Low Barrier Height Research Articles

Targeted Chemical Processing Initiating Biosome Action-Potential-Matched Artificial Synapses for the Brain-Machine Interface.

A great gap still exists between artificial synapses and their biological counterparts in operation voltage or stimulation duration. Here, an artificial synaptic device based on a thin-film transistor with an operating voltage (-50-50 mV) analogous to biological action potential is developed by targeted chemical processing with the help of supercritical fluids. Chemical molecules [hexamethyldisilazane (HMDS)] are elaborately chosen and brought into the target interface to form charge receptors through supercritical processing. These charge receptors with the ability of capturing electrons mimic neurotransmitter receptors in terms of mechanism and constitute key players accounting for the synaptic behaviors. The relatively lower electrical barrier height contributes to an action-potential-matched operating voltage and considerably low power consumption (∼1 pJ/synaptic event), minimizing the divide with biological synapse for a seamless linkage to the biosystem or brain-machine interface. The stable synaptic behaviors also lead to near-ideal accuracy in pattern recognition. Moreover, this methodology that introduces chemical groups into a target interface can be viewed as a platform technology that could be adapted to other conventional devices with suitable chemical molecules to reach designed synaptic behaviors. This environmentally friendly and low-temperature processing method, which can be performed even after device fabrication, has the potential to play an important role in the future development of bionic devices.

Conduction mechanism of Schottky contacts fabricated on etch pits originating from single threading dislocation in a highly Si-doped HVPE GaN substrate

This paper reports the relationship between a single dislocation structure and current leakage mechanism observed in a Pt/n+-GaN Schottky contacts with the combined use of transmission electron microscopy (TEM) and conductive atomic force microscopy (C-AFM). The contacts were fabricated on a substrate grown by hydride vapor phase epitaxy. TEM analysis clarified that large- and small-size etch pits are attributed to mixed/screw and edge dislocations, respectively. Schottky contacts formed on the surfaces of large and small etch pits (L-EPC, S-EPC), a dislocation-free flat area (FLAT), and a Schottky barrier diode with a diameter of 50 μm (SBD) were prepared and their temperature-dependent current-voltage characteristics were measured using C-AFM. Analysis of the forward bias characteristics revealed that the Schottky barrier height increased and the ideality factor approached unity with increasing temperature. Such temperature dependence of the barrier height was well explained by barrier inhomogeneity at the metal/semiconductor interface. From the analysis of the reverse bias characteristics, we found that field emission (FE) currents caused by the high donor concentration of the sample were the dominant conduction mechanism for S-EPC and FLAT. On the other hand, for L-EPC, the FE currents were overlapped by Poole-Frenkel emission (PFE) currents. For SBD, the leakage currents were successfully explained by the FE and thermionic field emission (TFE) mechanisms by assuming dislocation regions with a lowered barrier height. These findings suggest that mixed dislocations are associated with PFE and lower the Schottky barrier.

Oxygen partial pressure dependent sputtered copper oxide films for visible photodetectors

In this study, metal-semiconductor-metal (MSM) structured visible photodetectors based on Cu2-xO thin films are fabricated onto a SiO2/Si substrate via direct current (DC) reactive magnetron sputtering. The dependence of oxygen partial pressure (pO2) on the visible photodetector performance of the deposited Cu2-xO films has been studied. Here, Cu2-xO film thickness is found to decrease from 229 nm to 202 nm with increasing pO2 from 2.0% to 8.0%. X-ray diffraction (XRD) analysis confirmed the cubic structure of the Cu2O phase. The compositional analysis confirms that 6.0% pO2 film has more copper vacancies than 2.0% pO2 film. As-deposited films have grain-like morphology with a uniform surface. The energy bandgap of deposited films decreases with the increase of pO2 from 2.0% to 6.0% and further increases at 8.0% pO2. Current-voltage (I-V) curves of the Au/Cu2-xO/Au test-device showed ohmic contact between the metal (Au) and semiconductor (Cu2-xO) due to the lower barrier height. Among different pO2 samples, the film grown at 6.0% pO2 exhibited a maximum photocurrent of 20.23 µA and also quick rise and fall times (100 ms and 90 ms) even at lower power density (0.1283 mW/cm2), which are superior to other reported copper oxide-based visible photodetectors. The obtained results indicated that MSM planar structured Cu2-xO photodetector is suitable to serve for future low-cost optoelectronic applications.

