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Contamination-induced inhomogeneity of noise sources distribution in Al2O3-passivated quasi-free-standing graphene on 4H-SiC(0001)
Abstract
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
... Further processing of the digitized noise signal, i.e., fast Fourier transformation, is performed with the LabVIEW application. More details on the measurement setup have been reported elsewhere [32]. In LFN modeling, the distributed resistance of the sensor layer is modeled as a resistance network with local lumped parameters as illustrated in Fig. 1(c). ...
... Each resistor r α in the network is treated as a local microscopic noise source δ R α with power spectral density (PSD) S α in the branch number α. The total macroscopic PSDs S CT and S VT of signals δV CT and δV VT are given by the formulas [32], [33], [34]: S CT = α i 4 α S α /I 2 and S VT = α i 2 α j 2 α S α /I 2 , where i α and j α are the currents in the α-branch in the original and adjoint resistance network (the adjoint network is the circuit after switching the roles of current and voltage contacts). Following the methodology presented in our recent paper [32], the numerical value of the ratio S CT /S VT can be calculated assuming the homogeneous distributions of the local noise sources' magnitudes (S α = const.) ...
... The total macroscopic PSDs S CT and S VT of signals δV CT and δV VT are given by the formulas [32], [33], [34]: S CT = α i 4 α S α /I 2 and S VT = α i 2 α j 2 α S α /I 2 , where i α and j α are the currents in the α-branch in the original and adjoint resistance network (the adjoint network is the circuit after switching the roles of current and voltage contacts). Following the methodology presented in our recent paper [32], the numerical value of the ratio S CT /S VT can be calculated assuming the homogeneous distributions of the local noise sources' magnitudes (S α = const.) and resistances (r α = const.). ...
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.
... In this experiment, two measurements were made at the diagonal contacts, AC and BD, for each sample. Changing the contacts' role from driving to sensing should not change the measured noise magnitude if noise originated from resistance fluctuations [8][9][10]. This so-called reciprocity rule holds for both SL structures, which means that: (i) contact noise does not contribute to the total measured noise, (ii) fluctuations originate from mobility or carrier concentration. ...
... In the low-temperature region, the difference between the 1/f noise magnitudes of samples is more substantial and exceeds one order of magnitude. On this basis, it can be claimed that the noise measurement is more sensitive to local effects [9] (i.e., high local inhomogeneous current) than average quantities such as sheet resistance, which is comparable for both samples. More studies are needed, including a batch of samples with different doping and mobility, to reveal the origin of the fluctuations. ...
The paper reports 1/f noise properties of InAs/GaSb superlattice as a function of voltage bias and temperature. Noise measurements were compared with standard transport characteristics: mobility and carrier concentration. Interestingly, while these standard characteristics of the two samples are comparable, the 1/f noise is substantially different. The results suggest that low-frequency noise is a more sensitive electronic transport characterization tool than standard techniques based on average current/voltage analysis.
... Its exceptional electrical, mechanical, and thermal properties [1][2][3], stemming from its unique honeycomb lattice structure, have attracted attention for a plethora of powerful applications across various fields, including energy storage and conversion [4][5][6], catalysis [7][8][9], sensing [10][11][12], and electronics [13,14]. Moreover, recent advancements in sheet transfer and epitaxial growth have enabled transformative progress in graphene-based device fabrication [15][16][17][18][19]. Paired with refinements in characterization methods [20][21][22], these developments have significantly broadened the potential applications for graphenebased 2D/3D systems, propelling them towards even greater utility and innovation [23][24][25][26][27][28][29][30]. ...
Both intentional and unintentional doping of graphene is a common occurrence, as its carrier concentration can be modulated through various mechanisms. While extensively explored in electronics for achieving desirable conductivity, other aspects of doping remain largely untapped, presenting opportunities for further innovation. This study demonstrates that carrier concentration serves as a powerful and selective tool for modulating the interaction between molecular adsorbates and graphene. The effects are tunable and evident for both n-type and p-type doping, with low-to-medium modulation at doping levels of ±1012e/cm2, and substantial enhancements, with interaction strength increases exceeding 150% and hundreds of meV, at doping levels of ±1013e/cm2. These effects are also molecule specific, with significant enhancements for species such as water (H2O), ammonia (NH3), and aluminum chloride (AlCl3), while having minimal impact on species like hydrogen (H2). This finding not only elucidates the fundamental chemical behavior of graphene but also provides a versatile method to tailor its surface chemistry for applications in sensors, catalysis, and electronic devices. The insights from this research pave the way for advanced material design strategies, leveraging the tunable nature of graphene’s properties to optimize its interaction with various molecular species.
... Since HOPG and QFS graphene proved almost identical FWHM of their relative contact potential distributions and are closely related carbon materials, both underwent an aging experiment to assess their vulnerability to storage conditions and surface contamination [36]. Initially, the QFS-graphene/4H-SiC(0001) structure had hole density = 1.61 × 10 13 cm −2 and hole mobility = 1681 cm 2 /Vs. ...
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.
... Its exceptional electrical, mechanical, and thermal properties [1][2][3], stemming from its unique honeycomb lattice structure, have attracted attention for a plethora of powerful applications across various fields, including energy storage and conversion [4][5][6], catalysis [7][8][9], sensing [10][11][12], and electronics [13,14]. Moreover, recent advancements in sheet transfer and epitaxial growth have enabled transformative progress in graphene-based device fabrication [15][16][17][18][19]. Paired with refinements in characterization methods [20][21][22], these developments have significantly broadened the potential applications for graphenebased 2D/3D systems, propelling them towards even greater utility and innovation [23][24][25][26][27]. ...
