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Radiative Efficiency (radiative + efficiency)
Selected AbstractsHomoepitaxy of ZnTe on (100) oriented substrates: Technology issues and MOVPE growth aspectsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 10-11 2005M. Traversa Abstract The metalorganic vapour phase epitaxy of ZnTe on single crystal (100)ZnTe:P wafers is reported. The technological steps to prepare a substrate surface suitable for the high quality homoepitaxy of ZnTe are identified and optimised in terms of structural and morphological properties of overgrown epilayers. Removal of ,7 µm of material from the ZnTe:P wafers by chemical etching in 1% Br2 -methanol solution proved necessary to achieve a sufficiently smooth and homogeneous surface; in-situ H2 heat treatment of the wafers at 350°C immediately before growth ensures optimal desorption of residual oxides, allowing epilayer crystalline quality comparable to the substrate. However, the structure of epilayers degrades for growth temperatures (TG) above 350 °C due to the occurrence of stacking faults (SFs) within ,200-300 nm from the epilayer-substrate interface. Accordingly, the epilayer band-edge luminescence vanishes below 350 nm, indicating a worsening of the material radiative efficiency in very thin epilayers. The epilayer surface morphology is the result of a complex interplay between SF nucleation and Te:Zn ad-atom stoichiometry during growth. Almost featureless morphologies are obtained for growth at 350 °C, i.e. under nearly stoichiometric surface conditions. Pyramid-like hillocks develop instead for TG , 360 °C, corresponding to Te-rich surface conditions, their density rapidly increasing up to around 9 × 106 cm,2 at TG = 400 °C. Hillocks occur in close pairs on the epilayer surface, their nucleation being strongly reduced if a thin ZnTe buffer layer is grown at low (325 °C) temperature, i.e. if SFs do not occur at the epilayer-substrate interface. This demonstrates that hillocks form as a result of three-dimensional growth around partial dislocations pairs bounding SFs, the phenomenon being driven by Te ad-atoms experiencing a Schwoebel potential barrier at the surface step edges around the dislocations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Parallel tracks in infrared versus X-ray emission in black hole X-ray transient outbursts: a hysteresis effect?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007David M. Russell ABSTRACT We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the IR emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalization) between IR and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled IR and X-ray coverage: LNIR,L0.5,0.7X. In the high/soft state this slope is much shallower; LNIR,L0.1,0.2X, and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources. [source] A transition in the accretion properties of radio-loud active nucleiMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004Danilo Marchesini ABSTRACT We present evidence for the presence of a transition in the accretion properties of radio-loud sources. For a sample of radio galaxies and radio-loud quasars, selected based on their extended radio properties, the accretion rate is estimated from the black hole mass and nuclear luminosity. The inferred distribution is bimodal, with a paucity of sources at accretion rates, in Eddington units, of the order of ,10,2, assuming a radiative efficiency of 10 per cent , and possibly spanning 1,2 orders of magnitude. Selection biases are unlikely to be responsible for such behaviour. We discuss possible physical explanations, including a fast transition to low accretion rates, a change in the accretion mode/actual accretion rate/radiative efficiency, the lack of stable disc solutions at intermediate accretion rates or the inefficiency of the jet formation processes in geometrically thin flows. This transition might be analogous to spectral states (and jet) transitions in black hole binary systems. [source] Photo-induced improvement of radiative efficiency and structural changes in GaAsN alloysPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2006H. Yaguchi Abstract We have investigated the excitation power density and nitrogen concentration dependence of the changes in the radiative efficiency of GaAsN alloys to examine the mechanism of the photo-induced improvement of radiative efficiency. With increasing excitation power density, the radiative efficiency increased more rapidly. The measure of the improvement Iafter/Ibefore superlinearly increased with increasing nitrogen concentration x up to ,1%. This suggests that the nonradiative recombination centers eliminated by photoexcitation are not defects formed by a single nitrogen atom but complexes formed by gathering of several nitrogen atoms. Micro Raman study revealed that the GaAs-like LO mode phonon peak intensity increased with photoexcitation time in a similar way to the increase in the radiative efficiency. Considering that this phenomenon is in a time scale of several seconds, the photo-induced structural changes correspond not to long range inter-diffusion but to local changes in atomic configuration which lead to the decrease in the density of nonradiative recombination centers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||