Insights into factors that affect non-Arrhenius migration of a simulated incoherent Σ3 grain boundary

Non-Arrhenius grain boundary migration, sometimes referred to as antithermal migration where temperature and GB velocity values are inversely related to each other, is examined in an incoherent twin Σ3 [111] 60° (11 8 5) nickel grain boundary. Molecular dynamics is used to simulate migration and examine the effect of various factors on the migration, including interatomic potential, system size, driving force, and variation of atomic grain boundary structure. A classical model for grain boundary migration, in its unsimplified form, is used to analyze the results. The grain boundaries exhibit migration mechanisms with very low apparent barrier heights to migration. As a result, the boundaries migrate quickly but exhibit a velocity saturation similar to that of dislocations. The various interatomic potentials exhibit different migration velocities, but their similarities suggest they all predict similar overall behaviors of migration. The variation of atomic structure in the same incoherent twin grain boundary leads to diverse behaviors with barrier heights that vary from non-Arrhenius to Arrhenius migration. Facet nucleation is confirmed not to be a requirement for this boundary based on an examination of simulation cell size; however, the presence and/or interaction between numerous facets does suggest a slowing and increased barrier height to migration for larger boundaries.

Halogen atom-doped graphene/MoSe2 heterojunction Schottky barrier height modulation

Achieving lower Schottky barrier heights in logic devices remains a formidable challenge. In this paper, the effect of halogen atoms doping on the type and height of graphene/MoSe2 Schottky is investigated using density functional theory. For monolayer MoSe2, the doping of halogen atoms introduces impurity energy levels in the energy band and leads to a change in the position of the Fermi energy level. Graphene and MoSe2 bonding retains their respective intrinsic electrochemical properties and bond with weak van der Waals forces to form n-type Schottky contacts. The doping of halogen atoms effectively changes the Schottky type and height of graphene/MoSe2. The Schottky type of heterojunctions doped with F, Cl, and Br atoms changes from n-type contacts to ohmic contacts. I and At atoms effectively reduce the Schottky barrier height of graphene/MoSe2. The disparity charge density indicates that the electrons in graphene are transferred to MoSe2, forming a built-in electric field pointing from graphene to MoSe2. After the doping of halogen atoms, the interlayer charge transfer between graphene and MoSe2 is significantly reduced, and the Fermi energy level appears to be shifted downward, which eventually leads to the reduction of the Schottky barrier height.

Voltage-margin limiting mechanisms of AlScN-based HEMTs

In this work, the off-state characteristics of AlScN/GaN high electron mobility transistors (HEMTs) grown by metalorganic chemical vapor deposition (MOCVD) were studied and directly compared to an AlGaN- and an AlN-HEMT grown in the same MOCVD. Pinch-off instability and leaky capacitive measurements were observed for AlScN-based HEMTs, which was correlated with a higher ideality factor and lower effective potential barrier height than the AlGaN and AlN-HEMTs. However, the reverse bias characteristics exhibited a sudden drain-current increase without a significant increase in gate-leakage current. The drain-leakage current is assumed to be related to a parasitic channel across the AlScN-barrier as a result of trap-assisted carrier transport with a Poole–Frenkel characteristic. The demonstrated pinch-off instability led to significant gain expansion in load-pull measurements and early soft-breakdown, which, in turn, limits the achievable voltage-margin. The results demonstrate a key issue to reveal the full potential of AlScN-based HEMTs for mm-wave applications.

Equilibrium structures of selenium compounds: The torsionally flexible molecule of selenophenol.

