Electron Tunneling Current Research Articles

Radiation-Induced Surface Conductivity in an Alkaline-Earth Boroaluminosilicate Glass Measured with Elevated-Temperature Scanning Probe Microscopy

A 50 mW 325 nm He:Cd laser was used to irradiate the fracture surface of a commercial alkaline-earth boroaluminosilicate display glass (Corning Code 1737) held at temperatures above 800 K. After this treatment, the surface of the glass was found to be conductive for several hours in a temperature range between room temperature and 850 K. The conductivity shows an activation energy of 0.18 eV. Scanning tunneling spectroscopy measurements indicate that the glass surface is semiconducting with a bandgap of ∼4.4 eV. It is possible to form topographical images of the surface of the glass using the electron tunneling current; the observed topography corresponds to that acquired using atomic force imaging from the same area.

Interference, Fluctuation, and Alternation of Electron Tunneling in Protein Media. 1. Two Tunneling Routes in Photosynthetic Reaction Center Alternate Due to Thermal Fluctuation of Protein Conformation

Electron tunneling routes for the electron transfer from the bacteriopheophytin anion to the primary quinone in the bacterial photosynthetic reaction center of Rhodobactor sphaeroides are investigated by a combined method of molecular dynamics simulations for the protein conformation fluctuation and quantum chemical calculations for the electronic states of the donor, acceptor, and protein medium. The analysis of the tunneling route is made by mapping interatomic electron tunneling currents for each protein conformation. We found that there are two dominant routes mainly passing through Trp(M252) (Trp route) or mainly passing through Met(M218) (Met route). Actual electron tunneling pathways alternate between the two routes, depending on the protein conformation which varies with time. When either the Trp route or the Met route dominates, the electron tunneling matrix element /T(DA)/ becomes large. When both the Trp route and the Met route dominate, /T(DA)/ becomes very small due to the destructive interference of the electron tunneling currents between the two routes. We found that a linear relationship exists between the value of /T(DA)/ and the inverse of the degree of destructive interference Q for a wide range of values (ca. 3-10(3) for Q). A similar relationship was also found previously for electron transfer in ruthenium-modified azurins, suggesting that this relationship holds true in general. From these results, we are led to the conclusion that /T(DA)/ cannot exceed a maximum value at Q = 1, even if much variation of /T(DA)/ happens due to the fluctuation of protein conformation. We also conclude that the property of the electron transfer alternates between constructive and destructive interference, due to the fluctuation of protein conformation. It is impossible to keep a system in either constructive or destructive interference because thermal fluctuation of protein conformation takes place.

Tunneling Current through Ultrathin Silicon Dioxide Films under Light Exposure

Tunneling current through ultrathin silicon dioxide films with a thickness of approximately 3.1 nm, formed on n-Si (100) by controlling preoxide growth during heating, is examined using Al/oxide/n-Si structures. Electron tunneling current through the oxide from n-Si to Al is decreased and the dielectric breakdown voltage is increased by the preoxide growth control. Electron tunneling current from Al to n-Si is increased by light exposure. The increase in electron tunneling current can be explained by the increase in oxide voltage with an inversion layer formed by photoexcitation.

Electroluminescence spectra of an STM-tip-induced quantum dot

We analyze the electroluminescence spectrum of an STM-tip-induced quantum dot in a GaAs surface layer. A flexible model has been developed, that combines analytical and numerical methods and describes the key features of many-particle states in the STM-tip-induced quantum dot. The dot is characterized by its depth and lateral width, which are experimentally controlled by the bias and the tunneling current. We find, in agreement with experiment, that increasing voltage on the STM-tip results in a red shift of the electroluminescence peaks, while the peak positions as a function of the electron tunneling current through the STM-tip reveal a blue shift.

