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Role of Fermi-Level Pinning in Nanotube Schottky Diodes
Abstract
At semiconductor-metal junctions, the Schottky barrier height is generally fixed by "Fermi-level pinning." We find that when a semiconducting carbon nanotube is end contacted to a metal (the optimal geometry for nanodevices), the behavior is radically different. Even when the Fermi level is fully "pinned" at the interface, the turn-on voltage is that expected for an unpinned junction. Thus the threshold may be adjusted for optimal device performance, which is not possible in planar contacts. Similar behavior is expected at heterojunctions between nanotubes and semiconductors.
... where ϵs is the permittivity of the semiconductor, Nd is the doping concentration of the semiconductor, ϕB is the tunneling barrier height, and V is the bias voltage. The tunneling thickness was determined by calculating the depletion width of the metal-semiconductor contact using the formula = √((2 ( − ))/( )) , where Fermi-level pinning effects were disregarded due to the specific charge characteristics of one-dimensional nanowires [16]. The tunneling barrier height (ϕB) for both the axial and radial contacts was obtained using Equations (4) and (5), respectively. ...
... Fermi-level pinning effects were disregarded due to the specific charge characteristics of one-dimensional nanowires [16]. The tunneling barrier height (ϕ B ) for both the axial and radial contacts was obtained using Equations (4) and (5), respectively. ...
In this study, we investigated the influence of quasi-one-dimensional (Quasi-1D) characteristics on the source and drain contact resistances within vertical nanowire (NW) field-effect transistors (FETs) of diminutive diameter. The top contact of the NW is segregated into two distinct regions: the first encompassing the upper surface, designated as the axial contact, and the second encircling the side surface, known as the radial contact, which is formed during the top-contact metal deposition process. Quantum confinement effects, prominent within Quasi-1D NWs, exert significant constraints on radial transport, consequently inducing a noticeable impact on contact resistance. Notably, in the radial direction, electron tunneling occurs only through quantized, discrete energy levels. Conversely, along the axial direction, electron tunneling freely traverses continuous energy levels. In a meticulous numerical analysis, these disparities in transport mechanisms unveiled that NWs with diameters below 30 nm exhibit a markedly higher radial contact resistance compared to their axial counterparts. Furthermore, an increase in the overlap length (less than 5 nm) contributes to a modest reduction in radial resistance; however, it remains consistently higher than the axial contact resistance.
... 11,12 We treat the MIGS following Ref. 13, which models an interface dipole induced by MIGS by a charge, ...
... where the horizontal position x ¼ 0 is defined at the boundary between the metal covered and the extension regions as shown in of the MIGS. [11][12][13] The value of 1=q characterizes the average characteristic decay distance of the MIGS into semiconductor. A typical value of q $ 1 to 2 nm À1 has been previously used. ...
A multiscale simulation approach is developed to simulate the contact transport properties between semimetal and a monolayer two-dimensional transition metal dichalcogenide (TMDC) semiconductor. The results elucidate the mechanisms for low contact resistance between semimetal and TMDC semiconductor contacts from a quantum transport perspective. The simulation results compare favorably with recent experiments. Furthermore, the results show that the contact resistance of a bismuth-MoS 2 contact can be further reduced by engineering the dielectric environment and doping the TMDC material to [Formula: see text]. The quantum transport simulation indicates the possibility to achieve an ultrashort contact transfer length of ∼1 nm, which can allow aggressive scaling of the contact size.
... However, owing to the lack of simple and efficient doping approaches for SWCNTs, the S and D electrodes in a SWCNT-FET are metals and the S (or D)/SWCNTs contact exhibits typical Schottky contact properties. [15,[112][113][114] The Schottky barriers (SBs) at the metal electrodes/s-SWCNT interface plays a significant role in the operation of SWCNT-FETs. When a metal is in contact with a bulk semiconductor such as silicon, metal-induced gap states (MIGSs) form on the semi- . ...
... However, the MIGS induced Fermi level pinning plays a minor effect in the metal/SWCNT contact due to the 1D ultrathin SWCNT. [112,116,117] Self-consistent calculations show that the MIGS in SWCNT decay more rapidly than in bulk semiconductors. As a result, the barrier width at the metal/SWCNT interface is only a few nanometers so that electrons or holes can effectively transport across the contacts, with a negligible effect of the Fermi level pinning. ...
Single‐walled carbon nanotubes (SWCNTs) have been considered as one of the most promising electronic materials for the next‐generation electronics in the more Moore era. Sub‐10 nm SWCNT‐field effect transistors (FETs) have been realized with several performances exceeding those of Si‐based FETs at the same feature size. Several industrial initiatives have attempted to implement SWCNT electronics in integrated circuit (IC) chips. Here, the recent advances in SWCNT electronics are reviewed from in‐depth understanding of the fundamental electronic structures, the carrier transport mechanisms, and the metal/SWCNT contact properties. In particular, the subthreshold switching properties are highlighted for low‐power, energy‐efficient device operations. State‐of‐the‐art low‐power SWCNT‐based electronics and the key strategies to realize low‐voltage and low‐power operations are outlined. Finally, the essential challenges and prospects from the material preparation, device fabrication, and large‐scale ICs integration for future SWCNT‐based electronics are foregrounded.
... D 1 is forward-biased, whereas D 2 and D 3 are reverse-biased. The biasing of D 1 and D 3 is due to a metal-semiconductor junction [41], whereas D 2 is reverse biased as the PSiNWs act as an n-type semiconductor (compared to the p-type substrate) due to loss of dopant concentration during the fabrication process [42]. Figure 7a depicts the I-V curves of PSiNWs in the biasing voltage range of − 5 V to 5 V. ...
Si nanowires (SiNWs) are receiving tremendous attention due to their significant optical and electrical properties; however, porosity on the nanowires opens an opportunity for further improvement. The work establishes a connection between morphological changes in the optical and electrical characteristics of the porous SiNWs (PSiNWs) because of H2O2 concentration (0.1 M, 0.2 M, 0.3 M) variation during metal-assisted chemical etching. The directional etching controls the porosity; a modification to the classical gravimetric method is introduced to measure the porosity. PSiNWs fabricated at 0.2 M H2O2 concentration achieve the minimal average reflectance of 8.29% in the visible range and a band gap of 1.39 eV. The work discusses the effect of the decrease in saturation current and broad diode biasing voltage on the open-circuit voltage of solar cells, considering the optical and electrical properties of the PSiNWs. The optimization of H2O2 concentration to fabricate the PSiNWs for photovoltaic applications (photon absorption and antireflection properties) is illustrated.
... The I-V measurement requires the deposition of metal contacts at the front and rear surfaces to bias the sample. The metal-semiconductor junction behavior is analyzed by Schottky barrier height; in reality, Schottky barrier height (rather than the theoretical concept such that the difference between the metal work function and semiconductor electron affinity) is decided by interface states at metal-semiconductor interface, Fermi level pinning, surface defects [12,13]. A low-workfunction metal forms a Schottky contact on the p-type semiconductor surface, whereas a high-work fiction metal forms ohmic contact on the p-type semiconductor. ...
Optoelectronic applications prefer Si nanostructures over bulk-Si due to improved optical and electrical properties. However, tuning the electrical properties of Si nanostructures is a bottleneck for a broad range of applications. Metal-assisted chemical etching (MACE) is a cost-effective method to fabricate silicon nanowires (SiNWs) array and etched silicon (eSi) using bulk-Si and porous substrates, respectively. Among various fabrication parameters, MACE temperature is appropriate to tailor the nanostructure dimensions- length and diameter of SiNWs and thickness of the porous layer of eSi, on which the bandgap and the electrical biasing characteristic depend. The study addresses the dimensional change of nanostructures as the impact of MACE temperature variation on the bandgap and the DC bias characteristics. Increasing MACE temperature reduces the nanowire diameter and the porous layer thickness. As a result, the bandgap widening and the lowering of the DC bias current are characterized by the series diode-resistance equivalent circuits.
... A common feature of these devices is that the SB at the contacts between the source/drain and the channels could be modulated by the gate voltage in the subthreshold region, yielding a high intrinsic gain and an exponentially steep subthreshold switching behavior of the channel current. SWCNT-FETs have been experimentally and theoretically shown to function as SB-FETs [40][41][42]. The SB at the interface between the metal electrodes and the s-SWCNT channel plays a significant role in carrier injection and the operation of the SWCNT-FETs. ...