Both intentional and unintentional doping of graphene is a common occurrence, as its carrier concentration can be modulated through various mechanisms. While extensively explored in electronics for achieving desirable conductivity, other aspects of doping remain largely untapped, presenting opportunities for further innovation. This study demonstrates that carrier concentration serves as a powerful and selective tool for modulating the interaction between molecular adsorbates and graphene. The effects are tunable and evident for both n-type and p-type doping, with low-to-medium modulation at doping levels of ±1012 e/cm2 , and substantial enhancements, with interaction strength increases exceeding 150% and hundreds of meV, at doping levels of ±1013 e/cm2 . These effects are also molecule-specific, with significant enhancements for species such as water (H2O), ammonia (NH3), and aluminum chloride (AlCl3 ), while having minimal impact on species like hydrogen (H2 ). This finding not only elucidates the fundamental chemical behavior of graphene but also provides a versatile method to tailor its surface chemistry for applications in sensors, catalysis, and electronic devices. The insights from this research pave the way for advanced material design strategies, leveraging the tunable nature of graphene’s properties to optimize its interaction with various molecular species.
... Since the TEM technique requires the deposition of a thin Pt layer, a process potentially hazardous to graphene, the sample surface was intentionally passivated with an 85-nm-thick [18] amorphous aluminum oxide layer synthesized from trimethylaluminum and deionized water in the process of atomic layer deposition. The sole purpose of the dielectric film was to secure the structural composition of graphene and provide a favorable visual contrast in the HR-TEM image [39]. ...
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.
... Received 9 February 2023; Received in revised form 25 April 2023; Accepted 4 May 2023 layer deposition (ALD). The technique is well suited to grow highquality continuous films and has already found extensive use in the fabrication of electronic devices [54][55][56][57][58]. However, the main drawback of ALD use with TMDs is that it often requires coordinatively unsaturated surface sites to initiate the growth process. ...
Transition-metal dichalcogenide (TMD) layers have been a subject of widespread interest as platforms for electronic devices. However, the low chemical activity of their basal plane results in several technological bottlenecks, including high contact resistance at TMD–electrode interfaces, difficult growth of high-quality gate-oxide layers, and challenging functionalization. The simplest, and perhaps only, approach to overcoming those limitations may be to exploit dative bonding. The effect can enhance binding on TMDs, since their chalcogen nonbonding lone-pair orbitals can function as electron donors. Therefore, it should also be able to impact the surface catalysis for reactions that produce acceptors. This computational study seeks to investigate whether S -> P dative bonding may be an effective mechanism for catalysis on the surface of MoS2, and whether the sheet can be functionalized via chemical reactions enabled by the binding of PHn and PCln. The results show that the bonding facilitates the PH functionalization of MoS2. The interaction is strong (1.11 eV), making the whole process exothermic, and the activation energy notably reduced (from 2.08 to 0.5 eV). Furthermore, the mechanism is intrinsically selective, which could prove a vital feature for future advancements in TMD-based electronics, since it could steer selected processes towards surface functionalization or thin-film growth.
... Low frequency noise, particularly the 1/f type, is of unwavering interest, as confirmed by recently published articles related to, e.g., condensed matter physics, materials science, and electronics [1][2][3][4][5][6][7][8]. It carries information about the properties of the system under consideration and, simultaneously, constitutes an obstacle in the optimization of the signal-to-noise ratio of many semiconductor devices, particularly in the field of sensors/detectors the low frequency noise is extensively studied [9][10][11][12][13][14][15][16][17][18]. ...
... An alternative could be to employ atomic layer deposition (ALD). The technique is well suited to grow highquality continuous films and has already found extensive use in the fabrication of electronic devices [54][55][56][57][58]. However, the main drawback of ALD use with TMDs is that it often requires coordinatively unsaturated surface sites to initiate the growth process. ...
Transition-metal dichalcogenide (TMD) layers have been a subject of widespread interest as platforms for electronic devices. However, the low chemical activity of their basal plane results in several technological bottlenecks, including high contact resistance at TMDelectrode interfaces, difficult growth of high-quality gate-oxide layers, and challenging functionalization. The simplest, and perhaps only, approach to overcoming those limitations may be to exploit dative bonding. The effect can enhance binding on TMDs, since their chalcogen nonbonding lone-pair orbitals can function as electron donors. Therefore, it should also be able to impact the surface catalysis for reactions that produce acceptors. This computational study seeks to investigate whether S→P dative bonding may be an effective mechanism for catalysis on the surface of MoS2, and whether the sheet can be functionalized via chemical reactions enabled by the binding of PHn and PCln. The results show that the bonding facilitates the PH functionalization of MoS2. The interaction is strong (1.11 eV), making the whole process exothermic, and the activation energy notably reduced (from 2.08 to 0.5 eV). Furthermore, the mechanism is intrinsically selective, which could prove a vital feature for future advancements in TMD-based electronics, since it could steer selected processes toward surface functionalization or thin-film growth.
... 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.
Electronic noise has its roots in the fundamental physical interactions between matter and charged particles, carrying information about the phenomena that occur at the microscopic level. Therefore, Low-Frequency Noise Measurements (LFNM) are a well-established technique for the characterization of electron devices and materials and, compared to other techniques, they offer the advantage of being non-destructive and of providing a more detailed view of what happens in the matter during the manifestation of physical or chemical phenomena. For this reason, LFNM acquire particular importance in the modern technological era in which the introduction of new advanced materials requires in-depth and thorough characterization of the conduction phenomena. LFNM also find application in the field of sensors, as they allow to obtain more selective sensing systems even starting from conventional sensors. Performing meaningful noise measurements, however, requires that the background noise introduced by the measurement chain be much smaller than the noise to be detected and the instrumentation available on the market does not always meet the specifications required for reaching the ultimate sensitivity. Researchers willing to perform LFNM must often resort to the design of dedicated instrumentation in their own laboratories, but their cultural background does not necessarily include the ability to design, build, and test dedicated low noise instrumentation. In this review, we have tried to provide as much theoretical and practical guidelines as possible, so that even researchers with a limited background in electronic engineering can find useful information in developing or customizing low noise instrumentation.
