No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Low-noise epitaxial graphene on SiC Hall effect element for commercial applications
Applied Physics Letters
Abstract
In this report, we demonstrate a complete Hall effect element that is based on quasi-free-standing monolayer
graphene synthesized on a semi-insulating on-axis Si-terminated 6H-SiC substrate in an epitaxial
Chemical Vapor Deposition process. The device offers the current-mode sensitivity of 87 V/AT and low excess noise (Hooge's parameter αH < 2 × 10−3) enabling room-temperature magnetic resolution of 650 nT/Hz0.5 at 10 Hz, 95 nT/Hz0.5 at 1 kHz, and 14 nT/Hz0.5 at 100 kHz at the total active area of 0.1275 mm2. The element is passivated with a silicone encapsulant to ensure its electrical stability and environmental resistance. Its processing cycle is suitable for large-scale commercial production and it is available in large quantities through a single growth run on an up to 4-in SiC wafer.
... Directly after the deposition process, graphene RT transport properties were monitored at = 1 mA with a 0.55-T Ecopia HMS-3000 direct-current Hall effect measurement system to verify whether the charge carrier type and sheet concentration was consistent with the theoretically postulated [41][42][43] 6 ≈ 7.5×10 12 cm -2 and 4 ≈ 1.2×10 13 cm -2 and whether the charge carrier mobility was comparable with the one expected for conventional processes on unmodified substrates [44][45][46]. ...
... Necessary structural parameters, i.e., the coordinates, were assumed based on the Crystallography Open Database data [53][54][55][56][57][58][59][60][61] with lattice constants from a book chapter by Goldberg et al. [62] The design of the test device, although determined by the HRPITS apparatus, was conceptually convergent with the van der Pauw Hall effect structures that had historically helped describe the thermal activation of the double-carrier transport in QFS graphene on SI 6H-SiC:V [11] and SI HP 4H-SiC [12]. In these works, the cross-shaped [46] QFS graphene mesas had also been removed by etching in oxygen plasma, leaving bare SI SiC(0001) substrate between the contacts. The conceptual convergence lets us believe that the conclusions of this section are to be directly implementable in the Hall effect sensor technology. ...
... The pre-epitaxial ion modification was implemented in the technology of 1.4-mm × 1.4-mm Hall effect sensors, manufactured in the number of 96 out of each SI SiC substrate in the 20-mm × 20-mm standard, as previously introduced in Ref. [63]. Through a series of optical lithography-based steps involving metal electron-beam deposition, oxygen plasma etching and atomic layer deposition, the substrates were turned into van der Pauw structures, each featuring a cross-shaped [46] 100-μm × 300-μm QFS graphene mesa, Ti/Au (10 nm/90 nm) ohmic contacts, and a 100-nm-thick a-Al 2 O 3 passivation synthesized from trimethylaluminum (TMA) and deionized water at 770 K in a Picosun R200 Advanced ALD reactor [64]. Individual sensors were mounted onto and bonded to in-house-made 6.6-mm × 6.6-mm sapphire holders equipped with four Ti/Au (10 nm/190 nm) corner contacts enabling electrical characterization at = 1 mA in a 0.556-T direct-current Ecopia AHT55T5 automated Hall effect measurement system between 300 K and 770 K. Schematic of the Hall effect sensor is depicted in Fig. 3, while its real-life implementation evoked in the results and discussion section. ...
High-temperature electrical properties of p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadium-compensated on-axis 6H-SiC(0001) and high-purity on-axis 4H-SiC(0001) originate from the double-carrier system of spontaneous-polarization-induced holes in graphene and thermally activated electrons in the substrate. In this study, we pre-epitaxially modify SiC by implanting hydrogen (H+) and helium (He+) ions with energies ranging from 20 keV to 50 keV to reconstruct its post-epitaxial defect structure and suppress the thermally developed electron channel. Through a combination of dark current measurements and High-Resolution Photo-Induced Transient Spectroscopy between 300 K and 700 K, we monitor the impact of ion bombardment on the transport properties of SiC and reveal activation energies of the individual deep-level defects. We find that the ion implantation has a negligible effect on 6H-SiC. Yet in 4H-SiC, it shifts the Fermi level from ∼600 meV to ∼800 meV below the minimum of the conduction band and reduces the electron concentration by two orders of magnitude. Specifically, it eliminates deep electron traps related to silicon vacancies in the charge state (2-/-) occupying the h and k sites of the 4H-SiC lattice. Finally, we directly implement the protocol of deep-level defect engineering in the technology of amorphous-aluminum-oxide-passivated Hall effect sensors and introduce a mature sensory platform with record-linear current-mode sensitivity of approximately 80 V/AT with -0.03-%/K stability in a broad temperature range between 300 K and 770 K, and likely far beyond 770 K.
https://www.sciencedirect.com/science/article/pii/S2667056923000585
... This issue is of particular importance in magnetic field detection as LF noise constitutes the main limitation of the sensor's magnetic resolution or the minimum detectable field. For this reason, many studies devoted to graphene-based Hall effect sensors concern their LF noise properties [27][28][29][30][31]. It occurs that even ultra-clean graphene sensors may have their LF performance deteriorated by 1/f -like and/or random telegraph noise. ...
... Quasi-free-standing (QFS) hydrogen-intercalated [32] graphene was grown on a semi-insulating high-purity nominally on-axis 15 mm × 15 mm 4H-SiC(0001) substrate cut from a 4-in wafer purchased at Cree Inc., in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition (CVD) method [33] and thermally decomposed propane as carbon source. The sample was processed into a number of 1.6 mm × 1.6 mm Hall effect sensors featuring a cross-shaped [30] 100 μm × 300 μm graphene mesa and four Ti/Au (10 nm/60 nm) ohmic contacts, all passivated with a 100-nm-thick aluminum oxide layer synthesized from trimethylaluminum (TMA) and deionized water at 400 • C in a Picosun R200 Advanced ALD reactor. It is only within the L-shaped contact areas that the dielectric passivation was etched with ammonium-fluoride-buffered hydrofluoric acid (HF) to provide access for the bonding wires. ...
... Their distinctive noise properties are already evidenced by the dimensionless Hooge parameter defined as = ∕ ( 2 ), where is the bias voltage, is the sheet charge carrier concentration and is the sensor's effective surface area, a commonly used figure of merit for the comparison of noise properties of various materials. In the four sensors, the average Hooge parameter proves ≈ 0.6, which is more than two orders of magnitude higher than the typical = 2 × 10 −3 reported for bulk uncontaminated semiconductors [38,39] and one to five orders of magnitude higher than measured in various graphene-based devices (5 × 10 −6 to 10 −2 ) [28,30,40,41]. ...
