Resonant Tunneling Process Research Articles

Magnetic-field effects on undriven chaos in a weakly coupled GaAs/AlAs superlattice

We have investigated the effect of a perpendicular magnetic field on the spontaneous current oscillations in a weakly coupled GaAs/AlAs superlattice. At zero magnetic field, voltage regions of periodic as well as chaotic oscillations are observed. With increasing magnetic field B, the voltage regions for chaotic oscillations become larger and those for periodic oscillations smaller. At B=9 T, the whole voltage range consists of chaotic oscillations. At the same time, the time-averaged current–voltage characteristic hardly changes. The experimental observations demonstrate that the quantization by the perpendicular magnetic field results in an equalization of the sequential resonant tunneling process over the whole superlattice, suppressing scattering effects due to disorder.

Transport through a Metallic Quantum Dot in the Limit of Strong Coulomb Interaction

In the limit of strong interaction where a two-charge state approximation is applicable, the current through a metallic quantum dot is studied based on the Keldysh formalism. We performed diagrammatic analyses by describing the operators relevant to charge in terms of spin 1/2 operators and by describing the spin 1/2 operators in terms of the product of two Fermion operators. Two transport processes through a quantum dot are found: (i) The first one satisfies the current continuity independent of the configuration of the electrochemical potentials (ECPs) in the subsystems, and corresponds to the resonant tunneling process in double-barrier structures. Schoeller and Schon give an approximate expression for this process. (ii) The second one corresponding to the sequential process does not satisfy the current continuity regardless of the ECPs. It is also shown that, the currents through the left and the right junctions are continuous only in an appropriate configuration of the ECPs.

Negative Differential Resistance Behavior in Delta‐Doped AlInP Structure Grown by MOCVD

An AlInP delta‐doped schottky diode exhibiting negative differential resistance (NDR) behavior is demonstrated for the first time. The NDR characteristics with a peak to valley ratio of 5.5 and peak current density of 1KA/cm2 were achieved at room temperature. In addition, the maximum available power is estimated up to 5W/cm2 . The mechanism for such performance is phenomenologically analyzed by the combination of resonant interband tunneling (RIT) and thermionic emission processes associated with tunneling effect on the metal‐semiconductor (MS) interface.

Neutralization ofO+during scattering and ion-stimulated desorption from the TiC(100) surface

The mechanism of ion-stimulated ${\mathrm{O}}^{+}$ desorption from the oxygenated TiC(100) surface has been investigated, in comparison with the neutralization of ${\mathrm{O}}^{+}$ and ${\mathrm{Ne}}^{+}$ ions during scattering. The ${\mathrm{O}}^{+}$ ion desorbs from the surface due to the potential energy of ${\mathrm{He}}^{+}.$ This process is initiated by two valence holes resulting from Auger decay of ${\mathrm{He}}^{+}$ but due to a long-lived O $2s$ core-hole state with the antibonding character, which is formed by the quasiresonant charge exchange with ${\mathrm{He}}^{+}$ $1s.$ In the ion-scattering experiment, the ${\mathrm{O}}^{+}$ ion is neutralized almost completely whereas ${\mathrm{Ne}}^{+}$ survives neutralization considerably even at energy as small as below 10 eV. This is because the lifetime of the core hole is much longer than that of the valence holes. The ab initio molecular-orbital calculation also reveals that the O $2p$ hole diffuses immediately into the band via the resonant tunneling process. The O $2s$ (or Ne $2p)$ core hole decays with a much longer lifetime via the two-electron Auger process. The core-excited oxygen species, which undergoes the intra-atomic Auger decay during and after breakage of the chemisorptive bond, results in the ${\mathrm{O}}^{+}$ ion.

Capture and loss of valence electrons during low energy H + and H − scattering from LaB 6(100), Cs/Si(100), graphite and LiCl

The mechanism of electron attachment to low-energy (100 eV) protons has been investigated during scattering from graphite, Si(100), LaB 6(100), and LiCl. The primary H + ion survives neutralization when scattered from highly ionized target species, such as Cs +, La 3+, Li +, and Cl −, existing on the topmost surface layer. In terms of H − ion formation at the metal and semiconductor surfaces, multiple scattering from the solid is dominant relative to single surface scattering because the former has a much larger cross-section than the latter, indicating that H − is caused by the non-local resonant tunneling process. At the LiCl surface, on the other hand, a close atomic encounter with individual target ions is found to be important for H − ion formation. This is because the non-local resonance process is prohibited due to the existence of the large band gap and the charge state of scattered hydrogen is determined by the transient chemisorption state very close to the surface.

