Direct Tunneling Process Research Articles

1.43 kV GaN-based MIS Schottky barrier diodes

In this letter, we report on a quasi–vertical GaN-based metal–insulator–semiconductor (MIS) Schottky barrier diode (SBD) with an insertion of 2 nm thick Al2O3 dielectric layer. It shows a turn-on voltage of 0.7 V, a specific on-resistance of 3.5 mΩ·cm2, and a high on/off current ratio of 1011. The proposed structure enables a breakdown voltage of 1430 V, rendering a Baliga’s power figure-of-merit of 0.58 GW cm−2. The enhanced performance is attributed to defect-related leakage can be suppressed and the direct tunneling process dominates at the MIS-based Schottky contact interface.

Magnetic investigations of monocrystalline [Co(NCS)2(L)2]n: new insights into single-chain relaxations.

A large single crystal of a compound from the family of coordination polymer [Co(NCS)2(L)2]n chains was synthesized and its magnetic properties are reported. [Co(NCS)2(4-(3-phenylpropyl)pyridine)2]n is ferromagnetic with Tc = 3.39 K. Single-ion ab initio calculations predict an almost Ising-type magnetic anisotropy and the direction of the magnetic easy-axis nearly along the Co-Npy bond of the apical pyridine-based co-ligand. Both predictions are confirmed by single-crystal magnetic measurements. The magnetic relaxation of the single crystal sample significantly differs from the powder sample data, and clearly shows the presence of two separate relaxation processes. The process dominant below 3.2 K demonstrates a single chain magnet (SCM) behaviour, with a crossover between single-wall and two-wall processes, in spite of the fact that the system is ferromagnetically ordered. The faster process that dominates just below Tc is attributed to spin waves. Micromagnetic Monte Carlo simulations of the investigated compound show that the dipolar field cancels for some chains located at the border between 3-dimensional domains. Such chains are responsible for the measured ac signal, and demonstrate the SCM behaviour. The quantitative analysis of the SCM relaxation time is supported by preparing and examining a corresponding diamagnetically diluted compound, [CoxCd1-x(NCS)2(4-(3-phenylpropyl)pyridine)2]n (x = 0.013), which behaves as a field-induced single-ion magnet. The relaxation pathways for single Co(ii) spins are determined to be Raman, direct, and quantum tunneling processes, which were included in an improved approach to describe the magnetic relaxation in the Co(ii)-based SCM compound.

Transport properties of proximitized double quantum dots

We study the sub-gap spectrum and the transport properties of a double quantum dot coupled to metallic and superconducting leads. The coupling of both quantum dots to the superconducting lead induces a non-local pairing in both quantum dots by the Andreev reflection processes. Additionally, we obtain two channels of Cooper pair tunneling into a superconducting lead. In such a system, the direct tunneling process (by one of two dots) or the crossed tunneling process (by both quantum dots at the same time) is possible. We consider the dependence of the Andreev transmittance on an inter-dot tunneling amplitude and the coupling between a quantum dot and the superconducting lead. We also consider the occurrence of interferometric Fano-type line shapes in the linear Andreev conductance spectra.

Modeling the Reverse Gate-Leakage Current in GaN-Channel HFETs: Realistic Assessment of Fowler–Nordheim and Leakage at Mesa Sidewalls

A model considering different leakage paths for describing the reverse gate leakage in mesa-isolated polar GaN-channel heterostructure field-effect transistors (HFETs) is presented. For AlGaN/GaN HFETs, it is illustrated that for small negative values of gate–source bias, gate leakage happens from the gate-covered mesa sidewalls to the 2-D electron gas (2-DEG). The bias and temperature dependences of the gate current show that the sidewall path to the 2-DEG is associated with the Poole–Frenkel electron emission. As the gate–source bias becomes more negative, electrons choose a different path to the 2-DEG. In this case, results corroborate that the gate leakage is dominated by the Fowler–Nordheim (FN) direct tunneling process through the III-nitride barrier. The novel contribution of the present analysis is that it postulates that in absence of absolute uniformity, FN tunneling takes place through only a small portion of the surface of the barrier, which boasts the highest electric field or the smallest Schottky barrier height. By applying this hypothesis, the origin of the inconsistencies inherent to the previously presented models in selecting the value of the electron effective mass can be explained.