In this report, we introduce a novel method based on low-frequency noise analysis for the assessment of quality and pattern of inhomogeneity in intentionally-aged Hall effect sensors featuring hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene mesa on semi-insulating high-purity on-axis 4H-SiC(0001), all passivated with a 100-nm-thick atomic-layer-deposited Al2O3 layer. Inferring from the comparison of the measured noise and one calculated for a homogeneous sensor, we hypothesize about possible unintentional contamination of the sensors’ active regions. Following in-depth structural characterization based on Nomarski interference contrast optical imaging, confocal micro-Raman spectroscopy, high-resolution Transmission Electron Microscopy and Secondary Ion Mass Spectrometry, we find out that the graphene’s quasi-free-standing character and p-type conductance make the Al2O3/graphene interface exceptionally vulnerable to uncontrolled contamination and its unrestrained lateral migration throughout the entire graphene mesa, eventually leading to the blistering of Al2O3. Thus, we prove the method’s suitability for the detection of these contaminants’ presence and location, and infer on its applicability to the investigation of any contamination-induced inhomogeneity in two-dimensional systems. https://authors.elsevier.com/a/1eyP64xMlkIhhc
... These works include graphene grown on copper foil and transferred onto a SiO 2 /n-Si substrate [4], monolayer graphene epitaxially grown on SiC [5], graphene grown on platinum and transferred onto a SiO 2 /Si substrate [6], one-two layer epitaxial sublimated graphene grown on 4H-SiC(0001) [7], and mechanically exfoliated graphene encapsulated in hexagonal boron nitride (hBN) [8]. In the previous paper we have shown a complete Hall effect sensor [9] made from quasi-free-standing (QFS) monolayer graphene grown on semi-insulating (SI) vanadium-compensated 6H-SiC(0001) substrate in an epitaxial Chemical Vapor Deposition (CVD) process. That sensor was passivated with a silicone encapsulant to ensure its electrical stability and environmental resistance and its current-mode sensitivity of 87 V/AT was defined at room temperature. ...
... In Ref. [9] we described the application of a silicone as an encapsulant for graphene and verified its suitability for the Hall effect sensor operation at room temperature. In this report, we introduce an aluminum oxide passivation realized in a hightemperature atomic layer deposition process in the 4-in Picosun R200 Advanced reactor. ...
... At room temperature (300 K) the sensor passivated with the 100-nm-thick aluminum oxide layer offers S I~1 36 V/AT, which is higher than previously reported for the silicone-encapsulated Hall effect sensor [9]. At 80 K the sensor is characterized by S I~1 41 V/AT. ...
... It was grown at 1873 K in an Aixtron VP508 reactor on a 20 × 20 mm sample diced from a 4-in, 500-µm-thick, SI, vanadium-compensated, nominally on-axis 6H-SiC(0001) wafer (II-VI Inc.), using thermally decomposed propane [26], and processed into a standard batch of 96 devices [27]. The individual device was a 1.4 × 1.4 mm four-terminal van der Pauw structure [4] featuring an equal-arm, cross-shaped [28] 100 × 300 µm QFS graphene mesa (50 000 µm 2 ), Ti/Au (10/110 nm) ohmic contacts, and a 100-nm-thick, ALD, amorphous Al 2 O 3 passivation [29], [30] synthesized from trimethylaluminum and deionized water at 770 K in a Picosun R200 Advanced ALD reactor. It was mounted and bonded to in-house-made 6.6 × 6.6 mm sapphire holders equipped with four Ti/Au (10/190 nm) corner contacts enabling direct-current electrical characterization at I = 1 mA in a 0.556-T Ecopia AHT55T5 automated Hall effect measurement system between 300 and 770 K (on par with the upper limit of the projected DEMO-class reactor requirements). ...
... The RTP process breaks the homogeneous character of the sensor resistance. Since the entire experiment was framed in a timely manner, we hypothesize that the inhomogeneity be attributed to a local detachment of the dielectric encapsulation from the QFS graphene and, in consequence, the decline of the double polarization effect [28] rather than to contamination or environmental factors. The character of R(T ) as a function of RTP temperature differs for certain pairs of contacts. ...
We studied the effect of abnormal thermal stress on a graphene-on-SiC Hall effect sensor dedicated to elevated temperatures. After subsequent thermal stresses at 823, 873, and 923 K provided by rapid thermal processing (RTP), we monitored the transport parameters of a sensor (sheet resistance, mobility, and carrier concentration) and its low-frequency noise (LFN). We showed that RTP increases the average carrier concentration and widens its distribution across the device, as confirmed by Raman spectroscopy. We observed that the LFN magnitude significantly increases after subsequent stresses, much more than the average resistance, which decreases. The evaluation of thermal stress in electronic devices should include the noise-based method because it seems to be a much more sensitive indicator of thermal degradation than typically monitored electronic transport parameters. We showed that the RTP promotes nonhomogeneity within the sensor, the presence of which is directly exposed by the LFN measurements. Moderate thermal degradation (noise and resistances) up to 873 K suggests that the graphene-on-SiC Hall platform is promising for magnetic field detection at elevated temperatures. Furthermore, the methodology of 1/ f noise analysis is universally applicable whenever the noise model of the active layer can be represented by a resistance network.
... [30]. Plots are from the references shown in the bottom of the figure [9,30,41,42,48,49,[69][70][71][72][73][74][75][76]. Black, purple, and red lines are theoretical mobility limits [39,44]. ...
... As for interface modification, the most typical method is hydrogen intercalation at the interface. After growing a uniform buffer layer, hydrogen can be intercalated by heating in a hydrogen atmosphere, and then the buffer layer turns into graphene, the data for which are shown by red and orange triangles in Figure 5 [64,65,73,74,79]. As a result, a maximum mobility of 11,300 cm 2 /Vs at 7 × 10 11 cm −2 has been reported, as shown by green bars above [75]. ...
Graphene growth by thermal decomposition of silicon carbide (SiC) is a technique that produces wafer-scale, single-orientation graphene on an insulating substrate. It is often referred to as epigraphene, and has been thought to be suitable for electronics applications. In particular, high-frequency devices for communication technology or large quantum Hall plateau for metrology applications using epigraphene are expected, which require high carrier mobility. However, the carrier mobility of as-grown epigraphene exhibit the relatively low values of about 1000 cm2/Vs. Fortunately, we can hope to improve this situation by controlling the electronic state of epigraphene by modifying the surface and interface structures. In this paper, the mobility of epigraphene and the factors that govern it will be described, followed by a discussion of attempts that have been made to improve mobility in this field. These understandings are of great importance for next-generation high-speed electronics using graphene.
... There are several substrates for the growth of graphene [27,[36][37][38][39][40][41]. However, considering the requirements and the ease of sensor fabrication, we believe that the best candidate is the Si-face of semiinsulating on-axis silicon carbide (SiC). ...
... The substrate was processed into a batch of 25 1.6 mm × 1.6 mm Hall effect sensors [13,41] featuring a cross-shaped [39] 100 μm × 300 μm graphene mesa and four Ti/Au (10 nm/60 nm) ohmic contacts, all passivated with a 100-nm-thick aluminum oxide (Al 2 O 3 ) layer synthesized from trimethylaluminum (TMA) and deionized water at 670 K in a Picosun R200 Advanced ALD reactor [49]. The type of dielectric layer was selected for its noninvasive character, lack of energetic or reactive species, sub-nanometer precision, and layer uniformity. ...
In this paper, we report on the first experimental study on the impact of neutron radiation on quasi-free-standing (QFS) graphene. For this purpose, we have fabricated hydrogen-intercalated QFS graphene on semiinsulating high-purity 4H-SiC(0001), passivated it with an Al2O3 layer,and exposed it to a fast-neutron fluence of ≈6.6×1017 cm⁻². The results have shown that the graphene sheet is only moderately affected by the neutron radiation with the estimated defect density of ≈4×1010 cm⁻². The low structural damage allowed the Al2O3/graphene/SiC system to maintain its electrical properties and an excellent sensitivity to magnetic fields characteristic of QFS graphene. Consequently, our findings suggest that the system may be a promising platform for magnetic diagnostics in magnetic-confinement fusion reactors. However, the scope of its use should be a subject of further study. In this context, we have explored possible modes of damage and have concluded that the main factor that affects the electrical parameters of the structure is the impact of neutrons on the layer of hydrogen atoms saturating the SiC(0001) surface. We have shown, employing density functional theory (DFT) computations, that damage to the intercalating layer could lower hole concentration in graphene via reduced charge polarization and local coupling on the interface.
... The test elements were manufactured using graphene-on-SiC technology [32] under the geometry and type available with the GET ® platform [44]. Each of the structures was a 1.4 mm × 1.4 mm fourterminal van der Pauw device [34] featuring an oxygen-plasma-etched, equal-arm, cross-shaped 100-μm × 300-μm QFS graphene mesa [45], electron-beam-deposited Ti/Au (10 nm/110 nm) current feed and voltage readout contacts, and a 100-nm-thick, atomic-layer-deposited, amorphous, non-stoichiometric, oxygen-deficient [46] Al 2 O 3 encapsulation [47,48]. The graphene was transfer-free, in-situ fully [49] hydrogen-intercalated [50,51] at 1273 K (therefore quasi-free-standing and p-type), and epitaxial Chemical Vapor Deposition [30,31]. ...
This article reveals a unique self-healing ability of the amorphous-aluminum-oxide-passivated p-type hydrogenintercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating vanadiumcompensated nominally on-axis 6H-SiC(0001) system, exposed for 166 h to a destructive flux of 3.3 ×
E11 cm−2s−1 of mostly fast-neutrons (1–2 MeV), resulting in an accumulated fluence of 2.0 × E17 cm−2. Postirradiation room-temperature Hall effect characterization proves that the a-Al2O3/QFS-graphene/6H-SiC(0001) is n-type, which implies the loss of the quasi-free-standing character of graphene and likely damage to the SiC(0001)-saturating hydrogen layer. Micro-Raman spectroscopy suggests an average defect density in graphene of 𝑛𝐷 = 3.1 × 1010 cm−2 with an 𝐿𝐷 = 32-nm inter-defect distance. Yet, a thermal treatment up to 623 K eliminates defect-related Raman peaks and restores the original p-type conductance. At the same time, 623 K is not enough to recover the initial transport properties in a sample irradiated for 245 h with a total fluence of 2.0 × E18 cm−2. A Density Functional Theory model explains the self-healing phenomenon and restoration of the quasi-free-standing properties through thermally-activated lateral diffusion of the remaining population of hydrogen atoms and re-decoupling of the graphene sheet from the SiC(0001) surface. The thermal
regime of 623 K fits perfectly into the operational limits of the a-Al2O3/QFS-graphene/6H-SiC(0001) system, defined as 300 K to 770 K. The finding constitutes a milestone for two-dimensional, graphene-based diagnostic and control systems designed for operation in extreme environments
... Graphene Hall elements generally feature high sensitivity (Fig. 3a, b) 120,123,124 . The combination of high sensitivity and low noise of graphene devices results in the higher resolution of magnetic field ( Fig. 3c) 125,126 . In addition to silicon-based substrates, graphene Hall elements can also be fabricated on flexible substrates (Fig. 3d) 127 . ...
Graphene is being increasingly used as an interesting transducer membrane in micro- and nanoelectromechanical systems (MEMS and NEMS, respectively) due to its atomical thickness, extremely high carrier mobility, high mechanical strength and piezoresistive electromechanical transductions. NEMS devices based on graphene feature increased sensitivity, reduced size, and new functionalities. In this review, we discuss the merits of graphene as a functional material for MEMS and NEMS, the related properties of graphene, the transduction mechanisms of graphene MEMS and NEMS, typical transfer methods for integrating graphene with MEMS substrates, methods for fabricating suspended graphene, and graphene patterning and electrical contact. Consequently, we provide an overview of devices based on suspended and nonsuspended graphene structures. Finally, we discuss the potential and challenges of applications of graphene in MEMS and NEMS. Owing to its unique features, graphene is a promising material for emerging MEMS, NEMS and sensor applications.
... Then, through a series of optical lithography-based steps involving metal deposition and oxygen plasma etching, the surface of the sample was processed into 96 1.4-mm × 1.4-mm four-terminal van der Pauw elements [23], each featuring an oxygen-plasma-etched, crossshaped [28] 100-μm×300−μm (50 000 μm 2 ) QFS graphene mesa against plasma-exposed 4H-SiC(0001) and electron-beam-deposited Ti/Au (10 (a) Nomarski interference contrast optical image of the four-terminal element featuring a cross-shaped 100-μm × 300 − μm hydrogen-intercalated QFS epitaxial CVD graphene mesa against semi-insulating high-purity on-axis 4H-SiC(0001). For clarity, a black dashed line highlights the graphene mesa. ...
