Variation Of Barrier Height Research Articles

Pt/ZnGa₂O₄ Schottky Barrier Diodes Fabricated by Using Single Crystal n-ZnGa₂O₄ (111) Substrates

In this letter, we fabricated Schottky barrier diodes (SBDs) using n-type zinc gallium oxide (n-ZnGa2O4) single-crystal substrates, which were grown from the melt by the vertical gradient freeze (VGF) method. X-ray diffraction rocking curve and Laue diffraction results indicate the good crystal quality. The electrical properties of SBDs fabricated on substrates with two doping concentrations are systematically investigated. Heavily-doped (HD) SBDs exhibit lower differential specific on-resistance (~1.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula> cm2) and lower turn-on voltage (~0.16 V) while the lightly-doped (LD) SBDs exhibit better ideality factor (1.17), rectification characteristics (on/off ratio ~1011) and lower reverse leakage. In addition, we separately tested the electrical properties of the two devices at different temperatures (298 K-473 K). The results of varying temperature show that LD-SBDs have better temperature stability in smaller variation of ideality factor and barrier height with temperature. Based on these experimental results, this work demonstrates the great promise of ZnGa2O4 materials to fabricate high-performance power devices.

A high-performance quasi-vertical MoSe2 photodiode with ultra-low dark current

Van der Waals heterostructure based on 2D materials is a promising technology for high-performance optoelectronic devices because of its tunable bandgaps and optical properties. However, photodetectors with a low dark current and a fast response speed commonly lose their photoresponsivity. The recovery current induced by the Schottky barrier height variation cancels out the device's reverse bias current in this paper, resulting in a quasi-vertical MoSe2 photodiode with ultralow dark current (&amp;lt;1 pA). Simultaneously, rapid electron–hole pair separation occurs at the interface due to the large heterojunction area and the strong interlayer coupling of MoSe2/graphene heterojunction, resulting in a fast response time of 1.5 ms and a high photoresponsivity of 19.72 A/W. Furthermore, the Au/MoSe2 forms a Schottky contact, which is asymmetrical to the Ohmic contact formed by the MoSe2/graphene, enabling the proposed device to achieve high-performance self-powered photodetection. Our work shows an alternative approach to improve the performance of future electronic and optoelectronic applications.

A bipolar charge transport model to simulate the impact of nanometric scale processes on the space charge behaviour in polyethylene

Predicting the electric field distribution in polymers used as electrical insulating materials remains the Holy Grail, as the presence of charges disturbs the Laplacian electric field. Charges arising from the electrodes are one of the dominant mechanisms of charge generation, particularly in polyethylene-based materials. Hence, the nanometric scale processes at play at the interface exert a non-negligible impact on charge injection. In this study, a bipolar charge transport model developed in 2D is used to simulate the effects of several nanometric scale processes, such as the variation of the barrier height linked to the chemical structure of the material at the interface and surface roughness. Simulation results with regard to the net charge density. current and the recombination rate will be compared to the case where no specific supplementary hypothesis is set at the electrodes. Finally, simulations are performed for a combination of roughness and barrier height variations along the electrodes.

Angular Deviation of the Exchange Bias in Bilayer CoFe/IrMn Under Rotating Magnetic Field

A model of exchange bias in a polycrystalline bilayer based on thermal fluctuations was extended to account for an arbitrary magnetization angle of the ferromagnetic (FM) layer. It allows for a more accurate description of the thermally activated reversal of the Néel vector in antiferromagnetic (AFM) grains since the extended model takes into account the variation of barrier height resulting from the magnetization angle of the FM layer. Field-annealing (setting of the exchange bias) and further relaxation of the FM/AFM bilayer was simulated using a sequence of iterations based on this model. An array of AFM grains is generated based on a lognormal distribution of their volume. Chosen parameters of distribution are based on the fit to the experimentally measured temperature dependence of exchange bias in sputter-deposited CoFe/IrMn films. The angular deviation of the exchange bias and the degradation of its amplitude in an applied field, with relative angle varied from 0° to 180°, are calculated. Hysteresis of the exchange bias with field exposure is observed. A temperature–time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T$ </tex-math></inline-formula> – <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\tau$ </tex-math></inline-formula> ) diagram of the deviation angle, predicting the stability of the exchange bias under specific temperature and time, is presented.

