Direct Tunneling Research Articles

Biodegradable natural chitosan coating films-based flexible resistive switching memory for transient electronics

Three flexible transparent transient memristors based on degradable natural chitosan coating films are reported. The Mg/Ag-doped chitosan/ITO sandwich structures were prepared by spin coating and drop coating, respectively. The plane structure device was also prepared. Three kinds of device exhibit different memory behaviors, namely, bipolar resistive switching behavior and butterfly resistive switching behavior accompanied by capacitance and negative resistance effect. The elucidated behavior of the bipolar resistive switching can be attributed to filament conduction. The behavior of the butterfly resistive switching is attributed to interface effects. In addition, the three devices have a large on/off ratio and long data retention time, showing promising non-volatile memory characteristics in flexible memory applications. The switching mechanism was evaluated by Schottky emission, hopping and direct tunneling. The transient memristor device prepared in this paper has excellent transient characteristics and controllable performance. This work reveals the potential of biodegradable flexible electronic devices.

Early estimation method of rockburst and large deformation of surrounding rock based on the deep borehole test

In geotechnical engineering, several major catastrophic accidents occur frequently in areas of high in-situ stress. To study the influence of high in-situ stress on deep mining, the in-situ stress was tested using the hydraulic fracturing method in the mine. Based on the measured data of the initial stress, a comprehensive evaluation of the stress field of the deep surrounding rocks was carried out. Combining the physical and mechanical indexes of the surrounding rocks, field measurements, and theoretical analysis, the Russenes criterion and Turchaninov criterion were applied to evaluate the propensity of rockbursts of hard rocks in the mine area. In addition, the large deformation of the soft rock in the mine was predicted based on the large deformation classification criteria. The results demonstrate that the vertical stress is linearly related to the depth. The horizontal principal stress values tested in all holes except boreholes G and I are approximately linearly distributed with depth. The greater the depth, the greater the propensity for rockburst. For obvious deviation from the maximum horizontal main stress direction of the mining tunnel, the tendency of rockburst during construction is greater. Slight deformation occurs when the tunnel surrounding rock is shallow than 660 m; larger deformation occurs when the burial depth exceeds 660 m. Level-Ⅱ or level-Ⅲ deformations may occur near the bottom of holes F, G, and I due to the lower uniaxial compressive strength of the phyllites of these holes.

Realizing improved performance of metal-insulator-semiconductor diodes with high-k MgO/SiOx stack

With the development of metal-insulator-semiconductor (MIS) field-effect transistors, the thickness of silicon dioxide (SiO2) must be scaled down; however, it causes a large gate leakage current and a low on/off current ratio. Magnesium oxide (MgO) exhibits a dielectric constant as high as 11.2, which is about three times larger than that (3.9) of SiO2; thus the thickness of MgO can be three times greater than that of SiO2 and significantly reduces the gate leakage current. Nevertheless, a large lattice mismatch (22.5%) exists between MgO and Si materials. In the study, the MgO/SiOx stack dielectrics are employed to suppress the large lattice mismatch and improve the performance of MIS diodes. The MgO/SiOx stack largely reduces the oxygen vacancy of MgO from 75% to 52.3%; also the fixed oxide charge and interface trap charge density are drastically decreased from 1.3 × 1012 and 1.8 × 1013 to 8.2 × 1011 cm−2 and 7.8 × 1012 cm−2eV−1, respectively. Compared to the MIS diodes with a single MgO, the MgO/SiOx stack dielectrics suppress the gate leakage current by about two orders in MIS diodes; hence the rectification ratio is enhanced from 114 to 1032 at ±2V. Band-diagram shows that Fowler-Nordheim tunneling occurs at a high reverse-bias voltage, and the barrier height increases from 0.18 to 0.42 eV in the MIS diodes with single MgO and MgO/SiOx stack dielectrics, respectively. Nevertheless, the MIS diodes operate through a direct tunneling at low reverse-bias voltages.

