Intrinsic Tunnel Research Articles

Dielectric Pocket-Pocket Intrinsic Triple Gate TFET for Low Power Application: A Device Level Analysis

This report investigates a novel dielectric pocket—pocket intrinsic triple gate tunnel field effect transistor to magnify a device’s performance well suited for low-power applications. The source engineering which was adopted here using DP and PI has resulted in improved drive current of the device. A heterojunction was also formed between low bandgap energy materials and silicon, at the source side, to escalate the carrier tunneling at the interface of source and channel. Gate-drain overlapping technique, usage of high bandgap materials in drain region along with high-K dielectric materials in oxides ensured low ambipolar current, low off current, and a steeper subthreshold slope respectively. With these traits, the proposed device is believed to be suitable for low-power applications. The proposed structure is simulated, and the electrical parameters are analysed using the SILVACO TCAD ATLAS tool.

In-Situ TEM Observation of E-Beam Irradiation Induced Porous Molybdenum-Oxide Nanowires

The Mo5O14-type structure is an important and representative to the MoO-based catalyst in the selective oxidation process. In the petrochemical industries, the MoO-based catalyst has been widely doped with other elements like V, Nb and Te to have distinct properties with high activity, selectivity and conversion efficiency. The specific facet and bonding relationship would affect the catalyst properties and functionality. The catalyst is usually synthesized as nanoparticles, which have random configuration and uniformity. Therefore, we successfully synthesized the single-crystalline Mo5O14 nanowires, which have uniform structure and controllability about diameter and length by CVD process. Besides, nanowire catalyst with nanoporous structure could possess both advantages of nanoparticles and nanowires. Therefore, we aimed at systematically analyzing Mo5O14 nanowires and then fabricating nanoporous structure in nanowires by e-beam irradiation. Utilizing advanced TEM techniques could reveal the overall atomic structure, which have intrinsic tunnel structure and unique periodicity. In addition, the e-beam irradiation on the intrinsic tunnel structure is observed by in-situ TEM. During the e-beam irradiation process, the oxygen atoms would be removed and result in the structure transformation. Using this technology, the irradiation region could be definable and controllable by electron beam size to define the distribution of nanoporous in nanowires. These results would benefit to realizing the catalytic selective oxidation related with Mo5O14-type structure but also fabricating nanoporous nanowires with rapid and convenient methods. Figure 1

The role of the quadrupolar interaction in the tunneling dynamics of lanthanide molecular magnets

Quantum tunneling dominates the low temperature magnetization dynamics in molecular magnets and presents features that are strongly system dependent. The current discussion is focused on the terbium(III) bis(phtalocyanine) ([TbPc2]−1) complex that should serve as a prototypical case for lanthanide molecular magnets. We analyze numerically the effect of non-axial interactions on the magnitude of the intrinsic tunnel splitting and show that usual suspects like the transverse ligand field and Zeeman interaction fail to explain the experimentally observed dynamics. We then propose through the nuclear quadrupolar interaction a viable mechanism that mixes, otherwise almost degenerate hyperfine states.

An intrinsic magnetic tunnel junction

Magnetism An electrical current running through two stacked magnetic layers is larger if their magnetizations point in the same direction than if they point in opposite directions. These so-called magnetic tunnel junctions, used in electronics, must be carefully engineered. Two groups now show that

Intercalation pseudocapacitance in flexible and self-standing V2O3 porous nanofibers for high-rate and ultra-stable K ion storage

Transition metal oxides possess eminently high capacity based on the conversion reaction by delivering multiple electrons. However, they usually demonstrate limited cycle life and inferior rate capability due to the pulverization and aggregation of active materials. V2O3 is considered to be an attractive electrode material due to its intrinsic tunnel structure provided by 3D V-V framework. Herein, a flexible and self-standing electrode of V2O3 nanoparticles embedded in porous N-doped carbon nanofibers (V2O3@PNCNFs) has been fabricated through a facile electrospinning method and subsequent thermal treatment. Electrochemical kinetic analysis, in situ XRD, ab initio molecular dynamics (AIMD) and density functional theory (DFT) calculation suggest that the potassium storage of V2O3 is dominated by intercalation pseudocapacitance and only up to 1 mol K+ can insert into V2O3 crystal. Different from the conversion reaction in conventional transition metal oxides, in the mechanism of intercalation pseudocapacitance, K+ intercalate into the tunnels of V2O3 accompanied by a faradaic charge-transfer with no crystallographic phase change. This mechanism combines short charging/discharging times with long cycle life, which is reported in PIBs for the first time. This work may open up a new avenue for further development of pseudocapacitive nanomaterials for high-rate and ultra-stable energy storage.

Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling.

Magnetic insulators are a key resource for next-generation spintronic and topological devices. The family of layered metal halides promises varied magnetic states, including ultrathin insulating multiferroics, spin liquids, and ferromagnets, but device-oriented characterization methods are needed to unlock their potential. Here, we report tunneling through the layered magnetic insulator CrI3 as a function of temperature and applied magnetic field. We electrically detect the magnetic ground state and interlayer coupling and observe a field-induced metamagnetic transition. The metamagnetic transition results in magnetoresistances of 95, 300, and 550% for bilayer, trilayer, and tetralayer CrI3 barriers, respectively. We further measure inelastic tunneling spectra for our junctions, unveiling a rich spectrum consistent with collective magnetic excitations (magnons) in CrI3.

Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures.

Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here, we report multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance that is drastically enhanced with increasing CrI3 layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. Using magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of the atomically thin CrI3 Our work reveals the possibility to push magnetic information storage to the atomically thin limit and highlights CrI3 as a superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.

Comparison of laser-induced and intrinsic tunnel magneto-Seebeck effect inCoFeB/MgAl2O4and CoFeB/MgO magnetic tunnel junctions

We present a comparison of the tunnel magneto-Seebeck effect for laser induced and intrinsic heating. Therefore, Co$_{40}$Fe$_{40}$B$_{20}$/MgAl$_2$O$_4$ and Co$_{25}$Fe$_{55}$B$_{20}$/MgO magnetic tunnel junctions have been prepared. The TMS ratio of 3\,\% in case of the MAO MTJ agrees well with ratios found for other barrier materials, while the TMS ratio of 23\,\% of the MgO MTJ emphasizes the influence of the CoFe composition. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alternatively be explained by the Brinkman model and the given junction properties. Especially, we are able to demonstrate that the symmetric contribution is solely influenced by the barrier asymmetry. Thus, we conclude that the symmetry analysis used for the intrinsic method is not suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect.

A novel sub 20 nm single gate tunnel field effect transistor with intrinsic channel for ultra low power applications

We propose a nanoscale single gate ultra thin body intrinsic channel tunnel field effect transistor using the charge plasma concept for ultra low power applications. The characteristics of TFETs (having low leakage) are improved by junctionless TFETs through blending advantages of Junctionless FETs (with high on current). We further improved the characteristics, simultaneously simplifying the structure at a very low power rating using an InAs channel. We found that the proposed device structure has reduced short channel effects and parasitics and provides high speed operation even at a very low supply voltage with low leakage. Simulations resulted in IOFF of ∼ 9 × 10−16 A/μm, ION of ∼20 μA/μm, ION/IOFF of ∼2 × 1010, threshold voltage of 0.057 V, subthreshold slope of 7 mV/dec and DIBL of 86 mV/V for PolyGate/HfO2/InAs TFET at a temperature of 300 K, gate length of 20 nm, oxide thickness of 2 nm, film thickness of 10 nm, low-k spacer thickness of 10 nm and VDD of 0.2 V.

A structural health assessment method for shield tunnels based on torsional wave speed

Following recent rapid developments in tunnel engineering in China, the heavy structural maintenance work of the future is likely to pose a great challenge. Newly developed vibration-based health assessment and monitoring methods offer good prospects for large-scale structural monitoring, hidden surface detection and disease pre-judgment. However, because the dynamic properties of tunnels are sensitive to the coupling and damping effects of the surrounding soil, there is little relevant research on tunnel structures. Using the PiP (pipe in pipe) model, the intrinsic tunnel modes and their response characteristics are investigated in this paper, and the degree to which the identification of these characteristics is influenced by mode superposition and the soil coupling effect are also considered. The response features of these flexible wave modes are found to be barely recognizable in a tunnel-soil coupled system, while the phase velocity of the torsional wave can be determined by combining phase spectrum analysis and the HHT (Hilbert-Huang transformation) method. A new structural health assessment method based on the torsional wave speed is therefore proposed. In this method, the torsional wave speed is used to determine the tunnel structure’s global stiffness based on a newly developed dispersion algorithm. The calculated stiffness is then used to evaluate the tunnel’s structural service status. A field test was also carried out at a newly built tunnel to validate the proposed method; the tunnel structure’s Young’s modulus was obtained and was very close to the designed value. This indicates that this method is an effective way to assess tunnel service conditions, and also provides a theoretical basis for future applications to health assessment of shield tunnels.