Ultralow-power electronics is critical to wearable, portable, and implantable applications where the systems could only have access to very limited electrical power supply or even be self-powered. Here, we report on a type of Schottky barrier (SB) contacted single-walled carbon nanotube (SWCNT) network film field-effect-transistors (FETs) that are operated in the subthreshold region to achieve ultralow-power applications. The thin high-k gate dielectric and the overlap between the gate and the source electrodes offer highly efficient gate electrostatic control over the SWCNT channel and the SB at the source contact, resulting in steep subthreshold switching characteristics with a small subthreshold swing (~ 67 mV/dec), a large current on/off ratio (~ 106), and a low off-state current (~ 0.5 pA). A p-channel metal-oxide-semiconductor (PMOS) inverter built with the subthreshold SB-SWCNT-FETs exhibits a well-defined logic functionality and small-signal amplification capability under a low supply voltage (~ 0.5 V) and an ultralow power (~ 0.05 pW/µm). The low-voltage and deep subthreshold operations reported here could lay an essential foundation for high-performance and ultralow-power SWCNTs-based electronics.
... In a conventional Si MOSFET, metal induced gap states pins the Fermi-level deep into the bandgap 58 . However, a 1D conductor like CNT remains unaffected because of its molecular scale dipoles at the interface 59 . This is highly advantageous, in the sense that the Schottky barrier height can be essentially tuned by choosing the right materials as a metal contact [60][61][62] . ...
Electrically induced light emission from semiconducting carbon nanotubes (CNTs) results from the radiative recombination of excitons across the band gap. Large diameter CNTs, in particular, emit light in near-infrared (NIR) range which make them an attractive candidate for telecommunication applications. This thesis aims at the investigation of NIR electroluminescence (EL) of such large diameter CNTs in field effect transistor (FET) configuration. Single-tube and multi-tube devices are fabricated on Si/SiO2 substrate by dielectrophoretic assembly from multichiral dispersion. The spectra show well-defined emission wavelength, mainly dominated by the larger diameter species of the suspension. Single-tube device shows strong detectable emission with exceptionally narrow linewidth. Multi-tube devices, on the other hand, have relatively broad emission linewidth and exhibits EL as well as thermal emission. For both cases, noticeable red shift in the emission is observed. Furthermore, the effect of the device substrate, which has a crucial impact on the emission properties, is also investigated.
... Fermi-level pinning (FLP) has been identified to be a major factor in this problem. 7 FLP in contacts on lowdimensional materials has been linked to metal-induced gap states (due to orbital hybridization with the semiconductor), metal-induced defects, and intrinsic defects in the semiconductor. 8−11 Among the two-dimensional materials, layered monochalcogenides are a very interesting class of materials. ...
Germanium sulfide (GeS) is a layered monochalcogenide semiconductor with a band gap of about 1.6 eV. To verify the suitability of GeS for field-effect-based device applications, a detailed understanding of the electronic transport mechanisms of GeS-metal junctions is required. In this work, we have used conductive atomic force microscopy (c-AFM) to study charge carrier injection in metal-GeS nanocontacts. Using contact current-voltage spectroscopy, we identified three dominant charge carrier injection mechanisms: thermionic emission, direct tunneling, and Fowler-Nordheim tunneling. In the forward-bias regime, thermionic emission is the dominating current injection mechanism, whereas in the reverse-bias regime, the current injection mechanism is quantum mechanical tunneling. Using tips of different materials (platinum, n-type-doped silicon, and highly doped p-type diamond), we found that the Schottky barrier is almost independent of the work function of the metallic tip, which is indicative of a strong Fermi-level pinning. This strong Fermi-level pinning is caused by charged defects and impurities.
... We treat the MIGS following Ref. 13 , which models an interface dipole induced by MIGS by a charge, ...
A multiscale simulation approach is developed to simulate the contact transport properties between semimetal to a monolayer two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductor. The results elucidate the mechanisms for low contact resistance between semimetal and TMDC semiconductor contacts from a quantum transport perspective. The simulation results compare favorably with recent experiments. Furthermore, the results show that the contact resistance of a Bismuth-MoS2 contact can be further reduced by engineering the dielectric environment and doping the TMDC material to <100 ${\Omega}$.${\mu}$m. The quantum transport simulation indicates the possibility to achieve an ultrashort contact transfer length of ~1 nm, which can allow aggressive scaling of the contact size.
... This metal-carbon junction has been widely studied theoretically, demonstrating that the quality of the contact strongly depends on the metal type, 12,13 the nanotube chirality, 14,15 and how the nanotube is contacting the electrodes. 16 Experimentally, Seidel et al. 17 developed a multilayer structure for growing nanotubes in situ over electrodes, allowing very high on-currents of the order of several milliamperes and on/off ratios exceeding 500. Nosho et al. 18 investigated the response of SWNT field-effect transistors (FETs) with diverse metal contacts, demonstrating that the conduction type and the drain current are dependent on the metal-work function. ...
A straightforward protocol is presented to covalently bond gold nanoparticles exclusively at the tips of single-walled carbon nanotubes. This approach exploits the sterical hindrance given by a polymer and surfactant, preventing the attachment of not-functionalised gold nanoparticles onto the main nanotube body. These novel heterostructures have promising potential for applications in photonics and electronic devices.
... The properties of the Schottky barrier will depend on the band alignment at the interface ( Fig. 1.12a). In the absence of interface states believed to be a good approximation for the metal/tube interface [37] , the heights of the Schottky barriers for hole and electron injection are given by the work function of the metal contacts, φM, the work function of the nanotube, φNT, and its energy gap, Eg. ...
Printed electronics is a branch of electronics where functional materials, such as conductors, semiconductors and dielectrics, are deposited one by one in the forms of inks with high-throughput printing machines inherited by the graphical arts. Printed electronics allows to add electronic functionalities with a cost-effective process on a wide variety of substrates, including plastic and paper, and it allows to target large-area electronics applications. Rather than competing with established silicon electronics, it offers alternative applications where lightweight, flexibility and ease of integration are critical figure of merits. Organic semiconductors, and in particular conjugated polymers, are typically considered among the best options for printed electronics, because of their chemical tunability, which simplifies inks formulations, their advantageous mechanical properties and electronic and optoelectronic properties which have improved over the last few decades. However, semiconducting polymers still show limitations, both in terms of maximum field-effect charge carrier mobility, which is below 5 cm 2 V-1 s-1 for most of largely available materials, and in terms of contact resistance, owing to high energetic barriers for injection of carriers from most of printable electrodes, thus complicating downscaling of devices. Both limitations represent a strong bottleneck for improvement of switching performances of printed electronics devices, such as printed transistors, therefore strongly limiting the overall expected speed of a printed polymer circuit. With the aim to find alternatives to printed polymers to overcome their current limitations, this work has the goal of assessing the use of semiconducting single-walled carbon nanotubes (swCNTs) for the fabrication of fully printed field-effect transistors on plastic substrates. While CNTs offer in principle much better electronic performances than polymers, especially in terms of charge mobility, economically viable methodologies to produce them lead to mix of conducting and semi-conducting nanotubes. Moreover their dispersion in suitable solvents to formulate stable, printable inks is still a challenge. One possible solution to both criticalities is to adopt polymer wrapping, where a semiconducting polymer capable of selectively wrapping specific chiralities can be used both as a sorting agent and a dispersant. The formulations of polymer wrapped swCNTs studied here have been provided by the group of Prof. ii M.A. Loi at the University of Groningen. An optimization of inkjet printing of swCNTs formulations has been performed in order to control the formation of suitable nanotubes networks. These printed networks were integrated in field-effect transistors printed on plastic by combining only inkjet printing and bar-coating in ambient air, demonstrating working devices with holes field-effect mobility reaching 1.90 cm 2 V-1 s-1. The obtained result is a first promising step towards high performance, fully printed CNTs based flexible circuits, which can be achieved by further optimization of the quality of the printed CNTs networks, especially in terms of coverage.
... The abrupt change of geometry results in an energy level splitting at the interface. As a result, the metal allows states at all energies [24]. Sometimes its influence cannot be ignored. ...
Processing layer-dependent direct band gap and good absorption coefficient especially in the mid-infrared band, black phosphorous is believed to make a contribution superior to that of graphene in broadband photodetectors. The narrow band gap of 0.3 eV for bulk black phosphorous helps to absorb infrared radiation while a relatively large dark current under zero gate voltage is inevitable. Few layer black phosphorous sheets with asymmetrical thickness sealed in an insulator for protection is designed and explored for photosensitive mechanism in this work. Saturable absorption dominates the light harvesting process in visible light detection and thus limits maximum photocurrent to 3.3 and 1.4 μA for 520 and 650 nm lasers with a dark current of 0.7 μA. While in near-infrared wavelength, a responsivity of 0.12 A/W is inducted for 808 nm free of adsorption saturation even if the incident power is increased to 200 m W / c m 2 . Discrimination for the origin of the photo-response in short wavelength is conducted and the abnormal negative and nearly constant photocurrent in mid-infrared, irrelevant to inhomogeneous thickness, reveals the photothermal effect in a black phosphorous sheet. This work unravels various photoelectric features in black phosphorous and paves the way to designing more outstanding broadband photodetectors based on black phosphorous.