Scrutinization of non‒saturation behaviour of reverse current‒voltage characteristics in Ni/SiO2/p-Si/Al diodes

The present work is an endeavour to investigate non‒saturation behaviour of reverse current in Ni/SiO 2 / p -Si/Al over a wide low temperature range of 70–300 K at step of 10 K using well accepted models i.e., Poole-Frenkel emission, Schottky emission and Fowler‒Nordheim tunneling mechanisms. The results of the study revealed that Schottky emission has the dominance over Poole-Frenkel emission in the temperature range of 200–300 K with the trap state activation energy of 0.17 eV. In the remaining temperature range trap assisted tunnelling and involvement of other mechanisms were suggested. Further, the Fowler‒Nordheim tunneling mechanism was found to be effective above reverse bias of 0.5 V over the entire temperature range of 70–300 K. Thus, variation of barrier height with temperature was examined using Fowler‒Nordheim tunneling model and it was found to increase from 0.17 eV to 0.28 eV as temperature varied from 300 to 70 K. The increase in barrier height with decrease in temperature corroborates the decrease in reverse current with temperature. • Reverse current in Ni/SiO 2 /p-Si/Al MIS diodes was analyzed using various models. • Possible current conduction mechanism is predicted for the diode using various employed mechanisms. • Schottky emission process found to be dominating others mechanisms in the temperature range of 200–300 K at low bias range. • Whereas, Fowler-Nordheim tunneling was found to be effective in the bias range between 0.5 and 2.0 V. • The variation of barrier height with temperature was also examined using FNT.

Highly superlinear giant terahertz photoconductance in GaAs quantum point contacts in the deep tunneling regime

A highly superlinear in radiation intensity photoconductance induced by terahertz laser radiation with moderate intensities has been observed in quantum point contacts made of GaAs quantum wells operating in the deep tunneling regime. For very low values of the normalized dark conductance $G_{\rm dark}/ G_0 \approx 10^{-6}$, with the conductance quantum $G_0=2e^2/h$, the photoconductance scales exponentially with the radiation intensity, so that already at $ 100 \text{ mW}/\text{cm}^2$ it increases by almost four orders of magnitude. This effect is observed for a radiation electric field oriented along the source drain direction. We provide model considerations of the effect and attribute it to the variation of the tunneling barrier height by the radiation field made possible by local diffraction effects. We also demonstrate that cyclotron resonance due to an external magnetic field manifests itself in the photoconductance completely suppressing the photoresponse.

Room Temperature Hydrogen Sensor Using Schottky Contacted Zinc Oxide Thin-Film Transistor: A Comprehensive Investigation

A ZnO-based Schottky thin-film transistor (TFT) utilizing palladium-titanium (Pd-Ti) source/drain electrodes as a sensing medium is used to fabricate a room temperature (RT) hydrogen sensor. In the absence of any heating element, the fabricated sensor is capable of reducing the design complications and power consumption considerably. The RT sensor response was realized with respect to drain current rise owing to the Schottky barrier height reduction depending on the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> concentration (500-4500 ppm). For 500-ppm (minimum detection level) and 4500-ppm (maximum detection level) H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas concentrations, sensor responses of 18% and 78.2% with 65 s/35 s and 10.2 s/85 s response/recovery times are acquired, respectively. The Pd-Ti/ZnO Schottky barrier height variation is attempted to explain the incorporating energy band diagrams of the ZnO channel between source/drain electrodes at different gate and drain bias conditions. Performance analysis of the sensor in terms of repeatability, selectivity toward H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> over methane and acetone, hysteresis, reliability, and humidity exposure has also been examined.

Metallization of ZnSb and contact resistance

We present results on electrical resistance of metal contacts to ZnSb. We synthesized the thermoelectric semiconductor ZnSb with specific doping concentrations by adding Cu as an acceptor to the melt, followed by solidification, crushing, ball-milling, hot-pressing, sawing, and polishing yielding wafers suitable for substrates for further processing. Many batches were made yielding different doping concentrations. We defined transmission line geometries in deposited metal films for specific contact resistance measurements. We prepared sets of Cu, Ti, and Ni films, respectively. We measured the contact resistance vs annealing temperatures. For Cu/ZnSb samples, we observed a specific contact resistance from 5 × 10−7 to 4 × 10−5 Ω cm2. We also measured the carrier concentration of ZnSb. The measurement data of the specific contact resistance had systematic dependence on doping concentration and annealing temperature and were analyzed by a model incorporating different transport mechanisms across the energy barrier at the metal–semiconductor interface. The data were discussed in terms of systematic variation in barrier height and density of states effective mass. We proposed these arising as a consequence of interactions at the interface and a nonparabolic valence band. We have also monitored the interface of the ZnSb substrate and metal films with transmission electron microscopy.

