Optische und elektronische Eigenschaften von AlGaN/GaN-Heterostrukturen

The electronic material properties of AlGaN/GaN heterostructures were investigated. The analysis of optical spectra by complex models allowed for the first time to confirm the theoretically predicted dependence of the polarisation discontinuity (also called polarisation charge) on the Al content by reliable experiments. Furthermore, it is shown that the polarisation discontinuity is constant over the temperature range from 5 K up to room temperature. The method employed here is based on the analysis of electroreflectance (ER) spectra and exploits the specific dependence of the electric field strength within a layer on the applied electric voltage. In this work this method is consequently refined to surpass all alternative methods in accuracy. ER spectra of group-III-nitrides posses some general peculiarities: (i) In direct proximity to the band gap they can not be described by constant Seraphin coefficients in contrast to small gap semiconductors (e.g. GaAs). (ii) Although, the analysis of the Franz-Keldysh oscillations by Aspnes’ method yields the correct values of the electric field strength as for small gap semiconductors. Optical and especially ER spectra of group-III-nitrides can only be described completely by taking into account for excitons in electric fields. For this a model proposed by Blossey was applied to nitride semiconductors and implemented into a software program. The approach presented is unique since it allows for a quantitative description of excitons in inhomogeneous electric fields. The good agreement between experiment and simulation supports the reliability of the material properties presented in this work. Furthermore, it was found that the energetic position of the exciton main resonance as well as its spectral width depend linearly on the electric field strength. The fuction of AlGaN/GaN heterostructures as chemical sensors was investigated too. If Pt contacted samples were exposed to hydrogen the density of their two dimensional electron gases were raised by some 10^{12} e/cm^2 while the Schottky barrier heights were lowered by up to 0.85 V. Wetting of the free surface of not contacted samples by a polar liquid (acetone) led to an increase of the surface potential by 30 mV and a decrease of the net surface charge density by 10^{11} e/cm^2.Die vorliegende Arbeit behandelt die elektronischen Materialeigenschaften von AlGaN/GaN-Heterostrukturen. Die Auswertung optischer Spektren mit komplexen Modellen ermöglichte erstmals die Bestätigung des theoretisch vorausgesagten Verlaufes der Polarisationsdiskontinuität (auch Polarisationsladung genannt) in Abhängigkeit vom Al-Gehalt durch zuverlässige Experimente. Weiterhin wurde festgestellt, dass die Polarisationsdiskontinuität im Bereich von 5 K bis Raumtemperatur konstant ist.Das hier verwendete Verfahren basiert auf der Auswertung von Elektroreflexions- (ER-)spektren und nutzt die spezifische Abhängigkeit der Schichtfeldstärke von der angelegten elektrischen Spannung. In dieser Arbeit wurde das Verfahren konsequent weiterentwickelt und übertrifft so alle alternativen Methoden in der Genauigkeit. ER-Spektren von Gruppe-III-Nitriden weißen generelle Besonderheiten auf. In unmittelbarer Nähe zur Bandkante können ER-Spektren im Gegensatz zu schmallückigen Halbleitern (z.B. GaAs) nicht durch konstante Seraphinkoeffizienten beschrieben werden. Jedoch ergibt die Analyse der Franz-Keldysh-Oszillationen nach der Aspnes’schen Methode wie bei schmallückigen Halbleitern die korrekten Feldstärkebeträge. Optische und insbesondere ER-Spektren von Gruppe-III-Nitridschichten lassen sich nur vollständig durch Berücksichtigung der Exzitonen im elektrischen Feld beschreiben. Dazu wurde in dieser Arbeit ein von Blossey vorgeschlagenes Modell auf die Nitride angewandt und in einer Software umgesetzt. Der dargestellte Ansatz zur Spektrensimulation ist dadurch einzigartig, dass man mit ihm Exzitonen in inhomogenen elektrischen Feldern quantitativ beschreiben kann. Die gute Übereinstimmung von berechneten und experimentellen Spektren bekräftigt die Zuverlässigkeit der in dieser Arbeit bestimmten Materialgrößen. Weiterhin wurde festgestellt, dass die energetische Position der Exzitonenhauptresonanz und deren spektrale Breite näherungsweise einer linearen Abhängigkeit von der elektrischen Feldstärke folgen. Die Wirkungsweise von AlGaN/GaN-Heterostrukturen als chemische Sensoren wurde ebenfalls untersucht. Werden Pt-kontaktierten Proben Wasserstoff ausgesetzt, erhöht sich die Dichte des zweidimensionalen Elektronengases um einige 10^{12} e/cm^2 und die Schottkybarriere verringert sich um bis zu 0,85 V. Bei Proben mit unkontaktierter Oberfläche führt die Benetzung mit einer polaren Flüssigkeit (Azeton) zu einer Potenzialerhöhung um 30 mV und zu einer Verringerung der Oberflächennettoladung um 10^{11} e/cm^2