... Thus, the increase of the electrical conducting in the random-nanotube networks could be verified by the number of nanotubes in the channel, in contrast to the carrier mobility. The appearance of the rectifying behavior in such an asymmetrical Schottky contact structure of low-nanotube concentrations (0.01-0.11 μg ml −1 ) shows the existence of interface dipoles at metal-nanotube contacts, resulting in the depinning of the Fermi level at the metal-nanotube contact [36]. This is unlikely to happen in a bulk nanotube-metal contact since its barrier height is fixed by the neutrality level of the interface state ( -E E F N ). ...
We investigate the use of random networks of single-walled carbon nanotubes for near-infrared photodetection. By increasing the number of nanotubes between asymmetrical work-function electrodes using dielectrophoretic assembly, the effect of Fermi-level pinning of nanotube-Schottky contacts was revealed in the linear current-voltage characteristic. The extracted device resistance showed an abrupt drop when the numerous intertube junctions formed densely packed networks in the electrode channel. Under the excitation of a near-infrared laser, we performed the photocurrent measurement at ambient temperature at different light powers. Our devices with densely packed nanotube networks showed enhanced photoconductive detection of responsivity, detectivity, and detection response. This is attributed to the increase in the photoabsorption area, the decrease of the channel resistance, and the formation of continuous conducting paths for high-efficient charge percolation. The photoconductive responsivity of up to 8.0 μA W⁻¹ was found with a detectivity of about 4.9 × 10⁵ cm Hz1/2 W⁻¹, which is 4 orders of magnitude greater than that achieved in the channel with individual nanotubes deposited and comparable to that of suspended nanotube bolometers. The densely packed nanotube devices had a detection response of ∼ 4 ms under a finite bias that can be explained by the short-diffusion length of the photoexcited electrons and holes. However, the decrease in the photocurrent with time observed in our devices that exhibited photovoltaic characteristics indicates that electron-hole pair recombination in the nanotube networks occurs with differing characteristic time scales of the injected electrons and holes.
... Accordingly, we succeeded in producing an SBD with V BD that varied linearly with d ( Supplementary Fig. 6, Supplementary Table 1a). Such an optimized SBD was also confirmed to have an ideal Schottky barrier without pinning 16,17 as shown in Fig. 2c and d. ...
Power devices (PD) are ubiquitous elements of the modern electronics industry that must satisfy the rigorous and diverse demands for robust power conversion systems that are essential for emerging technologies including Internet of Things (IoT), mobile electronics, and wearable devices. However, conventional PDs based on “bulk” and “single-crystal” semiconductors require high temperature (> 1000 °C) fabrication processing and a thick (typically a few tens to 100 μm) drift layer, thereby preventing their applications to compact devices, where PDs must be fabricated on a heat sensitive and flexible substrate. Here we report next-generation PDs based on “thin-films” of “amorphous” oxide semiconductors with the performance exceeding the silicon limit (a theoretical limit for a PD based on bulk single-crystal silicon). The breakthrough was achieved by the creation of an ideal Schottky interface without Fermi-level pinning at the interface, resulting in low specific on-resistance Ron,sp (< 1 × 10–4 Ω cm²) and high breakdown voltage VBD (~ 100 V). To demonstrate the unprecedented capability of the amorphous thin-film oxide power devices (ATOPs), we successfully fabricated a prototype on a flexible polyimide film, which is not compatible with the fabrication process of bulk single-crystal devices. The ATOP will play a central role in the development of next generation advanced technologies where devices require large area fabrication on flexible substrates and three-dimensional integration.
... 204,304-306 However, these effects play a secondary role for SWNTs due to their one-dimensional structure and their high charge carrier density at the van Hove singularities as illustrated by previous theoretical and experimental studies. 160,[307][308][309] Hence, the Schottky-Mott model delivers a good estimation for the Schottky barrier heights in metal-SWNT contacts simply based on Φ M and the band edges of the SWNT. 309,310 Choosing a suitable electrode material whose work function coincides with the respective band edge is thus a widespread strategy to establish so-called ohmic contacts with negligible injection barriers. ...
The extraordinary mechanical and charge transport properties of semiconducting single-walled carbon nanotubes (SWNTs) make them a promising material for solution-processable, flexible and stretchable electronics. Many of these remarkable features are even obtained in randomly-oriented SWNT networks that are compatible with established large-scale thin-film processes based on printing techniques or optical lithography. Given the enormous progress in the purification of solely semiconducting nanotubes as well as in the preparation of SWNT networks with a uniform and defined morphology in recent years, their widespread application as active layers in field-effect transistors (FETs) has become feasible. Likewise, this progress raised subsequent questions of what key parameters determine the charge transport processes across these networks and how they can further be optimized. This thesis investigates charge transport and its limitations in polymer-sorted semiconducting SWNT networks with a focus on the precise nanotube network composition. The employed FET geometry enabled a reproducible and undistorted analysis of composition- and temperature- dependent transport parameters such as the charge carrier mobility. A comparison between nanotube networks with various selected or even precisely defined SWNT species distributions and average tube diameters reveals that additional energy barriers created at the junctions of adjacent nanotubes with different diameters result in inferior transport properties. While the network charge transport was formerly considered to be solely limited by the charge transfer across these inter-nanotube junctions, the results of this work imply that also the transport within each individual SWNT is important. The specific diameter dependence of this intra-nanotube transport can rationalize the substantially higher carrier mobilities observed for large-diameter networks with a certain SWNT bandgap distribution compared to monochiral networks that contain only a single small-diameter nanotube species. These findings suggest that composition optimizations for SWNT network FETs with maximum carrier mobilities should aim at monochiral large-diameter nanotubes. Aside from insights into the underlying transport mechanisms, this work demonstrates a novel approach to intentionally modify charge transport in semiconducting SWNT network FETs by adding photochromic spiropyran compounds to the dielectric layer. The strong impact of the spiropyran and its photoinduced isomerization to merocyanine on the charge carrier mobilities give these transistors the properties of basic optical memory devices. Upon UV illumination the carrier mobilities are severely reduced until their recovery is induced by annealing or illumination with visible light. This implemented light responsiveness illustrates the fundamental suitability of SWNT network FETs for multifunctional applications beyond integrated circuits.
... vdW top contact by metal transfer method was shown effectively to reduce FLP, [69] but the large vdW gap could introduce tunneling barrier leading to large contact resistance. For edge contact on monolayer 2D material, weak FLP has been demonstrated possibly due to dimensionality reduction of interface dipole effect, [74,75] which seems promising due to the absence of vdW gap compared with top contact structures. State-of-theart low contact resistances %180 and 750 Ω μm for monolayer MoS 2 [76] and WS 2 nFETs [77] with specific contact engineering was reported. ...
Down‐scaling of transistor size in the lateral dimensions must be accompanied by a corresponding reduction in the channel thickness to ensure sufficient gate control to turn off the transistor. However, the carrier mobility of 3D bulk semiconductors degrades rapidly as the body thickness thins down due to more pronounced surface scattering. Two‐dimensional‐layered materials with perfect surface structures present a unique opportunity as they naturally have atomically thin and smooth layers while maintaining high carrier mobility. To benefit from continuous scaling, the performance of the scaled 2D transistors needs to outperform Si technology nowadays. There are already quite a few reviews discussing on the material property of potential 2D materials. It is believed that rigorous analysis based on industrial perspectives is needed. Herein, an analysis on channel material selection is presented and arguments on the four selected 2D semiconductors are provided, which can possibly meet the needs of future transistors, including WS2, SnSe, PtSe2, and InSe. The challenges and recent related research progresses for each material are also discussed. To continue to produce tiny transistors without sacrificing device performance, new materials with perfect surface structures are needed. Two‐dimensional‐layered materials with extremely flat surfaces offer great potentials to further scale down the size of the transistor. The perspective on channel material selection is presented. Challenges of suggested potential material candidates are discussed.
... One more essential feature toward the interface among CNTs and metallic connections is Fermi level restraining. It had been noticed that a CNT is an edge communicated through metal, the comportment is entirely distinct as contrasted toward the metal-semiconductor boundary [90]. The edge within CNTs and metal also hinge on perceptively at heat, electrical field, and the vanishing aspects, including distinct wetting behavior. ...
