Tunneling Barrier Height Research Articles

Tunneling transport of mono- and few-layers magnetic van der Waals MnPS3

We have investigated the tunneling transport of mono- and few-layers of MnPS3 by using conductive atomic force microscopy. Due to the band alignment of indium tin oxide/MnPS3/Pt-Ir tip junction, the key features of both Schottky junction and Fowler-Nordheim tunneling (FNT) were observed for all the samples with varying thickness. Using the FNT model and assuming the effective electron mass (0.5 me) of MnPS3, we estimate the tunneling barrier height to be 1.31 eV and the dielectric breakdown strength as 5.41 MV/cm.

Metallic Functionalization of CdSe 2D Nanoplatelets and Its Impact on Electronic Transport

We explore the gold functionalization of 2D CdSe nanoplatelets (NPL) as a possible way to tune their electronic and transport properties. We demonstrate that the size and location of the gold tip can be controlled using light and temperature. The Au tip–CdSe NPL hybrid presents a large rise of the conductance compared to the pristine semiconductor (i.e., without gold functionalization). The role of the semiconductor in this transport remains unclear and needs to be better understood. We hypothesize four mechanisms: (i) a reduction of the band gap energy due to the formation of a gold–selenium compound, (ii) a charge transfer between the metal and the semiconductor leading to an increase in carrier concentration, (iii) a change in the inter-nanoparticle tunnel barrier height, and (iv) a simple percolation process between the metallic grain. X-ray photoelectron spectroscopy (XPS) shows that the CdSe NPL are unaffected by oxidation and that gold is in the metallic state Au0. We consequently exclude the formation of a narrow band gap Au2Se phase as the possible mechanism leading to the observed rise of conductance. Moreover, Kelvin probe force microscopy and XPS give evidence for an increase in work function upon gold tipping, which can be interpreted in terms of a shift of the Fermi level toward the valence band maximum. As hole conduction in CdSe NPLs is very unlikely to occur, we rather favor the hypothesis that the strong increase in conduction is largely driven by percolation between the metallic tips as the main mechanism responsible for transport in this hybrid system.

Ultrathin Epitaxial Ferromagnetic γ‐Fe2O3 Layer as High Efficiency Spin Filtering Materials for Spintronics Device Based on Semiconductors

In spintronics, identifying an effective technique for generating spin‐polarized current has fundamental importance. The spin‐filtering effect across a ferromagnetic insulating layer originates from unequal tunneling barrier heights for spin‐up and spin‐down electrons, which has shown great promise for use in different ferromagnetic materials. However, the low spin‐filtering efficiency in some materials can be ascribed partially to the difficulty in fabricating high‐quality thin film with high Curie temperature and/or partially to the improper model used to extract the spin‐filtering efficiency. In this work, a new technique is successfully developed to fabricate high quality, ferrimagnetic insulating γ‐Fe2O3 films as spin filter. To extract the spin‐filtering effect of γ‐Fe2O3 films more accurately, a new model is proposed based on Fowler–Nordheim tunneling and Zeeman effect to obtain the spin polarization of the tunneling currents. Spin polarization of the tunneled current can be as high as −94.3% at 2 K in γ‐Fe2O3 layer with 6.5 nm thick, and the spin polarization decays monotonically with temperature. Although the spin‐filter effect is not very high at room temperature, this work demonstrates that spinel ferrites are very promising materials for spin injection into semiconductors at low temperature, which is important for development of novel spintronics devices.

Leaky Integrate-and-Fire Neuron Circuit Based on Floating-Gate Integrator.

