Schottky Barrier Modification Research Articles

Full-Inorganic Flexible Ag2S Memristor with Interface Resistance-Switching for Energy-Efficient Computing.

Flexible memristor-based neural network hardware is capable of implementing parallel computation within the memory units, thus holding great promise for fast and energy-efficient neuromorphic computing in flexible electronics. However, the current flexible memristor (FM) is mostly operated with a filamentary mechanism, which demands large energy consumption in both setting and computing. Herein, we report an Ag2S-based FM working with distinct interface resistance–switching (RS) mechanism. In direct contrast to conventional filamentary memristors, RS in this Ag2S device is facilitated by the space charge-induced Schottky barrier modification at the Ag/Ag2S interface, which can be achieved with the setting voltage below the threshold voltage required for filament formation. The memristor based on interface RS exhibits 105 endurance cycles and 104 s retention under bending condition, and multiple level conductive states with exceptional tunability and stability. Since interface RS does not require the formation of a continuous Ag filament via Ag+ ion reduction, it can achieve an ultralow switching energy of ∼0.2 fJ. Furthermore, a hardware-based image processing with a software-comparable computing accuracy is demonstrated using the flexible Ag2S memristor array. And the image processing with interface RS indeed consumes 2 orders of magnitude lower power than that with filamentary RS on the same hardware. This study demonstrates a new resistance–switching mechanism for energy-efficient flexible neural network hardware.

First-principles investigation on Schottky barrier modification of graphene/CdSe heterojunction by the interlayer distance

Based on the first-principles calculation method of quantum mechanics, we systematically studied the geometric structure and electronic properties of graphene/CdSe heterojunction, and considered the influence of vertical strain on its performance. The calculated results show that the model II has the smallest binding energy of −1.09 eV when the interlayer distance is 3.4 Å. Graphene opens a small band gap of 3 meV, which indicates that it has a good application prospect in future micro-nano devices. Differential charge density found that the electrons and holes at the interface gathered on the surface of CdSe and graphene, respectively. When the interlayer distance increases from 2.6 Å to 4.0 Å, the band gap decreases from 19 meV to 1 meV. In addition, the most stable heterojunction has n- and p-type Schottky barrier heights of 0.224 eV and 1.324 eV, respectively, thus the system forms n-type Schottky contact. As the interlayer distance increases, the n- and p-type Schottky barrier heights continuously decrease and increase, respectively. When the interlayer distance increases from 2.6 Å to 2.8 Å, the heterojunction changes from p-type Schottky contact to n-type Schottky contact, and when the interlayer distance reaches 4.2 Å, the heterojunction can realize the transformation from n-type Schottky contact to n-type Ohmic contact. These findings may have potential guiding significance for the design and preparation of graphene/CdSe heterojunction nanodevices.

High Performance Full-Inorganic Flexible Memristor with Combined Resistance-Switching.

Flexible memristors hold great promise for flexible electronics applications but are still lacking of good electrical performance together with mechanical flexibility. Herein, we demonstrate a full-inorganic nanoscale flexible memristor by using free-standing ductile α-Ag2S films as both a flexible substrate and a functional electrolyte. The device accesses dense multiple-level nonvolatile states with a record high 106 ON/OFF ratio. This exceptional memristor performance is induced by sequential processes of Schottky barrier modification at the contact interface and filament formation inside the electrolyte. In addition, it is crucial to ensure that the cathode junction, where Ag+ is reduced to Ag, dominates the total resistance and takes the most of setting bias before the filament formation. Our study provides a comprehensive insight into the resistance-switching mechanism in conductive-bridging memristors and offers a new strategy toward high performance flexible memristors.

Schottky barrier modification of GaSSe/graphene heterojunctions based on density functional theory

Constructing van der Waals heterojunctions is an effective way to solve the defects of two-dimensional (2D) material properties. The crystal structure and electronic properties of 2D monolayer GaSSe have been studied using first-principles calculation. Based on density functional theory calculations, we designed two different GaSSe/graphene heterojunctions and computed their electrical and interfacial properties. The calculated results confirm that graphene and GaSSe can maintain the original band structure after compounding, and the interaction between the two layers shows a weak van der Waals effect. The energy band gap of graphene can be opened to 25 meV when the GaSSe/graphene heterojunctions are formed. Graphene can be used as an electrode, and electrons will flow from the graphene layer to the GaSSe channel. Moreover, the interlayer distance and the applied electric field can be adjusted to modulate the Schottky barrier height and the Schottky contact type (n-type and p-type) of GaSSe/graphene heterojunctions. The two different heterojunctions can achieve Schottky contact-type conversion at a layer spacing of 3.00 and 2.70 Å, respectively. The position of the Dirac point of GaSSe/graphene heterojunctions will move with the change of the applied electric field. The Dirac point gradually moves towards the semiconductor valence band as the forward electric field of the heterojunctions increases. In addition, the n-type contact formed at the interface of the GaSSe/graphene heterojunctions will be converted into an Ohmic contact when the reverse electric field is about 0.5 eV Å−1. All the theoretical results clarify the fundamental properties of GaSSe/graphene heterojunctions and predict that the GaSSe/graphene heterojunctions can be used to design high-performance field effect transistor devices.