A comparison of trapping the rubidium atoms in optical and magnetic lattices

In this paper, the critical parameters of quantum phase transition from the superfluid to the Mott insulator phase are studied for three-dimensional traps in 2D optical lattices and permanent magnetic lattices for trapping the 87Rb atoms. Using the harmonic oscillator wave function approximation, the on-site interaction U, the hopping matrix element J and the ratio J/U are calculated to determine the quantum phase transition point analytically. In a magnetic lattice two components of the external magnetic field B 1x and B 1y had been considered for studying quantum phase transition. Here, B 1z as the z- direction component of the bias magnetic field is also considered for further control of the magnetic lattice parameters. In addition, an optical lattice constructed from the interference of two pairs of orthogonal counter-propagating laser beams is investigated. In both optical and magnetic lattices, the ratio J/U and the critical point of the quantum phase transition can be calculated by variation of the potential barrier height between traps. Comparison of optical lattices with permanent magnetic lattices shows that in the critical point of quantum phase transition the potential barrier height of the magnetic potential is higher than that of the optical trap which suggests that the magnetic microtraps are deeper and more stable than the optical traps in similar structures.

The role of AlGaN/GaN heterostructure properties in barrier height variation of Au-free ohmic contacts

In this work, the temperature dependence of the specific contact resistance in Au-free ohmic contacts formed on AlGaN/GaN heterostructures is studied for different barrier layer characteristics, i.e. thickness and surface roughness of the AlGaN layer, and Al mole fraction. It is demonstrated that a minimum in specific contact resistance and Schottky barrier is reached with decreasing the AlGaN thickness to an optimum at which a maximum polarization field-induced carrier density ND-2DEG is formed. The Schottky barrier height at the metal/AlGaN interface is shown to be strongly linked to the heterostructure and interface properties. The dependence of the Schottky barrier height is found to be linear with Al mole fraction. The predictions from the physical interface induced gap states (IFIGS) and chemical electronegativity concepts are discussed.

Photoelectrical Properties Investigated on Individual Si Nanowires and Their Size Dependence

Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated with controllable diameters and lengths. Their photoconductive properties are investigated by photoconductive atomic force microscopy (PCAFM) on individual nanowires. The results show that the photocurrent of Si NWs increases significantly with the laser intensity, indicating that Si NWs have good photoconductance and photoresponse capability. This photoenhanced conductance can be attributed to the photoinduced Schottky barrier change, confirmed by I–V curve analyses. On the other hand, electrostatic force microscopy (EFM) results indicate that a large number of photogenerated charges are trapped in Si NWs under laser irradiation, leading to the lowering of barrier height. Moreover, the size dependence of photoconductive properties is studied on Si NWs with different diameters and lengths. It is found that the increasing magnitude of photocurrent with laser intensity is greatly relevant to the nanowires’ diameter and length. Si NWs with smaller diameters and shorter lengths display better photoconductive properties, which agrees well with the size-dependent barrier height variation induced by photogenerated charges. With optimized diameter and length, great photoelectrical properties are achieved on Si NWs. Overall, in this study the photoelectrical properties of individual Si NWs are systematically investigated by PCAFM and EFM, providing important information for the optimization of nanostructures for practical applications.

Quantum Monte Carlo calculations on dissociative chemisorption of H2 + Al(110): Minimum barrier heights and their comparison to DFT values.

Reactions of molecules on metal surfaces are notoriously difficult to simulate accurately. Density functional theory can be utilized to generate a potential energy surface, but with presently available functionals, the results are not yet accurate enough. To provide benchmark barrier heights with a high-quality method, diffusion Monte Carlo (DMC) is applied to H2 + Al(110). Barrier heights have been computed for six geometries. Our present goal is twofold: first, to provide accurate barrier heights for the two lowest lying transition states of the system, and second, to assess whether density functionals are capable of describing the variation of barrier height with molecular orientation and impact site through a comparison with DMC barriers. To this end, barrier heights computed with selected functionals at the generalized gradient approximation (GGA) and meta-GGA levels are compared to the DMC results. The comparison shows that all selected functionals yield a rather accurate description of the variation of barrier heights with impact site and orientation, although their absolute values may not be accurate. RPBE-vdW-DF and BEEF-vdW were found to perform quite well even in terms of absolute numbers. Both functionals provided barrier heights for the energetically lowest lying transition state that are within 1 kcal/mol of the DMC value.