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.
Featured Application
Graphene-based electronics and sensing.
Abstract
Due to its excellent physical properties and availability directly on a semiconductor substrate, epitaxial graphene (EG) grown on the (0001) face of hexagonal silicon carbide is a material of choice for advanced applications in electronics, metrology and sensing. The deposition of ultrathin high-k insulators on its surface is a key requirement for the fabrication of EG-based devices, and, in this context, atomic layer deposition (ALD) is the most suitable candidate to achieve uniform coating with nanometric thickness control. This paper presents an overview of the research on ALD of high-k insulators on EG, with a special emphasis on the role played by the peculiar electrical/structural properties of the EG/SiC (0001) interface in the nucleation step of the ALD process. The direct deposition of Al2O3 thin films on the pristine EG surface will be first discussed, demonstrating the critical role of monolayer EG uniformity to achieve a homogeneous Al2O3 coverage. Furthermore, the ALD of several high-k materials on EG coated with different seeding layers (oxidized metal films, directly deposited metal-oxides and self-assembled organic monolayers) or subjected to various prefunctionalization treatments (e.g., ozone or fluorine treatments) will be presented. The impact of the pretreatments and of thermal ALD growth on the defectivity and electrical properties (doping and carrier mobility) of the underlying EG will be discussed.
Graphene (Gr) with its distinctive features is the most studied two-dimensional (2D) material for the new generation of high frequency and optoelectronic devices. In this context, the Atomic Layer Deposition (ALD) of ultra-thin high-k insulators on Gr is essential for the implementation of many electronic devices. However, the lack of out-of-plane bonds in the sp² lattice of Gr typically hinders the direct ALD growth on its surface. To date, several pre-functionalization and/or seed-layer deposition processes have been explored, to promote the ALD nucleation on Gr. The main challenge of these approaches is achieving ultra-thin insulators with nearly ideal dielectric properties (permittivity, breakdown field), while preserving the structural and electronic properties of Gr. This paper will review recent developments of ALD of high k-dielectrics, in particular Al2O3, on Gr with “in-situ” seed-layer approaches. Furthermore, recent reports on seed-layer-free ALD onto epitaxial Gr on SiC and onto Gr grown by chemical vapor deposition (CVD) on metals will be presented, discussing the role played by Gr interaction with the underlying substrates.
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.
Atomic layer deposition (ALD) is the method of choice to obtain uniform insulating films on graphene for device applications. Owing to the lack of out‐of‐plane bonds in the sp² lattice of graphene, nucleation of ALD layers is typically promoted by functionalization treatments or predeposition of a seed layer, which, in turn, can adversely affect graphene electrical properties. Hence, ALD of dielectrics on graphene without prefunctionalization and seed layers would be highly desirable. In this work, uniform Al2O3 films are obtained by seed‐layer‐free thermal ALD at 250 °C on highly homogeneous monolayer (1L) epitaxial graphene (EG) (>98% 1L coverage) grown on on‐axis 4H‐SiC(0001). The enhanced nucleation behavior on 1L graphene is not related to the SiC substrate, but it is peculiar of the EG/SiC interface. Ab initio calculations show an enhanced adsorption energy for water molecules on highly n‐type doped 1L graphene, indicating the high doping of EG induced by the underlying buffer layer as the origin of the excellent Al2O3 nucleation. Nanoscale current mapping by conductive atomic force microscopy shows excellent insulating properties of the Al2O3 thin films on 1L EG, with a breakdown field > 8 MV cm⁻¹. These results will have important impact in graphene device technology.
In this paper, micro-Raman mapping and conductive atomic force microscopy (C-AFM) were jointly applied to investigate the structural and electrical homogeneity of quasi-free-standing monolayer
raphene (QFMLG), obtained by high temperature decomposition of 4H-SiC(0001) followed by hydrogen intercalation at 900 °C. Strain and doping maps, obtained by Raman data, showed the presence of sub-micron patches with reduced hole density correlated to regions with higher compressive strain, probably associated with a locally reduced hydrogen intercalation. Nanoscale resolution electrical maps by C-AFM also revealed the presence of patches with enhanced current injection through the QFMLG/SiC interface, indicating a locally reduced Schottky barrier height (ФB). The ФB values evaluated from local I-V curves by the thermionic emission model were in good agreement with the values calculated for the QFMLG/SiC interface using the Schottky-Mott rule and the graphene holes density from Raman maps. The demonstrated approach revealed a useful and non-invasive method to probe the structural and electrical homogeneity of QFMLG for future nano-electronics applications.
Noise spectra of Type-II Strained Layer Superlattice (T2SLS) mid-wave infrared photodetectors were compared pre- and post-irradiation with proton fluence of 7:5 x 10 cm (Total Ionizing Dose equivalent of 100 krad (Si)) and related to the shot noise limit at biases ranging from +200 mV to –800 mV and temperatures of 130 K and 160 K. Pre-irradiation dark current at 130 K was 7.5X Rule ’07 and increased to 59X Rule ’07 after irradiation. The pre-irradiation noise spectra were within one order of magnitude of the shot-noise prediction while post-irradiation noise spectra were close to two orders higher, indicating the introduction of non-shot-like noise sources.