Shot noise in double barrier structures

It is pointed out that the Lesovik theory, in which the scattering states are made use of as a basis set of functions, does not fulfill the completeness when the incident electron energy is higher than the scattering potential. The incompleteness influences the non-local noise and leads to a new contribution which is absent in the Lesovik theory. The difference between the local noise and the non-local noise is attributed to the real transitions in the sample. It is shown from exact analysis that the current through double barrier structures (DBS) is written in a separate form as the sum of contributions resulting from the sequential and the resonant tunneling processes. The shot noise is, on the other hand, also written as a sum of contributions resulting from the sequential and the resonant tunneling processes in addition to the correlation term between the sequential and the resonant tunnel process.

Nonlinear and oscillatory electronic transport in superlattices as a probe of structural imperfections

Abstract We present a theoretical analysis of the correlations between the macroscopic global transport properties and microscopic imperfections of a superlattice. High-field transport and domain formation is modelled using a microscopic quantum transport model which includes interface roughness and impurity scattering and describes resonant and nonresonant tunnelling processes in weakly coupled multiple quantum wells without adjustable parameters. Our analysis can be used on one hand to identify random fluctuations of the structural parameters in real samples like deviations from the perfect periodicity due to varying barrier widths, or doping densities in the individual wells. On the other hand, we demonstrate that the current–voltage characteristics can be tailored in a wide range to exhibit specific features like sharply rising steps at given voltages, arbitrarily modulated current maxima of multistable branches, or self-generated current oscillations.

Resonant tunneling through a quantum dot weakly coupled to quantum wires or quantum Hall edge states

Resonant tunneling through a quantum dot weakly coupled to Tomonaga-Luttinger liquids is discussed. The linear conductance due to sequential tunneling is calculated by solving a master equation for temperatures below and above the average level spacing in the dot. When the parameter $g$ characterizing the Tomonaga-Luttinger liquid is smaller than 1/2, the resonant tunneling process is incoherent down to zero temperature. At low temperature $T$ the height and width of the conductance peaks in the Coulomb blockade oscillations are proportional to ${T}^{1/g\ensuremath{-}2}$ and $T$, respectively. The contribution from tunneling via a virtual intermediate state (cotunneling) is also included. The resulting conductance formula can be applied for the resonant tunneling between edge states of fractional quantum Hall liquids with filling factor $\ensuremath{\nu}=1/(2m+1)=g$.

Resonant tunneling between transverseXstates in GaAs/AlAs double-barrier structures under elevated hydrostatic pressure

We present a model to describe the mechanisms involved in tunneling between the quasiconfined $X$ subbands of the AlAs layers in GaAs/AlAs double-barrier structures at pressures up to the type-II transition. The model involves self-consistent Schr\"odinger-Poisson calculation of the potential profiles within the device for a given relative alignment between the two $X$-like quantum wells, and thus allows prediction of the bias positions at which certain resonant tunneling processes will occur. By systematic variation of the parameters involved, in particular the charge distribution between the two AlAs layers, these predictions have been fitted to the measured vertical transport characteristics of a series of samples of different AlAs layer width. In the cases of those samples with $X$ ground states that are transverse in nature, very good agreement has been obtained. The level of insight afforded by the model opens up alternative methods for the determination of important band-structure parameters, such as the light effective mass of the $X$ minima in AlAs, and the ${\ensuremath{\Gamma}}_{\mathrm{GaAs}}{\ensuremath{-}X}_{\mathrm{AlAs}}$ conduction-band offset. It also proves to be an extremely sensitive probe of the degree of symmetry between the AlAs layer widths of near-symmetric devices, and we are thus able to measure an asymmetry of around one monolayer in each of our devices. This explains entirely the asymmetry exhibited by the low bias ${X}_{t}(1)\ensuremath{\rightarrow}{X}_{t}(1)$ resonances of some nominally symmetric double-barrier structures, which has been a source of some debate in recent years.