Kelvin Probe Force Microscopy is a method to assess the contact potential difference between a sample and the probe tip. It remains a relative tool unless a reference standard with a known work function is applied, typically bulk gold or cleaved highly oriented pyrolytic graphite. In this report, we suggest a verifiable, two-dimensional standard in the form of a photolithographically patterned, wire-bonded structure manufactured in the technology of transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity nominally on-axis 4H-SiC(0001). The particular structure has its hole density 𝑝𝑆 = 1.61 × 1013 cm−2 measured through a classical Hall effect, its number of the graphene
layers 𝑁 = 1.74 extracted from the distribution of the ellipsometric angle 𝛹, measured at the angle of incidence AOI = 50◦ and the wavelength 𝜆 = 490 nm, and its work function 𝜙𝐺𝑅 = 4.79 eV postulated by a Density Functional Theory model for the specific 𝑝𝑆 and 𝑁. Following the algorithm, the contact potential difference
between the structure and a silicon tip, verified at 𝛥𝑉𝐺𝑅−Si = 0.64 V, ought to be associated with 𝜙𝐺𝑅 = 4.79 eV and applied as a precise reference value to calculate the work function of an arbitrary material.
... The sample was processed into a batch of 96 van der Pauw structures [18]. Each structure was a 1.4-mm × 1.4-mm four-terminal device [12] featuring an oxygen-plasma-etched, equal-arm, cross-shaped [21] 100-µm × 300-µm graphene mesa, Ti/Au (10 nm/110 nm) current feed and voltage readout contacts, and a 100-nm-thick, atomic-layer-deposited, amorphous, non-stoichiometric [22] Al 2 O 3 passivation [23] synthesized from trimethylaluminum and deionized water at 770 K in the Picosun R-200 Advanced reactor. The choice of this specific geometry, rather than of an optimized Hall bar, was justified by the authors' will to elucidate the transport properties of the 5-keV H + -modified a-Al 2 O 3 /QFS-graphene/4H-SiC(0001) platform. ...
In this letter, we demonstrate a Hall effect sensor in the technology of amorphous-Al2O3-passivated transfer-free
p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semi-insulating high-purity on-axis 4H-SiC(0001), pre-epitaxially modified with 5-
keV hydrogen (H+) ions. The sensor operates between 305 K and 770 K, with a current-mode sensitivity of ∼75 V/AT and thermal stability below 0.15 %/K (⩽ 0.03 %/K in a narrower range between 305 K and 700 K). It is a promising two-dimensional platform for high-temperature magnetic diagnostics and plasma control systems for modern tokamak fusion reactors.
... Next, through a series of optical lithography-based steps involving metal deposition and oxygen plasma etching, the surface of the sample was processed into 63 van der Pauw structures, each featuring a cross-shaped [25] 100-μm × 300-μm QFS graphene mesa against bare 6H-SiC(0001). In this particular experiment, the metallic pads played no electrical role but helped localize the graphene crosses within the sample. ...
In this report, we present transfer-free p-type hydrogen-intercalated quasi-free-standing epitaxial Chemical
Vapor Deposition graphene on 15-mm × 15-mm semi-insulating vanadium-compensated on-axis 6H–SiC(0001),
characterized in that its room-temperature direct-current Hall-effect-derived hole mobility 𝜇p = 5019 cm2/Vs,
and its statistical number of layers (N), as indicated by the relative intensity of the SiC-related Raman-active
longitudinal optical A1 mode at 964 cm−1, equals N = 1.05. The distribution of the ellipsometric angle 𝛹
measured at an angle of incidence of 50◦ and 𝜆 = 490 nm points out to N = 0.97. The close-to-unity value of
N implies that the material under study is a close-to-perfect quasi-free-standing monolayer, which is further
confirmed by High-Resolution Transmission Electron Microscopy. Therefore, its spectroscopic properties, which
include the Si–H peak at 2131 cm−1, the histograms of 𝛹 and 𝛥, and the Raman G and 2D band positions,
widths, and the 2D-to-G band intensity ratios, constitute a valuable reference for this class of materials.
... QFS graphene [26][27][28][29] necessary for the graphene-based HTHS was grown on semiinsulating high-purity on-axis 4H-SiC(0001) (Cree Inc.) in a hot-wall Aixtron VP508 reactor using the epitaxial Chemical Vapor Deposition method [30], with thermally decomposed propane as a carbon source and in situ hydrogen atom intercalation [31]. The substrate was processed into a 1.6 mm × 1.6 mm Hall effect structure featuring a cross-shaped [32] 100 µm × 300 µm graphene mesa and four Ti/Au (10 nm / 60 nm) ohmic contacts, all passivated with a 100 nm-thick aluminum oxide (Al 2 O 3 ) layer synthesized from trimethylaluminum (TMA) and deionized water at 670 K in a Picosun R200 Advanced Atomic Layer Deposition (ALD) reactor [33,34]. Detailed information on the fabrication processes is included in Refs. ...
The ability to precisely measure magnetic fields under extreme operating conditions is
becoming increasingly important as a result of the advent of modern diagnostics for future magnetic confinement fusion devices. These conditions are recognized as strong neutron radiation and high temperatures (up to 350 °C). We report on the first experimental comparison of the impact of neutron radiation on graphene and indium antimonide thin films. For this purpose, a 2D-material-based structure was fabricated in the form of hydrogen-intercalated quasi-free-standing graphene on semi-insulating high-purity on-axis 4H-SiC(0001), passivated with an Al2O3 layer. InSb-based thin films, donor doped to varying degrees, were deposited on a monocrystalline gallium arsenide or a polycrystalline ceramic substrate. The thin films were covered with a SiO2 insulating layer. All samples were exposed to a fast-neutron fluence of 7 x 10^17 cm2. The results have shown that the graphene sheet is only moderately affected by neutron radiation compared to the InSb-based structures. The low structural damage allowed the graphene/SiC system to retain its electrical properties and excellent sensitivity to magnetic fields. However, InSb-based structures proved to have significantly more post-irradiation self healing capabilities when subject to proper temperature treatment. This property has been tested depending on the doping level and type of the substrate.
... The choice of the sensor's structure was grounded on three premises. First, it provides an oxygen-plasmaetched cross-shaped [21] graphene mesa that clearly distinguishes the graphene-covered region from bare SiC(0001) substrate, both areas being crucial for the method. Second, the dielectric passivation preserves graphene's electrical and chemical properties throughout the experiment, otherwise affected by ambient atmosphere. ...
In this report we demonstrate a method for direct determination of the number of layers of hydrogen-intercalated quasi-free-standing epitaxial Chemical Vapor Deposition graphene on semiinsulating vanadium-compensated on-axis 6H-SiC(0001). The method anticipates that the intensity of the substrate’s Raman-active longitudinal optical A1 mode at 964 cm^(−1) is attenuated by 2.3 % each time the light passes through a single graphene layer. Normalized to its value in a graphene-free region, the A1 mode relative intensity provides a greatly enhanced topographic image of graphene and points out to the number of its layers within the terraces and step edges, making the technique a reliable diagnostic tool for applied research.