Impact of particle size and surfactant on electrical resistivity and thermopower of silver nanoparticles

We report here the critical role of crystallite size and surfactant of Ag nanoparticles on electrical resistivity and thermopower in the temperature range of 5 K–300 K. The Debye temperature (θD) decreases as the crystallite size decreases; latter's decrease corresponds to increase in the trioctylphosphine quantity. The θD decrease is ~36% in the sample with crystallite size of 15.1 nm compared to the bulk Ag. The usual phonon drag peak in thermopower for the bulk Ag becomes phonon drag minimum (PDM) in these nanoparticles. The PDM positions get gradually suppressed and shifted towards lower temperature with decrease in the crystallite size. They are attributed to the modifications in the overall electron-phonon interactions due to spatial confinement of electrons, variation in barrier heights, charge-transfer among metal NPs and surfactant, and enhanced disorder.

Noble‐Metal‐Free Memristive Devices Based on IGZO for Neuromorphic Applications

AbstractAmorphous indium‐gallium‐zinc‐oxide (a‐IGZO) based memristive devices with molybdenum contacts as both top and bottom electrodes are presented aiming to be used in neuromorphic applications. Devices down to 4 µm2 are fabricated using conventional photolithography processes, with an extraordinary yield of 100%. X‐ray photoelectron spectroscopy and transmission electron microscopy performed on the developed structures confirm the presence of a thin intermixed oxide layer (4–5 nm) containing Mo6+ oxidation state at the interface with the bottom contact. This results in Schottky diode‐like characteristics at the pristine state with a rectification ratio of 3 orders of magnitude. The devices have electroforming‐free and area‐dependent analog resistive switching properties. Temperature analysis of resistive switching I–V data reveals barrier height variations of the junction. Several synaptic functions, such as synaptic potentiation and depression as response to programmed pulses, short‐ to long‐term plasticity transition (STP to LTP) and “learning experience” properties are presented. The Mo/IGZO/Mo memristive device shows potential application of an electronic synapse for brain‐inspired computing application. Integration in System‐on‐Panel architectures is possible at negligible cost, because all materials are used in commercial IGZO thin‐film transistor fabrication.

Collective dipole effects in ionic transport under electric fields

In the context of ionic transport in solids, the variation of a migration barrier height under electric fields is traditionally assumed to be equal to the classical electric work of a point charge that carries the transport charge. However, how reliable is this phenomenological model and how does it fare with respect to Modern Theory of Polarization? In this work, we show that such a classical picture does not hold in general as collective dipole effects may be critical. Such effects are unraveled by an appropriate polarization decomposition and by an expression that we derive, which defines the equivalent polarization-work charge. The equivalent polarization-work charge is not equal neither to the transported charge, nor to the Born effective charge of the migrating atom alone, but it is defined by the total polarization change at the transition state. Our findings are illustrated by oxygen charged defects in MgO and in SiO2.

Investigation of Parameters for Schottky Barrier (SB) Height for Schottky Barrier Based Carbon Nanotube Field Effect Transistor Device

The design and development of Schottky Barrier Carbon Nanotube Field Effect Transistor (SB CNTFET) is still in the primitive research phase for its utilization in digital design. There is an immediate requirement for the analysis of parametric relations with structural factors to benefit the researchers working in this field. This work helps in the improvement of SB based CNTFET devices to be used in the development of various circuit applications. In this work, investigation of Schottky Barrier height on the performance of SB CNTFET for various geometrical and physical design parameters at the device level has been reported. The analysis of various device parameters of carbon nanotube, i. e., chirality, diameter, band gap, oxide thickness and dielectric constant has been carried out viz. subthreshold conduction, and ION/IOFF ratio. The paper also reports the effect of high dielectric constant material in SB CNTFET with oxide thickness along with Schottky Barrier Height Variation. The performance of SB CNTFET with variation in Schottky Barrier height and temperature variation is also reported. The results obtained indicate that performance of SB based CNTFET can be modified by the proper choice of chirality, dielectrics, oxide thickness and operating temperature. The SB parameter can be optimized by proper choice of metal contact in case of CNTFET.

Research on Cs/O activation process of near-infrared In0.53Ga0.47As photocathodes

Abstract We report a systematic theoretical investigation on the activation mechanism of near-infrared In0.53Ga0.47As photocathodes. Adsorption energies, dipole moments, work functions, density of states, band structures and electron affinity of Cs/O co-adsorption on (001) surface of In0.53Ga0.47As photocathodes are investigated. First-principles calculation results indicate that the structural stability is greatly enhanced when oxygen atoms adsorb on the Cs-covered surface. The incorporation of oxygen atom helps lower the work function further to achieve the true NEA state. Moreover, two adatom-induced surface dipoles, namely [Csn+-In0.53Ga0.47Asn−] and [Cs+-O2--Cs+], are also adopted to explain the relationship between the adatoms and the substrate. The HOMO and LUMO levels can further move downward and the band bending region is enlarged after Cs/O co-activation. Meanwhile, new energy bands appear in the deep valence band due to the joint effect of Cs 5s, Cs 5p, O 2s and O 2p state electrons. Finally, Cs/O activation experiments are carried out and the photocurrent curves during activation procedure are recorded. The change of photocurrent is closely related to the work function variation, which affects the photoemission of the photocathodes. In order to clearly show the electron affinity, the variation of surface barrier height of InGaAs photocathodes with Cs/O adlayer is also given.

