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The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated using a combined experimental and theoretical approach. On the experimental side, GaN/AlN RTDs are grown by MBE. In-situ electron diffraction is employed to monitor the number of monolayers incorporated into each tunneling barrier. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current as a function of barrier thickness. Both the peak voltage and characteristic threshold bias exhibit a dependence on barrier thickness as a result of the intense electric fields present in the polar heterostructures. To get further insight into the asymmetric tunneling injection, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current with contributions from coherent and sequential tunneling processes is introduced. After applying this theory to the case of GaN/AlN RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities, with tunneling currents spanning several orders of magnitude. This agreement allows us to elucidate the role played by the internal polarization fields on the magnitude of the tunneling current and broadening of the resonant line shape. Under reverse bias, we identify new tunneling features originating from highly attenuated resonant tunneling phenomena, which are completely captured by our model. Our analytical model, provides a simple expression which reveals the connection between the polar RTD design parameters and its current-voltage characteristics. This new theory paves the way for the design of polar resonant tunneling devices exhibiting efficient resonant current injection and enhanced tunneling dynamics, as required in various practical applications., 13 pages, 3 figures

Resonant tunneling properties of finite Gaussian double quantum barrier structure are studied in the absence and presence of electric field bias. Non-equilibrium Green's function method based on the finite difference method is used. A detailed analysis of the resonant energy level is given and the importance of system parameters is discussed. The dependence of the transmission properties on the barriers and electric field bias are revealed. A comparison between different barrier shape data is presented.

Spin-dependent resonant tunneling in CdTe/CdMnTe double-barrier heterostructures was theoretically investigated. The transfer-matrix method was employed to study the spin transport properties under zero applied field. The effect of Dresselhaus spin- orbit coupling was considered in the symmetrical heterostructure. The influences of the barrier width and height on the polarization efficiency, barrier transparency and dwell time of light and heavy holes were observed and discussed.

The ballistic electron transport characteristics of hyperbolic Poschl-Teller double-barrier resonant tunneling structures are investigated. The transmission coefficient and the current density-voltage characteristic in electrically biased one dimensional hyperbolic Poschl-Teller double-barrier structures with different structure parameters have been calculated by using non-equilibrium Green's function method. Results for the transmission coefficients and the current densities are compared with the barrier shapes. The results show that the widths of the wells and heights of barriers have a significant effect on the transmission properties. The current density-voltage characteristic presents N-type negative differential resistance.

In this work, we have modelled and computed the transport properties of the double-barrier InGaAs/GaAsSb structure from the resonant tunnelling point of view. Based on the classical Tsu-Esaki formula for the tunnelling current, we have calculated the current density-voltage characteristic of a Al-free type-II $$\hbox {In}_{0.53}\hbox {Ga}_{0.47}\hbox {As/GaAs}_{0.51}\hbox {Sb}_{0.49}$$In0.53Ga0.47As/GaAs0.51Sb0.49 structure at different cryogenic and elevated temperatures. The tunnelling coefficient has been calculated in the framework of effective mass approximation with nonparabolicity included, using the transfer matrix approach. A good qualitative agreement of the position of resonant current peaks with the existing experimental data was achieved. Our calculation shows a very high sensitivity of the tunnelling current peak on monolayer-scale layer structure fluctuation which strongly affects peak to valley ratio in the resonant tunnelling structure.

This paper will cover the basic working of tunnel diode, its V-I characteristics curve, principal function of RTD, the Double Barrier Quantum well (DBQW) structure, and applications. This will enable the reader to get a fair idea about what tunnel diodes exactly are and understand the importance and scope of their applications.

The Seebeck coefficients are studied in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with the structures as CoFeB/MgO/ZnO/MgO/LSMO. In the linear response regime, oscillatory behavior is observed with change in the ZnO layer thickness because of the resonant tunneling in the DBMTJs. Effects of average temperature and magnetizations orientation are also discussed. It is found that the Seebeck coefficients are greater in the DBMTJs than asymmetric single-barrier MTJs (SBMTJs). Therefore, it is to be achieved the greater Seebeck coefficients by choosing a proper thickness for the ZnO layer in the proposed asymmetric DBMTJs.

