Decrease In Barrier Height Research Articles

Predicting the Formation of Sulfur-Based Brønsted Acids from the Reactions of SOx with H2O and H2S.

We have performed an extensive computational investigation of the potential energy surfaces for the reactions of SOx (x = 2 or 3) with H2S and H2O in the gas phase and in aqueous solution at the CCSD(T)/CBS level of theory plus a self-consistent reaction field approach. Formation of a gas-phase H2SO4 from the hydrolysis of SO3 at lower temperatures requires the presence of additional water molecules. When additional waters are introduced, the barrier for H2SO4 formation is significantly reduced, and a barrierless transition occurs with only three excess waters as well as in aqueous solution. In a mixture of H2O, H2S, and SO3, formation of H2S2O3 has a lower barrier in the gas phase than does the formation of H2SO4. In aqueous solution, no barrier is predicted, and both are likely to be formed. Introduction of an additional water to the SO3 + H2S reaction results in a decrease in barrier height, nearly identical to the ∼18 kcal/mol catalytic effect of the first additional water in SO3 hydrolysis, with the barrier for H2S2O3 formation disappearing altogether in aqueous solution. Formation of H2SO3 by the way of SO2 hydrolysis is unlikely. Excess waters reduce the barrier for SO2 hydrolysis; however, the overall endothermicity is increased as waters are added. The formation of H2S2O2 from SO2 and H2S via an isostructural pathway to SO2 hydrolysis is unlikely, with additional water molecules resulting in a small increase in the overall endothermicity and a catalytic effect smaller than that observed for the SO3 reactions. The results of this work have implications pertaining to the formation of H2SO4, H2S2O3, H2SO3, and H2S2O2 in the atmospheres of Earth and Venus. These results also question the existence of H2S2O2 as an intermediate in the Claus process.

Features of current transport in Schottky diodes with additional electric field

Abstract Schottky diodes with AEF is characterized by an equilibrium internal part with an electric field SCR and a non-equilibrium peripheral part with a resulting electric field. With a significant contribution of peripheral current, the forward I–V characteristic SD is shifted in the direction of increasing voltage by the value of the AEF voltage. The reverse I–V characteristic SD consists of the saturation current of the contact peripheral part. The peripheral barrier height Au - nGaAs SD becomes less than about 100 mV from the barrier height of the common contact. As the contact diameter increases from 5 to 500 μm, the AEF voltage decreases from 200 to 20 mV, and the difference in the saturation currents SD in opposite directions decreases by more than two orders of magnitude. Under the action of light, the AEF voltage increases by ~200 mV, and the peripheral barrier height decreases by the same amount, the barrier height of the common contact remains almost unchanged, and the difference in the saturation currents increases by more than three orders of magnitude. As the temperature decreases from ~300 K to 100 K, the barrier heights for the common contact and the periphery (Au + Ag) - nGaAs SD decrease from 0.956 eV to 0.690 eV–0.554 eV and 0.223 eV, respectively. At the same time, the ideality coefficient and the AEF voltage increase from 1.08 to 200 mV to 2.04 and 640 mV, respectively.

Humidity effect on electrical properties of graphene oxide back-to-back Schottky diode

A Schottky diode-based sensor is a promising structure for high sensitive and low power sensor. This paper investigates a device called back-to-back Schottky diode (BBSD) for humidity sensing operation. The BBSD provides simpler device configuration that can be fabricated using less complicated process. The current-voltage characteristic of the fabricated BBSD was measured at different relative humidity. From the obtained characteristics, series resistance, barrier height and ideality factor was analyzed. The device current increased at higher humidity level. The current increase could be associated to the decrease in series resistance, barrier height and ideality factor. When humidity decreased from 11 % to 97%, the barrier height showed reduction of 0.1 eV. The barrier height reduction was explained by considering electric field-induced reduction of graphene oxide. The observed result confirmed the device feasibility as promising simple and low cost humidity sensor.

