Showing papers on "Schottky barrier published in 2015"
TL;DR: In this article, the authors characterized fully encapsulated ultrathin (down to bilayer) black phosphorus field effect transistors fabricated under inert gas conditions by utilizing graphene as source-drain electrodes and boron nitride as an encapsulation layer.
Abstract: The presence of direct bandgap and high mobility in semiconductor few-layer black phosphorus offers an attractive prospect for using this material in future two-dimensional electronic devices. However, creation of barrier-free contacts which is necessary to achieve high performance in black phosphorus-based devices is challenging and currently limits their potential for applications. Here, we characterize fully encapsulated ultrathin (down to bilayer) black phosphorus field effect transistors fabricated under inert gas conditions by utilizing graphene as source–drain electrodes and boron nitride as an encapsulation layer. The observation of a linear ISD–VSD behavior with negligible temperature dependence shows that graphene electrodes lead to barrier-free contacts, solving the issue of Schottky barrier limited transport in the technologically relevant two-terminal field-effect transistor geometry. Such one-atom-thick conformal source–drain electrodes also enable the black phosphorus surface to be sealed, ...
450 citations
TL;DR: In this article, the structural and electronic properties of single-layer and bilayer phosphorene with graphene were studied and it was shown that both the properties of both the graphene and phosphorus are preserved in the composed heterostructure.
Abstract: In this Letter, we study the structural and electronic properties of single-layer and bilayer phosphorene with graphene. We show that both the properties of graphene and phosphorene are preserved in the composed heterostructure. We also show that via the application of a perpendicular electric field, it is possible to tune the position of the band structure of phosphorene with respect to that of graphene. This leads to control of the Schottky barrier height and doping of phosphorene, which are important features in the design of new devices based on van der Waals heterostructures.
446 citations
TL;DR: A review of the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications can be found in this article.
Abstract: In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.
409 citations
TL;DR: It is shown that the native oxide on the silicon presents a transport barrier for photogenerated holes and causes recombination current, which is responsible for causing the kink, and a simple semiconductor physics model is proposed that qualitatively captures the effect.
Abstract: The advent of chemical vapor deposition (CVD) grown graphene has allowed researchers to investigate large area graphene/n-silicon Schottky barrier solar cells. Using chemically doped graphene, efficiencies of nearly 10% can be achieved for devices without antireflective coatings. However, many devices reported in past literature often exhibit a distinctive s-shaped kink in the measured I/V curves under illumination resulting in poor fill factor. This behavior is especially prevalent for devices with pristine (not chemically doped) graphene but can be seen in some cases for doped graphene as well. In this work, we show that the native oxide on the silicon presents a transport barrier for photogenerated holes and causes recombination current, which is responsible for causing the kink. We experimentally verify our hypothesis and propose a simple semiconductor physics model that qualitatively captures the effect. Furthermore, we offer an additional optimization to graphene/n-silicon devices: by choosing the o...
384 citations
TL;DR: A new design is reported for synergizing the plasmonic effect with the Schottky junction by building two M–S interfaces based on the selection of semiconductor facets and metals that are synergized by Photocatalytic water splitting.
Abstract: DOI: 10.1002/adma.201501200 participate in oxidation reaction. As long as a semiconductor with appropriate bandgap (i.e., wide bandgap) is selected, the redox abilities of electrons or holes can be maintained as high as those in wide-bandgap semiconductors despite the use of incident visible light with relatively low energy. Unfortunately, the reported photocatalytic effi ciencies purely offered by the plasmonic hot carrier injection effect in the absence of semiconductor photoexcitation are negligible in contrast to those by semiconductor photoexcitation. [ 11,19 ] The major reason for this limitation is the lack of a driving force to steer the migration of injected electrons or holes to semiconductor surface for reduction or oxidation reactions. The low charge migration rates and uncertain charge diffusion directions make the charge carriers randomly walk in the semiconductor, so only a small portion of plasmonic hot carriers can arrive at the catalytic sites. We have thus decided to develop a new approach to better harness the utilization of plasmonic hot carriers. Thus far, use of a Schottky junction has been recognized as the most wellestablished strategy for steering the fl ow of the carriers that are photogenerated in semiconductor. It is well known that metal (especially for nonplasmonic metal, Pt and Pd) can serve as an sink for the photogenerated electrons or holes when forming a Schottky junction with n-type or p-type semiconductor, respectively (Figure S1, Supporting Information). [ 20,21 ] The formed Schottky barrier can inhibit the backfl ow of electrons or holes from metal to semiconductor. As a result, the charge “pump” role of the Schottky junction ensures the effi cient unidirectional transfer of charge carriers across the interface of metal– semiconductor (M–S) junction. Naturally we consider the possibility whether this Schottky-junction effect may be extended to the utilization of plasmonic hot carriers in photocatalysis through guiding their migration directions. However, this idea can be hardly accomplished by a single M–S junction between plasmonic metal and semiconductor. When a plasmonic metal is used for both the Schottky junction and hot carrier injection, the injection of plasmonic hot carriers would follow an opposite direction to the carrier fl ow driven by the Schottky junction (Figure S1, Supporting Information). [ 11,14,15,18,22 ] This severe competition dramatically reduces the effi ciency of carrier trapping on metal and e–h separation, particularly when metal and semiconductor are both excited under full-spectrum irradiation. In this communication, we report a new design for synergizing the plasmonic effect with the Schottky junction. The core concept of this work is to separate the Schottky junction from the plasmonic hot carrier injection by building two M–S interfaces based on the selection of semiconductor facets and metals. The functions of these two interfaces are synergized by Photocatalytic water splitting represents a highly important approach to addressing current energy and environmental demands. Photocatalysis requires effi cient separation of photo generated electron–hole (e–h) pairs in semiconductor to undergo redox reactions. [ 1 ] The reduction and oxidation capabilities of photogenerated electrons and holes in a semiconductor are determined by the positions of conduction band (CB) and valence band (VB) edges, respectively. Only when the CB edge lies at a higher position (more negative) than the redox potential of reduction half reaction, and meanwhile, the VB edge is at a lower position (more positive) than the potential of oxidation half reaction, can an overall photocatalytic reaction take place. [ 2 ] Thus wide-bandgap semiconductors with higher CB and lower VB edges generally show higher redox abilities as well as more promising photocatalytic performance in comparison with narrow-bandgap ones. However, semiconductors with wide bandgaps can only absorb light in the UV region which accounts for ≈5% of solar spectrum, thereby limiting their solar energy conversion effi ciency for practical applications. [ 3 ] For this reason, the relationship between absorption of long-wavelength light and high redox abilities of charge carriers is essentially an irreconcilable contradiction for bare semiconductor photocatalysts. Most recently, integration of surface plasmon into photocatalysis has been widely explored by composing hybrid structures between noble metals and semiconductors, which may potentially circumvent this situation. [ 4–10 ] As demonstrated by many research groups, [ 11–18 ] the metal with surface plasmon (e.g., Ag and Au) that directly contacts a semiconductor can be excited under visible light illumination to generate and inject hot carriers into the semiconductor. Specifi cally, hot electrons may fl ow into the CB of n-type semiconductor [ 13 ] and in the case of p-type semiconductor, instead hot holes are injected into the VB of semiconductor (Figure S1, Supporting Information). [ 16 ]
350 citations
TL;DR: It is shown that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector and shows multiple significant advances in overall performance for this class of devices.
Abstract: Atomically thin photodetectors based on 2D materials have attracted great interest due to their potential as highly energy-efficient integrated devices. However, photoinduced carrier generation in these media is relatively poor due to low optical absorption, limiting device performance. Current methods for overcoming this problem, such as reducing contact resistances or back gating, tend to increase dark current and suffer slow response times. Here, we realize the avalanche effect in a 2D material-based photodetector and show that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector. This is achieved by exploiting the large Schottky barrier formed between InSe and Al electrodes, enabling the application of a large bias voltage. Plasmonic enhancement of the photosensitivity, achieved by patterning arrays of Al nanodisks onto the InSe layer, further improves device efficiency. With an external quantum efficiency approaching 866%, a dark current in the pic...
247 citations
TL;DR: In this article, high-Hall-electron-mobility and high-performance Schottky barrier diodes for edge-defined β-Ga2O3 single crystals have been demonstrated.
Abstract: High-Hall-electron-mobility and high-performance Schottky barrier diodes for edge-defined fed-grown () β-Ga2O3 single crystals have been demonstrated. A high electron mobility of 886 cm2/(Vs) at 85 K was obtained. By theoretical specific scattering mechanisms, it was found that the electron mobility for >200 K is limited by optical phonon scattering and that for <100 K by ionized impurity scattering. On Schottky barrier diodes with Ni contacts, the current density for the forward voltage was 70.3 A/cm2 at 2.0 V, and a nearly ideal ideality factor of 1.01 was obtained.