Giant intrinsic tunnel magnetoresistance in manganite thin films etched with antidot arrays

Huge intrinsic tunnel magnetoresistance effects at low field are demonstrated in macroscopic La0.33Pr0.34Ca0.33MnO3 thin films etched with periodic antidot arrays, and a highest magnetoresistance ratio (about 1600%) is achieved at 58 K. Such giant tunnel magnetoresistance effect might originate from delicate phase separation and coherent transport under the applied periodic spatial confinement. Strong transport fluctuation is also revealed in such systems due to phase competition. Our findings pave a way to realize tunnel magnetoresistance devices based on electronically phase separated materials with spatial modulations.

Metastable resistivity states and conductivity fluctuations in low-doped La1−xCaxMnO3 manganite single crystals

Conductivity noise in dc current biased La0.82Ca0.18MnO3 single crystals has been investigated in different metastable resistivity states enforced by applying voltage pulses to the sample at low temperatures. Noise measured in all investigated resistivity states is of 1/f-type and its intensity at high temperatures and low dc bias scales as a square of the bias. At liquid nitrogen temperatures for under bias exceeding a threshold value, the behavior of the noise deviates from above quasi-equilibrium modulation noise and depends in a non monotonic way on applied bias. The bias range of nonequilibrium 1/f noise coincides with the range at which the conductance increases linearly with bias voltage. This feature is attributed to a broad continuity of states enabling indirect inelastic tunneling across intrinsic tunnel junctions. The nonequilibrium noise has been ascribed to indirect intrinsic tunneling mechanism while resistivity changes in metastable states to variations in the energy landscape for charge carriers introduced by microcracks created by the pulse procedures employed.

Inverse thermal hysteresis and peculiar transport properties of La0.9MnO3-δ film

Magnetic and transport properties of 100 nm thick La0.9MnO3-δ manganite films have been studied in the temperature range 5 − 300 K. The films exhibit a paramagnetic to ferromagnetic transition at TC ≈ 194 K. The temperature dependence of the resistivity shows a metal-insulator transition at 204 K and a strong resistivity increase at temperatures T < 160 K. Variations of resistivity with magnetic field and current are non-hysteretic, while the temperature dependence of the resistivity exhibits unusual inverse thermal hysteresis. The magnetic field independent inverse thermal hysteresis depends significantly on the thermal history of the sample. The data suggests that nonlinear transport characteristics are attributed to inelastic tunneling through intrinsic tunnel juctions formed by phase separated ferromagnetic metallic domains and insulating antiferromagnetic matrics.

Tunneling magnetoresistance in phase-separated manganite nanobridges

The manganite $(\text{La},\text{Pr},\text{Ca}){\text{MnO}}_{3}$ is well known for its micrometer-scale phase separation into coexisting ferromagnetic metallic (FMM) and insulating regions. Fabrication of bridges with widths smaller than the phase-separation length scale has allowed us to probe the magnetic properties of individual phase-separated regions. At the onset of phase separation, a magnetic field induced insulator-to-metal transition among a discrete number of domains within the narrow bridges gives rise to abrupt, low-field colossal magnetoresistance steps at well-defined switching fields. At lower temperatures when the FMM phase becomes energetically favorable, the insulating regions shrink to form thin insulating strips separating adjacent FMM regions with different coercive fields. Tunneling magnetoresistance is observed across the naturally occurring intrinsic insulating strips (tunnel barriers) spanning the width of the bridges. The presence of such intrinsic tunnel barriers introduces an alternative approach to fabricating novel nanoscale magnetic tunnel junctions.

Magnetic separation and inelastic tunneling in self-doped manganite films

Magnetic and transport properties of 100 nm thick La0.9MnO3−δ self-doped manganite films have been investigated in the temperature range 5–300 K. The films exhibit a paramagnetic to ferromagnetic transition at TC=194 K. The temperature dependence of the resistivity shows a metal-insulator transition at 204 K and a strong resistivity increase below 160 K. The magnetoresistance was always negative and slightly bias dependent. Variations in resistivity with magnetic field and current are nonhysteretic, while the temperature dependence of the resistivity exhibits unusual inverse thermal hysteresis. The magnetic field independent inverse thermal hysteresis is strongly influenced by a thermal history of the sample. The data suggest that nonlinear low temperature transport is dominated by inelastic tunneling through intrinsic tunnel junctions formed by phase-separated ferromagnetic metallic domains and insulating antiferromagnetic matrix.