Interface engineering by inserting Al2O3 tunneling layer to enhance the performance of graphene/GaAs heterojunction photodetector

Interface engineering is an effective way to improve the performance of graphene-based photodetectors. Here, a graphene/GaAs heterojunction photodetector is fabricated, and when inserting an Al2O3 tunneling layer, its performance is improved through direct tunneling (DT) and Fowler-Nordheim tunneling (FNT). According to the experimental results, it is found that the thickness of the tunneling layer has a great influence on the performance of the photodetector. Compared with graphene/GaAs photodetector, the performance of graphene/Al2O3(2 nm)/GaAs photodetector is significantly improved with the responsivity, detectivity, and external quantum efficiency value of 0.80 A/W, 3.02 × 1011 Jones and 306% under 1 mW/cm2 light intensity at 2 V bias. Meanwhile, fast response is also observed (rise/decay time of 3 ms/8.6 ms). The improvement of the photodetector's performance in this work is mainly attributed to the effective modification of the interface state by the Al2O3 tunneling layer and the effect of the two tunneling mechanisms based on DT and FNT.

Temperature Evolution of Charge Transport in Chitosan Based Bio‐Resistive Random‐Access Memory Device

This study reports on the temperature stability of the Ag/chitosan/fluorine‐doped tin oxide (Ag/chitosan/FTO)‐based bio‐resistive random‐access memory (bio‐RRAM) device through current–voltage (I–V) characteristics in the temperature range of 280–360 K. From I–V characteristics, it is affirmed that in the present device, the unipolar nature of resistive switching is highly stable and reproducible. The device is quite stable at 360 K. Activation energy is higher in the low resistance state (LRS) (≈0.096 eV) compared with the high resistance state (HRS) (≈0.076 eV) due to sufficient thermal energy to cross the barrier at high temperature. From 280 to 360 K, the conduction mechanism in the HRS of the chitosan device is followed by a direct tunneling mechanism, while the Schottky mechanism is dominated in the LRS. Barrier height calculated from Schottky mechanism in an LRS is found to increase with temperature from 0.50 eV (280 K) to 0.66 eV (360 K). Evidenced current values up to 200 pA obtained with a conducting atomic force microscope infer that conduction in the chitosan‐based device is due to filaments formed by oxygen defects. It is believed that the present results are helpful for the development of future bio‐RRAM devices.

Longitudinal Seismic Analysis of Tunnels with Nonuniform Strata Considering the Effect of Karst

The longitudinal direction of shield tunnels is prone to seismic damage due to excessive deformation under seismic action, especially in nonuniform stratum. In this paper, the longitudinal dynamic response of the tunnel under different seismic effects is calculated based on the longitudinal equivalent stiffness model using the stratigraphic load model, and the seismic indexes such as longitudinal corner, tube sheet and joint bolt stresses are verified. The calculation results show that the longitudinal seismic weakness of the shield tunnel is in the interface between soft and hard strata and karst development. The longitudinal axial force of the structure is larger during the longitudinal excitation of seismic waves, and the maximum bending moment is mainly in the vertical plane, i.e., the vertical bending moment. The axial force of the tunnel is smaller during the transverse excitation of seismic waves. The maximum bending moment is mainly the bending moment in the horizontal plane, i.e., the transverse moment. The South Lake section of the Two Lakes Tunnel has good seismic performance in the event of a rare earthquake with a 50-year exceedance probability of 2%. The investigation can guide the seismic design of the South Lake section of the Two Lakes Tunnel.

Numerical study on the effect of protective wall depth in reducing structure deformations caused by tunnelling

To reduce tunnelling induced damage of buildings, a ‘protective wall’ is sometimes constructed between the location of the new tunnel and existing foundations. One of the key design questions relates to the optimal wall depth, where further increase has a limited benefit in reducing structural distortions. This paper presents the results from finite element analyses used to study how protective walls reduce tunnelling induced damage to adjacent structures. A scenario where the protective is located very close to a tunnel is studied, which is considered to be of practical interest and a critical case concerning potential for building damage. The analyses consider a tunnel with a row of 4 piles (in sand) supporting a 5-storey steel frame running transverse to the tunnel direction, with a wall located between the tunnel and the first pile. Results show that the protective wall’s depth affects soil and pile settlements, the redistribution of loads applied to the piles (due to structure stiffness), and ultimately structural deformations and building damage. For the case considered, the optimal depth of the wall is approximately 1.25 times the tunnel axis depth; marginal reductions in pile settlements and building damage are obtained as wall depth is increased further.