2ʹ-Deoxyadenosine 5ʹ-diphosphoribose is an endogenous TRPM2 superagonist

Transient receptor potential melastatin 2 (TRPM2) is a ligand-gated Ca2+-permeable nonselective cation channel. Whereas physiological stimuli, such as chemotactic agents, evoke controlled Ca2+ signals via TRPM2, pathophysiological stimuli such as reactive oxygen species and genotoxic stress result in prolonged TRPM2-mediated Ca2+ entry and, consequently, apoptosis. To date, adenosine 5'-diphosphoribose (ADPR) has been assumed to be the main agonist for TRPM2. Here we show that 2'-deoxy-ADPR was a significantly better TRPM2 agonist, inducing 10.4-fold higher whole-cell currents at saturation. Mechanistically, this increased activity was caused by a decreased rate of inactivation and higher average open probability. Using high-performance liquid chromatography (HPLC) and mass spectrometry, we detected endogenous 2'-deoxy-ADPR in Jurkat T lymphocytes. Consistently, cytosolic nicotinamide mononucleotide adenylyltransferase 2 (NMNAT-2) and nicotinamide adenine dinucleotide (NAD)-glycohydrolase CD38 sequentially catalyzed the synthesis of 2'-deoxy-ADPR from nicotinamide mononucleotide (NMN) and 2'-deoxy-ATP in vitro. Thus, 2'-deoxy-ADPR is an endogenous TRPM2 superagonist that may act as a cell signaling molecule

Unconventional double-bended saturation of carrier occupation in optically excited graphene due to many-particle interactions

Saturation of carrier occupation in optically excited materials is a well-established phenomenon. However, so far, the observed saturation effects have always occurred in the strong-excitation regime and have been explained by Pauli blocking of the optically filled quantum states. On the basis of microscopic theory combined with ultrafast pump-probe experiments, we reveal a new low-intensity saturation regime in graphene that is purely based on many-particle scattering and not Pauli blocking. This results in an unconventional double-bended saturation behaviour: Both bendings separately follow the standard saturation model exhibiting two saturation fluences; however, the corresponding fluences differ by three orders of magnitude and have different physical origin. Our results demonstrate that this new and unexpected behaviour can be ascribed to an interplay between time-dependent many-particle scattering and phase-space filling effects

Hypoxia Induces Late Preconditioning in the Rat Heart In Vivo

Background Although hypoxic late preconditioning (LPC) limits ischemia-reperfusion injury in vitro, its cardioprotective effect is not established in vivo Methods In part 1, rats were exposed to 4 h of hypoxia (16% 12%, 8% oxygen) before 24 h of reoxygenation In part 2, normoxic rats received early preconditioning with sevoflurane (1 minimum alveolar concentration [MAC] for 3 X 5 min) continuous administration of 1 MAC sevoflurane, or 11 mg kg h propofol Thereafter, all rats underwent 25 min of regional myocardial ischemia and 120 min of reperfusion After reperfusion, hearts were excised for infarct staining The expression of protein kinase C (PKC)alpha and PKC epsilon was assessed by Western blot analysts and the expression of heme oxygenase-1 and vascular endothelial growth factor by reverse transcriptase polymerase chain reaction Results In normoxic control rats, infarct size was 62 +/- 6% of the area at risk Hypoxic LPC reduced infarct size (LPC16 36 +/- 11%, LPC12 38 +/- 10%, LPC8 39 +/- 11%, each P <0 001) to approximately the same magnitude as sevoflurane-preconditioning (40 8%, P <0 001) Combined LPC16 and sevoflurane preconditioning was not superior to either substance alone Continuous sevoflurane or propofol was not protective The PKC inhibitor calphostin C abolished the cardioprotective effects of LPC16 PKC epsilon, but not PKC alpha, expression was increased 6 and 28 h after hypoxic LPC Heme oxygenase-1 and vascular endothelial growth factor were transiently up-regulated after 6 h Conclusion Hypoxic LPC at 8%, 12%, and 16% oxygen reduces infarct size in the rat heart in vivo This effect is as powerful as sevoflurane-preconditioning PKC epsilon is a key player in mediating hypoxic LP

ARTICLE Microscopic origins of the terahertz carrier relaxation and cooling dynamics in graphene

The ultrafast dynamics of hot carriers in graphene are key to both understanding of fundamental carrier-carrier interactions and carrier-phonon relaxation processes in two-dimensional materials, and understanding of the physics underlying novel high-speed electronic and optoelectronic devices. Many recent experiments on hot carriers using terahertz spectroscopy and related techniques have interpreted the variety of observed signals within phenomenological frameworks, and sometimes invoke extrinsic effects such as disorder. Here, we present an integrated experimental and theoretical programme, using ultrafast timeresolved terahertz spectroscopy combined with microscopic modelling, to systematically investigate the hot-carrier dynamics in a wide array of graphene samples having varying amounts of disorder and with either high or low doping levels. The theory reproduces the observed dynamics quantitatively without the need to invoke any fitting parameters, phenomenological models or extrinsic effects such as disorder. We demonstrate that the dynamics are dominated by the combined effect of efficient carrier-carrier scattering, which maintains a thermalized carrier distribution, and carrier-optical-phonon scattering, which removes energy from the carrier liquid