We successfully improved the tunnel magnetoresistance ratio (TMR) of an MgO/CoFeB/MgO-based double-barrier magnetic tunnel junction (DMTJ) by inserting ultra-thin Fe into a CoFeB middle electrode. The Fe played as an absorber of B diffused out of the CoFeB and thereby facilitated the crystallization of the CoFeB at the interface. Changes in resistivity of the middle electrode with Fe insertion calculated by the modified current-in-plane tunneling method suggested that they can be explained by the formation of Fe borides, which is ascribed to the fact that Fe indeed promoted the diffusion of B by providing places for the B diffusion. As a result, Fe insertion improved the (001) texture of the middle electrode and thereby increased the TMR of the junction due to enhanced coherent tunneling. However, when inserted Fe was thick, the tunnel properties of DMTJs were degraded because of the absence of strong (001) texture of MgO. Materials and their thickness for a middle electrode are key parameters to successful application of DMTJs for spin devices.

Spin-dependent thermoelectric (TE) effects are studied in MgO-based double-barrier magnetic tunnel junctions with a non-magnetic metal spacer (DBMTJs-NM) in the linear response regime. Using non-equilibrium Green’s function (NEGF) formalism, the results show resonant peaks at specific thicknesses of the NM spacer because of the resonant tunneling effect through the DBMTJs-NM. Effects of average temperature and magnetization alignment are also described. Therefore, thermally charge and spin voltage lead to large spin-dependent TE effects by the DBMTJs-NM structures.

Charge current and thermal spin-transfer torque (TSTT) in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer as CoFeB/MgO/NM/MgO/LSMO are studied by non-equilibrium Green’s function (NEGF) formalism. It is found that the results show oscillatory behavior with an increase of the NM spacer thickness due to the resonant tunneling effect through the DBMTJs structures. About the dependence of the temperature difference and angle between the magnetizations, the charge current and TSTT are larger at the specific thicknesses (resonant positions) of the NM spacer than the others. The results also illustrate a magnitude enhancement for asymmetric DBMTJs compared with asymmetric single-barrier MTJs (SBMTJs). Therefore, it is possible to achieve a large TSTT for magnetization dynamics by new asymmetric DBMTJs.

The charge and spin thermopower in double-barrier magnetic tunnel junctions with a semiconductor (e.g. GaAs) spacer (DBMTJs-SC) are studied using non-equilibrium Green’s function (NEGF) formalism in the linear response regime. The results show oscillatory behavior with an increase of the SC spacer thickness due to the resonant tunneling effect through the DBMTJs-SC. Effects of the average temperature of the junctions and the spin-splitting of the ferromagnetic (FM) contacts on the charge and spin thermopower are also described. It is found that the results show a magnitude enhancement at the specific thicknesses of the SC spacer (resonant positions) compared with the other thicknesses.

The substitution effect of a Ferro-magnet (FM) electrode by a half-metallic FM material La0.7Sr0.3MnO3 (LSMO) on charge current and spin-transfer torque (STT) components is studied in MgO-based double barrier magnetic tunnel junctions (DBMTJs) with a middle non-magnetic metal (NM) layer. Using non-equilibrium Green’s function (NEGF) formalism, it is observed that the current and STT components show oscillatory behavior due to quantum well states in the middle NM layer and resonant tunneling effect. We also study effect of difference in the thickness of the MgO insulators. Bias dependence demonstrate the magnitude enhancement of the current and in-plane STT in new asymmetric DBMTJs (A-DBMTJs) compared with symmetric DBMTJs (S-DBMTJs), however, perpendicular STT decreases in the A-DBMTJs. Results also show different behavior compared with conventional asymmetric MTJs and spin valves (SVs). Therefore, one can design new memory devices by means of suitable insulator and FM electrodes with proper thicknesses.

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH

The hot carrier cell (HCSCs) is a promising advanced photovoltaic concept. It aims to minimize major losses in p-n junction solar cells and is predicted to have energy conversion efficiency over 65% at one sun. Energy selective contacts (ESCs) is a critical component of the HCSC. In this paper, potential of ESCs based on quantum dot and quantum well double barrier resonant tunneling structures is presented. Results of I-V measurements these show negative differential resistance (NDR) characteristics both at low temperature and room temperature, thus indicating that these structures can be suitable for ESC application.

We study the quantum size effects of an MgO-based double barrier magnetic tunnel junction with a nonmagnetic-metal (DBMTJ-NM) (semiconductor (DBMTJ-SC)) spacer on the charge current and the spin-transfer torque (STT) components using non-equilibrium Green's function (NEGF) formalism. The results show oscillatory behavior due to the resonant tunneling effect depending on the structure parameters. We find that the charge current and the STT components in the DBMTJ-SC demonstrate the magnitude enhancement in comparison with the DBMTJ-NM. The bias dependence of the STT components in a DBMTJ-NM shows different behavior in comparison with spin valves and conventional MTJs. Therefore, by choosing a specific SC spacer with suitable thickness in a DBMTJ the charge current and the STT components significantly increase so that one can design a device with high STT and faster magnetization switching.