Field-Induced Single-Ion Magnet Behaviour in Two New Cobalt(II) Coordination Polymers with 2,4,6-Tris(4-pyridyl)-1,3,5-triazine

We herein reported the syntheses, crystal structures, and magnetic properties of a two-dimensional coordination polymer {[CoII(TPT)2/3(H2O)4][CH3COO]2·(H2O)4}n (1) and a chain compound {[CoII(TPT)2(CHOO)2(H2O)2]}n (2) based on the 2,4,6-Tris(4-pyridyl)-1,3,5-triazine (TPT) ligand. Structure analyses showed that complex 1 had a cationic hexagonal framework structure, while 2 was a neutral zig-zag chain structure with different distorted octahedral coordination environments. Magnetic measurements revealed that both complexes exhibit large easy-plane magnetic anisotropy with the zero-field splitting parameter D = 47.7 and 62.1 cm−1 for 1 and 2, respectively. This magnetic anisotropy leads to the field-induced slow magnetic relaxation behaviour. However, their magnetic dynamics are quite different; while complex 1 experienced a dominating thermally activated Orbach relaxation at the whole measured temperature region, 2 exhibited multiple relaxation pathways involving direct, Raman, and quantum tunneling (QTM) processes at low temperatures and Orbach relaxation at high temperatures. The present complexes enlarge the family of framework-based single-ion magnets (SIMs) and highlight the significance of the structural dimensionality to the final magnetic properties.

Electrical mechanisms of bi-stable memory devices based on an Al/PVK:ZnO NPs/ITO structure with different ZnO NPs annealing temperatures

We reported on the influence of the structural properties of zinc oxide nanoparticles (ZnO NPs) on electrical bi-stable device based on an aluminum/poly(9-vinylcarbazole):ZnO NPs/indium-tin oxide structure. ZnO NPs were synthesized by simple precipitation method with different annealing temperatures. When annealing temperature was increased, the ZnO NPs crystallite increased and the ZnO NPs gradually changed shape from nanostar-like to nanospherical. The structural parameters, such as the crystallite size of ZnO NPs and dislocation density, can be calculated from the XRD spectra. The effect of ZnO NPs annealing temperature on memory properties can be measured by current-voltage characteristics and retention time measurements. The maximum ON/OFF current ratio for the memory devices was about 103 and the devices exhibited the Write-Once, Read-Many (WORM) memory type. The operating mechanisms of the memory device were analyzed using theoretical models, in which electrons trapping in the ZnO NPs and electrons tunneling among a PVK matrix by direct tunneling process were discussed.

Decoherence of an entangled state of a strongly correlated double quantum dot structure through tunneling processes

We consider two quantum dots described by the Anderson-impurity model with one electron per dot. The goal of our work is to study the decay of a maximally entangled state between the two electrons localized in the dots. We prepare the system in a perfect singlet and then tunnel-couple one of the dots to leads, which induces the non-equilibrium dynamics. We identify two cases: if the leads are subject to a sufficiently large voltage and thus a finite current, then direct tunneling processes cause decoherence and the entanglement as well as spin correlations decay exponentially fast. At zero voltage or small voltages and beyond the mixed-valence regime, virtual tunneling processes dominate and lead to a slower loss of coherence. We analyze this problem by studying the real-time dynamics of the spin correlations and the concurrence using two techniques, namely the time-dependent density matrix renormalization group method and a master-equation method. The results from these two approaches are in excellent agreement in the direct-tunneling regime for the case in which the dot is weakly tunnel-coupled to the leads. We present a quantitative analysis of the decay rates of the spin correlations and the concurrence as a function of tunneling rate, interaction strength, and voltage.

