Showing papers on "Schottky barrier published in 2014"

The physics and chemistry of the Schottky barrier height

Abstract: The formation of the Schottky barrier height (SBH) is a complex problem because of the dependence of the SBH on the atomic structure of the metal-semiconductor (MS) interface. Existing models of the SBH are too simple to realistically treat the chemistry exhibited at MS interfaces. This article points out, through examination of available experimental and theoretical results, that a comprehensive, quantum-mechanics-based picture of SBH formation can already be constructed, although no simple equations can emerge, which are applicable for all MS interfaces. Important concepts and principles in physics and chemistry that govern the formation of the SBH are described in detail, from which the experimental and theoretical results for individual MS interfaces can be understood. Strategies used and results obtained from recent investigations to systematically modify the SBH are also examined from the perspective of the physical and chemical principles of the MS interface.

Tunable Transport Gap in Phosphorene

Abstract: In this article, we experimentally demonstrate that the transport gap of phosphorene can be tuned monotonically from ∼0.3 to ∼1.0 eV when the flake thickness is scaled down from bulk to a single layer. As a consequence, the ON current, the OFF current, and the current ON/OFF ratios of phosphorene field effect transistors (FETs) were found to be significantly impacted by the layer thickness. The transport gap was determined from the transfer characteristics of phosphorene FETs using a robust technique that has not been reported before. The detailed mathematical model is also provided. By scaling the thickness of the gate oxide, we were also able to demonstrate enhanced ambipolar conduction in monolayer and few layer phosphorene FETs. The asymmetry of the electron and the hole current was found to be dependent on the layer thickness that can be explained by dynamic changes of the metal Fermi level with the energy band of phosphorene depending on the layer number. We also extracted the Schottky barrier heigh...

Defect-Dominated Doping and Contact Resistance in MoS2

Abstract: Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. We find that intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Furthermore, we show that MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. We identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resolution. We find that these variations in doping are defect-chemistry-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.

The Unusual Mechanism of Partial Fermi Level Pinning at Metal–MoS2 Interfaces

Abstract: Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2–6.1 eV. Except for the p-type Schottky contact with platinum, the Fermi levels in all of the studied metal–MoS2 complexes are situated above the midgap of MoS2. The mechanism of the Fermi level pinning at metal–MoS2 contact is shown to be unique for metal–2D-semiconductor interfaces, remarkably different from the well-known Bardeen pinning effect, metal-induced gap states, and defect/disorder induced gap states, which are applicable to traditional metal–semiconductor junctions. At metal–MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function modification by interface dipole formation due to the charge redistribution, and second by the production of gap states mainly of Mo d-orbitals character by the ...

High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors

Abstract: Trace chemical detection is important for a wide range of practical applications. Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of Schottky-contacted chemical vapor deposition grown monolayer MoS2 as high-performance room temperature chemical sensors. The Schottky-contacted MoS2 transistors show current changes by 2–3 orders of magnitude upon exposure to very low concentrations of NO2 and NH3. Specifically, the MoS2 sensors show clear detection of NO2 and NH3 down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS2 sensors. This study show...

Chloride molecular doping technique on 2D materials: WS2 and MoS2.

Abstract: Low-resistivity metal–semiconductor (M–S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kΩ·μm and 0.5 kΩ·μm, respectively. The significant reduction of the Rc is attributed to the achieved high electron-doping density, thus a significant reduction of Schottky barrier width. As a proof-of-concept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 μA/μm, an on/off ratio of 4 × 106, and a peak field-effect mobility of 60 cm2/(V·s). This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nanoelectronic devices.

MoS2 P-type Transistors and Diodes Enabled by High Work Function MoOx Contacts

Abstract: The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TM...

Device perspective for black phosphorus field-effect transistors: contact resistance, ambipolar behavior, and scaling.