Leakage Current Reduction in β-Ga2O3 Schottky Barrier Diodes by CF4 Plasma Treatment

This letter reports on the suppression of reverse leakage current (Ir) in β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Schottky barrier diodes (SBDs) through Schottky barrier modification by a low power CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -plasma treatment prior to Schottky metal deposition. Revealed by an X-ray photoelectron spectroscopy (XPS) analysis, the fluorine-plasma treatment brought an incorporation of fluorine ions and depletion of silicon donors in the near surface region of the β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and thus raised its surface potential by around 0.14 eV. Furthermore, insulating GaFx was likely created at the Schottky interface. Attributed to the fluorine-plasma modified Schottky barrier, a reduced Ir by around four orders of magnitude and enhanced blocking voltage (Vblock) from 150 V to 470 V at Ir = 100μ A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> have been achieved without degrading the forward characteristics. Different from the untreated device whose Ir was purely governed by the thermionic field emission (TFE), the fluorine-plasma-treated SBD showed a greatly suppressed TFE-current until a space-chargelimited current (SCLC) started to dominate at around -500 V.

Plasmon-Induced Heterointerface Thinning for Schottky Barrier Modification of Core/Shell SiC/SiO2 Nanowires.

In this work, plasmon-induced heterointerface thinning for Schottky barrier modification of core/shell SiC/SiO2 nanowires is conducted by femtosecond (fs) laser irradiation. The incident energy of polarized fs laser (50 fs, 800 nm) is confined in the SiO2 shell of the nanowire due to strong plasmonic localization in the region of the electrode-nanowire junction. With intense nonlinear absorption in SiO2, the thickness of the SiO2 layer can be thinned in a controllable way. The tuning of the SiO2 barrier layer allows the promotion of electron transportation at the electrode-nanowire interface. The switching voltage of the rectifying junction made by the SiC/SiO2 nanowire can be significantly tuned from 15.7 to 1 V. When selectively thinning at source and drain electrodes and leaving the SiO2 barrier layer at the gate electrode intact, a metal/oxide/semiconductor (MOS) device is fabricated with low leakage current. This optically controlled interfacial engineering technology should be applicable for MOS components and other heterogeneous integration structures.

Fully Aerosol-Jet Printed, High-Performance Nanoporous ZnO Ultraviolet Photodetectors

We report fully aerosol-jet printed ultraviolet photodetectors based on zinc oxide nanocrystal networks having a porous morphology. The developed photodetectors exhibit selective optical photoresponse in the ultraviolet spectrum at wavelengths ranging from 250 nm up to 400 nm. These devices show a high ON/OFF ratio of ∼106 and short response times. The characteristic time constants for the rise and fall edges were measured to be ∼0.4 and ∼1.3 s, respectively. In this work, we propose a direct approach for aerosol-jet printing of presynthesized nanocrystals to overcome the limitations of high post-annealing temperatures and tedious fabrication methods to obtain high aspect ratio nanostructures. The high performance characteristics of these devices are attributed to Schottky barrier modification under the influence of oxygen, which is enhanced by the porosity of the semiconductor material. The random orientation of the crystals aids the formation of air traps in the network, thereby enhancing the surface ar...

Interface Engineering of Organic Schottky Barrier Solar Cells and Its Application in Enhancing Performances of Planar Heterojunction Solar Cells.

In this work, we describe the performance of organic Schottky barrier solar cells with the structure of ITO/molybdenum oxide (MoOx)/boron subphthalocyanine chloride (SubPc)/bathophenanthroline (BPhen)/Al. The SubPc-based Schottky barrier solar cells exhibited a short-circuit current density (Jsc) of 2.59 mA/cm2, an open-circuit voltage (Voc) of 1.06 V, and a power conversion efficiency (PCE) of 0.82% under simulated AM1.5 G solar illumination at 100 mW/cm2. Device performance was substantially enhanced by simply inserting thin organic hole transport material into the interface of MoOx and SubPc. The optimized devices realized a 180% increase in PCE of 2.30% and a peak Voc as high as 1.45 V was observed. We found that the improvement is due to the exciton and electron blocking effect of the interlayer and its thickness plays a vital role in balancing charge separation and suppressing quenching effect. Moreover, applying such interface engineering into MoOx/SubPc/C60 based planar heterojunction cells substantially enhanced the PCE of the device by 44%, from 3.48% to 5.03%. Finally, we also investigated the requirements of the interface material for Schottky barrier modification.

Large and robust resistive switching in co-sputtered Pt-(NiO-Al2O3)-Pt devices

We have systematically investigated the resistive switching and electroresistance behavior in Pt-[NiO-Al2O3]-Pt (PNAP) capacitor-like structures. The PNAP devices show a large ON-OFF ratio (∼107), which is strongly dependent on the rate of the voltage sweep. Interestingly, the devices exhibit a robust electroresistance behavior in the high resistance OFF state and show an intriguing change of sign of rectification with increasing end voltage. Our direct measurement of the surface temperature of the sample during resistive switching indicates that RESET process is assisted by Joule heating effects. The results are explained on the basis of plausible interplay between Schottky barrier modification due to the trapped charge carriers at the metal–oxide interface and percolation effects of conducting nanofilaments.

Compliance current dependence of conversion between bipolar, unipolar, and threshold resistance switching in Mn3O4 films

We report deterministic conversion between bipolar, unipolar and threshold resistance switching in Pt/Mn3O4/Pt memory devices via tuning compliance current. The conversion between bipolar and unipolar switching is reversible, while that between memory switching and threshold switching is irreversible. The nonvolatile bipolar resistance switching behaviors could be attributed to modification of Schottky barrier at Pt/Mn3O4 interface due to the migration of positively charged oxygen vacancies. With the increase of current, the incomplete filament formed in the set operation of bipolar switching could continue to grow and until completely form. The subsequent rupture and formation of filament consisting of oxygen vacancies under electric field are responsible for the unipolar resistance switching. Further increase of compliance current causes the volatile threshold switching behavior in the Pt/Mn3O4/Pt devices, which could be originated from formation and rupture of filament consisting of Mn ions due to the high Joule heat generated by large current.

Layer-dependent Band Alignment and Work Function of Few-Layer Phosphorene

Using first-principles calculations, we study the electronic properties of few-layer phosphorene focusing on layer-dependent behavior of band gap, work function band alignment and carrier effective mass. It is found that few-layer phosphorene shows a robust direct band gap character, and its band gap decreases with the number of layers following a power law. The work function decreases rapidly from monolayer (5.16 eV) to trilayer (4.56 eV), and then slowly upon further increasing the layer number. Compared to monolayer phosphorene, there is a drastic decrease of hole effective mass along the ridge (zigzag) direction for bilayer phosphorene, indicating a strong interlayer coupling and screening effect. Our study suggests that 1). Few-layer phosphorene with a layer-dependent band gap and a robust direct band gap character is promising for efficient solar energy harvest. 2). Few-layer phosphorene outperforms monolayer counterpart in terms of a lighter carrier effective mass, a higher carrier density and a weaker scattering due to enhanced screening. 3). The layer-dependent band edges and work functions of few-layer phosphorene allow for modification of Schottky barrier with enhanced carrier injection efficiency. It is expected that few-layer phosphorene will present abundant opportunities for a plethora of new electronic applications.

Structural analysis of SiC Schottky diodes failure mechanism under current overload

1.2 kV–10 A tungsten Schottky diodes (W-SBD) have been aged and tested at limit under current overload (surge current pulses) to determine their structural weakest spots. All devices showed no ageing at 40 A amplitudes and a surge current capability higher than 60 A. Infrared lock-in measurements have located the weakest spots on the surface of failed chips and allowed us to non-invasively infer their origin: Schottky barrier modification by metal contact change. After, a focused ion beam coupled with scanning electron microscope has been used to analyse the physical signature at these locations. These inspections have revealed that the destruction mechanism responsible for their failure was the electromigration and thermomigration of tungsten into aluminum, locally modifying the electrical behaviour of the Schottky barrier (loss of blocking capability).

The effects of switching time and SrTiO3−xNy nanostructures on the operation of Al/SrTiO3−xNy/Al memristors

The nanostructure and switching properties of the Al/SrTiO3−xNy/Al memristors have been investigated. It was found that the formation of stacking faults defects in the perovskite structure during plasma nitridation facilitates resistivity switching in Al/SrTiO3−xNy/Al memristors. From thermal emission measurements we observed local heating of the Al/SrTiO3−xNy anode interface (285°C) during switching when the interface becomes rectified due to the formation of locally undoped region. A model for resistivity switching and parameters affecting the memristor operation is discussed considering the electromigration theory. The I–V curves are analyzed taking into account the tunnel barrier formations and an inhomogeneous Schottky barrier modification at the anode interface.