Carrier Transport Mechanisms of the Programming and Retention Characteristics for TaN–Al2O3–Si3N4–SiO2–Si Flash Memory Devices

Carrier transport mechanisms of the programming and retention characteristics for TaN–Al2O3–Si3N4–SiO2–Si (TANOS) flash memory devices were theoretically investigated by using the one-band model taking into account Shockley–Reed statistics, the continuity equation, and the Pöisson equation. The simulation results showed that the dominant tunneling mechanism during the programming operation in the TANOS memory devices was varied from the Fowler–Nordheim (FN) tunneling to the direct tunneling (DT) processes and from the DT to the modified FN tunneling processes. The dominant tunneling mechanism in TANOS memory devices at the retention operation mode without gate bias voltage was DT process. Simulation results showed that the retention time increased with increasing tunneling oxide thickness. The threshold voltage shifts, as determined from the theoretical calculation during the programming and retention process, were in reasonable agreement with the threshold voltage shifts obtained from experimental results.

Theoretical studies on nonvolatile holographic recording for LiNbO3:Cu:Ce crystals

The steady-state nonvolatile two-step, two-color holographic recording performance is studied theoretically for LiNbO3:Cu:Ce based on the two-center model, with taking into account the direct electron transfer between the deep-trap center Cu+/Cu2+ and the shallow-trap center Ce3+/Ce4+ due to the tunneling effect. The results show that the total space-charge field is determined by the space-charge field on the deep-trap center, and the direct electron exchange between the Cu+/Cu2+ and the Ce3+/Ce4+ levels through the tunneling effect dominates the charge-transfer process in the two-step, two-color holographic recording. Therefore, the material parameters related to this direct tunneling process play a key role in the two-step, two-color holography performance.

Investigation of Electronic Transport in Lateral NiFe/Al$_{2}$O$_{3}$/p-Si/Al$_{2}$O$_{3}$/NiFe Junctions

A series of lateral junctions consisting of NiFe/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /p-Si/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /NiFe have been investigated for the understanding of manipulating the electronic spin in semiconducting channel. Devices with various conducting channel lengths between two electrodes were fabricated using a top-down technique. An ion beam sputtering was used for the required film stack and an electron beam lithography in combination with ion beam etching technique were adopted for patterning. The current-voltage measurements reveal a remarkable phenomenon of current enhancement, indicating a formation of the interface state in such devices. This transport behavior associated with trapping assistance has also been demonstrated with a fitting based on Poole-Frenkel effect. The results also show that a transition from trapping assisted process to direct tunneling process occurs when the semiconducting channel length is below 1 μm. In addition, the current enhancement may be suppressed with increasing the thickness of the oxide layer.

Resonant electron tunneling through defects in GaAs tunnel diodes

Current-voltage characteristics of GaAs tunnel diodes are studied experimentally and theoretically. In theoretical calculations contributions of three different transport mechanisms are considered: direct tunneling processes, nonresonant multiphonon tunneling processes via defects, and resonant tunneling processes through defects. The comparison between theoretical results and experimental data reveals resonant tunneling as the dominant transport mechanism at voltages corresponding to the peak current. At higher voltages this mechanism is replaced by nonresonant tunneling, which is in its turn replaced by over-barrier transport at even larger voltages.

Modeling and simulation for the enhancement of electron storage in a stacked multilayer nanocrystallite silicon floating gate memory

In this article, we investigate the storage enhancement mechanism of stacked multilayer nanocrystallite silicon (nc-Si) structures in a master-equation-based equivalent circuit model. As a theoretical extension from our previous experimental works, we reveal the detail injection sequence of electrons into the multilayer nc-Si structure via a direct tunneling process, and how the retention property is enhanced by the stacked structures. Seeking a further improvement in the multilayer nc-Si-based nonvolatile memory structure, we compare two major approaches for that purpose, i.e. (1) by further increasing the number of stacked layers or (2) by adopting an asymmetric double-layer structure. It is shown that the latter is more promising for achieving better nonvolatile storage property and shows a more effective threshold shifting, while retaining the virtues of direct tunneling process like fast write/erase and low operation voltage. We suggest that these results provide important guides for practical design of memory devices based on multilayer nc-Si floating gate structures.

