Showing papers on "Schottky barrier published in 2012"

Graphene Barristor, a Triode Device with a Gate-Controlled Schottky Barrier

Abstract: Despite several years of research into graphene electronics, sufficient on/off current ratio I(on)/I(off) in graphene transistors with conventional device structures has been impossible to obtain. We report on a three-terminal active device, a graphene variable-barrier "barristor" (GB), in which the key is an atomically sharp interface between graphene and hydrogenated silicon. Large modulation on the device current (on/off ratio of 10(5)) is achieved by adjusting the gate voltage to control the graphene-silicon Schottky barrier. The absence of Fermi-level pinning at the interface allows the barrier's height to be tuned to 0.2 electron volt by adjusting graphene's work function, which results in large shifts of diode threshold voltages. Fabricating GBs on respective 150-mm wafers and combining complementary p- and n-type GBs, we demonstrate inverter and half-adder logic circuits.

High Efficiency Graphene Solar Cells by Chemical Doping

Abstract: We demonstrate single layer graphene/n-Si Schottky junction solar cells that under AM1.5 illumination exhibit a power conversion efficiency (PCE) of 8.6%. This performance, achieved by doping the graphene with bis(trifluoromethanesulfonyl)amide, exceeds the native (undoped) device performance by a factor of 4.5 and is the highest PCE reported for graphene-based solar cells to date. Current–voltage, capacitance–voltage, and external quantum efficiency measurements show the enhancement to be due to the doping-induced shift in the graphene chemical potential that increases the graphene carrier density (decreasing the cell series resistance) and increases the cell’s built-in potential (increasing the open circuit voltage) both of which improve the solar cell fill factor.

Channel Length Scaling of MoS2 MOSFETs

Abstract: In this article, we investigate electrical transport properties in ultrathin body (UTB) MoS2 two-dimensional (2D) crystals with channel lengths ranging from 2 μm down to 50 nm. We compare the short channel behavior of sets of MOSFETs with various channel thickness, and reveal the superior immunity to short channel effects of MoS2 transistors. We observe no obvious short channel effects on the device with 100 nm channel length (Lch) fabricated on a 5 nm thick MoS2 2D crystal even when using 300 nm thick SiO2 as gate dielectric, and has a current on/off ratio up to ∼109. We also observe the on-current saturation at short channel devices with continuous scaling due to the carrier velocity saturation. Also, we reveal the performance limit of short channel MoS2 transistors is dominated by the large contact resistance from the Schottky barrier between Ni and MoS2 interface, where a fully transparent contact is needed to achieve a high-performance short channel device.

Nature and Light Dependence of Bulk Recombination in Co-Pi-Catalyzed BiVO4 Photoanodes

Abstract: BiVO4 is considered to be a promising photoanode material for solar water splitting applications. Its performance is limited by two main factors: slow water oxidation kinetics and poor charge separation. We confirm recent reports that cobalt phosphate (Co-Pi) is an efficient water oxidation catalyst for BiVO4 and report an AM1.5 photocurrent of 1.7 mA/cm2 at 1.23 V vs RHE for 100 nm spray-deposited, compact, and undoped BiVO4 films with an optimized Co-Pi film thickness of 30 nm. The charge separation of these films depends strongly on light intensity, ranging from 90% at low light intensities to less than 20% at intensities corresponding to 1 sun. These observations indicate that the charge separation efficiency in BiVO4 is limited by poor electron transport and not by the presence of bulk defect states, interface traps, or the presence of a Schottky junction at the back-contact.

Device-Quality β-Ga2O3 Epitaxial Films Fabricated by Ozone Molecular Beam Epitaxy

Abstract: N-type Ga2O3 homoepitaxial thick films were grown on β-Ga2O3(010) substrates by ozone molecular beam epitaxy. The epitaxial growth rate was increased by more than ten times by changing from the (100) plane to the (010) plane. The carrier concentration of the epitaxial layers could be varied within the range of 1016–1019 cm-3 by changing the Sn doping concentration. Platinum Schottky barrier diodes (SBDs) on 1.4-µm-thick β-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited a reverse breakdown voltage of 100 V, an on-resistance of 2 mΩ cm2, and a forward voltage of 1.7 V (at 200 A/cm2).

