A. Stemmann

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.

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.

Optical Modes Excited by Evanescent-Wave-Coupled PbS Nanocrystals in Semiconductor Microtube Bottle Resonators

We report on optical modes in rolled-up microtube resonators that are excited by PbS nanocrystals filled into the microtube core. Long ranging evanescent fields into the very thin walled microtubes cause strong emission of the nanocrystals into the resonator modes and a mode shift after a self-removal of the solvent. We present a method to precisely control the number, the energy and the localization of the modes along the microtube axis.

Time-resolved studies of a rolled-up semiconductor microtube laser

We report on lasing in rolled-up microtube resonators. Time-resolved studies on these semiconductor lasers containing GaAs quantum wells as optical gain material reveal particularly fast turn-on-times and short pulse emissions above the threshold. We observe a strong red-shift of the laser mode during the pulse emission which is compared to the time evolution of the charge-carrier density calculated by rate equations.

Local etching of nanoholes and quantum rings with InxGa1−x droplets

We study the formation of nanoholes and quantum rings in GaAs and AlGaAs surfaces by local droplet etching with InxGa1−x. The rings are crystallized from droplet material and surround the nanohole openings. In particular, the influence of the In content x on density, diameter, and depth of the nanoholes is investigated. Our data establish an exponential dependence of these quantities on x, which is quantitatively reproduced by a model that considers different surface diffusion energy barriers for Ga and In. By etching with pure In, hole densities as low as 5×106 cm−2 have been achieved. In addition, for low In content incompletely removed initial droplets are visible on the surface. These droplets are not visible on samples with x>0.5 which indicates a higher desorption rate of In compared to Ga. As a consequence, even in the case of etching with InGa the quantum rings consist of nearly pure GaAs. This is confirmed by photoluminescence experiments of quantum rings overgrown with AlGaAs barrier material.

Rolled-up nanotechnology for the fabrication of three-dimensional fishnet-type GaAs-metal metamaterials with negative refractive index at near-infrared frequencies

We propose and demonstrate the fabrication of a three-dimensional fishnet metamaterial by utilizing rolled-up nanotechnology. It consists of 6 alternating layers of silver and (In)GaAs with an array of subwavelength holes “drilled” by focused ion beams. By means of finite-integration technique simulations, we show that the fabricated structure is a single-negative material possessing a negative real part of the refractive index in the near-infrared regime. We show that the fabricatedmaterial can be made double negative by slightly changing the size of the holes.

Single-dot Spectroscopy of GaAs Quantum Dots Fabricated by Filling of Self-assembled Nanoholes

We study the optical emission of single GaAs quantum dots (QDs). The QDs are fabricated by filling of nanoholes in AlGaAs and AlAs which are generated in a self-assembled fashion by local droplet etching with Al droplets. Using suitable process parameters, we create either uniform QDs in partially filled deep holes or QDs with very broad size distribution in completely filled shallow holes. Micro photoluminescence measurements of single QDs of both types establish sharp excitonic peaks. We measure a fine-structure splitting in the range of 22–40μeV and no dependence on QD size. Furthermore, we find a decrease in exciton–biexciton splitting with increasing QD size.

Optical Properties of GaAs Quantum Dots Fabricated by Filling of Self-Assembled Nanoholes

Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines.