Proximity effect, Andreev reflections, and charge transport in mesoscopic superconducting/semiconducting heterostructures

In the quasi-two-dimensional (Q2D) electron gas of an InAs channel between an AlSb substrate and superconducting niobium layers, the proximity effect induces a pair potential so that a Q2D mesoscopic superconducting/normal/superconducting (SNS) junction forms in the channel. The pair potential is calculated with quasiclassical Green’s functions in the clean limit. For such a junction, alternating Josephson currents and current–voltage characteristics (CVCs) are computed, using the nonequilibrium quasiparticle wavefunctions which solve the time-dependent Bogoliubov–de Gennes equations. The CVCs exhibit features found experimentally by the Kroemer group: a steep rise of the current at small voltages (‘foot’) changes at a ‘corner current’ to a much slower increase of current with higher voltages, and the zero-bias differential resistance increases with temperature. Phase-coherent multiple Andreev reflections and the associated Cooper pair transfers are the physical mechanisms responsible for the oscillating Josephson currents and the CVCs. Additional experimental findings not reproduced by the theory require model improvements, especially a consideration of the external current leads which should give rise to hybrid quasiparticle/collective-mode excitations.     

Technology aspects of GaN-based diodes for high-field operation

This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.     

DFT-NEGF study of transport properties and NDR behavior in fused furan and thiophene dimmers

The nonequilibrium Green's function approach in combination with density-functional theory is used to perform quantum mechanical calculations of the electron transport properties of furan and thiophene dimmers. Both the molecular systems have two S-linker and translated into the Gold junction with (1 1 1) surfaces. The studied molecular junctions at zero bias voltage are HOMO-based junctions and currents through these systems are driven by hole transport. The current–voltage characteristics of the both studied molecular junctions illustrate that negative differential resistance (NDR) feature is observed over the bias voltage of 2.0 V. Higher conductivity of fused furan dimmer and NDR character have been explained by the monitoring of the transmission resonance peak across the bias window against varying bias voltages.     

S-shaped current bistability in a bipolar resonant tunneling diode

The bipolar tunneling transport through p–i–n double barrier structures has been studied by means of simultaneous electrical transport measurements and electroluminescence spectroscopy. An “inverted” hysteresis loop is observed at the onset of the first electronic resonance in the current–voltage characteristics with an electrical ON/OFF ratio of more than two orders of magnitude. Relating the different branches of the current–voltage characteristic to the space charges accumulated throughout the structure the inverted hysteresis loop is interpreted in terms of an S-shaped current bistability. The S-shaped current bistability is similar to the current driven negative differential resistivity as known for instance from thyristor action. This analogy between the bipolar double barrier structure with alloyed n-type emitter and the thyristor will be briefly discussed.     

Conductance studies in a double-bend quantum structure

Transport phenomena in a double-bend quantum structure fabricated in the two-dimensional electron gas of a modulation doped GaAs/AlGaAs structure, are studied experimentally. The structure consists of an electrostatically defined quantum dot with two one-dimensional wires connected on opposite corners of the dot. The current–voltage characteristics of such devices exhibit quantized conductance breakdown (non-linear behavior), conductance variation with confinement, and non-linear and asymmetric behavior at high bias condition. Low temperature conductance of this structure shows evidence of resonant tunneling, while the peaks of the conductance vary with temperature.     

Suppression of Josephson currents in ballistic junctions by an injection current

Control of the critical current in a superconductor/two-dimensional electron gas Josephson junction by means of an injection current is reported. The control mechanism is explained by a theoretical model, which takes ballistic transport across the junction and diffusive transport through the semiconductor wire structure into account. Measurements on a Nb-AlGaSb/InAs-Nb junction show that the strong suppression of the critical current can, in principle, be explained by the theoretical model. Deviations are due to the nonlinear current–voltage characteristics of the superconductor/two-dimensional electron gas interface and the two-dimensionality of the supercurrent transport.     

Charging effects in biased molecular devices

The influence of charging effects on the transport characteristics of a molecular wire bridging two metallic electrodes in the limit of weak contacts is studied by the generalized Breit–Wigner formula. Molecule is modeled as a quantum dot with discrete energy levels, while the coupling to the electrodes is treated within a broad-band theory. Owing to this model we find self-consistent occupation of particular energy levels and orbital energies of the wire in the presence of transport. The nonlinear conductance and current–voltage characteristics are investigated as a function of bias voltage in the case of symmetric and asymmetric coupling to the electrodes. It is shown that the shape of those curves are determined by the combined effect of the electronic structure of the molecule and by electron–electron repulsion.     

