Suppression of the quantum-confined Stark effect in polar nitride heterostructures

Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Large internal dipole moment in InGaN/GaN quantum dots

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 97, 063103 (2010) and may be found at https://doi.org/10.1063/1.3477952.Direct observation of large permanent dipole moments of excitonic complexes in InGaN/GaN quantum dots is reported. Characteristic traces of spectral diffusion, observed in cathodoluminescence of InGaN/GaN quantum dots, allow deducing the magnitude of the intrinsic dipole moment. Our experimental results are in good agreement with realistic calculations of quantum dot transition energies for position-dependent external electric fields.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Nanofocusing with aberration-corrected rotationally parabolic refractive X-ray lenses

Wavefront errors of rotationally parabolic refractive X-ray lenses made of beryllium (Be CRLs) have been recovered for various lens sets and X-ray beam configurations. Due to manufacturing via an embossing process, aberrations of individual lenses within the investigated ensemble are very similar. By deriving a mean single-lens deformation for the ensemble, aberrations of any arbitrary lens stack can be predicted from the ensemble with \bar{\sigma} = 0.034λ. Using these findings the expected focusing performance of current Be CRLs are modeled for relevant X-ray energies and bandwidths and it is shown that a correction of aberrations can be realised without prior lens characterization but simply based on the derived lens deformation. The performance of aberration-corrected Be CRLs is discussed and the applicability of aberration-correction demonstrated over wide X-ray energy ranges

