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We fabricate self-aligned vertically stacked GaAs quantum dot molecules (QDMs) by filling of self-assembled nanoholes in AlGaAs. The tunable nanoholes are created using local droplet etching (LDE) combining conventional molecular beam epitaxy with self-assembled, lithography-free patterning. The optical emission from single, strain-free QDMs shows clear excitonic features with linewidths below 150 µeV after optimizations of the fabrication process. This allows investigations of the coupling among the individual dots forming a QDM. In electric fields oriented along the axis of the QDM, luminescence emission from direct and indirect transitions can be clearly distinguished. Furthermore, an anti-crossing behaviour demonstrates inter-dot coupling in the QDM.

The density of nanoholes created by self-assembled Al droplet etching of AlAs surfaces during molecular beam epitaxy is studied. We find a clear decrease of the hole density with increasing etching temperature T up to a threshold temperature T = 620 ° C . At T > 620 ° C , the hole density saturates at a minimum of 2 × 10 8 cm − 2 . We attribute this saturation to a change of the AlAs surface reconstruction. On the other hand, at reduced T , hole densities up to 2 × 10 9 cm − 2 have been achieved. However, this hole density increase is accompanied by a reduction of the hole depth. To generate high density holes with larger depth suited for quantum dot fabrication, we have studied the effect of repeated etching steps.

We apply the local droplet etching (LDE) technique in a conventional molecular beam epitaxy system for self-organized nanohole formation on AlGaAs surfaces. By optimizing the arsenic flux during the Al droplet deposition the hole density can be reduced down to the 10 6 cm − 2 regime. The nanohole shape and density are tunable in a wide range by the amount of droplet material deposited and the initial holes are customized by overgrowth with AlGaAs of varied thickness. Nanoholes with controlled depths from 30 down to a few nanometers are fabricated, while the ultra-low density is sustained. These nanoholes are filled with GaAs to form well separated GaAs QDs, with size precisely controlled by the filling level. Studies of the optical emission of single dots exhibit sharp excitonic lines at low and a clear shell structure at high excitation power. In particular, the quantization energies in dependence of the QD size are measured.

GaAs nanopillars are fabricated using a combination of in situ self-organized local droplet etching of nanoholes in a semiconductor surface, nanohole filling with a different material, and ex situ material selective etching. The structural properties of the pillars are studied with atomic-force microscopy. Experimental pillar densities are analyzed using a scaling law. Furthermore, the thermal transport through ensembles of pillars is measured. The results show a clear correlation between the pillar density and the thermal conductivity which is several orders of magnitude reduced in comparison to the bulk.

We investigate strain-free GaAs quantum dots (QDs) fabricated by filling of nanoholes in semiconductor surfaces. The nanoholes are created in a self-organized fashion by local droplet etching with Al droplets as etchants. High resolution transmission electron microscopy (TEM) demonstrates that the quantum dots are free of extended defects. Elemental mapping using local electron energy loss spectroscopy (EELS) shows that the walls surrounding the nanohole openings consist of AlAs. This result confirms that the walls are optically inactive.

This paper reports on the application of a thin MgO interlayer as a diffusion barrier between a Ni 2 MnIn Heusler film and the substrate consisting of either (0 0 1)InAs or a high electron mobility transistor structure with an InAs channel layer. The functionality of the MgO interlayers is studied in dependence of their layer thicknesses. Our studies reveal that MgO interlayers are effective diffusion barriers, which in conjunction with post-growth annealing significantly improve the structural and magnetic properties of the Heusler films. For all as-grown samples, a Curie temperature of 170 K was found indicating that the Ni 2 MnIn films are crystallized in the B2 phase. Post-growth annealing for 15 h at 350 °C of samples with MgO layer thicknesses smaller than 3 nm leads to a strong decrease in magnetisation. This film degradation may be attributed to the intermixing of the Heusler films with substrate material through not-completely closed MgO films. For samples with a MgO interlayer thickness of 3 nm, the Curie temperature increases up to 300 K. This Curie temperature is close to the value reported for bulk Ni 2 MnIn films in the desired L2 1 phase. Furthermore, an increase in saturation magnetisation by a factor of 2.4 was observed.

This paper reports on the influence of deposition temperature on structure and morphology of uncapped Ni 2 MnIn Heusler films grown by molecular beam epitaxy on InAs(0 0 1). Deposition temperatures between 80 and 360 °C and layer thicknesses between 20 and 100 nm have been used. Our studies reveal that during growth beyond 80 °C an intermixing layer arises at the Heusler/substrate interface that is formed by diffusion of arsenic from the substrate. The intermixing process, which is found to take place via interstitial sites, becomes increasingly severe with increasing growth temperature. Furthermore, we find that the films grown at 80 °C are polycrystalline with crystallites in the B2 phase. At 250 °C the films are found to be single-crystalline and pseudomorphically strained in the B2 phase. The desired L2 1 phase and single-crystalline Heusler films are observed at a growth temperature of 300 °C. Interestingly, Heusler films in L2 1 phase are found to have a (1 1 0) surface orientation in contrast to the InAs(0 0 1) substrate crystal. At temperatures higher than 300 °C, strong intermixing and a morphological degradation of the L2 1 Heusler films is observed.

A1. Atomic force microscopy ; A1. Diffusion ; A1. Low-dimensional structures ; A3. Molecular beam epitaxy ; B2. Semiconducting III-V materials We demonstrate the self-assembled generation of nanoholes on AlGaAs surfaces by local droplet etching (LDE). For the etching process, Ga is deposited on the surface, where liquid droplets are formed in a Volmer-Weber-like growth mode. The etching takes place locally at the interface between droplets and substrate and removes a significant amount of substrate material. The structural properties of the LDE nanoholes are studied with atomic force microscopy as function of etching temperature and Ga coverage. A bimodal depth distribution with flat and deep holes is observed. The formation of flat holes can be almost suppressed by optimized etching parameters. The depth of deep holes was adjusted by the process parameters up to a maximum depth of 15 nm. The density of deep holes is in the range 5 × 10 -7 -1 × 10 -8 cm -2 and depends only slightly on the etching parameters. However, the density can be significantly increased by repeated etching.

Spontaneous formation of self-assembled InAs quantum dots (QDs) is strongly influenced by growth parameter-dependent intermixing of the deposit with substrate material. This paper describes an analytical approach to model InAs QD formation that in particular considers intermixing. Intermixing is modelled based on thermally activated vertical exchange processes where Ga atoms from the GaAs substrate transmit the wetting layer via a two-step process. With the model, the intermixing-induced QD composition as well as the critical coverage of QD formation is calculated. Calculated values of the critical coverage at various growth parameters are found to agree well with our experimental electron diffraction data. In particular, the calculation results exhibit a flux-dependent critical temperature limiting QD formation that is found in experiments as well.

We study the shape transformation of self-assembled InAs quantum dots on (0 0 1) GaAs during overgrowth with AlAs and subsequent thermal annealing. InAs dots are grown in Stranski–Krastanow growth mode at various growth temperatures, thickness of the AlAs cap layers, as well as post-growth annealing times and temperatures. Dependent on these parameters, we observe shape transitions of the initial InAs quantum dots e.g. into ring-like structures or a splitting into lateral double-dot molecules.

The influence of growth parameters and substrate material on the basic processes during strain-induced InAs-quantum dot (QD) formation and the resulting structural properties of the QDs is studied with reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), and X-ray diffraction. The process of strain-induced QD formation is strongly influenced by the intermixing with substrate material and the desorption of indium at high temperatures. From the experimental results, we conclude that the strain-energy modification due to temperature-dependent intermixing has a larger influence on QD formation than temperature-dependent kinetic processes at the surface. Interestingly, we also find that the maximum growth temperature limited by desorption depends on the growth speed.

Intermixing with Ga from the substrate, and In over Ga segregation during strain-induced formation of InGaAs quantum dots are studied experimentally with electron diffraction (RHEED) and theoretically with a kinetic rate equations-based growth model. Our growth model yields the temperature-dependent critical time for quantum dot formation. The results reproduce our RHEED data very well. Furthermore, the quantum dot volume calculated with the model agrees with independent measurements. Calculations of the time-dependent strain energy inside the quantum dots establish strain-relaxation by quantum dot formation.

In situ re#ection high energy electron di!raction (RHEED) has been used to study the time evolution during self-assembled molecular beam epitaxy (MBE) growth of InAs quantum dots on GaAs. Using a special data acquisition technique, two characteristic time constants are determined very precisely: the time t # up to the "rst appearance of InAs dots and the time t & it takes to complete the 2D}3D transition of all islands. Surprisingly, we "nd that t # increases with temperature which disagrees with a thermally activated process. In contrast to this, t & behaves Arrhenius-like and an activation energy of E & K0.39 eV is determined. Furthermore, the sum t # #t & does not depend signi"cantly on temperature and corresponds to an InAs coverage of K2.0 monolayers. A second focus of this paper is the study of dissolution of InAs dots after interruption of the As #ux. From the experiments, an activation energy of 3.2 eV for desorption of In located on top of the wetting layer is determined, whereas direct desorption from the wetting layer corresponds to an activation energy of 3.4 eV. ( 2000 Elsevier Science B.V. All rights reserved.

We study the formation of GaAs quantum dots (QDs) by droplet epitaxy on various GaAs surfaces. The fabrication process consists of two steps. First, liquid Ga droplets are grown in a Volmer-Weber-like mode. This is followed by crystallization under As pressure. We demonstrate fabrication of droplet epitaxial GaAs QDs on (0 0 1), vicinal (0 0 1), (1 1 0), and (3 1 1)A GaAs surfaces. On (3 1 1)A GaAs, QDs are formed with higher density and smaller height compared to (0 0 1) and (1 1 0), which is attributed to a higher energy barrier for surface diffusion. Values of the surface diffusion barrier are determined and its influence on temperature-dependent QD density is studied by means of a QD growth model. On vicinal (0 0 1) surfaces, step bunches are found to act as preferred nucleation sites for GaAs QDs which opens the possibility for a lateral positioning of the QDs by pre-patterning.

We investigate electron systems in single- (SPQW) and coupled double parabolic quantum wells (DPQW). The samples are grown with molecular beam epitaxy using the analogue growth technique. The DPQW sample is designed so that in the regions occupied by electrons the bare potential of the wells remains parabolic to a great extent even if the electron occupation is altered. In the experiments the electron density is controlled by a gate. Far-infrared transmission and capacitance experiments demonstrate the nearly parabolic potential of the SPQW. In the DPQW the coupling of the wells is reflected in the occurrence of additional resonances with strong gate voltage dependence of the resonance positions.

Shape transformations of self-assembled InAs quantum dots are observed after overgrowth with GaAs or AlAs and a subsequent growth interruption. The structural, optical, and electronic properties of shape transformed self-assembled InAs quantum dots (STQD) are studied with atomic force microscopy, photoluminescence (PL), and deep level transient spectroscopy, respectively. We find a strong influence of the capping procedure on both the shape of the STQDs as well as the resulting energy level structure.