Double-barrier Tunnel Junctions Research Articles

The impact of quasiparticle tunnel injection on the proximity effect in Pb/Sn sandwich

The proximity effect in Pb/Sn sandwich under the impact of quasiparticle tunnel injection has been studied. The estimation of the γ and γBN proximity effect parameters has been made; the dependence of the γ and γBN parameters on the Sn thickness has been built. It has been shown by studying the current-voltage characteristics of the double-barrier tunnel junction with a Pb/Sn intermediate proximity electrode that an increase of the γm parameter of the strength of the proximity effect occurs as a result of quasiparticle tunnel injection into the Pb/Sn proximity electrode.

Gold Nanoparticles on Oxide-Free Silicon–Molecule Interface for Single Electron Transport

Two different organic monolayers were prepared on silicon Si(111) and modified for attaching gold nanoparticles. The molecules are covalently bound to silicon and form very ordered monolayers sometimes improperly called self-assembled monolayers (SAM). They are designed to be electrically insulating and to have very few electrical interface states. By positioning the tip of an STM above a nanoparticle, a double barrier tunnel junction (DBTJ) is created, and Coulomb blockade is demonstrated at 40 K. This is the first time Coulomb blockade is observed with an organic monolayer on oxide-free silicon. This work focuses on the fabrication and initial electrical characterization of this double barrier tunnel junction. The organic layers were prepared by thermal hydrosilylation of two different alkene molecules with either a long carbon chain (C11) or a shorter one (C7), and both were modified to be amine-terminated. FTIR and XPS measurements confirm that the Si(111) substrate remains unoxidized during the whole chemical process. Colloidal gold nanoparticles were prepared using two methods: either with citrate molecules (Turkevich method) or with ascorbic acid as the surfactant. In both cases AFM and STM images show a well-controlled deposition on the grafted organic monolayer. I-V curves obtained by scanning tunneling spectroscopy (STS) are presented on 8 nm diameter nanoparticles and exhibit the well-known Coulomb staircases at low temperature. The curves are discussed as a function of the organic layer thickness and silicon substrate doping.

Shot noise in magnetic double-barrier tunnel junctions

This work is supported by the National Science Center in Poland as a research project in the years 2011–2014 and by the Polish National Center of Research and Development within the framework of the European project Era.Net.Rus “SpinBarrier” for the years 2012–2014. The work in Madrid has been supported by the Spanish MINECO (Grant No. MAT2012-32743, CONSOLIDER CSD2007-00010) and Comunidad de Madrid (Grant No. P2009/MAT-1726)

Symmetry Dependence of Vibration-Assisted Tunneling

We present spatially resolved vibronic spectroscopy of individual pentacene molecules in a double-barrier tunneling junction. It is observed that even for this effective single-level system the energy dissipation associated with electron attachment varies spatially by more than a factor of 2. This is in contrast to the usual treatment of electron-vibron coupling in the Franck-Condon picture. Our experiments unambiguously prove that the local symmetry of initial and final wave function determines the dissipation in electron transport.

Growth and characterization of GaDyN/GaN double barrierstructures

GaDyN/GaN double-barrier magnetic tunneling junctions (DB-MTJs) structures are grown on GaN templates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). X-ray diffraction (XRD) θ/2θ scan curves exhibited a clear peak of GaDyN at low angle side of the GaN (0002) diffraction peak. From analyzing extended x-ray absorption fine structure (EXAFS), majority of Dy atoms are found to be incorporated into substituting the Ga sites in GaN with wurtzite structure, and GaDyN has longer bond length than that of GaN. From XRD and EXAFS results, the lattice constant of the GaDyN is larger than that of GaN due to large radius of Dy ion. Photoluminescence from GaDyN DB-MTJs structure was found, which is related to the Dy ions. Ferromagnetism is confirmed in the magnetization versus magnetic field curves at room temperature for the all samples. It is found that the magnetization per unit volume and the coercivity show change with increasing the thickness of the middle GaDyN layer. This implies that the interlayer interaction exists between the GaDyN layers.