Electron conduction mechanisms in magnetic tunnel junctions fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier

CoFeB/ Si-Zn-Sn-O /CoFeB magnetic tunnel junctions (MTJs) have been fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier. In the low bias voltage range (up to ∼0.2 V), direct tunneling is found to be the dominant transport mechanism in MTJs. Tunneling conduction is further verified by simulation of tunnel current density and differential conductance using Simmon’s and Brinkmann model, respectively. Simulated results provided valuable insights into the barrier properties, including interfacial barrier height, thickness, and barrier asymmetry. Above the direct tunneling regime, electron transport in MTJs is governed by Pool Frenkel emission, which possibly arises due to the presence of high-density localized tail states below the conduction band of amorphous Si-Zn-Sn-O. Tunnelling magnetoresistance value of MTJs is found to be very low, which is attributed to the presence of various inelastic conduction channels. The results of this study might be useful to explore the potential of amorphous Si-Zn-Sn-O for fabricating low-resistive MTJ based spintronic devices.

Regulating optoelectronics of carbon dots with redox-active dopamine

The electron transfer (ET) processes in carbon-based quantum dots (CDs) have laid to extensive research endeavors due to their tunable optoelectronic properties with alteration in surface functionalization, doping, surface charge, etc. Here, we proffer shreds of evidence on pH-dependent electron transfer and transport properties of undoped CDs (UCDs) and amine-doped CDs (ACDs) with a redox-active neurotransmitter, dopamine (DA). A pronounced pH-dependent photoluminescence (PL) quenching is depicted for both the CDs with DA, where the quenching efficiency substantially increases in alkaline solvents. Taking advantage of the structural transformation of DA from hydroquinone to quinone, the electron accepting capacity can be improved for alkaline pH, which enhances the ET efficiency. The pH-regulated conductance measurements across the metal-CD-metal junction reveal a hike in the conductivity for acidic and alkaline pH relative to the neutral one. The I-V traces of UCDs and ACDs are contributed by an initial linear current rise and non-linear growth at higher bias explicating both direct and Fowler-Nordheim (F-N) tunneling mechanisms. In addition, the incorporation of DA into these CDs, not only significantly increases conductance in all pH media but also enables them to tunnel the potential barrier directly in acidic media, reducing F-N tunneling. These results could provide new avenues for developing reliable optoelectronic devices and sensing probes for a wide range of applications.

Experimental Study on Deformation Behavior of Shield Tunnel Subjected to Riverbed Scour Based on DOFS

Exploring the deformation behavior of shield tunnels is significant to design and safety assessment for shield tunnel structures. This paper carries out a reduced scale model test of shield tunnel subjected to riverbed scour for the first time. A new loading device system is designed to simulate the initial stress field of stratum and effect of riverbed scour. Distributed optical fiber sensing (DOFS) technology is adopted as measuring method to achieve the surrounding soil deformation and structural deformation, ovality, and radius of curvature of shield tunnel. The test results show that uneven rebound deformation of stratum and shield tunnel structure occurs in both transverse and longitudinal directions under the action of riverbed scour. Variations in ovality differences exist along the longitudinal direction of the shield tunnel, indicating that the transverse and longitudinal deformation of the shield tunnel affects each other. The radius of curvature of the longitudinal deformation of the shield tunnel at the vault is much smaller than that of the bottom. The results demonstrate the broad application value of DOFS technology in shield tunnel structure monitoring and reveal the deformation mechanism of shield tunnels subjected to riverbed scour, which can provide important reference values for the structure design and safety monitoring of shield tunnels.

Exploring optoelectronic properties of undoped and amine-doped carbon dots: Impact of surface functionalization and pH

Tuning optoelectronic properties through surface modifications and doping in low dimensional carbon dots (CDs) has been an asset in designing them for versatile applications. In this article, we explore the modulations in photophysical and charge transport characteristics of doped and undoped CDs functionalized with redox-active dopamine (DA). The pH-dependent DA chemistry on the surface of CDs is observed to have deterring effect on excited state dynamics with superior charge transfer efficiency for pristine CDs compared with the doped one. Charge transport studies through conducting measurements across electrode-CDs-electrode junction report a pH-induced current enhancement with DA functionalization resulting from a shifted equilibrium among the reduced (hydroquinone) and oxidized (quinone) isomers of DA. While the changes in the optical and electronic features are significantly perturbed with pH for the DA-treated undoped CDs, nominal alterations in spectral features are reported for the doped ones. The current-voltage conductance across the junctions is influenced by surface-treated CDs for acidic pH, with signatures of direct tunneling for a low bias regime, curtailing the Fowler-Nordheim (FN) tunneling. These features can have immediate optoelectronic-based applications in pH-based sensors and molecular electronics.

Ultrahigh Rectification Ratio in an Asymmetric Metal/Semiconductor/Metal Nanoscale Tunneling Junction: Implications for High-Frequency Rectifiers

The ultrafast quantum-based electron-transport mechanism with a typical tunneling time of femtoseconds is appealing to fabricate high-frequency rectifier diode devices for applications such as solar energy harvesting, THz mixers, and infrared detectors. Here, we demonstrate an ultrahigh rectification of more than 104 and a large forward current density of 13.8 A/cm2 at −2 V in the structure of a metal/semiconductor/metal (MSM, Pt/ZnO/TiN) nanoscale tunneling junction. Technology computer-aided design (TCAD) simulations demonstrate that high rectification originates from the switching of the electron-transport mechanism between direct tunneling (DT) and Fowler–Nordheim tunneling (FNT) under positive and negative voltage biases, respectively. The quantum-based FNT mechanism gives very slight temperature dependence of currents in a wide temperature range of 100–400 K. This work opens an avenue for high-frequency rectifier diodes based on MSM nanoscale tunneling junctions.

Control over Molecular Orbital Gating and Marcus Inverted Charge Transport in Molecular Junctions with Conjugated Molecular Wires (Adv. Electron. Mater. 2/2023)

Marcus Inverted Region In article number 2200637, Christian A. Nijhuis, Damien Thompson, Enrique Del Barco, and co-workers demonstrate that the mechanism of charge transport can be deliberately switched between the normal and Marcus Inverted regions by controlling the molecule–electrode coupling strength in a series of molecules. The cover shows two almost identical junctions. The only difference is one CH2 which lowers molecule-electrode coupling strength pushing the junctions from direct coherent tunnelling to hopping in the Marcus Inverted region. This ability can improve the energy efficiency of molecular devices. Image credit: Niels van der Velde.

The seismic damage mechanism of Daliang tunnel by fault dislocation during the 2022 Menyuan Ms6.9 earthquake based on unidirectional velocity pulse input

Fault dislocation may cause severe damage to tunnel structures. A representative example is the severe damage of the Daliang Tunnel caused by the dislocation of the Lenglongling Fault in the 2022 Menyuan, Qinghai, China, Ms6.9 earthquake. The relative displacement on both sides of the fault during an earthquake is often assumed to be a quasi-static dislocation load, ignoring the dynamic effect. In this study, the dislocation load was considered as a unidirectional harmonic velocity pulse and the damage mechanism of the Daliang tunnel was studied. First, the background of this earthquake and the damage of Daliang tunnel is introduced. The characteristics of surface displacement in this earthquake are analyzed by finite fault model and Okada dislocation theory model, and the dislocation vector of Daliang tunnel is calculated. On this basis, a fault-tunnel FEM nonlinear dynamic model is established, in which unidirectional velocity pulse is used as input of dislocation displacement to consider the dynamic effect of fault dislocation. The study found that the Daliang tunnel was located in one of the regions with the largest dislocation displacement in this earthquake, and the tunnel structure was damaged as a result. The analysis of FEM model reveals that when a unidirectional velocity pulse is used as the load, the tunnel damage is consistent with reality, and the conventional method using the quasi-static dislocation as the load only reflects the dislocation effect of the fault displacement and ignores the impact effect. When the dislocation displacement is certain, the duration of the unidirectional velocity pulse is an important parameter; the shorter the duration, the greater the amplitude of unidirectional velocity pulse, resulting in more serious tunnel damage. The tunnel’s failure mechanism near the fault can be divided into dislocation failure and impact failure. The dislocation failure is manifested by non-uniform dislocation displacement along the longitudinal direction of tunnel, which is mainly related to the amount of dislocation. The impact failure is manifested by crushing and invert uplift of the lining in cross-section, and it is related to the pulse duration.

Crossover from direct to trap assisted Fowler Nordheim (FN) tunneling in CoFeB/MgO/CoFeB magnetic tunnel junctions

Electron conduction mechanisms in CoFeB (0.8–10 nm)/MgO (3 nm)/CoFeB (4.2 nm) magnetic tunnel junctions (MTJs) have been investigated in detail. A clear crossover from direct tunnelling to trap assisted Fowler Nordheim (FN) tunneling is observed with increasing bias voltage in all the pristine MTJs. In contrast, FN-like tunneling is completely suppressed in vacuum annealed MTJs, which plausibly indicates diffused interfaces and enhanced trap state density in the MgO barrier. In annealed MTJs, beyond the direct tunneling regime, bulk-limited Pool Frenkel emission followed by interface-limited Schottky emission are found to be the dominant transport mechanisms. Simulations of tunnel current density J(V) and differential conductance (dJ/dV) of MTJs have been carried out using Simmons (symmetric rectangular barrier) and Brinkman (asymmetric trapezoidal barrier) model, which provides valuable insights into the barrier height and interface property at the MgO/CoFeB interface. Results of this study might be helpful to further improve CoFeB/MgO based MTJs for efficient implementation in sensors and memory devices.

Implementation of high-performance and high-yield nanoscale hafnium zirconium oxide based ferroelectric tunnel junction devices on 300 mm wafer platform

In this work, hafnium zirconium oxide (HZO)-based 100 × 100 nm2 ferroelectric tunnel junction (FTJ) devices were implemented on a 300 mm wafer platform, using a baseline 65 nm CMOS process technology. FTJs consisting of TiN/HZO/TiN were integrated in between metal 1 (M1) and via 1 (V1) layers. Cross-sectional transmission electron microscopy and energy dispersive x-ray spectroscopy analysis confirmed the targeted thickness and composition of the FTJ film stack, while grazing incidence, in-plane x-ray diffraction analysis demonstrated the presence of orthorhombic phase Pca21 responsible for ferroelectric polarization observed in HZO films. Current measurement, as a function of voltage for both up- and down-polarization states, yielded a tunneling electroresistance (TER) ratio of 2.28. The device TER ratio and endurance behavior were further optimized by insertion of thin Al2O3 tunnel barrier layer between the bottom electrode (TiN) and ferroelectric switching layer (HZO) by tuning the band offset between HZO and TiN, facilitating on-state tunneling conduction and creating an additional barrier layer in off-state current conduction path. Investigation of current transport mechanism showed that the current in these FTJ devices is dominated by direct tunneling at low electric field (E < 0.4 MV/cm) and by Fowler–Nordheim (F–N) tunneling at high electric field (E > 0.4 MV/cm). The modified FTJ device stack (TiN/Al2O3/HZO/TiN) demonstrated an enhanced TER ratio of ∼5 (2.2× improvement) and endurance up to 106 switching cycles. Write voltage and pulse width dependent trade-off characteristics between TER ratio and maximum endurance cycles (Nc) were established that enabled optimal balance of FTJ switching metrics. The FTJ memory cells also showed multi-level-cell characteristics, i.e., 2 bits/cell storage capability. Based on full 300 mm wafer statistics, a switching yield of >80% was achieved for fabricated FTJ devices demonstrating robustness of fabrication and programming approach used for FTJ performance optimization. The realization of CMOS-compatible nanoscale FTJ devices on 300 mm wafer platform demonstrates the promising potential of high-volume large-scale industrial implementation of FTJ devices for various nonvolatile memory applications.