236 citations
TL;DR: The experimental observation indicates that the conductivity of MoS2 devices can be actively modulated by the piezoelectric charge polarization-induced built-in electric field under strain variation, providing evidence for strain-gating monolayer MoS3 piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical–electronic nanodevices.
Abstract: High-performance piezoelectricity in monolayer semiconducting transition metal dichalcogenides is highly desirable for the development of nanosensors, piezotronics and photo-piezotransistors. Here we report the experimental study of the theoretically predicted piezoelectric effect in triangle monolayer MoS2 devices under isotropic mechanical deformation. The experimental observation indicates that the conductivity of MoS2 devices can be actively modulated by the piezoelectric charge polarization-induced built-in electric field under strain variation. These polarization charges alter the Schottky barrier height on both contacts, resulting in a barrier height increase with increasing compressive strain and decrease with increasing tensile strain. The underlying mechanism of strain-induced in-plane charge polarization is proposed and discussed using energy band diagrams. In addition, a new type of MoS2 strain/force sensor built using a monolayer MoS2 triangle is also demonstrated. Our results provide evidence for strain-gating monolayer MoS2 piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical-electronic nanodevices.
224 citations
TL;DR: In this paper, the authors reviewed the mechanism focusing on how Schottky barrier and SPR phenomena help to improve a photoreaction, as well as the paradox between the SBS and SPR in the matter of the direction of electron flow in the metal/semiconductor system.
222 citations
TL;DR: Multilayered graphene and single-layered graphene are assembled onto perovskite films in the form of Schottky junctions and ohmic contacts, respectively, for the production of a graphene-based hole transporting material-free perovSKite solar cell.
Abstract: Multilayered graphene and single-layered graphene are assembled onto perovskite films in the form of Schottky junctions and ohmic contacts, respectively, for the production of a graphene-based hole transporting material-free perovskite solar cell. Multilayered graphene extracts charge selectively and efficiently, delivering a higher efficiency of 11.5% than single-layered graphene (6.7%).
218 citations
TL;DR: In this article, a high power conversion efficiency of 13.7% with a device area of 0.8 cm2 has been achieved for organic-nanostructured Si hybrid solar cells by inserting a cesium carbonate (Cs2CO3) layer between Si and the rear electrode aluminium.
Abstract: A high doping technique has been widely used for record-efficiency crystalline silicon (Si) solar cells to minimize the series resistance losses and to form a back surface field. However, it requires high temperatures (up to 1000 °C) and involves toxic gases, which may not be compatible for hybrid organic–silicon solar cells. Here, we report that a high power conversion efficiency (PCE) of 13.7% with a device area of 0.8 cm2 has been achieved for organic-nanostructured Si hybrid solar cells by inserting a cesium carbonate (Cs2CO3) layer between Si and the rear electrode aluminium (Al), which is realized by a solution process under low-temperature annealing (<150 °C). Transient and constant current–voltage, capacitance–voltage, and scanning Kelvin probe microscope measurements are used to characterize the effect of the Cs2CO3 layer on the device performance. The insertion of Cs2CO3 not only decreased the contact resistance, but also generated a built-in electric field on the rear electrode. The recombination rates are suppressed at the back surface due to the deflection of minority carriers. These findings show a promising strategy to achieve high performance organic–silicon solar cells with a simple, low temperature and cost effective process.
TL;DR: A simple and inexpensive electrodeposition method is introduced to produce an efficient n-Si/SiOx/Co/ Co/CoOOH photoanode for the photoelectrochemical oxidation of water to oxygen.
Abstract: The photoelectrochemical splitting of water into hydrogen and oxygen requires a semiconductor to absorb light and generate electron-hole pairs, and a catalyst to enhance the kinetics of electron transfer between the semiconductor and solution. A crucial question is how this catalyst affects the band bending in the semiconductor, and, therefore, the photovoltage of the cell. We introduce a simple and inexpensive electrodeposition method to produce an efficient n-Si/SiOx/Co/CoOOH photoanode for the photoelectrochemical oxidation of water to oxygen. The photoanode functions as a solid-state, metal-insulator-semiconductor photovoltaic cell with spatially non-uniform barrier heights in series with a low overpotential water-splitting electrochemical cell. The barrier height is a function of the Co coverage; it increases from 0.74 eV for a thick, continuous film to 0.91 eV for a thin, inhomogeneous film that has not reached coalescence. The larger barrier height leads to a 360 mV photovoltage enhancement relative to a solid-state Schottky barrier.
