Bibliothek

Notizen: The paper reports a theoretical and experimental study of the dependence of the radiative recombination efficiency (ηi) on the GaAs quantum well width (Lz) in AlGaAs/GaAs quantum well structures with binary/binary superlattice confinement. Values of ηi(approximately-greater-than)60% at room temperature have been obtained for quantum wells with Lz≥40 A(ring). It is shown that structures with Lz〈40 A(ring) exhibit a sharp decrease in ηi associated with nonradiative recombination of the high energy part of nonequilibrium carriers in the confining layers even in the presence of comparatively high potential barriers for Γ electrons.

Notizen: Aggregation of casein micelles after addition of the proteolytic enzyme chymosin has been studied by static and dynamic light scattering at three different concentrations of casein corresponding to dilutions 1:100, 1:500, and 1:1000 of native milk. The static light scattering data have been analyzed by an indirect Fourier transformation method which gives the distance distributions as a function of time. From these curves radius of gyration and an average number of casein micelles in the aggregates have been derived as a function of time. The dynamic light scattering experiments give the hydrodynamic radius as a function of time after the addition of rennet. The initial radius of gyration for the intact casein micelles is 140 nm. The corresponding hydrodynamic radius is also 140 nm. This shows that the casein micelles are not solid spheres. Inspection of a plot of relative mass versus radius of gyration for the aggregates appearing after the addition of chymosin shows that two processes take place. First extended linear aggregates are built up to a relative mass of the aggregates of about 10 and then restructuring of aggregates occurs such that increasingly compact objects are formed. Whereas the first process exhibits a relatively fast growth in size, the aggregates grow slowly in size during the second process. Further evidence of the formation of linear aggregates followed by more dense aggregates was obtained by forming the ratio between the radius of gyration and the hydrodynamic radius.This ratio increases to values of about 2.5 (indicating that linearly extended molecules are present followed) by a decrease to about 1. The log–log plot of mass versus radius of gyration is linear up to relative masses of about 10 with a slope of about 2. This extends up to sizes of 1 μm in diameter. The slope then increases to values indicating branching and thereby the formation of more compact aggregates. For relative masses below 10 and sizes below 1 μm sedimentation is unlikely to occur and information about the mechanism of aggregation can be obtained. The aggregation number as a function of time has been analyzed in terms of Smoluchowski's equations with a rate constant including both functionality and a changing barrier height as a function of the extent of proteolysis. The functionality obtained from Smoluchowski's equations is about 2.1. © 1995 American Institute of Physics.

Notizen: Summary. Using a one-channel neuromagnetometer adjusted to a special site on the mother's abdomen, we succeeded in recording prenatally, for the first time, human fetal brain activity in late pregnancy. It was possible to record both the fetal auditory-evoked neuromagnetic field and to detect fetal brain activity by analysis of the frequency spectrum. Such measurements may soon prove valuable for the non-invasive investigation of human brain function.

Notizen: The extraction of high-Z high-charge-state ions up to U90+ from a high-energy electron-beam ion trap, the SuperEBIT at Lawrence Livermore National Laboratory, is reported. The SuperEBIT provides a 240 mA electron beam with up to 200 keV of energy. Depending on the operating conditions (pulsed, continuous) and charge state, the number of ions extracted from the SuperEBIT varies between 102 and 105 ions per second under the tested conditions. The ions produced in SuperEBIT are extracted at potentials ranging from 0.5 to 20 keV (continuously variable) to provide highly charged low-emittance ion beams with energies between a few keV and several MeV. The performance of the SuperEBIT as an ion source is described and aspects for future developments and potential applications are discussed. © 2002 American Institute of Physics.

Notizen: The recombination dynamics of GaAs multiple quantum wells as a function of barrier widths LB are studied in a semiconductor in the range between LB=0.87 nm (superlattice) and LB=18.1 nm (uncoupled wells) by means of cathodo- and photoluminescence. With decreasing LB the nonradiative recombination rate is found to be drastically enhanced, where-as the radiative recombination probability decreases. Thus a pronounced decrease of the quantum efficiency results. The controlled variation of the barrier width is found to be decisive for an unambiguous identification of the origin of the traps which are responsible for the nonradiative processes: Comparison with a theoretical calculation shows that they are localized at the heterointerfaces and not in the barriers.

