Electron Tunneling Transport Research Articles

Tunneling electron transport of silicon nanochains studied by in situ scanning electron microscopy

Electron transport and field emission properties of silicon nanochains are studied by in situ scanning electron microscopy at bias voltages up to 120V using a micromanipulator system. The current-voltage (I-V) characteristics follow the Fowler-Nordheim law when the anode is in contact with the silicon nanochains as well as when separated by about 1μm. This result suggests that the field-induced tunneling current is dominant even when the microprobe is in contact with the silicon nanochains.

Conductance fluctuation and degeneracy in nanocontact between a conductive AFM tip and a granular surface under small-load conditions

Abstract Conductive-tip atomic force microscope (c-AFM) has been extensively used in measuring electrical properties of surface nanostructures, but the electrical conduction in c-AFM tip–sample contacts in nanometer scale is not well understood. In the present work, we experimentally investigated the electrical properties of the nanocontact between a W2C-coated c-AFM tip and granular gold film under small-load (∼5 nN) at ambient air conditions. We found that under a constant bias voltage (10 V), the electrical current passing through the tip–sample junction at fixed location of sample surface dramatically fluctuated and degenerated. By quantitatively estimating the mechanical and electrical aspects of the nanocontact, we explained the observed phenomena as mechanical instabilities, electron tunneling transport and atomic rearrangements at the contact junction. We think that our results are important for the realistic application of c-AFM in nanoelectronic measurement.

Room temperature tunneling transport through Si nanodots in silicon rich silicon nitride

Devices containing Si nanodots (NDs) were made in an α-SiNx:H∕Si NDs∕α-SiNx:H structure to explore the transport characteristics. The Si NDs were embedded in a silicon nitride matrix and were produced by using a plasma enhanced chemical vapor deposition technique. Room temperature Si NDs related electron and hole tunneling transport were observed in these devices. Negative differential resistance in the current–voltage characteristics was observed for the hole tunneling. The peak-to-valley ratio was as high as 13.9. Transmission electron microscopy reveals that only one Si ND exists in the current transport direction. The Si NDs are ∼5nm in diameter. A model with a double barrier band diagram is suggested to explain the Si NDs related transport.

Structure and magnetotransport properties of Fe3O4–SiO2 composite films reactively sputtered at room temperature

( Fe 3 O 4 ) 1−x –( SiO 2 ) x composite films have been prepared by reactive sputtering iron and SiO2 targets in Ar+O2 mixture at room temperature. Transmission electron microscopy bright field images show that with the increase of SiO2 addition, uniform Fe3O4 grains are well separated by the amorphous SiO2 matrix, forming a well-defined granular structure. Temperature dependence of resistivity ρ(T) indicates that the electron tunneling mechanism featured by log ρ∝T−1/2 dominates the transport properties of the films, which smears out the Verwey transition intrinsic to Fe3O4. This tunneling transport of electrons causes a spin-dependent magnetoresistance {=(ρH−ρ0)/ρ0} of about −4.7% for Fe3O4 films and −1.8% for (Fe3O4)0.6(SiO2)0.4 composite films under a 46 kOe magnetic field at room temperature. Magnetic and magnetoresistance measurements reveal that the antiferromagnetically coupled Fe3O4 grains are decoupled and show the behavior of superparamagnetism at x⩾0.4.

Temperature dependence of conductance of electrostatically disordered quasi-2D semiconductor systems near an insulator-metal percolation transition

The experimental temperature dependence (4.2–300 K) of the conductance of mesoscopic quasi-2D electronic systems under conditions of the insulator-metal percolation transition are discussed for the case of metal-nitride-oxide-semiconductor silicon transistor structures with an inversion n channel and an extremely high (≥1013 cm−2) built-in charge density (source of electrostatic fluctuation potential). Saddle domains of the fluctuation potential are analyzed within the framework of the Landauer-Buttiker formalism. These domains, being point quantum contacts between wells in the chaotic potential distribution, determine both the nature of electron transport and the conditions of the insulator-metal transitions. The results of analyzing the dependence of the conductivity on temperature and field (on the gate voltage) are shown to be consistent. The shape of the effective potential barrier for the electron tunneling transport across the saddle domains is reconstructed.

Inelastic electron tunneling across magnetically active interfaces in cuprate and manganite heterostructures modified by electromigration processes

We report a study of the electron tunneling transport in point-contact junctions formed by a sharp Ag tip and two different highly correlated oxides, namely, a magnetoresistive manganite La0.66Ca0.34MnO3 and a superconducting cuprate LaBa2Cu3O7−x. Strong chemical modifications of the oxide surface (supposedly, oxygen ion displacements) caused by applying high voltages to the junctions have been observed. This effect is believed to be responsible for an enormous growth of inelastic tunneling processes across a transition region that reveals itself in an overall V-shaped conductance background, with a strong temperature impact. The mechanism of the inelastic scattering is ascribed to charge transmission across magnetically active interfaces between two electrodes forming the junction. To support the latter statement, we have fabricated planar junctions between Cr and Ag films with an antiferromagnetic chromium oxide Cr2O3 as a potential barrier and at high-bias voltages have found an identical conductance trend with a similar temperature effect.

Resonant tunneling through a single self-assembled InAs quantum dot in a micro-RTD structure

By forming a micron-scale Schottky diode on a GaAs/n +–GaAs wafer with low density ( ∼4×10 8 cm −2 ) self-assembled InAs quantum dots (QDs) embedded in the top GaAs layer, we have studied resonant tunneling transport of electrons via single QDs. As the diode defined by electron-beam lithography and chemical etching has a size of about 1 μm 2 and the edges of each diode are depleted, the number of active InAs QD contained in each microdiode can be reduced to less than unity in average. Current–voltage measurements were performed using conductive probe atomic force microscope, and current peaks caused by electrons resonantly tunneling through quantum levels of single QDs have been observed at temperatures as high as ∼130 K .

