97 research outputs found
Some aspects of hydrogen plasma treatment of anti-modulation doped near surface GaAs/AIGaAs single quantum well structures/ Yu. A. Bumai, G. Gobsch, R. Goldhahn, N. Stein, A. Golombek, V. Nakov, T. S. Cheng
No full textThe MBE grown anti-modulation doped GaAs/AIGaAs structures with near surface single quantum wells (QWs) were exposed to a DC hydrogen plasma (~400eV) and investigated using PL, PLE and PR spectroscopy at 5 K. Strong acceptor related free to bound transition (FB) dominates for QW related PL but excitonic features are still observed in PLE spectra. After hydrogen plasma treatment the PL intensity of FB transition from QW was strongly increased for above AIGaAs band gap excitation and was unchanged for below AIGaAs one. These results are consistent with atomic hydrogen passivation of deep defects in Al-GaAs barriers. At the same time radiative excitonic recombination was quenched by hydrogenation. PLE and PR spectra indicate on a strong increase of electric field in subsurface region of the structure after hydrogenation. The increase of electric field GaAs barriers. At the same time radiative excitonic recombination was quenched by hydrogenation. PLE and PR spectra indicate on a strong increase of electric field in subsurface region of the structure after hydrogenation. The increase of electric fieldin anti-modulation doped structure after hydrogen plasma treatment is supposed to be due to passivation by atomic hydrogen of surface states that leads to unpinning of Fermi level from mid gap to carbon acceptor level position in GaAs cap layer. It causes the further band bending and surface electric field increase that strongly suppress excitonic recombination in near surface QWs
Metal–Insulator Transition and Lifshits Instability in the Hubbard Model. II. Stationary Point Approximation for the Functional Integral
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Laboratory experiments on the microphysics of charged cloud droplets
No full textManifold studies indicate a correlation between solar activity and
terrestrial climate proxies. But within, the coupling link between solar
variability and terrestrial weather and climate is missing. For this reason
the German Research Foundation (DFG) took part in “CAWSES” (Climate And
Weather of the Sun-Earth-System). Within this priority program the present
thesis gives an experimental input.A hypothesis therein proposes the solar
induced modulation of the terrestrial global electric circuit[1]. This can
alter the charging state of comprised clouds and cloud droplets and
therefore influence their weather- and climate relevant behavior.Research
was focused on dominant microphysical cloud processes and their
quantification. An experiment was set up which enables automated
levitation, supercooling and manipulation of individual droplets of up to
90µm in diameter.One field of investigation was the homogeneous ice
nucleation and the evaporation of super cooled cloud droplets. As results
there was no increased probability of ice formation, but appreciable
reduced evaporation rates (up to stagnancy) in the presence of net
charges.A further series of investigations was dedicated to the interaction
of charged cloud droplets with aerosol particles. A method to determine the
scavenging efficiency from experimental conditions was developed and
validated. It has been demonstrated that polarization induced charges
assume a crucial role in the characterization of
cloud-aerosol-interaction.For demonstration of a climate relevant impact of
the charge state of cloud droplets, the enhanced formation of ice due to
contact freezing was emulated. Surrogates of mineral dust particles (pure
Illite and Kaolinite aerosols) were provided to the supercooled droplets.
The presence of electrostatic enhanced concentrations of potential nuclei
raises the probability for ice nucleation by a multiple. An implication is
the advancing glaciation of clouds which influences the formation of
precipitation and the radiative budget.The therefore used experimental
setup combines individual droplet experiments and aerosol generation and
classification systems. It establishes a new and ongoing used facility for
laboratory experiments on the microphysics of clouds.[1]: Tinsley, B. A.
and G. W. Deen (1991), Journal of Geophysical Research (JGR) No. 96 (D12),
S. 22,283-22,296, doi:10.1029/91JD02473.Vielfältigste Studien zeigen eine Korrelation der solaren Aktivität mit
terrestrischen Klimaanzeigern. Dabei ist der verbindende Mechanismus,
welcher die solare Variabilität mit dem Wettergeschehen in den untersten
Atmosphärenschichten der Erde koppelt, bisher unidentifiziert. Aus diesem
Grund wurde das DFG-Schwerpunktprogramm “CAWSES“ initiiert, zu welchem die
vorliegende Arbeit einen experimentellen Beitrag liefert.Eine der
vorgeschlagenen Hypothesen [1] folgt dabei dem Ansatz, dass solare
Ereignisse den globalen, terrestrischen Stromkreis modulieren. Der dadurch
veränderbare Ladungszustand atmosphärischer Wolken, beziehungsweise
Wolkentropfen, kann sich wetter- und somit klimarelevant auswirken.Zur
Untersuchung wurde eine Anlage aufgebaut, welche dominante
wolkenmikrophysikalische Effekte an geladenen Mikrotropfen nachbildet und
quantifiziert. Dazu werden einzelne Tropfen von bis zu 90µm Größe
automatisiert in einer elektrodynamischen Falle berührungslos gespeichert,
unterkühlt und manipuliert.Untersucht wurden unter anderem die homogene
Eisnukleation unterkühlter Wolkentropfen sowie deren Verdunstungsverhalten.
Mit dem Ergebnis, dass keine erhöhte Gefrierwahrscheinlichkeit, aber eine
deutliche Verzögerung der Verdunstung (bis hin zum Erliegen) in Anwesenheit
von Nettoladungen festgestellt wurde.In einer zweiten Serie von
Experimenten wurde die Wechselwirkung geladener Wolkentropfen mit den
allgegenwärtigen Aerosolen, welche ebenfalls geladen vorkommen, untersucht.
Dabei wird eine Möglichkeit entwickelt und validiert, die Sammeleffizienz
aus den experimentellen Bedingungen quantitativ zu bestimmen. Es konnte
gezeigt werden, dass der Ladungsinfluenz hierbei eine maßgebliche Rolle
zukommt. Zur Demonstration einer klimarelevanten Wirkungskette wurde anhand
zweier atmosphärisch relevanter Mineralstaubspezis (Illit- und
Kaolinitaerosol) die vermehrte Eispartikelbildung durch unterkühlte
Wolkentropfen im Kontaktgefriermodus nachgebildet. Hierbei wird die
Gefrierwahrscheinlichkeit durch das elektrostatisch erhöhte Eiskeimangebot
erheblich gesteigert. Das dafür geschaffene System, welches
Einzeltropfenuntersuchung und Aerosolerzeugung vereint, eröffnet eine neue
und weiterhin genutzte Möglichkeit wolkenphysikalischer Experimente im
Labormaßstab. [1]: Tinsley, B. A. und G. W. Deen (1991),JGR Nr. 96 (D12),
S. 22,283-22,296, doi:10.1029/91JD0247
The g‐Tensor of the Interstitial Oxygen and the Broken SiO Bond in Silicon Dioxide ‐ A MO Model Calculation
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Self-Consistent Calculation of the Electronic Structure of an Inversion Layer in a Perpendicular Magnetic Field
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