Coulomb blockade phenomena observed in supported metallic nanoislands

ORIGINAL RESEARCH article Front. Phys., 27 September 2013 | https://doi.org/10.3389/fphy.2013.00013

Charge carrier identification in tunneling spectroscopy of core-shell nanocrystals

Semiconductor PbSe/CdSe core-shell nanocrystals (NCs) in a double barrier tunnel junction have been investigated by means of scanning tunneling spectroscopy at low temperature. From the analysis of the differential conductance peak position as a function of the potential distribution in both potential barriers, we demonstrate a unipolar transport regime for a large amount of NCs. The same charge carriers are injected on both sides of the zero-conductance gap, and the peaks observed at higher energy arise from the charging of the NCs. Similar results are obtained for CdSe/CdS dot-in-rod NCs, indicating that the addition of a shell favors transitions between different charge states rather than single particle excited states. Further characterization of the PbSe/CdSe core-shell NCs by x-ray photoemission spectroscopy reveals that the variations in the transport properties from NC to NC are explained by the occurrence of unprotected PbSe facets that have different orientations in the junction.

Influence of confinement on single-electron charging in a network of nanoparticles

We investigated the single-electron tunneling (SET) behavior in a network of ligand stabilized Au nanoparticles (NPs) that are self-organized on an Au(111) surface by means of low-temperature scanning tunneling microscopy and spectroscopy. We demonstrate that for a proper combination of ligand chain length and NP radius the ligand shell is able to isolate a particle from the neighboring ones. This results in SET spectra with a clear Coulomb blockade and a regular staircase, similar to SET spectra obtained for isolated particles. A fraction of the investigated particles exhibits additional fine structure on top of the Coulomb charging peaks in the tunneling conductance spectra. The origin of the fine structure can be related to quantum size effects due to the very small NP size rather than to inter-particle capacitive coupling. Our findings indicate the possibility of using an individual particle in the self-organized network as the central Coulomb island in a double-barrier tunnel junction configuration, similar to the case of an isolated particle.

Single Electron Tunneling through a Tailored Arylthio-coronene

The possibility to control the charge transport properties in molecular scale devices strongly depends upon the nature of the molecule―metal interfaces, causing an intense request to engineer at molecular level the interface properties. Here, we report on single electron tunneling effects observed in a STM-tip/single molecule/substrate device at room temperature using a molecule with a three-dimensional aromatic system. The molecule consists of a coronene core per-substituted with arylthio groups which are tailored in such a way that the aromatic system is efficiently decoupled from the metal substrate, and thus a double-barrier tunnel junction is created by means of a built-in insulating spacer. Comparing ab initio calculations with the experimental observations allows us to identify the specific arrangement of the substituents in the most stable conformer of this molecule. The tailored molecular structure in combination with the identified adsorption geometry controls the electron transport behavior and results in single electron transport features.

Single-Electron Transistor Fabricated by Two Bottom-Up Processes of Electroless Au Plating and Chemisorption of Au Nanoparticle

Coulomb diamonds were clearly observed on single-electron transistors (SETs) fabricated by bottom-up processes of electroless plating of Au nanogap electrodes and chemisorption of a Au nanoparticle at 80 K. In the drain current–drain voltage characteristics, Coulomb staircases were modulated by the side gate voltage. Tunneling resistances and source/drain/gate capacitances of the SET were evaluated by fitting the theoretical Coulomb staircase determined on the basis of the full orthodox theory in a double-barrier tunneling junction to the experimental results of Coulomb blockade under the application of side gate voltages. The theoretical results for the Coulomb diamond are in good agreement with the experimental results.