Abstract: Although monolayer black phosphorus (BP), or phosphorene, has been successfully exfoliated and its optical properties have been explored, most of the electrical performance of the devices is demonstrated on few-layer phosphorene and ultrathin BP films. In this paper, we study the channel length scaling of ultrathin BP field-effect transistors (FETs) and discuss a scheme for using various contact metals to change the transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted with the transfer length method (TLM). With different contact metals, we find out that the metal/BP interface has different Schottky barrier heights, leading to a significant difference in contact resistance, which is quite different from previous studies of transition metal dichalcogenides (TMDs), such as MoS2, where the Fermi level is strongly pinned near the conduction band edge at the metal/MoS2 interface. The nature of BP transistors is Schottky barrier F...

High Mobility WSe2 p- and n-Type Field-Effect Transistors Contacted by Highly Doped Graphene for Low-Resistance Contacts

Abstract: We report the fabrication of both n-type and p-type WSe2 field-effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal–insulator transition at a characteristic conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107 at 170 K, and large electron and hole mobility of μ ≈ 200 cm2 V–1 s–1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ∼330 cm2 V–1 s–1 and that of holes to ∼270 cm2 V–1 s–1. We attribute our ability to observe the intrinsic, phonon-limited conduction in both the electron and hole channels to the drastic reduction of the Schottky barriers between the channel and the graphene contact electrodes using IL gating. We elucidate this process by studying a Schottky diode consisting of a single graphene/WSe2 Schottky junction. Our results indicate the possibility to utilize chemically or electrostatically high...

Development of gallium oxide power devices

Abstract: Gallium oxide (Ga2O3) is a strong contender for power electronic devices. The material possesses excellent properties such as a large bandgap of 4.7–4.9 eV for a high breakdown field of 8 MV cm−1. Low cost, high volume production of large single-crystal β-Ga2O3 substrates can be realized by melt-growth methods commonly adopted in the industry. High-quality n-type Ga2O3 epitaxial thin films with controllable carrier densities were obtained by ozone molecular beam epitaxy (MBE). We fabricated Ga2O3 metal-semiconductor field-effect transistors (MESFETs) and Schottky barrier diodes (SBDs) from single-crystal Ga2O3 substrates and MBE-grown epitaxial wafers. The MESFETs delivered excellent device performance including an off-state breakdown voltage (Vbr) of over 250 V, a low leakage current of only few μA mm−1, and a high drain current on/off ratio of about four orders of magnitude. The SBDs also showed good characteristics such as near-unity ideality factors and high reverse Vbr. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh-voltage power switching applications.

Ambipolar MoTe2 Transistors and Their Applications in Logic Circuits

Abstract: We report ambipolar charge transport in α-molybdenum ditelluride (MoTe2 ) flakes, whereby the temperature dependence of the electrical characteristics was systematically analyzed. The ambipolarity of the charge transport originated from the formation of Schottky barriers at the metal/MoTe2 contacts. The Schottky barrier heights as well as the current on/off ratio could be modified by modulating the electrostatic fields of the back-gate voltage (Vbg) and drain-source voltage (Vds). Using these ambipolar MoTe2 transistors we fabricated complementary inverters and amplifiers, demonstrating their feasibility for future digital and analog circuit applications.

Ambipolar phosphorene field effect transistor.

Abstract: In this article, we demonstrate enhanced electron and hole transport in few-layer phosphorene field effect transistors (FETs) using titanium as the source/drain contact electrode and 20 nm SiO2 as the back gate dielectric. The field effect mobility values were extracted to be ∼38 cm2/Vs for electrons and ∼172 cm2/Vs for the holes. On the basis of our experimental data, we also comprehensively discuss how the contact resistances arising due to the Schottky barriers at the source and the drain end effect the different regime of the device characteristics and ultimately limit the ON state performance. We also propose and implement a novel technique for extracting the transport gap as well as the Schottky barrier height at the metal–phosphorene contact interface from the ambipolar transfer characteristics of the phosphorene FETs. This robust technique is applicable to any ultrathin body semiconductor which demonstrates symmetric ambipolar conduction. Finally, we demonstrate a high gain, high noise margin, che...

Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in β-Ga2O3 solar-blind ultraviolet photodetectors

Abstract: β-Ga2O3 epitaxial thin films were deposited using laser molecular beam epitaxy technique and oxygen atmosphere in situ annealed in order to reduce the oxygen vacancy. Metal/semiconductor/metal structured photodetectors were fabricated using as-grown film and annealed film separately. Au/Ti electrodes were Ohmic contact with the as-grown films and Schottky contact with the annealed films. In compare with the Ohmic-type photodetector, the Schottky-type photodetector takes on lower dark current, higher photoresponse, and shorter switching time, which benefit from Schottky barrier controlling electron transport and the quantity of photogenerated carriers trapped by oxygen vacancy significant decreasing.

Device Perspective for Black Phosphorus Field-Effect Transistors: Contact Resistance, Ambipolar and Scaling

Abstract: Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With different contact metals, we find out that the metal/BP interface has different Schottky barrier heights, leading to a significant difference in contact resistance, which is quite different from previous studies of transition metal dichalcogenides (TMDs) such as MoS2 where Fermi-level is strongly pinned near conduction band edge at metal/MoS2 interface. The nature of BP transistors are Schottky barrier FETs, where the on and off states are controlled by tuning the Schottky barriers at the two contacts. We also observe the ambipolar characteristics of BP transistors with enhanced n-type drain current and demonstrate that the p-type carriers can be easily shifted to n-type or vice versus by controlling the gate bias and drain bias, showing the potential to realize BP CMOS logic circuits.

High-performance MoS2 transistors with low-resistance molybdenum contacts

Abstract: In this Letter, molybdenum (Mo) is introduced and evaluated as an alternative contact metal to atomically-thin molybdenum disulphide (MoS2), and high-performance field-effect transistors are experimentally demonstrated. In order to understand the physical nature of the interface and highlight the role of the various factors contributing to the Mo-MoS2 contacts, density functional theory (DFT) simulations are employed, which reveal that Mo can form high quality contact interface with monolayer MoS2 with zero tunnel barrier and zero Schottky barrier under source/drain contact, as well as an ultra-low Schottky barrier (0.1 eV) at source/drain-channel junction due to strong Fermi level pinning. In agreement with the DFT simulations, high mobility, high ON-current, and low contact resistance are experimentally demonstrated on both monolayer and multilayer MoS2 transistors using Mo contacts. The results obtained not only reveal the advantages of using Mo as a contact metal for MoS2 but also highlight the fact that the properties of contacts with 2-dimensional materials cannot be intuitively predicted by solely considering work function values and Schottky theory.

Switching mechanism in single-layer molybdenum disulfide transistors: an insight into current flow across Schottky barriers.

Abstract: In this article, we study the properties of metal contacts to single-layer molybdenum disulfide (MoS2) crystals, revealing the nature of switching mechanism in MoS2 transistors. On investigating transistor behavior as contact length changes, we find that the contact resistivity for metal/MoS2 junctions is defined by contact area instead of contact width. The minimum gate dependent transfer length is ∼0.63 μm in the on-state for metal (Ti) contacted single-layer MoS2. These results reveal that MoS2 transistors are Schottky barrier transistors, where the on/off states are switched by the tuning of the Schottky barriers at contacts. The effective barrier heights for source and drain barriers are primarily controlled by gate and drain biases, respectively. We discuss the drain induced barrier narrowing effect for short channel devices, which may reduce the influence of large contact resistance for MoS2 Schottky barrier transistors at the channel length scaling limit.

Ligand-exchange assisted formation of Au/TiO2 Schottky contact for visible-light photocatalysis.