Carbon nanotube (CNT)-doped transparent conductive films (TCFs) is an encouraging option toward generally utilized indium tin oxide-depended TCFs for prospective stretchable optoelectronic materials. Industrial specifications of TCFs involve not just with high electrical performance and transparency but also amidst environmental resistance and mechanical characteristic; those are usually excused within the research background. Though the optoelectronic properties of these sheets require to be developed to match the necessities of various strategies. While, the electrical stability of single-walled CNT TCFs is essentially circumscribed through the inherent resistivity of single SWCNTs and their coupling confrontation in systems. The main encouraging implementations, CNT-doped TCFs, is a substitute system during approaching electronics to succeed established TCFs, that utilize indium tin oxide. Here we review, a thorough summary of CNT-based TCFs including an overview, properties, history, synthesis protocol covering patterning of the films, properties and implementation. There is the attention given on the optoelectronic features of films and doping effect including applications for sophisticated purposes. Concluding notes are given to recommend a prospective investigation into this field towards real-world applicability.
Graphic abstract
This graphical abstract shows the overview of different properties (mechanical, electrical, sensitivity and transportation), synthesis protocols and designing (dry and wet protocol, designing by surface cohesive inkjet-printed and the support of polymers), doping effect (general doping, metal halides, conductive polymers and graphene for transparent electrodes) and implementations (sensing panels, organic light-emitting diodes devices, thin-film transistors and bio-organic interface) of carbon nanotubes transparent conductive films.
... Article Next, we focus on attributes that especially benefit the construction of ICs in a flexible form. Unlike traditional metal−semiconductor contacts, the contact between a CNT and a metal is largely unaffected by Fermi-level pinning at the interface, 51 which allows the choice of the metal contacts with different work functions to control the contact behavior and thus the contact-controlled polarity in transistors. As a result, p-type and n-type transistors with ohmic contacts have been successfully realized by choosing high-work-function (Pd) 52 and low-work-function (Sc) 53 metals to inject holes and electrons into the channel, respectively. ...
Flexible integrated circuits, working as the core unit of information processing, have been a subject of extensive research; they are essential to realize fully flexible electronic systems, which possess advantages over the current hybrid flexible systems (part or all based on rigid silicon chips) because of their extended application forms, better adaptability, and greater ability to operate at biotic/abiotic interfaces. To a great extent, the deliverable functionality of the ultimate integrated system is determined by the information handling capability of flexible integrated circuits, which is highly dependent on their performance and integration scale. Additionally, as the applications of flexible electronic systems become increasingly favorable in portable, wearable, or remote forms with a restrained power supply, the power dissipation of flexible integrated circuits becomes crucial, as they consume considerable energy in a system. Due to the restricted fabrication circumstances on flexible substrates with a low thermal budget and complex working environments (from the perspectives of both mechanical and electrical conditions), pursuing flexible integrated circuits with high performance, decent integration scale, and low power consumption is very challenging, but it is necessary for advanced, fully flexible systems. Other characteristics, such as biocompatibility, degradability, and configurability, would add value to flexible integrated circuits and introduce unconventional forms and new deliverables for flexible electronics.
... For this junction, the electric field corresponding to the contact potential difference is directed from Bi 4 O 7 to Bi 3.33 (VO 4 ) 2 O 2 , and its decrease, because of direct polarization, means a decrease in the potential barrier height for the electrons in Bi 4 O 7 ; this scenario allows an easier motion of the electrons from Bi 4 O 7 to Bi 3.33 (VO 4 ) 2 O 2 and, therefore, an increase in the photogenerated current. This behavior is similar to that from a Schottky diode 61,62 . ...
Electrochemical cells for direct conversion of solar energy to electricity (or hydrogen) are one of the most sustainable solutions to meet the increasing worldwide energy demands. In this report, a novel and highly-efficient ternary heterojunction-structured Bi4O7/Bi3.33(VO4)2O2/Bi46V8O89 photoelectrode is presented. It is demonstrated that the combination of an inversion layer, induced by holes (or electrons) at the interface of the semiconducting Bi3.33(VO4)2O2 and Bi46V8O89 components, and the rectifying contact between the Bi4O7 and Bi3.33(VO4)2O2 phases acting afterward as a conventional p–n junction, creates an adjustable virtual p–n–p or n–p–n junction due to self-polarization in the ion-conducting Bi46V8O89 constituent. This design approach led to anodic and cathodic photocurrent densities of + 38.41 mA cm–2 (+ 0.76 VRHE) and– 2.48 mA cm–2 (0 VRHE), respectively. Accordingly, first, this heterojunction can be used either as photoanode or as photocathode with great performance for artificial photosynthesis, noting, second, that the anodic response reveals exceptionally high: more than 300% superior to excellent values previously reported in the literature.
... The diagnostics of electric surface potentials in semiconductors, arising as a result of the charged states at the surface of the material, together with the measurement of the dynamics of the surface carriers, has been at the heart of the development of many modern technologies [31]. In particular, in semiconductor technology, the map of surface carrier dynamics is of paramount importance in a number of industrial steps; for example, it is well established that the surface states strongly influence the potential of Schottky junctions [46]. Many rising technologies, such as photovoltaics and tuneable metamaterials, rely on thin-layer semiconductors and surface engineering [47,48] requiring fast and insitu monitoring of the surface properties, capable of evaluating the dynamics of carriers and static potentials at the surface. ...
With the massive advantages of THz radiation and the current technical di�culties in
mind, I have chosen to undertake research into terahertz surface phenomena, which is the
focal point of my thesis. Ultrathin surface terahertz emitters have many advantages as
they have an extremely thin active region, typically hundreds of atomic layers. In this
framework, III-V semiconductors, such as InAs and InSb, have record-breaking conversion
e�ciencies per unit thickness. In addition, the phase mismatch, which commonly limits
the generation of terahertz from optical crystal, is negligible and so there is an opportunity
for enhancing the emitted bandwidth.
My thesis is born as the core of many research interests of my research lab (Emergent
Photonics), which enabled the appropriate availability of resources that made my results
possible. It also created several spin-out research lines. All the work presented is my work
(with the exception of the background research). Parts of chapters have been published
in journals and publications which see me as the �rst author.
The structure of this thesis is as follows. First I discuss optical pump recti�cation
emission, and the saturation of InAs terahertz emissions. Then I introduce my work on
terahertz enhancement emission through graphene. Finally, I present my work on an
exotic terahertz emission mechanism, namely the all-optical surface optical recti�cation
and I place my concluding remarks.
... [43,44] The height of Schottky barrier is usually not depended on the difference of work functions between the semiconductor and the metal. [44][45][46][47][48] For a semiconductor with good piezoelectric property, thanks to the partial shielding of piezocharges, the remnant piezocharges can affect tremendously the nature of Schottky barrier. Due to the positive piezoelectric polarization under a tensile strain, the Schottky barrier height (SBH) is decreased, and the space charge zone is compressed (Figure 3a). ...
For third generation semiconductors, for example, wurtzite ZnO and GaN, a piezopotential will be induced in these non‐centrosymmetric structures through applying a stationery deformation. The synchronous occupancy of semiconductor engineering, piezoelectric property, and optical excitation processes in these materials brings about outstanding device performances, such as promoting carrier creation, transfer, separation, or suppressing carrier recombination. Enormous research interests have been sparked in this emerging field known as the piezo‐phototronic effect. This manuscript reviews the fundamental research progress in enhanced photovoltaic efficiency by this effect, which not only provides a comprehensive coverage of the theoretical and experimental works illustrating the basic physics for understanding the enhanced solar cells when an external mechanical strain is applied, but also gives new insight into designing high‐performance solar cells. Recent progress in the applications of the piezo‐phototronic effect in tuning the performance of various solar cells is reviewed. Using this emerging effect, this paper not only provides a comprehensive coverage of the theoretical and experimental works illustrating the basic physics for understanding the characteristics of the solar cells, but also sheds light on designing future high‐performance solar cells.
Two-dimensional (2D) semiconductors, especially transition metal dichalcogenides (TMDCs), have been proven to be excellent channel materials for the next-generation integrated circuit (IC). However, the contact problem between 2D TMDCs and metal electrodes has always been one of the main factors restricting their development. In this review, we summarized recent work on 2D TMDCs contact from the perspective of compatible integration with silicon processes and practical application requirements, including the contact performance evaluation indicators, special challenges encountered in 2D TMDCs, and recent optimization methods. Specifically, we sorted out and highlighted the performance indicators of 2D TMDCs contacts, including contact resistance (RC), contact scaling, contact stability, and contact electrical/thermal conductivity. Special challenges of 2D TMDCs and metal contact, such as severe Fermi level pinning, large RC and difficult doping, are systematically discussed. Furthermore, typical methods for optimizing 2D TMDCs RC, edge contact strategies for scaling contact lengths, and solutions for improving contact stability are reviewed. Based on the current research and problems, the development direction of 2D TMDCs contacts that meet the silicon-based compatible process and application performance requirements is proposed.