Shape-controlled crystallisation pathways in dense fluids of ccp-forming hard polyhedra

Emergent directional entropic forces that favour facet-to-facet alignment of neighbouring colloidal particles – arising from multi-body interactions upon crowding – should cause the process by which colloidal crystals form from fluids of hard polyhedra to be different than that from fluids of hard spheres. We compute nucleation-free energy barriers for a family of shapes that all self-assemble the same cubic close-packed (ccp) crystal structure formed by hard spheres and find that changing rhombicuboctahedra into rhombic dodecahedra by successively removing facets that compete with the ccp structure cause a systematic decrease in barrier heights. We show that this decrease arises from the increased prominence of facets aligned with neighbouring shapes in the target crystal, which produce local environments in the fluid that facilitate crystallisation.

Study of ZnO/NiO heterojunction I–V characteristics measured in-situ during 200 MeV Ag ion irradiation

Heterojunction of ZnO/NiO films each of thickness 100 nm was irradiated with 200 MeV Ag ions at room temperature. The films were deposited on Si(100) by PLD method. Evolution of electrical property with ion fluence probed though in-situ I–V measurement showed that ion irradiation suppressed the rectifying nature of the pristine heterojunction and the junctions became Ohmic at high ion fluences. The reverse biased leakage current showed a monotonic increase, while the ideality factor of this diode structure, showed an increase followed by a decrease and then saturation with increasing irradiation fluence. The mechanism of electrical conduction seems to follow the power law model. The exponential decrease of barrier height with ion fluence could be attributed to the implantation of ion track in the sample. The close matching of the radius of ion tracks obtained from various parameters of the heterojunction suggests that mixing at the interface of ZnO/NiO layer lead to the variation of these parameters with ion fluence.

Structural and dynamic responses of calcium ion binding loop residues in metallo-proteins

Conformational changes in bio-molecular systems are fundamental to several biological processes. It is important to study changes in responses of underlying microscopic variables, like dihedral angles as conformational change takes place. We perform all-atom simulations and modelling via Langevin equation to illustrate the changes in structural and dynamic responses of dihedral angles of calcium ion binding residues of different proteins in metal ion free (apo) and bound (holo) states. The equilibrium distributions of dihedral angles in apo- and holo-states represent structural response. Our studies show the presence of dihedrals with multiple peaks (isomeric states) separated by barrier heights is more frequent in apo- than in holo-state. The relaxation time-scale of dihedral fluctuations is found to increase linearly with decreasing barrier height due to more frequent barrier re-crossing events. The slow kinetic response of the dihedrals also contributes to slowing down of macro-scale fluctuations, which may be useful to understand kinetics of various bio-molecular processes.

The effects of γ-ray irradiation on graphene/n-Si Schottky diodes

The effects of γ-ray irradiation on graphene/n–Si Schottky diodes are investigated. The electrical characteristics are measured before and after serial radiation doses in a dark environment. Experimental results show an increase of reverse current and ideality factor, while the Schottky barrier height decreases. The XPS results indicate that γ-ray irradiation mainly leads to a break in Si–O and C=C bonds; the C dangling bonds can combine with dangling O on the surface of SiO2 while leaving Si dangling bonds in SiO2. These Si dangling bonds seriously degrade the electrical performance of the device.

Synergetic Enhancement of Thermoelectric Performance by Selective Charge Anderson Localization-Delocalization Transition in n-Type Bi-Doped PbTe/Ag2Te Nanocomposite.

Considerable efforts have been devoted to enhancing thermoelectric performance, by employing phonon scattering from nanostructural architecture, and material design using phonon-glass and electron-crystal concepts. The nanostructural approach helps to lower thermal conductivity but has limited effect on the power factor. Here, we demonstrate selective charge Anderson localization as a route to maximize the Seebeck coefficient while simultaneously preserving high electrical conductivity and lowering the lattice thermal conductivity. We confirm the viability of interface potential modification in an n-type Bi-doped PbTe/Ag2Te nanocomposite and the resulting enhancement in thermoelectric figure-of-merit ZT. The introduction of random potentials via Ag2Te nanoparticle distribution using extrinsic phase mixing was determined using scanning tunneling spectroscopy measurements. When the Ag2Te undergoes a structural phase transition ( T > 420 K) from monoclinic β-Ag2Te to cubic α-Ag2Te, the band gap in the α-Ag2Te increases due to the p -d hybridization. This results in a decrease in the potential barrier height, which gives rise to partial delocalization of the electrons, while wave packets of the holes are still in a localized state. Using this strategic approach, we achieved an exceptionally high thermoelectric figure-of-merit in n-type PbTe materials, a ZT greater than 2.0, suitable for waste heat power generation.

