GaN-basierte Laserdioden : Epitaxie und Simulation

The demand for higher recording densities in optical storage devices requires the development of semiconductor lasers with short wavelengths. This thesis deals with the realisation and simulation of GaN-based laser diodes with an emission wavelength of 400 nm. One of the key issues is the reduction of the resistivity of the Magnesium doped epitaxial layers in order to achieve lasing in such devices. The p-type doping of GaN utilizing both metalorganic vapor phase deposition (MOVPE) and molecular beam epitaxy (MBE) is investigated. High room temperature hole concentrations of 4y1018 1/cm3 have been achieved in samples grown by MBE. Compared to published data, these data belong to the best. The incorporation of Magnesium in these samples is limited by the desorption of Magnesium during growth. Therefore the doping is sensitive to the growth temperature. It could be shown that the incorporation of Magnesium can be enhanced by the use of a Hydrogen-Nitrogen mixed plasma. Nevertheless, these layers are compensated due to the formation of both Magnesium related planary defects and Hydrogen induced point defects. Therefore these layers exhibited high resistivities of 130 z cm. The formation of pyramidal defects was realised to be the cruicial factor in the limitation of p-type doping in MOVPE grown samples. The segregation of Magnesium on the growth surface is identified by transmission electron microscopy to be the driving force of defect formation. A rate equation model is employed and a defect formation criterion is established. Gain guided GaN-based laser diodes were produced on sapphire substrates with a Magnesium doping level just below defect formation. These laser structures showed a light output power of up to 263 mW in pulsed operation mode. However, the driving parameters are limited due to the generation of joule heat to pulse lengths and duty cycles of 100 ns and 2 %, respectively. The heat dissipation in the devices was simulated. These simulations are in good agreement with time dependent electroluminescence data collected in pulsed operation. The decay of luminescence intensity during the pulses is identified as a thermal activation process. Different laser structures were simulated and it was found, that the thickness of both pcontact metallization and substrate has a major influence on the thermal resistivity of the laser diodes. The heat dissipation model has been extended by an electrical model based on the laplace equation. The influence of current spreading on the current densities in the active region of gain guided laser structures is investigated. The activation of carriers in the p-type layers and the reduction of the conductivity by phonon scattering in the n-type layers at elevated temperatures during operation has been identified to be the main reason for high threshold current densities of the gain-guided laser diodes

Optical Properties and Modal Gain of InGaN Quantum Dot Stacks

We present investigations of the optical properties of stacked InGaN quantum dot layers and demonstrate their advantage over single quantum dot layer structures. Measurements were performed on structures containing a single layer with quantum dots or threefold stacked quantum dot layers, respectively. A superlinear increase of the quantum dot related photoluminescence is detected with increasing number of quantum dot layers while other relevant GaN related spectral features are much less intensive when compared to the photoluminescence of a single quantum dot layer. The quantum dot character of the active material is verified by microphotoluminescence experiments at different temperatures. For the possible integration within optical devices in the future the threshold power density was investigated as well as the modal gain by using the variable stripe length method.Comment: 9 Pages, 4 Figure

GaN based laser diodes : epitaxy and simulation

The demand for higher recording densities in optical storage devices requires the development of semiconductor lasers with short wavelengths. This thesis deals with the realisation and simulation of GaN-based laser diodes with an emission wavelength of 400 nm. One of the key issues is the reduction of the resistivity of the Magnesium doped epitaxial layers in order to achieve lasing in such devices. The p-type doping of GaN utilizing both metalorganic vapor phase deposition (MOVPE) and molecular beam epitaxy (MBE) is investigated. High room temperature hole concentrations of 4y1018 1/cm3 have been achieved in samples grown by MBE. Compared to published data, these data belong to the best. The incorporation of Magnesium in these samples is limited by the desorption of Magnesium during growth. Therefore the doping is sensitive to the growth temperature. It could be shown that the incorporation of Magnesium can be enhanced by the use of a Hydrogen-Nitrogen mixed plasma. Nevertheless, these layers are compensated due to the formation of both Magnesium related planary defects and Hydrogen induced point defects. Therefore these layers exhibited high resistivities of 130 z cm. The formation of pyramidal defects was realised to be the cruicial factor in the limitation of p-type doping in MOVPE grown samples. The segregation of Magnesium on the growth surface is identified by transmission electron microscopy to be the driving force of defect formation. A rate equation model is employed and a defect formation criterion is established. Gain guided GaN-based laser diodes were produced on sapphire substrates with a Magnesium doping level just below defect formation. These laser structures showed a light output power of up to 263 mW in pulsed operation mode. However, the driving parameters are limited due to the generation of joule heat to pulse lengths and duty cycles of 100 ns and 2 %, respectively. The heat dissipation in the devices was simulated. These simulations are in good agreement with time dependent electroluminescence data collected in pulsed operation. The decay of luminescence intensity during the pulses is identified as a thermal activation process. Different laser structures were simulated and it was found, that the thickness of both pcontact metallization and substrate has a major influence on the thermal resistivity of the laser diodes. The heat dissipation model has been extended by an electrical model based on the laplace equation. The influence of current spreading on the current densities in the active region of gain guided laser structures is investigated. The activation of carriers in the p-type layers and the reduction of the conductivity by phonon scattering in the n-type layers at elevated temperatures during operation has been identified to be the main reason for high threshold current densities of the gain-guided laser diodes

Zur Schätzung der Intelligenzhöhe mit dem Rorschach-Versuch

Schmidt HD, Figge HH, Gehlen ST. Zur Schätzung der Intelligenzhöhe mit dem Rorschach-Versuch. Diagnostica. 1967;13:60-61

Sind gültige Aussagen über die Intelligenzhöhe aufgrund des Rorschach-Versuchs möglich?

Schmidt HD, Figge HH, Gehlen ST. Sind gültige Aussagen über die Intelligenzhöhe aufgrund des Rorschach-Versuchs möglich? