Randolph Elmquist - Academia.edu (original) (raw)

Papers by Randolph Elmquist

Research paper thumbnail of Rapid characterization of wafer-scale 2D material: Epitaxial graphene and graphene nanoribbons on SiC

arXiv: Applied Physics, 2017

We demonstrate that the confocal laser scanning microscopy (CLSM) provides a non-destructive, hig... more We demonstrate that the confocal laser scanning microscopy (CLSM) provides a non-destructive, highly-efficient characterization method for large-area epitaxial graphene and graphene nanostructures on SiC substrates, which can be applied in ambient air without sample preparation and is insusceptible to surface charging or surface contamination. Based on the variation of reflected intensity from regions covered by interfacial layer, single layer, bilayer, or few layer graphene, and through the correlation to the results from Raman spectroscopy and SPM, CLSM images with a high resolution (around 150 nm) reveal that the intensity contrast has distinct feature for undergrown graphene (mixing of dense, parallel graphene nanoribbons and interfacial layer), continuous graphene, and overgrown graphene. Moreover, CLSM has a real acquisition time hundreds of times faster per unit area than the supplementary characterization methods. We believe that the confocal laser scanning microscope will b...

Research paper thumbnail of Onsager-Casimir frustration from resistance anisotropy in graphene quantum Hall devices

Physical Review B, 2021

We report on nonreciprocity observations in several configurations of graphene-based quantum Hall... more We report on nonreciprocity observations in several configurations of graphene-based quantum Hall devices. Two distinct measurement configurations were adopted to verify the universality of the observations (i.e., two-terminal arrays and four-terminal devices). Our findings determine the extent to which epitaxial graphene anisotropies contribute to the observed asymmetric Hall responses. The presence of backscattering induces a device-dependent asymmetry rendering the Onsager-Casimir relations limited in their capacity to describe the behavior of such devices, except in the low-field classical regime and the fully quantized Hall state. The improved understanding of this quantum electrical process broadly limits the applicability of the reciprocity principle in the presence of quantum phase transitions and for anisotropic two-dimensional materials.

Research paper thumbnail of Transport Behavior and Characterization of Precision 1 Resistors for International Comparisons

The U. S. National Institute of Standards and Technology (NIST) and the National Measurement Inst... more The U. S. National Institute of Standards and Technology (NIST) and the National Measurement Institute of Australia (NMIA) have recently examined two types of precision transportable 1  resistors that are based on different alloys and construction principles to determine characteristics that can reduce the uncertainty of international comparisons. This work focuses on standards manufactured from 1970 through 2000 by the NMIA, and Thomas-type resistors designed in the 1930s by James L. Thomas and manufactured commercially through about 1980. The effects of temperature, barometric pressure, humidity, power loading, and heat dissipation in oil are described in these two types of transportable wire-wound 1  resistance standards, and the process of characterization of these resistors for use as transport standards is described.

Research paper thumbnail of Confocal laser scanning microscopy: A tool for rapid optical characterization of 2D materials

Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characte... more Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characterization method for large-area analysis of graphene on different substrates, which can be applied in ambient air, does not require additional sample preparation, and is insusceptible to surface charging and surface contamination. CLSM leverages optical properties of graphene and provides greatly enhanced optical contrast and mapping of thickness down to a single layer. We demonstrate the effectiveness of CLSM by measuring mechanically exfoliated and chemical vapor deposition graphene on Si/SiO2, and epitaxial graphene on SiC. In the case of graphene on Si/SiO2, both CLSM intensity and height mapping is powerful for analysis of 1-5 layers of graphene. For epitaxial graphene on SiC substrates, the CLSM intensity allows us to distinguish features such as dense, parallel 150 nm wide ribbons of graphene (associated with the early stages of the growth process) and large regions covered by the ...

Research paper thumbnail of Comparison between NIST Graphene and AIST GaAs Quantized Hall Devices

IEEE Transactions on Instrumentation and Measurement

Several graphene quantized Hall resistance (QHR) devices manufactured at the National Institute o... more Several graphene quantized Hall resistance (QHR) devices manufactured at the National Institute of Standards and Technology (NIST) were compared to GaAs QHR devices and a 100 Ω standard resistor at the National Institute for Advanced Industrial Science and Technology (AIST). Measurements of the 100 Ω resistor with the graphene QHR devices agreed within 5 nΩ/Ω of the values for the 100 Ω resistor obtained through GaAs measurements. The electron density of the graphene devices was adjusted at AIST to restore device properties such that operation was possible at low magnetic flux densities of 4 T to 6 T. This adjustment was accomplished with a functionalization method utilized at NIST, allowing for consistent tunability of the graphene QHR devices with simple annealing. Such a method replaces older and less predictable methods for adjusting graphene for metrological suitability. The milestone results demonstrate the ease with which graphene can be used to make resistance comparison measurements among many National Metrology Institutes.

Research paper thumbnail of Final report on SIM.EM-S5: Voltage, current and resistance comparison, June 2007–October 2009

Metrologia, 2014

This is a report of the results of the second Interamerican Metrology System (SIM) comparison on ... more This is a report of the results of the second Interamerican Metrology System (SIM) comparison on calibration of digital multimeters, performed for strengthening the interaction among National Metrology Institutes (NMIs) and for establishing the degree of equivalence between those laboratories in accordance with the CIPM Mutual Recognition Agreement. From June 2007 to October 2009, four multimeters were used as traveling standards for measurements in eleven countries, with NIST-USA acting as pilot laboratory. Results for nine measurement points are presented as errors relative to a comparison reference value together with their uncertainty.

Research paper thumbnail of Characteristics of precision 1 Ω standard resistors influencing transport behaviour and the uncertainty of key comparisons

Metrologia, 2009

National measurement institutes (NMIs) participate in international key comparisons organized by ... more National measurement institutes (NMIs) participate in international key comparisons organized by the Bureau International des Poids et Mesures (BIPM), the Regional Metrology Organizations (RMOs) or the Consultative Committees of the Comité International des Poids et Mesures (CIPM) in order to provide evidence of equivalent reference standards and measurement capabilities. The US National Institute of Standards and Technology (NIST) and the National Measurement Institute of Australia (NMIA) have recently examined power loading and several other influences on the value of precision transportable 1 resistors that can increase the uncertainty of key comparisons. We have studied the effects of temperature, barometric pressure, humidity, power loading and heat dissipation in oil on transportable wire-wound 1 resistance standards that are based on different alloys and construction principles. This work focuses on standards manufactured from 1970 through 2000 by the NMIA made of Evanohm alloy and on Thomas-type resistors designed in the 1930s and made of Manganin alloy. We show that the relative standard uncertainty related to transport can be less than 0.01 µ −1 when using certain resistors of these two types that are characterized and selected for stability. We describe the characterization process, and relate the environmental influences to the physical design, as well as to the mechanical properties and condition of the standards.

Research paper thumbnail of Cryogenic current comparator measurements at 77 K using thallium-2223 thick-film shields

IEEE Transactions on Instrumentation and Measurement, 1999

Several different magnetic shield geometries were used to study cryogenic current comparator (CCC... more Several different magnetic shield geometries were used to study cryogenic current comparator (CCC) operation with thallium-based thick film superconducting shields at 77 K. These shields are found to have good magnetic shielding properties and to support low-level persistent currents. This study investigates geometric properties of the magnetic coupling between the CCC and one type of superconducting quantum interference device (SQUID) sensor. The sensitivity of the SQUID and its coupling to the shielding currents limit ten-to-one CCC resistance ratio measurements to approximately 0.2 = in relative uncertainty.

Research paper thumbnail of Next-generation crossover-free quantum Hall arrays with superconducting interconnections

Metrologia

This work presents precision measurements of quantized Hall array resistance devices using superc... more This work presents precision measurements of quantized Hall array resistance devices using superconducting, crossover-free, multiple interconnections as well as graphene split contacts. These new techniques successfully eliminate the accumulation of internal resistances and leakage currents that typically occur at interconnections and crossing leads between interconnected devices. As a result, a scalable quantized Hall resistance array is obtained with a nominal value that is as precise and stable as that from single-element quantized Hall resistance standards.

Research paper thumbnail of Confocal laser scanning microscopy for rapid optical characterization of graphene

Communications Physics

Two-dimensional (2D) materials such as graphene have become the focus of extensive research effor... more Two-dimensional (2D) materials such as graphene have become the focus of extensive research efforts in condensed matter physics. They provide opportunities for both fundamental research and applications across a wide range of industries. Ideally, characterization of graphene requires non-invasive techniques with single-atomic-layer thickness resolution and nanometer lateral resolution. Moreover, commercial application of graphene requires fast and large-area scanning capability. We demonstrate the optimized balance of image resolution and acquisition time of non-invasive confocal laser scanning microscopy (CLSM), rendering it an indispensable tool for rapid analysis of mass-produced graphene. It is powerful for analysis of 1-5 layers of exfoliated graphene on Si/SiO 2 , and allows us to distinguish the interfacial layer and 1-3 layers of epitaxial graphene on SiC substrates. Furthermore, CLSM shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning electron microscopy and Raman mapping.

Research paper thumbnail of Surface reconstruction and graphene formation on face-to-face 6H-SiC at 2000$^{\ circ} $ C

Bulletin of the American …, 2012

... 11:15 AM–2:15 PM, Wednesday, February 29, 2012. Room: 210C Sponsoring Units: DMP DCMP Chair: ... more ... 11:15 AM–2:15 PM, Wednesday, February 29, 2012. Room: 210C Sponsoring Units: DMP DCMP Chair: Kevin McCarty, Sandia National Laboratory. ... Brian G. Bush (National Institute of Standards and Technology). Tian Shen (National Institute of Standards and Technology). ...

Research paper thumbnail of Development of gateless quantum Hall checkerboard p–n junction devices

Journal of Physics D: Applied Physics

Research paper thumbnail of Accessing ratios of quantized resistances in graphene p–n junction devices using multiple terminals

AIP Advances

The utilization of multiple current terminals on millimeter-scale graphene p-n junction devices h... more The utilization of multiple current terminals on millimeter-scale graphene p-n junction devices has enabled the measurement of many atypical, fractional multiples of the quantized Hall resistance at the ν = 2 plateau (R H ≈ 12 906 Ω). These fractions take the form a b R H and can be determined both analytically and by simulations. These experiments validate the use of either the LTspice circuit simulator or the analytical framework recently presented in similar work. Furthermore, the production of several devices with large-scale junctions substantiates the approach of using simple ultraviolet lithography to obtain junctions of sufficient sharpness.

Research paper thumbnail of Nanostructured graphene for nanoscale electron paramagnetic resonance spectroscopy

Journal of Physics: Materials

The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive ph... more The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive photodetectors, with electrical noise equivalent power lower than 10 −15 W Hz −0.5 at temperatures below 3 K, for detection of radiation in a very broad frequency range, including ultraviolet, visible and terahertz. Here we demonstrate the operation of these detectors in the presence of magnetic field as high as 7 T, paving the way to in situ spectroscopy of molecular nanomagnets.

Research paper thumbnail of A Self-Assembled Graphene Ribbon Device on SiC

ACS Applied Electronic Materials

Graphene ribbons, which may be fabricated by a wide variety of experimental techniques such as ch... more Graphene ribbons, which may be fabricated by a wide variety of experimental techniques such as chemical processing, unzipping or etching of carbon nanotubes, molecular precursors, ion implantation, and so on, can find promising applications in interconnects, terahertz sensors, and plasmonic devices. Here we report measurements on self-assembled graphene ribbons that are prepared by a controlled high-temperature sublimation technique. The epitaxial graphene ribbons on SiC can be readily and efficiently located by confocal laser scanning microscopy for device fabrication using a removable metal protection layer to avoid contamination of the graphene and hexagonal boron nitride to serve as a top-gate dielectric spacer. These self-assembled graphene ribbons have smooth edges, and the observation of a magnetoresistance side peak in such a structure is consistent with diffusive boundary scattering in the quasi-ballistic regime. In contrast, graphene ribbons defined by electron-beam lithography and subsequent conventional reactive ion etching on the same SiC wafer only show pronounced negative magnetoresistance due to strong disorder in the edge structures (chemical dopants, the resolution of electron-beam lithography, etc.). Our experimental approaches are applicable to wafer-scale, graphene-based integrated circuits.

Research paper thumbnail of Quantum transport in graphene p−n junctions with moiré superlattice modulation

Physical Review B

We present simulations of quantum transport in graphene p-n junctions (pnJs) in which moiré super... more We present simulations of quantum transport in graphene p-n junctions (pnJs) in which moiré superlattice potentials are incorporated to demonstrate the interplay between pnJs and moiré superlattice potentials. It is shown that the longitudinal and Hall resistivity maps can be strongly modulated by the pnJ profile, junction height, and moiré potentials. Device resistance measurements are subsequently performed on graphene/hexagonalboron-nitride heterostructure samples with accurate alignment of crystallographic orientations to complement and support the simulation results.

Research paper thumbnail of Manetoresistance of Ultralow-Hole-Density Monolayer Epitaxial Graphene Grown on SiC

Materials

Silicon carbide (SiC) has already found useful applications in high-power electronic devices and ... more Silicon carbide (SiC) has already found useful applications in high-power electronic devices and light-emitting diodes (LEDs). Interestingly, SiC is a suitable substrate for growing monolayer epitaxial graphene and GaN-based devices. Therefore, it provides the opportunity for integration of high-power devices, LEDs, atomically thin electronics, and high-frequency devices, all of which can be prepared on the same SiC substrate. In this paper, we concentrate on detailed measurements on ultralow-density p-type monolayer epitaxial graphene, which has yet to be extensively studied. The measured resistivity ρxx shows insulating behavior in the sense that ρxx decreases with increasing temperature T over a wide range of T (1.5 K ≤ T ≤ 300 K). The crossover from negative magnetoresistivity (MR) to positive magnetoresistivity at T = 40 K in the low-field regime is ascribed to a transition from low-T quantum transport to high-T classical transport. For T ≥ 120 K, the measured positive MR ratio...

Research paper thumbnail of The units for mass, voltage, resistance, and electrical current in the SI

IEEE Instrumentation & Measurement Magazine

Research paper thumbnail of Epitaxial graphene for quantum resistance metrology

Metrologia

Graphene-based quantised Hall resistance standards promise high precision for the unit ohm under ... more Graphene-based quantised Hall resistance standards promise high precision for the unit ohm under less exclusive measurement conditions, enabling the use of compact measurement systems. To meet the requirements of metrological applications, national metrology institutes developed large-area monolayer graphene growth methods for uniform material properties and optimized device fabrication techniques. Precision measurements of the quantised Hall resistance showing the advantage of graphene over GaAs-based resistance standards demonstrate the remarkable achievements realized by the research community. This work provides an overview over the state-of-the-art technologies in this field.

Research paper thumbnail of Quantum Hall device data monitoring following encapsulating polymer deposition

Data in brief, 2018

The information provided in this data article will cover the growth parameters for monolayer, epi... more The information provided in this data article will cover the growth parameters for monolayer, epitaxial graphene, as well as how to verify the layer homogeneity by confocal laser scanning and optical microscopy. The characterization of the subsequently fabricated quantum Hall device is shown for example cases during a series of environmental exposures. Quantum Hall data acquired from a CYTOP encapsulation is also provided. Data from Raman spectroscopy, atomic force microscopy, and other electrical property trends are shown. Lastly, quantum Hall effect data are presented from devices with deposited Parylene C films measuring 10.7 μm and 720 nm. All data are relevant for Rigosi et al. [1].

Research paper thumbnail of Rapid characterization of wafer-scale 2D material: Epitaxial graphene and graphene nanoribbons on SiC

arXiv: Applied Physics, 2017

We demonstrate that the confocal laser scanning microscopy (CLSM) provides a non-destructive, hig... more We demonstrate that the confocal laser scanning microscopy (CLSM) provides a non-destructive, highly-efficient characterization method for large-area epitaxial graphene and graphene nanostructures on SiC substrates, which can be applied in ambient air without sample preparation and is insusceptible to surface charging or surface contamination. Based on the variation of reflected intensity from regions covered by interfacial layer, single layer, bilayer, or few layer graphene, and through the correlation to the results from Raman spectroscopy and SPM, CLSM images with a high resolution (around 150 nm) reveal that the intensity contrast has distinct feature for undergrown graphene (mixing of dense, parallel graphene nanoribbons and interfacial layer), continuous graphene, and overgrown graphene. Moreover, CLSM has a real acquisition time hundreds of times faster per unit area than the supplementary characterization methods. We believe that the confocal laser scanning microscope will b...

Research paper thumbnail of Onsager-Casimir frustration from resistance anisotropy in graphene quantum Hall devices

Physical Review B, 2021

We report on nonreciprocity observations in several configurations of graphene-based quantum Hall... more We report on nonreciprocity observations in several configurations of graphene-based quantum Hall devices. Two distinct measurement configurations were adopted to verify the universality of the observations (i.e., two-terminal arrays and four-terminal devices). Our findings determine the extent to which epitaxial graphene anisotropies contribute to the observed asymmetric Hall responses. The presence of backscattering induces a device-dependent asymmetry rendering the Onsager-Casimir relations limited in their capacity to describe the behavior of such devices, except in the low-field classical regime and the fully quantized Hall state. The improved understanding of this quantum electrical process broadly limits the applicability of the reciprocity principle in the presence of quantum phase transitions and for anisotropic two-dimensional materials.

Research paper thumbnail of Transport Behavior and Characterization of Precision 1 Resistors for International Comparisons

The U. S. National Institute of Standards and Technology (NIST) and the National Measurement Inst... more The U. S. National Institute of Standards and Technology (NIST) and the National Measurement Institute of Australia (NMIA) have recently examined two types of precision transportable 1  resistors that are based on different alloys and construction principles to determine characteristics that can reduce the uncertainty of international comparisons. This work focuses on standards manufactured from 1970 through 2000 by the NMIA, and Thomas-type resistors designed in the 1930s by James L. Thomas and manufactured commercially through about 1980. The effects of temperature, barometric pressure, humidity, power loading, and heat dissipation in oil are described in these two types of transportable wire-wound 1  resistance standards, and the process of characterization of these resistors for use as transport standards is described.

Research paper thumbnail of Confocal laser scanning microscopy: A tool for rapid optical characterization of 2D materials

Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characte... more Confocal laser scanning microscopy (CLSM) is a non-destructive, highly-efficient optical characterization method for large-area analysis of graphene on different substrates, which can be applied in ambient air, does not require additional sample preparation, and is insusceptible to surface charging and surface contamination. CLSM leverages optical properties of graphene and provides greatly enhanced optical contrast and mapping of thickness down to a single layer. We demonstrate the effectiveness of CLSM by measuring mechanically exfoliated and chemical vapor deposition graphene on Si/SiO2, and epitaxial graphene on SiC. In the case of graphene on Si/SiO2, both CLSM intensity and height mapping is powerful for analysis of 1-5 layers of graphene. For epitaxial graphene on SiC substrates, the CLSM intensity allows us to distinguish features such as dense, parallel 150 nm wide ribbons of graphene (associated with the early stages of the growth process) and large regions covered by the ...

Research paper thumbnail of Comparison between NIST Graphene and AIST GaAs Quantized Hall Devices

IEEE Transactions on Instrumentation and Measurement

Several graphene quantized Hall resistance (QHR) devices manufactured at the National Institute o... more Several graphene quantized Hall resistance (QHR) devices manufactured at the National Institute of Standards and Technology (NIST) were compared to GaAs QHR devices and a 100 Ω standard resistor at the National Institute for Advanced Industrial Science and Technology (AIST). Measurements of the 100 Ω resistor with the graphene QHR devices agreed within 5 nΩ/Ω of the values for the 100 Ω resistor obtained through GaAs measurements. The electron density of the graphene devices was adjusted at AIST to restore device properties such that operation was possible at low magnetic flux densities of 4 T to 6 T. This adjustment was accomplished with a functionalization method utilized at NIST, allowing for consistent tunability of the graphene QHR devices with simple annealing. Such a method replaces older and less predictable methods for adjusting graphene for metrological suitability. The milestone results demonstrate the ease with which graphene can be used to make resistance comparison measurements among many National Metrology Institutes.

Research paper thumbnail of Final report on SIM.EM-S5: Voltage, current and resistance comparison, June 2007–October 2009

Metrologia, 2014

This is a report of the results of the second Interamerican Metrology System (SIM) comparison on ... more This is a report of the results of the second Interamerican Metrology System (SIM) comparison on calibration of digital multimeters, performed for strengthening the interaction among National Metrology Institutes (NMIs) and for establishing the degree of equivalence between those laboratories in accordance with the CIPM Mutual Recognition Agreement. From June 2007 to October 2009, four multimeters were used as traveling standards for measurements in eleven countries, with NIST-USA acting as pilot laboratory. Results for nine measurement points are presented as errors relative to a comparison reference value together with their uncertainty.

Research paper thumbnail of Characteristics of precision 1 Ω standard resistors influencing transport behaviour and the uncertainty of key comparisons

Metrologia, 2009

National measurement institutes (NMIs) participate in international key comparisons organized by ... more National measurement institutes (NMIs) participate in international key comparisons organized by the Bureau International des Poids et Mesures (BIPM), the Regional Metrology Organizations (RMOs) or the Consultative Committees of the Comité International des Poids et Mesures (CIPM) in order to provide evidence of equivalent reference standards and measurement capabilities. The US National Institute of Standards and Technology (NIST) and the National Measurement Institute of Australia (NMIA) have recently examined power loading and several other influences on the value of precision transportable 1 resistors that can increase the uncertainty of key comparisons. We have studied the effects of temperature, barometric pressure, humidity, power loading and heat dissipation in oil on transportable wire-wound 1 resistance standards that are based on different alloys and construction principles. This work focuses on standards manufactured from 1970 through 2000 by the NMIA made of Evanohm alloy and on Thomas-type resistors designed in the 1930s and made of Manganin alloy. We show that the relative standard uncertainty related to transport can be less than 0.01 µ −1 when using certain resistors of these two types that are characterized and selected for stability. We describe the characterization process, and relate the environmental influences to the physical design, as well as to the mechanical properties and condition of the standards.

Research paper thumbnail of Cryogenic current comparator measurements at 77 K using thallium-2223 thick-film shields

IEEE Transactions on Instrumentation and Measurement, 1999

Several different magnetic shield geometries were used to study cryogenic current comparator (CCC... more Several different magnetic shield geometries were used to study cryogenic current comparator (CCC) operation with thallium-based thick film superconducting shields at 77 K. These shields are found to have good magnetic shielding properties and to support low-level persistent currents. This study investigates geometric properties of the magnetic coupling between the CCC and one type of superconducting quantum interference device (SQUID) sensor. The sensitivity of the SQUID and its coupling to the shielding currents limit ten-to-one CCC resistance ratio measurements to approximately 0.2 = in relative uncertainty.

Research paper thumbnail of Next-generation crossover-free quantum Hall arrays with superconducting interconnections

Metrologia

This work presents precision measurements of quantized Hall array resistance devices using superc... more This work presents precision measurements of quantized Hall array resistance devices using superconducting, crossover-free, multiple interconnections as well as graphene split contacts. These new techniques successfully eliminate the accumulation of internal resistances and leakage currents that typically occur at interconnections and crossing leads between interconnected devices. As a result, a scalable quantized Hall resistance array is obtained with a nominal value that is as precise and stable as that from single-element quantized Hall resistance standards.