In recent years, the demand for high data rate wireless communications has increased dramatically, which requires larger bandwidth to sustain multi-user accessibility and quality of services. This can be achieved at millimeter wave frequencies. Graphene is a promising material for the development of millimeter-wave electronics because of its outstanding electron transport properties. Up to now, due to the lack of high quality material and process technology, the operating frequency of demonstrated circuits has been far below the potential of graphene. Here, we present monolithic integrated circuits based on epitaxial graphene operating at unprecedented high frequencies (80–100 GHz). The demonstrated circuits are capable of encoding/decoding of multi-gigabit-per-second information into/from the amplitude or phase of the carrier signal. The developed fabrication process is scalable to large wafer sizes.
We report on the identification of a deep level trap centre which contributes to generation-recombination noise. A n-GaN epilayer, grown by MOCVD on sapphire, was measured by deep level transient spectroscopy (DLTS) and noise spectroscopy. DLTS found 3 well documented deep levels at E[subscript c] − 0.26 eV, E[subscript c] − 0.59 eV, and E[subscript c] − 0.71 eV. The noise spectroscopy identified a generation recombination centre at E[subscript c] − 0.65 ± 0.1 eV with a recombination lifetime of 65 μs at 300 K. This level is considered to be the same as the one at E[subscript c] − 0.59 eV measured from DLTS, as they have similar trap densities and capture cross section. This result shows that some deep levels contribute to noise generation in GaN materials.
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 sensitivity of graphene based devices to surface adsorbates and charge traps at the
graphene/dielectric interface requires proper device passivation in order to operate them reproducibly under ambient conditions. Here we report on the use of atomic layer deposited aluminum oxide as passivation layer on graphene field effect devices (GFETs). We show that successful passivation produce hysteresis free DC characteristics, low doping level GFETs stable over weeks though operated and stored in ambient atmosphere. This is achieved by selecting proper seed layer prior to deposition of encapsulation layer. The passivated devices are also demonstrated to be robust towards the exposure to chemicals and heat treatments, typically used during device fabrication. Additionally, the passivation of high stability and reproducible characteristics is also shown for functional devices like integrated graphene based inverters.
A thin Al2O3 dielectric film was directly grown onto graphene layers without any surface treatment prior to H2O-based atomic layer deposition for the first time. The growth mechanism of Al2O3 dielectric film has been studied by changing the growth temperature and purge time. We found that the film morphology was influenced by the amount and distribution of physically adsorbed precursor molecules on the graphene, especially by physically adsorbed H2O molecules. Within an optimal temperature window, conformal and uniform Al2O3 thin films were obtained as confirmed by atomic force microscopy and transmission electron microscopy results. Raman spectroscopy revealed that no extra defects are generated in the graphene layers. Furthermore, the low leakage current and interface traps in dual-gated graphene field-effect transistors demonstrate the high-quality dielectric/graphene stack.
Hall elements were fabricated based on high quality chemical vapor deposition grown graphene, and their performance limit was explored. The as-fabricated graphene Hall element exhibits current-related sensitivity of up to 2093 V/AT under 200 μA, and magnetic resolution of around 1 mG/Hz0.5 at 3 kHz. This ultrahigh sensitivity and resolution stem from high carrier mobility, small Dirac point voltage of 3 V, and low carrier density of about 3 × 1011 cm−2 in graphene device. The current sensitivity is found to decrease with increasing current bias at large bias, and this phenomenon is attributed to the drain induced Dirac point shift effect in graphene channel.
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 passivation stability and blister formation during firing at 800°C of an Al2O3/SiNx stack deposited on p-type float zone silicon at different Al2O3 deposition set temperatures ranging from 170°C to 400°C. The actual wafer temperatures during Al2O3 deposition in the FlexAL reactor are determined using spectroscopic ellipsometry. After the firing step blistering can be observed for stacks featuring 15 nm thick Al2O3 layers grown at 170°C set temperature. We show that the deposition of the layer at higher set temperatures of 250°C, 300°C and 400°C reduces blister formation significantly. After firing, stacks with 15 nm thick Al2O3 layers deposited at set temperatures of 250°C and 300°C show the best passivation resulting in effective surface recombination velocities below 5 cm/s without significant blister formation.
This work proves that blistering is the partial delamination of a thick enough Al2O3 layer caused by gaseous desorption in the Al2O3 layer upon thermal treatments above a critical temperature: the Al2O3 layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al2O3 samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H2O, illustrates that gaseous desorption from Al2O3 and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al2O3 layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (< 10 nm) Al2O3 film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al2O3 film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al2O3 layers can still be adequate passivation layers after being annealed in N2 environment at temperatures up to 500–700 °C: (i) interface trap densities (Dit) can remain below 1×1011 cm−2 and (ii) fixed charge densities (Qf) stay negative and in the order of −3×1012 cm−2 Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al2O3 passivation layers and performing an annealing step prior to capping and co-firing. Al2O3/SiNx passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.
We present results of the experimental investigation of the low-frequency noise in bilayer graphene transistors. The back-gated devices were fabricated using the electron beam lithography and evaporation. The charge neutrality point for the transistors was around +10 V. The noise spectra at frequencies f > 10-100 Hz were of the 1/f type with the spectral density on the order of S1 ~ 10-23-10-22 A2/Hz at the frequency of 1 kHz. The deviation from the 1/f spectrum at f < 10-100 Hz suggests that the noise is of the carrier-number fluctuation origin due to the carrier trapping by defects. The Hooge parameter was determined to be as low as ~ 10-4. The gate dependence of the normalized noise spectral density indicates that it is dominated by the contributions from the ungated parts of the device and can be reduced even further. The obtained results are important for graphene electronic and sensor applications.