Notizen: The distribution of Fe implanted at medium (1–4×1014 cm−2) and low (2×1012 cm−2) doses into InGaAs and annealed with or without a cap is investigated and the degree of compensation of such implanted regions is assessed. Secondary ion mass spectrometry profiles of low dose implanted Fe reveal a substantial role of the capping layer. Fe concentrations below as well as above the estimated metal vacancy concentration produced by implantation are observed. The effect of the cap strongly depends on the wet chemical surface preparation before insulator deposition. A correlation of the magnitude of the Fe accumulation at the InGaAs surface with defect related photoluminescence intensity is established. On the basis of the substitutional-interstitial diffusion model the barrier effects of the various caps for host and dopant atoms are analyzed. The best semi-insulating properties were obtained for plasma enhanced chemical vapor deposition SiO2 caped samples using a H2SO4:H2O2:H2O=1:1:125 surface preparation before deposition resulting in a 53% incorporation of Fe. A high electrical activation is proved directly by capacity-voltage profiles.

Notizen: We present results of a comparative study of the time-dependent luminescence properties of multiple quantum well structures with varying barrier widths which are embedded in the active area of a light-emitting device. The carrier kinetics is investigated by different experimental approaches: Cathodoluminescence and electroluminescence experiments where excitation is on/off-modulated for the purpose of time-resolved measurements and time-resolved electroluminescence experiments in the small signal regime which allow for observation of the carrier kinetics under flatband conditions. Due to the exact determination of the excess carrier density the latter technique provides a sensitive tool for a precise estimation of the mono- and bimolecular recombination coefficients. Comparison with light output data yields radiative and nonradiative parts. We find that coupling of quantum wells dramatically favors nonradiative interface recombination as expected from a theoretical model accounting for the superlattice wavefunctions. On the other hand, the bimolecular recombination rate remains unaltered even when the barrier width is lowered from 18 to 0.9 nm. In contrast, on/off modulated experiments reveal that luminescence decay is strongly influenced by carrier drift out of the active area. A barrier width dependent carrier mobility in growth direction accounts for these results if phonon assisted hopping rather than Bloch transport is presumed. Thus, an estimation of device quality of quantum well light emitters by conventional time-resolved cathodo- (or photo-) luminescence experiments is found to be possible if internal field induced carrier drift processes are taken into account.

Notizen: Growth of Ti-doped In0.53Ga0.47As using liquid phase epitaxy is reported. The energy position of the Ti4+/Ti3+ deep donor level in In0.53Ga0.47As p-n diodes is precisely identified for the first time by deep level transient spectroscopy. The near midgap location of this level at EC−ET =0.37 eV and the potential of InGaAs:Ti of superior thermal stability make it a promising dopant for growing semi-insulating In0.53Ga0.47As. A recent model suggesting the energy positions of transition metals not to vary across heterojunctions is tested and found to be at least qualitatively valid for this low spin transition metal.

Notizen: The band gap of Al0.48In0.52As lattice matched to InP is determined with high precision at 1.5 and 300 K as 1.511 and 1.439 eV, respectively. This determination, which resolves a long lasting dispute on the most fundamental material parameter of this semiconductor, is based on a comparative study of temperature-dependent photoluminescence, wavelength-dispersive x-ray analysis, and triple-crystal x-ray diffractometry.

Notizen: A simultaneous reduction of interface roughness and of impurity and trap incorporation in GaAs/AlGaAs quantum wells is observed for a decrease of the molecular beam epitaxy growth rate below its standard value 1 μm/h, down to 0.1 μm/h, at a substrate temperature of 620 °C. Thus, layer quality is drastically improved at low, nonstandard growth rates. Incorporation of impurities from the background is observed to induce a transition from two- to three-dimensional growth. The conclusions are based on a detailed study and line shape analysis of quantum well luminescence.