Single Electron Tunneling Effects Research Articles

Low temperature scanning tunneling microscopy studies of granular metal films

Cryogenic scanning tunneling microscopy is used to study local electrical transport properties of thin granular Au/Al2O3 films in the vicinity of the percolation threshold. The current–voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. The characteristics often exhibit single electron tunneling effects such as the Coulomb blockade and the Coulomb staircase. This behavior is similar to that observed for tunneling into a single isolated nanometer size metallic particle which was explained in terms of a double-barrier tunnel junction model. Some of the characteristics show, however, novel Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel junction model, where the electron tunnels through two metallic particles along its path, accounts quantitatively for the experimental results.

A fascinating new field in colloid science: small ligand-stabilized metal clusters and their possible application in microelectronics

Small metal clusters, like Au 55 (PPh 3 ) 12 Cl 6 , which fall in the size regime of 1 - 2 nm are colloidal nanoparticles with quantum properties in the transitional range between metals and semiconductors. These chemically tailored quantum dots show by the Quantum Size Effect (QSE) a level splitting between 20 and 100 meV, increasing from small particle sizes to the molecular state. The organic ligand shell surrounding the cluster acts like a dielectric «spacer» generating capacitances between neighboring clusters down to 10 -18 F. Therefore, charging effects superposed by level spacing effects can be observed. The ligand-stabilized colloidal quantum dots in condensed state can be described as a novel kind of artificial solid with extremely narrow mini or hopping bands depending on the chemically adjustable thickness of the ligand shell and its properties. Since its discovery, the Single Electron Tunneling (SET) effect has been recognized to be the fundamental concept for ultimate miniaturization in microelectronics. The controlled transport of charge carriers in arrangements of ligand-stabilized clusters has been observed already at room temperature through Impedance Spectroscopy (IS) and Scanning Tunneling Spectroscopy (STS). This reveals future directions with new concepts for the realization of simple devices for Single Electron Logic (SEL). Part II presents models and connections between microscopic and macroscopic level, regardless of whether there already exist suitable nanoscale metal cluster compounds, and is aimed at the ultimate properties for a possible application in microelectronics

A fascinating new field in colloid science: small ligand-stabilized metal clusters and possible application in microelectronics

Small metal clusters, like Au55(PPh3)12Cl6, which fall in the size regime of 1–2 nm are colloidal nanoparticles with quantum properties in the transitional range between metals and semiconductors. These chemically tailored quantum dots show regarding the Quantum Size Effect (QSE) a level splitting between 20 and 100 meV, increasing from small particle sizes to the molecular state. The organic ligand shell surrounding the cluster acts like a dielectric “spacer” generating capacitances between neighboring clusters down to 10−18 F. Therefore, charging effects superposed by level spacing effects can be observed. The ligand-stabilized colloidal quantum dots in condensed state can be described as a novel kind of artificial solid with extremely narrow mini or hopping bands depending on the chemically adjustable thickness of the ligand shell and its properties. Since its discovery, the Single Electron Tunneling (SET) effect has been recognized to be the fundamental concept for ultimate miniaturization in microelectronics. The controlled transport of charge carriers in arrangements of ligand-stabilized clusters has been observed already at room temperature through Impedance Spectroscopy (IS) and Scanning Tunneling Spectroscopy (STS). This reveals future directions with new concepts for the realization of simple devices for Single Electron Logic (SEL).

New potentiometry method in scanning tunneling microscopy: Exploiting the correlation of fluctuations

We developed a new scanning tunneling microscopy technique to measure the surface potential. The new method exploits the tunneling voltage dependence of the tip-sample separation. The indirect measurement of the potential together with a differential measurement technique makes the new potentiometry insensitive to errors of the electronic setup and provides submicrovolt sensitivity limited by approximately thermal noise of the tunneling resistance. We illustrate the new technique by basic measurements performed under ultrahigh-vacuum conditions. In addition we present the coherence of tunneling current fluctuations and potential fluctuations which underlines the quality of the new technique: the coherence differs significantly from unity. We conclude that the tunneling resistance does not have thermal voltage fluctuations predicted by Nyquist’s formula. Possible sources of residual voltage fluctuations such as single electron tunneling effects are discussed.

Single-electron tunneling effects in granular metal films.

Cryogenic scanning tunneling microscopy is used to study local electrical-transport properties of thin granular Au/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ films in the vicinity of the percolation threshold. The current-voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. These characteristics often exhibit interesting Coulomb-staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple-barrier tunnel-junction model accounts quantitatively for the experimental results.

Single-electron tunneling and Coulomb charging effects in ultrasmall double-barrier heterostructures

An extensive study of charge transport through submicron-diameter double barrier heterostructure diodes is reported. The occupation of the quantum well with single electrons, starting from zero, is observed in the form of sharp steps in the current-voltage characteristics. The magnitude of the current steps can be controlled by changing the barrier thicknesses and thus their transparency for tunneling electrons. The plateau width of the current steps is related to the energies of the electron states in the quantum well that are affected by the lateral quantum confinement, and by Coulomb charging effects. Diameter dependent studies of the tunneling current suggest that the lateral quantum confinement can result from the surface depletion potential, potential fluctuations, or single impurities. High magnetic field studies confirm this conclusion. The contribution of the Coulomb charging energy is investigated by using an asymmetric double barrier profile. It is shown that tunneling through submicron-diameter double barrier heterostructures provides valuable insight into the electronic properties of quantum boxes containing a few electrons.

Single-electron tunneling and Coulomb charging effects in aysmmetric double-barrier resonant-tunneling diodes.

Resonant tunneling is studied in an ultrasmall asymmetric GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double-barrier diode at low temperatures. In reverse bias, spikelike current-voltage characteristics are observed and assigned to electrons tunneling from zero-dimensional (0D) states in the accumulation layer to 0D states in the well. The 0D-0D tunneling reflects the single-electron spectrum without Coulomb charging effects. In forward bias, steplike characteristics are observed and ascribed to tunneling from one-dimensional subbands in the emitter contacts through 0D states in the well, accompanied by Coulomb charging effects. Moderate magnetic fields (B\ensuremath{\approxeq}4 T) parallel to the current improve the flatness of the steps.

Coulomb blockade effects in STM-type tunnel junctions

Experiments on single electron tunneling effects induced by the Coulomb blockade in small capacitance tunnel junctions are described. The tunnel junctions are of the point contact type, produced by means of scanning tunneling microscope techniques. Both single junctions and double junctions have been studied. The observed behaviour is compared with calculations based on Monte Carlo simulations.

Single electron tunneling oscillations in one-dimensional arrays of ultrasmall tunnel junctions

Time correlation of Single Electron Tunneling events i.e. SET-oscillations has been observed in one dimensional arrays of Al/AlxOy/Al tunnel junctions with very small areas (0.006μm2). They are detected by phase locking to external microwaves with frequency fext between 0.7GHz and 5GHz. Peaks in the differential resistance occur at currents corresponding to l=efext. Good agreement is obtained with theoretical data.

Tunneling in artificial Al2O3 tunnel barriers and Al2O3-metal multilayers.

We report on the material and electron-tunneling properties of thin ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ films deposited onto room-temperature substrates by rf magnetron sputtering of a pressed aluminum oxide target in a pure argon atmosphere. X-ray photoelectron spectroscopy shows the films to be composed of >99% ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$. The electrical properties have been investigated by tunneling in junctions of the form: Cu/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/counterelectrodes with counterelectrodes of Cu, Pb, and Pb-Bi. The Cu/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ bilayers were deposited in situ with ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ thicknesses ranging from 8 to 20 A\r{}. These junctions have been found to exhibit excellent tunneling characteristics including low zero-bias conduction (typically below 1% at 4.2 K) and large effective barrier heights (typically above 1 eV). We have observed full, clean superconducting gap structure, and strong, clear phonon structure for junctions with Pb-Bi counterelectrodes. The expected exponential rise of junction resistance with increasing barrier thickness was observed, giving an average barrier height of 1.65 eV and an effective tunneling length of 0.82 A\r{}. We observe a steplike increase in junction yield as barrier thickness exceeds 12--15 A\r{}, the dependence of which has been successfully modeled as a statistical buildup of barrier molecules on the base-electrode surface. We have also investigated the tunneling characteristics of metal/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/intermediate metal/${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$/metal multilayer junctions wherein the properties of the intermediate metal films can be studied and have provided a confirmation of single-electron tunneling effects.