Vertical spin transport in MnAs/GaMnAs heterostructures

We have studied the effect of barrier strength on the tunneling magnetoresistance of MnAs/GaMnAs heterostructures with double AlAs barriers. The epitaxial structures were grown by low-temperature molecular beam epitaxy. The vertical magnetotransport properties were studied for various GaAs spacer thicknesses. We find that the junction resistivities of double barrier samples increase exponentially as the barrier strength increases, implying that direct tunneling process governs the transport properties. In contrast, the tunneling magnetoresistance depends primarily on the number of interfaces rather than on the barrier strength.

CdTe/Zn 1−x Mg x Te self-assembled quantum dots: Towards room temperature emission

We report the dependence of the growth and the optical properties of self-assembled CdTe/Zn1−xMgxTe quantum dots on the barrier Mg content x (0⩽x⩽0.3). Due to the decrease of the lattice mismatch between CdTe and Zn1−xMgxTe with increasing x, we use a technique for inducing dot formation, based on efficient reduction of the surface energy by deposition of amorphous Te, which is then desorbed. Mg incorporation in the barriers leads to a better heavy-hole confinement along the growth axis, which is manifested in photoluminescence (PL) studies by both an extension of the radiative regime temperature range (up to 150 K for 30% Mg) and a strong increase of the activation energy for the nonradiative recombination. However, the in-plane confinement is less enhanced, which allows observation of interdot carrier transfer with increasing temperature, as evidenced directly by the analysis of PL intensities for different single dots. Our temperature-dependent data (time-resolved and microphotoluminescence) suggest that this transfer consists of a thermally activated process via the two-dimensional wetting-layer states rather than a direct tunneling (hopping) process.

Emission lifetime of polarizable charge stored in nano-crystalline Si based single-electron memory

The lifetime of the emission of a single electron stored in a nanocrystalline Si (nc-Si) dot has been studied in order to understand the physical processes for memory applications. A small active area field effect transistor channel (50×25 nm) is defined by electron-beam lithography on a thin (20 nm) silicon-on-insulator channel and allows for the electrical isolation of a single nc-Si dot. Remote plasma enhanced chemical vapor deposition is used to form 8±1 nm diameter nc-Si dots in the gas phase from a pulsed SiH4 source. Electrons stored in a dot results in an observed discrete threshold shift of 90 mV. Analysis of lifetime as a function of applied potential and temperature show the dot to be an acceptor site with nearly Poisson time distributions. An observed 1/T2 dependence of lifetime is consistent with a direct tunneling process, and interface states are not the dominant mechanism for electron storage in this device structure. Median emission lifetimes as a function of applied gate bias are readily modeled by the polarizability of an electron in a delocalized bound state over the entire semiconducting dot.

Theoretical modeling of dark current and photo-response for quantum well and quantum dot infrared detectors

Dark currents (due to direct tunneling process) and photo-response for both quantum well (QW) and quantum dot (QD) systems are calculated. A stabilization method (SM) is used to study the dynamic behavior and photo-response of an electron in an isolated QW, coupled QW or QD (with conical shape) under uniform electric field. A stabilization graph is obtained by plotting the eigenvalues of a QW or QD embedded in a confining box made of barrier material as functions of the size of the box. The eigenvalues of the system are calculated within the effective mass approximation via the Rayleigh–Ritz variational method. Density of states (DOSs) and time-dependent wave functions associated with the quasi-bound state are constructed via the SM. It is shown that the DOSs have a Lorentzian profile and the width of the Lorentzian profile gives the tunneling rate. The photo-response for QWs and conical QDs are studied and implications for device applications are discussed.

Current transport in two-dimensional HTS NID junctions

Motivated by existing difficulties in the experimental observation of the zero bias anomaly (ZBA) in HTS NID and DID structures with thick dielectric barriers we have studied theoretically the effect of scattering of electrons on localized states (LS) within a barrier on width and amplitude of the ZBA. Our model structure consists of thick dielectric barrier with predominantly two-dimensional (2D) character of the electron transport (e.g. PBCO), separated normal metal (N), and an HTS superconductor with d-wave superconducting correlation (D). The barrier is modeled by a 2D rectangular potential with δ-functional cavities describing LS. To study conductance spectra the concept of coherent transport through systems with complex barrier developed by Büttiker and Belogolovskii et al. has been generalized to the case of breaking of the conservation of the quasiparticle momentum component parallel to the interfaces in the tunneling process. We have shown that transport properties of NID structures essentially differ from the transport in contacts with conventional superconductors (NIS) and principally cannot be described in a one-dimensional approximation. Direct analytical calculations give that tunneling via LS does not contribute to ZBA independently on their space position in the barrier and on the relation between the metals Fermi energy and LS energy level. It is also shown that the interference current component, which usually was not taken into account in the description of NIS contacts, suppresses the amplitude of ZBA provided by the direct tunneling process. The resultant ZBA peak should be very narrow and its amplitude is effectively suppressed by any scatters located in the barrier. This makes the experimental ZBA observation in NID structures with thick barriers very questionable.

Degradation of direct-tunneling gate oxide under hot-hole injection

The degradation of silicon-dioxide thin films induced by the hole direct-tunneling process is investigated using a substrate hot-hole (SHH) injection technique. Hot holes from the substrate, as well as cold holes in the inversion layer, are injected into the gate oxide in p-channel metal–oxide–semiconductor field-effect transistor with a p+ polycrystalline-silicon gate. The gate bias used is kept low enough to avoid simultaneous electron injection from the gate. During the SHH stress, in contrast to the case of thicker oxide films, a strong correlation is observed between the oxide film degradation and the injected hole energy, while no degradation occurs due to the direct tunneling of holes from the inversion layer. These experimental findings indicate the existence of threshold energy for the trap creation process, which has been predicted by the theoretical study of the hole-injection-induced structural transformation of oxygen vacancies in silicon dioxide.

Charge Storage Mechanism in Nano-Crystalline Si Based Single-Electron Memories

AbstractA memory device sensitive to a single electron stored in a nanocrystalline Si dot has been synthesized allowing for the study of charge retention lifetime. A 50 nm by 20 nm transistor channel is synthesized by E-beam lithography followed by reactive ion etching of thin (20nm) Silicon-on-Insulator (SOI) channel. This small area of the narrow channel allows for the isolation of a single nano-crystalline Si dot and elimination of channel percolation paths around the screening charge. Remote Plasma Enhanced CVD is used to form 6±2nm diameter nc-Si dots in the gas phase from a pulsed SiH4 source. Electrons stored in a dot results in an observed discrete threshold shift of 90 mV. Analysis of lifetime as a function of applied potential and temperature show the dot to be an acceptor site with nearly Poisson lifetime distributions. An observed 1/T2 dependence of lifetime is consistent with a direct tunneling process, thus interface states are not the dominant mechanism for electron storage in this device structure. Median lifetimes are modeled by a direct tunneling process, which is influenced by gate bias and dot size.

A quantitative analysis of stress-induced leakage currents and extraction of trap properties in 6.8 nm ultrathin silicon dioxide films

An analytical model for the quantitative analysis of stress-induced leakage currents (SILC) in ultrathin silicon dioxide films is described, which enables the extraction of trap parameters, e.g., trap site location. Assuming a two-step trap-assisted inelastic tunneling mechanism, the conduction of electrons through the silicon dioxide film proceeds as follows: First, electrons tunnel from the cathode into neutral trap sites followed by an energy relaxation into the lowest available energy state of these trap sites. Finally, the electrons reach the anode by a direct tunneling process. We applied this model to the SILC characteristics of a stressed 6.8 nm ultrathin silicon dioxide film. The following parameters could be deduced: The trap sites are located at 4.47 nm relative to the cathode interface with a trapped sheet charge density of |6.54×10−8| C/cm2, and a trap state energy of 2.3 eV relative to the conduction band edge of the silicon dioxide.