W. Hansen

LATERAL SUPERLATTICES AS VOLTAGE-CONTROLLED TRAPS FOR EXCITONS

We demonstrate the localization of quantum-well excitons in a periodic array of linear traps using photoluminescence experiments. The excitonic traps are induced by applying spatially alternating external voltages via interdigitated metal gates. The localization originates from the periodical modulation of the strength of the quantum-confined Stark effect in the plane of the quantum well. In our experiments, the trap depth is easily tuned by the voltages applied to the interdigitated gates. Furthermore, we find that a perpendicular magnetic field reduces the exciton diffusion length. In short-period lateral superlattices, we observe a magnetic-field-induced stabilization of excitons in the presence of strong in-plane electric fields.

Franz–Keldysh effect in a two‐dimensional system

We report luminescence and photocurrent experiments on an InGaAs/GaAs quantum well in strong lateral electric fields. The fields are imposed with an interdigitated gate on the crystal surface that consists of an array of metal stripes with a 250 nm period. The small period allows generation of electric fields of about 105 V/cm by application of only 1.5 V at the gate electrodes. Strong subgap absorption and oscillations of the interband absorption as function of radiation energy are observed and discussed in view of the Franz–Keldysh effect. The resulting photo‐I‐V characteristic shows high potential for electro‐optic applications.

Distribution and shape of self-assembled InAs quantum dots grown on GaAs (001)*

Grazing incidence small angle X-ray scattering (GISAXS) and atomic force microscopy (AFM) experiments are employed to study self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on GaAs (001). The GISAXS spectra show pronounced non-specular diffuse scattering satellite peaks with high diffraction orders up to ±3 along [110], [1-10], and [100] sample azimuthal orientations with respect to the incoming beam, indicating a lateral ordering of the InAs QDs. The correlation lengths of the lateral dot distribution are found to be identical along [110] and [1-10] but smaller along [100] direction. The ratio of the mean dot-dot distances along [100] and [1-10] azimuths is determined to be 1.13, indicating the anisotropic ordering of QD distribution. Broad diffraction peaks are observed at larger scattering angles and associated to dot facet crystal truncation rods (CTR). We determine {111}-like facets along [110] and [1-10] sample azimuths, and {101}-like facets along [100] azimuth.

Widely tuneable quantum wire arrays in MISFET-type heterojunctions with a stacked gate

Abstract A stacked gate technique is applied to study electronic excitations in electron quantum wire, quantum dot and antidot arrays in GaAs/AlAs MISFET-type heterojunctions at far-infrared frequencies. The gate configuration consists of a finely (period a ⩾ 250 nm) patterned bottom electrode on the surface of the heterostructure, a dielectric spacer and a homogeneous top gate. An electron grating is induced at the heterostructure interface either beneath the strips or beneath the gaps of the grating gate. With the fields induced by the top gate the confining potential can be controlled nearly independently of the electron density in quantum wires below the bottom gate. This is demonstrated by far-infrared studies of the dimensional resonances in the wires. In a perpendicular magnetic field we observe a very pronounced splitting of the dimensional resonance. We study this behaviour as a function of wire separation and potential form and find this splitting to reflect the anharmonicity of the confining potential.

Spectroscopy of field-effect-induced superlattices

Interdigitated gates are employed on heterostructure surfaces to induce strong and tunable lateral superlattice potentials of type II, i.e. of purely field-effect origin. The design of the heterostructures is optimized in a way that avoids statistical potential fluctuations even at very low electron densities where screening of carriers is ineffective. Single-particle electron states confined to one-dimensional electron wires are investigated with capacitance spectroscopy. The precise control of the potential as well as the homogeneity of the potential allow us to investigate true quantum wire arrays, i.e. wires in which only the lowest one-dimensional subband is occupied. Furthermore, we employ interdigitated gates to investigate the optical properties of quantum wells in a type II potential superlattice. The lateral fields are sufficiently high to field-ionize optically excited electron-hole pairs. Absorption spectra of the quantum well under such strong lateral fields show the typical signatures of the Franz-Keldysh effect.

Interacting edge states in quantum wires

Abstract We investigate the longitudinal and transversal magneto resistance in quantum wires prepared on high mobility AlGaAs-GaAs heterojunctions. In the quantum Hall regime the extension of edge states is so large that in our narrow channels states of opposing edges are interacting. In the experiment plateau-like regions are still discernible in the Hall resistance although the resistance values are significantly larger than the plateau values h/e 2 n, where n is an integer. The experimental observations are discussed with a model that introduces phenomenological transfer times between adjacent edge states. From our experimental data we evaluate a rate for transfer across the wire that is of the same order as the transfer rate between edge states on the same side.

Strong localization effect in magnetic two-dimensional hole systems

We report an extensive study of the magnetotransport properties of magnetically doped two-dimensional hole systems. Inverted manganese modulation doped InAs quantum wells with localized manganese ions providing a magnetic moment of S=5/2 were grown by molecular beam epitaxy. Strong localization effect found in low-field magnetotransport measurements on these structures can either be modified by the manganese doping density or by tuning the two-dimensional hole density p via field effect. The data reveal that the ratio between p and manganese ions inside or in close vicinity to the channel enlarges the strong localization effect. Moreover, asymmetric broadening of the doping layer due to manganese segregation is significantly influenced by strain in the heterostructure.

Far‐infrared photocurrent of quantum wires

We study the photocurrent of quantum capacitor devices consisting of an electron wire array induced in an undoped AlAs/GaAs heterojunction with an interdigitated gate electrode. The photocurrent measures the collective one‐dimensional intersubband resonance in the electron wires. The new technique to measure fundamental excitations of nanostructured electron systems yields strong signals even at very low electron densities so that only a single one‐dimensional subband is occupied. Compared to previous far‐infrared transmission studies the length and the number of the wires averaged in our technique is substantially smaller. The intersubband resonance in our devices is tunable in a wide range of energies up to 10.5 meV.

Anomalous magnetotransport and cyclotron resonance of high mobility magnetic 2DHGs in the quantum Hall regime

Low-temperature magnetotransport measurements and far-infrared transmission spectroscopy are reported in molecular beam epitaxial grown two-dimensional hole systems confined in strained InAs quantum wells with magnetic impurities in the channel. The interactions of the free holes spin with the magnetic moment of 5/2 provided by manganese features intriguing localization phenomena and anomalies in the Hall and the quantum Hall resistance. In magnetic field-dependent far-infrared spectroscopy measurements well-pronounced cyclotron resonance and an additional resonance are found that indicates an anti-crossing with the cyclotron resonance.

X-Ray Interface Characterization of Buried InAs Layers on GaAs (001)

Ultra thin buried InAs layers on GaAs (001) crystals prepared by molecular beam epitaxy are structurally characterized using synchrotron radiation. Grazing incidence X-ray reflectivity and crystal truncation rods were utilized to determine the average layer thickness, interface roughness, and the stoichiometry of the layers. From X-ray standing wave experiments the In lattice site and vertical distribution are determined. We discuss our results in view of the structural transition of the layer system with In deposition from 1.0 to 2.1 ML.