Metal--Semiconductor Schottky Barrier Junctions and Their Applications

Abstract: 1. Physics of Schottky Barrier Junctions.- 1. Introduction.- 2. Origins of Barrier Height.- 2.1. Schottky-Mott Theory of Ideal Metal-Semiconductor Contact.- 2.2. Modifications to Schottky Theory.- 2.3. Classifications of Metal-Semiconductor Interfaces.- 2.4. Contacts on Reactive Interfaces.- 2.5. Contacts with Surface States and an Insulating Interfacial Layer.- 2.6. Contacts on Vacuum Cleaved Surfaces.- 3. Measurement of Barrier Height.- 3.1. Capacitance-Voltage Measurement.- 3.2. Current-Voltage Measurement.- 3.3. Photoelectric Measurement.- 4. Results of Barrier Height Measurements.- 4.1. Chemically Prepared Surfaces.- 4.2. Vacuum Cleaved Surfaces.- 4.3. Concluding Remarks.- 5. Capacitance-Voltage Characteristics.- 5.1. Electric Field and Potential Distribution in the Depletion Region.- 5.2. Depletion Region Capacitance.- 5.2.1. Ideal Schottky Barrier.- 5.2.2. Effect of Minority Carriers.- 5.2.3. Effect of Interfacial Layer.- 5.2.4. Effect of Deep Traps.- 6. Current-Voltage Characteristics.- 6.1. Transport Mechanisms.- 6.1.1. Diffusion and Thermionic Emission over the Barrier.- 6.1.2. Tunneling through the Barrier.- 6.1.3. Carrier Generation and Recombination in the Junction Depletion Region.- 6.1.4. Minority Carrier Injection.- 6.2. Forward Characteristics.- 6.3. Reverse Characteristics.- 7. Transient Behavior.- 8. Low-Resistance Schottky Barrier Contacts.- References.- 2. Interface Chemistry and Structure of Schottky Barrier Formation.- 1. Introduction.- 2. Perspectives on Schottky Barrier Formation.- 2.1. Introduction.- 2.2. Brief Review of Phenomenological Schottky Barrier Data.- 3. The Chemistry and Structure of the Interfacial Layer.- 3.1. Synopsis of the Layer-by-Layer Evolution.- 3.2. Some Techniques for Studying the Stages of Interface Formation.- 4. Evolution of the Interfacial Layer.- 4.1. Stage 0: The Clean Semiconductor Surface.- 4.1.1. Silicon (100) and (111) Surfaces.- 4.1.2. GaAs (110) and GaAs (100) Surfaces.- 4.2. Stage 1: The Dilute Limit (< 1/2 Monolayer).- 4.3. Stage 2: Monolayer Formation-Metal Film Nucleation.- 4.4. Stage 3: Additional Monolayers and Interdiffusion.- 4.5. Some Specific Characteristics of the Interfacial Layers.- 5. Formation of Interface States.- 5.1. Intrinsic Interface States Derived from the Metal and Semiconductor.- 5.2. Localized Defect and Impurity Related States.- 5.3. Interface States and the Stages of Interface Formation.- 6. Case Studies of the Chemistry and Structure of Schottky Barrier Formation.- 6.1. Case Studies of Silicon Schottky Barriers.- 6.1.1. Al, Ag, Cu, and Au Schottky Barriers.- 6.1.2. Silicide-Silicon Interfaces.- 6.2. Case Studies of III-V and II-VI Compound Semiconductor Schottky Barriers.- 6.2.1. The Ga-Al-As System.- 6.2.2. The GaAlAs Ternary System with Au Schottky Barriers.- 6.2.3. InP.- 6.2.4. Some II-VI Examples.- 7. Summary.- References.- 3. Fabrication and Characterization of Metal-Semiconductor Schottky Barrier Junctions.- 1. Introduction.- 2. Selection of Semiconductor Materials.- 3. Metal-Semiconductor Systems.- 3.1. Metal-Silicon Systems.- 3.2. Metal-GaAs Systems.- 3.3. Multilayer Metallization Systems.- 4. Design Considerations.- 5. Fabrication Technology.- 5.1. Surface Processing.- 5.2. Dielectric Film Deposition.- 5.3. Ohmic Contact Formation.- 5.4. Metal Deposition.- 5.5. Other Steps.- 6. Characterization.- References.- 4. Schottky-Barrier-Type Optoelectronic Structures.- 1. Introduction.- 2. Barrier Formation in Schottky-Barrier-Type Junctions.- 3. Transport in Schottky-Barrier-Type Structures.- 3.1. MS and MIS Structures.- 3.2. SIS Structures.- 4. Schottky-Barrier-Type Optoelectronic Structures.- 4.1. Schottky-Barrier-Type Light-Emitting Structures.- 4.2. Schottky-Barrier-Type Photodiodes.- 4.3. Schottky-Barrier-Type Photovoltaic Devices.- 4.3.1. MS and MIS Photovoltaic Devices.- 4.3.2. SIS Photovoltaic Devices.- 3. Summary.- References.- 5. Schottky Barrier Photodiodes.- 1. Introduction.- 2. General Parameters of Photodiodes.- 2.1. Signal-to-Noise Ratio (S/N).- 2.2. Noise Equivalent Power (NEP).- 2.3. Detectivity (D).- 2.4. Normalized Detectivity (D*).- 2.5. Detectivity Normalized Also with Respect to the Field of View(D**).- 2.6. Resistance Area Product.- 2.7. Response Time.- 3. Selection of Materials.- 3.1. Metal Systems.- 3.2. Semiconducting Materials.- 4. Fabrication Technology.- 5. Techniques for Evaluating Device Parameters.- 5.1. Current-Voltage Characteristics.- 5.2. Capacitance-Voltage Characteristics.- 5.3. Photoelectric Measurements.- 5.4. Electron Beam Induced Current Technique.- 6. Applications.- 7. Conclusions.- References.- 6. Microwave Schottky Barrier Diodes.- 1. Introduction.- 2. Diode Design Considerations.- 2.1. Equivalent Circuit.- 2.2. Frequency Conversion.- 2.3. Basic Mixer Diode RF Parameters.- 2.3.1. Conversion Loss Theory.- 2.3.2. Noise-Temperature Ratio.- 2.3.3. Overall Receiver Noise Figure.- 2.3.4. Mixer Noise Temperature.- 2.3.5. RF Impedance.- 2.3.6. IF Impedance.- 2.3.7. Receiver Sensitivity.- 2.3.8. Doppler Shift.- 2.3.9. Typical Doppler Radar System.- 2.4. Basic Detector RF Parameters.- 2.4.1. Video Resistance (Rv).- 2.4.2. Voltage Sensitivity.- 2.4.3. Current Sensitivity ?.- 2.4.4. Minimum Detectable Signal (MDS).- 2.4.5. Tangential Signal Sensitivity (TSS).- 2.4.6. Nominal Detectable Signal (NDS).- 2.4.7. Noise Equivalent Power (NEP).- 2.4.8. Video Bandwidth.- 2.4.9. Superheterodyne vs. Single Detection.- 2.5. Mixer Configurations.- 2.5.1. Single-Ended Mixer.- 2.5.2. Single-Balanced Mixer.- 2.5.3. Double-Balanced Mixer.- 2.5.4. Image Rejection Mixer.- 2.5.5. Image Enhanced or Image Recovery Mixer.- 3. Properties of Schottky Barrier Diodes.- 3.1. Diode Theory.- 3.2. DC Parameters.- 3.2.1. Junction Capacitance.- 3.2.2. Overlay Capacitance.- 3.2.3. Series Resistance.- 3.2.4. Figure of Merit.- 3.3. Semiconductor Materials.- 3.4. Epitaxial GaAs.- 3.5. Barrier Height Lowering.- 3.6. Fabrication.- 4. Microwave Performance.- 4.1. Mixer Diodes.- 4.2. Detector Diodes.- 5. RF Pulse and CW Burnout.- 5.1. Introduction.- 5.2. Factors Affecting RF Burnout.- 5.3. Experimental Results.- 5.4. Physical Analysis of RF Pulsed Silicon Schottky Barrier Failed Diodes.- 5.5. Physical Analysis of RF Pulsed Millimeter GaAs Schottky Barrier Failed Diodes.- 5.6. Electrostatic Failure of Silicon Schottky Barrier Diodes.- 6. Conclusions.- References.- 7. Metal-Semiconductor Field Effect Transistors.- 1. Introduction.- 2. Small-Signal FET Theory.- 3. Design Parameters of a Low-Noise Device.- 4. Practical Small-Signal FET Fabrication Techniques.- 4.1. Material Growth Techniques.- 4.2. FET Fabrication Technology.- 5. GaAs Power Field Effect Transistors.- 5.1. Principle of Power FET Operation.- 5.2. Thermal Impedance.- 5.3. Power FET Technology.- 6. Conclusions.- References.- 8. Schottky Barrier Gate Charge-Coupled Devices.- 1. Introduction.- 2. Schottky Gate CCDs.- 3. Potential-Charge Relationships.- 3.1. Surface Channel CCD.- 3.2. Bulk Channel CCD.- 3.3. Schottky Gate CCD.- 4. Charge Storage Capacity.- 4.1. Surface Channel CCD.- 4.2. Bulk Channel CCD.- 4.3. Schottky Gate CCD.- 5. Charge Transfer.- 5.1. Charge Transfer Efficiency.- 5.2. Charge Transfer Mechanisms.- 5.2.1. Surface Channel CCD.- 5.2.2. Bulk Channel CCD.- 5.2.3. Schottky Gate CCD.- 6. Input-Output Circuits.- 7. Schottky Gate Heterojunction CCDs.- 8. Experimental Results.- 8.1. High-Frequency Devices.- 8.2. Heterojunction Devices.- 9. Applications.- References.- 9. Schottky Barriers on Amorphous Si and their Applications.- 1. Introduction.- 2. Properties of Amorphous Si.- 2.1. Deposition Methods.- 2.2. Structural Properties.- 2.3. Electronic Properties.- 2.4. Surfaces.- 3. The Schottky Barrier on ?-Si:H.- 3.1. Current-Voltage Measurements.- 3.2. Capacitance Measurements.- 3.3 Internal Photoemission.- 4. Interface Kinetics and Its Effect on the Schottky Barrier.- 5. Applications.- 5.1. Drift Mobility.- 5.2. Deep Level Transient Spectroscopy.- 5.3. Solar Cells.- 5.4. Thin Film Transistors.- 6. Concluding Remarks.- References.