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.
In this report, we demonstrate a method for the enhancement of Raman active modes of hydrogen-intercalated quasi-free-standing epitaxial chemical vapor deposition graphene and the underlying semi-insulating 6H–SiC(0001) substrate through constructive signal interference within atomic-layer-deposited amorphous Al2O3 passivation. We find that an optimum Al2O3 thickness of 85 nm for the graphene 2D mode and one of 82 nm for the SiC longitudinal optical A1 mode at 964 cm–1 enable a 60% increase in their spectra intensities. We demonstrate the method’s efficiency in Raman-based determination of the dielectric thickness and high-resolution topographic imaging of a graphene surface.
In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H-SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 10¹⁵ cm⁻². We find that the material’s electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.
https://authors.elsevier.com/a/1cvqR5aLOStCsP
Graphene has attracted huge attention due to its unique electronic properties, however, when supported those are significantly dependent on the interface interactions. One of the methods of decoupling graphene sheets from a substrate is hydrogen intercalation, which has been shown to produce quasi-free-standing (QFS) layers on a SiC (0001) surface. Still, the effects of incomplete H termination of SiC remain mostly unknown. This work in vestigates, employing density functional theory calculations, the impact of partial termination on the structural, and electronic properties of graphene. It is predicted that interfaces with partially damaged H layer or produced under a lower technological standard could still benefit from the intrinsic, however, quantitatively reduced, properties of QFS graphene.
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The fundamental origin of low-frequency noise in graphene field effect transistors (GFETs) has been widely explored but a generic engineering strategy towards low noise GFETs is lacking. Here, we systematically study and eliminate dominant sources of electrical noise to achieve ultralow noise GFETs. We find that in edge contacted, high-quality hexagonal boron nitride (hBN) encapsulated GFETs, the inclusion of a graphite bottom gate and long (≳1.2 μm) channel-contact distance significantly reduces noise as compared to global Si/SiO2 gated devices. From the scaling of the remaining noise with channel area and its temperature dependence, we attribute this to the traps in hBN. To further screen the charge traps in hBN, we place few layers of MoS2 between graphene and hBN, and demonstrate that the noise is as low as ∼5.2 × 10⁻⁹ μm² Hz⁻¹ (corresponding to minimum Hooge parameter ∼5.2 × 10⁻⁶) in GFETs at room temperature, which is an order of magnitude lower than the earlier reported values.
The nucleation and growth mechanism of aluminum oxide (Al2O3) in the early stages of atomic layer deposition (ALD) on monolayer epitaxial graphene (EG) on silicon carbide (4H-SiC) has been investigated by atomic force microscopy (AFM), conductive-atomic force microscopy (C-AFM) and Raman spectroscopy. Differently than for other types of graphene, a large and uniform density of nucleation sites was observed in the case of EG and ascribed to the presence of the buffer layer at EG/SiC interface. The deposition process was characterized by Al2O3 island growth in the very early stages, followed by the formation of a continuous Al2O3 film (∼2.4 nm thick) after only 40 ALD cycles due to the islands coalescence, and subsequent layer-by-layer growth. The electrical insulating properties of the deposited ultrathin Al2O3 films were demonstrated by nanoscale current mapping with C-AFM. Raman spectroscopy analyses showed low impact of the ALD process on the defect’s density of EG. The EG strain was also almost unaffected by the deposition in the regime of island growth and coalescence, whereas a significant increase was observed after the formation of a compact Al2O3 film. The obtained results can have important implications for device applications of epitaxial graphene requiring ultra-thin high-k insulators.
Subthreshold leakage currents and threshold-voltage shifts due to total-ionizing-dose (TID) irradiation are reviewed briefly for highly scaled devices in emerging MOS technologies. When isolation oxides of digital and analog MOS devices and ICs exhibit satisfactory performance, failure doses often are 100 krad(SiO
2
) to 1 Mrad(SiO
2
) or higher. Oxygen vacancies in SiO
2
and/or high-K gate dielectrics and/or O-vacancy complexes with hydrogen are typically the dominant border traps before and after irradiation. Low-frequency noise measurements can provide significant insight into effective border-trap microstructures, densities, and energy distributions, especially when combined with complementary measurements and density-functional theory calculations. Illustrative examples are presented for past, present, and emerging MOS technologies with SiO
2
and/or high-K gate dielectrics. These include FinFETs, MOS devices with alternative channels to Si, MOS devices based on 2-D materials, and SiC MOS devices. Traps in regions of MOS isolation oxides under strong gate control can also contribute to low-frequency noise, especially for multifinger, multiedge devices irradiated to high doses. The effects of defects on the 1/
f
noise of GaN-based HEMTs and thin metal lines are illustrated for comparison.
We measure the low-frequency noise in epitaxial β-Ga2O3 grown by MOVPE. Both 1/f and generation-recombination noise components are well resolved. The Hooge parameters characterizing the 1/f noise are 3 × 10–4 at room temperature and 2 × 10–5 at temperatures near 200 K. Mid bandgap trap states result in generation-recombination noise that is analyzed using temperature dependent low-frequency deep-level noise spectroscopy. Trap levels with energies of 165, 127, and 37 meV below the conduction band minimum are characterized in terms of density and activation energy.
