to a maximum sensitivity with a thermopower of ∼73 µV/K. This agrees well with the working principle of thermocouple, and it proves the good compatibility and functionality of the graphene thermocouple on the EVA/PET substrate. Our findings suggest possible applications for producing scalable and reliable graphene-based electronic devices on flexible and transparent substrates in a simple way.
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| 24. |
- Nam, Youngwoo, 1983, et al.
(författare)
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Quantum Hall effect in graphene decorated with disordered multilayer patches
- 2013
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 103:233, s. 233110-
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Tidskriftsartikel (refereegranskat)abstract
- Quantum Hall effect (QHE) is observed in graphene grown by chemical vapour deposition using platinum catalyst. The QHE is even seen in samples which are irregularly decorated with disordered multilayer graphene patches and have very low mobility (
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| 25. |
- Nam, Youngwoo, 1983, et al.
(författare)
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The Aharonov-Bohm effect in graphene rings with metal mirrors
- 2012
-
Ingår i: Carbon. - : Elsevier BV. - 0008-6223. ; 50:15, s. 5562-5568
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Tidskriftsartikel (refereegranskat)abstract
- We measured the Aharonov-Bohm (AB) effect in graphene rings with superconducting-(Al) and normal-metal (Au) mirrors. The mirrors were deposited either on additional stubs connected to the rings in the transverse direction or on the ring bias lines. A significant enhancement of the visible phase coherence was observed in the latter case, in which we observed even the third harmonic of the AB oscillations. The superconductivity of the mirrors appears to be unimportant for the improved coherence in graphene. A large Fermi energy mismatch between graphene and the mirror material is sufficient for this effect. In addition, a transport gap was observed in our graphene structures at the gate voltage close to the Dirac point. The value of the gap can be reproduced by assuming the occurrence of Coulomb blockade effects in graphene.
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