Electron tunneling properties of Al2O3 tunnel barrier made using atomic layer deposition in multilayer devices

As metal/insulator/metal tunnel junctions (MIMTJs), such as magnetic tunnel junctions and Josephson tunnel junctions, push the insulating tunnel barrier (TB) towards the ultrathin regime (<1 nm) defects inherent in current physical vapor deposition methods become a fundamental obstacle to create pinhole-free and defect-free MIMTJs. Atomic layer deposition (ALD) could offer a solution by providing a conformal, leak-free tunnel barrier with low defect density and atomic thickness as demonstrated recently in ALD Al2O3 tunnel barriers. A question arises on the viability of the ALD TBs in practical circuits of multilayer structures on which increased roughness may occur. To answer this question, this work investigates electron tunneling properties of ALD Al2O3 tunnel barriers of 1.1 –1.2 Å in thickness on half-cell MIMTJs of Al/Fe/Nb fabricated on multilayer structures of different surface roughness using in situ scanning tunneling spectroscopy. Remarkably, the tunnel barriers grown on the raised multilayer device analogue only show a moderate decrease in barrier height from 1.63 eV, to 1.51 eV and to 1.27 eV as the surface roughness increases from 0.9 nm to 2.3 nm, and to 15 nm, alongside a slight decrease in ALD coverage from ∼96%, to ∼93% and 84% on these samples. Overall, these results validate the ALD TBs of atomic thickness for future 3D arrays of devices.

Formation of non-alloyed Ti/Al/Ni/Au low-resistance ohmic contacts on reactively ion-etched n-type GaN by surface treatment for GaN light-emitting diodes applications

Ti/Al/Ni/Au (30/120/20/100 nm) non-alloyed ohmic contacts on reactively ion-etched n-GaN have been obtained by thermal annealing in N2 + O2 ambient prior to metal contacts deposition. After surface treatment, contacts show the linear current–voltage (I–V) characteristics with specific contact resistance (SCR) of 2.91 × 10−5 Ω-cm2 without any post-deposition annealing, whereas, SCR of untreated sample has been found 1.02 × 10−4 Ω-cm2. The improvement in SCR of Ti/Al/Ni/Au contacts with this process may be attributed to formation of n+-highly doped layer due to presence of oxygen during thermal annealing. A further increase in SCR of non-treated sample from 1.02 × 10−4 Ω-cm2 to 3.54 × 10−4 Ω-cm2, has been observed on further annealing of contacts at 550 °C in N2 + O2 ambient for 5 min, which is generally an essential condition for formation of low-resistance highly transparent p-contact on p-GaN during fabrication of conventional GaN light-emitting diodes (LEDs). Based on X-ray photoelectron spectroscopy results, we found that there is increase in Ga 3d photoelectron binding energy with this surface treatment, which indicates a shift of Fermi level closure to conduction band, resulting in decrease of Schottky barrier height (SBH). The reduction in SBH is responsible for formation of low-resistance Ohmic contacts on n-GaN. Such non-alloyed n-contacts formed on thermally annealed reactively etched n-GaN surface may open for fabrication of more efficient conventional GaN LEDs. In addition, the forward voltage of GaN LED fabricated by present suggested process lowered by an amount of 0.37 V at 100 mA compared to standard process.

