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Showing papers on "Schottky barrier published in 2016"


Journal ArticleDOI
TL;DR: In this article, a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices is presented, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy.
Abstract: This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt–growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal–semiconductor field-effect transistors, metal–oxide–semiconductor field-effect transistors and Schottky barrier diodes.

742 citations


Journal ArticleDOI
TL;DR: 2D metals are used, which are bounded with 2D semiconductors through van der Waals interactions, and the Schottky barrier at the metal-semiconductor junction becomes tunable and can vanish with proper 2D metals.
Abstract: Two-dimensional (2D) semiconductors have shown great potential for electronic and optoelectronic applications. However, their development is limited by a large Schottky barrier (SB) at the metal-semiconductor junction (MSJ), which is difficult to tune by using conventional metals because of the effect of strong Fermi level pinning (FLP). We show that this problem can be overcome by using 2D metals, which are bounded with 2D semiconductors through van der Waals (vdW) interactions. This success relies on a weak FLP at the vdW MSJ, which is attributed to the suppression of metal-induced gap states. Consequently, the SB becomes tunable and can vanish with proper 2D metals (for example, H-NbS2). This work not only offers new insights into the fundamental properties of heterojunctions but also uncovers the great potential of 2D metals for device applications.

514 citations


Journal ArticleDOI
TL;DR: Optoelectronic analysis reveals that the heterojunction device is virtually blind to light illumination with wavelength longer than 280 nm, but is highly sensitive to 254 nm light with very good stability and reproducibility.
Abstract: A deep UV light photodetector is assembled by coating multilayer graphene on beta-gallium oxide (β-Ga2 O3 ) wafer. Optoelectronic analysis reveals that the heterojunction device is virtually blind to light illumination with wavelength longer than 280 nm, but is highly sensitive to 254 nm light with very good stability and reproducibility.

491 citations


Journal ArticleDOI
TL;DR: Understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials is enhanced, and predictions for improving 2D electronics are given.
Abstract: Two-dimensional (2D) metal carbides and nitrides, called MXenes, have attracted great interest for applications such as energy storage. We demonstrate their potential as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. On the basis of first-principles calculations, we find that the surface chemistry strongly affects Fermi level of MXenes: O termination always increases the work function with respect to that of bare surface, OH always decreases it, whereas F exhibits either trend depending on the specific material. This phenomenon originates from the effect of surface dipoles, which together with the weak Fermi level pinning, enable Schottky-barrier-free hole (or electron) injection into 2D semiconductors through van der Waals junctions with some of the O-terminated (or all the OH-terminated) MXenes. Furthermore, we suggest synthetic routes to control surface terminations based on calculated formation energies. This study e...

386 citations


Journal ArticleDOI
TL;DR: High-performance MoS2 transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS1.
Abstract: High-performance MoS2 transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS2 . Benefiting from the ultrathin tunneling layer within 0.6 nm, the Schottky barrier is significantly reduced from 158 to 31 meV with small tunneling resistance.

380 citations


Journal ArticleDOI
TL;DR: A systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures, carrier densities, and contact dimensions finds that Au deposited in ultra-high vacuum yields three times lower RC than under normal conditions.
Abstract: The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (RC). Here we present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (1012 to 1013 cm–2), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (∼10–9 Torr) yields three times lower RC than under normal conditions, reaching 740 Ω·μm and specific contact resistivity 3 × 10–7 Ω·cm2, stable for over four months. Modeling reveals separate RC contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ∼35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the “14 nm” technology node.

369 citations


Journal ArticleDOI
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.