The existence of a reduced Schottky barrier at the nanoscale junction between semiconductor and metal domains has yet to be acknowledged among the photocatalysis community, despite its critical role in dictating the quality and functionality of the hybrid photocatalytic system.
Carbon nanotubes (CNTs) are promising materials for gas sensing because of their large specific area and high sensitivity to charge differentiation. In CNT-based field-effect transistors (FETs) for gas sensing, both CNT potential modulation in the channels and Schottky barrier height modulation at the CNT/metal electrode contact influence the current properties. However, researchers have not used Schottky barrier height modulation for gas detection. To investigate and compare the effects of Schottky barrier height modulation and CNT channel potential modulation on NO 2 gas exposure, we fabricated ambipolar CNT FETs by the dielectrophoretic assembly. We exposed CNT FET gas sensors to N 2 gas containing 100-ppb NO 2 and observed two different responses in the electric properties: a steady current shift in the positive direction in the hole-conduction region because of the channel potential modulation, and an abrupt decrease in transconductance in the electron-conduction region because of the Schottky barrier modulation. The CNT channels and CNT/metal contact both contributed to the sensor response, and the modulation rate of the Schottky barrier was higher than that of the CNT potential shift in the channel.
In the past 60 years, silicon-based semiconductor technology has triggered off the profound change of our information society, but it is also gradually approaching to the physical limit and engineering limit as well. Thus, the global semiconductor industry has entered into the post-Moore era. Carbon nanotube has many excellent electronic properties such as high mobility and ultra-thin body, so it has become a hopeful candidate for the new semiconductor material in the post-Moore era. After more than 20 years of development, carbon based electronic technology has made fundamental breakthroughs in many basic problems such as material preparation, ohmic metal-semiconductor contact and gate engineering. In principle, there is no insurmountable obstacle in its industrialization process now. Therefore, in this paper the intrinsic advantages of carbon based electronic technology in the post-Moore era is introduced, the basic problems, progress and optimization direction of carbon based electronic technology are summarized, the application prospects in the fields of digital circuits, RF electronics, sensing and detection, three-dimensional integration and chips for special applications are presented. Finally, the comprehensive challenges to the industrialization of carbon based electronic technology are analyzed, and its future development is also prospected.
The contact barrier formed between a metal and crystalline silicon during wire electrical discharge machining (wire-EDM) hinders improvements in the processing efficiency; thus, this paper proposes a workpiece pretreatment method that increases the doping concentration of the crystalline silicon surface. First, a silver layer was coated on a silicon surface by silk screen printing, then a laser marking machine was used to irradiate the coating surface. The mechanism of this method to form a heavily-doped layer was analyzed, and the current-voltage (I–V) characteristic curves under different pretreatment processes were compared. Finally, the influence of this method on the machining performance was studied by wire-EDM experiments. The results showed that the doping concentration of the silicon surface increased, and the contact resistance decreased. When the open voltage was 120 V, the cutting efficiency of the workpiece treated with laser irradiation and screen printing was 60% higher than that without pretreatment, and it was 28% higher than the workpiece with only the metal coating treatment; however, the treatment method had little effect on the kerf width or surface roughness. It is hoped that this work will provide useful guidance for improving the EDM efficiency of crystalline silicon.
Carbon nanotube field-effect transistor (CNTFET) shows great potential for digital and analog applications due to the unique 1-D ballistic carrier transport in carbon nanotubes (CNTs). A new compact physical model for CNT Schottky-barrier transistors is proposed in this article. The Schottky barrier between the contact metal and CNTs is described by using an effective barrier height, and more importantly, the influence of using CNT arrays or networks rather than a single CNT as the device channel is considered in this model. Moreover, the trapping effect is investigated and modeled by adding
RC
delay element to the equivalent circuit. The model is proposed in a concise form to avoid complex operations such as integration and is compatible with circuit simulation. The simulation results of
${I}-{V}$
characteristics are verified to be in agreement with experimental data.
Over the last two decades, carbon nanomaterials including two-dimensional graphene, one-dimensional carbon nanotubes (CNTs), and zero-dimensional carbon quantum dots, fullerenes have gained tremendous attention from researchers due to their unique optical, electronic, mechanical, chemical, and thermal properties. Furthermore, to enhance the properties of pristine carbon nanomaterials, their hybrid materials have been synthesized. Even though tremendous advancement in carbon nanomaterials-based electronic devices and sensors has been achieved, a few challenges need to be addressed before the commercialization of carbon nanomaterials-based devices. Apart from the improvements, the device to device variations, and extrinsic factors like dielectric layers, metal contact resistance remain an issue. Strategies such as chemically tuning and enhancing the properties of carbon nanomaterials are important for the further improvement of carbon nanomaterial-based device performance. This chapter focuses on understanding the basic electronic properties of graphene, CNT. and carbon quantum dots/fullerenes and their applications in electronic devices (field-effect transistors, diodes, etc.), optoelectronics, and various chemical and physical sensors.
Electronic transport through a metal|semiconductor (M|S) heterojunction is largely determined by its Schottky barrier. In 3D M|S junctions, the barrier height determines the turn-on voltage and is often pinned by the interface states, causing Fermi level pinning (FLP). The pinning strength in 3D depends on the ratio C
i
/CM between the interface quantum capacitance C
i
and the metal surface capacitance CM. In 2D, the interface dipole does not influence the band alignment, but still affects the Schottky barrier and transport. In light of the general interest in building 2D electronics, in this work we discover the relevant material parameters which dictate the behavior and strength of FLP in 2D M|S contacts. Using a multiscale model combining first-principles, continuum electrostatics, and transport calculations, we study a realistic Gr|MoS2 interface as an example with high interface state density (C
i
/CM ≫ 1). Transport calculations show partial pinning with a strength P ∼ 0.6, while a 3D junction with similar heterogeneity gives full pinning with P = 1 as expected. We further show that in 2D M|S contacts the turn-on voltage and pinning strength are affected by a physical parameter l/λD, the ratio between the interface width l, and the thermal de Broglie wavelength λD. Pinning is absent for ideal line-contacts (l/λD = 0), but increases for realistic l/λD values.
In this paper, the effect of annealing on the electrical properties of graphene/Al-Zr ZnO Schottky contacts was studied in detail. The results showed that the grain size of Al-Zr doped ZnO films grew with the increasing of annealing temperature. Meanwhile, the defective oxygen in the films decreased. In addition, the leakage current of graphene/Al-Zr ZnO Schottky contact decreased and barrier height increased. This phenomenon can be explained by the reduction of oxygen vacancies and the weakening of Fermi level pinning.
Ohmic back contact is one of the keys to fabricate high efficient CdTe solar cells, as it can improve fill factor FF and open-circuit voltage Voc. In this work, antimony-containing reduced graphene oxide (RGO) was prepared and used as a back contact for CdTe thin film solar cells, in which antimony is an effective dopant for CdTe and RGO acts as a hole transport layer and diffusion barrier of antimony in the solar cells. Post deposition annealing was performed to activate the back contact layer and passivate the interface defect states. Electrical performance and apparent quantum efficiency characterization were performed to investigate device performance of CdTe solar cells with antimony-containing RGO back contacts. The results indicate that, the introduction of extrinsic doping near the CdTe back surface serves to reduce the back contact depletion width and enhance the built-in potential to enable the tunnelling of majority carrier holes through the barrier, and the RGO is beneficial for the formation of Ohmic back contact on CdTe.
As a newly developed and powerful analytical method, photoelectrochemical (PEC) biosensors open up new opportunities to provide wide applications in early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much efforts on PEC sensing with semiconductor materials are highlighted. Thus, we propose a systematical introduction of recent progress in nanostructures-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in PEC field with an emphasis on charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensor from the perspective of basic principle, and give a brief overview of the main advances in versatile sensing pattern of nanostructures-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructures-based PEC analysis field.