Abstract: Plasmonic noble metal nanoparticles have emerged as a promising material in sensitizing wide-bandgap semiconductors for visible-light photocatalysis. Conventional methods in constructing such heterocatalysts suffer from either poor control over the size of the metal nanoparticles or inefficient charge transfer through the metal/semiconductor interface, which limit their photocatalytic activity. To solve this problem, in this work we construct Au/TiO2 photocatalysts by depositing presynthesized colloidal Au nanoparticles with well-controlled sizes to TiO2 nanocrystals and then removing capping ligands on the Au surface through a delicately designed ligand-exchange method, which leads to close Au/TiO2 Schottky contact after a mild annealing process. Benefiting from this unique synthesis strategy, the obtained photocatalysts show superior activity to conventionally prepared photocatalysts in dye decomposition and water-reduction hydrogen production under visible-light illumination. This study not only opens ...

Tuning On–Off Current Ratio and Field-Effect Mobility in a MoS2–Graphene Heterostructure via Schottky Barrier Modulation

Abstract: Field-effect transistor (FET) devices composed of a MoS2–graphene heterostructure can combine the advantages of high carrier mobility in graphene with the permanent band gap of MoS2 for digital applications. Herein, we investigate the electron transfer, photoluminescence, and gate-controlled carrier transport in such a heterostructure. We show that the junction is a Schottky barrier, whose height can be artificially controlled by gating or doping graphene. When the applied gate voltage (or the doping level) is zero, the photoexcited electron–hole pairs in monolayer MoS2 can be split by the heterojunction, significantly reducing the photoluminescence. By applying negative gate voltage (or p-doping) in graphene, the interlayer impedance formed between MoS2 and graphene exhibits an 100-fold increase. For the first time, we show that the gate-controlled interlayer Schottky impedance can be utilized to modulate carrier transport in graphene, significantly depleting the hole transport, but preserving the electr...

Schottky barrier heights for Au and Pd contacts to MoS2

Abstract: The search of a p-type metal contact on MoS2 has remained inconclusive, with high work function metals such as Au, Ni, and Pt showing n-type behavior and mixed reports of n as well as p-type behavior for Pd. In this work, we report quantitative Schottky barrier heights for Au and Pd contacts to MoS2 obtained by analysing low temperature transistor characteristics and contact resistance data obtained using the transfer length method. Both Au and Pd exhibit n-type behavior on multilayer as well as monolayer MoS2 transistors with Schottky barrier heights of 0.126 eV and 0.4 eV, and contact resistances of 42 Ω.mm and 18 × 104 Ω.mm respectively. Scanning photocurrent spectroscopy data is in agreement with the resulting energy band alignment in Au-MoS2-Pd devices further reinforcing the observation that the Fermi-level is pinned in the upper half of MoS2 bandgap.

Ideal Graphene/Silicon Schottky Junction Diodes

Abstract: The proper understanding of semiconductor devices begins at the metal–semiconductor interface. The metal/semiconductor interface itself can also be an important device, as Schottky junctions often forms when the doping in the semiconductors is low. Here, we extend the analysis of metal–silicon Schottky junctions by using graphene, an atomically thin semimetal. We show that a fundamentally new transport model is needed to describe the graphene–silicon Schottky junction. While the current–voltage behavior follows the celebrated ideal diode behavior, the details of the diode characteristics is best characterized by the Landauer transport formalism, suggesting that the injection rate from graphene ultimately determines the transport properties of this new Schottky junction.

GaAs/AlGaAs nanowire photodetector.

Abstract: We demonstrate an efficient core–shell GaAs/AlGaAs nanowire photodetector operating at room temperature. The design of this nanoscale detector is based on a type-I heterostructure combined with a metal–semiconductor–metal (MSM) radial architecture, in which built-in electric fields at the semiconductor heterointerface and at the metal/semiconductor Schottky contact promote photogenerated charge separation, enhancing photosensitivity. The spectral photoconductive response shows that the nanowire supports resonant optical modes in the near-infrared region, which lead to large photocurrent density in agreement with the predictions of electromagnetic and transport computational models. The single nanowire photodetector shows a remarkable peak photoresponsivity of 0.57 A/W, comparable to large-area planar GaAs photodetectors on the market, and a high detectivity of 7.2 × 1010 cm·Hz1/2/W at λ = 855 nm. This is promising for the design of a new generation of highly sensitive single nanowire photodetectors by con...