Schottky barrier height of Au on the transparent semiconducting oxide β-Ga2O3

Abstract: The Schottky barrier height of Au deposited on (100) surfaces of n-type β-Ga2O3 single crystals was determined by current-voltage characteristics and high-resolution photoemission spectroscopy resulting in a common effective value of 1.04 ± 0.08 eV. Furthermore, the electron affinity of β-Ga2O3 and the work function of Au were determined to be 4.00 ± 0.05 eV and 5.23 ± 0.05 eV, respectively, yielding a barrier height of 1.23 eV according to the Schottky-Mott rule. The reduction of the Schottky-Mott barrier to the effective value was ascribed to the image-force effect and the action of metal-induced gap states, whereas extrinsic influences could be avoided.

Schottky solar cells based on CsSnI3 thin-films

Abstract: We describe a Schottky solar cell based on the perovskite semiconductor CsSnI3 thin-film. The cell consists of a simple layer structure of indium-tin-oxide/CsSnI3/Au/Ti on glass substrate. The measured power conversion efficiency is 0.9%, which is limited by the series and shunt resistance. The influence of light intensity on open-circuit voltage and short-circuit current supports the Schottky solar cell model. Additionally, the spectrally resolved short-circuit current was measured, confirming the unintentionally doped CsSnI3 is of p-type characteristics. The CsSnI3 thin-film was synthesized by alternately depositing layers of SnCl2 and CsI on glass substrate followed by a thermal annealing process.

Layered semiconductor molybdenum disulfide nanomembrane based Schottky-barrier solar cells

Abstract: We demonstrate Schottky-barrier solar cells employing a stack of layer-structured semiconductor molybdenum disulfide (MoS2) nanomembranes, synthesized by the chemical-vapor-deposition method, as the critical photoactive layer An MoS2 nanomembrane forms a Schottky-barrier with a metal contact by the layer-transfer process onto an indium tin oxide (ITO) coated glass substrate Two vibrational modes in MoS2 nanomembranes, E12g (in-plane) and A1g (perpendicular-to-plane), were verified by Raman spectroscopy With a simple stacked structure of ITO–MoS2–Au, the fabricated solar cell demonstrates a photo-conversion efficiency of 07% for ∼110 nm MoS2 and 18% for ∼220 nm MoS2 The improvement is attributed to a substantial increase in photonic absorption The MoS2 nanomembrane exhibits efficient photo-absorption in the spectral region of 350–950 nm, as confirmed by the external quantum efficiency A sizable increase in MoS2 thickness results in only minor change in Mott–Schottky behavior, indicating that defect density is insensitive to nanomembrane thickness attributed to the dangling-bond-free layered structure

High-Efficiency Ferroelectric-Film Solar Cells with an n-type Cu2O Cathode Buffer Layer

Abstract: Becasue of the existence of interface Schottky barriers and depolarization electric field, ferroelectric films sandwiched between top and bottom electrodes are strongly expected to be used as a new kind of solar cells. However, the photocurrent with a typical order of μA/cm2 is too low to be practical. Here we demonstrate that the insertion of an n-type cuprous oxide (Cu2O) layer between the Pb(Zr,Ti)O3 (PZT) film and the cathode Pt contact in a ITO/PZT/Pt cell leads to the short-circuit photocurrent increasing 120-fold to 4.80 mA/cm2 and power conversion efficiency increasing of 72-fold to 0.57% under AM1.5G (100 mW/cm2) illumination. Ultraviolet photoemission spectroscopy and dark J–V characteristic show an ohmic contact on Pt/Cu2O, an n+–n heterojunction on Cu2O/PZT and a Schottky barrier on PZT/ITO, which provide a favorable energy level alignment for efficient electron-extraction on the cathode. Our work opens up a promising new method that has the potential for fulfilling cost-effective ferroelectri...

Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector

Abstract: time, and recovery speed of our UV detectors are 8 � 10 2 , 07s, and 05s, respectively, which are significantly improved compared to the conventional ZnO NWs photodetectors The improved performance is attributed to the existence of Schottky barriers between ZnO NW and graphene electrodes The graphene/ZnO NW/graphene vertical sandwiched structures may be promising candidates for integrated optoelectronic sensor devices V C 2012 American Institute of Physics [http://dxdoiorg/101063/14724208] ZnO, as a wide direct band gap (337eV) compound semiconductor with large exciton binding energy (60meV), has been widely investigated for its potential applications in optoelectronic devices, gas and chemical sensors 1,2 Due to large surface-to-volume ratio, ZnO nanowires (NWs) exhibit highly susceptible photoelectric properties by means of electron-hole generation or recombination during ultraviolet (UV) illumination Therefore, ZnO NWs have great potential in high sensitivity and fast-response UV sensors, 3 environmental monitors, and optical communications 4 Recently, Hu et al 5 reported ZnO NW based UV sensors using Schottky contact formed between ZnO and Pt electrode and the device performance such as the sensitive and UV response, is much higher than that of the traditional ZnO NW photoconductivity based UV sensors The UV detectors based on Schottky barriers formed between ZnO NW and other metal electrodes, such as gold electrodes, have also been studied 6,7 Nevertheless, metal electrodes are poor in transparency and can dramatically influence the absorption efficiency of the UV sensors Graphene, a monolayer sp 2 carbon atoms with unique physical properties, such as high mobility and conductivity, 8 high optical transparency 9 and mechanical flexibility, 10 etc, has attracted great research interest recently The high conductive and optical transparent properties make graphene an ideal candidate for the application in transparent electrode The Schottky barrier is also expected to be existed at the interface between ZnO nanowire and graphene, and it has been utilized for light-emitting diodes 11 and transparent nanogenerators 12 In this letter, we have fabricated a vertical sandwich structure of graphene/ZnO NW/graphene We demonstrate the high performance of our ZnO NW based vertical UV photodetector due to the existence of Schottky barriers between graphene electrodes and ZnO NW The current on-off ratio of the UV detector is up to 8 � 10 2 at a illumination power density of 50lw/lm 2 , the photocurrent

Piezo-phototronic effect enhanced visible and ultraviolet photodetection using a ZnO-CdS core-shell micro/nanowire.

Abstract: The piezo-phototronic effect is about the use of the piezoelectric potential created inside some materials for enhancing the charge carrier generation or separation at the metal-semiconductor contact or pn junction. In this paper, we demonstrate the impact of the piezo-phototronic effect on the photon sensitivity for a ZnO-CdS core-shell micro/nanowire based visible and UV sensor. CdS nanowire arrays were grown on the surface of a ZnO micro/nanowire to form a ZnO-CdS core-shell nanostructure by a facile hydrothermal method. With the two ends of a ZnO-CdS wire bonded on a polymer substrate, a flexible photodetector was fabricated, which is sensitive simultaneously to both green light (548 nm) and UV light (372 nm). Furthermore, the performance of the photon sensor is much enhanced by the strain-induced piezopotential in the ZnO core through modulation of the Schottky barrier heights at the source and drain contacts. This work demonstrates a new application of the piezotronic effect in photon detectors.

Waveguide based compact silicon Schottky photodetector with enhanced responsivity in the telecom spectral band

Abstract: We experimentally demonstrate an on-chip compact and simple to fabricate silicon Schottky photodetector for telecom wavelengths operating on the basis of internal photoemission process. The device is realized using CMOS compatible approach of local-oxidation of silicon, which enables the realization of the photodetector and low-loss bus photonic waveguide at the same fabrication step. The photodetector demonstrates enhanced internal responsivity of 12.5mA/W for operation wavelength of 1.55µm corresponding to an internal quantum efficiency of 1%, about two orders of magnitude higher than our previously demonstrated results [22]. We attribute this improved detection efficiency to the presence of surface roughness at the boundary between the materials forming the Schottky contact. The combination of enhanced quantum efficiency together with a simple fabrication process provides a promising platform for the realization of all silicon photodetectors and their integration with other nanophotonic and nanoplasmonic structures towards the construction of monolithic silicon opto-electronic circuitry on-chip.

Ultrahigh sensitive piezotronic strain sensors based on a ZnSnO3 nanowire/microwire.