The equilibrium structure of selenophenol has been investigated using rotational spectroscopy and high-level quantum mechanical calculations, offering electronic and structural insight into the scarcely studied selenium compounds. The jet-cooled broadband microwave spectrum was measured in the 2-8GHz cm-wave region using broadband (chirped-pulse) fast-passage techniques. Additional measurements up to 18GHz used narrow-band impulse excitation. Spectral signatures were obtained for six isotopic species of selenium (80Se, 78Se, 76Se, 82Se, 77Se, and 74Se), together with different monosubstituted 13C species. The (unsplit) rotational transitions associated with the non-inverting μa-dipole selection rules could be partially reproduced with a semirigid rotor model. However, the internal rotation barrier of the selenol group splits the vibrational ground state into two subtorsional levels, doubling the dipole-inverting μb transitions. The simulation of the double-minimum internal rotation gives a very low barrier height (B3PW91: 42cm-1), much smaller than for thiophenol (277cm-1). A monodimensional Hamiltonian then predicts a huge vibrational separation of 72.2GHz, justifying the non-observation of μb transitions in our frequency range. The experimental rotational parameters were compared with different MP2 and density functional theory calculations. The equilibrium structure was determined using several high-level abinitio calculations. A final Born-Oppenheimer (reBO) structure was obtained at the coupled-cluster CCSD(T)_ae/cc-wCVTZ level of theory, including small corrections for the wCVTZ → wCVQZ basis set enlargement calculated at the MP2 level. The mass-dependent method with predicates was used to produce an alternative rm(2) structure. The comparison between the two methods confirms the high accuracy of the reBO structure and offers information on other chalcogen-containing molecules.

Molecular Dynamics Simulations of High-Concentration Li[TFSA] Sulfone Solution: Effect of Easy Conformation Change of Sulfolane on Fast Diffusion of Li Ion.

The parameters of the polarizable force field used for molecular dynamics simulations of Li diffusion in high-concentration lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) sulfone (sulfolane, dimethylsulfone, ethylmethylsulfone, and ethyl-i-propylsulfone) solutions were refined. The densities of the solutions obtained by molecular dynamics simulations reproduced well the experimental values. The calculated concentration, temperature, and solvent dependencies of self-diffusion coefficients of ions and solvents in the mixtures well reproduce the experimentally observed dependencies. Ab initio calculations show that the intermolecular interactions between Li ions and four sulfones are not largely different. Conformational analyses show that sulfolane can change the conformation more easily owing to lower barrier height for pseudorotation compared to the rotational barrier heights of diethylsulfone and ethylmethylsulfone. Molecular dynamics simulations indicate that the easy conformation change of solvent affects the rotational relaxation of the solvent and the diffusion of Li ion in the mixture. The easy conformation change of sulfolane is one of the causes of faster diffusion of Li ion in the mixture of Li[TFSA] and sulfolane compared to the mixtures of smaller dimethylsulfone and ethylmethylsulfone.

Energetics and diffusion kinetics of point defects in MnCr2O4 spinel from first principles

MnCr2O4 spinel is a component of the multilayered oxide film which forms atop austenitic steel alloys under high-temperature corrosion conditions. In this work, the thermodynamics and kinetics of cationic and anionic point defects in this spinel are examined through density functional theory calculations. To model the physical conditions of the corrosion process more closely, temperature is accounted for by adjusting the chemical potential of each species to fit experimentally determined values. We find that manganese is the most mobile species - migrating through the lattice via a vacancy mediated mechanism. Oxygen migration can occur via vacancy mediated or three-body interstitial mechanisms with similarly low barrier heights, highlighting the possibility of oxide growth at the film/alloy interface due to the high oxygen permeability. Furthermore, a number of more complex migration mechanisms involving substitution and antisite defects are evaluated for their potential contribution to cation mobility.

Photolytic Decarbonylation of Oxalyl Diisothiocyanate in Solid Argon Matrices tosyn‐antiCarbonyl Diisothiocyanate and Its Isomerization

AbstractOxalyl diisothiocyanate, ((CO)NCS)2, has been studied in solid argon matrices at 4.2 K with the aid of infrared (IR) spectroscopy. The spectra show mainly signals attributed to the most stableanti‐anticonformer, which is corroborated by comparison to computed anharmonic fundamental IR transitions. Upon irradiation with 254 nm UV light, oxalyl diisothiocyanate eliminates carbon monoxide under formation of carbonyl diisothiocyanate, CO(NCS)2. This reaction is only slightly exothermic by 0.4 kcal mol−1at the DLPNO‐CCSD(T)/def2‐QZVPP//B3LYP‐D3/def2‐TZVPP level of theory. Remarkably, photolysis produces mostly the less stablesyn‐anticonformer of carbonyl diisothiocyanate. Subsequent annealing at 30 K for two minutes results in a structural relaxation to the 0.7 kcal mol−1more stablesyn‐synconformer confirming a low torsional barrier height between the isomers.