Titanium silicide islands on atomically clean Si(100): Identifying single electron tunneling effects

Titanium silicide islands have been formed by the ultrahigh vacuum deposition of thin films of titanium (<2 nm) on atomically clean Si(100) substrates followed by annealing to ∼800 °C. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy have been performed on these islands to record current-voltage (I-V) curves. Because each island forms a double barrier tunnel junction (DBTJ) structure with the STM tip and the substrate, they would be expected to exhibit single electron tunneling (SET) according to the orthodox model of SET. Some of the islands formed are small enough (diameter <10 nm) to exhibit SET at room temperature and evidence of SET has been identified in some of the I-V curves recorded from these small islands. Those curves are analyzed within the framework of the orthodox model and are found to be consistent with that model, except for slight discrepancies of the shape of the I-V curves at current steps. However, most islands that were expected to exhibit SET did not do so, and the reasons for the absence of observable SET are evaluated. The most likely reasons for the absence of SET are determined to be a wide depletion region in the substrate and Schottky barrier lowering due to Fermi level pinning by surface states of the clean silicon near the islands. The results establish that although the Schottky barrier can act as an effective tunnel junction in a DBTJ structure, the islands may be unreliable in future nanoelectronic devices. Therefore, methods are discussed to improve the reliability of future devices.

Measuring charge trap occupation and energy level in CdSe/ZnS quantum dots using a scanning tunneling microscope

We use a scanning tunneling microscope to probe single-electron charging phenomena in individual CdSe/ZnS (core/shell) quantum dots (QDs) at room temperature. The QDs are deposited on top of a bare Au thin film and form a double-barrier tunnel junction (DBTJ) between the tip, QD, and substrate. Analysis of room-temperature hysteresis in the current-voltage $(IV)$ tunneling spectra, is consistent with trapped charge(s) presenting an additional potential barrier to tunneling, a measure of the Coulomb blockade. The paper describes the first direct electrical measurement of the trap-state energy on individual QDs. Manipulation of the charge occupation of the QD, verified by measuring the charging energy, $(61.4\ifmmode\pm\else\textpm\fi{}2.4)\text{ }\text{meV}$, and analysis of the DBTJ, show trap states $\ensuremath{\sim}1.09\text{ }\text{eV}$ below the QD conduction-band edge. In addition, the detrapping time, a measure of the tunneling barrier thickness, is determined to have an upper time limit of 250 ms. We hypothesize that the charge is trapped in a quantum-dot surface state.

Mesoscopic phenomena in Au nanocrystal floating gate memory structure

A resonant tunneling process is demonstrated in the HfAlO/Au nanocrystals/HfAlO trilayer nonvolatile memory (NVM) structure on Si, where the electrons tunnel back and forth to the Au nanocrystals due to the various mesoscopic behaviors. The electron tunneling behavior in this trilayer structure exhibits dissimilar resemblance to those in double-barrier tunnel junctions taking into account of the correlation of Coulomb blockade effect. The observed specific tunneling process is beneficial in studying the interplays of various mesoscopic physics and application of single electron devices into NVM.

I−Vcurves of Fe/MgO (001) single- and double-barrier tunnel junctions

In this work, we calculate with ab initio methods the current-voltage characteristics for ideal single- and double-barrier Fe/MgO 001 magnetic tunnel junctions. The current is calculated in the phase-coherent limit by using the recently developed SMEAGOL code, combining the nonequilibrium Green function formalism with density-functional theory. In general we find that double-barrier junctions display a larger magnetoresistance, which decays with bias at a slower pace than their single-barrier counterparts. This is explained in terms of enhanced spin filtering from the middle Fe layer sandwiched in between the two MgO barriers. In addition, for double-barrier tunnel junctions, we find a well defined peak in the magnetoresistance at a voltage of V =0.1 V. This is the signature of resonant tunneling across a majority quantum well state. Our findings are discussed in relation to recent experiments.

Phonon-assisted spin-polarized tunneling through an interacting quantum dot

Using the nonequilibrium Green function technique we study theoretically spin-polarizedtransport in double-barrier tunneling junctions based on a single-level quantum dotinteracting with a local phonon mode. Phonon emission and absorption spectra have beencalculated for arbitrary Coulomb correlations on the dot and for different temperatures. Itis shown that in the nonlinear response regime the electron–phonon interaction gives rise tocurrent suppression in symmetric junctions as well as to oscillations of the tunnelmagnetoresistance (TMR). In asymmetric junctions, the same mechanism may leadeffectively to enhancement of the diode-like characteristics. We have also found that atsufficiently low temperatures additional phonon-induced resonance peaks appear in thelinear spectral function on both sides of the main resonance peaks corresponding to thequantum dot energy levels. The case of negative effective charging energy is also analyzednumerically. A significant enhancement of electric current (or suppression of TMR) abovethe threshold bias voltages at which the dot energy level enters the tunneling window isobserved. The gate voltage-controlled rectification effect of the tunneling current inasymmetric junctions with positive and negative effective Coulomb correlations is alsodiscussed.

Controlling Single-Molecule Negative Differential Resistance in a Double-Barrier Tunnel Junction

We observed the transition from negative differential resistance (NDR) to the absence of NDR in the differential conductance (dI/dV) spectra of single copper-phthalocyanine (CuPc) molecules adsorbed on one, two, and three atomic layers of NaBr grown on a NiAl(110) substrate. Through numerical simulation, this transition is attributed to two phenomena in the double-barrier tunnel junction: (i) the opposite bias dependence of the vacuum and NaBr barrier heights, and (ii) the changing barrier widths for CuPc molecules adsorbed on different layers of NaBr.

Current-driven magnetization reversal at extremely low threshold current density in (Ga,Mn)As-based double-barrier magnetic tunnel junctions

Current-driven magnetic orientation reversal at an extremely low threshold current density, as low as 2.0×104A∕cm2, has been achieved in (Ga,Mn)As-based double-barrier magnetic tunneling junctions (MTJs) sandwiched between top and bottom MTJs. The middle magnetic free layer thickness dependence clearly demonstrates that the low threshold current density is owing not only to the small magnetization of the magnetic free layer but also the enhancement of the spin torque caused by a spin-polarized current through the top and bottom MTJs.

ハイブリッド型微細加工技術を用いた金ナノ粒子の単電子トンネル効果の観測

We would like to share our recent results from a research on the single electron tunneling effect in a double-barrier tunneling junction (DBTJ) structure. Single Au nanoparticles were trapped in a nanogap electrodes, whose gap size was precisely controlled using a combination of electron beam lithography and molecular ruler technique. Here, we introduce how the novel combination technique was utilized to obtain the nanogap electrode. We also introduce the size effect of Au particles on the single electron tunneling behavior as a Coulomb island. Finally, we introduce the relation between the geometrical and electrical properties of the DBTJ.

Single-electron tunneling phenomena on preformed gold clusters deposited on dithiol self-assembled monolayers

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on preformed nanometer sized gold clusters. The clusters are deposited on Au(1 1 1) films that are coated with a self-assembled monolayer of 1,4-benzenedimethanethiol molecules, thereby creating a double barrier tunnel junction. In this configuration electrons can only move between the scanning tunneling microscope tip and the substrate through tunneling via the cluster. Due to the very small cluster size this results in pronounced single-electron tunneling effects. Anomalies in the I( V) curves measured above a single cluster can be consistently described in terms of charging of the double barrier junction for the majority of the particles. Both Coulomb blockade and Coulomb staircase with step widths in the 200–600 meV range were observed on various individual clusters. The results agree with the commonly used semiclassical theory of charging effects in a double barrier tunnel junction. Furthermore, for a limited number of clusters, additional conductance peaks are observed which are attributed to strong electron confinement inside the nanoscale clusters.

Oscillations of tunnel magnetoresistance induced by spin-wave excitations in ferromagnet-ferromagnet-ferromagnet double-barrier tunnel junctions

The possibility of oscillations of the tunnel conductance and magnetoresistance induced by spin wave excitations in a ferromagnet-ferromagnet-ferromagnet double-barrier tunnel junction, when the magnetizations of the two side ferromagnets are aligned antiparallel to that of the middle ferromagnet, is investigated in a self-consistent manner by means of Keldysh nonequilibrium Green's function method. It has been found that owing to the $s\text{\ensuremath{-}}d$ exchange interactions between conduction electrons and the spin density induced by spin accumulation in the middle ferromagnet, the differential conductance and the tunnel magnetoresistance indeed oscillate with the increase of bias voltage, being qualitatively consistent with the phenomenon that is observed recently in experiments. The effects of magnon modes, the energy levels of electrons, as well as the molecular field in the central ferromagnet on the oscillatory transport property of the system are also discussed.