Abstract: We demonstrated a flexible strain sensor based on ZnSnO3 nanowires/microwires for the first time. High-resolution transmission electron microscopy indicates that the ZnSnO3 belongs to a rhombohedral structure with an R3c space group and is grown along the [001] axis. On the basis of our experimental observation and theoretical calculation, the characteristic I–V curves of ZnSnO3 revealed that our strain sensors had ultrahigh sensitivity, which is attributed to the piezopotential-modulated change in Schottky barrier height (SBH), that is, the piezotronic effect. The on/off ratio of our device is ∼587, and a gauge factor of 3740 has been demonstrated, which is 19 times higher than that of Si and three times higher than those of carbon nanotubes and ZnO nanowires.

Fast-Switching GaN-Based Lateral Power Schottky Barrier Diodes With Low Onset Voltage and Strong Reverse Blocking

Abstract: GaN-based heterostructure lateral Schottky barrier diodes (SBDs) grown on n-SiC substrate are investigated in this letter. These SBDs own very low onset voltage VF = 0.43 V, high reverse blocking VBR >; 1000 V, very low capacitive charge of 0.213 nC/A, and a very fast recovery time of 10 ps. These unique qualities are achieved by combining lateral topology, GaN:C back-barrier epitaxial structure, fully recessed Schottky anode (φB = 0.43 eV), and slanted anode field plate in a robust and innovative process. Diode operation at elevated temperature up to 200 °C was also characterized.

Back-to-back Schottky diodes: the generalization of the diode theory in analysis and extraction of electrical parameters of nanodevices

Abstract: We report on the analysis of nonlinear current?voltage characteristics exhibited by a set of blocking metal/SnO2/metal. Schottky barrier heights in both interfaces were independently extracted and their dependence on the metal work function was analyzed. The disorder-induced interface states effectively pinned the Fermi level at the SnO2 surface, leading to the observed Schottky barriers. The model is useful for any two-terminal device which cannot be described by a conventional diode configuration.

High-Sensitivity p–n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots

Abstract: Chemically synthesized nanocrystal quantum dots (NQDs) are promising materials for applications in solution-processable optoelectronic devices such as light emitting diodes, photodetectors, and solar cells. Here, we fabricate and study two types of p-n junction photodiodes in which the photoactive p-layer is made from PbS NQDs while the transparent n-layer is fabricated from wide bandgap oxides (ZnO or TiO 2). By using a p-n junction architecture we are able to significantly reduce the dark current compared to earlier Schottky junction devices without reducing external quantum efficiency (EQE), which reaches values of up to ∼80%. The use of this device architecture also allows us to significantly reduce noise and obtain high detectivity (>10 12 cm Hz 1/2 W -1) extending to the near infrared past 1 μm. We observe that the spectral shape of the photoresponse exhibits a significant dependence on applied bias, and specifically, the EQE sharply increases around 500-600 nm at reverse biases greater than 1 V. We attribute this behavior to a "turn-on" of an additional contribution to the photocurrent due to electrons excited to the conduction band from the occupied mid-gap states.

Nanowire Piezo-Phototronic Photodetector: Theory and Experimental Design

Abstract: one-dimensional structures of these materials are ideal for fabricating strain-controlled piezo-phototronic devices. The strain applied to cause the deformation of the nanowires is mainly through shape change of the fl exible substrate that supports the device. Such devices can be the basis for active fl exible electronics, which uses the mechanical actuation from the substrate for inducing new electronic/optoelectronic effects. As the piezopotential is controlled by externally applied mechanical deformation with different orientation and magnitude, the piezo-phototronics effect can be combined with fl exible optoelectronics to promote new device functions. Previously, we have demonstrated the enhancement of the sensitivity of UV photodetector, [ 6 ] the response of photocells, [ 7 ] and the emission effi ciency of light emitting diodes. [ 8 ] In these reports, the coupling between piezoelectric effect and photoexcitation has been investigated experimentally. Theoretical calculation of the piezopotential along ZnO nanowires under different strain has been carried out, [ 9‐11 ] and a theoretical framework has been built for the two-way coupling between the piezoelectric effect and semiconductor transport properties. [ 12 ] Theoretical study for the three-way coupling in piezophototronics remains to be investigated. Constructing such a model will not only provide an in-depth understanding about the experimental results, but also explore the core phenomena and build high performance devices. Besides piezo-phototronic effect, other factors such as piezoresistance effect and change of contact area or contact condition can also affect the device performance. It is important to distinguish the contribution made by the piezo-phototronics effect from these other factors through theoretical analysis. In this paper, we have constructed a theoretical model and fabricated corresponding experimental devices to study the piezo-phototronic photodetectors based on single-Schottky and double-Schottky contacted metal‐semiconductor‐metal (MSM) structures. We have coupled the photoexcitation and piezoelectric terms into basic current equations to study their infl uence on the fi nal device performance. Theoretically predicted results have been quantitatively verifi ed by photodetectors based on CdS nanowires for visible light and ZnO nanowires for UV light. Our experimental results show that the piezo-phototronic effect dominates the performance of the photodetector rather than other experimental factors. It is shown that the piezophototronic effect is signifi cantly pronounced at low light intensities, which is important for extending the sensitivity and application range of the photodetector. The conclusions drawn on Schottky contacts present the core properties of the effect and can easily be extrapolated to other structures like p-n junctions. Finally, based on the theoretical model and experimental results, we have proposed three criteria for describing the contribution made by the piezo-phototronic effect to the performance of the photodetectors, which are useful for distinguishing this effect from other factors in governing the performance of the photodetector. The theoretical model for two-way coupling in piezotronics has been developed in a previous report. [ 12 ] Here we adopt the same assumptions and follow similar methods, as schematically shown in Figure 1 . The depletion approximation is assumed for the Schottky contact. Piezoelectric polarization is induced in a semiconductor nanowire when it is subjects to strain, it is reasonable to assume that the piezo charges are distributed in a layer in the depletion zone, which tune the Schottky barrier height. The formation of an inner potential will drive the free charge carriers to redistribute. If there is no external bias, the inner electric fi eld and net charges should only exist in the depletion zone at static or quasi-static state. The Schottky contact current equation will be used as the basic starting point. The infl uence of photoexcitation and piezo-charges on the material band structure will be discussed, and the fi nal coupled term will be integrated into the current transport equation. To give more intuitive perspective of the piezo-phototronic effect, we have also carried out numerical simulations. A one-dimensional model and other simplifi cations are adopted for easy understanding. The core equations and conclusions are shown in the anayltical model below. Current Density for a Forward Schottky Contact : For a piezophototronic photodetector, a measurement of the photoninduced current is an indication of photon intensity. The coupling effect of piezoelectricity and photon excitation is also

Self-powered and fast-speed photodetectors based on CdS:Ga nanoribbon/Au Schottky diodes

Abstract: Self-powered photodetectors based on CdS:Ga nanoribbons (NR)/Au Schottky barrier diodes (SBDs) were fabricated. The as-fabricated SBDs exhibit an excellent rectification characteristic with a rectification ratio up to 106 within ±1 V in the dark and a distinctive photovoltaic (PV) behavior under light illumination. Photoconductive analysis reveals that the SBDs were highly sensitive to light illumination with very good stability, reproducibility and fast response speeds at zero bias voltage. The corresponding rise/fall times of 95/290 μs represent the best values obtained for CdS based nano-photodetectors. It is expected that such self-powered high performance SBD photodetectors will have great potential applications in optoelectronic devices in the future.

Piezotronic effect on the transport properties of GaN nanobelts for active flexible electronics.

Abstract: The transport properties of GaN nanobelts (NBs) are tuned using a piezotronic effect when a compressive/tensile strain is applied on the GaN NB. This is mainly due to a change in Schottky barrier height (SBH). A theoretical model is proposed to explain the observed phenomenon.

Improved transfer of graphene for gated Schottky-junction, vertical, organic, field-effect transistors.

Abstract: An improved process for graphene transfer was used to demonstrate high performance graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces disorder and eliminates the polymeric residue that typically plagues transferred films. The method also allows for purposely creating pores in the graphene of a controlled areal density. Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene source electrode is attributed to modulation of the contact barrier height between the graphene and organic semiconductor due to a gate field induced Fermi level shift in the low density of electronic-states graphene electrode. Pores introduced in the graphene source electrode are shown to boost the G-VFET performance, which scales with the areal pore density taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 106 and 200 mA/cm2, respectivel...

Platinum Nanoparticles Supported on Anatase Titanium Dioxide as Highly Active Catalysts for Aerobic Oxidation under Visible Light Irradiation

Abstract: Visible light irradiation (λ >450 nm) of platinum (Pt) nanoparticles supported on anatase titanium dioxide (TiO2) promotes efficient aerobic oxidation at room temperature. This occurs via the electronic excitation of Pt particles by visible light followed by the transfer of their electrons to anatase conduction band. The positively charged Pt particles oxidize substrates, whereas the conduction band electrons are consumed by the reduction of molecular oxygen. The activity of this photocatalysis depends on the height of Schottky barrier and the number of perimeter Pt atoms created at the Pt/anatase heterojunction, which are affected by the amount of Pt loaded and the size of Pt particles. The catalyst loaded with 2 wt % Pt, containing 3–4 nm Pt particles, creates a relatively low Schottky barrier and a relatively large number of perimeter Pt atoms and, hence, facilitates smooth Pt→anatase electron transfer, resulting in very high photocatalytic activity. This catalyst is successfully activated by sunlight ...