Origin of Schottky Barrier Modification by Hydrogen on Diamond

The origin of the Schottky barrier modification on diamond (100) surfaces by hydrogen is studied based on simplified interface models with the first-principles calculation. It is revealed that the estimated S-factor is rather negative for the ideal contact with no interfacial dangling bonds, while it approaches 1 when the interface is terminated by just hydrogen. The negative S-factor is thought to come from the formation of interfacial chemical bonds between the metal and the diamond. It is suggested that hydrogen-termination prevents such interfacial bond formation and makes the general metal induced gap state theory work, as well as passivates the interfacial dangling bond states which are the strong sources of the pinning of the Schottky barrier height.

Measurements of current-voltage-induced heating in the Al/SrTiO3−xNy/Al memristor during electroformation and resistance switching

Heating of the Al/SrTiO3−xNy/Al memristor is characterized during electroformation and switching of the resistances. The electrode with the higher voltage potential is heated to higher temperatures than the electrode with the lower potential, suggesting a reversible (nonstable) displacement of the anions in a low voltage region (|V|&amp;lt;±3 V). Application of a threshold voltage appropriate for resistance switching (|V|≥±3 V) facilitates migration of anions to the anode interface and increases the local anode temperature to a maximum of 285 °C. The hysteretic I-V curves are discussed taking into account tunnel barrier formation/break and inhomogeneous Schottky barrier modification at the anode interface.

Fermi Level Unpinning and Schottky Barrier Modification by Ti, Sc and V Incorporation at NiSi2/Si Interface

A new method is proposed to modify the Schottky barrier height (SBH) for nickel silicide/Si contact. Chemical and electrical properties for NiSi2/Si interface with titanium, scandium and vanadium incorporation are investigated by first-principles calculations. The metal/semiconductor interface states within the gap region are greatly decreased, which is related to the diminutions of junction leakage when Ti-cap is experimentally used in nickel silicide/Si contact process. It leads to an unpinning metal/semiconductor interface. The SBH obeys the Schottky-Mott theory. Compared to Ti substitution, the SBH for electrons is reduced for scandium and increases for vanadium.

Schottky barrier modification and electrical characterization of low energy He-ion bombardment induced defects in n- and p-type GaAs

Schottky barrier height modification (BHM) resulting from 0.5 keV He-ion bombardment of n- and p-GaAs (doping ∼10 16 cm −3) is investigated for doses between 5 × 10 10 and 5 × 10 14 cm −2. Current–voltage measurements reveal a decrease (0.95–0.63 eV) and an increase (0.63–0.70) in barrier height, for n- and p-GaAs, respectively, for the dose above. Electronic properties of defects introduced are investigated through DLTS, current–voltage ( I– V) and capacitance–voltage ( C– V) measurements. Furthermore, we compare defects from 0.5 keV and MeV He-ion irradiation in both n- and p-GaAs. Annealing n-GaAs reveal a correlation between the BHM and the density of He-ion induced defects.

Low-energy hydrogen implantation for silicon Schottky barrier modification

Low-energy (≲0.4 keV) atomic hydrogen has been found to passivate defects created in Si by ion bombardment as under dopant ion implantation or ion-assisted dry etching. The interaction of atomic hydrogen with crystalline Si is, however, more intricate involving also further surface damage and formation of complexes with dopant atoms. The application of hydrogen implant following dopant and inert gas implants for Schottky barrier modification is discussed.

Silicon Schottky-barrier modification by ion-implantation damage

Low-energy (5-keV) high-dose (1015cm-2) argon implantation has been carried out on n- and p-type silicon to confirm the role of ion damage on the characteristics of subsequently formed Schottky barrier diodes. The electrical behavior of the diodes is similar to that observed recently on inert-gas ion-etched and reactive-ion-etched silicon surfaces, thus unambiguously attributing the Schottky-barrier modification to ion-induced surface damage.

Modification of Schottky barriers in silicon by reactive ion etching with NF3

Reactive ion etching of silicon with NF3 gas has been found to alter the silicon surface such that the Schottky barrier height is systematically changed with ion energy. The energetic ions introduce a net positive surface charge which increases the barrier height on p-Si and decreases it on n-Si. The Schottky barrier modification is found to be a function of ion energy as well as gas plasma used.