91 research outputs found
Multi-excitonic complexes in single InGaN quantum dots
Cathodoluminescence spectra employing a shadow mask technique of InGaN layers
grown by metal organic chemical vapor deposition on Si(111) substrates are
reported. Sharp lines originating from InGaN quantum dots are observed.
Temperature dependent measurements reveal thermally induced carrier
redistribution between the quantum dots. Spectral diffusion is observed and was
used as a tool to correlate up to three lines that originate from the same
quantum dot. Variation of excitation density leads to identification of exciton
and biexciton. Binding and anti-binding complexes are discovered.Comment: 3 pages, 4 figure
Transmission electron microscopy investigation of segregation and critical floating-layer content of indium for island formation in InGaAs
We have investigated InGaAs layers grown by molecular-beam epitaxy on
GaAs(001) by transmission electron microscopy (TEM) and photoluminescence
spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and
respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 C. The
parameters were chosen to grow layers slightly above and below the transition
between the two- and three-dimensional growth mode. In-concentration profiles
were obtained from high-resolution TEM images by composition evaluation by
lattice fringe analysis. The measured profiles can be well described applying
the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557].
Calculated photoluminescence peak positions on the basis of the measured
concentration profiles are in good agreement with the experimental ones.
Evaluating experimental In-concentration profiles it is found that the
transition from the two-dimensional to the three-dimensional growth mode occurs
if the indium content in the In-floating layer exceeds 1.1+/-0.2 monolayers.
The measured exponential decrease of the In-concentration within the cap layer
on top of the islands reveals that the In-floating layer is not consumed during
island formation. The segregation efficiency above the islands is increased
compared to the quantum wells which is explained tentatively by
strain-dependent lattice-site selection of In. In addition, In0.25Ga0.75As
quantum wells were grown at different temperatures between 500 oC and 550 oC.
The evaluation of concentration profiles shows that the segregation efficiency
increases from R=0.65 to R=0.83.Comment: 16 pages, 6 figures, 1 table, sbmitted in Phys. Rev.
Active sites in heterogeneous ice nucleation-the example of K-rich feldspars
Ice formation on aerosol particles is a process of crucial importance to Earth's climate and the environmental sciences but it is not understood at the molecular level. This is so partly because the nature of active sites, local surface features where ice growth commences, is still unclear. Here we report direct electron-microscopic observations of deposition growth of aligned ice crystals on feldspar, an atmospherically important component of mineral dust. Our molecular-scale computer simulations indicate that this alignment arises from the preferential nucleation of prismatic crystal planes of ice on high-energy (100) surface planes of feldspar. The microscopic patches of (100) surface, exposed at surface defects such as steps, cracks, and cavities, are thought to be responsible for the high ice nucleation efficacy of K-feldspar particles
Gravity compensation in complex plasmas by application of a temperature gradient
Micron sized particles are suspended or even lifted up in a gas by
thermophoresis. This allows the study of many processes occurring in strongly
coupled complex plasmas at the kinetic level in a relatively stress-free
environment. First results are presented. The technique is also of interest for
technological applications.Comment: 4 pages, 4 figures, final version to be published in Phys. Rev. Let
Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion
Epitaxially strained LaCoO3 (LCO) thin films were grown with different film
thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT)
substrates. After initial pseudomorphic growth the films start to relieve their
strain partly by the formation of periodic nano-twins with twin planes
predominantly along the direction. Nano-twinning occurs already at the
initial stage of growth, albeit in a more moderate way. Pseudomorphic grains,
on the other hand, still grow up to a thickness of at least several tenths of
nanometers. The twinning is attributed to the symmetry lowering of the
epitaxially strained pseudo-tetragonal structure towards the relaxed
rhombohedral structure of bulk LCO. However, the unit-cell volume of the
pseudo-tetragonal structure is found to be nearly constant over a very large
range of t. Only films with t > 130 nm show a significant relaxation of the
lattice parameters towards values comparable to those of bulk LCO.Comment: 31 pages, 10 figure
In situ control of the structure formation of magnetron sputtered Vanadium carbide coatings: periodic modulation of the microstructure
Electron microscopic and optical investigations of the indium distribution GaAs capped InxGa1-xAs islands
Results from a structural and optical analysis of buried InxGa1-xAs islands carried out after the process of GaAs overgrowth are presented. It is found that during the growth process, the indium concentration profile changes and the thickness of the wetting layer emanating from a Stranski-Krastanow growth mode grows significantly. Quantum dots are formed due to strong gradients in the indium concentration, which is demonstrated by photoluminescence and excitation spectroscopy of the buried InxGa1-xAs islands. (C) 1997 American Institute of Physics
Investigation of pre-structured GaAs surfaces for subsequent site-selective InAs quantum dot growth
In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination
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