Heat Capacity Anomalies Induced by Magnetization Quantum Tunneling in a Mn12O12-Acetate Single Crystal

A particularly sensitive heat capacity measuring device has allowed us to measure the tunneling process of ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}$-acetate single crystals (mass: $1$ and $20\ensuremath{\mu}\mathrm{g}$) from the irreversible tunneling process below the blocking temperature ${T}_{B}$ to the reversible resonant tunneling process above ${T}_{B}$. Above the ${T}_{B}$ (typically $3.5\mathrm{K}$) we find specific heat anomalies at the magnetic field values that correspond to the crossing of spin up and spin down levels of different magnetic quantum numbers. Below ${T}_{B}$, heat relaxation pulses at the crossing of crystal field levels are observed for fields applied antiparallel to the initial magnetization. These measurements give a new scope to ${\mathrm{Mn}}_{12}{\mathrm{O}}_{12}$-acetate investigations and show the great interest of nanocalorimetry for studies of big magnetic molecules.

Optical probing of interface roughness in resonant tunneling structures

We report on photoluminescence (PL) and photoluminescence-excitation measurements in GaAs/In0.1Ga0.9As/AlAs double-barrier tunneling structures as a function of bias voltage and temperature. We have observed a splitting in the quantum well photoluminescence due to island formation in the quantum well. We have a good correlation between the variation of integrated PL intensity, linewidth, and tunnel current bias for both lines. The temperature dependence of photoluminescence spectra shows that transfer of carrier between islands can be tuned by the applied bias and that states in different islands are populated by electrons in the resonant tunneling process.

Tunnelling barriers for high temperature superconductor devices

In this contribution we will discuss the requirements to be imposed on the barriers of HTS Josephson junctions as well as quasi particle injection devices. Factors that play a role are noise immunity, operating margins, injection efficiency etc., leading to the need for barriers in which the charge transport is dominated by direct tunnelling. So far, it is unclear if this has ever been observed in any practical device. We will demonstrate that localised states, that are probably ubiquitous in the materials used, are of prime importance in understanding device properties. In the model system we investigated, i.e. DyBCO/PrBCO/DyBCO, gallium-doping on the copper chain sites reduces the density of localised states appreciably without affecting the barrier height. Furthermore, it seems that due to an on-site Coulomb repulsion, pair transport proceeds via direct tunnelling, at least in the temperature region T< T c/2. Quasi particles, in contrast, tunnel resonantly via one or more localised states. At higher temperatures and bias voltages inelastic processes may dominate. These resonant tunnelling processes reduce the normal state resistance, and hence the I c R n-product. The consequences of these findings will be discussed and directions for future work set.

Quantum tunneling of magnetization in mesoscopic systems

Mesoscopic systems provide a unique opportunity for the study of the phenomenon of quantum tunneling of magnetization (QTM). This phenomenon is a macroscopic quantum tunneling effect in which the magnetization vector undergoes quantum underbarrier transitions which manifest themselves as temperature independent magnetic relaxation. The study of this relaxation at very low temperatures affords the opportunity to study the phenomenon experimentally. Low temperature relaxation will be discussed in a variety of mesoscopic systems. Data will be presented for logarithmic and exponential relaxation to demonstrate the occurrence of QTM. At high temperatures the relaxation shows the temperature dependence. Below a certain temperature, called the crossover temperature, T C, the relaxation becomes independent of temperature. These data agree qualitatively with the recent theoretical work on QTM. We will conclude the paper with the recent work on polynuclear high spin molecules where we have observed thermally assisted resonant tunneling processes between degenerate spin levels of opposite orientation.

Coherence and phase sensitive measurements with a quantum dot

The subject of coherent versus sequential resonant tunnelling (RT) process has been controversial for a long time (T. Weil and B. Winter, Appl. Phys. Lett. B 50 (1987) 1281). We devised a novel interference experiment which determines conclusively the nature of transport - with some added surprises - through a submicron RT structure: a quantum dot in the Coulomb blockade regime (M.A. Kastner, Phys. Today 46 (1993) 24). In principle, this method allows the measurement of both the magnitude and the phase of the transmission amplitude through any coherent mesoscopic structure.

Evidence of Γ–X sequential resonant tunneling in GaAs/AlAs superlattices

We have studied the sequential resonant tunneling of doped weakly coupled GaAs/AlAs superlattices under hydrostatic pressure up to 4.5 kbar. The pressure coefficient obtained from the experiment, 15.3 meV/kbar, provides a strong evidence for the formation of the electric field domain due to Γ–X sequential resonant tunneling. At the same time, we have observed the transition between two kinds of sequential resonant tunneling processes within the pressure range from 0 to 4.5 kbar, where the transition pressure between Γ–Γ and Γ–X sequential resonant tunneling is Pt∼1.6 kbar. For P&amp;lt;Pt, the electric field domain is formed by Γ–Γ sequential resonant tunneling, while for P≳Pt, the electric field domain is preferably formed by Γ–X sequential resonant tunneling.

Reentrant resonant tunneling.

We study the effect of electron-electron interactions on the resonant-tunneling spectroscopy of the localized states in a barrier. Using a simple model of three localized states, we show that, due to the Coulomb interactions, a single state can give rise to two resonant peaks in the conductance as a function of gate voltage, G(Vg). We also demonstrate that an additional higher-order resonance with Vg-position in between these two peaks becomes possibile when interactions are taken into account. The corresponding resonant-tunneling process involves two-electron transitions. We have observed both these effects in GaAs transistor microstructures by studying the time evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an isolated impurity (modulator). The heights of the two stronger peaks exibit in-phase fluctuations. The phase of fluctuations of the smaller middle peak is opposite. The two stronger peaks have their origin in the same localized state, and the third one corresponds to a co-tunneling process.

Dark conductivity and photoconductivity of asymmetric gold/ (Langmuir-Blodgett film)/gold tunnel junctions

We have performed measurements of the dark conductivity and photoconductivity on organic tunnel junctions. The junctions incorporate only a few monolayers of two different organic substances, transferred to the substrate by the Langmuir-Blodgett technique. Current/voltage measurements at liquid helium temperature reveal a rectifying behavior which is manifested in characteristic onsets in current, arising from resonant tunneling processes through molecular states. To emphasize that the observed effects are actually originating from the molecules, we record the photoaction spectra of the tunnel junctions and compare the results with absorption spectra measured on identical organic structures without the presence of electrodes. The photoaction spectra and the absorption spectra show characteristic bands at the same wavelengths. From this result we conclude that both organic layers in the tunnel junction are intact. Therefore, the application of electrodes affects only a small number of the outer monolayers of the organic film and does not destroy the heterostructure itself.

EFFECTS OF SINGLE ELECTRON TUNNELING IN STM/STS STUDIES USING Fe TIPS

We have used scanning tunneling microscopy working at 4.2 K to study the transition metal dichalcogenide 1T-TaSe2. The charge density wave (CDW) superstructure (131/2α0×131/2α0) and atomic modulation are well resolved, as well as the associated CDW energy gap. In particular we used high purity iron as tip material and demonstrated clearly the existence of single electron tunnel-ing (so-called Coulomb blocade) effects, further experiments with stainless steel tips also confirmed this observation. It is pointed out that during STM/STS studies tips covered with oxides are very likely to display Coulomb blocade effects due to their small capacitance. Therefore, extra cautious are needed when analyzing spectroscopic features in the area that could be affected by Coulomb blocade effects. At the mean time, high quality scanning images can still be regularly obtained through reso-nant tunneling process.

An assessment of potential nonlinear circuit models for the characterization of resonant tunneling diodes

The intrinsically fast process of resonant tunneling through double barrier heterostructures along with the existence of negative differential resistance in the current-voltage characteristic of these structures has led to their implementation as sources for high frequency electromagnetic energy. While sources based upon resonant tunneling diodes (RTDs) have produced frequency of oscillations up to 712 GHz, only microwatt levels of performance has been achieved above 100 GHz. Since stability criteria plays critical role in determining the deliverable power of any oscillator, a physically accurate equivalent-circuit model for the RTD is extremely important for optimizing the dynamics of the device-cavity package. This study identifies a distinctly new equivalent circuit model for characterizing the modes of oscillation in RTD-based sources. Specifically, in order to exhibit the fundamental self-oscillations and the overall I-V characteristics (plateau structure and hysteresis) observed experimentally, an accurate circuit model of the RTD must incorporate: (i) a quantum-well inductance which directly chokes the nonlinear conductance and, (ii) a nonlinear access resistance, associated with the accumulation of charge in the injection region of the double barriers, with a nonlocal dependence on the bias across the double barrier structure.

Aharonov-Bohm oscillations and resonant tunneling in strongly correlated quantum dots.

We investigate Aharonov-Bohm oscillations of the current through a strongly correlated quantum dot embedded in an arbitrary scattering geometry. Resonant-tunneling processes lead to a flux-dependent renormalization of the dot level. As a consequence we obtain a fine structure of the current oscillations which is controlled by quantum fluctuations. Strong Coulomb repulsion leads to a continuous bias voltage dependent phase shift and, in the nonlinear response regime, destroys the symmetry of the differential conductance under a sign change of the external flux.