TL;DR: Sub-bandgap photocurrent originating from the injection of hot electrons into MoS2 as well as photoamplification that yields a photogain of 10(5) results in a photoresponsivity of 5.2 A/W at 1070 nm, which is far above similar silicon-based hot electron photodetectors in which no photo amplification is present.
Abstract: Recently, there has been much interest in the extraction of hot electrons generated from surface plasmon decay, as this process can be used to achieve additional bandwidth for both photodetectors and photovoltaics. Hot electrons are typically injected into semiconductors over a Schottky barrier between the metal and semiconductor, enabling generation of photocurrent with below bandgap photon illumination. As a two-dimensional semiconductor single and few layer molybdenum disulfide (MoS2) has been demonstrated to exhibit internal photogain and therefore becomes an attractive hot electron acceptor. Here, we investigate hot electron-based photodetection in a device consisting of bilayer MoS2 integrated with a plasmonic antenna array. We demonstrate sub-bandgap photocurrent originating from the injection of hot electrons into MoS2 as well as photoamplification that yields a photogain of 105. The large photogain results in a photoresponsivity of 5.2 A/W at 1070 nm, which is far above similar silicon-based hot ...
IBM1
TL;DR: A single-walled carbon nanotube (SWNT) transistor technology with an end-bonded contact scheme that leads to size-independent contact resistance to overcome the scaling limits of conventional side- bonded or planar contact schemes is reported.
Abstract: Moving beyond the limits of silicon transistors requires both a high-performance channel and high-quality electrical contacts. Carbon nanotubes provide high-performance channels below 10 nanometers, but as with silicon, the increase in contact resistance with decreasing size becomes a major performance roadblock. We report a single-walled carbon nanotube (SWNT) transistor technology with an end-bonded contact scheme that leads to size-independent contact resistance to overcome the scaling limits of conventional side-bonded or planar contact schemes. A high-performance SWNT transistor was fabricated with a sub–10-nanometer contact length, showing a device resistance below 36 kilohms and on-current above 15 microampere per tube. The p-type end-bonded contact, formed through the reaction of molybdenum with the SWNT to form carbide, also exhibited no Schottky barrier. This strategy promises high-performance SWNT transistors, enabling future ultimately scaled device technologies.
Posted Content•
TL;DR: An ab initio quantum transport device simulation better reproduces the observed SBH in 2D MoS2-Sc interface and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study.
Abstract: Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in the two types of contacts and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.
TL;DR: In this article, the Schottky barrier for high work function (≳4.7 eV) metals was shown to be 0.1-0.3 eV below the conduction band edge of MoS 2 for low work function metals, due to metal-MoS 2 interaction.
Abstract: Making a metal contact to the two-dimensional semiconductor MoS 2 without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function (≳4.7 eV) metals, the Fermi level is pinned at 0.1–0.3 eV below the conduction band edge of MoS 2 for low work function metals, due to the metal-MoS 2 interaction. Inserting a boron nitride (BN) monolayer between the metal and the MoS 2 disrupts this interaction, and restores the MoS 2 electronic structure. Moreover, a BN layer decreases the metal work function of Co and Ni by ∼2 eV, and enables a lineup of the Fermi level with the MoS 2 conduction band. Surface modification by adsorbing a single BN layer is a practical method to attain vanishing Schottky barrier heights.
TL;DR: A new type of graphene/InAs nanowire (NW) vertically stacked heterojunction infrared photodetector is reported, with a large photoresponsivity and I(light)/I(dark) ratio, which are promising for the development of graphene electronic and optoelectronic applications.
Abstract: Graphene is a promising candidate material for high-speed and ultra-broadband photodetectors. However, graphene-based photodetectors suffer from low photoreponsivity and I(light)/I(dark) ratios due to their negligible-gap nature and small optical absorption. Here, a new type of graphene/InAs nanowire (NW) vertically stacked heterojunction infrared photodetector is reported, with a large photoresponsivity of 0.5 AW(-1) and I(light)/I(dark) ratio of 5 × 10(2), while the photoresponsivity and I(light)/I(dark) ratio of graphene infrared photodetectors are 0.1 mAW(-1) and 1, respectively. The Fermi level (E(F)) of graphene can be widely tuned by the gate voltage owing to its 2D nature. As a result, the back-gated bias can modulate the Schottky barrier (SB) height at the interface between graphene and InAs NWs. Simulations further demonstrate the rectification behavior of graphene/InAs NW heterojunctions and the tunable SB controls charge transport across the vertically stacked heterostructure. The results address key challenges for graphene-based infrared detectors, and are promising for the development of graphene electronic and optoelectronic applications.