348 citations


Journal ArticleDOI
TL;DR: A Au/β-Ga2O3 nanowires array film vertical Schottky photodiode is successfully fabricated by a simple thermal partial oxidation process and exhibits a very low dark current of 10 pA at -30 V with a sharp cutoff at 270 nm, which is much quicker than any other previously reported β-Ga 2O3-based photodetectors.
Abstract: Because of the direct band gap of 4.9 eV, β-Ga2O3 has been considered as an ideal material for solar-blind photodetection without any bandgap tuning. Practical applications of the photodetectors require fast response speed, high signal-to-noise ratio, low energy consumption and low fabrication cost. Unfortunately, most reported β-Ga2O3-based photodetectors usually possess a relatively long response time. In addition, the β-Ga2O3 photodetectors based on bulk, the individual 1D nanostructure, and the film often suffer from the high cost, the low repeatability, and the relatively large dark current, respectively. In this paper, a Au/β-Ga2O3 nanowires array film vertical Schottky photodiode is successfully fabricated by a simple thermal partial oxidation process. The device exhibits a very low dark current of 10 pA at −30 V with a sharp cutoff at 270 nm. More interestingly, the 90–10% decay time of our device is only around 64 μs, which is much quicker than any other previously reported β-Ga2O3-based photodet...

309 citations


Journal ArticleDOI
TL;DR: In this paper, the temperature-dependent electrical properties of Pt/Ga2O3 Schottky barrier diodes (SBDs) fabricated on n−-Ga 2O3 drift layers grown on single-crystal n+-G 2O 3 (001) substrates by halide vapor phase epitaxy were investigated.
Abstract: We investigated the temperature-dependent electrical properties of Pt/Ga2O3 Schottky barrier diodes (SBDs) fabricated on n–-Ga2O3 drift layers grown on single-crystal n+-Ga2O3 (001) substrates by halide vapor phase epitaxy. In an operating temperature range from 21 °C to 200 °C, the Pt/Ga2O3 (001) Schottky contact exhibited a zero-bias barrier height of 1.09–1.15 eV with a constant near-unity ideality factor. The current–voltage characteristics of the SBDs were well-modeled by thermionic emission in the forward regime and thermionic field emission in the reverse regime over the entire temperature range.

251 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts.
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 2D MoS2-Sc interface and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts generally have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.

236 citations


Journal ArticleDOI
TL;DR: The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (Rc), as shown in this article, where the authors present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions.
Abstract: The scaling of transistors to sub-10 nm dimensions is strongly limited by their contact resistance (Rc). Here we present a systematic study of scaling MoS2 devices and contacts with varying electrode metals and controlled deposition conditions, over a wide range of temperatures (80 to 500 K), carrier densities (10^12 to 10^13 1/cm^2), and contact dimensions (20 to 500 nm). We uncover that Au deposited in ultra-high vacuum (~10^-9 Torr) yields three times lower Rc than under normal conditions, reaching 740 {\Omega}.{\mu}m and specific contact resistivity 3x10^-7 {\Omega}.cm2, stable for over four months. Modeling reveals separate Rc contributions from the Schottky barrier and the series access resistance, providing key insights on how to further improve scaling of MoS2 contacts and transistor dimensions. The contact transfer length is ~35 nm at 300 K, which is verified experimentally using devices with 20 nm contacts and 70 nm contact pitch (CP), equivalent to the "14 nm" technology node.

Journal ArticleDOI
TL;DR: In this article, deep level optical spectroscopy (DLOS) and deep level transient spectroscopic (DLTS) measurements performed on Ni/β-Ga2O3 Schottky diodes fabricated on unintentionally doped (010) substrates prepared by edge-defined film-fed growth revealed a rich spectrum of defect states throughout the 4.84
Abstract: Deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements performed on Ni/β-Ga2O3 Schottky diodes fabricated on unintentionally doped (010) substrates prepared by edge-defined film-fed growth revealed a rich spectrum of defect states throughout the 4.84 eV bandgap of β-Ga2O3. Five distinct defect states were detected at EC − 0.62 eV, 0.82 eV, 1.00 eV, 2.16 eV, and 4.40 eV. The EC − 0.82 eV and 4.40 eV levels are dominant, with concentrations on the order of 1016 cm−3. The three DLTS-detected traps at EC − 0.62 eV, 0.82 eV, and 1.00 eV are similar to traps reported in Czochralski-grown β-Ga2O3, [K. Irmscher et al., J. Appl. Phys. 110, 063720 (2011)], suggesting possibly common sources. The DLOS-detected states at EC − 2.16 eV and 4.40 eV exhibit significant lattice relaxation effects in their optical transitions associated with strongly bound defects. As a consequence of this study, the Ni/β-Ga2O3 (010) Schottky barrier height was determined to be 1.55 eV, with good c...

Journal ArticleDOI
Ting Yu1, Feng Wang1, Yang Xu1, Ma Lingling1, Xiaodong Pi1, Deren Yang1 
TL;DR: Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector that combines the electrical and optical contributions of Si QDs to enable a superior performance.
Abstract: Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector.

Journal ArticleDOI
TL;DR: In this paper, the contact properties of monolayer (ML) phosphorene with a series of commonly used metals in a transistor were investigated by using both ab initio electronic structure calculations and more reliable quantum transport simulations.
Abstract: Recently, phosphorene electronic and optoelectronic prototype devices have been fabricated with various metal electrodes. We systematically explore for the first time the contact properties of monolayer (ML) phosphorene with a series of commonly used metals in a transistor by using both ab initio electronic structure calculations and more reliable quantum transport simulations. ML phosphorene undergoes a metallization under the checked metals, and the metallized ML phosphorenes have an unnegligible coupling with channel ML phosphorene. ML phosphorene forms an n-type Schottky contact with Au, Cu, Cr, Al, and Ag electrodes and a p-type Schottky contact with Ti, Ni, and Pd electrodes upon inclusion of such a coupling. The calculated Schottky barrier heights are in good agreement with the available experimental data with Ni and Ti as electrodes. Our findings not only provide an insight into the ML phosphorene–metal interfaces but also help in ML phosphorene based device design.

Journal ArticleDOI
21 Oct 2016-Science
TL;DR: A Schottky-barrier indium-gallium-zinc-oxide thin-film transistor operating in the deep subthreshold regime at low supply voltages and ultralow power is reported, minimizes power consumption by operating near the off-state limit.
Abstract: The quest for low power becomes highly compelling in newly emerging application areas related to wearable devices in the Internet of Things. Here, we report on a Schottky-barrier indium-gallium-zinc-oxide thin-film transistor operating in the deep subthreshold regime (i.e., near the OFF state) at low supply voltages ( 400) that was both bias and geometry independent. The transistor reported here is useful for sensor interface circuits in wearable devices where high current sensitivity and ultralow power are vital for battery-less operation.

Journal ArticleDOI
TL;DR: A novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer is reported, and design principles for simultaneous optimization of built-in field, interface quality, and hole extraction are formulated to maximize the photvoltage of oxide-protected water-splitting anodes.
Abstract: Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal-insulator-semiconductor Schottky junction cells compared to buried junction p(+)n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

Journal ArticleDOI
TL;DR: It is experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast.
Abstract: Finding alternative optoelectronic mechanisms that overcome the limitations of conventional semiconductor devices is paramount for detecting and harvesting low-energy photons. A highly promising approach is to drive a current from the thermal energy added to the free-electron bath as a result of light absorption. Successful implementation of this strategy requires a broadband absorber where carriers interact among themselves more strongly than with phonons, as well as energy-selective contacts to extract the excess electronic heat. Here we show that graphene-WSe2-graphene heterostructure devices offer this possibility through the photo-thermionic effect: the absorbed photon energy in graphene is efficiently transferred to the electron bath leading to a thermalized hot carrier distribution. Carriers with energy higher than the Schottky barrier between graphene and WSe2 can be emitted over the barrier, thus creating photocurrent. We experimentally demonstrate that the photo-thermionic effect enables detection of sub-bandgap photons, while being size-scalable, electrically tunable, broadband and ultrafast. The detection of low-energy photons may be enabled by devices that make use of the excess thermal energy from photoexcited carriers as a result of light absorption. Here the authors demonstrate a vertical graphene-WSe2-graphene heterostructure that takes advantage of the photo-thermionic effect.

Journal ArticleDOI
TL;DR: In this article, the hole barrier between WSe2 and Ti2C(OH)xFy is estimated to be ≈ 0.23 eV and the electron barrier between the MoS2 band and Ti 2C(O(OH), which is a member of the MXene family, was determined to extract their Schottky barrier heights.
Abstract: Recently, MXenes, which are 2D early transition metal carbides and carbonitrides, have attracted wide attention because of their excellent conductivities. Here, the electrode applications of Ti2C(OH)xFy, one member of the MXene family, in WSe2 and MoS2 field effect transistors (FETs) are assessed. Kelvin probe force microscopy analysis is performed to determine its work function, which is estimated to be ≈4.98 eV. Devices based on WSe2/Ti2C(OH)xFy and MoS2/Ti2C(OH)xFy heterostructures are fabricated with the mechanical transfer method and their electronic performances evaluated. The temperature-dependent current–voltage transfer characteristics of the devices are determined to extract their Schottky barrier heights. The hole barrier between WSe2 and Ti2C(OH)xFy is estimated to be ≈0.23 eV and the electron barrier between the MoS2 band and Ti2C(OH)xFy is ≈0.19 eV, which indicates that the pinning effect occurs at the MoS2/Ti2C(OH)xFy interface but not at the WSe2/Ti2C(OH)xFy interface; this difference arises because of the difference between the band structures of WSe2 and MoS2. A complementary metal–oxide–semiconductor inverter based on these electrode properties of Ti2C(OH)xFy with MoS2 (n-channel) and WSe2 (p-channel) is fabricated, which demonstrates that Ti2C(OH)xFy is a promising electrode for future nanoelectronics applications.

Journal ArticleDOI
TL;DR: Graphene nanoribbons show promise for high-performance field-effect transistors, however they often suffer from short lengths and wide band gaps, so a bottom-up synthesis approach is used to fabricate 9- and 13-atom wide ribbons enabling short-channel transistors with 105 on-off current ratio.
Abstract: Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) devices with a thin, high-k gate dielectric and a 9-atom wide (0.95 nm) armchair GNR as the channel material, we demonstrate FETs with high on-current (Ion >1 uA at Vd = -1 V) and high Ion/Ioff ~10^5 at room temperature. We find that the performance of these devices is limited by tunnelling through the Schottky barrier (SB) at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high performance short-channel FETs with bottom-up synthesized armchair GNRs.

Journal ArticleDOI
TL;DR: It is reported that near-unity light absorption is possible in extremely thin (<15 nm) van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures.
Abstract: We demonstrate near-unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (<15 nm) van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency.

Journal ArticleDOI
TL;DR: In this paper, thin-film corundum-structured gallium oxide (α-Ga2O3) Schottky barrier diodes were fabricated by growing α-Ga 2O3 layers on sapphire substrates by the safe, low-cost, and energy-saving MIST EPITAXY® technique.
Abstract: Thin-film corundum-structured gallium oxide (α-Ga2O3) Schottky barrier diodes (SBDs) were fabricated by growing α-Ga2O3 layers on sapphire substrates by the safe, low-cost, and energy-saving MIST EPITAXY® technique, followed by lifting off the α-Ga2O3 layers from the substrates. The SBDs exhibited on-resistance and breakdown voltage of 0.1 mΩcm2 and 531 V (SBD1) or 0.4 mΩcm2 and 855 V (SBD2), respectively. These results will encourage the future evolution of low-cost and high-performance SBDs with α-Ga2O3.

Journal ArticleDOI
TL;DR: Schottky barrier heights of metal-MoS2 contacts to devices fabricated from CVD synthesized MoS2 films were reduced by inserting a thin tunneling Ta2O5 layer between MoS 2 and metal contacts to directly correlate with exponential reductions in contact resistance.
Abstract: Creating high-quality, low-resistance contacts is essential for the development of electronic applications using two-dimensional (2D) layered materials. Many previously reported methods for lowering the contact resistance rely on volatile chemistry that either oxidize or degrade in ambient air. Nearly all reported efforts have been conducted on only a few devices with mechanically exfoliated flakes which is not amenable to large scale manufacturing. In this work, Schottky barrier heights of metal-MoS2 contacts to devices fabricated from CVD synthesized MoS2 films were reduced by inserting a thin tunneling Ta2O5 layer between MoS2 and metal contacts. Schottky barrier height reductions directly correlate with exponential reductions in contact resistance. Over two hundred devices were tested and contact resistances extracted for large scale statistical analysis. As compared to metal-MoS2 Schottky contacts without an insulator layer, the specific contact resistivity has been lowered by up to 3 orders of magni...