Research paper thumbnail of Confocal laser scanning microscopy for rapid optical characterization of graphene

Communications Physics

Two-dimensional (2D) materials such as graphene have become the focus of extensive research effor... more Two-dimensional (2D) materials such as graphene have become the focus of extensive research efforts in condensed matter physics. They provide opportunities for both fundamental research and applications across a wide range of industries. Ideally, characterization of graphene requires non-invasive techniques with single-atomic-layer thickness resolution and nanometer lateral resolution. Moreover, commercial application of graphene requires fast and large-area scanning capability. We demonstrate the optimized balance of image resolution and acquisition time of non-invasive confocal laser scanning microscopy (CLSM), rendering it an indispensable tool for rapid analysis of mass-produced graphene. It is powerful for analysis of 1-5 layers of exfoliated graphene on Si/SiO 2 , and allows us to distinguish the interfacial layer and 1-3 layers of epitaxial graphene on SiC substrates. Furthermore, CLSM shows excellent correlation with conventional optical microscopy, atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, scanning electron microscopy and Raman mapping.

Research paper thumbnail of Surface reconstruction and graphene formation on face-to-face 6H-SiC at 2000$^{\ circ} $ C

Bulletin of the American …, 2012

... 11:15 AM–2:15 PM, Wednesday, February 29, 2012. Room: 210C Sponsoring Units: DMP DCMP Chair: ... more ... 11:15 AM–2:15 PM, Wednesday, February 29, 2012. Room: 210C Sponsoring Units: DMP DCMP Chair: Kevin McCarty, Sandia National Laboratory. ... Brian G. Bush (National Institute of Standards and Technology). Tian Shen (National Institute of Standards and Technology). ...

Research paper thumbnail of Development of gateless quantum Hall checkerboard p–n junction devices

Journal of Physics D: Applied Physics

Research paper thumbnail of Accessing ratios of quantized resistances in graphene p–n junction devices using multiple terminals

AIP Advances

The utilization of multiple current terminals on millimeter-scale graphene p-n junction devices h... more The utilization of multiple current terminals on millimeter-scale graphene p-n junction devices has enabled the measurement of many atypical, fractional multiples of the quantized Hall resistance at the ν = 2 plateau (R H ≈ 12 906 Ω). These fractions take the form a b R H and can be determined both analytically and by simulations. These experiments validate the use of either the LTspice circuit simulator or the analytical framework recently presented in similar work. Furthermore, the production of several devices with large-scale junctions substantiates the approach of using simple ultraviolet lithography to obtain junctions of sufficient sharpness.

Research paper thumbnail of Nanostructured graphene for nanoscale electron paramagnetic resonance spectroscopy

Journal of Physics: Materials

The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive ph... more The opening of a quantum confinement gap in nanostructured graphene yields extremely sensitive photodetectors, with electrical noise equivalent power lower than 10 −15 W Hz −0.5 at temperatures below 3 K, for detection of radiation in a very broad frequency range, including ultraviolet, visible and terahertz. Here we demonstrate the operation of these detectors in the presence of magnetic field as high as 7 T, paving the way to in situ spectroscopy of molecular nanomagnets.

Research paper thumbnail of A Self-Assembled Graphene Ribbon Device on SiC

ACS Applied Electronic Materials

Graphene ribbons, which may be fabricated by a wide variety of experimental techniques such as ch... more Graphene ribbons, which may be fabricated by a wide variety of experimental techniques such as chemical processing, unzipping or etching of carbon nanotubes, molecular precursors, ion implantation, and so on, can find promising applications in interconnects, terahertz sensors, and plasmonic devices. Here we report measurements on self-assembled graphene ribbons that are prepared by a controlled high-temperature sublimation technique. The epitaxial graphene ribbons on SiC can be readily and efficiently located by confocal laser scanning microscopy for device fabrication using a removable metal protection layer to avoid contamination of the graphene and hexagonal boron nitride to serve as a top-gate dielectric spacer. These self-assembled graphene ribbons have smooth edges, and the observation of a magnetoresistance side peak in such a structure is consistent with diffusive boundary scattering in the quasi-ballistic regime. In contrast, graphene ribbons defined by electron-beam lithography and subsequent conventional reactive ion etching on the same SiC wafer only show pronounced negative magnetoresistance due to strong disorder in the edge structures (chemical dopants, the resolution of electron-beam lithography, etc.). Our experimental approaches are applicable to wafer-scale, graphene-based integrated circuits.

Research paper thumbnail of Quantum transport in graphene p−n junctions with moiré superlattice modulation

Physical Review B

We present simulations of quantum transport in graphene p-n junctions (pnJs) in which moiré super... more We present simulations of quantum transport in graphene p-n junctions (pnJs) in which moiré superlattice potentials are incorporated to demonstrate the interplay between pnJs and moiré superlattice potentials. It is shown that the longitudinal and Hall resistivity maps can be strongly modulated by the pnJ profile, junction height, and moiré potentials. Device resistance measurements are subsequently performed on graphene/hexagonalboron-nitride heterostructure samples with accurate alignment of crystallographic orientations to complement and support the simulation results.

Research paper thumbnail of Manetoresistance of Ultralow-Hole-Density Monolayer Epitaxial Graphene Grown on SiC

Materials

Silicon carbide (SiC) has already found useful applications in high-power electronic devices and ... more Silicon carbide (SiC) has already found useful applications in high-power electronic devices and light-emitting diodes (LEDs). Interestingly, SiC is a suitable substrate for growing monolayer epitaxial graphene and GaN-based devices. Therefore, it provides the opportunity for integration of high-power devices, LEDs, atomically thin electronics, and high-frequency devices, all of which can be prepared on the same SiC substrate. In this paper, we concentrate on detailed measurements on ultralow-density p-type monolayer epitaxial graphene, which has yet to be extensively studied. The measured resistivity ρxx shows insulating behavior in the sense that ρxx decreases with increasing temperature T over a wide range of T (1.5 K ≤ T ≤ 300 K). The crossover from negative magnetoresistivity (MR) to positive magnetoresistivity at T = 40 K in the low-field regime is ascribed to a transition from low-T quantum transport to high-T classical transport. For T ≥ 120 K, the measured positive MR ratio...

Research paper thumbnail of The units for mass, voltage, resistance, and electrical current in the SI

IEEE Instrumentation & Measurement Magazine

Research paper thumbnail of Epitaxial graphene for quantum resistance metrology

Metrologia

Graphene-based quantised Hall resistance standards promise high precision for the unit ohm under ... more Graphene-based quantised Hall resistance standards promise high precision for the unit ohm under less exclusive measurement conditions, enabling the use of compact measurement systems. To meet the requirements of metrological applications, national metrology institutes developed large-area monolayer graphene growth methods for uniform material properties and optimized device fabrication techniques. Precision measurements of the quantised Hall resistance showing the advantage of graphene over GaAs-based resistance standards demonstrate the remarkable achievements realized by the research community. This work provides an overview over the state-of-the-art technologies in this field.

Research paper thumbnail of Quantum Hall device data monitoring following encapsulating polymer deposition

Data in brief, 2018

The information provided in this data article will cover the growth parameters for monolayer, epi... more The information provided in this data article will cover the growth parameters for monolayer, epitaxial graphene, as well as how to verify the layer homogeneity by confocal laser scanning and optical microscopy. The characterization of the subsequently fabricated quantum Hall device is shown for example cases during a series of environmental exposures. Quantum Hall data acquired from a CYTOP encapsulation is also provided. Data from Raman spectroscopy, atomic force microscopy, and other electrical property trends are shown. Lastly, quantum Hall effect data are presented from devices with deposited Parylene C films measuring 10.7 μm and 720 nm. All data are relevant for Rigosi et al. [1].