157 research outputs found
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
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
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
Three-Dimensionally Confined Optical Modes in Quantum Well Microtube Ring Resonators
We report on microtube ring resonators with quantum wells embedded as an
optically active material. Optical modes are observed over a broad energy
range. Their properties strongly depend on the exact geometry of the microtube
along its axis. In particular we observe (i) preferential emission of light on
the inside edge of the microtube and (ii) confinement of light also in
direction of the tube axis by an axially varying geometry which is explained in
an expanded waveguide model.Comment: 5 pages, 4 figure
Congruent evaporation temperature of molecular beam epitaxy grown GaAs (001) determined by local droplet etching
The congruent evaporation temperature Tc of GaAs (001) is critical for many technological processes and is fundamental to the control and stability of Ga droplets for quantum structure fabrication. We apply the technique of local droplet etching (LDE) to measure Tc for technologically important molecular beam epitaxy (MBE) grown GaAs (001). Below Tc, Ga droplets deposited on the surface shrink and form nanoholes via LDE and thermal widening. Above Tc, droplets grow by capturing excess Ga. From the transition between both regimes, we determine Tc = 680 ± 10 °C. Additionally, we find that the nanohole/droplet densities follow an Arrhenius-type temperature dependence with an activation energy of 1.31 eV. The method probes the stability of pre-existing droplets formed by deposition and so avoids the complication of nucleation barriers and readily allows the measurement of Tc for technologically important planar GaAs surfaces in any standard MBE system.
The authors thank S. Schnüll for MBE growth and the Deutsche Forschungsgemeinschaft for financial support via HA 2042/6-1. D.E.J. acknowledges the support from a Marie Curie International Incoming Fellowship
Quantized Dispersion of Two-Dimensional Magnetoplasmons Detected by Photoconductivity Spectroscopy
We find that the long-wavelength magnetoplasmon, resistively detected by
photoconductivity spectroscopy in high-mobility two-dimensional electron
systems, deviates from its well-known semiclassical nature as uncovered in
conventional absorption experiments. A clear filling-factor dependent
plateau-type dispersion is observed that reveals a so far unknown relation
between the magnetoplasmon and the quantum Hall effect.Comment: 5 pages, 3 figure
The Investigation of Intermediate Stage of Template Etching with Metal Droplets by Wetting Angle Analysis on (001) GaAs Surface
In this work, we study metal droplets on a semiconductor surface that are the initial stage for both droplet epitaxy and local droplet etching. The distributions of droplet geometrical parameters such as height, radius and volume help to understand the droplet formation that strongly influences subsequent nanohole etching. To investigate the etching and intermixing processes, we offer a new method of wetting angle analysis. The aspect ratio that is defined as the ratio of the height to radius was used as an estimation of wetting angle which depends on the droplet material. The investigation of the wetting angle and the estimation of indium content revealed significant materials intermixing during the deposition time. AFM measurements reveal the presence of two droplet groups that is in agreement with nanohole investigations. To explain this observation, we consider arsenic evaporation and consequent change in the initial substrate. On the basis of our analysis, we suggest the model of droplet evolution and the formation of two droplet groups
Zero-field thermopower of a thin heterostructure membrane with a 2D electron gas
We study the low-temperature thermopower of micron sized, free-standing
membranes containing a two-dimensional electron system. Suspended membranes of
320 nm thickness including a high electron mobility structure in Hall bar
geometry of 34 {\mu}m length are prepared from GaAs/AlGaAs heterostructures
grown by molecular beam epitaxy. Joule heating on the central region of the
membrane generates a thermal gradient with respect to the suspension points
where the membrane is attached to cold reservoirs. Temperature measurements on
the membrane reveal strong thermal gradients due to the low thermal
conductivity. We measure the zero-field thermopower and find that the
phonon-drag contribution is suppressed at low temperatures up to 7 K.Comment: 5 page
Far-infrared photo-conductivity of electrons in an array of nano-structured antidots
We present far-infrared (FIR) photo-conductivity measurements for a
two-dimensional electron gas in an array of nano-structured antidots. We
detect, resistively and spectrally resolved, both the magnetoplasmon and the
edge-magnetoplasmon modes. Temperature-dependent measurements demonstrates that
both modes contribute to the photo resistance by heating the electron gas via
resonant absorption of the FIR radiation. Influences of spin effect and phonon
bands on the collective excitations in the antidot lattice are observed.Comment: 5 pages, 3 figure
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