The artificial spiking neural network (SNN) is promising and has been brought to the notice of the theoretical neuroscience and neuromorphic engineering research communities. In this light, we propose a new type of artificial spiking neuron based on leaky integrate-and-fire (LIF) behavior. A distinctive feature of the proposed FG-LIF neuron is the use of a floating-gate (FG) integrator rather than a capacitor-based one. The relaxation time of the charge on the FG relies mainly on the tunnel barrier profile, e.g., barrier height and thickness (rather than the area). This opens up the possibility of large-scale integration of neurons. The circuit simulation results offered biologically plausible spiking activity (<100 Hz) with a capacitor of merely 6 fF, which is hosted in an FG metal-oxide-semiconductor field-effect transistor. The FG-LIF neuron also has the advantage of low operation power (<30 pW/spike). Finally, the proposed circuit was subject to possible types of noise, e.g., thermal noise and burst noise. The simulation results indicated remarkable distributional features of interspike intervals that are fitted to Gamma distribution functions, similar to biological neurons in the neocortex.

Revealing buried hydrogen bonds

Surface Science One way to stabilize self-assembly monolayers of alkane thiols on gold surfaces is to introduce amide groups into the chain near the thiol groups to form hydrogen-bonding networks. This subsurface location has made direct structural studies challenging, but Thomas et al. have used scanning tunneling microscopy and local tunneling barrier height studies of 3-mercapto- N -nonylpropionamide on Au{111} (gold films on mica) at cryogenic temperatures to map the spatial orientation of the hydrogen bond. These networks can cross the boundaries between the domains that form with different rotational orientations. ACS Nano 10.1021/acsnano.6b01717 (2016).

Enhanced Tunneling Electroresistance in Ferroelectric Tunnel Junctions due to the Reversible Metallization of the Barrier.

Realizing a large tunneling electroresistance (TER) effect is crucial for device application of ferroelectric tunnel junctions (FTJs). FTJs are typically composed of a thin ferroelectric layer sandwiched by two metallic electrodes, where TER generally results from the dependence of the effective tunneling barrier height on the ferroelectric polarization. Since the resistance depends exponentially not only on barrier height but also on barrier width, TER is expected to be greatly enhanced when one of the electrodes is a semiconductor where the depletion region near the interface can be controlled via ferroelectric polarization. To explore this possibility, we perform studies of SrRuO_{3}/BaTiO_{3}/n-SrTiO_{3} FTJs, where n-SrTiO_{3} is an electron doped SrTiO_{3} electrode, using first-principles density functional theory. Our studies reveal that, in addition to modulation of the depletion region in n-SrTiO_{3}, the BaTiO_{3} barrier layer becomes conducting near the interface for polarization pointing into n-SrTiO_{3}, leading to dramatic enhancement of TER. The effect is controlled by the band alignment between the semiconductor and the ferroelectric insulator and opens the way for experimental realization of enhanced TER in FTJs through the choice of a semiconducting electrode and interface engineering.

Contact doping, Klein tunneling, and asymmetry of shot noise in suspended graphene

The inherent asymmetry of the electric transport in graphene is attributed to Klein tunneling across barriers defined by $\textit{pn}$-interfaces between positively and negatively charged regions. By combining conductance and shot noise experiments we determine the main characteristics of the tunneling barrier (height and slope) in a high-quality suspended sample with Au/Cr/Au contacts. We observe an asymmetric resistance $R_{\textrm{odd}}=100-70$ $\Omega$ across the Dirac point of the suspended graphene at carrier density $|n_{\rm G}|=0.3-4 \cdot 10^{11}$ cm$^{-2}$, while the Fano factor displays a non-monotonic asymmetry in the range $F_{\textrm{odd}} \sim 0.03 - 0.1$. Our findings agree with analytical calculations based on the Dirac equation with a trapezoidal barrier. Comparison between the model and the data yields the barrier height for tunneling, an estimate of the thickness of the $\textit{pn}$-interface $d < 20$ nm, and the contact region doping corresponding to a Fermi level offset of $\sim - 18$ meV. The strength of pinning of the Fermi level under the metallic contact is characterized in terms of the contact capacitance $C_c=19 \times 10^{-6}$ F/cm$^2$. Additionally, we show that the gate voltage corresponding to the Dirac point is given by the work function difference between the backgate material and graphene.

Semiquantitative Submolecular Barrier Height Measurements of 4-Aza-8-fluorotryptanthrin Monolayers on HOPG: Orbital-Mediated Tunneling

4-Aza-8-fluoroindolo[2,1-b]quinazoline-6,12-dione (4-aza-8-fluorotryptanthrin) was imaged via scanning tunneling microscopy and barrier height tunneling spectroscopy at the graphite–solution interface. Submolecular resolution was achieved for the apparent barrier height (ϕap) images and revealed that ϕap displays significant variation over the area of the molecule. Topographic effects and electrostatic potential effects were found to be insufficient to explain the behavior of the measured ϕap variations. Specifically, where barrier height images were collected over regions of large molecular orbital (MO) wave function amplitudes, the measured ϕap did not agree with either affecting phenomenon. The authors attribute the disparity to the fact that electron tunneling is orbital-mediated for these systems. Thus, the ϕap is dominated by the spatial disposition of the mediating MO in locations of large wave function amplitude. This hypothesis is corroborated by excellent agreement with a computational treatment...

Ballistic transfer of spin momentum in multiferroic tunnel junction

The field dependence of critical voltages of switching of magnetic states of a synthetic multiferroic structure is studied based on a bifurcation analysis of Landau–Lifshitz–Gilbert equations with the torques caused by a tunneling spin transport, taking into account the voltage dependence of transferred spin momenta at the variations in the magnitude and direction of electric polarization. The voltage dependences of transferred spin momenta are determined based on the free electron model, taking into account exchange splitting of electron energy subbands in magnetic beaches and the effect of changing the tunnel barrier height at the variations in the polarization magnitude and state.

The influence of interfacial tensile strain on the charge transport characteristics of MoS2-based vertical heterojunction devices

We demonstrate the charge transport characteristics of MoS2-based vertical heterojunction devices through the formation of interfacial strain. Atomically thin MoS2 bilayers were directly synthesized on a p-type Si substrate by using chemical vapor deposition to introduce an interfacial tensile strain in the vertical heterojunction diode structure, which was confirmed by Raman, X-ray and ultraviolet photoelectron spectroscopy techniques. The electrical and optoelectronic properties of the heterojunction devices with the as-grown MoS2 (A-MoS2) on p-Si were compared with those of transferred MoS2 (T-MoS2)/p-Si devices. To clearly understand the charge transport characteristics induced by the interfacial tensile strain, the Fowler-Nordheim (FN) analysis of the electrical properties of the diode devices was conducted with the corresponding energy band diagrams. All of the fabricated MoS2-based vertical diodes exhibited clearly rectifying behaviors, but the photoresponse properties of the A-MoS2-based and T-MoS2-based heterojunctions exhibited distinct differences. Interestingly, we found that the tunneling barrier heights of the A-MoS2-based heterojunction devices were relatively higher than those of the T-MoS2-based devices and were almost the same before and after illumination due to the interfacial tensile strain, whereas those of the T-MoS2-based devices were lowered after illumination. Our study will help further understand the charge transport properties of 2D material-based heterojunction devices in the presence of interfacial strain, ultimately enabling the design of electronic and optoelectronic devices with novel functionalities.

Semiquantitative Electron Tunneling Barrier Height Measurements of Molecular Monolayers at the Solution–Graphite Interface: Nonorbital-Mediated Tunneling

Semiquantitative apparent tunneling barrier measurements were collected of stearic acid adsorbed onto highly oriented pyrolytic graphite from 1-phenyloctane. A detailed investigation of the topography revealed an interesting geometric phenomenon, namely, the observation of different conformers within a single monolayer; this phenomenon has not been previously reported, despite the large volume of work on these and similar systems in the literature. Apparent barrier height images showed two separate effects. The apparent barrier height map is dominated by the distribution of surface charge (as tracked by electrostatic potential) in regions where significant polarity exists (i.e., near the carboxyl groups). Conversely, in regions where there is little variation in electrostatic potential (i.e., near the interlaced alkane chains), the manifestation of the topography dominates the apparent barrier height. The electron tunneling involved in imaging these types of monolayers is not mediated by adlayer molecular...