TL;DR: The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.
Abstract: Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5–20 cm2/V·s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 105) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm2/V·s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.
TL;DR: In this article, a novel fuel cell device based on integrating the Schottky junction effect with the electrochemical principle is designed, constructed, and verified through experiments, and it is found that it is possible to use Schott...
Abstract: A novel fuel cell device based on integrating the Schottky junction effect with the electrochemical principle is designed, constructed, and verified through experiments. It is found that the Schott ...
TL;DR: In this article, the Schottky barrier of transition metal dichalcogenides MoSe2, MoTe2, WS2, WSe2 and WTe2 was predicted to suffer less from Fermi level pinning by chalcogene vacancies than does MoS2, because their vacancy formation energies are larger.
Abstract: It is predicted that Schottky barriers of the transition metal dichalcogenides MoSe2, MoTe2, WS2, WSe2, and WTe2 will suffer less from Fermi level pinning by chalcogen vacancies than does MoS2, because their vacancy formation energies are larger. The reduction in vacancy numbers will allow a greater degree of Schottky barrier height tuning by varying metal work function of the contacts in these compounds. The vacancy levels of WS2, WSe2 and MoSe2, and MoTe2 are also calculated to lie nearer midgap, so that ambipolar conduction will be easier in these compounds than in MoS2.
TL;DR: Graphene-MoS2 heterostructure exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths, and the peak in the photocurrent around zero gate voltage is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage.
Abstract: Lateral and vertical two-dimensional heterostructure devices, in particular graphene–MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene–MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which de...
TL;DR: In this article, the fundamental theories of piezotronics and piezo-phototronics, forming their basis for electromechancial devices, sensors and energy sciences, were derived starting from the basic equations for piezoelectricity, semiconductor and photoexcitation, and analytical equations for describing the strain-tuned device current.
TL;DR: A simple modelling approach is proposed to quantitatively describe the transfer characteristics of Schottky barrier-MOSFETs from ultra-thin body materials accurately in the device off-state and successfully applied the approach to extract Schotky barrier heights for electrons and holes in black phosphorus devices for a large range of body thicknesses.
Abstract: Owing to the difficulties associated with substitutional doping of low-dimensional nanomaterials, most field-effect transistors built from carbon nanotubes, two-dimensional crystals and other low-dimensional channels are Schottky barrier MOSFETs (metal-oxide-semiconductor field-effect transistors). The transmission through a Schottky barrier-MOSFET is dominated by the gate-dependent transmission through the Schottky barriers at the metal-to-channel interfaces. This makes the use of conventional transistor models highly inappropriate and has lead researchers in the past frequently to extract incorrect intrinsic properties, for example, mobility, for many novel nanomaterials. Here we propose a simple modelling approach to quantitatively describe the transfer characteristics of Schottky barrier-MOSFETs from ultra-thin body materials accurately in the device off-state. In particular, after validating the model through the analysis of a set of ultra-thin silicon field-effect transistor data, we have successfully applied our approach to extract Schottky barrier heights for electrons and holes in black phosphorus devices for a large range of body thicknesses.
TL;DR: The presented work clearly shows that PEDOT:PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.
Abstract: We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/PEDOT:PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that PEDOT:PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.
TL;DR: In this article, a comparative study of the interfacial properties between ML/BL WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts is presented, with the spin-orbital coupling (SOC) effects and quantum transport simulations.
Abstract: Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of 2D WSe2 devices. We present the first comparative study of the interfacial properties between ML/BL WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band calculations with inclusion of the spin-orbital coupling (SOC) effects and quantum transport simulations. The interlayer coupling tends to reduce both the electron and hole Schottky barrier heights (SBHs) and alters the polarity for WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, Pd contact has the smallest hole SBH with a value no less than 0.22 eV. Dramatically, Pt contact surpasses Pd contact and becomes p-type Ohmic or quasi-Ohmic contact with inclusion of the SOC. Our study provides a theoretical foundation for the selection of favorable metal electrodes in ML/BL WSe2 devices.
TL;DR: It is found that the SBHs without defects are quite strongly pinned, with a pinning factor S of about S = 0.3, a similar value for both top and edge contact geometries, despite the weak interlayer bonding in the isolated materials.