Electroluminescence spectra of an STM-tip-induced quantum dot

We analyze the electroluminescence spectrum of an STM-tip-induced quantum dot in a GaAs surface layer. A flexible model has been developed that combines analytical and numerical methods and describes the key features of many-particle states in the STM-tip-induced quantum dot. The dot is characterized by its depth and lateral width, which are experimentally controlled by the bias and current. We find, in agreement with experiment, that increasing the voltage on the STM tip results in a redshift of the electroluminescence peaks, while the peak positions as a function of electron tunneling current through the STM tip reveal a blueshift.

Theoretical study of carotene as a molecular wire

Carotenoid molecules have important photo-chemical properties and may serve as molecular wires in a molecular electronic circuit. We have theoretically studied the intrinsic conducting properties of sulfur-terminated carotene between gold contacts using local orbital density functional theory. The dependence of the tunneling decay parameter “ β” on the degree of single–double bond alternation within the polyene backbone is determined from the polyene complex band-structure. The electron tunneling current–voltage characteristics is calculated using the Landauer–Büttiker formalism. The calculations are in qualitative agreement with experiments.

Indirect quantum measurement of a single quantum spin

We establish the conditions to observe a single localized quantum spin 1 2 using a tunneling current of electrons as the probe in an indirect quantum measurement. We show that a strong tunneling current, due to the shot noise, suppresses the spin dynamics, such as the spin precession in an external magnetic field (quantum Zeno effect), while a weak tunneling current preserves the spin precession but the oscillatory component of the current is of the same order as the noise. Within our semi-classical analysis we describe the physical requisites to observe a signature of quantum oscillations in the current–current correlation function.

Tunnel emission of electrons in photo-field detectors and in an auger transistor in very strong electric fields

The effect of a strong electric field [(5–7)×107 V cm−1] on the electron emission from a semiconductor to vacuum in photo-field detectors and in MIS structures with a tunnel-transparent insulator layer (Al-SiO2-n-Si Auger transistor) has been studied. It is shown for the first time that the existence of deep self-consistent quantum wells on the semiconductor surface in a strong electric field provides a possibility of controlling the energy of fast electrons responsible for the impact ionization near the base of the Auger transistor and changes the photosensitivity of narrow-gap photo-field cathodes in the IR spectral range due to the formation of a transistor structure at the semiconductor-vacuum interface. It is also demonstrated that, both in photo-field detectors and in tunnel Al-SiO2-n-Si transistor structures, only the electron tunnel current should be taken into account and the hole current, disregarded. The reason for this is that only the electron current exists in vacuum, and the tunneling of holes in Al-SiO2-n-Si from the semiconductor into the metal is unlikely because of the large hole effective mass in the valence band of SiO2.

Tunneling measurement of a single quantum spin.

Measurement of the tunneling current of spin-polarized electrons via a molecule with a localized spin provides information on the orientation of that spin. We show that a strong tunneling current due to the shot noise suppresses the spin dynamics, such as the spin precession in an external magnetic field, and the relaxation due to the environment (quantum Zeno effect). A weak tunneling current preserves the spin precession with the oscillatory component of the current of the same order as the noise. We propose an experiment to observe the Zeno effect in a tunneling system and describe how the tunneling current may be used to read a qubit represented by a single spin 1/2.

Electron Tunneling and Electrochemical Currents through Interfacial Water Inside an STM Junction

The apparent barrier height for charge transfer through an interfacial water layer between a Pt/Ir tip and a gold surface has been measured using STM technique. The average thickness of the interfacial water layer inside an STM junction was controlled by the amount of moisture. A thin water layer on the surface was formed when relative humidity was in the range of 10 to 80%. In such a case, electron tunneling through the thin water layer became the majority of charge transfers. The value of the barrier height for the electron tunneling was determined to be 0.95 eV from the current vs. distance curve, which was independent of the tip-sample distance. On the other hand, the apparent barrier height for charge transfer showed a dependence on tip-sample distance in the bias range of 0.1-0.5 V at a relative humidity of approximately 96%. The non-exponentiality for current decay under these conditions has been explained in terms of electron tunneling and electrochemical processes. In addition, the plateau current was observed at a large tip-sample distance, which was caused by electrochemical processes and was dependent on the applied voltage.

Carrier separation measurement of leakage current under prebreakdown in ultrathin SiO2 films

The anomalously large gate leakage current observed prior to dielectric breakdown in electrically stressed n+ gate p metal–oxide–semiconductor field-effect transistors is investigated. Carrier separation measurements reveal that the leakage currents are electron tunneling current, and in some cases are accompanied by noticeable hole-related current at low gate voltages. Experimental results demonstrate the close correlation between this phenomenon and soft breakdown in terms of current–voltage characteristics.

Current components analysis of MIS/IL solar cells for different fabrication parameters

A model has been developed to simulate the performance of MIS/IL silicon solar cells taking into consideration the effect of cell surface parameters, namely, positive fixed oxide density, surface states density and mobile charge density on the current components. We consider a MIS/IL device on p-type semiconductor substrate. A complete set of equations is used which include the diffusion current ( J diff), the hole and the electron tunneling currents through the oxide layer ( J ht, J et), the surface states current components ( J hs, J es and J ss), the generation–recombination current in the transition region ( J RG), and the photocurrent generated in the bulk in the transition region ( J LN). The results show that the photogenerated current, surface state current and the tunneling current have the dominant effects on the cell performance.

Reduction of electron tunneling current due to lateral variation of the wave function

Electron tunneling in solids is usually envisioned in terms of a simple barrier model based on free electrons tunneling through a region of homogeneous potential. We point out that this model neglects the variation of the wave function in the plane of the interface and show that oscillations of the wave function parallel to the interface increase its rate of decay perpendicular to the interface. This simple observation has important implications for spin-dependent tunneling and may explain why $``s$ electrons'' seem to tunnel much more readily than $``d$ electrons.''

DFT calculation of electron tunneling currents: Real-space (grid) molecular orbitals vs. Gaussian-type molecular orbitals

Long-range electron tunneling occurs in many biological systems. Detailed quantitative information about the tunneling process can be obtained from the analysis of electron tunneling currents. However, ab initio tunneling current calculation is a challenging problem since it demands an accurate spatial representation of molecular orbitals. Conventional Gaussian-type molecular orbitals give relatively inaccurate results according to continuity equation for current. We have applied a newly developed grid-based numerical method for the density functional theory (DFT) calculation of molecular orbitals that describes the tunneling electron in the medium. The method yields excellent results for the spatial distribution of tunneling currents and for the tunneling matrix element. The method employs the full approximation scheme of multigrid technique to solve both the Kohn-Sham equations and the Poisson equation. c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 80: 591-597, 2000

Dynamic behavior of electron tunneling and dark current in quantum well systems under an electric field

A time evolution method is used to study the time dependence of electron tunneling in a quantum well under an electric field. A comparison of phase-shift analysis and the full time-dependent calculation is presented. Good agreement between the two on a large time scale is found, but discrepancy exists on the small time scale especially at high fields. Our studies confirm that the direct tunneling rate $(1/{\ensuremath{\tau}}_{t})$ evaluated by the phase-shift analysis is reliable at low fields. The phonon-assisted tunneling rate $(1/{\ensuremath{\tau}}_{p})$ is also calculated. We found that $1/{\ensuremath{\tau}}_{p}$ dominates at low fields, while $1/{\ensuremath{\tau}}_{t}$ increases exponentially with increasing field and becomes dominant at high fields. Contributions due to the thermionic emission, direct tunneling as well as phonon-assisted tunneling to the dark current are also evaluated.

THE ROLE OF THE SURFACE ELECTRONIC STRUCTURE IN THE OXIDATION OF IRON

The Fromhold–Cook coupled-currents theory of metal oxidation states that below ~ 150° C , a saturation thickness is reached, which is independent of temperature, as a result of the diminution of the electron tunnel current. However, Fe and Co show a strong temperature dependence of the oxide saturation thickness. In this study, we combine ellipsometry, X-ray Photoelectron Spectroscopy (XPS) and the high energy ion beam technique Elastic Recoil Detection (ERD) for the detailed determination of oxidation rate and oxide thickness, composition, and electronic structure on Fe(100) in O 2. At room temperature, first a 1 nm thin FeO film forms, followed by the growth of a 1 nm mixed Fe 2+/ Fe 3+ layer, which forms the effective barrier against further oxidation. By vacuum annealing at 200°C, the Fe 3+ is reduced to Fe 2+ and the oxidation kinetics through a single Fe 2+ containing layer can be studied, yielding results in excellent agreement with the FC theory. At T<150° C , the transport of metal ions to the surface is rate-limiting and the saturation thickness is independent of temperature (3.2 nm of FeO). At T ≥ 150° C , the thermionic emission of electrons determines the oxidation rate. Thus, a complete and consistent picture of the initial thin oxide layer growth on Fe single crystals is obtained.

Semiclassical description of resonant tunneling

We derive a semiclassical formula for the tunneling current of electrons trapped in a potential well which can tunnel into and across a wide quantum well. The tunneling current is measured at the second interface of this well and our calculations idealized an experimental situation where a strong magnetic field tilted with respect to an electric field was used. It is shown that the contribution to the tunneling current, due to trajectories which begin at the first interface and end on the second, is dominant for special periodic orbits which hit both walls of the quantum well and are perpendicular to the first wall. The resulting formula, which is a semiclassical expression for the total tunneling current, differs in a few important aspects from the Gutzwiller trace formula for the density of states. The Miller-type modification of the obtained semiclassical formula for stable orbits is also discussed.

Two-Band Tunneling Currents and Stress-Induced Leakage in Ultra-Thin SiO2 Films

ABSTRACTThe conduction band and valence band electron tunneling currents in ultra-thin SiO2 films at the transition from direct to Fowler-Nordheim tunneling regimes are studied. The slopes of the current voltage characteristics agree well with the simulations performed. The Stress-Induced Leakage Current (SILC) behavior is quite similar for both conduction and valence band currents even if the amplitude of the valence band SILC is much lower. We show that a linear dependence exists between the stress-induced interface trap density and both valence and conduction band SILC. A new model of SILC is also proposed.

Fowler–Nordheim tunneling of holes through thermally grown SiO2 on p+ 6H–SiC

Fowler–Nordheim tunneling of holes through thermally grown silicon dioxide on 6H–silicon carbide is reported. Oxides of 5.2, 10, and 14.2 nm thickness were grown on the p+ face of a p+n SiC junction. The p+n junction served to separate the electron and hole tunneling currents. Hole tunneling was found to be the dominant current mechanism through the oxide. Fowler–Nordheim analysis, using a parabolic E–K relationship, was performed to extract a barrier height–effective mass product, ΦB3/2(mox/m0)1/2, for electrons and holes of 2.88%±4.9% and 2.38%±3.8% (V3/2) respectively. An estimate for the effective mass of holes within the oxide was made using both the parabolic and Franz dispersion relations.

The magnetic-field influence on the inelastic electron tunnel current mediated by a molecular wire

The magnetic-field influence on the inelastic interelectrode tunnel current mediated by a molecular wire is studied theoretically for the case in which the wire includes paramagnetic ions with frozen angular momenta. The vibrational and spin relaxation within the terminal sites of the wire (adjacent to the electrodes) are assumed to be much faster than the related jump of the tunneling electron. For a wire containing a pair of paramagnetic ions, a low-temperature blocking of the current is demonstrated for the cases of ions both coupled by an exchange interaction and largely separated in space. The blocking is shown to appear if the paramagnetic ion reduces its electronic ground-state spin S to S−12 during the formation of the intermediate bound state with the transferred electron. At low temperatures the current formation is predominantly originated thus by tunnel channels for which the magnetic energy of paramagnetic ions has a minimal value. In the presence of an exchange spin–spin interaction or a single-ion anisotropy, just these predominant channels are manifested to form a steplike behavior of the current before the current itself is completely blocked by the increasing magnetic field.