Andreev tunneling, Coulomb blockade, and resonant transport of nonlocal spin-entangled electrons

We propose and analyze a spin-entangler for electrons based on an s-wave superconductor coupled to two quantum dots each of which is tunnel-coupled to normal Fermi leads. We show that in the presence of a voltage bias and in the Coulomb blockade regime two correlated electrons provided by the Andreev process can coherently tunnel from the superconductor via different dots into different leads. The spin-singlet coming from the Cooper pair remains preserved in this process, and the setup provides a source of mobile and nonlocal spin-entangled electrons. The transport current is calculated and shown to be dominated by a two-particle Breit-Wigner resonance which allows the injection of two spin-entangled electrons into different leads at exactly the same orbital energy, which is a crucial requirement for the detection of spin entanglement via noise measurements. The coherent tunneling of both electrons into the same lead is suppressed by the on-site Coulomb repulsion and/or the superconducting gap, while the tunneling into different leads is suppressed through the initial separation of the tunneling electrons. In the regime of interest the particle-hole excitations of the leads are shown to be negligible. The Aharonov-Bohm oscillations in the current are shown to contain single- and two-electron periods with amplitudes that both vanish with increasing Coulomb repulsion albeit differently fast.

Tunneling competition of photoexcited carriers in a system of monolithically integrated dual multiple InGaAs/AlGaAs and GaAs/AlGaAs quantum wells

Vertical transport of photoexcited carriers has been studied in a p–i–n diode whose intrinsic layer contains two different multiple quantum wells (MQW), GaAs/Al0.15Ga0.85As (MQW1) and strained In0.15Ga0.85As/Al0.15Ga0.85As (MQW2) isolated by a thick Al0.15Ga0.85As barrier. Pseudo-negative photocurrent (PC) peaks are observed at exciton resonance wavelengths of MQW1 located far from the n-electrode under low electric fields, while the normal positive excitonic peaks recover with increasing the electric field. Moreover, the PC intensity of MQW1 as a function of inverse electric field shows linear dependence due to Fowler–Nordheim-type tunneling with a slope change. The observed PC intensity crossover is rigorously explained by tunneling probability calculations, because of differences in the thickness and height of the transport limiting tunneling barriers, assuming dominance of electron tunneling transport for the PC responses.

Negative peaks in photocurrent spectra of thick barrier GaAs/AlAs multiple quantum wells

We have experimentally studied photocurrent (PC) spectral features of a relatively thick barrier multiple quantum well (MQW) p-i-n diode at 18 and 80 K as a function of axial electric field. It is found that PC spectra do not always reflect the photoabsorption spectral line shape under the low field condition and show negative peaks at the exciton resonance wavelengths. These PC dips are qualitatively explained by considering the distribution of photogenerated carriers within the intrinsic region and competition between carrier transit and recombination times. Furthermore, the dominance of electron-tunneling transport for the low temperature PC mechanism explains enhanced negative peaks in the PC spectra at 80 K when the tunneling assisted drift is reduced.

Photocurrent spectra of monolithically integrated dual multiple quantum wells

We have experimentally studied the vertical transport mechanism of photoexcited carriers in a p-i-n diode whose intrinsic layer contains two different quantum wells, GaAs/AlGaAs (MQW1) and InGaAs/AlGaAs (MQW2) isolated by a thick barrier by photocurrent (PC) spectroscopy. When the MQW2 layer located near the n-type substrate is illuminated from the n-side at wavelength below the GaAs band gap, the PC signal is more enhanced at the exciton resonances than the p-side illumination. For the MQW1 layer, on the other hand, pseudonegative PC peaks are observed at the exciton resonance wavelengths under low electric fields, while the normal positive excitonic peaks recover with increasing the electric field. These results are rigorously explained by considering that electron tunneling transport is dominant and that the transport from MQW1 to the n-electrode through MQW2 is blocked by the central thick barrier at low fields because shadowing effects of the front MQW1 layer decrease the number of photons absorbed in the MQW2 layer at the MQW1 resonances.

Quantum Point Contact Switches

Switching behavior between electron tunneling and ballistic transport states was induced by repeatedly bringing a sharpened nickel wire into contact with a gold surface. The high-conductivity ballistic state had a quantized conductance of 0.977 +/- 0.015 (2e(2)/h). Switching was accomplished by moving the electrodes with a piezoelectric actuator over a distance of 2 angstroms. The two electrodes and the actuator form a three-terminal device that is demonstrated to be a reliable digital and analog switch; it shows good discrimination between high and low states and possesses the important property of power gain. The conductance channel is most likely only one atom wide and possibly consists of a single atom.

Growth and Properties of (Al, Ga)As / NiAl / (Al, Ga)As: An Epitaxical Semiconductor / Metal / Semiconductor System

The epitaxical and thermally stable NiAI/(AI, Ga)As system is shown to meet all of the basic materials criteria for buried metal/compound semiconductor heterostructures. We describe the growth of these heterostructures by molecular beam epitaxy. Even the thinnest buried NiAI films grown thus far (1.5 nm) are electrically continuous and metallic. Electron tunneling and lateral transport measurements provide strong evidence for size quantization in NiAl films thinner than 3.5 nm. The merging of compound semiconductor tunneling barriers with epitaxical metallic quantum wells and the ability to selectively contact buried metallic quantum wells is expected to yield novel three-terminal devices.