This paper deals with design and fabrication of graphene micro-Hall devices along with driving and processing electronics to construct a highly sensitive graphene magnetometer system. Graphene micro-Hall elements were fabricated by implementing microfabrication techniques. Custom driving and processing circuitry was designed and built on a printed circuit board (PCB) and integrated with fabricated graphene Hall devices; reducing the offset equivalent magnetic field and greatly improving the sensitivity. The performance parameters were quantitatively analyzed for characterizing the sensitivity in terms of linearity, current-related sensitivity, and magnetic field resolution. The potential of graphene magnetometers for detection capability of low fields has been demonstrated with a current related sensitivity of 2540 V/AT and magnetic field resolution of 162 nT/√Hz.
The functionalization of graphene is important in practical applications of graphene, such as in catalysts. However, the experimental study of the interactions of adsorbed molecules with functionalized graphene is difficult in ambient conditions at which catalysts are operated. Here, the adsorption of CO2 on an oxygen-functionalized epitaxial graphene surface was studied at near-ambient conditions using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). The oxygen-functionalization of graphene is achieved in-situ by the photo-induced dissociation of CO2 with X-rays on graphene in a CO2 gas atmosphere. The oxygen species on the graphene surface is identified as the epoxy group by XPS binding energies and thermal stability. Under near-ambient conditions of 1.6 mbar CO2 gas pressure and 175 K sample temperature, CO2 molecules are not adsorbed on the pristine graphene, but are adsorbed on the oxygen-functionalized graphene surface. The increase in the adsorption energy of CO2 on the oxygen-functionalized graphene surface is supported by first-principles calculations with the van der Waals density functional (vdW-DF) method. The adsorption of CO2 on the oxygen-functionalized graphene surface is enhanced by both the electrostatic interactions between the CO2 and the epoxy group and the vdW interactions between the CO2 and graphene. The detailed understanding of the interaction between CO2 and the oxygen-functionalized graphene surface obtained in this study may assist in developing guidelines for designing novel graphene-based catalysts.
We demonstrate a full-fledged millimeter-wave graphene-based six-port receiver frontend at 90 GHz employing graphene power detectors. Exploiting the high responsivity and wide dynamic range reported for the state-of-the-art graphene field-effect transistors (GFETs), graphene power detectors are demonstrated beyond the maximum oscillation frequency, fmax, of the graphene transistor. The proposed circuit is fabricated on thinned SiC substrate and its functionality is verified by demodulation of 10-Mbps ON-OFF keying (OOK) digitally modulated signal.
Graphene is a two dimensional material with extraordinary properties, which make it an interesting material for many optical and electronic devices. The integration of graphene in these devices often requires the deposition of thin dielectric layers on top of graphene. Atomic layer deposition (ALD) is the method of choice to deposit these layers due to its ability to deposit ultra-thin, high quality films with sub-monolayer thickness control. ALD on graphene however, is a challenge due to the lack of reactive surface sites on graphene. This leads to the selective growth on grain boundaries, wrinkles and defect sites present in the graphene. In this review an overview of the different methods to achieve uniform deposition of ALD on graphene is presented. The advantages and disadvantages of each method are discussed from the perspective of ALD together with the opportunities for further research. Special emphasis is given to the recent advancements in the understanding of the ALD process conditions and their influence on the deposition uniformity on graphene. Particularly, improving the quality of the dielectric layers deposited by ALD while maintaining the pristine properties of graphene, will prove vital for the device integration of graphene.
The low-frequency noise in high voltage Ni/4H-SiC Schottky diodesirradiated with high energy (0.9 MeV) electrons was studied in the frequency range from 1 Hz to 50 kHz, temperature interval 295–410 K, and irradiation dose Φ from 0.2 × 1016 cm⁻² to 7 × 1016 cm⁻². The noise amplitude was found monotonically increasing with the irradiation dose. With the irradiation dose increase, the noise spectra on the linear part of the current voltagecharacteristic transform from the 1/f noise to the generation recombination noise of at least two trap levels. One of these levels can be classified as Z1/2 with the capture cross section determined from the noise measurements to be ∼10⁻¹⁵ cm².
This letter presents the design, fabrication and characterization of the first graphene based monolithic microwave integrated circuit (MMIC) in microstrip technology operating in W-band. The circuit is a resistive mixer in a 250 nm graphene field effect transistor (G-FET) technology on a SiC substrate. A conversion loss of 18 dB is achieved which is limited by the available local oscillator (LO) power. The mixer exhibits a flat response over radio frequency (RF) range of 90-100 GHz. The RF bandwidth is also limited by the measurement setup.
High quality thin insulating films on graphene (Gr) are essential for field effect transistors (FETs) and other electronics applications of this material. Atomic Layer Deposition (ALD) is the method of choice to deposit high-κ dielectrics with excellent thickness uniformity and conformal coverage. However, to start the growth on the sp(2) Gr surface a chemical pre-functionalization or the physical deposition of a seed layer are required, which can effect, to some extent, the electrical properties of Gr. In this paper we report a detailed morphological, structural and electrical investigation of Al2O3 thin films grown by a two-steps ALD process on large area Gr membranes residing on an Al2O3/Si substrate. This process consists of the H2O-activated deposition of few nanometer Al2O3 seed layer performed in-situ at 100 °C, followed by ALD thermal growth of Al2O3 at 250°C. The optimization of the low-temperature seed layer allowed to obtain a uniform, conformal and pinhole free Al2O3 film on Gr by the second ALD step. Nanoscale resolution mapping of the current through the dielectric by conductive atomic force microscopy (CAFM) demonstrated an excellent laterally uniformity of the film. Raman spectroscopy measurements indicated that the ALD process does not introduce defects in Gr, whereas it produces a partial compensation of Gr unintentional p-type doping, as confirmed by a threefold increase of Gr sheet resistance (from ~300 Ω/sq in pristine Gr to ~1100 Ω/sq after Al2O3 deposition). Analysis of the transfer characteristics of Gr field effect transistors (GFETs) allowed to evaluate the relative dielectric permittivity (ε=7.45) and the breakdown electric field (EBD=7.4 MV/cm) of the Al2O3 film, as well as the transconductance and the holes field effect mobility (~1200 cm(2)V(-1)s(-1)). A special focus has been given to the electrical characterization of the Al2O3/Gr interface by the analysis of high frequency capacitance-voltage measurements, which allowed to elucidate the charge trapping/detrapping phenomena due to near-interface and interface oxide traps.
