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We have studied the magneto-photo luminescence (PL) of triply stacked layers of self-assembled InP quantum dots, For a small separation of the uppermost dot in (he triple stack from the other two closely stacked layers, only one PL peak is observed, whereas for a large separation two PL peaks are present: a broad low-energy and a narrow high-energy peak. We propose a model in which the low-energy peak is associated with the uppermost dot where alignment faults result in enhanced size fluctuations, broadening the line, The narrow high-energy peak originates from charges located in the double stack. In the large separation regime, an increased broadening of the PL line-width indicates that the size fluctuations are augmented as the layer separation is increased from 15 to 30 nm. (C) 2002 Elsevier Science B.V. All rights reserved.

The ripples at the interfaces of five-period Si/Ge multilayer samples, grown on 0.3° miscut (0 0 1) Si are studied systematically. Five samples with Si spacer layer thicknesses ranging from 12.6 to 102.7 nm and 6 monolayer Ge were investigated. From the X-ray reflectivity investigations a characteristic step bunching morphology is found, with a ripple period which increases by more than 30% if the Si spacer thickness is doubled from about 13 to 25 nm , but does not change further with increasing spacer. These results shed light on the ongoing discussion about the relative importance of kinetic versus strain-related origin of the ripple pattern.

We have measured photoluminescence (PL) and inelastic light scattering spectra of GaAs/AlGaAs double quantum wells (DQW). The structures contain a high-mobility two-dimensional (2D) electron gas produced by modulation doping. The electron density as well as the intersubband spacings of the DQWs are changed by applying a DC voltage between a top and a bottom contact. Band-gap renormalization effects are observed in PL spectra when an excited subband becomes populated with electrons, as the 2D density increases. Inelastic light scattering yields information on the bias dependence of collective intersubband excitations of the 2D electron gas. The applied voltage reduces the coupling between layers leading to an energy shift of the excitations in each well in different directions.

The optical transitions of the wetting layers in two-fold self-assembled InAs/GaAs quantum dot samples are studied as a function of GaAs spacer thickness by various methods. The absorption related studies by photoreflectance and selective photoluminescence excitation spectroscopy reveal already for thick barriers, for which coupling effects can be excluded, two energetically separated heavy-hole transitions. This splitting indicates the formation of two wetting layers during growth with a 10% difference in width and reflects strain field interaction between the island layers. Thin spacer layer samples show in addition the expected wetting layer coupling as confirmed by subband calculations.

We determined the wave vector dispersion of elementary excitations of the two-dimensional electron gas (2DEG) like plasmons and intersubband excitations of the charge (CDE) and spin density (SDE) by inelastic light scattering using a grating coupler. The 2DEG forms in a modulation-doped AlGaAs/GaAs single quantum well sample on top of which we evaporated a metallic grating. In this way, in backscattering geometry with incident light perpendicular to the plane of the 2DEG, light can couple to elementary excitations with finite wave vectors in the range from 104 to 10 6 cm −1 .

Magneto-tunnelling for holes was studied in SiGe/Si/SiGe heterostructures with Ge quantum dots of the ‘hut cluster’ type in the middle of the Si-layer. At temperatures below 10 K two different transport regimes can be distinguished in the current–voltage characteristic. In high magnetic fields a dramatic instability develops in the non-linear current–voltage characteristics. The influence of different layer structures is discussed.

We demonstrate the stepwise fabrication of parallel double quantum dots in GaAs/AlGaAs-heterostructures. The atomic force microscope serves as a direct lithographic tool for the processing of our samples. The devices are characterized by transport measurements. Coulomb-blockade oscillations and diamonds with different periods for the two quantum dots are observed.

We have measured the magnetoresistance of a trench-like modulated two-dimensional electron gas in a parallel magnetic field in comparison with an unstructured reference sample. We observe a step-like increase in the magnetoresistance of the trench sample that is not present in the reference. We attribute this to different numbers of edge channels being transported through the trench structure.

We perform microwave spectroscopy on a single quantum dot using a compact pulsed spectrometer. Although DC transport shows strong elastic cotunneling in the Coulomb blockade regime and inelastic tunneling in the single-electron tunneling (SET) regime, these effects are suppressed under picosecond impulses whose energies are commensurate with those of the dot. Instead, under non-zero bias we find excited-state resonances in the induced complex photoconductance as well as strong deviations of the dot capacitance compared to equilibrium values, a signature of the effective relaxation times of SET.

Scanning force microscope measurements of the Hall potential distribution of a two-dimensional electron system (2DES) in the quantum Hall regime reveal the presence of a partial depletion along the border between the 2DES and the good ohmic metal contact: incompressible strips are formed in high magnetic fields along this border, decoupling the bulk region of the 2DES from the contact. This finding clarifies the role of potential probe contacts in Hall bar devices and might affect the interpretation of conductance measurements on Corbino devices.

We observe Aharonov–Bohm oscillations in an interferometer containing one quantum dot in each arm. The two quantum dots are laterally defined in a two-dimensional electron gas and are both brought to the few electron limit. By the results we verify that at least part of the charge transport is coherent in such a mesosopic system.

The transition of two-dimensional electron systems (2DES) with intentional disorder in form of a δ-layer doped with beryllium into quantum Hall insulators composed of separate electron droplets is examined by magnetotransport and far-infrared spectroscopy. We observe the percolation threshold at filling factors clearly below ν c = 1 2 that is expected for statistical disorder. The far-infrared excitations bear strong resemblance to magnetoplasmons of density-modulated 2DES and antidot systems in regular arrays. Thus, a perceptible degree of short-range order between the sites of the beryllium acceptors is uncovered.