Share Link: https://authors.elsevier.com/c/1dMSw4xMlkEtr3
... Graphene was epitaxially grown through the chemical vapor deposition (CVD) from hydrocarbons on the silicon face of a semi-insulating on-axis 6H-SiC substrate [38,40] (see also [41][42][43]). First an electrically inactive buffer layer was synthesized and only later in situ turned into a QFS monolayer through hydrogen atom intercalation [41] (see also [44][45][46]). ...
The structure of magnetoresistance curves as a function of magnetic field from 0 to 14 T at temperatures from 0.4 to 6.0 K for macroscopic samples of the quasi-free-standing (QFS) graphene monolayer on SiC substrate, are observed and analyzed, and also the spatial and depth frequency distribution of phonons have been measured using the micro-Raman spectroscopy (MRS). That one enables us to interpret the obtained resonance magnetoresistance curves based on the electron-phonon (e-p) interaction taking into account the actually observed phonon spectrum in researched samples: in the case of a linear e-p interaction the observation of the corresponding peaks on the Rxx(B) curves is difficult because an uninterrupted background is created. While nonlinear MPR with simultaneous G-phonon emission and D-phonon absorption occur in magnetic fields below 5 T against the background of MPR due to linear e-p interaction as well as Shubnikov–de Haas oscillations.
... At room temperature (300 K), the 23 elements' arithmetical mean current-mode sensitivity is S Imean ≈ 83 V/AT with the standard deviation σ ≈ 3.8 V/AT. It is lower than in the above-cited publications on graphene grown on copper and transferred onto SiO 2 /Si (310 V/AT [2], 2540 V/AT [3]), graphene grown on platinum and transferred onto SiO 2 /Si (800 V/AT [4], 1200 V/AT [5]), exfoliated or copper grown graphene transferred onto hexagonal boron nitride (5700 V/AT [6], 1986 V/AT [7]), epitaxial graphene on SiC (310 V/AT [8], 1000 V/AT [9]) yet higher than in the permalloy (30 V/AT [1]) and consistent with other reports on epitaxial CVD graphene on SiC (83 V/AT [11], 50 V/AT [12]). At 573 K, the elements offer S Imean ≈ 79 V/AT with σ ≈ 2.9 V/AT and these values are greatly stable up to 770 K, where S Imean ≈ 67 V/AT and σ ≈ 4.5 V/AT (Fig. 2). ...
In this report, we demonstrate a novel high-temperature Hall effect sensor that is based on quasi-free-standing monolayer graphene epitaxially grown on high-purity semiinsulating (SI) on-axis 4H-SiC(0001) substrate in a chemical vapor deposition process. To ensure statistical perspective, characteristics of 23 elements are determined as a function of temperature ranging from 300 to 770 K. Passivated with a 100-nm-thick atomic-layer-deposited aluminum oxide, the sensor offers current-mode sensitivity of 80 V/AT with thermal stability of -0.02%/K within the range between 300 and 573 K, and -0.06%/K between 573 and 770 K. The sensor's room-temperature output voltage is monitored in the magnetic field from -300 to +300 mT and its offset voltage at 0 T is assessed. Its high-temperature electrical properties are explained through a double-carrier transport involving spontaneous-polarization-induced holes in the graphene layer and thermally activated electrons emitted from a deep acceptor level related to silicon vacancy VSi^1-/2- occupying the k site of the 4H-SiC lattice. The sensor is compared with a previously reported one on vanadium-compensated SI on-axis 6H-SiC(0001). The new sensor's applicability to magnetic field detection at high temperatures is verified.
... The significant doping of graphene in the all-CVD heterostructure devices is another major reason behind the reduced Hall sensitivities and it could be due to contaminations introduced during wet-transfer process of large area graphene and h-BN layers. This could explain also the significantly lower minimum magnetic field resolution of Hall sensors based on all-CVD h-BN/graphene/h-BN heterostructures compared to previous reports on graphene Hall sensors 17,[19][20][21][22] . These are currently the practical challenges for the development of graphene and 2D materials science and technology. ...
The two-dimensional (2D) material graphene is highly promising for Hall sensors due to its potential of having high charge carrier mobility and low carrier concentration at room temperature. Here, we report the scalable batch-fabrication of magnetic Hall sensors on graphene encapsulated in hexagonal boron nitride (h-BN) using commercially available large area CVD grown materials. The all-CVD grown h-BN/graphene/h-BN van der Waals heterostructures were prepared by layer transfer technique and Hall sensors were batch-fabricated with 1D edge metal contacts. The current-related Hall sensitivities up to 97 V/AT are measured at room temperature. The Hall sensors showed robust performance over the wafer scale with stable characteristics over six months in ambient environment. This work opens avenues for further development of growth and fabrication technologies of all-CVD 2D material heterostructures and allows further improvements in Hall sensor performance for practical applications.
... Graphene is a highly promising material for Hall sensing applications due to its beneficial properties such as low charge carrier concentration and high mobility. Recent experimental reports [12][13][14][15][16]24,25 have already proven the feasibility of graphene Hall sensors with current-related sensitivity more than 60 times higher than in silicon-based sensors. 13 Furthermore, graphene's 2D nature allows its application in flexible and transparent electronics. ...
Graphene is an excellent material for Hall sensors due to its atomically thin structure, high carrier mobility, and low carrier density. However, graphene devices need to be protected from the environment for reliable and durable performance in different environmental conditions. Here we present magnetic Hall sensors fabricated on large area commercially available chemical vapor deposited (CVD) graphene protected by exfoliated hexagonal boron nitride (h-BN). To connect the graphene active regions of Hall samples to the outputs, 1D edge contacts were utilized which show reliable and stable electrical properties. The operation of the Hall sensors shows the current-related sensitivity up to 345 V/(AT). By changing the carrier concentration and type in graphene by the application of gate voltage, we are able to tune the Hall sensitivity.
... Excitation wavelength of 488 nm. [42,47,48]. ...
In this paper we present the CMOS-compatible procedure for graphene-based planar electronic devices (i.e.
magnetic field sensor) fabrication in the hundreds of micrometer scale by using of: combined photolithography,
Ar+ ion sputtering and DC magnetron metal evaporation processes. In our approach, the conductive channels
based on the single graphene layer on the SiC substrate have been fabricated by Ar+ ion sputtering, instead of
reactive - ion etching techniques. The proposed solution prevents against semiconductor surface oxidation and
therefore is much more compatible with the standard CMOS technology. The processed graphene channels and
devices demonstrate similar properties as fabricated by means of the early used procedures. That underline
usefulness of the presented approach in many fields of applications, i.e.: the devices which contain graphene
channels on standard reactive, semiconductor surfaces.