Tellurium can form nanowires of helical atomic chains. With their unique one-dimensional van der Waals structure, these nanowires are expected to show physical and electronic properties that are remarkably different from those of bulk tellurium. Here, we show that few-chain and single-chain van der Waals tellurium nanowires can be isolated using carbon nanotube and boron nitride nanotube encapsulation. With this approach, the number of atomic chains can be controlled by the inner diameter of the nanotube. The Raman response of the structures suggests that the interaction between a single-atomic tellurium chain and a carbon nanotube is weak, and that the inter-chain interaction becomes stronger as the number of chains increases. Compared with bare tellurium nanowires on SiO2, nanowires encapsulated in boron nitride nanotubes exhibit a dramatically enhanced current-carrying capacity, with a current density of 1.5 × 108 A cm−2 that exceeds that of most semiconducting nanowires. We also use our tellurium nanowires encapsulated in boron nitride nanotubes to create field-effect transistors with a diameter of only 2 nm. By isolating one-dimensional tellurium nanowires in boron nitride nanotubes, the electronic properties of the atomic chains can be measured and the structures used to create field-effect transistors.
Tellurium can form nanowires of helical atomic chains. Given their unique one-dimensional van der Waals structure, these nanowires are expected to show remarkably different physical and electronic properties than bulk tellurium. Here we show that few-chain and single-chain van der Waals tellurium nanowires can be isolated using carbon nanotube and boron nitride nanotube encapsulation. With the approach, the number of atomic chains can be controlled by the inner diameter of the nanotube. The Raman response of the structures suggests that the interaction between a single-atomic tellurium chain and a carbon nanotube is weak, and that the inter-chain interaction becomes stronger as the number of chains increases. Compared with bare tellurium nanowires on SiO2, nanowires encapsulated in boron nitride nanotubes exhibit a dramatically enhanced current-carrying capacity, with a current density of 1.5*10^8 A cm-2, which exceeds that of most semiconducting nanowires. We also use our tellurium nanowires encapsulated in boron nitride nanotubes to create field-effect transistors that have a diameter of only 2 nm.
Cambridge Core - Materials Science - Introduction to Graphene-Based Nanomaterials - by Luis E. F. Foa Torres
In recent years, carbon nanotube (CNT) emerged as one of the promising materials that shows various advantages over silicon material (e.g., aggressive channel length scaling due to absence of mobility degradation, variable bandgap with single material, ultra-thin body device that is possible due to smaller diameter [1-3nm], and compatibility of CNT with high-k materials resulting in high ION). Moreover, CNTs show both metallic and semiconducting properties; hence, by using metallic CNTs, interconnects can be realized to fabricate a circuit purely consisting of CNTs. This chapter will provide introduction to carbon nanotubes field effect transistor (CNTFETs) starting from material properties of carbon nanotubes and followed by how it can be used as semiconductor channel in field effect transistor (MOSFET) to form CNTFET. The different types of CNTFETs will be discussed based on the type of CNT used along with their advantages and disadvantages.
The unique electrostatic properties of semiconductor nanowires enable the realization of novel transistor types by the possibility to use surround gate architectures resembling ideal gate electrostatic control. Nevertheless one fundamental issue of semiconducting nanowire channels is the reliable control of doping to adjust the charge carrier concentration. Indeed, as dimensions scale down the surrounding media and the interfaces become more important. In this study we experimentally investigate the role of surface depletion and dielectric mismatch on the electronic charge transport of highly arsenic doped and bottom-up grown silicon nanowires. Electrical characterization of silicon nanowires (SiNWs) synthesized by Au catalyzed vapour-liquid-solid (VLS) growth and in-situ arsine (AsH3) doping is reported for the first time. We demonstrate that high n-type doping is possible by adjusting the dopant precursor flow ratio during growth. Based on electrical measurements of individual nanowires, reproducible donor concentrations of up to 5.2 × 10¹⁹ cm⁻³ could be revealed. By measuring the electrical characteristics for individual nanowires in dependence of their radius, we show that the electrically active carrier density drastically reduces for small nanowires at radii much larger than those at which quantization or dopant surface segregation effects are expected to occur. Furthermore, enhancement of the contact transparency for small radii nanowires is demonstrated through dopant segregation upon metal silicidation. Size dependent measurement of electrical characteristics revealed improved contact resistivities as low as 1.4 × 10⁻¹¹ Ωm².
The contact properties of van der Waals (vdW) layered semiconducting materials are not adequately understood, particularly for edge contact. Edge contact is extremely helpful in the case of graphene, for producing efficient contacts to vertical heterostructures as well as for improving the contact resistance through strong covalent bonding. Herein, we report on edge contacts to MoS2 of various thicknesses. The carrier-type conversion is robustly controlled by changing the flake thickness and metal workfunctions. Regarding the ambipolar behaviour, we suggest that the carrier injection is segregated in relatively thick MoS2 channel, i.e., electrons are in the uppermost layers, and holes are in the inner layers. Calculations reveal that the strength of the Fermi-level pinning (FLP) varies layer-by-layer, owing to the inhomogeneous carrier concentration, and particularly, there is negligible FLP in the inner layer, supporting the hole injection. The contact resistance is large despite the significantly reduced contact resistivity normalized by the contact area, which is attributed to the current-crowding effect arising from the narrow contact area.
The La-Zr-ZnO/n-Si(111) schottky heterojunction was prepared by sol-gel method. The results showed that the average grain size of La-Zr-ZnO films decreases by increasing Zr ion doping concentration. Based on XPS analysis, it is found that O1s have been composed to two peaks, and the surface defect oxygen of La-Zr-ZnO films decreases. By the PL testing, La-Zr-ZnO films have two excitation peaks at 377.78 nm and 389.17 nm, respectively. As for La-Zr-ZnO/n-Si (111) schottky heterojunction, barrier height increased and ideality factor decreased with the increasing of La ion and Zr ion doping, indicating that the rectifying characteristics of ZnO/n-Si (111) schottky contacts was enhanced due to ion co-doping. It can be explained that oxygen vacancies of La-Zr-ZnO films decreases due to the ion doping and weakens Fermi level pinning.
A given review describes models based on Wentzel-Kramers-Brillouin approximation, which are used to obtain I-V characteristics for ballistic CNTFETs with Schottky-Barrier (SB) contacts. The SB is supposed to be an exponentially, linearly, or parabolically decaying function along the channel. The probability of electrons to tunnel through the SB is obtained in the framework of both non-parabolic two-band and parabolic one-band approximations. The electron transmission probability for non-ballistic SB-CNTFETs is shortly discussed.
The band offsets occurring at the abrupt heterointerfaces of suitable material combinations offer a powerful design tool for high performance or even new kinds of devices. Due to a large variety of applications for metal-semiconductor heterostructures and the promise of low-dimensional systems to present exceptional device characteristics, nanowire heterostructures gained particular interest over the last decade. However, compared to those achieved by mature 2D processing techniques, quasi 1D heterostructures often suffer from low interface and crystalline quality. For GaAs-Au system, we demonstrate exemplarily a new approach to generate epitaxial and single crystalline metal-semiconductor nanowire heterostructures with atomically sharp interfaces using standard semiconductor processing techniques. Spatially resolved Raman measurements exclude any significant strain at the lattice mismatched metal-semiconductor heterojunction. Based on experimental results and simulation work, a novel self-assembled mechanism is demonstrated which yields one-step reconfiguration of a semiconductor-metal core-shell nanowire to a quasi 1D axially stacked heterostructure via flash lamp annealing. Transmission electron microscopy imaging and electrical characterization confirm the high interface quality resulting in the lowest Schottky barrier for the GaAs-Au system reported to date. Without limiting the generality, this novel approach will open up new opportunities in the syntheses of other metal-semiconductor nanowire heterostructures and thus facilitate the research of high-quality interfaces in metal-semiconductor nanocontacts.
Black phosphorus (BP) is a semiconducting material with a direct finite band gap in its monolayer, attracting intensive attention in the application of field-effect transistors. However, strong Fermi level pinning (FLP) has been observed for contacts between BP and high work function metals, e.g., Cu. Such FLP presents an undesirable huddle to prevent the achievement of high performance of field-effect devices. In this regard, there is a crucial need to understand the FLP occurring at the metal-BP interfaces and explore the possibility to reduce it. The present work studied atomic passivation in reducing FLP for the Cu-BP system using density functional theory calculations. The passivation by H, N, F, S, and Cl atoms on the Cu(111) surface has been considered. The results showed that the passivated atoms can shield direct contact between Cu(111) and BP, thus reducing FLP at Cu-BP interfaces. In particular, S and Cl atoms were found to be highly effective agents to achieve a significant reduction of FLP, leading to Cu-BP contacts with ultralow Schottky barrier height (SBH) and suggesting the possibility of ohmic contact formation. Our findings demonstrate surface passivation as an effective route towards depinning Fermi level at the metal-BP interface and subsequently the control of SBH for BP-based electronic devices.