We have performed time-resolved measurements of the far-infrared photoconductivity of the two-dimensional electron system in GaAs/AlGaAs heterostructures in the integer quantum Hall regime (near filling factor ν=2) by the application of a p–Ge laser radiation. We have found, that the photoresponse (PR) signal has two components, a bolometric component with a decay time not longer than 1– 2 μs , and a cyclotron resonance-related component of opposite polarity with a decay time of about 4– 5 μs . We have observed a strong dependence of the PR signal on the filling factor, sample current and laser intensity. Surprisingly, no pronounced dependence of the PR decay times on these parameters and on the electron mobility was found. Accordingly, we have found a decreasing influence of the mobility on the critical drift length of the quantum Hall effect breakdown with increasing amplitude when applying short electric pulses.

Kondo resonances have recently been observed in transport measurements through quantum dots. In certain Coulomb blockade regions a peak in the differential conductance at zero source–drain voltage develops with lowering the temperature. The Anderson impurity model, used to explain these observations, requires a degeneracy. For a slightly lifted degeneracy, the Kondo resonances are expected to split. Here we present measurements in magnetic fields showing split Kondo resonances which occur in a regular pattern as a function of gate voltage and magnetic field. This pattern is related to the electronic spectrum of the quantum dot and it is found that differences in the tunnel coupling are essential to understand these experimental results.

The formation of self-assembling quantum dots for the different strained material systems Ge/Si, InAs/GaAs and InP/GaInP in molecular beam epitaxy is discussed. Intense 1.55 and 1.3μm room temperature photoluminescence is achieved for stacked Ge dots in Si and single InAs dot layers in GaAs, respectively. Small InP/Ga0.52In0.48P quantum dots emit in the visible red spectrum. Strain-induced vertical alignment, size modification and interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for relatively large size stacked Ge and InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For extremely small and densely stacked InP and InAs dots the stronger confinement causes a red shift of the ground state emission. InP/Ga0.52In0.48P quantum dot injection lasers with coupled three-fold stacked InP dots are presented. Ground state lasing is observed at room temperature with a maximum output power up to 250 mW. The threshold current density at 300 K is jthr=2.3kA/cm2, and an external quantum efficiency of ηext=8.5% is measured for a 2 mm long laser cavity.

We have measured the photoluminescence of stacked layers of self-assembled InP quantum dots in magnetic fields up to 50 T. In the samples with three stacked layers of dots strong electronic coupling was clearly observed for layer separations of 2 and 4 nm. For a sample with only two stacked layers of dots (separated by 4 nm) no coupling was observed. We attribute this difference in behaviour to a model in which the coupling is via holes that are weakly bound in the GaInP layers between the dots. Since the doubly stacked layers have only one such layer, no coupling is possible. (C) 2000 Elsevier Science B.V. All rights reserved.

We report about optical and structural investigations of a self-aligned single electron transistor (SET) structure using cathodoluminescence-(CL) and transmission electron microscopy (TEM). The SET structures were fabricated by MBE growth of GaAs/AlAs on different prepatterned GaAs (1 0 0) substrates. This technique for the in situ formation of nanoscopic semiconductor heterostructures is presently a widely used and promising approach for the fabrication of low-dimensional systems like quantum wires and quantum dots (QD). The active region of the SET structure consists of a GaAs/AlGaAs-QD formed by thickness modulation of a single quantum well (SQW) during the MBE growth. The position and the size of the QD is defined by the design of the substrate pattern. The thickness modulation of the GaAs-SQW is evidenced by TEM investigations. The lateral confinement potential given by the thickness modulation of GaAs-SQW is directly imaged by CL microscopy.

Room temperature (RT) I – V characteristics of epitaxially grown Si / Si 0.5 Ge 0.5 / Si p + –i– n + Esaki tunneling diodes are presented. A detailed investigation of the tunneling mechanism shows the importance of the incorporated Ge for an enhancement of the tunneling probability, due to a reduction of the tunneling barrier. The variation of the deposited P at the i– n + interface, to provide a δ -doping layer, results in a monotonic increase of the peak current with increasing doping concentration. The optimization of the post-growth annealing process results in a peak-to-valley current ratio (PVCR) of 4.2.

For investigating the compressible and incompressible strips in the depletion region near the edge of a two-dimensional electron system (2DES) in the integer and fractional quantum Hall regime at T

The Kondo resonances observed in quantum dots are studied as a function of the tunnel coupling to the leads. It is observed that a Kondo resonance develops in one Coulomb-blockade region as the coupling is increased. This is taken as an indication that there is a degeneracy on the dot for this electron number, which is a prerequisite of the Anderson impurity model predicting the Kondo resonances. The nature of this degeneracy is studied in tilted magnetic fields and the generally accepted picture, that a spin degeneracy is responsible for the Kondo resonance, is confirmed.

We have measured the generation and relaxation of excited carriers along their drift direction near the breakdown of the quantum Hall effect (QHE). The dissipative resistivity ρ xx ( x ) at current densities close to the critical value for the QHE breakdown was measured as a function of the distance x from the electron injection at x =0. By injecting “cold” electrons into constrictions at supercritical current levels, the evolution of the breakdown along the drift direction was monitored. After a smooth increase of the resistivity with the drifting distance, an avalanche-like rise towards a saturation value occurs. Drastic changes of the resistivity profiles with the applied current were found in a narrow range around the critical current. The observed behavior is attributed to impurity-assisted tunneling between Landau levels. By injecting hot electrons (excited in a periodic set of constrictions) into a region with subcritical current density, the relaxation process was analyzed. Inelastic relaxation lengths with typical values in the range from 0.3 to 4 μm were found, which agree within 10% with the elastic mean free path determined from the Hall mobility at zero magnetic field. We conclude that the energy relaxation process is triggered by scattering at impurity potentials.