... Many successes in this field were reported, showing impressive results concerning mostly hall sensors and geometrical magnetoresistors based on the EMR effect. 11,12,[14][15][16][17] However, until the time when the EMR effect was discovered, the common form of the geometrical magnetoresistor was a simple, twocontact device, consisting of rectangular modules of a highmobility semiconductor combined together and equipped with two metallic electrodes. 18 This kind of geometry is shown schematically in Fig. 1(a) and is well-known as the raster pattern geometry (RPG). ...
The idea of a raster pattern magnetoresistor made of thin films of III–V compounds and a metallic
layer has been known for over fifty years. Based on this knowledge, we present the construction of
a magnetoresistor made of combined graphene and metallic strip patterns. The presented device is
implemented using a monolayer of graphene epitaxially grown on a semi-insulating substrate. A
graphene strip pattern magnetoresistor gives a promising wide range of practical applications due
to its very high sensitivity combined with the mono-atomic thickness of the sensitive layer, the sim-
plicity of realization, and a very convenient principle of sensor operation assuming only the usage
of two electrical terminals.
The two-dimensional (2D) material graphene is highly promising for Hall sensors due to its potential of having high charge carrier mobility and low carrier concentration at room temperature. Here, we report the scalable batch-fabrication of magnetic Hall sensors on graphene encapsulated in hexagonal boron nitride (h-BN) using commercially available large area CVD grown materials. The all-CVD grown h-BN/graphene/h-BN van der Waals heterostructures were prepared by layer transfer technique and Hall sensors were batch-fabricated with 1D edge metal contacts. The current-related Hall sensitivities up to 97 V/AT are measured at room temperature. The Hall sensors showed robust performance over the wafer scale with stable characteristics over six months in ambient environment. This work opens avenues for further development of growth and fabrication technologies of all-CVD 2D material heterostructures and allows further improvements in Hall sensor performance for practical applications.
Graphene is being increasingly used as an interesting transducer membrane in micro- and nanoelectromechanical systems (MEMS and NEMS, respectively) due to its atomical thickness, extremely high carrier mobility, high mechanical strength, and piezoresistive electromechanical transductions. NEMS devices based on graphene feature increased sensitivity, reduced size, and new functionalities. In this review, we discuss the merits of graphene as a functional material for MEMS and NEMS, the related properties of graphene, the transduction mechanisms of graphene MEMS and NEMS, typical transfer methods for integrating graphene with MEMS substrates, methods for fabricating suspended graphene, and graphene patterning and electrical contact. Consequently, we provide an overview of devices based on suspended and nonsuspended graphene structures. Finally, we discuss the potential and challenges of applications of graphene in MEMS and NEMS. Owing to its unique features, graphene is a promising material for emerging MEMS, NEMS, and sensor applications.
There is a constant need for Hall-effect magnetic sensors with lower noise and lower offset for the high accuracy analog applications driven primarily by industrial and automotive markets, for example, high dynamic range current sensing for battery management in electric vehicles. Graphene-based Hall sensors (GHSs) demonstrate low thermal noise due to their high carrier mobility but suffer from large low-frequency flicker noise and offset limiting the prevalent low-frequency applications. In this paper, we present the results of a recently proposed gate modulation scheme for GHSs that uses dynamic ambipolar operation to up-convert the input magnetic signal to higher modulating frequencies while the offset and flicker noise remain spectrally unchanged and thus enabling thermal noise limited performance. We implement a prototype GHS using a scalable fabrication process with CVD graphene transferred on to silicon-oxide and followed by standard thin film and subsequent photolithographic processes. The measurement results confirm the effectiveness of dynamic ambipolar gate modulation technique in producing low offset of
in conjunction with three orders of magnitude estimated improvement in the low frequency noise performance.
Hall sensors have become one of the most used magnetic sensors in recent decades, performing the vital function of providing a magnetic sense that is naturally absent in humans. Various electronic applications have evolved from circuit-integrated Hall sensors due to their low cost, simple linear magnetic field response, ability to operate in a large magnetic field range, high magnetic sensitivity and low electronic noise, in addition to many other advantages. Recent developments in the fabrication and performance of graphene Hall devices promise to open up the realm of Hall sensor applications by not only widening the horizon of current uses through performance improvements, but also driving Hall sensor electronics into entirely new areas. In this review paper we describe the evolution from the traditional selection of Hall device materials to graphene Hall devices, and explore the various applications enabled by them. This includes a summary of the selection of materials and architectures for contemporary micro- to nanoscale Hall sensors. We then turn our attention to introducing graphene and its remarkable physical properties and explore how this impacts the magnetic sensitivity and electronic noise of graphene-based Hall sensors. We summarise the current state-of-the art of research into graphene Hall probes, demonstrating their record-breaking performance. Building on this, we explore the various new application areas graphene Hall sensors are pioneering such as magnetic imaging and non-destructive testing. Finally, we look at recent encouraging results showing that graphene Hall sensors have plenty of room to improve, before then discussing future prospects for industry-level scalable fabrication.
We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a ‘hands-on’ approach, providing practical details and procedures as derived from literature as well as from the authors’ experience, in order to enable the reader to reproduce the results.
Section I is devoted to ‘bottom up’ approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour.
Section II covers ‘top down’ techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers’ and modified Hummers’ methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach.
The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step.
Sections IV–VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning.
Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V.
Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resource-consuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer.
There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.
Section VIII discusses advances in GRM functionalization. A broad range of organic molecules can be anchored to the sp² basal plane by reductive functionalization. Negatively charged graphene can be prepared in liquid phase (e.g. via intercalation chemistry or electrochemically) and can react with electrophiles. This can be achieved both in dispersion or on substrate. The functional groups of GO can be further derivatized. Graphene can also be noncovalently functionalized, in particular with polycyclic aromatic hydrocarbons that assemble on the sp² carbon network by π–π stacking. In the liquid phase, this can enhance the colloidal stability of SLG/FLG. Approaches to achieve noncovalent on-substrate functionalization are also discussed, which can chemically dope graphene. Research efforts to derivatize CNMs are also summarized, as well as novel routes to selectively address defect sites. In dispersion, edges are the most dominant defects and can be covalently modified. This enhances colloidal stability without modifying the graphene basal plane. Basal plane point defects can also be modified, passivated and healed in ultra-high vacuum. The decoration of graphene with metal nanoparticles (NPs) has also received considerable attention, as it allows to exploit synergistic effects between NPs and graphene. Decoration can be either achieved chemically or in the gas phase. All LMs, can be functionalized and we summarize emerging approaches to covalently and noncovalently functionalize MoS2 both in the liquid and on substrate.
Section IX describes some of the most popular characterization techniques, ranging from optical detection to the measurement of the electronic structure. Microscopies play an important role, although macroscopic techniques are also used for the measurement of the properties of these materials and their devices. Raman spectroscopy is paramount for GRMs, while PL is more adequate for non-graphene LMs (see section IX.2). Liquid based methods result in flakes with different thicknesses and dimensions. The qualification of size and thickness can be achieved using imaging techniques, like scanning probe microscopy (SPM) or transmission electron microscopy (TEM) or spectroscopic techniques. Optical microscopy enables the detection of flakes on suitable surfaces as well as the measurement of optical properties. Characterization of exfoliated materials is essential to improve the GRM metrology for applications and quality control. For grown GRMs, SPM can be used to probe morphological properties, as well as to study growth mechanisms and quality of transfer. More generally, SPM combined with smart measurement protocols in various modes allows one to get obtain information on mechanical properties, surface potential, work functions, electrical properties, or effectiveness of functionalization. Some of the techniques described are suitable for ‘in situ’ characterization, and can be hosted within the growth chambers. If the diagnosis is made ‘ex situ’, consideration should be given to the preparation of the samples to avoid contamination. Occasionally cleaning methods have to be used prior to measurement.
In this paper, we show that the very large offset voltage observed in Hall sensors exploiting the epitaxial graphene on an SiC substrate can be reduced quite effectively with the help of the double Hall sensor structure (DHSS). A record offset reduction by four orders of magnitude to the DC microvolt level is achieved. The strongly reduced offset is thermally stable, provided that the single Hall sensors of the DHSS have equal temperature coefficients of resistance and the heating/cooling procedure is performed under isothermal conditions to avoid the generation of thermoelectric voltages that add to the reduced offset.
Regarding the improvement of current quantized Hall resistance (QHR) standards, one promising avenue is the growth of homogeneous monolayer epitaxial graphene (EG). A clean and simple process is used to produce large, precise areas of EG. Properties like the surface conductivity and dielectric loss tangent remain unstable when EG is exposed to air due to doping from molecular adsorption. Experimental results are reported on the extraction of the surface conductivity and dielectric loss tangent from data taken with a noncontact resonance microwave cavity, assembled with an air-filled, standard R100 rectangular waveguide configuration. By using amorphous boron nitride (a-BN) as an encapsulation layer, stability of EG's electrical properties under ambient laboratory conditions is greatly improved. Moreover, samples are exposed to a variety of environmental and chemical conditions. Both thicknesses of a-BN encapsulation are sufficient to preserve surface conductivity and dielectric loss tangent to within 10% of its previously measured value, a result which has essential importance in the mass production of millimeter-scale graphene devices demonstrating electrical stability.
The effects of humidity on the electronic properties of quasi-free standing one layer graphene (QFS 1LG) are investigated via simultaneous magneto-transport in the van der Pauw geometry and local work function measurements in a controlled environment. QFS 1LG on 4H-SiC(0001) is obtained by hydrogen intercalation of the interfacial layer. In this system, the carrier concentration experiences a two-fold increase in sensitivity to changes in relative humidity as compared to the as-grown epitaxial graphene. This enhanced sensitivity to water is attributed to the lowering of the hydrophobicity of QFS 1LG, which results from spontaneous polarization of 4H-SiC(0001) strongly influencing the graphene. Moreover, the superior carrier mobility of the QFS 1LG system is retained even at the highest humidity. The work function maps constructed from Kelvin probe force microscopy also revealed higher sensitivity to water for 1LG compared to 2LG in both QFS 1LG and as-grown systems. These results point to a new field of applications for QFS 1LG, i.e., as humidity sensors, and the corresponding need for metrology in calibration of graphene-based sensors and devices.
The encapsulation of graphene in hexagonal boron nitride provides graphene on
substrate with excellent material quality. Here, we present the fabrication and
characterization of Hall sensor elements based on graphene boron nitride
heterostructures, where we gain from high mobility and low charge charier
density at room temperature. We show a detailed device characterization
including Hall effect measurements under vacuum and ambient conditions. We
achieve a current- and voltage-related sensitivity of up to 5700 V/AT and 3
V/VT, respectively, outpacing state-of- the-art silicon and III/V Hall sensor
devices. Finally, we extract a magnetic resolution limited by low frequency
electric noise of less than 50 nT/pHz making our graphene sensors highly
interesting for industrial applications.
The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by (i) the spontaneous polarization of the substrate, and (ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC (Ristein et al 2012 Phys. Rev. Lett. 108 [http://dx.doi.org/10.1103/PhysRevLett.108.246104] 246104 ).
The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps ∼10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be ∼190 Ωμm for step height hS = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications.
We have performed sensitivity mapping of graphene Hall devices with the width of 0.6–15 micron
operating in the diffusive regime under non-uniform, local magnetic and electric fields induced by a
scanning metallic magnetic probe. The transverse voltage was recorded, while tuning the magnitude
and orientation of the bias current, the probe-sample distance, and orientation of the probe
magnetization. A strong two-fold symmetry pattern has been observed, as a consequence of
capacitive coupling between the probe and the sample. The effect is particularly pronounced in small
devices (<1 micron), where the dominating electric field contribution significantly lowers the effective
area of the magnetic sensor. We show that implementation of the Kelvin probe feedback loop in the
standard scanning gate microscopy setup drastically reduces parasitic electric field effects and
improves magnetic sensitivity.
Hall elements fabricated on chemical vapor deposited graphene exhibited high current- and voltage-related sensitivities due to its low intrinsic carrier density and high mobility about 5000 cm2/V s. Electric noise of the Hall elements was measured at room temperature and found to be largely Flicker noise at low frequency which can be well described by Hooge's empirical relation with a low noise parameter of about 1.8 × 10-4. The combination of high sensitivity and low noise in graphene Hall elements leads to a high room temperature magnetic resolution of about 5 × 10-3 G/Hz0.5 at 3 kHz.
We investigate the magnetotransport properties of epitaxial graphene on
SiC(0001), which exhibits a stepped surface with regular terraces and
step edges. We observe an anisotropic behavior in the magnetoresistance
measured at high magnetic fields for narrow Hall bars patterned on
stepped surfaces. If the Hall bar is aligned parallel to the terraces,
the conventional quantum Hall effect behavior is observed. In contrast,
a predominantly positive magnetoresistance arises if the current crosses
many surface steps. The results can be explained by a model which takes
into account inter-Landau-level scattering, which is enhanced in the
step edge surface region and may enable electron backscattering from one
channel to another on the opposite side, resulting in a positive
magnetoresistance.
