In situ reduction of charge noise in GaAs/AlGaAs Schottky-gated devices

We show that an insulated electrostatic gate can be used to strongly suppress ubiquitous background charge noise in Schottky-gated GaAs/AlGaAs devices. Via a 2-D self-consistent simulation of the conduction band profile we show that this observation can be explained by reduced leakage of electrons from the Schottky gates into the semiconductor through the Schottky barrier, consistent with the effect of "bias cooling". Upon noise reduction, the noise power spectrum generally changes from Lorentzian to $1/f$ type. By comparing wafers with different Al content, we exclude that DX centers play a dominant role in the charge noise.Comment: 4 pages, 3 figure

Temperature dependence of the visibility in an electronic Mach-Zehnder interferometer

We performed the conductance and the shot noise measurements in an electronic Mach-Zehnder interferometer. The visibility of the interference is investigated as a function of the electron temperature that is derived from the thermal noise of the interferometer. The non-equilibrium noise displays both h/e and h/2e oscillations vs. the modulation gate voltage.Comment: 19 pages, 4 figure

Vier- und Multiwellenmischen an II-VI-Halbleiternanostrukturen

Diese Arbeit beschäftigt sich mit dem polarisations-, intensitäts- und temperaturabhängigen Dephasierungsverhalten kohärent angeregter Exzitonen, Biexzitonen und Trionen in Quantentrögen, Quantendrähten und Quanteninseln. Die Experimente, die an II-VI-Halbleiternano- strukturen durchgeführt wurden, basieren auf einer nichtlinear optischen Methode der Ultrakurzzeitspektroskopie, der Vierwellenmischung. Phasenrelaxation und Wechselwirkungsprozesse der resonant angeregten Quasiteilchen werden durch erweiterte optische Blochgleichungen theoretisch beschrieben. Um die Bedeutung von Korrelationen höherer Ordnung von Exzitonen in ZnSe Quantentrögen zu ermitteln, wird die Methode der Multiwellenmischung entwickelt. Durch Anwendung von modernen Halbleiterblochgleichungen bis zur 5. Ordnung in den anregenden Feldern gelingt die Beschreibung der beobachteten Spektren.In this theses the polarization, intensity and temperature dependent dephasing behavior of coherently excited excitons, biexcitons, and trions are investigated in quantum wells, quantum wires and quantum islands. The experiments, applied to II-VI semiconductor nanostructures, base on a nonlinear optical method of ultrashorttime spectroscopy, the four-wave mixing. Phase relaxation and interaction processes of the resonantly excited quasi-particles are described by extended optical bloch equations. The importance of higher order correlations of excitons in ZnSe quantum wells is investigated. Therefore a new method has been developed, the so called multi-wave mixing. The application of modern semiconductor bloch equations up to fifth order in the exciting fields results in a successful description of the observed spectra

Electric-field stabilization in a high-density surface superlattice

By application of the cleaved-edge overgrowth technique, we realize a two-channel superlattice (SL) device. The structure combines the parallel transport through a low-density SL under almost homogeneous electric field conditions with that through a surface SL (SSL) with large carrier density, which is, without parallel transport, subject to pronounced field instabilities. Direct control of the SSL density allows a separation of both transport contributions. With parallel transport through the low-density SL, the current carried by the SSL is characteristic for a SL with homogeneous field distribution. In particular, it exhibits negative differential conductivity over a wide range of applied electric fields. In contrast, for current only through the SSL clear electric-field instabilities, typical for SLs at high densities are observed. Thus, by means of the parallel transport channel, field instabilities are avoided and transport in high-density SLs with a homogeneous field distribution becomes accessible

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

Oscillations of the magnetoresistance of a two-dimensional electron gas under microwave irradiation: Influence of the irradiation frequency

We present measurements of the magnetoresistance of a two-dimensional electron gas (2DEG) under continuous microwave as a function of the irradiation frequency. In a previous work by Simovič et al. [Phys. Rev. B 71 (2005) 233303], the magnetoresistance under microwave was shown to be modulated by oscillations of large amplitude that are periodic with magnetic field, their period and phase depending strongly on the electron density. Here we show that the phase and the amplitude of the microwave-induced oscillations also depend on the frequency of irradiation and the sign of the magnetic field

Two beam Aharonov–Bohm interference in the integer quantum Hall regime

We study an electronic Mach–Zehnder interferometer employing the edge channels of a two-dimensional electron gas (2DEG) in the quantum Hall regime and quantum point contacts (QPC) as beam splitters. The linear conductance of the QPC as a function of gate voltage shows resonances superimposed on quantized conductance steps. It was found that the visibility depends strongly on magnetic field and is highest near a filling factor of 1.5 in the interferometer arm

Current-induced heating in quantum well and quantum wire intersubband emitter structures

We discuss the influence of current-induced heating on the current-voltage (I-V) characteristics and the spectral behavior in quantum well and quantum wire intersubband emitter structures. A conventional quantum cascade laser structure in the AlGaAs/GaAs material system with undoped cladding layers and an undoped active region is examined. This heterostructure serves as a first growth step for quantum wire intersubband emitters fabricated by the cleaved-edge overgrowth technique. We discuss the influence of electrons supplied by a remote delta-silicon doping. Duty-cycle dependent measurements on the quantum wire structures confirm the influence of current-induced heating on the I-V characteristics as well as on the emission spectra

Nanometre spaced electrodes on a cleaved AlGaAs surface

We present a novel technique for fabricating nanometre spaced metal electrodes on a smooth crystal cleavage plane with precisely predetermined spacing. Our method does not require any high-resolution nanolithography tools, all lateral patterning being based on conventional optical lithography. Using molecular beam epitaxy we embedded a thin gallium arsenide (GaAs) layer in between two aluminium gallium arsenide (AlGaAs) layers with monolayer precision. By cleaving the substrate an atomically flat surface is obtained exposing the AlGaAs–GaAs sandwich structure. After selectively etching the GaAs layer, the remaining AlGaAs layers are used as a support for deposited thin film metal electrodes. We characterized these coplanar electrodes by atomic force microscopy and scanning electron microscopy; this revealed clean, symmetric and macroscopically flat surfaces with a maximum corrugation of less than 1.2 nm. In the case of a device with a 20 nm thick GaAs layer the measured electrode distance was 22.5 nm with a maximum deviation of less than 2.1 nm. To demonstrate the electrical functionality of our device we positioned single colloidal gold nanoparticles between the electrodes by an alternating voltage trapping method; this resulted in a drop of electrical resistance from ~11 G Ω to ~1.5 k Ω at 4.2 K. The device structure has large potential for the manipulation of nanosized objects like molecules or more complex aggregates on flat surfaces and the investigation of their electrical properties in a freely suspended configuration

A comparison: 2D electron- and hole systems in the fractional quantum Hall regime

We investigated two-dimensional electron- (2DES) and hole systems (2DHS) in the fractional quantum Hall regime for filling factors ν=1/3 and ν=2/3. Due to a metallic top gate we are able to vary the electron-/hole density of the samples over a wide range. Measuring activated transport on these systems with perpendicular magnetic fields up to 18 T and temperatures down to about 30 mK allows to get an insight into the excitation spectra. Although the bandstructure of holes is complicated compared to those of electrons, the theoretical description of the fractional quantum Hall effect in the composite Fermion picture should hold for both types of particles. We analyzed and compared the results for the different filling factors and systems