H. van Houten

Coherent Electron Focussing in a Two-Dimensional Electron Gas.

The first experimental realization of ballistic point contacts in a two-dimensional electron gas for the study of transverse electron focussing by a magnetic field is reported. Multiple peaks associated with skipping orbits of electrons reflected specularly by the channel boundary are observed. At low temperatures fine structure in the focussing spectra is seen.

Single-Electron Tunnelling Observed At Room Temperature by Scanning-Tunnelling Microscopy

Ultrasmall ( 5 nm in lateral diameter) double-barrier tunnel junctions have been realized using a scanning tunnelling microscope, and an optimized metal particle-oxide-metallic substrate system. Three electrical transport effects, all in good agreement with the semi-classical theory of single-electron tunnelling, have been found at room temperature: the Coulomb gap, the Coulomb staircase and zero-bias conductance oscillations as a function of tip-particle distance.

Submicron conducting channels defined by shallow mesa etch in GaAs-AlGaAs heterojunctions

A new approach to the lateral confinement of electrons in the two‐dimensional electron gas of GaAs‐AlGaAs heterojunctions has been developed. The electrons are electrostatically confined by a shallow mesa structure etched in the upper n‐doped AlGaAs layer. This structure is fabricated using electron beam lithography and reactive ion etching. The undoped AlGaAs spacer layer is not removed in order to avoid mobility degradation and channel depletion. Long narrow channels have been made for the study of electrical transport properties. The effective channel width in the submicron range is smaller than the width of the mesa structure. Preliminary low‐temperature magnetoresistance data are presented.

Coulomb-Blockade Oscillations in the Thermopower of a Quantum Dot.

The thermopower of a quantum dot, defined in the two-dimensional electron gas in a GaAs-AlxGa1-xAs heterostructure, is investigated using a current heating technique. At lattice temperatures kBT much smaller than the charging energy e2/C, and at small heating currents, sawtoohlike thermopower oscillations are observed as a function of gate voltage, in agreement with a recent theory. In addition, a remarkable sign reversal of the amplitude of the thermopower oscillations is found in the non-linear regime at large heating currents.

Thermo-electric properties of quantum point contacts

The conductance, the thermal conductance, the thermopower and the Peltier coefficient of a quantum point contact all exhibit quantum size effects. The authors review and extend the theory of these effects. In addition, they review their experimental work on the quantum oscillations in the thermopower, observed using a current heating technique. New data are presented showing evidence for quantum steps in the thermal conductance, and (less unequivocally) for quantum oscillations in the Peltier coefficient. For these new experiments the authors have used a quantum point contact as a miniature thermometer.

Coulomb-Blockade Oscillations in Semiconductor Nanostructures

A resume is given of recent theoretical and experimental work on single-electron tunneling in the size quantized regime in semiconductor quantum dots and wires. In addition, the quantum Hall effect regime is discussed.

Single-electron tunneling up to room temperature

Ultrasmall (5 nm in lateral diameter) double-barrier tunnel junctions have been realized using metal particles (2-5 nm in diameter) sandwiched in between a metallic substrate and the metallic tip of a scanning tunneling microscope (STM). The particles were either grown by e-beam evaporation or prepared using colloid chemistry. Two electrical transport effects, in good agreement with the semi-classical theory of single-electron tunneling, have been found at room temperature: the Coulomb gap and the Coulomb staircase. The interpretation in terms of single-electron tunneling is further supported by liquid helium temperature STM measurements on identical samples.

Skipping Orbits, Traversing Trajectories, and Quantum Ballistic Transport in Microstructures

Three topics of current interest in the study of quantum ballistic transport in a two-dimensional electron gas are discussed, with an emphasis on correspondences between classical trajectories and quantum states in the various experimental geometries. We consider the quantized conductance of point contacts, the quenching of the Hall effect in narrow channels, and coherent electron focusing in a double-point contact geometry.

Single-electron tunneling in double-barrier junctions by scanning tunneling microscopy

Abstract Ultrasmall (≲5 nm in lateral diameter) double-barrier tunnel junctions have been realized using a scanning tunneling microscope and an optimized metal-particle-oxide-metallic substrate system. The effects of single-electron charging (single-electron tunneling) on the current-voltage characteristics are studied experimentally at 4.2 K. The circumstances under which a Coulomb staircase is observable for this kind of model systems is investigated systematically, using the orthodox theory of single-electron tunneling to analyze the data.

Anomalous mobility and photo‐Hall effect in ZnSe‐GaAs heterostructures

The Hall and photo‐Hall effect are studied experimentally in pure, unintentionally doped n‐ZnSe epilayers grown by molecular‐beam epitaxy on semi‐insulating GaAs substrates. An anomalous depression of the dark values of the room‐temperature and peak mobility in the n‐ZnSe layer is observed in a substantial fraction of the samples studied, while normal values for the mobility are recovered under illumination with above‐band‐gap light. This is accompanied by an increase in the effective areal electron density. These results point to space‐charge scattering causing the dark mobility reduction. The photo‐Hall effect in ZnSe‐GaAs heterostructures is discussed in terms of simple models considering both misfit dislocations at the heterostructure interface and extended defects in the bulk of the ZnSe epilayer. Additional photo‐Hall experiments have been performed for longer‐wavelength laser lines. In this case, the areal electron concentration decreases strongly under intense illumination, while a large room‐temp...