Abstract: The transition metal dichalcogenides (TMDs) are two-dimensional layered solids with van der Waals bonding between layers. We calculate their Schottky barrier heights (SBHs) using supercell models and density functional theory. It is found that the SBHs without defects are quite strongly pinned, with a pinning factor S of about S = 0.3, a similar value for both top and edge contact geometries. This arises because there is direct bonding between the contact metal atoms and the TMD chalcogen atoms, for both top and edge contact geometries, despite the weak interlayer bonding in the isolated materials. The Schottky barriers largely follow the metal induced gap state (MIGS) model, like those of three-dimensional semiconductors, despite the bonding in the TMDs being largely constrained within the layers. The pinning energies are found to be lower in the gap for edge contact geometries than for top contact geometries, which might be used to obtain p-type contacts on MoS2.
TL;DR: In this paper, the electronic characteristics of arsenene-graphene van der Waals (vdW) heterostructures were studied by using first-principles methods.
Abstract: The electronic characteristics of arsenene-graphene van der Waals (vdW) heterostructures are studied by using first-principles methods. The results show that a linear Dirac-like dispersion relation around the Fermi level can be quite well preserved in the vdW heterostructures. Moreover, the p-type Schottky barrier (0.18 eV) to n-type Schottky barrier (0.31 eV) transition occurs when the interlayer distance increases from 2.8 to 4.5 A, which indicates that the Schottky barrier can be tuned effectively by the interlayer distance in the vdW heterostructures.
TL;DR: In this article, first-principles calculations are performed to study the structural and electronic properties of hybrid graphene and phosphorene nanocomposites, and it is shown that weak van der Waals interactions dominate between graphene and phosphorus with their intrinsic electronic properties preserved.
Abstract: Combining the electronic structures of two-dimensional monolayers in ultrathin hybrid nanocomposites is expected to display new properties beyond their single components. Here, first-principles calculations are performed to study the structural and electronic properties of hybrid graphene and phosphorene nanocomposites. Our calculations show that weak van der Waals interactions dominate between graphene and phosphorene with their intrinsic electronic properties preserved. Furthermore, we found that as the interfacial distance decreases, the Dirac point of graphene moves from the conduction band to the valence band of phosphorene in hybrid graphene and phosphorene nanocomposites, inducing a transition from an n-type Schottky contact to a p-type Schottky contact at the graphene/phosphorene interface.
TL;DR: A chemical doping technique to achieve low contact resistance by keeping the intrinsic properties of few layers WS2 by using LiF doping in contact engineering of TMDs is reported.
Abstract: The development of low resistance contacts to 2D transition-metal dichalcogenides (TMDs) is still a big challenge for the future generation field effect transistors (FETs) and optoelectronic devices. Here, we report a chemical doping technique to achieve low contact resistance by keeping the intrinsic properties of few layers WS2. The transfer length method has been used to investigate the effect of chemical doping on contact resistance. After doping, the contact resistance (Rc) of multilayer (ML) WS2 has been reduced to 0.9 kΩ·μm. The significant reduction of the Rc is mainly due to the high electron doping density, thus a reduction in Schottky barrier height, which limits the device performance. The threshold voltage of ML-WS2 FETs confirms a negative shift upon the chemical doping, as further confirmed from the positions of E(1)2g and A1g peaks in Raman spectra. The n-doped samples possess a high drain current of 65 μA/μm, with an on/off ratio of 1.05 × 10(6) and a field effect mobility of 34.7 cm(2)/(V·s) at room temperature. Furthermore, the photoelectric properties of doped WS2 flakes were also measured under deep ultraviolet light. The potential of using LiF doping in contact engineering of TMDs opens new ways to improve the device performance.
TL;DR: Black phosphorus (BP) has been recently unveiled as a promising 2D direct bandgap semiconducting material and ambipolar field-effect transistor behavior of nanolayers of BP with ferromagnetic tunnel contacts is reported.
Abstract: Black phosphorus (BP) has been recently unveiled as a promising 2D direct bandgap semiconducting material Here, ambipolar field-effect transistor behavior of nanolayers of BP with ferromagnetic tunnel contacts is reported Using TiO2/Co contacts, a reduced Schottky barrier <50 meV, which can be tuned further by the gate voltage, is obtained Eminently, a good transistor performance is achieved in the devices discussed here, with drain current modulation of four to six orders of magnitude and a mobility of μh ≈ 155 cm2 V−1 s−1 for hole conduction at room temperature Magnetoresistance calculations using a spin diffusion model reveal that the source–drain contact resistances in the BP device can be tuned by gate voltage to an optimal range for injection and detection of spin-polarized holes The results of the study demonstrate the prospect of BP nanolayers for efficient nanoelectronic and spintronic devices