Multibarrier Tunneling Research Articles

An In-Depth Study of Redox-Induced Conformational Changes in Charge Transport Characteristics of a Ferrocene-Alkanethiolate Molecular Electronic Junction: Temperature-Dependent Transition Voltage Spectroscopy Analysis

We studied the temperature-dependent electrical properties of ferrocene-alkanethiolate molecular electronic junctions fabricated by a conventional solid-state device technique using a conducting polymer poly(3,4-ethylenedioxythiophene) stabilized with a poly(4-styrenesulfonic acid) (known as PEDOT:PSS) interlayer. We had previously observed unusual temperature-dependent electrical characteristics at a voltage polarity larger than +0.6 V and at temperatures higher than ∼220 K (see Adv. Funct. Mater. 2014, 24, 2472). The origin of this distinctive behavior was ascribed to redox-induced conformational changes of the ferrocene-alkanethiolate molecules. We performed temperature-dependent transition voltage spectroscopy analysis based on the multibarrier tunneling model to support the proposed redox-induced conformational changes observed in the ferrocene-alkanethiolate molecular junctions. The obtained results were consistent with the unusual thermal characteristics of the ferrocene-alkanethiolate molecular junctions predicted by the proposed redox-induced conformational changes of the molecules.

Electrical conductivity of silicon quantum nanowires

We suggest a new theory for the description of electrical conductivity of semiconductor quantum nanowires. We take into account that oscillations of quantum nanowires lead to their self-similar deformation, and because of interaction between nanowires they form fractal clusters. Electrical potential of these structures is described via nonlinear fractal measures. We conclude that current-voltage characteristics of quantum nanowires contain hysteresis loops with oscillations. This fact corresponds to existence of negative differential resistance due to multi barrier tunneling effect in the described fractal structures.Our theoretical results have been confirmed by results of corresponding specific experimental study of nanoscale wire-like structures in silicon.

Electrical conduction through self-assembled monolayers in molecular junctions: Au/molecules/Au versus Au/molecule/PEDOT:PSS/Au

We fabricated and characterized a large number of octanedithiol (denoted as DC8) molecular devices as vertical metal–molecule–metal structure with or without using an intermediate conducting polymer layer of poly (3,4-ethylenedioxythiophene) stabilized with poly(4-styenesulfonic acid) (called as PEDOT:PSS). The electronic transport properties of DC8 molecular devices with and without PEDOT:PSS layer were statistically compared in terms of current density and device yield. The yields of the working molecular devices were found to be ~ 1.75% (84 out of 4800 devices) for Au/DC8/Au junctions and ~ 58% (74 out of 128 devices) for Au–DC8/PEDOT:PSS/Au junctions. The tunneling decay constants were obtained with the Simmons tunneling model and a multibarrier tunneling model for two kinds of molecular devices with and without PEDOT:PSS layer.

Theory of nonequilibrium quantum transport and energy dissipation in terahertz quantum cascade lasers

We analyze theoretically several crucial performance aspects of terahertz quantum cascade lasers, such as the impact of doping on the threshold current, the relative importance of the various scattering mechanisms, and the balance of coherent transport and realistic energy dissipation. We have developed a fully self-consistent model for stationary charge transport based on nonequilibrium Green's function theory that takes into account incoherent scattering with phonons, impurities, and rough interfaces as well as electron-electron scattering in the Hartree approximation, but does not a priori assume the electron distributions to follow the periodicity of the quantum cascade laser (QCL) structure. The theoretical results show excellent quantitative agreement with experimental data. We find scattering at rough interfaces to strongly affect electronic transport and efficiently limit the optical gain. Our results also indicate that a large portion of the current is maintained by coherent multibarrier tunneling. We show that this dominant coherent transport may lead to electron distributions that do not follow the periodicity of the QCL.

Enhancement of Field Emission Transport by Molecular Tilt Configuration in Metal−Molecule−Metal Junctions

We studied the molecular configuration-dependent charge transport of alkyl metal-molecule-metal junctions using conducting atomic force microscopy (CAFM). The inflection point (or transition voltage V(T)) on the plot of ln(I/V(2)) versus 1/V shifted to a lower voltage with increasing CAFM tip-loading force and decreasing molecular length. Our results indicate that the reduction of gap distance by molecular tilt configuration enhances the transition of the electronic transport mechanism from direct tunneling to field emission transport through molecules. The obtained results are consistent with a barrier height decrease, as affected by the enhancement of the intermolecular chain-to-chain tunneling as molecular tilt, predicted by a multibarrier tunneling model.

Effects of Metal−Molecule Contact and Molecular Structure on Molecular Electronic Conduction in Nonresonant Tunneling Regime: Alkyl versus Conjugated Molecules

The effect of metal−molecule contacts and molecular structures on the electronic conduction of alkyl and conjugated molecular junctions in nonresonant tunneling regime was investigated based on a proposed multibarrier tunneling (MBT) model, where the molecular junction was decomposed into three individual barriers through the molecular body and metal−molecule contacts on either side of the molecule. The resistance−area product (RA), specific contact resistance (RC), and decay coefficients (βC, βP, βBody, and βo) were estimated for alkyl and two selected conjugated molecules (oligoacene and oligophenylene) between Au contacts, and the result of RA (alkyl) > RA (oligoacene) ≥ RA (oligophenylene) for a given molecular length was obtained from different decay coefficients for different molecules. By assuming the tunneling around the highest occupied molecular orbital of alkyl molecules (like a hole type tunneling), RC for alkanemonothiol and alkanedithiol junctions with various metallic (Ag, Cu, and Au) conta...

