933 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
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
On the magnetic nature of electron transport barriers in tokamaks
The formation of internal transport barriers in the vicinity of rational magnetic surfaces in tokamaks with braided magnetic fields is studied for a simplified model of the perturbed magnetic field with a broad spatial spectrum and a monotonous shear profile. The island overlap criterion is used to derive a condition for barrier formation. This condition links the amplitude and the spectral width of the perturbation with the shear parameter. Numerical experiments with the MHD Monte-Carlo code E3D, where the problem of plasma heat conductivity is solved in 3D, confirm this formation of transport barriers in the case of a monotonous shear profile. Assuming that experimentally observed electron internal transport barriers are the result of local reduction of electron heat transport due to the magnetic field braiding, the amplitude and spectral width of magnetic perturbations are estimated for the tokamak RTP
Guided Neuronal Growth on Arrays of Biofunctionalized GaAs/InGaAs Semiconductor Microtubes
We demonstrate embedded growth of cortical mouse neurons in dense arrays of
semiconductor microtubes. The microtubes, fabricated from a strained
GaAs/InGaAs heterostructure, guide axon growth through them and enable
electrical and optical probing of propagating action potentials. The coaxial
nature of the microtubes -- similar to myelin -- is expected to enhance the
signal transduction along the axon. We present a technique of suppressing
arsenic toxicity and prove the success of this technique by overgrowing
neuronal mouse cells.Comment: 3 pages, 4 figure
Kinetic estimate of the shielding of resonant magnetic field perturbations by the plasma in DIII-D
Kinetic modelling of bifurcation conditions of resonant magnetic perturbations in ASDEX Upgrade experiments
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
Calculation of Self-consistent Radial Electric Field in Presence of Convective Electron Transport in a Stellarator *
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