Electro-opto-mechano driven reversible multi-state memory devices based on photocurrent in Bi0.9Eu0.1FeO3/La0.67Sr0.33MnO3/PMN-PT heterostructures.

A single device with extensive new functionality is highly attractive for the increasing demands for complex and multifunctional optoelectronics. Multi-field coupling has been drawing considerable attention because it leads to materials that can be simultaneously operated under several external stimuli (e.g. magnetic field, electric field, electric current, light, strain, etc.), which allows each unit to store multiple bits of information and thus enhance the memory density. In this work, we report an electro–opto–mechano-driven reversible multi-state memory device based on photocurrent in Bi0.9Eu0.1FeO3 (BEFO)/La0.67Sr0.33MnO3 (LSMO)/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) heterostructures. It is found that the short-circuit current density (Jsc) can be switched by the variation of the potential barrier height and depletion region width at the Pt/BEFO interface modulated by light illumination, external strain, and ferroelectric polarization reversal. This work opens up pathways toward the emergence of novel device design features with dynamic control for developing high-performance electric–optical–mechanism integrated devices based on the BiFeO3-based heterostructures.

Scanning tunneling Andreev microscopy of titanium nitride thin films

We report scanning tunneling microscopy results on 25- and 50-nm-thick films of superconducting TiN that show Andreev tunneling behavior at 0.5 K. At most locations on the topographically rough surfaces, we observe tip-sample current-voltage characteristics with a clear superconducting gap, as expected for superconductor-normal (S-N) tunneling through a low-transparency barrier, while in some places, we find a zero-voltage conductance peak, as expected for S-N Andreev tunneling through a highly transparent barrier. Fitting the Blonder-Tinkham-Klapwijk model to the conductance data allows an accurate extraction of the TiN superconducting gap $\mathrm{\ensuremath{\Delta}}$, by accounting for local variations in the tip-sample barrier height $Z$ and junction temperature $T$. From spatial maps of the model parameters, we find that both films show a strong inhomogeneity, with $\mathrm{\ensuremath{\Delta}}$ varying by as much as a factor of 2 from grain to grain. In the thicker film, however, correlations between $T,Z$, and $\mathrm{\ensuremath{\Delta}}$ suggest the grains are thermally isolated, perhaps due to internal stress. We discuss possible mechanisms that could produce these large correlated variations, including local heating and surface contamination, and consider some of the implications for devices made from such films.

Sensitivity of the Power Spectra of Thermal Magnetization Fluctuations in Low Barrier Nanomagnets Proposed for Stochastic Computing to In-Plane Barrier Height Variations and Structural Defects

Nanomagnets with small in-plane shape anisotropy energy barriers on the order of the thermal energy have unstable magnetization that fluctuates randomly in time. They have recently emerged as promising hardware platforms for stochastic computing and machine learning because the random magnetization states can be harnessed for probabilistic bits. Here, we have studied how the statistics of the magnetization fluctuations (e.g., the power spectral density) is affected by (i) moderate variations in the barrier height of the nanomagnet (caused by small size variations) and (ii) the presence of structural defects — both localized and delocalized — in order to assess how robust the stochastic computing platform based on Low Barrier Nanomagnets (LBM) is. We found that the power spectral density is relatively insensitive to moderate barrier height change and also relatively insensitive to the presence of small localized defects. However, extended (delocalized) defects, such as thickness variations over a significant fraction of the nanomagnet, affect the power spectral density very noticeably. That means extended defects can significantly alter the fluctuation rate of the magnetization in low barrier nanomagnets. Since the fluctuation rate is crucial for stochastic computing applications, this has very serious implications for the latter. Thickness variations are difficult to avoid in real nanomagnets with in-plane anisotropy since they must be thin to keep the barrier height small and the substrate on which they are fabricated may have surface roughness comparable to the nanomagnet thickness. This raises questions about the viability of stochastic computing with low barrier nanomagnets possessing in-plane anisotropy. Our results establish that small variations in the shape (causing small variations in the barrier height), or small localized defects, are relatively innocuous and tolerable but extended defects are not. The latter must be avoided for stochastic computing applications.