Intrinsic Tunneling in Phase Separated Manganites

We present evidence of direct electron tunneling across intrinsic insulating regions in submicrometer wide bridges of the phase-separated ferromagnet (La,Pr,Ca)MnO3. Upon cooling below the Curie temperature, a predominantly ferromagnetic supercooled state persists where tunneling across the intrinsic tunnel barriers (ITBs) results in metastable, temperature-independent, high-resistance plateaus over a large range of temperatures. Upon application of a magnetic field, our data reveal that the ITBs are extinguished resulting in sharp, colossal, low-field resistance drops. Our results compare well to theoretical predictions of magnetic domain walls coinciding with the intrinsic insulating phase.

Giant positive magnetoresistance in ultrathin films of mixed phase manganites

Magnetic tunnel junctions (MTJs) based on fully spin polarized ferromagnetic manganites have generated a lot of interest due to their enhanced field sensitivity at low temperatures. However, the tunneling magnetoresistance (TMR) drops rapidly with increasing temperature due to the reduction of spin polarization at the manganite-insulator interface. We have devised a method for creating intrinsic tunnel barriers by tuning the phase competition in manganites using substrate induced strain. Ultrathin films (7.5nm) of the mixed phase manganite (La0.5Pr0.5)0.67Ca0.33MnO3 (LPCMO) grown on the substrate (110) NdGaO3 using pulsed laser deposition show positive magnetoresistance (MR) of about 30% at magnetic fields less than 1T. Unlike the fabricated MTJ devices, this MR effect has its maximum value close to the insulator to metal transition temperature and reduces with decreasing temperature. To find out the mechanism leading to this positive MR, the effect of three orientations of the magnetic field on the LPCMO thin films were studied: (1) perpendicular to the plane of the film, (2) parallel to the plane of the film and applied current, and (3) parallel to the plane of the film but perpendicular to the applied current. The effect of field orientation suggests that a possible mechanism for the positive MR is tunneling magnetoresistance due to the spin conserving tunneling process across the insulating regions separating the ferromagnetic metallic regions. The voltage dependence of the MR also supports this mechanism. Our results suggest a novel method for obtaining enhanced TMR in manganite based MTJs by creating strain induced intrinsic tunnel barriers.

Intrinsic Reliability of AlOx-Based Magnetic Tunnel Junctions

We present a detailed investigation into the intrinsic tunnel barrier reliability in AlOx-based magnetic tunnel junctions (MTJs) as a function of aluminum thickness and oxidation time. The intrinsic reliability is measured as the ramped breakdown voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(V _ bd)$</tex> at room temperature for both positive and negative polarity. We find that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ V_ bd$</tex> generally increases with the resistance-area (RA) product of the MTJ. While this dependence is quite strong at low RA, it gradually weakens for higher RA. At fixed RA, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_ bd$</tex> also depends on the original Al film thickness with better properties for thicker Al. Finally, we observe a polarity dependence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$V_ bd$</tex> which changes sign as the MTJ goes from thin Al to thick Al. We attribute the polarity dependence to the different quality of the top and bottom interfaces and conclude that the interface emitting the tunneling electrons primarily governs the barrier reliability.

Intrinsic tunnelling effects in self-doped La0.89MnO3 single crystals

Transport properties of self-doped La0.89MnO3 single crystals with Neel temperature of TN ≈139 K have been investigated in wide temperature range 10–300 K. Data suggests that current at low temperature is conducted through a strongly temperature-dependent, but almost bias independent channel operating in parallel with a bias controlled but temperature independent channel. The first channel is associated with transport across an insulating antiferromagnetic matrix while the latter one represents tunnel conductivity through intrinsic tunnel junctions appearing due to interruption of conducting percolating paths by phase separated insulating inclusions. Tunnel character of the conductivity manifests itself in nonlinear current-voltage characteristics and appearance of a zero-bias anomaly in the form of a prominent conductance peak in the vicinity of zero bias. Zero bias anomaly and V-shaped characteristics of the differential conductance at high voltages are ascribed to the formation of local magnetic states in the insulating region of the tunneling junction.

Inhomogeneous superconductivity in deeply underdoped Bi 2Sr 2CaCu 2O 8+ δ

To study electronic states in deeply underdoped cuprates, we have measured the interlayer tunneling spectra of Bi 2Sr 2CaCu 2O 8+ δ ( T c = 44 K) realized by partially substituting Sr with La. A mesa structure is manufactured which is composed of 10 or less stacked intrinsic tunnel Josephson junctions. The tunneling characteristics exhibits a totally unknown behavior that the differential conductance increases significantly with decreasing temperature below T c. This anomalous behavior observed only for deeply underdoped samples strongly suggest that inhomogeneous superconductivity or phase separation in CuO 2 planes is essential bulk property of deeply underdoped cuprates.