F. Hirler

Spatially direct and indirect optical transitions in shallow etched GaAs/AlGaAs quantum wires

Electrons and photoexcited holes are separated into adjacent quantum wires in remote doped GaAs/AlGaAs quantum wells, using a shallow etching technique. This spatial separation results in a drastic change of the photoluminescence lineshape and an additional, broad feature shifted up to 30 meV below the luminescence energy of the unstructured sample. The two relevant recombination processes are shown to be due to spatially direct and indirect transitions, respectively. A variation of the temperature indicates, that both plasma-like and exciton-like recombinations contribute to the spatially direct luminescence.

Confined plasmons in shallow etched quantum wires

Resonant Raman scattering and FIR transmission spectroscopy is used to detect confined plasmons in shallow etched GaAs/AlGaAs single quantum well wires. Different samples with varying period lengths and wire widths are investigated. The lateral potential modulation can be estimated by analysing spatially direct and indirect luminescence. We find deviations from the simple model that uses a 2D plasmon dispersion relation and wavevectors corresponding to integral numbers of half wavelengths in the confined region.

One-dimensional charge density determination in shallow etched quantum wires

A numerical method is developed to determine the subband energies and electron concentrations in shallow etched quantum wires from magnetotransport data. Solving the two-dimensional Schrodinger equation self-consistently, the results are compared with the electron densities and subband spacings obtained from a harmonic oscillator model. Due to the non-parabolic wire potential, the numerically calculated electron densities are systematically smaller than the values obtained by a harmonic oscillator model.

Spatially direct and indirect optical transitions in shallow etched GaAs/AlGaAs wires, dots and antidots

By applying a shallow etching technique, the electrons in a remote doped GaAs/AlGaAs quantum well and the photogenerated holes are separated into adjacent lateral regions. The dominant photoluminescence of these carriers is shown to be indirect in real space. In addition, a spatially direct luminescence is observed. Its origin depends on the type of structuring and is discussed for wires, dots and antidots in comparison with unstructured samples.

Tunnelling processes between quantum wires and a two-dimensional electron gas

The authors have studied tunnelling processes between multiple quantum wires and a two-dimensional electron gas, separated by a barrier of 195 AA. Both the subband energies and the selection rules are derived from the tunnelling characteristics. Subband spacings of 4.5 meV are observed, which are the largest values presently reported on GaAs-AlGaAs quantum wires. These data are also verified by the results of a two-dimensional numerical model.

Information on the confinement potential in GaAsAlGaAs wires from magnetoluminescence experiments

Abstract Shallow etched wires, dots and antidots have been prepared from remote-doped GaAs AlGaAs quantum wells. The luminescence of electrons and holes separated into adjacent lateral regions was studied with a magnetic field applied in the growth direction. The spatially indirect luminescence was found to shift in energy and become direct at high magnetic fields. Details of the shape of the confinement potential were obtained by simulating the energy shift numerically.

Tunneling processes between quantum wires and two-dimensional electron states

Abstract We have studied tunneling processes between two independently contacted two-dimensional electron gas systems, which are separated by a barrier of 195 A. Resonant tunneling processes between the quantized states in the different channels. Structuring the upper channel into quantum wires, we also investigate tunneling processes between states of different dimensionality. By analyzing the transition rules, we explain the features of the tunneling characteristics and determine quantitatively the subband energies. Subband spacings up to 4.5 meV are obtained from the experimental data and are verified by the results of a two-dimensional self-consistent model. These extremely high values are a consequence of a Ge diffusion during the contact formation process, which suggests that an appropriate surface treatment after quantum wire fabrication can be used to increase the quantization energies.

Interplay of one- and two-dimensional systems

The authors have studied the influence of a weak magnetic field on the energy levels in a lateral surface superlattice fabricated on GaAs-GaAlAs heterostructures. In the transition region between a two-dimensional electron gas and a multiple quantum wire system, a new set of equidistant magnetoresistance oscillations is observed These structures can be explained in terms of a locally modulated density of states, which leads to a charge transfer between regions of high and low electron mobility.

Luminescence Properties of GaAs Quantum Wells, Wires, Dots, and Antidots

Luminescence properties of doped and undoped GaAs/(AlGa)As quantum wells, wires, dots, and antidots are discussed. The complex lineshape of high quality modulation doped single quantum wells can be understood in terms of wave vector conserving and nonconserving transitions. A shallow etching technique together with holographic optical lithography results in a spatial separation of electrons and photogenerated holes. The luminescence features of wires, dots, and antidots show both direct and indirect character in real space. Isolated single quantum dots have been fabricated by local interdiffusion of undoped GaAs/AlGaAs quantum wells. Due to the inherent exclusion of inhomogeneous broadening in single dot structures very sharp luminescence lines are observed in originally 3 nm wide quantum wells.

Tunneling Between Constrained Dimensionality Systems

Systems of reduced dimensionality became a topic of increased interest during the last few years. To induce a one-dimensional system, a two-dimensional electron gas can be constrained to a thin stripe (“quantum wire”) by etching processes or electrostatic confinement. This confinement results in an additional set of quantized states. The knowledge of these one dimensional (ID) subband energies is one of the basic requirements to understand the physical properties of quantum wires such as the quenching of the quantum hall effect (Roukes et al., 1987), mobility modulations (Ismail et al., 1989) or boundary scattering (Thornton et al., 1989). One effect widely used to determine the ID-energy levels is the magnetic depopulation of the lD-subbands. This was done first by (Berggren et al., 1986) on a split-gate field effect transistor structure. Assuming a parabolic electrostatic confinement, the influence of an additional magnetic field can be analyzed analytically. It was shown (Berggren et al., 1986), that the additional magnetic field increases the ID subband spacing, so that lD-subbands are shifted above the Fermi energy if the magnetic field is increased. Consequently, these subbands are depopulated, resulting in an oscillating behavior of the magneto resistance. In contradiction to the 2D-case, a plot of the oscillation index versus inverse magnetic field (Landau-plot) is not linear and saturates at low magnetic fields. By fitting the experimental results, both the ID electron concentration n1D and the subband energies are determined. From this, the widths of the conducting channels can be calculated.