Strain-Gated Piezotronic Transistors Based on Vertical Zinc Oxide Nanowires

Abstract: Strain-gated piezotronic transistors have been fabricated using vertically aligned ZnO nanowires (NWs), which were grown on GaN/sapphire substrates using a vapor-liquid-solid process. The gate electrode of the transistor is replaced by the internal crystal potential generated by strain, and the control over the transported current is at the interface between the nanowire and the top or bottom electrode. The current-voltage characteristics of the devices were studied using conductive atomic force microscopy, and the results show that the current flowing through the ZnO NWs can be tuned/gated by the mechanical force applied to the NWs. This phenomenon was attributed to the piezoelectric tuning of the Schottky barrier at the Au-ZnO junction, known as the piezotronic effect. Our study demonstrates the possibility of using Au droplet capped ZnO NWs as a transistor array for mapping local strain. More importantly, our design gives the possibility of fabricating an array of transistors using individual vertical nanowires that can be controlled independently by applying mechanical force/pressure over the top. Such a structure is likely to have important applications in high-resolution mapping of strain/force/pressure.

Ohmic contacts to n-type germanium with low specific contact resistivity

Abstract: A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (23 ± 18) × 10−7 Ω-cm2 for anneal temperatures of 340 °C The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge

Role of Carbon Nanotubes in Dye-Sensitized TiO2-Based Solar Cells

Abstract: Incorporation of low-dimensional carbon nanostructures such as carbon nanotubes (CNTs) and graphene sheets into the semiconductor electrodes is a common approach to improve the charge collection and photovoltaic performance of dye-sensitized solar cells. In this work, we clarify the role of CNTs in the semiconductor electrodes by investigating and comparing the electronic process in the dye-sensitized TiO2-based photovoltaic devices. The results show that the formed CNT–TiO2 Schottky junction plays a crucial role in the photovoltaic characteristics. According to the thermionic emission theory, the variation of the photocurrent over the voltage of the cells strongly depends on the height of the Schottky barrier. When the output voltage is low, the intrinsic one-dimensional carbon nanostructures can facilitate electron transport. With the voltage of the cell increasing, the energy dissipation on the Schottky junction increases dramatically and CNTs gradually lose the role of electron transport channels. At ...

Multifunctional devices and logic gates with undoped silicon nanowires.

Abstract: We report on the electronic transport properties of multiple-gate devices fabricated from undoped silicon nanowires. Understanding and control of the relevant transport mechanisms was achieved by means of local electrostatic gating and temperature-dependent measurements. The roles of the source/drain contacts and of the silicon channel could be independently evaluated and tuned. Wrap gates surrounding the silicide-silicon contact interfaces were proved to be effective in inducing a full suppression of the contact Schottky barriers, thereby enabling carrier injection down to liquid helium temperature. By independently tuning the effective Schottky barrier heights, a variety of reconfigurable device functionalities could be obtained. In particular, the same nanowire device could be configured to work as a Schottky barrier transistor, a Schottky diode, or a p-n diode with tunable polarities. This versatility was eventually exploited to realize a NAND logic gate with gain well above one.

Strain-Gated Piezotronic Transistors Based on Vertical Zinc Oxide

Abstract: Strain-gated piezotronic transistors have been fabricated using vertically aligned ZnO nanowires (NWs), which were grownon GaN/sapphire substrates using a vaporliquidsolid process. The gate electrode of the transistor is replaced by the internal crystal potential generated by strain, and the control over the transported current is at the interface between the nanowire and the top or bottom electrode. The currentvoltage characteristics of the devices were studied using conductive atomic force microscopy, and the results show that the current flowing through the ZnO NWs can be tuned/gated by the mechanical force applied to the NWs. This phenomenon was attributed to the piezoelectric tuning of the Schottky barrier at the AuZnO junction, known as the piezotronic effect. Our study demonstrates the possibility of using Au droplet capped ZnO NWs as a transistor array for mapping local strain. More importantly, our design gives the possibility of fabricating an array of transistors using individual vertical nanowires that can be controlled independently by applying mechanical force/pressure over the top. Such a structure is likely to have important applications in high- resolution mapping of strain/force/pressure.