Journal ArticleDOI
TL;DR: In this paper, a self-trapped hole formation near the Schottky barrier in reverse bias was found to produce photoconductive gain, which can explain the operation of a variety of β-Ga2O3 photodetectors.
Abstract: Solar-blind photodetection and photoconductive gain >50 corresponding to a responsivity >8 A/W were observed for β-Ga2O3 Schottky photodiodes. The origin of photoconductive gain was investigated. Current-voltage characteristics of the diodes did not indicate avalanche breakdown, which excludes carrier multiplication by impact ionization as the source for gain. However, photocapacitance measurements indicated a mechanism for hole localization for above-band gap illumination, suggesting self-trapped hole formation. Comparison of photoconductivity and photocapacitance spectra indicated that self-trapped hole formation coincides with the strong photoconductive gain. It is concluded that self-trapped hole formation near the Schottky diode lowers the effective Schottky barrier in reverse bias, producing photoconductive gain. Ascribing photoconductive gain to an inherent property like self-trapping of holes can explain the operation of a variety of β-Ga2O3 photodetectors.

Journal ArticleDOI
29 Nov 2016
TL;DR: In this article, the authors demonstrate tunable Schottky barrier height and record photo-responsivity in a new concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer.
Abstract: We demonstrate tunable Schottky barrier height and record photo-responsivity in a new-concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer. The original layout, where nano-sized graphene/Si heterojunctions alternate to graphene areas exposed to the electric field of the Si substrate, which acts both as diode cathode and transistor gate, results in a two-terminal barristor with single-bias control of the Schottky barrier. The nanotip patterning favors light absorption, and the enhancement of the electric field at the tip apex improves photo-charge separation and enables internal gain by impact ionization. These features render the device a photodetector with responsivity (3 A/W for white LED light at 3 mW/cm2 intensity) almost an order of magnitude higher than commercial photodiodes. We extensively characterize the voltage and the temperature dependence of the device parameters and prove that the multi-junction approach does not add extra-inhomogeneity to the Schottky barrier height distribution. This work represents a significant advance in the realization of graphene/Si Schottky devices for optoelectronic applications.

Journal ArticleDOI
TL;DR: In this article, a comparative study of the interfacial properties between monolayer/bilayer (ML/BL) WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts is presented.
Abstract: Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of two dimensional WSe2 devices. We present the first comparative study of the interfacial properties between monolayer/bilayer (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 the WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, the Pd contact has the smallest hole SBH. Dramatically, the Pt contact surpasses the Pd contact and becomes the p-type ohmic or quasi-ohmic contact with inclusion of the SOC. Therefore, p-type ohmic or quasi-ohmic contact exists in WSe2-metal interfaces. Our study provides a theoretical foundation for the selection of favorable metal electrodes in ML/BL WSe2 devices.

Journal ArticleDOI
TL;DR: A vertical field-effect transistor geometry based on a graphene/BP van der Waals heterostructure is presented and two distinct charge transport mechanisms are identified, which are dominant for different regimes of temperature and gate voltage.
Abstract: Black phosphorus (BP) has recently emerged as a promising narrow band gap layered semiconductor with optoelectronic properties that bridge the gap between semimetallic graphene and wide band gap transition metal dichalcogenides such as MoS2. To date, BP field-effect transistors have utilized a lateral geometry with in-plane transport dominating device characteristics. In contrast, we present here a vertical field-effect transistor geometry based on a graphene/BP van der Waals heterostructure. The resulting device characteristics include high on-state current densities (>1600 A/cm2) and current on/off ratios exceeding 800 at low temperature. Two distinct charge transport mechanisms are identified, which are dominant for different regimes of temperature and gate voltage. In particular, the Schottky barrier between graphene and BP determines charge transport at high temperatures and positive gate voltages, whereas tunneling dominates at low temperatures and negative gate voltages. These results elucidate out...

