W. Schlapp

Interface Roughness Scattering and Electron Mobilities in Thin GaAs Quantum Wells

The mobility of the two-dimensional electron gas in thin GaAs multiple quantum well structures is studied. At low temperatures a strong decrease is found for well widths between 60 A and 40 A. This can be explained by interface roughness scattering. The measured electron density dependence of the mobility is used to determine the roughness parameters Λ ≈ 65 A an Δ ≈ 2.4 A. The importance of the penetration of the electron wave function into the barrier is discussed.

Electronic excitations in narrow GaAs/AlxAlxGa−1xAs quantum well structures

Abstract Electric subbands of narrow, modulation doped GaAs/Al x Ga − x As quantum well structures are studied by inelastic light, infrared absorption, time resolved spectroscopy, and by self-consistent subband calculations. Intersubband excitations are observed in the energy range from 100 to 180 meV for quantum well thickness ranging from 7 to 4 nm. For the thinnest layers a loss of confinement of the higher subband is observed. Intersubband relaxation times of the order of 10 ps are obtained from time resolved infrared absorption experiments at room temperature.

Internal Photoemission in GaAs/(AlxGa1−x)As Heterostructures

Band offsets in (AlxGa1−x)As/GaAs heterostructures are determined using internal photoemission experiments. Onsets in the photocurrent are observed for photon energies exceeding the fundamental energy gaps of GaAs and (AlxGa1−x)As. Additional onsets occur at photon energies in the infrared region due to internal photoemission from the conduction band in GaAs over the barrier into the conduction band of (AlxGa1−x)As and in the near red region where excitations from the GaAs valence band into the (AlxGa1−x)As conduction band are involved. From the measured energies we determine ΔEc/ΔEg = 0.8 ± 0.03 for x = 0.2.

Magnetic-Field-Induced Intersubband Resonances in AlGaAs/GaAs Quantum Wells

We present a mechanism to excite intersubband transitions at normal incidence in quantum wells with a magnetic field applied parallel to the two-dimensional electron gas. In the Voigt configuration the light polarization perpendicular to the magnetic field induces intersubband transitions whose oscillator strength is proportional to the square of the field. This dependence allows us to determine experimentally the intersubband matrix elements with great accuracy.

Picosecond intersubband spectroscopy

Abstract A review of the experimental techniques for the study of the dynamics of intersubband transitions in multiple quantum well structures is presented. The investigations of samples with various parameters give time constants between a few picoseconds and 500 ps. In most cases, electron - polar LO phonon scattering is found to be the most relevant relaxation mechanism. Coupling between the upper subband and barrier states is discussed as a rate determining interaction in thin quantum wells.

Edge magnetoplasmons in the quantum Hall effect regime

Abstract Edge magnetoplasmons are studied in GaAs/AlGaAs heterojunctions of a typical size of 3×3 mm2 in the regime of the quantized Hall effect at frequencies below 500 MHz and low temperatures. The resonance frequency scales with integer Landau level filling factor. With increasing temperature, in the 0.4–4 K temperature range, the resonance frequency decreases slightly, whereas the linewidth increases about linearly with temperature. The experimental results are compared to theoretical predictions.

Plasmon excitations in layered 2D electron gas systems with large in-plane wave vector

Abstract Intrasubband plasmons in modulation doped multi-quantum well structures are investigated by low temperature micro Raman spectroscopy. The in-plane plasmon dispersion is measured up to about 7×105cm−1, which is four times the range acessable in earlier experiments. The results agree well with the predictions for an infinite 2DEG superlattice. A new resonance enhancement between the E0 and E0+Δ0 quantum well energy gaps is observed.

Grating coupler effects on inelastic light scattering by plasmons in micro structured GaAs MQW systems

Abstract Plasmons in modulation doped MQW systems are studied by micro Raman spectroscopy on unstructured samples and after preparation of a grating in the upper part of the sample. The structured samples show a series of additional peaks in the energy range of the plasmon dispersion measured on the unstructured sample using different scattering geometries. They are attributed to plasmons excited by the grating coupler effect of the wire system.

Negative infrared photoconductivity in narrow GaAs/AlGaAs multiple quantum well structures

Abstract Intersubband excitations in narrow modulation doped GaAs/Al x Ga 1− x As multi quantum well structures are studied by infrared photoconductivity parallel to the layers. The conductivity is decreased due to real space transfer of resonance excited electrons from the GaAs wells to the Al x Ga 1− x As barriers. This effect can be tuned strongly by photon-induced ionization of DX centers in the barriers.

Relevant scattering processes, band gap renormalization and moss-burstein shift in modulation doped narrow GaAs/AlGaAs multiple quantum wells

Abstract A set of GaAs/AlGaAs multiple quantum wells with systematic variations of the well width Lz and the electron density ns is investigated by magnetotransport and luminescence excitation spectroscopy. The single-particle relaxation time τs and the transport scattering time τt are extracted from Shubnikov-de Haas oscillations and zero-field mobility, respectively. The influence of different scattering processes is determined as a function of Lz and ns by comparing both τs and τt with calculations of interface roughness, remote impurity and allow disorder scattering of the 2DEG. Using photoluminescence and photoluminescence excitation spectroscopy the effect of band gap renormalization as well as the Moss-Burstein shift of the absorption edge are studied for the same set of samples.