The ability to tune the band-edge energies of bottom-up graphene nanoribbons (GNRs) via edge dopants creates new opportunities for designing tailor-made GNR heterojunctions and related nanoscale electronic devices. Here we report the local electronic characterization of type II GNR heterojunctions composed of two different nitrogen edge-doping configurations (carbazole and phenanthridine) that separately exhibit electron-donating and electron-withdrawing behavior. Atomically-resolved structural characterization of phenanthridine/carbazole GNR heterojunctions was performed using bond-resolved STM (BRSTM) and non-contact AFM (nc-AFM). Scanning tunneling spectroscopy (STS) and first-principles calculations reveal that carbazole and phenanthridine dopant configurations induce opposite upward and downward orbital energy shifts owing to their different electron affinities. The magnitude of the energy offsets observed in carbazole/phenanthridine heterojunctions is dependent on the length of the GNR segments comprising each heterojunction, with longer segments leading to larger heterojunction energy offsets. Using a new on-site energy analysis based on Wannier functions, we find that the origin of this behavior is a charge transfer process that reshapes the electrostatic potential profile over a long distance within the GNR heterojunction.
The dynamic spray-gun deposition method was developed in 2006 to fabricate field effect transistors based on random arrays of carbon nanotubes (CNTs) field effect transistors for gas sensing applications. Thanks to this deposition method, we were able to fabricate hundreds of operational devices in a reproducible way that were integrated in electronic chips. Following this first implementation, we decided to widen the application of the deposition technique to the field of Energy and specifically to the fabrication of supercapacitors. In this context, we demonstrated in 2012 the fabrication of nanostructured electrodes for supercapacitors, using mixtures of graphene/graphite and CNTs increasing the device capacitance and the power delivered of a factor 2.5 compared to CNT based electrochemical-double-layer-capacitors. Indeed, with high quality graphene we could reach a value of around 100 W Kg ⁻¹ . This value is extremely promising also considering that it has been obtained with an industrially suitable technique. This dynamic spray-gun deposition has been also exploited for the fabrication of resistance based random access memories, making use of thin layers of graphene oxide and of oxidized carbon nanofibers. In the first case, 5000 cycles of ‘write’ and ‘read’ phases were demonstrated. These results pave the way for the fabrication of very low cost memories that can be embedded in smart-cards, patches for health monitoring (e.g. diabetes), ID cards, RFID tags and more generally smart packaging. Finally we are also working on the utilization of this technique for the fabrication of layers for electro-magnetic interference shielding application. Thanks to a new machine with four nozzles, developed within the frame of the Graphene Flagship project, we are able to deposit four different nanomaterials at the same time or alternatively on a large surface (30 cm × 30 cm) creating specific nano-structuration and therefore ad hoc architectures allowing the smart absorption of specific frequencies (e.g. X-band). All these applications demonstrate the extreme versatility of this technique that constitutes a real breakthrough for exploiting the nanomaterials characteristics in real devices, using an industrial suitable fabrication method that can be implemented using roll-to-roll technique.
Carbon nanotubes (CNTs) have evolved into one of the most investigated nanostructures in the last decade for a wide range of applications. CNTs can be identified as helical microtubules of graphene sheets rolled around the chiral vector. The quasi-one-dimensional (1D) structure imparts to CNTs’ unique physical and chemical properties that have naturally led to their use in many nanoelectronic device applications. However, these properties of CNTs are determined by their synthesis methods, and this in turn determines their applicability. Their nanoscale size, unique structure, compositional elements, robustness, and immense surface area for functionalization are a few of the properties which give CNTs interesting prospects to be used in many varied applications. This chapter discusses the classification of CNTs based on their structural and electrical properties along with their fascinating applications in the development of biological, chemical and gas sensors, and field-effect transistors (FETs) and in integrated device fabrication such as memristors.
Carbon nanotubes are predicted to be metallic or semiconducting depending on their diameter and the helicity of the arrangement of graphitic rings in their walls. Scanning tunnelling microscopy (STM) offers the potential to probe this prediction, as it can resolve simultaneously both atomic structure and the electronic density of states. Previous STM studies of multi-walled nanotubes and single-walled nanotubes (SWNTs) have provided indications of differing structures and diameter-dependent electronic properties, but have not revealed any explicit relationship between structure and electronic properties. Here we report STM measurements of the atomic structure and electronic properties of SWNTs. We are able to resolve the hexagonal-ring structure of the walls, and show that the electronic properties do indeed depend on diameter and helicity. We find that the SWNT samples exhibit many different structures, with no one species dominating.
Contact resistances of multiwalled nanotubes deposited on gold contact fingers are very large. We show that the contact resistances decrease by orders of magnitudes when the contact areas are selectively exposed to the electron beam in a scanning electron microscope. The focused electron beam enables the selection of one particular nanotube for electrical measurement in a four-terminal configuration, even if a loose network of nanotubes is deposited on the gold electrodes. For all measured nanotubes, resistance values lie in a narrow range of 0.35–2.6 kΩ at room temperature. © 1998 American Institute of Physics.
Introduction of pentagon-heptagon pair defects into the hexagonal network of a single carbon nanotube can change the helicity of the tube and alter its electronic structure. Using a tight-binding method to calculate the electronic structure of such systems we show that they behave as nanoscale metal/semiconductor or semiconductor/semiconductor junctions. These junctions could be the building blocks of nanoscale electronic devices made entirely of carbon. Helicity has a profound effect on the electronic structure of carbon nanotubes [1,2]. All nonchiral, armchair ï¿¿n, nï¿¿ tubes are metals. Excepting those of very small radius [3], all ï¿¿n, mï¿¿ tubes with n 2 m a nonzero multiple of three are small gap semiconductors or semimetals [1]. The remaining tubes are semiconductors with band gaps roughly proportional to the reciprocal of the tube ra-dius [4]. Instead of comparing the electronic structures of tubes with different helicities, we consider changes in helic-ity within a single tube. The chirality of a tube can be changed by introducing topological defects into the hexagonal bond network [5]. The defects must induce zero net curvature to prevent the tube from flaring or closing. Minimal local curvature is desirable to mini-mize the defect energy. The smallest topological defect with minimal local curvature and zero net curvature is a pentagon-heptagon pair. A pentagon-heptagon defect pair with symmetry axis nonparallel to the tube axis changes the chirality of a nanotube by one unit from ï¿¿n, mï¿¿ to ï¿¿n 6 1, m 7 1ï¿¿. Figure 1 shows an (8,0) tube joined to a (7,1) tube. The highlighted atoms comprise the defect. We denote this structure by (8,0)/(7,1), in analogy with in-terfaces of bulk materials. Within a tight-binding model, far from the interface the (7,1) half tube is a semimetal and the (8,0) half tube is a moderate gap semiconductor. The full system forms a quasi-1D semiconductor/metal junction. Unlike most semiconductor/metal junctions [6], the (8,0)/(7,1) junction is composed of a single element. We use a tight-binding model with one p orbital per atom along with the surface Green function matching method (SGFM) [7] to calculate the local density of states (LDOS) in different regions of two archetypal ï¿¿n 1 , m 1 ï¿¿ï¿¿ï¿¿n 2 , m 2 ï¿¿ systems. In particular, we examine the (8,0)/(7,1) semiconductor/metal junction and the (8,0)/ (5,3) semiconductor/semiconductor junction formed with three heptagon-pentagon pairs. In both cases the unit cells of the perfect tubes match at the interface without the addition of extra atoms. The unit cells of the perfect (7,1) and (8,0) half tubes may be matched uniquely with a single pentagon-heptagon pair. The interface between the unit cells of the (8,0) and (5,3) half tubes contains three heptagons, three pentagons, and two hexagons. Two different matching orientations are possible: one with the two hexagons adjacent, the other without. We choose to study the configuration in which the hexagons are separated from each other. The sequence of n-fold rings around the circumference is then 6-7-5-6-7-5-7-5. In the tight-binding p-electron approximation [8], the (8,0) tube has a 1.2 eV gap [1] and the (7,1) tube is a semimetal. Within tight binding, these tubes form an
We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations—in our case a collapsed tube—can make them operate as field-effect transistors.
Carbon nanotubes have been regarded since their discovery1 as potential molecular quantum wires. In the case of multi-wall nanotubes, where many tubes are arranged in a coaxial fashion, the electrical properties of individual tubes have been shown to vary strongly from tube to tube2,3, and to be characterized by disorder and localization4. Single-wall nanotubes5,6 (SWNTs) have recently been obtained with high yields and structural uniformity7. Particular varieties of these highly symmetric structures have been predicted to be metallic, with electrical conduction occurring through only two electronic modes8-10. Because of the structural symmetry and stiffness of SWNTs, their molecular wavefunctions may extend over the entire tube. Here we report electrical transport measurements on individual single-wall nanotubes that confirm these theoretical predictions. We find that SWNTs indeed act as genuine quantum wires. Electrical conduction seems to occur through well separated, discrete electron states that are quantum-mechanically coherent over long distance, that is at least from contact to contact (140nm). Data in a magnetic field indicate shifting of these states due to the Zeeman effect.
It is argued that the absolute hydrostatic deformation potentials recently calculated for tetrahedral semiconductors with the linear muffin-tin-orbital method must be screened by the dielectric response of the material before using them to calculate electron-phonon interaction. This screening can be estimated by using the midpoint of an average dielectric gap evaluated at special (Baldereschi) points of the band structure. This dielectric midgap energy (DME) is related to the charge-neutrality point introduced by Tejedor and Flores, and also by Tersoff, to evaluate band offsets in heterojunctions and Schottky-barrier heights. We tabulate band offsets obtained with this method for several heterojunctions and compare them with other experimental and theoretical results. The DME’s are tabulated and compared with those of Tersoff’s charge-neutrality points.
At any metal-carbon nanotube interface there is charge transfer and the induced interfacial field determines the position of the carbon nanotube band structure relative to the metal Fermi-level. In the case of a single-wall carbon nanotube (SWNT) supported on a gold substrate, we show that the charge transfers induce a local electrostatic potential perturbation which gives rise to the observed Fermi-level shift in scanning tunneling spectroscopy (STS) measurements. We also discuss the relevance of this study to recent experiments on carbon nanotube transistors and argue that the Fermi-level alignment will be different for carbon nanotube transistors with low resistance and high resistance contacts. Comment: 4 pages, 3 ps figures, minor corrections, accepted by Phys. Rev. Lett
The level spectrum of a single-walled carbon nanotube rope, studied by transport spectroscopy, shows Zeeman splitting in a magnetic field parallel to the tube axis. The pattern of splittings implies that the spin of the ground state alternates by 1/2 as consecutive electrons are added. Other aspects of the Coulomb blockade characteristics, including the current-voltage traces and peak heights, also show corresponding even-odd effects. Comment: Preprint, pdf format only, 4 pages including figures
A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes
(SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission
electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces.
The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube,
represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important
role in the future fabrication of hybrid nanodevices.
The properties of metal-to-semiconductor junctions and of free semiconductor surfaces are usually explained on the basis of surface states. The theory of the metal contacts is discussed critically, because strictly speaking localized surface states cannot exist in such junctions. However, it is shown that virtual or resonance surface states can exist which behave for practical purposes in the same way. They are really the tails of the metal wave functions rather than separate states. In the past, the length of this tail has often been ignored. Some estimates of its length are made and its consequences pointed out. A semiquantitative discussion is given of various recent data, including the effect of an oxide layer on barrier height, the variation of barrier height with the metal, the work function of a free surface at high doping, and the effect of a cesium layer on the work function.
We calculate the properties of p-n junctions, n-i junctions, and Schottky barriers made on a single-wall carbon nanotube. In contrast to planar bulk junctions, the depletion width for nanotubes varies exponentially with inverse doping. In addition, there is a very long-range (logarithmic) tail in the charge distribution, extending over the entire tube. These effects can render traditional devices unworkable, while opening new possibilities for device design. Our general conclusions should apply to a broad class of nanotube heterojunctions, and to other quasi-one-dimensional ``molecular wire'' devices.
Simple physical considerations of local charge neutrality suggest that near a metal-semiconductor interface, the Fermi level in the semiconductor is pinned near an effective gap center, which is simply related to the bulk semiconductor band structure. In this way "canonical" Schottky barrier heights are calculated for several semiconductors. These are in excellent agreement with experiment for interfaces with a variety of metals.
Carbon nanotubes can be considered as single graphene sheets wrapped up into cylinders. Theoretical studies have shown that nanotubes can be either metallic or semiconducting, depending on minor differences in wrapping angle and diameter. We have obtained scanning tunneling microscopy and spectroscopy results on individual nanotubes which verify this prediction( J.W.G.Wildöer, L.C. Venema, A.G. Rinzler, R.E. Smalley, and C. Dekker, Nature, aimed for publication in Januari 1998). The combination of spectroscopy measurements and atomically resolved images allow to relate the electronic spectra to the wrapping angle and diameter. Tubes with various wrapping angles appear to be either metallic or semiconducting. Carbon nanotubes are expected to be one-dimensional conductors. Sharp peaks in the tunneling density of states that can be associated with the onsets of one-dimensional subbands are indeed observed. Furthermore, we are able to control the length of carbon nanotubes by STM nanostructuring( L.C. Venema, J.W.G. Wildöer, H.J. Temminck Tuinstra, A.G. Rinzler, R.E. Smalley and C. Dekker, Appl. Phys. Lett. (1997)). By applying voltage pulses to the STM tip above a nanotube, the tube can cut into a shorter section. In this way the electronic properties of nanotubes of various lengths can be investigated. footnotetext[0] email: venema@qt.tn.tudelft.nl Work done in collaboration with J.W.G. Wildöer, A.G. Rinzler, R.E. Smalley and C. Dekker.
The electronic structure of a jellium-Si interface is calculated using a jellium density corresponding to Al and self-consistent Si pseudopotentials. Local densities of states and charge densities are used to study states near the interface. At the Si surface, a high density of extra states is found in the energy range of the Si fundamental gap. These states are bulklike in jellium and decay into Si with a high concentration of charge in the dangling-bond free-surface-like Si state. Truly localized interface states are also found but at lower energies. The calculated barrier height is in excellent agreement with recent experimental results.
The metal-semiconductor interface is analysed by means of a simple method, which includes both the effects of virtual surface states and the many-electron interaction. This model is used to obtain the barrier height phi Bn for the junctions of Si (111) with Al and Na; the results are in good agreement with experimental data. The trend on going from covalent to increasingly ionic semiconductors is also studied for junctions of zincblende (110) compounds for different metals. The results do not show the transition from Barden-like to Schottky-like behaviour displayed by experimental data.
A tight-binding theory of semiconductor heterojunction band lineups is presented. Interface dipoles are shown to play a crucial role in determining lineups, so that lineups obtained by using the vacuum level as a reference (e.g., the electron affinity rule) are not reliable. Instead, the self-consistent lineup can be obtained approximately by aligning the average sp
3 hybrid energies in the respective semiconductors. Numerical results are provided and compared with experiment, and the approximations and accuracy in this approach are discussed. The application of these ideas to Schottky barriers is also considered.
A review is presented in which existing theories of the formation of Schottky barriers are analyzed. The list includes macroscopic dielectric approaches and various microscopic quantum mechanical treatments. The central role of interface states and their different physical origins are assessed. Simple concepts, able to predict general trends in barrier heights, are examined along with detailed microscopic theories applied to individual contacts.
Various models of Schottky-barrier formation suggest Fermi-level pinning in midgap. Elemen- tary band-structure considerations indicate that, for diamond-structure semiconductors, the physically relevant gap is the indirect gap, corrected for spin-orbit splitting. Schottky-barrier heights for elemental and III-V compound semiconductors can be predicted to 0.1 eV from measured indirect gaps and splittings. The dimensionless pinning strength S¯ is given by the optical dielectric constant. Chemical trends are thus simply explained.
A method based on a controlled solid-solid reaction was used to fabricate heterostructures between single-walled carbon nanotubes (SWCNTs) and nanorods or particles of silicon carbide and transition metal carbides. Characterization by high-resolution transmission electron microscopy and electron diffraction indicates that the heterostructures have well-defined crystalline interfaces. The SWCNT/carbide interface, with a nanometer-scale area defined by the cross section of a SWCNT bundle or of a single nanotube, represents the smallest heterojunction that can be achieved using carbon nanotubes, and it can be expected to play an important role in the future fabrication of hybrid nanodevices.
Electronic properties of heterojunctions between metallic and semiconducting single-wall carbon nanotubes are investigated. Ineffective screening of the long-range Coulomb interaction in one-dimensional nanotube systems drastically modifies the charge transfer phenomena compared to conventional semiconductor heterostructures. The length of depletion region varies over a wide range sensitively depending on the doping strength. The Schottky barrier gives rise to an asymmetry of the I-V characteristics of heterojunctions, in agreement with recent experimental results by Yao et al. and Fuhrer et al. Dynamic charge buildup near the junction results in a steplike growth of the current at reverse bias.
- J. Tersoff
- M. Cardona