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.
The use of 1/f noise measurements is explored for the purpose of finding faster techniques for electromigration (EM) characterization in advanced microelectronic interconnects, which also enable a better understanding of its underlying physical mechanisms. Three different applications of 1/f noise for EM characterization are explored. First, whether 1/f noise measurements during EM stress can serve as an early indicator of EM damage. Second, whether the current dependence of the noise power spectral density (PSD) can be used for a qualitative comparison of the defect concentration of different interconnects and consequently also their EM lifetime t50. Third, whether the activation energies obtained from the temperature dependence of the 1/f noise PSD correspond to the activation energies found by means of classic EM tests. In this paper, the 1/f noise technique has been used to assess and compare the EM properties of various advanced integration schemes and different materials, as they are being explored by the industry to enable advanced interconnect scaling. More concrete, different types of copperinterconnects and one type of tungsteninterconnect are compared. The 1/f noise measurements confirm the excellent electromigration properties of tungsten and demonstrate a dependence of the EM failure mechanism on copper grain size and distribution, where grain boundary diffusion is found to be a dominant failure mechanism.
We demonstrate the demodulation of a multi-Gb/s ON-OFF keying (OOK) signal on a 96 GHz carrier by utilizing a 250-nm graphene field-effect transistor as a zero bias power detector. From the eye diagram, we can conclude that the devices can demodulate the OOK signals up to 4 Gb/s.
We report on the identification of a deep level trap centre which contributes to generation-recombination noise. A n-GaN epilayer, grown by MOCVD on sapphire, was measured by deep level transient spectroscopy
(DLTS) and noise spectroscopy.
DLTS found 3 well documented deep levels at Ec − 0.26 eV, Ec − 0.59 eV, and Ec − 0.71 eV. The noise spectroscopy identified a generation recombination centre at Ec − 0.65 ± 0.1 eV with a recombination lifetime of 65 μs at 300 K. This level is considered to be the same as the one at Ec − 0.59 eV measured from DLTS, as they have similar trap densities and capture cross section. This result shows that some deep levels contribute to noise generation in GaN
materials.
Epitaxial graphene is grown on semi-insulating (SI) 4H-SiC in a hot wall CVD reactor by graphitization and in-situ intercalation with (H)ydrogen. A holistic material characterization is performed in order to ascertain the number of layers, layer uniformity, and electron transport properties of the epi-layers via electronic test structures and Raman spectroscopy. Bilayer graphene field effect transistors (GFETs) are fabricated using a full electron beam lithography (EBL) process which is optimized for low contact resistances of . Mobilities of order are achieved on bilayer samples after fabrication. The devices demonstrate high transconductance and high current density . The output conductance at the bias of maximum transconductance is . The GFETs demonstrate an extrinsic and of 20 and , respectively and show power gain at in a system, which is the highest reported to date.
Different kinds of post-deposition annealing (PDA) by a rapid thermal process (RTP) are used to enhance the field-effect passivation of film in crystal Si solar cells. To characterize the effects of PDA on and the interface, metal-insulator semiconductor (MIS) devices were fabricated. The effects of PDA were characterized as functions of RTP temperature from and RTP time from 30~120 s. A high temperature PDA can retard the passivation of thin film in c-Si solar cells. PDA by RTP at results in better passivation than a PDA at in forming gas ( 4% in ) for 30 minutes. A high thermal budget causes blistering on film, which degrades its thermal stability and effective lifetime. It is related to the film structure, deposition temperature, thickness of the film, and annealing temperature. RTP shows the possibility of being applied to the PDA of film. Optimal PDA conditions should be studied for specific films, considering blistering.
The linear four-point probe method is useful to measure the resistivity, by passing a current I14 through the outer probes and by measuring the voltage V23 between the inner probes. The contacts are on a line and denoted by 1, 2, 3, 4, respectively. The sheet resistance for thin layers with thickness t is Rsh = ρ/t (Ω). The sheet resistance is measured as Rsh = (V23/I14). C23 where the correction factor is C23 = π/ln2 = 4.53 for t smaller than s, the distance between probe points. In order to characterize the 1/f noise of a film with a four-point probe we need noise correction factors for the calculation of the conductance noise from the voltage noise between the pair of points, where the current is not passed through. Our calculations of the noise correction factors are based on the general theory for conductance noise investigations with four arbitrarily shaped and placed electrodes. Analytical expressions for noise correction factors Fij and fij are derived and compared with experimental results for the three cases. The subscripts of F and f denote the voltage sensor contacts. The following cases are investigated: 1) I14 and V23, 2) I13 and V24 and 3) I12 and V34. The subscripts in I and V denote the current driver contacts and voltage sensor contacts, respectively. From the calculations and experimental results it follows that: i) case1 (I14 and V23) is the best choice in order to suppress a non-intentional noise contribution from the interface between probe tip and layer, ii) the reciprocity relation is applicable to voltage noise due to conductance fluctuations. The voltage noise SV23 that is observed by passing a constant current I14 through the pair of probes 1, 4 is equal to the voltage noise SV14 if we pass the same current through the contacts 2, 3. The four-point probe method with the noise correction factors can be recommended to investigate e.g., the conductance noise of conductive polymer layers even with an insulating top layer and without the preparation of rectangular samples with perfect noise-free line contacts. The four-point probe can punch an insulating top layer and allows the measurement of sheet resistance and noise. Several tests show that the noise contributions from the interface between probe tip and layer are negligible. The ratio of the conductance (1/f) noise, Cus, normalized for frequency and area and the sheet resistance gives an indication of the degree of percolation in composite conductive layers.
Deposition of high-quality dielectric on a graphene channel is an essential technology to overcome structural constraints for the development of nano-electronic devices. In this study, we investigated a method for directly depositing aluminum oxide (Al2O3) on a graphene channel through nitrogen plasma treatment. The deposited Al2O3 thin film on graphene demonstrated excellent dielectric properties with negligible charge trapping and de-trapping in the gate insulator. A top-gate-structural graphene transistor was fabricated using Al2O3 as the gate dielectric with nitrogen plasma treatment on graphene channel region, and exhibited p-type transistor characteristics.
Fluorine functionalization, using XeF2, was investigated as a way to enhance atomic layer deposition (ALD) of thin, high-κ dielectrics on epitaxial graphene, which would enable the realization of graphene-based device technologies. The XeF2 dosage time was correlated with oxide coverage and morphology as well as its overall effect on the underlying graphene properties. An optimum XeF2 dose time of 120 s (PXeF2=1torr, PN2 = 35 torr) was found to form C–F bonds on 6–7% of the graphene surface, which are presumed to act as additional ALD reaction sites facilitating conformal Al2O3 films only 15 nm thick. Under these optimal conditions, the graphene lattice remained essentially undisturbed and the Hall mobility exhibited a 10–25% increase after oxide deposition. These results indicate that this novel technique is a viable path to obtaining ultrathin high-κ dielectrics by ALD on epitaxial graphene.
Experimental studies on 1/f noise are reviewed with emphasis on experiments that may be decisive in finding the correct theoretical model for this type of noise. The experimental results are confronted with two theories: McWhorter's (1959) surface state theory and Clarke and Voss's (1974) theory of local temperature fluctuations. The applicability of either theory turns out to be very limited. The validity of an empirical relation is investigated. Its application to electronic devices proves successful. Experiments show that 1/f noise obeying the empirical relation ( alpha noise) is a fluctuation in the part of the mobility that is due to lattice scattering.
Because of its superior stretchability, graphene exhibits rich structural deformation behaviours and its strain engineering has proven useful in modifying its electronic and magnetic properties. Despite the strain-sensitivity of the Raman G and 2D modes, the optical characterization of the native strain in graphene on silica substrates has been hampered by excess charges interfering with both modes. Here we show that the effects of strain and charges can be optically separated from each other by correlation analysis of the two modes, enabling simple quantification of both. Graphene with in-plane strain randomly occurring between -0.2% and 0.4% undergoes modest compression (-0.3%) and significant hole doping on thermal treatments. This study suggests that substrate-mediated mechanical strain is a ubiquitous phenomenon in two-dimensional materials. The proposed analysis will be of great use in characterizing graphene-based materials and devices.
Epitaxial graphene (EG) grown by solid‐state decomposition of SiC is a promising material for future graphenebased electronics. On EG, the fabrication of a field‐effect device requires the deposition of suitable gate insulator. Atomic layer deposition (ALD) of aluminum oxide might be useful for this purpose. Therefore, we investigated the growth of ALD‐Al 2 O 3 on highly oriented pyrolytic graphite (HOPG) and EG by atomic force microscopy (AFM), photoelectron spectroscopy (XPS) and Raman spectroscopy.
Water as oxidant in the ALD process leads to inhomogeneous nucleation, whereas ozone leads to the formation of closed oxide layers. However, significant degradation of the EG takes place at higher temperatures and ozone exposure as witnessed by XPS and Raman spectroscopy. Careful adjustment of the process parameters allows reducing the damage to a level undetectable in XPS while providing enough nucleation centers for the formation of a closed Al 2 O 3 film potentially suitable as a gate insulator (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A general relation is derived for the spectral noise density of the voltage between two arbitrarily shaped and placed sensor
electrodes on a conductor, when a constant current or voltage is applied to another pair of arbitrarily shaped and placed
driver electrodes. The general relation is based on the sensitivity calculation in linear electrical networks. The relation
is elaborated for conductivity fluctuations due to 1/f noise by using empirical 1/f noise relations. The influence of spot radii of the sensor electrodes on the noise is demonstrated.
The theoretical results for 2 and 3-dimensional conductors are in agreement with our experimental results for carbon resistance
sheets and on silicon and germanium in the resistivity range of 1Ωcm to 400Ωcm. The possibility of using the four-point probe
for measuring the fluctuations in the resistivity has been demonstrated.
We explore the effect of high-κ dielectric seed layer and overlayer on carrier transport in epitaxial graphene. We introduce a novel seeding technique for depositing dielectrics by atomic layer deposition that utilizes direct deposition of high-κ seed layers and can lead to an increase in Hall mobility up to 70% from as-grown. Additionally, high-κ seeded dielectrics are shown to produce superior transistor performance relative to low-κ seeded dielectrics and the presence of heterogeneous seed/overlayer structures is found to be detrimental to transistor performance, reducing effective mobility by 30-40%. The direct deposition of high-purity oxide seed represents the first robust method for the deposition of uniform atomic layer deposited dielectrics on epitaxial graphene that improves carrier transport.