Effect of surface treatments on electrical properties of β-Ga2O3

The effect of various combinations of gaseous (ultraviolet/O3), liquid (HCl, buffered oxide etch, and H2O2), or plasma (CF4 and O2) treatments of the surface of β-Ga2O3 was quantified by current–voltage and capacitance–voltage measurements of rectifier structures. Plasma exposure (13.56 MHz, 24 kW/cm2) always led to significant degradation of the surface, as evidenced by large increases in rectifier reverse current and ideality factor (from 1.01 in control samples to ∼3.8 in plasma exposed samples, indicating additional defect-related carrier transport mechanisms) and lowering of the Schottky barrier height (from 1.21 eV in control samples to 0.75–0.86 eV in plasma exposed samples) and diode rectification ratio, with degraded reverse recovery characteristics. This was true of both CF4 and O2, even though it is known that fluorine incorporation in the near-surface leads to donor compensation and an increase in barrier height. Damage from the plasma exposure was not fully recovered by annealing at 500 °C. The O3 and liquid chemical cleans did lead to reduced reverse current in rectifiers, with no measurable decrease in barrier height, increase in ideality factor, or degradation of reverse recovery characteristics. Surfaces treated in this manner did not significantly change for anneals up to 500 °C; however, the Ni/Au contacts already show degradation after annealing at 350 °C.

Influence of Mg ion concentration in ZrO2 gate dielectric layered silicon based MOS capacitors for memory applications: Thorough understanding of conduction processes

Various high-κ metal-oxide-semiconductor (MOS) capacitors with different concentrations of Mg doped ZrO2 (Mg:ZrO2) gate dielectric on p-type silicon (100) substrates were fabricated by using electron beam evaporation. A uniform and crack free dielectric layers of Mg:ZrO2 having RMS surface roughness in the range from 0.25 to 0.83 nm were achieved. The dielectric layers were structurally and electrically studied. Structural analysis from GIXRD and Williamson-Hall plots revealed that Mg incorporation into ZrO2 lattice has stabilized the tetragonal phase and the residual strain in the tetragonal stabilized Mg:ZrO2 layers had increased with increasing Mg concentration. Elemental composition studies of different layers in the MOS capacitors using Rutherford backscattering spectrometry (RBS) revealed the presence of very thin, unintentional and non-stoichiometric SiOx interfacial layer between dielectric and semiconductor interfaces. Thickness of the Mg:ZrO2 dielectric layers in the MOS capacitors measured by using RBS and cross-sectional field-emission scanning electron microscopy was in the range from 44.7 to 96.6 nm. A high dielectric constant of 29.8 with the equivalent oxide thickness of 6.5 nm was achieved in the 5 mol% Mg:ZrO2 dielectric layer. The hysteresis in the C-V characteristics of the MOS capacitors has been explained in detail based on the interactions of Mg ions with oxygen vacancies in the dielectric layer. Various conduction mechanisms have been elucidated to explain the leakage currents in the MOS capacitors. The decrease in Schottky and Poole-Frenkel barrier heights at the lower voltages (0 to − 3V) was found responsible for the increase in the leakage current as the Mg ion concentration was increased. Space charge limited conduction mechanism has been found to be dominant in larger applied voltage region (− 3V to − 10V). The Fowler-Nordheim tunneling and trap-assisted tunneling were dominant at the higher negative voltage regions greater than − 10V.

Resistive switching effect of YBa2Cu3O7–x/Nb:SrTiO3 heterostructure

In this study, YBa2Cu3O7―x/Nb:SrTiO3 (YBCO/Nb:STO) planar heterostructure was prepared on Nb:SrTiO3 (Nb:STO) substrate by sol-gel method. The I–V curves revealed that the structure exhibited excellent resistive switching (RS) effect. Current transmission characteristics of the heterostructure prepared using different electrodes and Nb:STO were analyzed. Results indicated that the RS effect was caused by the capture and escape of the electrons from the charge trap at the interface between electrode and Nb:STO. Schottky barrier was a prerequisite for the RS effect. Further investigation of the current transport properties of the YBCO/Nb:STO heterostructure at low temperature revealed that the current transport was affected by its own carriers and charge traps when the heterostructure was under low-resistance state, and the current decreased with decreasing temperature. Owing to the decrease in barrier height, the current increased with decreasing temperature under high-resistance state, with current transport properties exhibiting Schottky emission characteristics.