Anisotropic charge transfer and gate tuning for p-SnS/n-MoS2 vertical van der Waals diodes.

2D-material-based van der Waals heterostructures (vdWhs) have shown great potential in next-generation multi-functional microelectronic devices. Thanks to their sharp interface and ultrathin thickness, 2D p-n junctions with high rectification properties have been established by combining p-type monochalcogenides with n-type transition metal dichalcogenides. However, the anisotropic rectification together with the charge transfer and gate effect has not been clarified. Herein, the electrical anisotropy of p-SnS/n-MoS2 diodes was studied. Optimum ideality factors within 1.08-1.18 have been achieved for the diode with 6.6 nm thick SnS on monolayer MoS2, and a high rectification ratio of 3.1 × 104 with strong in-plane anisotropy is observed along the zigzag direction of SnS. A strong gate effect on the anisotropic series resistance has been verified and an effective tuning over the transport length of the SnS channel can be established through adjustment of the current orientation and gate voltage. A thickness-dependent minority carrier transport mechanism has also been demonstrated for the reverse drain current, and Fowler-Nordheim tunneling and direct tunneling are proposed for the increase of the reverse current of the thicker and thinner diodes, respectively. This work will provide another strategy for high-performance diodes based on vdWhs via the control of the current orientation and the gate effect.

A Physics-Based Analytical Formulation for the Tunneling Current Through the Base of Bipolar Transistors Operating at Cryogenic Temperatures

A physics-based analytical solution for the direct tunneling current through the base region of bipolar transistors operating at cryogenic temperatures (CTs) is derived. The obtained formulation is continuously differentiable over the entire bias region and contains only few experimentally determinable model parameters, which makes it well-suited for compact circuit modeling. Very good agreement of the new formulation with both a numerical evaluation of the tunneling current integral and experimental data of two silicon–germanium (SiGe) heterojunction bipolar transistor (HBT) process generations is demonstrated.

Probing the Effect of the Density of Active Molecules in Large-Area Molecular Junctions.

The effect of the density of active molecules in molecular junctions (MJs) has been investigated by using a host/guest strategy. Mixed layers consisting of oligothiophene (BTB) encapsulated by β-cyclodextrin (BTB@β-CD) were generated. Cyclodextrins were then removed, and the pinholes generated were filled with BTB to obtain BTB@BTB films. MJs based on mixed BTB@β-CD and BTB@BTB layers, as well as single-component BTB MJs, were compared. The variation of ln J vs thickness is similar for all systems while the Jo of BTB@β-CD MJs is 20 times lower than that of BTB MJs. After β-cyclodextrin has been removed, and the pinholes filled, Jo increases and reaches the same value as for the BTB MJs, showing that the conductance scales with the number of active molecules. This strategy provides a unique method for investigating molecular interactions in direct tunneling MJs as well as the possibility of fabricating new functionalized MJs based on mixed layers.

Chemical Conformation Induced Transport Carrier Switching in Molecular Junction based on Carboxylic-Terminated Thiol Molecules

The paper demonstrates the effect of the chemical conformation of the -COOH group on the transport characteristic including conductance, rectification, and length effect in molecular junctions (MJs) formed by self-assembled monolayers of carboxylic-terminated thiol molecules. For an alkyl chain shorter than C11, the transport mechanism was attributed to a direct off-resonant tunneling of a hole carrier, located at the Au-S interface, whereas a hopping mechanism was assigned to the alkyl chain longer than the C11 chain located at the -COOH group. The hopping mechanism may be operated by electron transport associated with the breaking of the -OH bonding likely driven by a voltage. Importantly, at the C11 alkyl chain, we observed that the transport carrier operating in MJs could change from a hole carrier into an electron carrier. The result strongly proves that the chemical conformation should be considered in analyzing molecular electronics and provides a basis for the rational design of molecular electronic devices.