Annealing effect on the barrier characteristics and interface properties of Au/Pt/Ti/n-InAlAs Schottky contacts

A comprehensive study of the annealing effect (300–400 °С) on the electrical properties, morphology and chemical composition of the Au/Pt/Ti/n-InAlAs interface (Schottky contact) is carried out. It is shown that the Schottky contact pre-annealing during the formation or primary short (∼1 min) annealing significantly increases the barrier height to the standard 0.68–0.7 eV with an ideality factor close to 1.1 due to the formation of a homogeneous amorphous TiAs layer with a small metallic (elemental) indium content. A further annealing at temperatures 300–350 °С for up to 20 min does not lead to significant changes in the morphology and electrical parameters of the Schottky contact. The annealing at the temperature of 400 °С (∼10 min) leads to an increase in the barrier height and the ideality factor to the values of 0.73 and 1.3, respectively. In this case, the formation of an about 20 nm thick TiAs layer and indium clusters shaped as a pyramids at the Ti/InAlAs interface is also observed. Analysis of the temperature dependences of Schottky barrier parameters within the Tung model showed that only structural changes at the interface after the 400 °С annealing lead to a significant increase in the Ti/InAlAs Schottky contact homogeneity, reducing the density of local regions with a lowered barrier height.

Numerical demonstration of SiC trench MOSFET with integrated heterojunction diode for enhanced performance

In this paper, a novel SiC trench-gate MOSFET with integrated heterojunction diode (HD-TG-MOS) is proposed and studied according to TCAD simulations. The n-type polysilicon/n-type SiC HD is introduced into the groove by the direct contact between the polysilicon and semiconductor. As a result, significant improvements of device performance in the first and third quadrants are observed as compared to the conventional SiC trench-gate MOSFET (C-TG-MOS). Better yet, aided by the extremely low barrier height across the heterojunction, the knee voltage reduces to only 0.5 V with comparison of that of ∼2.7 V for the body diode, when used for reverse freewheeling. These promotions make SiC HD-TG-MOS more advantageous for HF power conversion applications. In addition, another cell architecture variant adopting a similar concept is presented, and the according process implementation is addressed as well from viewpoint of manufacture.

Self-powered ultraviolet photodetector with very short and balanced response/recovery time based on ZnO/Si heterojunction with complex interface structure

A ZnO/Si heterojunction characterized by nonplanar complex interface structure was prepared by growing ZnO on silicon nanoporous pillar array (Si-NPA). Based on the heterojunction, a self-powered ultraviolet photodetector (UV PD) was structured and its UV detection properties were tested. Under UV irradiation of 360 nm laser, the on-off ratio of the detector increases with whereas the responsivity and specific detectivity decrease with the irradiation power density. This feature facilitates the flexibility to satisfy different measuring occasions or requirements simply through alternating the power density of UV irradiation. And most important of all, the PD can achieve UV detections with very short and balanced response/recovery time (∼50 ms) unchanging with the power density. The characteristics provide more options for optimizing the measuring condition selection in practical applications. The mechanism is attributed jointly to the enlarged specific interface area and the lowered barrier height of the junction, both of which are brought about by the unique microstructure of Si-NPA.

Influence of Oxygen Atoms and Ring Strain on the Low-Temperature Oxidation Pathways of 1,3-Dioxolane

Biohybrid fuels are a promising solution for making the transportation sector more environmentally friendly. One such interesting fuel candidate is 1,3-dioxolane, which can be produced from inedible biomass. However, very little kinetics data are available for the low-temperature oxidation of this fuel molecule. To remedy this, we present the reaction kinetics of O2 addition to 1,3-dioxolanyl radicals in this work. All energies have been calculated at the DLPNO-CCSD(T)/CBS//B2PLYPD3BJ/6-311+g(d,p) level of theory. Temperature- and pressure-dependent reaction rate constants have been calculated with the RRKM/master equation method. The effects of heterocyclic oxygen atoms and ring strain on the low-temperature oxidation of 1,3-dioxolane are also compared to that of similar fuel molecules containing five heavy atoms: cyclopentane, tetrahydrofuran, and diethyl ether (DEE). The ring-opening β-scission reactions of the dioxolane hydroperoxy species are found to be the most dominant pathways following the oxidation of 1,3-dioxolanyl radicals. The heterocyclic oxygen atoms in 1,3-dioxolane weaken its C-O bonds, which leads to low barrier heights of the ring-opening reactions. Ring strain in 1,3-dioxolane increases the barriers for isomerization reactions of peroxy radicals compared to the similar reactions of DEE, which has a chain structure.