Ideal Graphene/Silicon Schottky Junction Diodes

Abstract: The proper understanding of semiconductor devices begins at the metal-semiconductor interface. The metal/semiconductor interface itself can also be an important device, as Schottky junctions often forms when the doping in the semiconductors is low. Here, we extend the analysis of metal-silicon Schottky junctions by using graphene, an atomically thin semimetal. We show that a fundamentally new transport model is needed to describe the graphene-silicon Schottky junction. While the current-voltage behavior follows the celebrated ideal diode behavior, the details of the diode characteristics is best characterized by the Landauer transport formalism, suggesting that the injection rate from graphene ultimately determines the transport properties of this new Schottky junction.

High-Performance Molybdenum Disulfide Field-Effect Transistors with Spin Tunnel Contacts

Abstract: Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nanoelectronic, optoelectronic, and spintronic applications. Here, we investigate the field-effect transistor behavior of MoS2 with ferromagnetic contacts to explore its potential for spintronics. In such devices, we elucidate that the presence of a large Schottky barrier resistance at the MoS2/ferromagnet interface is a major obstacle for the electrical spin injection and detection. We circumvent this problem by a reduction in the Schottky barrier height with the introduction of a thin TiO2 tunnel barrier between the ferromagnet and MoS2. This results in an enhancement of the transistor on-state current by 2 orders of magnitude and an increment in the field-effect mobility by a factor of 6. Our magnetoresistance calculation reveals that such integration of ferromagnetic tunnel contacts opens up the possibilities for MoS2-based spintronic devices.

Designing p-Type Semiconductor–Metal Hybrid Structures for Improved Photocatalysis†

Abstract: A practical strategy is proposed to facilitate the migration of holes in semiconductor (the low rate of which limits photocatalytic efficiency) by taking advantage of the Schottky barrier between p-type semiconductor and metal. A high work function is found to serve as an important selection rule for building such desirable Schottky junction between semiconductor surface facets and metal. The intrinsic charge spatial distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Importantly, the facets have a high work function, the same characteristic required for the formation of Schottky junction in a p-type semiconductor-metal hybrid structure. As a result, the semiconductor crystals in the hybrid design may be better enclosed by single facets with high work function, so as to synergize the two effects: Schottky barrier versus charge spatial separation.

Toward Low-Power Electronics: Tunneling Phenomena in Transition Metal Dichalcogenides

Abstract: In this article, we explore, experimentally, the impact of band-to-band tunneling on the electronic transport of double-gated WSe2 field-effect transistors (FETs) and Schottky barrier tunneling of holes in back-gated MoS2 FETs. We show that by scaling the flake thickness and the thickness of the gate oxide, the tunneling current can be increased by several orders of magnitude. We also perform numerical calculations based on Landauer formalism and WKB approximation to explain our experimental findings. Based on our simple model, we discuss the impact of band gap and effective mass on the band-to-band tunneling current and evaluate the performance limits for a set of dichalcogenides in the context of tunneling transistors for low-power applications. Our findings suggest that WTe2 is an excellent choice for tunneling field-effect transistors.

Electrical characteristics of multilayer MoS2 FET's with MoS2/graphene heterojunction contacts.

Abstract: The electrical properties of multilayer MoS2/graphene heterojunction transistors are investigated. Temperature-dependent I–V measurements indicate the concentration of unintentional donors in exfoliated MoS2 to be 3.57 × 1011 cm–2, while the ionized donor concentration is determined as 3.61 × 1010 cm–2. The temperature-dependent measurements also reveal two dominant donor levels, one at 0.27 eV below the conduction band and another located at 0.05 eV below the conduction band. The I–V characteristics are asymmetric with drain bias voltage and dependent on the junction used for the source or drain contact. I–V characteristics of the device are consistent with a long channel one-dimensional field-effect transistor model with Schottky contact. Utilizing devices, which have both graphene/MoS2 and Ti/MoS2 contacts, the Schottky barrier heights of both interfaces are measured. The charge transport mechanism in both junctions was determined to be either thermionic-field emission or field emission depending on bi...