Interface injection-limited carrier-transport properties of Alq<Subscript>3</Subscript>

We have studied the carrier-transport properties of the amorphous organic material tris (8-hydroxyquinoline) aluminium (Alq₃) for Al and LiF/Al cathodes at room temperature. The investigation was made by the current–voltage characteristic measurements for different Alq₃ film thicknesses. It is found that the current–voltage characteristic has a linear dependence on the thickness of the Alq₃ film. The current density increases by several orders for a LiF/Al cathode over that of Al at a given bias voltage. The carrier-injection processes at the metal/organic contact dominate the current–voltage characteristics. The carrier injection seems to be limited by the charge hopping of interfacial molecular sites. PACS 73.50.Gr; 73.40.-c; 73.61.Ph; 78.45.+h; 42.70.Jk     

Nonlinear resonance between a soliton and Josephson plasma waves: experiment and theory

Resonant interaction of a soliton (Josephson fluxon) with its self-generated Josephson plasma waves is studied experimentally, numerically, and analytically. An externally applied magnetic field H forms a cos-like potential relief for the soliton in the annular junction. Soliton motion under the influence of the bias current leads to an emission of plasma waves, which gives rise to a resonance at a certain soliton velocity. This resonance on the current–voltage characteristics shows a clear backbending accompanied by a negative differential resistance. Our analysis quantitatively explains the observed effect.     

Improving performance of resonant tunneling devices in asymmetric structures

Based on the global coherent tunneling model, we present a self-consistent calculation and show that structural asymmetry of double barrier resonant tunneling structures (DBRTSs) significantly modifies the current–voltage characteristics compared to the symmetric structures. Within the framework of the dielectric continuum model, we further investigate the phonon-assisted tunneling (PAT) current in symmetric and asymmetric DBRTSs. Both the interface modes and the confined bulk-like longitudinal-optical phonons are considered. The results indicate that the four higher-frequency interface phonon modes (especially the one which has the largest electron–phonon interaction at either interface of the emitter barrier) dominate the PAT processes. We show that a suitably designed asymmetric structure can produce much larger peak current and absolute value of the negative differential conductivity than its commonly used symmetric counterpart.     

Conductance anomalies of small tunnel junctions inside the Coulomb gap

A small-capacitance normal tunnel deviates significantly from equilibrium because each tunneling event turns the junction voltage almost upside-down. If such a sudden perturbation occurs locally, Fermi liquid theory guarantees that infinitely many electron-hole pairs should be created near the Fermi surface. It is predicted that such an infrared-divergent shake-up combined with the electromagnetic environment leads to subgap conductance anomalies for two categories of junctions. For symmetric junctions whose electrodes have the same electronic properties, a nonvanishing subgap conductance is shown to be inevitable even if the environmental impedance is infinite. This effect smoothes the current-voltage (I–V) characteristic and shifts the Coulomb offset extrapolated back from the high-voltage part of theI–V curve. For asymmetric junctions, whose electrodes have different electronic affinities, tunneling conductance is enhanced in one direction and suppressed in the other; that is, the junctions exhibit a diode effect. In particular, when the tunneling resistance is much smaller than the resistance quantum and the current flows in the favorable direction, a strong tendency towards establishing phase coherence is shown to emerge, as in Josephson junctions, resulting in infinite differential conductance at zero bias voltage.     

Transport in a shunted surface superlattice with a perpendicular magnetic field

We experimentally investigate the transport through a shunted surface superlattice under the influence of a magnetic field applied perpendicular to the current direction. The current–voltage characteristics of these surface superlattices exhibit a peak which is followed by a wide region of negative differential resistance. The application of a transverse magnetic field has a profound influence on the position and height of this peak. The recorded shifts are compared to the predictions of different superlattice transport theories. Since these theories predict a different dependence on the magnetic field strength, the transport mechanism in the surface superlattice structures can be uniquely determined.     

Variation of co-tunneling and Kondo effects by control of the strength of coupling between a vertical dot and a two-dimensional electron gas

We have fabricated a vertical quantum dot with lateral coupling, modulated by a split gate voltage, to a two-dimensional electron. We thereby control not only electron configurations but also the strength of coupling between the dot and the lateral lead, by applying gate voltages. We have measured the conductance enhancement when the applied bias exceeds the single-electron excitation energy, in the Coulomb blockade regime. This conductance enhancement disappears as the split gate voltage decreases (reducing the coupling). This indicates that this enhancement is caused by inelastic co-tunneling. Furthermore, we observed a conductance enhancement at zero source–drain bias with stronger coupling. An anomaly is observed that we attribute to Kondo resonance between the dot and the leads.     