Interface-Assisted Room-Temperature Magnetoresistance in Cu-Phenalenyl-Based Magnetic Tunnel Junctions

Delocalized carbon-based radical species with unpaired spin, such as the phenalenyl (PLY) radical, have opened avenues for developing multifunctional organic spintronic devices. Using direct laser writing and in situ deposition, we successfully fabricated Cu-PLY- and Zn-PLY-based organic magnetic tunnel junctions (OMTJs) with improved morphology and a reduced junction area of 3 × 8 μm2. The nonlinear and weakly temperature-dependent current–voltage (I–V) characteristics in combination with the low organic barrier height suggest tunneling as the dominant transport mechanism in the structurally and dimensionally optimized OMTJs. Cu-PLY-based OMTJs show significant magnetoresistance up to 14% at room temperature due to the formation of hybrid states at the metal–molecule interfaces called “spinterface”, which reveals the importance of spin-dependent interfacial modification in OMTJs’ design. Additionally, at high bias, in the absence of a magnetic field, OMTJ shows stable voltage-driven resistive switching. Cu-PLY having spin 1/2 with net magnetic moment demonstrates magnetic hardening between the surface molecule at the Co interface and gives rise to stable MR, which suggests its use as a feasible and scalable platform for building molecular-scale quantum memristors and processors.

Oxygen ion migration induced polarity switchable SrFeOx memristor for high-precision handwriting recognition

This paper reports a repeatable polarity reversal behavior in Au/SrFeOx (SFO)/SrRuO3 (SRO) memristor. The set and reset voltage direction can be inverted by applying a suitable reverse bias voltage, and the SFO device always exhibits fine cycling stability and retention performance regardless of operation polarity. The cross-sectional transmission electron microscope (TEM) images of the SFO memristor on positive and negative set statuses display apparent perovskite SrFeO3-δ (PV-SFO) conductive filaments and brownmillerite SrFeO2.5 (BM-SFO) barrier layer, respectively. The surface oxygen atomic ratio characterized by X-ray photoelectron spectroscopy (XPS) demonstrates the oxygen ions migration behaviors during the polarity reversal processes. Moreover, the energy band structure evaluated by first-principles calculation suggests that insufficient oxygen injection into the BM-SFO barrier layer could reduce the band gap of BM-SFO and form a PV-SFO/BM-SFO+ interface observed by TEM with the lower Schottky barrier height, facilitating Schottky emission and conduction. Furthermore, the SFO device on negative set status shows preferable handwritten digit recognition accuracy of 98.81 % and faster convergence speed than positive set status due to the better synaptic weight update linearity. Hence, the alternated polarity reversal features of the SFO memristor may provide a new reference for resistive memory and synaptic memristors.

Halogenation as a novel scheme for enhanced photocatalytic activity in AlSb monolayers: a first-principles study

The novel scheme of halogenating an AlSb monolayer was proposed to enhance photocatalytic activity. The calculated solar-to-hydrogen efficiency was high with low barrier height in the Gibbs free energy profile and favorable band alignment.