Influence of metal-molecule contacts on decay coefficients and specific contact resistances in molecular junctions

The effect of metal-molecule contacts in molecular junctions is studied based on the analysis of a statistically significant number of devices and a proposed multibarrier tunneling (MBT) model, where a metal-molecule-metal junction is divided into three individual barriers: a molecular-chain body and metal-molecule contacts on either side of molecule. Using the MBT model with the statistical analysis, we could derive and distinguish decay coefficients for contact barriers $({\ensuremath{\beta}}_{1},{\ensuremath{\beta}}_{2})$, contact-dependent and contact-independent decay coefficients (${\ensuremath{\beta}}_{0}$ vs ${\ensuremath{\beta}}_{\text{body}}$), and specific contact resistances in terms of different molecular length and different natures of metal-molecule contacts.

Ballistic magnetoresistance of electroplated nickel devices

Electroplated nickel nanojunctions (NJs) showing ballistic magnetoresistance up to 100 000% could be the potential workhorse to drive the next generation of magnetic storage beyond one trillion bits per square inch. Understanding the mechanism of this kind of device is extremely important for both academia and industry. In this paper, the detailed oxidation process and the structures of junctions after deposition are analyzed. Depending on the details of structures within the junction, a device of this kind can work as a multibarrier tunneling magnetoresistance (MB-TMR) device, Coulomb blockade enhanced MB-TMR, or even a spin-dependent atomiclike quantum dot. In terms of device fabrication, better control of the oxidation after plating is crucial.

Multibarrier tunneling.

We study the tunneling through an arbitrary number of finite rectangular opaque barriers and generalize earlier results by showing that the total tunneling phase time depends neither on the barrier thickness nor on the interbarrier separation. We also predict peculiar features of the system considered, namely the independence of the transit time (for nonresonant tunneling) and the resonant frequency on the number of barriers crossed, which can be directly tested in photonic experiments. A thorough analysis of the role played by interbarrier multiple reflections and a physical interpretation of the results obtained is reported, showing that multibarrier tunneling is a highly nonlocal phenomenon.

Design optimization of multi-barrier tunneling devices using the transfer-matrix method

We present simulations of a recently published multi-barrier phase-state low electron device memory cell. For the proper consideration of tunneling through the insulating barriers we implemented a one-dimensional Schrödinger solver based on the transfer-matrix formalism into the device simulator MINIMOS-NT. We investigate the effect of barrier size and position on the I on/ I off ratio of the memory cell. We find that the position and thickness of the central shutter barrier can be used for device tuning. For high I on/ I off ratios the central shutter barrier (CSB) should be placed near the upper contact. Furthermore, a reduction in the stack width leads to increasing I on/ I off ratios. Although the use of the transfer-matrix method in a device simulator requires a number of assumptions, it turns out to be a viable tool for deepening the understanding of tunneling effects in devices where other tunneling models fail.

Resonance splitting effect in multibarrier tunneling.

An exact analytical expression for the transmission probability of electron transport through the superlattice (SL) modeled by a finite number of periodic rectangular multibarrier structures (PRMBS's) in one dimension is presented. The explicit correlation between the resonance properties and the SL potential structure as well as miniband structure is obtained. It is shown that (1) the resonances in multibarrier tunneling are confined in energy into the minibands of the corresponding SL; (2) the number of resonance splittings in each miniband is equal to the number of the wells in the structure; (3) in each set of splittings the peaks of the resonances are distributed equally about the energy asymptotically coinciding with the energy of the single-well structure, at which there is also a resonance peak if and only if the number of the barriers in the structure is even; and (4) for PRMBS's each of the resonances provides an ideal energy window of 100% transparency for the electron with the resonant energy.

Description of overlapping resonances in multibarrier tunneling structures.

A resonance formalism is used to obtain an expression for the transmission coefficient T that is appropriate to describe resonances that are arbitrarily close to each other in multibarrier resonant tunneling structures. The resulting expressions are compared rvith exact numerical calculations of T for a number of structures. For an isolated miniband an excellent description is obtained by using only the sum of Breit-Wigner-type expressions plus the overlapping resonance terms belonging to the miniband

Absorbing boundary conditions for the finite-difference time-domain calculation of the one-dimensional Schrödinger equation.

The absorbing boundary conditions for the finite-difference calculation of the time-dependent one-dimensional Schr\"odinger equation are presented. These are the local boundary conditions that approximate the one-way wave equation of a wave function. The conditions minimize undesirable reflections at the artificial boundaries of the area of computation, thus enabling us to limit the computation area efficiently. The calculations of the transmission coefficient of multibarrier tunneling show the validity of the method.