Surface-field induced interband tunneling in InAs

Metal/insulator/semiconductor junctions are prepared on degeneratep-type InAs substrates with hole concentrations ranging from 2.3×10¹⁷ cm^–3 to 2.7×10¹⁸ cm^–3. The low work function of the top metal Yb, Al, or Au and charged interface states influence a two-dimensional (2D) electron inversion layer at the InAs surface. The insulator barrier that is formed by thermal oxidation is designed sufficiently thin, so that the bias voltage applied at the metal electrode mainly drops across the depletion layer separating the electron channel from the bulk. The current-voltage (I–V) characteristics exhibit strong negative differential conductance due to interband, tunneling from the 2D subband into the 3D valence band with peak-to-valley current ratios up to 3.1, 18, and 32 at 300 K, 77 K, and 4.2 K, respectively. In agreement with a theoretical model based on coherentelastic tunneling, the form of the I–V curves resembles those of double-barrier resonant tunnel devices rather than those of 3D Esaki diodes. The series resistance is obtained from the saturation of the differential conductance dI/dV at high forward bias and from the shift of structures in d² I/dV ² arising from phonon assisted tunneling.Dedicated to G. Lautz on the occasion of his 65th birthday     

Resonant states in heterostructures of graphene nanoribbons

We study the transport properties of heterostructures of armchair graphene nanoribbons (AGNR) forming a double symmetrical barrier configuration. The systems are described by a single-band tight-binding Hamiltonian and Green's functions formalism, based on real-space renormalization techniques. We present results for the quantum conductance and the current for distinct configurations, focusing our analysis on the dependence of the transport with geometrical effects such as separation, width and transverse dimension of the barriers. Our results show the apparition of a series of resonant peaks in the conductance, showing a clear evidence of the presence of resonant states in the conductor. Changes in the barrier dimensions allow the modulation of the resonances in the conductance, making possible to obtain a complete suppression of electron transmission for determined values of the Fermi energy. The current–voltage curves show the presence of a negative differential resistance effect with a threshold voltage that can be controlled by varying the separation between the barriers and by modulating its confinement potential.     

A new analytical model of GaAs MESFET's

In this paper, a simple model for the calculation of the current—voltage characteristics of a short channel GaAs MESFET is first proposed. The new model can describe GaAs behavior for both high pinchoff and low pinchoff voltages. Test results show that it is more efficient and simpler than the previous models[1–7].     

Room-temperature observation of current bistability and fine structures in germanium quantum dots/SiO2 resonant tunneling diodes

The steady-state and time-dependent current–voltage (I–V) characteristics are experimentally investigated in Ge quantum dot (QD)/SiO₂ resonant tunneling diodes (RTDs). Ge QDs embedded in a SiO₂ matrix are naturally formed by thermal oxidation of Si_0.9Ge_0.1 nanowires (30 nm×50 nm) on silicon-on-insulator substrates. The average dot size and spacing between dots are 9±1 and 25 nm, respectively, from TEM observations, which indicate that one or two QDs are embedded between SiO₂ tunneling barriers within the nanowires. Room-temperature resonant oscillation, negative differential conductance, bistability, and fine structures are observed in the steady-state tunneling current of Ge-QD/SiO₂ RTDs under light illumination. Time-dependent tunneling current characteristics display periodic seesaw features as the Ge-QDs RTD is biased within the voltage regime of the first resonance peak while they exhibit harmonic swing behaviors as the RTD is biased at the current valleys or higher-order current peaks. This possibly originates from the interplay of the random telegraph signals from traps at the QD/SiO₂ interface as well as the electron wave interference within a small QD due to substantial quantum mechanics effects.     

Electronic transport properties in benzene-based heterostructure: Effects of anchoring groups

Using density functional theory (DFT) combined with nonequilibrium Green?s functions (NEGF), the electronic transport properties of benzene-based heterostructure molecular devices have been investigated. We focus on the contact geometry between molecules and electrodes, and several different anchoring groups have been considered. The current–voltage characteristics were calculated for positive and negative bias voltages, and discussed in terms of transmission spectra, transferred charges, and molecular projected self-consistent Hamiltonian (MPSH) states. Our results show that the anchoring groups play a crucial role in determining the overall conductivity of the molecular devices. Negative differential resistance (NDR) and rectifying effect can be observed.     

Fabrication of quantum dot transistors incorporating a single self-assembled quantum dot

We report on the fabrication and the characterization of quantum dot transistors incorporating a single self-assembled quantum dot. The current–voltage characteristics exhibit clear staircase structures at room temperature. They are attributed to electron tunneling through the quantized energy levels of a single quantum dot.     

Theory for electron hopping through nanometer-scale contacts: From tunneling regime to ballistic regime

Using the recursion-transfer-matrix (RTM) method combined with nonequilibrium Green's function (NEGF) method, we study the electronic states and current–voltage (I–V) characteristics of junction systems with atomic-scale nanocontacts as a function of the distance between electrodes. We observe a strong nonlinear behavior in the I–V characteristics and correspondingly a gap structure appears in conductance. We find that such a nonlinear behavior emerges when the transport properties change from tunneling to ballistic regimes.