Ch. Heyn

Highly uniform and strain-free GaAs quantum dots fabricated by filling of self-assembled nanoholes

We demonstrate the self-assembled creation of a novel type of strain-free semiconductor quantum dot (QD) by local droplet etching (LDE) with Al to form nanoholes in AlGaAs or AlAs surfaces and subsequent filling with GaAs. Since the holes are filled with a precisely defined filling level, we achieve ultrauniform LDE QD ensembles with extremely narrow photoluminescence (PL) linewidth of less than 10 meV. The PL peaks agree with a slightly anisotropic parabolic potential. Small QDs reveal indications for transitions between electron and hole states with different quantization numbers. For large QDs, a very small fine-structure splitting is observed.

Local droplet etching of nanoholes and rings on GaAs and AlGaAs surfaces

We study the formation of nanoholes and rings on GaAs and AlGaAs surfaces by local droplet etching (LDE) with gallium and indium. The nanohole properties are tuned by variation in etching temperature and time as well as by the etchant. Nanoholes fabricated by In LDE are larger and have an about ten times lower density compared to Ga LDE, which allows the fabrication of nanoholes with ultralow density of less than 5×106 cm−2. Furthermore, the nanohole borders are surrounded by distinct walls. The walls are crystallized from droplet material and serve as quantum rings with tunable size and band gap.

Dynamics of self-assembled droplet etching

We study the self-assembled local droplet etching of nanoholes in AlGaAs surfaces with Ga droplets. The data establish an unexpected delay of both the hole drilling process as well as the removal of the liquid material after etching. Furthermore, coarsening by Ostwald ripening is found to reduce the droplet density before drilling. Basing on these findings, we propose a growth, coarsening, drilling, and removal mechanism for the droplet etching process.

Lithographically defined metal-semiconductor-hybrid nanoscrolls

We report on two-layer metal-semiconductor-hybrid scrolls fabricated from rolled-up strained metal∕InGaAs-layers. As the central approach, the metallic layer itself acts as a stressor in contact with the semiconductor. Position and length of the scrolls can be precisely tuned by patterning the e-beam-evaporated metallic stressor with conventional lithographic techniques. The thickness of the metallization determines the radius of the resulting scrolls. This fabrication technique significantly improves the reliability and simplifies the fabrication of metal∕semiconductor three-dimensional objects which employ bending up layers. Even more important, using this technique the bending radius of such three-dimensional objects can easily be downsized to very small radii in the nanometer scale, e.g. in order to build nano-electro-mechanical systems.

Transport properties of modulation-doped InAs-inserted-channel In0.75Al0.25As/In0.75Ga0.25As structures grown on GaAs substrates

We report on gate voltage dependent electron transport in modulation-doped In0.75Al0.25As/In0.75Ga0.25As heterostructures with strained InAs-inserted-channels grown on GaAs substrates. At temperatures of T=4.2 K we achieve mobilities of up to μ=215 000 cm2(V s)−1 and electron densities of nS=1.2×1012 cm−2 for the highest measured gate voltage of Vg=20 V. The electron effective mass m*=0.036 me is determined by temperature dependent Shubnikov–de Haas measurements. The observation of an anisotropic mobility when the first excited subband becomes populated proves interface scattering to be the limiting mechanism for the electron mobility.

Strain status of self-assembled InAs quantum dots

Grazing incidence x-ray diffraction experiments employing the asymmetric (202) Bragg diffraction have been performed to characterize self-assembled InAs quantum dots grown by molecular-beam epitaxy. We find that the strain is elastically relaxed with different components. The volume distribution of partially strained InAs inside islands is peaked at intermediate strain values. The fraction of both almost fully strained and totally relaxed InAs is found to be small. In addition, a small volume fraction of relaxed InxGa1−xAs is found.

Faceting during GaAs quantum dot self-assembly by droplet epitaxy

Strain-free GaAs quantum dots (QDs) are grown in a self-assembled fashion by applying Ga droplet epitaxy. The QDs are studied using electron diffraction and atomic force microscopy. Two distinct regimes are observed for the QD shape. QDs whose volume exceeds approximately 3×105 Ga atoms are shaped like truncated pyramids with side facets having an angle of about 55°. Smaller QDs are pyramidlike with 25° facets.

Influence of Ga coverage and As pressure on local droplet etching of nanoholes and quantum rings

We study the formation of nanoholes and quantum rings in GaAs and AlGaAs surfaces by local droplet etching (LDE) with Ga and In. The quantum rings are formed by the droplet etching process and surround the nanohole openings. Our data show that a low As pressure is essential for LDE and that process conditions with high Ga coverage yield formation of additional hillocks or large hills. With atomic force microscopy we establish that the amount of material removed from the nanoholes is equal to the amount of material stored in the quantum rings. Basing on the experimental observations, we propose a model of nanohole and quantum ring formation.

Extraordinary magnetoresistance effect in a microstructured metal–semiconductor hybrid structure

We have fabricated hybrid structures consisting of a metallic thin film and of a microstructured two-dimensional electron system in an InAs heterostructure. The devices are found to exhibit a huge magnetoresistance (MR) effect in magnetic fields ⩽1 T. At low temperature, a value of ΔR/R=[R(B=1 T)−R(B=0)]/R(B=0) as high as 115 000% is measured. The value of ΔR/R has been studied as a function of the electron mobility, the electron density and the lateral width of the semiconductor. We find that the MR effect can be tailored by these different parameters and technological relevant devices can be realized.

Magnetization of small arrays of interacting single-domain particles

We have prepared tailored small arrays of single-domain Ninanomagnets exhibiting an easy-axis perpendicular to the substrate surface. Using ballistic Hall micromagnetometry we probe the magnetization under different tilt angles. For an array of 13 nanomagnets with 700 nm spacing, we observe distinct plateau values in the hysteresis loop indicating the stepwise switching of these particles. The observed hierarchy of reversal comes close to a model on the basis of the magnetostatic interparticle interaction.