Tunneling current spectroscopy of a nanostructure junction involving multiple energy levels

A multilevel Anderson model is employed to simulate the system of a nanostructure tunnel junction with any number of one-particle energy levels. The tunneling current, including both shell-tunneling and shell-filling cases, is theoretically investigated via the nonequilibrium Green's function method. We obtain a closed form for the spectral function, which is used to analyze the complicated tunneling current spectra of a quantum dot or molecule embedded in a double-barrier junction. We also show that negative differential conductance can be observed in a quantum dot tunnel junction when the Coulomb interactions with neighboring quantum dots are taken into account.     

Single electron tunneling rates in multijunction circuits

The rate of electron tunneling through normal metal tunnel junctions is calculated for the case of ultrasmall junction capacitances. The so-called Coulomb blockade of electron tunneling at low temperatures is shown to be strongly affected by the external electrical circuit. Under the common experimental condition of a low impedance environment the Coulomb blockade is suppressed for single tunnel junctions. However, a Coulomb gap structure emerges for junctions embedded in a high impedance environment. For a double junction setup a Coulomb blockade of tunneling arises even for low impedance environments due to the charge quantization on the metallic island between the junctions. An approach using circuit analysis is presented which allows to reduce the calculation of tunneling rates in multijunction circuits to those of a single junction in series with an effective capacitance. The range of validity of the socalled local rule and global rule rates is clarified. It is found that the tunneling rate tends towards the global rule rate as the number of junctions is increased. Some specific results are given for a one-dimensional array of tunnel junctions.     

压缩应变载荷下氮化镓隧道结微观压电特性及其巨压电电阻效应

电子器件可控性研究在日益追求器件智能化和可控化的当今社会至关重要. 基于第一性原理和量子输运计算, 本文研究了压缩应变载荷对氮化镓(GaN)隧道结基态电学性质和电流输运的影响, 在原子尺度上窥视了氮化镓隧道结的微观压电性, 验证了其内在的巨压电电阻(GPR)效应. 计算结果表明, 压缩应变载荷可以调节隧道结内氮化镓势垒层的电势能降、内建电场、电荷密度和极化强度, 进而实现对隧道结电流输运和隧穿电阻的调控. 在-1.0 V的偏置电压下, -5%的压缩应变载荷将使氮化镓隧道结的隧穿电阻增至4倍. 本研究展现了氮化镓隧道结在可控电子器件中的应用潜力, 也展现了应变工程在调控电子器件性能方面的光明前景.     

Tunneling spectroscopy of voltage tunable quantum wires

Resonant tunneling spectroscopy is used to investigate the tuning range for the one-dimensional subband spacing of side-gated quantum wires. We introduce a simplified selective depletion scheme for the implementation of a resonant tunneling device. From the analysis of the differential tunneling conductance obtained for a single-wire device we conclude that the energetic spacing for the one-dimensional subbands can be varied from effectively 0 to about 6 meV. Measurements in magnetic fields directed parallel and perpendicular to the tunnel current confirm the one-dimensional nature of the tunneling processes as well as the order of magnitude of the subband spacing by comparison of the tunneling characteristics with a model calculation that assumes a parabolic confinement.     

STM studies on quantum wire structures in air and liquid helium

In this work, low temperature scanning tunneling microscopy (STM) studies on quantum wires are reported, which were fabricated by laser holography and wet chemical etching. Inverted heterostructures with thin and highly doped cap layers were used as substrates in order to keep the total tunneling barrier as small as possible. Current—voltage curves were measured on the wires and in the depleted areas between them. Between the wires, significant current is only observed for electrons which tunnel from the GaAs valence band into the STM tip, whereas symmetric curren voltage curves are observed on the wires. This behavior is ascribed to the influence of surface depletion and thus, a comparison of current imaging spectroscopy data taken at 300 K and in liquid helium directly yields the edge depletion width of the quantum wires.     

Quantum mechanical tunneling phenomena in metal–semiconductor junctions

The impact of the quantum mechanical tunneling effect on the operation of MESFET device structure has been investigated. Due to the presence of a Schottky barrier in a highly doped semiconductor, the depletion region is so narrow that electrons can tunnel through the barrier and contribute to the gate leakage current. This, in turn, facilitates current gain of the Schottky junction transistor (SJT) in the subthreshold region. In a simulation of a SJT we have used 2D Monte Carlo particle-based simulations. Quantum mechanical tunneling effects have been accounted for by using the Airy function transfer matrix approach, valid for piecewise linear potential barriers.     

Design and modeling of an efficient metamorphic dual-junction InGaP/GaAs solar cell

In this work the impact of variation in mole fraction of tunnel junction and doping concentration of top window layer are investigated on the photovoltaic performance of dual junction InGaP/GaAs solar cell on silicon substrate. How does the Si substrate help this structure to act as a low cost concentrator cell for terrestrial application is also discussed. The detailed analysis of the cell is carried out through the performance measurement such as external quantum efficiency, internal quantum efficiency, fill factor, open circuit voltage, short circuit current density, spectral density and reflectance. This simulation model provides efficiency of 30.40 % at AM1.5G spectrum under 1 sun. It provides a path to the researcher for the development of III–V multi junction solar cell at a low cost.     

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p-n junction is reported.According to the well established light-to-electricity conversion theory,quantum wells(QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels,owing to quantum confinement,and cannot form a photocurrent.We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent,indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs.We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions.Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors.     

Calibrated numerical model of a GaInP–GaAs dual‐junction solar cell

In this letter a calibrated numerical model of a III–V dual‐junction solar cell including tunnel diode and Bragg reflector is presented. The quantum efficiencies of the subcells are computed by using the principle of current‐limitation in monolithic multi‐junction solar cells. A special procedure with bias‐illumination and bias‐voltage was implemented. Numerical simulations are used to study the influence of the top cell thickness on the cells' quantum efficiency and on the current‐matching condition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental evidence and consequences of rare events in quantum tunneling

Tiny spatial fluctuations of tunnel barrier parameters are shown to have dramatic consequences on the statistical properties of quantum tunneling. A direct experimental evidence is provided that the tunnel current through metal-oxide junctions, imaged at a nanometric scale, exhibits broad statistical distributions extending over more than 4 orders of magnitude. Striking effects of broad current distributions are shown: the total tunnel transmission is dominated by few highly transmitting sites and the typical current density varies strongly with the size of the junction. Moreover, self-averaging of the tunnel current fluctuations occurs only for unexpectedly large junction areas. Received 1 April 1999     

A new proposal for Si tandem solar cell: Significant efficiency enhancement in 3C–SiC/Si

In order to considerable enhancement of the efficiency of silicon solar cells, in this paper, for the first time, we present a new proposal for silicon based tandem solar cells. For investigation of this idea, we have evaluated the characteristics of 3C–SiC/Si crystalline tandem solar cells connected series by a tunneling junction, under air mass 1.5 global irradiance spectrums. A 2D simulation including the effects of surface passivation, back surface field (BSF), and carrier tunneling have been performed to obtain the optical and electrical characteristics of single junction silicon, 3C–SiC, and finally the tandem cells. The obtained data illustrate that the best design parameters considering the experimental limitations can be obtained. High energy conversion efficiency for the proposed structure of 26.09% has been achieved for 3C–SiC/Si tandem structure driven by 20.49% and 17.86% conversion efficiencies of single junction Si and 3C–SiC solar cells, respectively. Our results justifies that the higher conversion efficiency of the Si-based tandem structure compared with 3C–SiC and Si cells stems from enhancement of open circuit voltage and fill factor parameter at the hands of decrease in short circuit current limited by the top 3C–SiC cell.     

Intersubband photocurrent from double barrier resonant tunneling structures

Simple models of semiconductor-based double barrier resonant tunneling structures predict a large accumulation of charge carriers in the structure. These carriers can be excited optically from one subband to another generating photocurrent. In this work we have investigated the photo-induced current due to intersubband excitation in double barrier structures. We have found that the origin of the photocurrent is accumulation of quantized carriers in the emitter-barrier junction of the structure, rather than accumulation of carriers in the double barrier quantum well. This photon assisted tunneling process in double barrier structures may be used for infrared detection.     

Detection of local-moment formation using the resonant interaction between coupled quantum wires

We study the influence of many-body interactions on the transport characteristics of a pair of quantum wires that are coupled to each other by means of a quantum dot. Under conditions where a local magnetic moment is formed in one of the wires, tunnel coupling to the other gives rise to an associated peak in its density of states, which can be detected directly in a conductance measurement. Our theory is therefore able to account for the key observations in the recent study of T. Morimoto et al. [Appl. Phys. Lett., ()]], and demonstrates that coupled quantum wires may be used as a system for the detection of local magnetic-moment formation.     

Photon-assisted mesoscopic transport through a quantum dot–carbon nanotube system perturbed by microwave fields

We have investigated the coherent mesoscopic transport through the system with a quantum dot coupled to single-wall carbon nanotubes (CN–QD–CN) interfered by microwave fields (MWFs). The investigation focuses on the tunneling behaviors induced by the double coherent MWFs and the nature of CN leads. The incoherent fields induce the tunneling current possessing symmetric resonant behaviors. The coherent fields induce the asymmetric tunneling current resulting from the interference of tunneling current branches to form asymmetric photon-assisted net current. The quantum leads possess specific density of state (DOS) structure, and the matching–mismatching behavior takes important role in the mesoscopic transport. The feature of coupled MWFs and the connected quantum wires together control the characteristics of the mesoscopic system.     

Nonequilibrium Electron Transport Through a Quantum Dot from Kubo Formula

Based on the Kubo formula for an electron tunneling junction, we revisit the nonequilibrium transport properties through a quantum dot. Since the Fermi level of the quantum dot is set by the conduction electrons of the leads, we calculate the electron current from the left side by assuming the quantum dot coupled to the right lead as another side of the tunneling junction, and the other way round is used to calculate the current from the right side. By symmetrizing these two currents, an effective local density states on the dot can be obtained, and is discussed at high and low temperatures, respectively.     

Single-electron transistor effect in a two-terminal structure

A peculiarity of the single-electron transistor effect makes it possible to observe this effect even in structures lacking a gate electrode altogether. The proposed method can be useful for experimental study of charging effects in structures with an extremely small central island confined between tunnel barriers (like an ≃1 nm quantum dot or a macromolecule probed with a tunneling microscope), where it is impossible to provide a gate electrode for control of the tunnel current. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 507–511 (10 October 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Tunneling characteristics of a double-barrier magnetic junction

The spin-polarized current through a planar double-barrier magnetic tunnel junction has been calculated using the quasi-classical model. The coefficients of electron transmission through the barriers have been calculated in terms of the quantum theory. The dependences of the transmission coefficients, spinpolarized currents, and tunneling magnetoresistance on the applied voltage under resonant conditions have been shown. Under non-resonant conditions, the tunneling magnetoresistance has been compared with the experimental data.     

A lateral resonant tunneling FET

A lateral resonant tunneling FET (RTFET) is proposed. The RTFET has three closely spaced gates. The outer gates control the barrier heights, and the inner gate controls the potential of the quantum well. These gates are capacitively coupled to the barriers and the well, therefore, the gate currents are very small. Modeling and computer simulation show that the RTFET should have an improved peak-to-valley ratio, narrower current peak widths, and more uniform distribution of peak currents than that of a conventional resonant tunneling diode with the same structure. Furthermore, a unique feature of this device is that the barrier height can be adjusted, which allows the current peak, the peak-to-valley ratio, and the peak positions to be tuned.     

Application of Crystal Elements for Charged Particle Beam Steering and Radiation Beam Generation on the U-70 Synchrotron

Dependences of the tunnel magnetoresistance and in-plane component of the spin transfer torque on the applied voltage in a magnetic tunnel junction have been calculated in the approximation of ballistic transport of conduction electrons through an insulating layer with embedded magnetic or nonmagnetic nanoparticles. A single-barrier magnetic tunnel junction with a nanoparticle embedded in an insulator forms a double-barrier magnetic tunnel junction. It has been shown that the in-plane component of the spin transfer torque in the double-barrier magnetic tunnel junction can be higher than that in the single-barrier one at the same thickness of the insulating layer. The calculations show that nanoparticles embedded in the tunnel junction increase the probability of tunneling of electrons, create resonance conditions, and ensure the quantization of the conductance in contrast to the tunnel junction without nanoparticles. The calculated dependences of the tunnel magnetoresistance correspond to experimental data demonstrating peak anomalies and suppression of the maximum magnetoresistances at low voltages.