Exfoliated multilayer MoTe2 field-effect transistors

Abstract: The properties of multilayer exfoliated MoTe2 field-effect transistors (FETs) on SiO2 were investigated for channel thicknesses from 6 to 44 monolayers (MLs). All transistors showed p-type conductivity at zero back-gate bias. For channel thicknesses of 8 ML or less, the transistors exhibited ambipolar characteristics. ON/OFF current ratio was greatest, 1 × 105, for the transistor with the thinnest channel, 6 ML. Devices showed a clear photoresponse to wavelengths between 510 and 1080 nm at room temperature. Temperature-dependent current-voltage measurements were performed on a FET with 30 layers of MoTe2. When the channel is turned-on and p-type, the temperature dependence is barrier-limited by the Au/Ti/MoTe2 contact with a hole activation energy of 0.13 eV. A long channel transistor model with Schottky barrier contacts is shown to be consistent with the common-source characteristics.

Near-Infrared Light Photovoltaic Detector Based on GaAs Nanocone Array/Monolayer Graphene Schottky Junction

Abstract: Near infrared light photodiodes have been attracting increasing research interest due to their wide application in various fields. In this study, the fabrication of a new n-type GaAs nanocone (GaAsNCs) array/monolayer graphene (MLG) Schottky junction is reported for NIR light detection. The NIR photodetector (NIRPD) shows obvious rectifying behavior with a turn-on voltage of 0.6 V. Further device analysis reveals that the photovoltaic NIRPDs are highly sensitive to 850 nm light illumination, with a fast response speed and good spectral selectivity at zero bias voltage. It is also revealed that the NIRPD is capable of monitoring high-switching frequency optical signals (∼2000 Hz) with a high relative balance. Theoretical simulations based on finite difference time domain (FDTD) analysis finds that the high device performance is partially associated with the optical property, which can trap most incident photons in an efficient way. It is expected that such a self-driven NIRPD will have potential application in future optoelectronic devices.

On the plasmonic photovoltaic.

Abstract: The conversion of sunlight into electricity by photovoltaics is currently a mature science and the foundation of a lucrative industry. In conventional excitonic solar cells, electron-hole pairs are generated by light absorption in a semiconductor and separated by the "built in" potential resulting from charge transfer accompanying Fermi-level equalization either at a p-n or a Schottky junction, followed by carrier collection at appropriate electrodes. Here we report a stable, wholly plasmonic photovoltaic device in which photon absorption and carrier generation take place exclusively in the plasmonic metal. The field established at a metal-semiconductor Schottky junction separates charges. The negative carriers are high-energy (hot) electrons produced immediately following the plasmon's dephasing. Some of the carriers are energetic enough to clear the Schottky barrier or quantum mechanically tunnel through it, thereby producing the output photocurrent. Short circuit photocurrent densities in the range 70-120 μA cm(-2) were obtained for simulated one-sun AM1.5 illumination with devices based on arrays of parallel gold nanorods, conformally coated with 10 nm TiO2 films and fashioned with a Ti metal collector. For the device with short circuit currents of 120 μA cm(-2), the internal quantum efficiency is ∼2.75%, and its wavelength response tracks the absorption spectrum of the transverse plasmon of the gold nanorods indicating that the absorbed photon-to-electron conversion process resulted exclusively in the Au, with the TiO2 playing a negligible role in charge carrier production. Devices fabricated with 50 nm TiO2 layers had open-circuit voltages as high as 210 mV, short circuit current densities of 26 μA cm(-2), and a fill factor of 0.3. For these devices, the TiO2 contributed a very small but measurable fraction of the charge carriers.