Resonant tunneling quantum structures consist of asymmetric wells and barriers have been investigated to find their optimized geometrical parameters and potential profile by the numerical calculations. The results show that the widths and the depths of the asymmetric wells have a significant effect on the transmission coefficient and the dwell time. The properties exhibited in this work may establish guidance to the device applications.

We report large tunnel magnetoresistance (TMR) ratios of up to 219% at 300 K and 366% at 3.7 K obtained for a high-quality fully epitaxial double-barrier magnetic tunnel junction (MTJ) composed of Fe/MgO/Fe/γ-Al2O3/Nb-doped SrTiO3. The obtained TMR ratios are among the highest values reported in Fe/MgO/Fe structures. This result may be attributed to the small in-plane wave vectors of the tunneling electrons injected from the Nb-doped SrTiO3 electrode with a small carrier density, demonstrating good compatibility between the Fe-based MTJ and SrTiO3.

Using the model of rectangular potential and smooth position-dependent electron effective mass, which is a linear function of coordinate in near-interface regions between wells and barriers of open double-barrier resonant tunneling nano-structure, the solutions of Schrodinger equation are obtained and transmitting coefficient of the structure is analytically calculated. The influence of the sizes of inner and outer near-interface regions on transmitting coefficient of the structure is investigated. The comparative analysis of resonance energies and resonance widths of sub-barrier electron quasi-stationary states as functions of these sizes is performed. It is shown that the relative differences of resonance energies and resonance widths of the same state of electron in the model of linearly-dependent effective mass and simplified abrupt one do not exceed 1% and 5%, respectively, in physically correct interval of the sizes of outer and inner near-interface regions.

Shot noise reveals spin dependent transport properties in a magnetic tunnel junction. We report measurement of shot noise in CoFeB/MgO/CoFeB/MgO/CoFeB double barrier magnetic tunnel junctions, which shows a strong temperature dependence. The Fano factor used to characterize shot noise increases with decreasing temperature. A sequential tunneling model can be used to account for these results, in which a larger Fano factor results from larger spin relaxation length at lower temperatures.

The results of the fabrication and study of a double-barrier resonant tunneling structure grown on the basis of GaN and AlN wide-gap materials on a (0001)-oriented sapphire substrate are given. It is shown that, at voltages of ∼3 V, the current-voltage characteristics of resonant tunneling diode samples exhibit a region of negative differential conductivity that disappears upon multiple cyclic measurements. It is also shown that the reversal of sign of the applied voltage restores the initial shape of the current-voltage characteristics and an increase in the temperature of the structure from room temperature to 200°C yields irreversible degradation of the device and a shift of the region of negative differential conductivity to lower voltages

We study the effect of the magnetic field on the spin-dependent hole transport through a diluted magnetic semiconductor double barrier heterostrucutre such as Cd1-x Mn x Te/CdTe. Using the transfer matrix, the barrier transparency, polarization efficiency and tunneling lifetime of a light hole and a heavy hole are calculated. The spin separation is found out and is observed that it is the same for both the light and the heavy hole. The heavy hole takes more time to travel in the heterostrucutre than the light hole. The effect of the magnetic field influences the tunneling lifetime. The results give the basic idea about the spin-dependent transport of light and heavy holes.

Thin film transistors with nanoparticles silicon floating-gate are fabricated by plasma enhanced chemical vapor deposition. It should be noted that SiO2acts as both a tunneling and a blocking layer. Meanwhile, some np-Si dots are embedded within SiO2layers. The electrical characteristic of the devices are measured by semiconductor parameter analyzer at room temperature. These Thin film transistors show a good device performance with a high charge-carrier mobility of 33 cm2/vs and a large on/off ratio of 1.2×106. Moreover, the capability of written and erasing was demonstrated. This indicates that thin film transistors can be operated as rewritable nonvolatile floating gate memory devices.

The simulation is done for electron tunneling through the insulating layer of the magnetic tunnel junction with non-magnetic embedded nanoparticles (npMTJs). The distribution of the nanoparticles by size (DNPS) was important part of the solution for tunnel magnetoresistance (TMR) which was calculated on the basement of the double barrier tunneling model. It was found that theoretical agreement with experimental data can be influenced not only from DNPS but also due to different cases of the quantization. A few cases of the quantization related with a redistribution over quantum well (QW) states of the nanoparticles (NPs) (with diameter d) were considered (one of the possible DNPS and related TMR is shown in Fig. 1a and Fig. 1b).

We consider the one dimensional problem of a non-relativistic quantum particle scattering off a double barrier built from two generalized point interactions (each one characterized as a member of the four parameter family of point interactions). The properties of the double point barrier under parity transformations are investigated, using the distributional approach, and the constraints on the parameters necessary for the interaction to have a well-defined parity are obtained. We show that the limit of zero interbarrier distance of a renormalized odd arrangement with two $\delta'$ is either trivial or does not exist as a generalized point interaction. Finally, we specialize to double barriers with defined parity, calculate the phase and Salecker-Wigner-Peres clock times and argue that the emergence of the generalized Hartman effect is an artifact of the extreme opaque limit.

We study the tunneling of Dirac fermions in graphene through a double barrier potential allowing the carriers to have an effective mass inside the barrier as generated by a lattice miss-match with the boron nitride substrate. The consequences of this gap opening on the transmission are investigated. The realization of resonant tunneling conditions is also analyzed., Comment: 17 pages, 6 figures

In this paper, we study the value of the tunnel magnetoresistance (TMR) as a function of the applied voltage in double barrier magnetic tunnel junctions (DMTJs) with the left and right ferromagnetic (FM) layers being pinned and numerically estimate the possible difference of the TMR curves for negative and positive voltages in the homojunctions (equal barriers and electrodes). DMTJs are modeled as two single barrier junctions connected in series with consecutive tunneling (CST). We investigated the asymmetric voltage behavior of the TMR for the CST in the range of a general theoretical model. Significant asymmetries of the experimental curves, which arise due to different annealing regimes, are mostly explained by different heights of the tunnel barriers and asymmetries of spin polarizations in magnetic layers.

The paper discusses some issues of modeling the MOS tunnel structure with a gate stack containing a semiconductor quantum well (double barrier MOS system). The considerations are illustrated by simulations with the use of a theoretical model. Results of simulations are compared with experimental characteristics of fabricated DB MOS diodes.

Combining an extended Julliere model with transfer matrix method, we study the spin-polarized resonant tunneling in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double barrier ferromagnetic semiconductor (FS) tunnel junctions with the arbitrary angle θ between the magnetic directions of two FS's. It is shown that tunneling magnetoresistance (TMR) ratio linearly varies with sin 2 ( θ / 2 ) . We also demonstrate that for the heavy and light holes, the properties of the spin-polarized resonant tunneling are obviously different. The present results are expected to be instructive for manufacturing the relevant semiconductor spintronic devices.

Double barrier Si-based Resonant Tunneling Diode (RTD) structures were fabricated by ultra high vacuum (UHV) wafer bonding. 300 mm ultra thin body silicon-on-insulator (SOI) wafers with 10 nm top Si layers were developed and used for the integration of Si/SiO2((

We have investigated the vertical electronic transport in AlN/GaN/AlN double barrier structures grown by plasma-assisted molecular-beam epitaxy, with AlN barrier thickness of 0.5 nm and a GaN well thickness varying from 0.5 nm to 2 nm. Two different current levels were observed in different devices in all the samples, the highest one being attributed to leakage through dislocations, and the lower one to tunnelling transport through the barriers. In the sample with a well thickness of 0.5 nm, we observed a bi-stable behaviour, with an apparent negative differential resistance in the current-voltage characteristic when scanning the voltage from negative to positive bias. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the tunneling magnetoresistance (TMR) in GaMnAs-based double barrier magnetic tunneling junctions (DB-MTJs) to clarify the origin of extremely low threshold current density for current-driven magnetic orientation reversal achieved in the systems. The TMR characteristics including minor loop behavior are well explained by assuming that three magnetic layers in the DB-MTJ have different coercive forces and that the uniaxial magnetic anisotropy in the middle magnetic layer is enhanced with decreasing the layer thickness. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Appearances and disappearances of Gd islands grown on top of a W(1 1 0) substrate were observed in time scales of hours after exposing the surface to a few Langmuirs of hydrogen. The phenomenon is presented and explained in terms of (temporary) creation of electrically floating islands, due to electrical decoupling of the island and substrate by the hydrogen that diffuses into the island/substrate interface. The disappearance of such an island is explained by forming a double barrier junction consisting of two tunneling barriers in series, causing, by charging, the potential of the island to become equal to that of the tip. The island then becomes “invisible” and the tip follows the corrugation of the surface under the substrate. The reappearance follows hydrogen mobility that retains the electrical conductivity of the island–substrate interface.

Resonant tunneling characteristics of triangular double-barrier diodes have been investigated systematically in this Letter, using Airy function approach to solve time-independent Schrodinger function in triangular double-barrier structures. Originally, the exact analytic expressions of quasi-bound levels and quasi-level lifetime in symmetrical triangular double-barrier structures have been derived within the effective-mass approximation as a function of structure parameters including well width, slope width and barrier height. Based on our derived analytic expressions, numerical results show that quasi-bound levels and quasi-level lifetime vary nearly linearly with the structure parameters except that the second quasi-level lifetime changes parabolically with slope width. Furthermore, according to our improved transmission coefficient of triangular double-barrier structures under external electric field, the current densities of triangular double-barrier diodes with different slope width at 0 K have been calculated numerically. The results show that the N-shaped negative differential resistance behaviors have been observed in current–voltage characteristics and current–voltage characteristics depend on the slope width.

We report on the electrical characterization of AlN/GaN/AlN double-barrier resonant tunnelling diodes (RTDs) using steady-state current-voltage and capacitance-voltage (C-V) characteristics in a wide frequency range with 2 kHz steps. The C-V characteristics of a double-barrier RTD show different behaviour under forward and reverse polarities and a strong dependence on frequency. The monotonous growth of capacitance at forward bias was registered, while a more complicated dependence was observed at reverse voltages. In order to analyse this dependence, a self-consistent calculation of the potential profile of the structure was performed taking into account polarization effects at the AlN/GaN interfaces. The peculiarities are analysed in the model of possible charge trapping at the interface states. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A time-asymptotic analytical expression of the wave function transmitted by a double-barrier potential is presented. This analytical expression is validated by comparing it with the complete numerical solution of the Schroedinger equation. In the resonant regime, where the asymptotic expression is not valid, an approximation of the wave function, valid for all times, is given.

The resonant and non-resonant dynamies of a Gaussian quantum wave packet travelling through a double barrier system is studied as a function of the initial characteristics of the spectrum and of the parameters of the potential. The behaviour of the tunnelling time shows that there are situations where the Hartman effect occurs, while, when the resonances are dominant, and in particular for b>π/Δk (b being the inter-barrier distance and Δk the spectrum width), the tunnelling time becomes very large and the Hartman effect does not take place.

We have modeled the optical properties of the ZnMgO/ZnO/ZnMgO system and correlated them to its transport properties by employing an effective mass equation method. By its construction our method was sensitive to the barrier thickness of the system. From our results we deduce that there are more states in the well than resonant tunneling peaks in the transport data. The “missing” states in the electrical characteristics are not seen because of their much lower localization in the well. However these states are responsible for the UV activity of the system. Reducing the barrier thickness changes dramatically the localization of the states in the well without affecting significantly the energy levels. Hence the electrical properties can be changed without changing significantly the optical properties.

We present a systematic study of vertical transport in AlGaN/GaN heterostructures. The influence of barrier thickness and Al-concentration on the current across AlGaN barriers is investigated experimentally and compared to model calculations. The effect of polarization fields on the electronic properties of single- and double barrier heterostructures is discussed and experimental results are reviewed. AlN/GaN double barrier RTD structures are fabricated under optimized growth conditions. Experimental analysis of their electronic properties reveals indications for resonant tunneling in the low forward bias regime. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is shown, by solving numerically the time-dependent Schrodinger equation, that the resonant transmission of electrons through double barrier structures can significantly be modified and controlled by intense laser fields. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN/AlGaN double barrier resonant tunnelling structures grown by molecular beam epitaxy on GaN templates have been studied. Peaks in the I(V) characteristics are observed, which are similar to resonant peaks seen in conventional III–V based devices. However, the behaviour of the peaks in I(V) depend upon the previous charge-state of the device produced by electrical bias. Current instabilities are also observed at low voltages. The possible origin of the peaks in the I(V) at room temperature and 4 K is discussed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent studies have indicated that spin polarization of the conductance shows strong oscillation and zero average for electrons resonant tunnelling through a magnetic double-barrier and double-well quantum structure. Our studies indicate that strong oscillation of the polarization of the conductance is not physically intrinsic but inaccurate due to problems of numeration and insufficient numerical accuracy. We also indicate that its average of the spin polarization is not zero. The reasonable explanation is presented in details.