Hall sensors with the width range from 0.5 to 20.0 μm have been fabricated out of a monolayer graphene epitaxially grown on SiC. The sensors have been studied at room temperature using transport and noise spectrum measurements. The minimum detectable field of a typical 10-μm graphene sensor is ≈2.5 μT/√Hz, making them comparable with state of the art semiconductor devices of the same size and carrier concentration and superior to devices made of CVD graphene. Relatively high resistance significantly restricts performance of the smallest 500-nm devices. Carrier mobility is strongly size dependent, signifying importance of both intrinsic and extrinsic factors in the optimization of the device performance.
We studied the in-plane magnetoresistance R(B,T) anisotropy in epitaxial multilayer graphene films grown on the Si face of a 6H-SiC substrate that originates from steplike morphology of the SiC substrate. To enhance the anisotropy, a combination of argon atmosphere with graphite capping was used during the film growth. The obtained micro-Raman spectra demonstrated a complex multilayer graphene structure with the smaller film thickness on terraces as compared to the step edges. Several Hall bars with different current/steps mutual orientations have been measured. A clear anisotropy in the magnetoresistance has been observed, and attributed to variations in electron mobility governed by the steplike structure. Our data also revealed that (i) the graphene-layer stacking is mostly Bernal type, (ii) the carriers are massive, and (iii) the carriers are confined to the first 2–4 graphene layers following the buffer layer.
A new microsystem with the ability to detect magnetic microstructures based on the non-invasive Hall principle is presented. The micro-Hall plate embedded in the microsystem and scaled down to the physical limits of the employed CMOS technology has an active area of only 2.4 μm × 2.4 μm. The microsystem exhibits an output sensitivity of 7.5 V/T in perpendicular direction to the chip surface and a magnetic field resolution of 300 nT/√Hz at 1 Hz. A two-dimensional magnetic scanner was developed to demonstrate the performances of the developed microsystem.
We explain the robust p-type doping observed for quasi-free standing graphene
on hexagonal silicon carbide by the spontaneous polarization of the substrate.
This mechanism is based on a bulk property of SiC, unavoidable for any
hexagonal polytype of the material and independent of any details of the
interface formation. We show that sign and magnitude of the polarization are in
perfect agreement with the doping level observed in the graphene layer. With
this mechanism, models based on hypothetical acceptor-type defects as they are
discussed so far are obsolete. The n-type doping of epitaxial graphene is
explained conventionally by donor-like states associated with the buffer layer
and its interface to the substrate which overcompensate the polarization
doping.
We fabricated a large number of single and bilayer graphene transistors and carried out a systematic experimental study of their low-frequency noise characteristics. Special attention was given to determining the dominant noise sources in these devices and the effect of aging on the current-voltage and noise characteristics. The analysis of the noise spectral density dependence on the area of graphene channel showed that the dominant contributions to the low-frequency electronic noise come from the graphene layer itself rather than from the contacts. Aging of graphene transistors due to exposure to ambient conditions for over a month resulted in substantially increased noise, attributed to the decreasing mobility of graphene and increasing contact resistance. The noise spectral density in both single and bilayer graphene transistors either increased with deviation from the charge neutrality point or depended weakly on the gate bias. This observation confirms that the low-frequency noise characteristics of graphene transistors are qualitatively different from those of conventional silicon metal-oxide-semiconductor field-effect transistors.
Quasi-free standing graphene (QFG) obtained from the intercalation of a
hydrogen layer between a SiC surface and the graphene is recognized as an
excellent candidate for the development of graphene based technology. In
addition, the recent proposal of a direct equivalence between the p-type
doping typically found for these systems and the spontaneous polarization (SP)
associated to the particular SiC polytype, opens the possibility of tuning the
number of carriers in the Dirac cones without the need of external gate
voltages. However, first principles calculations which could confirm at the
atomic scale the effect of the SP are lacking mainly due to the difficulty of
combining a bulk property such as the SP with the surface confined graphene
doping. Here we develop an approach based on standard density functional theory
(DFT) slab calculations in order to quantify the effect of the SP on the QFG
doping level. First, we present a novel and accurate scheme to estimate the SPs
by exploiting the dependence of the slab's dipole moment with its thickness.
Next, and in order to circumvent the DFT shortcomings associated to polar slab
geometries, a double gold layer is attached at the C-terminated bottom of the
slab which introduces a metal induced gap state that pins the chemical
potential inside the gap thus allowing a meaningful comparison of the QFG
dopings among different polytypes. Furthermore, the slab dipole can be removed
after adjusting the Au-Au interlayer distances. Our results confirm that the SP
does indeed induce a substantial p-doping of the Dirac cones which can be as
large as a few hundreds of meV depending on the hexagonality of the polytype.
The evolution of the doping with the slab thickness or, equivalently, with the
number of stacking defects, reveals that at least ten SiC bilayers are required
to fully develop the SP and recover the macroscopic regime.
Intercalation of H between the SiC surface and graphene is known to largely reduce the graphene–substrate interaction thus leaving a so called quasi-free-standing graphene monolayer (QFG) which preserves most of the properties of free-standing graphene (FG). Here, we investigate via large-scale density functional theory (DFT) based calculations point defects in FG and QFG in the form of single vacancies passivated by additional H atoms. For QFG our results reveal that the intercalated H layer interacts strongly with the defects attracting unsaturated C atoms but repelling the H-passivated ones thus leading to large reconstructions which, in turn, may induce drastic changes on the electronic and magnetic properties when compared against FG. We conclude that QFG with defect concentrations larger than 0.3% cannot be regarded in general as quasi-free-standing any more.
We investigate the magnetotransport properties of epitaxial graphene on SiC(0001), which exhibits a stepped surface with regular terraces and step edges. We observe an anisotropic behavior in the magnetoresistance measured at high magnetic fields for narrow Hall bars patterned on stepped surfaces. If the Hall bar is aligned parallel to the terraces, the conventional quantum Hall effect behavior is observed. In contrast, a predominantly positive magnetoresistance arises if the current crosses many surface steps. The results can be explained by a model which takes into account inter-Landau-level scattering, which is enhanced in the step edge surface region and may enable electron backscattering from one channel to another on the opposite side, resulting in a positive magnetoresistance.
We have investigated the transport and noise properties of a micron-sized Hall probe, fabricated on chemical vapor deposited (CVD) graphene, from 300 K to 4.2 K. The field sensitivity of the Hall probe was tunable within ∼0.031-0.12 Ω/G, while the field resolution could reach ∼0.43-0.09 G/Hz1/2 at room temperature. The characteristics of graphene Hall probes (GHPs) were found to be comparable to present Hall sensors. Our results indicate that the fundamental limitation of the field sensitivity and the field resolution are respectively restricted by intrinsic and extrinsic defects. Our study paves the way for the use of CVD GHPs for scanning Hall probe with high field sensitivity and submicron spatial resolution at room temperature.
We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ∼1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.