Contact optimisation strategy for wafer-scale field-effect transistors based on two-dimensional semiconductors

• Wafer-scale multilayered MoS2 films were fabricated by vacuum transferring technique. • A novel electrical contact architecture was proposed to improve the contact capability between 3D metal and 2D semiconductors. • A novel edge-treatment method for the MoS2 side was developed by employing a mature magnetron sputtering system. • Approximately 16 times lower contact resistivity (22.8 kΩ μm) and 3 times lower Schottky barrier height (34.6 meV) than that of the conventional top contacted devices were achieved. • Simplified resistor network model and energy-band diagrams were elaborately illustrated to discuss the mechanisms. Two-dimensional (2D) semiconductors can be utilized to continually miniaturize nanoscale electronic devices. However, achieving a practical solution for satisfying electrical contact with 2D semiconductors remains challenging. In this study, we developed a novel contact structure with transferred multilayer (t-ML) MoS 2 by integrating both edge and top contact. After in-situ plasma treatment for the edge of the MoS 2 channel and successive metal deposition, we achieved 16 times lower contact resistivity (22.8 kΩ µm) than that of the top contacted devices. The thickness-dependent electrical measurement indicates that edge contact is highly effective with thick MoS 2 due to the alleviated current-crowding effect resulting from the small contact area. The temperature-dependent transport measurement further confirms the effective minimization of the influence from the Schottky barrier and tunnelling barrier. Finally, the simplified resistor network model and energy-band diagram were proposed to understand the carrier transport mechanism. Our work provides a practical strategy for achieving excellent electrical contact between bulk metals and 2D semiconductors, paving the way for future large-scale 2D electronic devices.

Advances in solar energy harvesting integrated by van der Waals graphene heterojunctions.

Graphene has garnered increasing attention for solar energy harvesting owing to its unique features. However, limitations hinder its widespread adoption in solar energy harvesting, comprising the band gapless in the molecular orbital of graphene lattice, its vulnerability to oxidation in oxidative environments, and specific toxic properties that require careful consideration during development. Beyond current challenges, researchers have explored doping graphene with ionic liquids to raise the lifespan of solar cells (SCs). Additionally, they have paid attention to optimizing graphene/Si Schottky junction or Schottky barrier SCs by enhancing the conductivity and work function of graphene, improving silicon's reflectivity, and addressing passivation issues at the surface/interface of graphene/Si, resulting in significant advancements in their power conversion efficiency. Increasing the functional area of graphene-based SCs and designing efficient grid electrodes are also crucial for enhancing carrier collection efficiency. Flaws and contaminants present at the interface between graphene and silicon pose significant challenges. Despite the progress of graphene/Si-based photovoltaic cells still needs to catch up to the efficiency achieved by commercially available Si p-n junction SCs. The low Schottky barrier height, design-related challenges associated with transfer techniques, and high lateral resistivity of graphene contribute to this performance gap. To maximize the effectiveness and robustness of graphene/Si-based photovoltaic cells, appropriate interlayers have been utilized to tune the interface and modulate graphene's functionality. This mini-review will address ongoing research and development endeavors using van der Waals graphene heterojunctions, aiming to overcome the existing limitations and unlock graphene's full potential in solar energy harvesting and smart storage systems.

Low leakage current and high breakdown voltage of AlGaN/GaN Schottky barrier diodes with wet-etching groove anode

AlGaN/GaN heterojunction epitaxies with wide bandgap, high critical electric field as well as high density and high mobility two-dimensional electron gas have shown great potential applications in the next-generation high-power and high-frequency devices. Especially, with the development of Si-based GaN epitaxial technique with big size, GaN devices with low cost also show great advantage in consumer electronics. In order to improve the rectification efficiency of AlGaN/GaN Schottky barrier diode (SBD), low leakage current and low turn-on voltage are important. The GaN Schottky barrier diode with low work-function metal as anode is found to be very effective to reduce turn-on voltage. However, the low Schottky barrier height makes the Schottky interface sensitive to damage to groove surface, which leads to a high leakage current. In this work, a novel wet-etching technique with thermal oxygen oxidation and KOH corrosion is used to prepare the anode groove, and the surface roughness of groove decreases from 0.57 to 0.23 nm, compared with that of the dry-etching surface of groove. Meanwhile, the leakage current is suppressed from 1.5 × 10<sup>–6</sup> to 2.6 × 10<sup>–7</sup> A/mm. Benefiting from the great corrosion selectively of hot KOH solution to AlGaN barrier layer and GaN channel layer after thermal oxygen oxidation, the spikes of the edge of groove region caused by the nonuniform distribution of plasma in the cavity is improved, and the breakdown voltage of the fabricated AlGaN/GaN SBDs is raised from –1.28 to –1.73 kV.