MgO magnetic tunnel junctions of enduring F-type upon annealing

The authors performed magnetotransport experiments to determine whether annealing alters the oxygen vacancy-mediated tunnelling potential landscape of the central portion of a MgO ultrathin film within sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions. Using the Îrel method reveals a temperature-dependent tunnelling barrier height for a non-annealed barrier that arises from single oxygen vacancies (F centres) and is qualitatively identical to that found for its partly and fully annealed counterparts. Thus these MTJs with F centres remain of F-type upon annealing. This explicitly confirms that the large tunnel-magnetoresistance (TMR) increase upon annealing results mainly from structural modifications of MgO and CoFeB and not from vacancy pairing within the barrier. Photoluminescence spectra performed on both annealed and non-annealed thin MgO films grown on CoFeB electrodes support this conclusion. This work should promote renewed scrutiny over the precise impact of annealing on tunnelling magnetotransport across MgO.

Investigation of local tunneling current noise spectra on the silicon crystal surfaces by means of STM/STS

We report on a careful analysis of the local tunneling conductivity by means of ultra-high vacuum scanning tunneling microscopy/spectroscopy (STM/STS) technique in the vicinity of low-dimensional structures on the Si(111)–(7 × 7) and Si(110)–(16 × 2) surfaces. The power-law exponent α of low-frequency tunneling current noise spectra is investigated for different values of the tunneling contact parameters: relaxation rates, the localized state coupling, and the tunneling barrier width and height.

A co-assembled organic thin film for nanoscale data storage with lowered threshold voltage

By means of photoelectric effect, nanoscale data storage with lowered pulsed voltage was achieved in an ordered co-assembled organic thin film using the scanning tunneling microscopy (STM) lithography method. The local I–V characteristics of the self-assembled thin film showed that the ultraviolet light-induced charge transfer could increase the value of the tunneling current and consequently induce the effective reduction in the tunneling barrier height. Furthermore, the macroscopic I–V characteristics of the thin film also demonstrated that the photoelectric effect can be used to lowering the threshold voltage for memory device. The results indicate that the tunable optoelectrical property of a material is of significance for highly efficient STM-based data storage and reduces the power consumption of storage device.

Impact of Uniaxial Strain on Random Telegraph Noise in High-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$k$ &lt;/tex-math&gt;&lt;/inline-formula&gt;/Metal Gate pMOSFETs

The random telegraph noise (RTN) characteristics of high- $k$ (HK)/metal gate (MG) pMOSFETs with uniaxial compressive strain have been investigated. The configuration-coordinate diagram and band diagram are both established by extracting trap parameters, including capture and emission time, activation energy for capture and emission, trap energy level, and trap location in gate dielectric. Through a comparison of RTN results and gate-leakage current density ( $J_{G})$ between HK/MG pMOSFETs with and without uniaxial compressive strain in channel, it is found that the trap position from the insulator/semiconductor interface is reduced in the uniaxial compressive strained HK/MG pMOSFETs. This is reasonably attributed to the strain-increased tunneling barrier height and out-of-plane effective mass, which brings about the reduction in the tunneling attenuation length. Meanwhile, it can also be demonstrated by the lower $J_{G}$ in uniaxial compressive strained HK/MG pMOSFETs.

Tuning the Ge(Sn) Tunneling FET: Influence of Drain Doping, Short Channel, and Sn Content

We present experimental results on the realization of p-channel mode Ge(Sn) heterojunction band-to-band tunneling field effect transistors. We investigate the influence of three device parameters (drain doping, channel length, and tunnel barrier height at source side) of the semiconductor body of the devices on the device performance. We achieve a complete suppression of the n-channel mode in p-type operating conditions by systematically reducing the p-type drain doping from 1·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> to 2 · 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , examined in sample series A. In the second sample series B, we investigate the influence of a reduction of the channel length down to 15 nm on transistor performance. To improve the ON current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> without degrading the OFF current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> , we introduce a 10-nm delta layer of a Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy at the source/channel junction in the third sample series C. We demonstrate an improved ON current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> compared with the reference sample without a GeSn delta layer.

Fluoride binding to an organoboron wire controls photoinduced electron transfer.

We demonstrate that the rates for long-range electron transfer can be controlled actively by tight anion binding to a rigid rod-like molecular bridge. Electron transfer from a triarylamine donor to a photoexcited Ru(bpy)32+ acceptor (bpy = 2,2'-bipyridine) across a 2,5-diboryl-1,4-phenylene bridge occurs within less than 10 ns in CH2Cl2 at 22 °C. Fluoride anions bind with high affinity to the organoboron bridge due to strong Lewis base/Lewis acid interactions, and this alters the electronic structure of the bridge drastically. Consequently, a large tunneling barrier is imposed on photoinduced electron transfer from the triarylamine to the Ru(bpy)32+ complex and hence this process occurs more than two orders of magnitude more slowly, despite the fact that its driving force is essentially unaffected by fluoride addition. Electron transfer rates in proteins could potentially be regulated via a similar fundamental principle, because interactions between charged amino acid side chains and counter-ions can modulate electronic couplings between distant redox partners. In artificial donor-bridge-acceptor compounds, external stimuli have been employed frequently to control electron transfer rates, but the approach of exploiting strong Lewis acid/Lewis base interactions to regulate the tunneling barrier height imposed by a rigid rod-like molecular bridge is conceptually novel and broadly applicable, because it is largely independent of the donor and the acceptor, and because the effect is not based on a change of the driving-force for electron transfer. The principle demonstrated here can potentially be used to switch between conducting and insulating states of molecular wires between electrodes.

Chemically doped three-dimensional porous graphene monoliths for high-performance flexible field emitters.

Despite the recent progress in the fabrication of field emitters based on graphene nanosheets, their morphological and electrical properties, which affect their degree of field enhancement as well as the electron tunnelling barrier height, should be controlled to allow for better field-emission properties. Here we report a method that allows the synthesis of graphene-based emitters with a high field-enhancement factor and a low work function. The method involves forming monolithic three-dimensional (3D) graphene structures by freeze-drying of a highly concentrated graphene paste and subsequent work-function engineering by chemical doping. Graphene structures with vertically aligned edges were successfully fabricated by the freeze-drying process. Furthermore, their number density could be controlled by varying the composition of the graphene paste. Al- and Au-doped 3D graphene emitters were fabricated by introducing the corresponding dopant solutions into the graphene sheets. The resulting field-emission characteristics of the resulting emitters are discussed. The synthesized 3D graphene emitters were highly flexible, maintaining their field-emission properties even when bent at large angles. This is attributed to the high crystallinity and emitter density and good chemical stability of the 3D graphene emitters, as well as to the strong interactions between the 3D graphene emitters and the substrate.

Electrical Properties of p-type 3C-SiC/Si Heterojunction Diode Under Mechanical Stress

The current mechanism and effects of external transverse stress in the [110] orientation on the electrical properties of a single crystal (100) p-3C-SiC/p-Si heterojunction diode are reported for the first time. It has been observed that the current flow in the heterojunction is due to tunneling through the triangular potential barrier formed due to valence band offset between Si and SiC. The applied stress produces small changes in tunneling current when stress is increased from 0 to 308 MPa. The observed increase in current at 0.24 V is 10% at maximum stress of 308 MPa. The increase of tunneling current when applying stress is explained in terms of stress, which alters the out-of-plane effective mass, and the effective tunneling barrier height of holes in top subbands of p-type Si.

Tunneling transport in a few monolayer-thick WS2/graphene heterojunction

This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed that WS2 exhibits tunnel barrier characteristics when its thickness is between 2 and 5 monolayers, whereas MoS2 experiences a transition from tunneling to thermionic emission transport with increasing thickness within the same range. Tunnel resistance in a graphene/WS2/metal heterostructure therefore increases exponentially with the number of WS2 layers, revealing the tunnel barrier height of WS2 to be 0.37 eV.