(Invited) Influence of Post Deposition Annealing on the Electrical Properties of Pt-Ti/HfO2/TiN/Sion/n-Si Capacitor for Flash Memory Applications

To address the cell-to-cell interference in conventional floating gate (FG) flash memory, a fully planar structure demands continuous scaling of poly silicon floating gate thickness. But the reduced FG thickness causes reliability issues due to ballistic current through inter-poly dielectric (IPD) and also decreases the gate coupling ratio (GCR). The introduction of metal FG in flash memory eliminates the problem of poly depletion and ballistic transport even with ultrathin floating gate (<10 nm) and excellent memory performance with 16 V P/E window for 1 nm metal floating gate. Numerous studies on flash memory devices reported that a hybrid FG, transition metal dichalcogenides and multilayer graphene as FG results in improved memory performance such as fast program speed, excellent data retention and endurance characteristics. According to ITRS scaling projections for FG NAND flash, GCR of 0.6-0.7 would be desirable for satisfactory operation. However, in order to obtain adequate GCR, high-k based IPD and/or tunnel barrier has been proposed as mandatory alternatives. The improvement in GCR owed to greater coupling between the control and floating gates as a result of increased dielectric constant. The incorporation of nitrogen into tunnel oxide of NVM through nitridation can enhance the reliability performance of tunnel oxide of charge storage NVM. In this paper, we report the fabrication of Pt-Ti/HfO2/TiN/SiON/n-Si capacitor for flash memory applications with gate coupling ratio (GCR) of 0.68 and 0.71 for as-deposited and annealed SiON respectively. The XRD, AFM, FTIR, FESEM and XPS characterizations have been performed for structural and morphological studies. Program/erase and retention characteristics reveal the largest memory window of approx. 11.2 V after post deposition annealing and excellent data retention at room temperature under ±10 V. The enhanced program/erase speed was due to the fact that annealed SiON tunnelling oxide provides stronger interface against Si dangling bond generation and the decrease in barrier height of both electrons and holes. These results indicate that the SiON tunnelling oxide can be engineered by post deposition annealing for the improvement of metal floating gate memory performance.

An improved model to assess temperature-dependent DC characteristics of submicron GaN HEMTs

A modified analytical model for the current–voltage (I–V) characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) is presented, considering the temperature-dependent: (a) Schottky barrier height, (b) energy bandgap discontinuity, (c) carrier mobility, and (d) saturation velocity. It is demonstrated that the Schottky barrier height and energy bandgap discontinuity decrease with increase of the temperature. The effective mobility of the two-dimensional electron gas (2-DEG) also decreases with increasing temperature, causing a reduction in the output current of the device. The model was tested over a wide range of temperatures (300–500 K) and bias, and it was observed that the developed model can successfully predict the I–V characteristic of the device with reasonable accuracy, especially at high temperatures ( $$\sim 500$$ K). It is shown that the developed model offers, on average, a 39 % improvement for the temperature variation, from 300–500 K, relative to the best model reported in literature.

Hydrogen sensors based on TFT’s with catalytic source/drain electrodes: IGZO vs. pentacene

A novel H2 sensor based on an InGaZnO thin-film transistor (IGZO TFT) with Pd source/drain electrodes as sensing medium is fabricated. Without the need of heating, rapid and concentration-dependent response (in the form of drain-current increase) of the sensor is observed under various cycles of H2 exposure/evacuation with H2 concentration ranging from 200 to 15 000 ppm. The drain-current increase is due to a decrease in barrier height at the interface between the Pd source and IGZO channel induced by the formation of a hydrogen dipole layer at the interface. By comparing this sensor with its counterpart based on pentacene OTFT having an opposite response, it can be claimed that the same hydrogen dipole layer has opposite effect on the two TFTs: it creates an electric field that can assist the flow of electrons from the source electrode to the IGZO channel, but oppose the flow of holes from the source electrode to the pentacene channel. As a result, the IGZO TFT can work in an OFF-to- ON mode during sensing, and so the standby power of sensing circuit could be reduced. Although the OTFT sensor shows higher sensitivity and faster response due to the higher flexibility of the pentacene layer, the proposed sensor has a better stability at high temperature due to the larger bandgap of IGZO and so is suitable for hydrogen-sensing applications in a wider range of operating temperature.