Multi particle states in semiconductor quantum dots

Die Verknüpfung zweier Halbleitermaterialien in Form von Heterostrukturen ermöglicht das Reduzieren der Bewegungsfreiheit von Elektronen und Löchern in bis zu drei Raumdimensionen. Bei hinreichend kleinem Durchmesser reduziert sich die Zustandsdichte einzelner, in der Doppel-Heterostruktur eingeschlossener Ladungsträger. Es entsteht ein Quantenpunkt mit einer Orbitalstruktur ähnlich der des Elektrons im Wasserstoffatom. Elektrisch betriebene Quantenpunkte sind überlegene Bausteine opto-elektronischer Systeme wie Laser-Kavitäten oder Emitter einzelner und verschränkter Photonen. Mit steigender Quantenpunktbefüllung durch Elektronen und Löcher entstehen räumlich gebundene Exzitonen oder noch größere exzitonische Komplexe. Ihre Dissoziation wird durch die umschließenden Potentialstufen an den Materialgrenzen des Quantenpunkts unterbunden. Das Ziel dieser Arbeit ist eine detaillierte Beschreibung von Mehrteilchenwechselwirkungen zwischen mehreren Elektronen und Löchern, die sich zusammen in einem einzelnen Quantenpunkt befinden. Hierfür werden zwei 8-Band-k · p-basierte Berechnungsmethoden implementiert, das Hartree-Fock- sowie das Konfigurationswechselwirkungsverfahren. Im Gegensatz zu anderen theoretischen Studien, welche sich ausschließlich der Konfigurationswechselwirkungsmethode auf Basis von Einteilchenzuständen bedienen, generiert das selbstkonsistente Hartree-Fock-Verfahren visualisierbare Mehrteilchenorbitale, die ein tieferes Verständnis der Teilchenwechselwirkungen erlauben. Hierbei wird der Effekt einer Wellenfunktionsrenormierung aufgrund der Wechselwirkungen greifbar. Letztgenannter Effekt ist der Hauptgrund für das Versagen konfigurationswechselwirkungsbasierter Studien auf Grundlage von Einteilchenzuständen bei der Erklärung experimenteller Ergebnisse an Gruppe-III-nitridbasierten Quantenpunkten mit starken inhärenten Polarisationsfeldern. Hartree-Fock-Zustände repräsentieren hier eine überlegene Basis für anschließende Konfigurationswechselwirkungsrechnungen zur realistischen Beschreibung von Mehrteilchenkomplexen in derartigen Quantenpunkten. Die Berechnungen erklären das inverse, emissionsenergieabhängige Skalierungsverhalten der bis zu zehnfach größeren Feinstrukturaufspaltung von Exzitonen in GaN/AlN-Quantenpunkten mit Wurtzit-Kristallstruktur im Vergleich zu arsenidbasierten Quantenpunkten. Die Kenntnis der energetischen Aufspaltung zwischen den exzitonischen Zuständen und ihrer verschiedenen Oszillatorstärken ermöglicht den experimentellen Nachweis effizienter phononenassistierter Spin-Umklappprozesse. Diese erzeugen eine thermische Besetzung orthogonal polarisierter Exzitonenzustände. Die theoretische Untersuchung größerer exzitonischer Komplexe deckt die Existenz einer neuen Gruppe von Quasi-Teilchen auf. Die als Hybrid-Teilchen bezeichneten Mehrteilchenzustände besitzen orbitalstrukturierte Elektronen in Verbindung mit einem System kristallisierter Lochdichten mit parallel ausgerichteten Spins. Das Hybrid-Biexziton als ein Vertreter dieser Gruppe besitzt eine reduzierte Gesamtenergie. Hierdurch wird das Rätsel um die experimentell beobachteten negativen und positiven Biexzitonenbindungsenergien in GaN/AlN Quantenpunkten sowie das Fehlen einer feinstrukturaufgespaltenen Biexzitonenlumineszenz gelöst. Das Hybrid-Biexziton zerfällt in die dark-Exzitonenzustände, was ein zusätzliches Niveau in der Zerfallskaskade des Biexzitons über das Exziton in den leeren Quantenpunktzustand erzeugt und dadurch die Zweiphotonenstatistik dieser Kaskade temperaturabhängig macht. Die zusätzliche Berücksichtigung der Kopplung an externe Ladungen erlaubt eine detaillierte Simulation der Lumineszenz einzelner Quantenpunkte. Die Ergebnisse stimmen sowohl qualitativ als auch quantitativ mit experimentell aufgezeichneten Spektren überein. Darüber hinausgehend werden diverse Eigenschaften prognostiziert, die unter anderem die Entwicklung eines neuen experimentellen Verfahrens zur Identifikation unterschiedlicher, emittierender Mehrteilchenkomplexe ermöglichen. Dies beinhaltet die Identifikation der unterschiedlichen Kopplungsstärken von Mehrteilchenkomplexen an externe Defektladungen in der Umgebung der Quantenpunkte. Sie führt zu voneinander abweichenden Amplituden in energetischen Fluktuationen der Emissionslinien sowie zu unterscheidbaren Emissionslinienbreiten.By assembling two semiconductor materials with conduction and valence band offsets at their interfaces it is possible to confine electrons and holes in up to three dimensions in space forming a so called quantum dot heterostructure. A discrete density of states constitutes and the single particle (electron or hole) states form an orbital system with individual energies as it is known from the electron of the hydrogen atom. Therefore, electrically driven quantum dots are superior building blocks in opto-electronic devices like laser cavities or single and entangled photon emitters as compared to non-solid atoms. With increased filling of quantum dots with electrons and holes, spatially bound excitons or larger excitonic complexes constitute while their dissociation is prevented by the surrounding quantum dot potential steps at the interfaces to the matrix material. The aim of the present work is a detailed description of the multiparticle interactions of several electrons and holes confined together within one quantum dot. Two 8-band-k · p-based calculation methods are implemented for this purpose, which are the Hartree-Fock method and the configuration interaction method. In contrast to several other studies solely based on the configuration interaction of single particle states, the self-consistent Hartree-Fock method provides visualizable multiparticle oribtals and therefore generates a deeper insight by calculating the mean interaction between the particle states. Therefore, the effect of interaction-based renormalization of wave functions becomes tangible. As the latter is the main reason for recent configuration interaction based studies on single particle states failing to explain measurements especially on wurtzite III-Nitride quantum dot systems with their large built-in polarization fields, Hartree-Fock states represent a superior configuration interaction basis for the realistic description of multiparticle complexes in such quantum dots. The calculations identify the reason for the inverse scaling behavior of the up to ten times larger fine structure splitting of the bright exciton states in GaN/AlN QDs with wurtzite crystal structure compared to arsenide-based quantum dots. The knowledge of the energetic separations between the exciton states and their individual oscillator strengths allows the experimental proof of highly efficient phonon-assisted spin-flip processes generating a thermal occupation of the orthogonally polarized exciton states. The theoretical investigation of larger excitonic complexes reveals the existence of a novel group of quasi-particle states, called hybrid-complexes comprising a regularly occupied orbital structure for electrons coupled to a crystallized system of hole densities with parallel aligned spins. The hybrid-biexciton as one representative has a drastically reduced energy solving the mystery about the observed negative and positive biexciton binding energies in GaN/AlN quantum dots as well as the lack of a fine-structure splitted biexciton luminescence. Due to its total spin of +/-2, the hybrid-biexciton decays into the dark exciton states, therefore introducing an additional level in the biexciton to exciton to empty quantum dot emission cascade, drastically changing the two-photon statistics, which becomes temperature dependent. By additionally including the coupling to external charges, a complete understanding of the multiparticle interaction processes in wurtzite III-Nitride quantum dots is generated enabling the detailed simulation of the luminescence of single quantum dots, which qualitatively and quantitatively matches experimentally recorded spectra. Beyond reproducing experimental spectra, the theoretic results enabled several forecasts, which for example were used to develop novel experimental identification techniques for the various emitting multiparticle complexes. These include the identification of differing coupling strengths of multiparticle complexes to external defect charges in the vicinity of the quantum dot leading to different amplitudes of energetic emission line variations as well as differing emission line widths

Activation and repassivation of stainless steels in artificial brines as a function of pH

When planning oil wells with stainless steel components, two possible reasons for depassivation have to be considered-chemical depassivation caused by acidizing jobs and mechanical depassivation caused by various tools and hard particles. The study explores conditions causing chemical activation of investigated steels and circumstances under which repassivation occurs after activation. The main focus of the study is to determine, how quickly various steels can repassivate under different conditions and to find pH values where repassivation will occur after depassivation. The investigated steels were ferritic (martensitic or bainitic) in the cases of 13Cr, 13Cr6Ni2Mo, and 17Cr4Ni2Mo, austenitic in the case of 17Cr12Ni2Mo, and duplex (austenitic and ferritic) in the case of 22Cr5Ni3Mo. Potentiodynamic experiments were employed to obtain electrochemical properties of investigated steels, followed by immersion tests to find ultimate conditions, where the steels still retain their passivity. After obtaining this information, scratch tests were performed to study the repassivation kinetics. It was found that repassivation times are similar for nearly all investigated steels independent of their chemical composition and microstructure

Depassivation and repassivation of stainless steels by stepwise pH change

Immersion tests with different stainless steels have been performed, while the pH was stepwise decreased and then increased again. During 8.5-day exposure, the depassivation and repassivation pH values as a function of pitting resistance equivalent number were determined. There is always a gap between both pH values (depassivation and repassivation), indicating that for every steel, there are conditions where an existing passive layer can be maintained but cannot be rebuilt after depassivation. In such environments, the passive layer is thicker, consisting mainly of molybdenum and iron rich oxides, while chromium is dissolved. Usually, depending on conditions, the passive layer is more chromium-rich, especially the inner layer. This is relevant, for example, for acidizing jobs in oil and gas industry, proving that repassivation after acidizing will happen promptly, when the pH is increased again

Activation and Repassivation of Stainless Steels in Artificial Brines as a Function of pH

When planning oil wells with stainless steel components, two possible reasons for depassivation have to be considered—chemical depassivation caused by acidizing jobs and mechanical depassivation caused by various tools and hard particles. The study explores conditions causing chemical activation of investigated steels and circumstances under which repassivation occurs after activation. The main focus of the study is to determine, how quickly various steels can repassivate under different conditions and to find pH values where repassivation will occur after depassivation. The investigated steels were ferritic (martensitic or bainitic) in the cases of 13Cr, 13Cr6Ni2Mo, and 17Cr4Ni2Mo, austenitic in the case of 17Cr12Ni2Mo, and duplex (austenitic and ferritic) in the case of 22Cr5Ni3Mo. Potentiodynamic experiments were employed to obtain electrochemical properties of investigated steels, followed by immersion tests to find ultimate conditions, where the steels still retain their passivity. After obtaining this information, scratch tests were performed to study the repassivation kinetics. It was found that repassivation times are similar for nearly all investigated steels independent of their chemical composition and microstructure

Hard X-ray Nanobeam Characterization by Ptychographic Imaging

As liver fibrosis is the result of persistent necroinflammation in the liver, pro-inflammatory cytokines secreted in response to cell injury have a central role in the pathogenesis of liver fibrosis. We aimed to investigate the association of cytokine gene polymorphism and liver fibrosis among Chinese patients with chronic hepatitis B.Polymorphisms at interleukin-10 (IL-10-627, -1117), interleukin-1-beta (IL-1beta-511, -31, -3964), interleukin-1 receptor antagonist (IL-1RN), and tumor necrosis factor-alpha (TNF-alpha-308, -238) among Chinese chronic hepatitis B patients were determined. Severe liver fibrosis was defined as Ishak fibrosis score = 4 (of 6).Fifty-nine of 273 (22%) patients had severe fibrosis. The distribution of genotypes for IL-10-627 was CC (11%), CA (41%), and AA (48%). The CC genotype at IL-10-627 was protective against severe fibrosis (odds ratio (OR) 0.11; 95% CI 0.014-0.82; P = 0.032). After adjusted for baseline variables, the adjusted OR of CC genotypes at IL-10-627 for severe fibrosis was 0.063 (95% CI 0.06-0.64; P = 0.063). Other gene polymorphisms at IL-1beta, IL-1RN, TNF-alpha, and IL-10 had no significant association with severe fibrosis. Weak linkage disequilibrium was observed between IL-10-627 and IL-10-1117 with linkage disequilibrium coefficient of 0.12 (P < 0.001). The distribution of haplotypes of IL-10-1117 and IL-10-627 was A-A (69%), A-C (26%), and G-C (5%). High and intermediate IL-10 production (A-C and G-C) haplotypes were protective against severe fibrosis (OR 0.62; 95% CI 0.39-0.99; P = 0.046).High production genotype and haplotypes of IL-10 were associated with less severe liver fibrosis in chronic hepatitis B in Chinese