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate tunable Schottky barrier height and record photo-responsivity in a new concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer.
Abstract: We demonstrate tunable Schottky barrier height and record photo-responsivity in a new-concept device made of a single-layer CVD graphene transferred onto a matrix of nanotips patterned on n-type Si wafer. The original layout, where nano-sized graphene/Si heterojunctions alternate to graphene areas exposed to the electric field of the Si substrate, which acts both as diode cathode and transistor gate, results in a two-terminal barristor with single-bias control of the Schottky barrier. The nanotip patterning favors light absorption, and the enhancement of the electric field at the tip apex improves photo-charge separation and enables internal gain by impact ionization. These features render the device a photodetector with responsivity (3 A/W for white LED light at 3 mW/cm2 intensity) almost an order of magnitude higher than commercial photodiodes. We extensively characterize the voltage and the temperature dependence of the device parameters and prove that the multi-junction approach does not add extra-inhomogeneity to the Schottky barrier height distribution. This work represents a significant advance in the realization of graphene/Si Schottky devices for optoelectronic applications.

Journal ArticleDOI
TL;DR: Interestingly, GQD sensitized nanorod-based device demonstrated high EQE value of 13,161% at -1 V bias (wavelength ∼340 nm), which is eight times higher than pristine ZnO NR-based detector, which revealed high external quantum efficiency (EQE) more than 100%.
Abstract: Graphene quantum dot (GQD)-sensitized ZnO nanorods/poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) Schottky junction has been fabricated for visible-blind ultraviolet (UV) photodetector applications. Schottky diode parameters such as ideality factor, effective work function, and series resistance have been studied for GQD-modified and pristine ZnO nanorod-based devices. Under illumination of broadband light of intensity 80 mW/cm2, GQD-sensitized samples showed 11 times higher photocurrent value compared to pristine ZnO at −0.75 V external bias. GQD-modified detector demonstrated maximum photocurrent at UV region (wavelength ∼340 nm) for all reverse bias voltages. ZnO nanorods/polymer Schottky junction UV detectors revealed high external quantum efficiency (EQE) more than 100%. Interestingly, GQD sensitized nanorod-based device demonstrated high EQE value of 13,161% at −1 V bias (wavelength ∼340 nm), which is eight times higher than pristine ZnO NR-based detector. GQD-modified detectors a...

Journal ArticleDOI
TL;DR: A systematic comparison of the impact of selective doping with the TiO2 layer under the source-Drain metal relative to that on top of the MoS2 channel shows a larger benefit for transistor performance from the reduction in source-drain contact resistance.
Abstract: We demonstrate a low and constant effective Schottky barrier height (ΦB ∼ 40 meV) irrespective of the metal work function by introducing an ultrathin TiO2 ALD interfacial layer between various metals (Ti, Ni, Au, and Pd) and MoS2. Transmission line method devices with and without the contact TiO2 interfacial layer on the same MoS2 flake demonstrate reduced (24×) contact resistance (RC) in the presence of TiO2. The insertion of TiO2 at the source-drain contact interface results in significant improvement in the on-current and field effect mobility (up to 10×). The reduction in RC and ΦB has been explained through interfacial doping of MoS2 and validated by first-principles calculations, which indicate metallic behavior of the TiO2-MoS2 interface. Consistent with DFT results of interfacial doping, X-ray photoelectron spectroscopy (XPS) data also exhibit a 0.5 eV shift toward higher binding energies for Mo 3d and S 2p peaks in the presence of TiO2, indicating Fermi level movement toward the conduction band (...

Journal ArticleDOI
TL;DR: The important role of interfacial energetics played in photoelectrochemical conversion is identified and an unprecedentedly low onset potential of 0.9 V (vs RHE) is realized for water oxidation among n-Si photoanodes.
Abstract: The photoanodes with heterojunction behavior could enable the development of solar energy conversion, but their performance largely suffers from the poor charge separation and transport process through the multiple interfacial energy levels involved. The question is how to efficiently manipulate these energy levels. Taking the n-Si Schottky photoanode as a prototype, the undesired donor-like interfacial defects and its adverse effects on charge transfer in n-Si/ITO photoanode are well recognized and diminished through the treatment on electronic energy level. The obtained n-Si/TiOx/ITO Schottky junction exhibits a highly efficient charge transport and a barrier height of 0.95 eV, which is close to the theoretical optimum for n-Si/ITO Schottky contact. Then, the holes extraction can be further facilitated through the variation of surface energy level, with the NiOOH coated ITO layer. This is confirmed by a 115% increase in surface photovoltage of the photoanodes. Eventually, an unprecedentedly low onset po...