Enhanced Hole Injection Into Single Layer WSe2

Electronic devices with light-emitting regions composed of single layer transition metal dichalcogenides, such as tungsten diselenide (WSe2), are of great interest due to the formation of a direct bandgap at the single layer limit. Furthermore, increasing injected hole current into the single layer WSe2 is of great importance due to the resulting increase in electroluminescence. In this paper, we demonstrate a $1000\times $ increase in injected hole current into the channel region of an FET composed of a single layer WSe2 channel by incorporation of a thicker (20–30 nm) multilayer WSe2 film under the metal source contact only. By fabrication and analysis of FETs composed of single layer, bi-layer, and tri-layer WSe2 channel regions, we correlate the increase in injected hole current to a decrease in the hole Schottky barrier height at the source contact due to incorporation of the multilayer WSe2 film.

Effect of the Substrate Nature on Electron Transport in Ga Doped ZnO Thin Films Grown by RF Sputtering

The influence of substrate nature on structure and the electrical properties of 2% Ga-doped ZnO thin films deposited on different substrates by rf magnetron sputtering is presented. XRD, AFM and XPS studies evidenced that thin films have a highly textured hexagonal structure, with a surface morphology dependent on substrate nature. The conduction mechanism is discussed in terms of grain boundary (GB) and nearest-neighbor hopping (NNH) conduction models. The decrease in the potential barrier height is considered to be caused by the increase in the crystallite size as a consequence of charge carrier scattering diminution at the grain boundaries. Surface trap density was found to be influenced by the substrate nature, the smallest value of being obtained for the SiO2/Si substrate.

Electric field tuned MoS2/metal interface for hydrogen evolution catalyst from first-principles investigations

Understanding the interfacial properties of catalyst/substrate is crucial for the design of high-performance catalyst for important chemical reactions. Recent years have witnessed a surge of research in utilizing MoS2 as a promising electro-catalyst for hydrogen production, and field effect has been employed to enhance the activity (Wang et al 2017 Adv. Mater. 29, 1604464; Yan et al 2017 Nano Lett. 17, 4109–15). However, the underlying atomic mechanism remains unclear. In this paper, by using the prototype MoS2/Au system as a probe, we investigate effects of external electric field on the interfacial electronic structures via density functional theory (DFT) based first-principles calculations. Our results reveal that although there is no covalent interaction between MoS2 overlayer and Au substrate, an applied electric field efficiently adjusts the charge transfer between MoS2 and Au, leading to tunable Schottky barrier type (n-type to p-type) and decrease of barrier height to facilitate charge injection. Furthermore, we predict that the adsorption energy of atomic hydrogen on MoS2/Au to be readily controlled by electric field to a broad range within a modest magnitude of field, which may benefit the performance enhancement of hydrogen evolution reaction. Our DFT results provide valuable insight into the experimental observations and pave the way for future understanding and control of catalysts in practice, such as those with vacancies, defects, edge states or synthesized nanostructures.

Analysis of the Conduction Mechanism and Copper Vacancy Density in p-type Cu2O Thin Films

A quantitative and analytical investigation on the conduction mechanism in p-type cuprous oxide (Cu2O) thin films is performed based on analysis of the relative dominance of trap-limited and grain-boundary-limited conduction. It is found that carrier transport in as-deposited Cu2O is governed by grain-boundary-limited conduction (GLC), while after high-temperature annealing, GLC becomes insignificant and trap-limited conduction (TLC) dominates. This suggests that the very low Hall mobility of as-deposited Cu2O is due to significant GLC, and the Hall mobility enhancement by high-temperature annealing is determined by TLC. Evaluation of the grain size and the energy barrier height at the grain boundary shows an increase in the grain size and a considerable decrease in the energy barrier height after high-temperature annealing, which is considered to be the cause of the significant reduction in the GLC effect. Additionally, the density of copper vacancies was extracted; this quantitatively shows that an increase in annealing temperature leads to a reduction in copper vacancies.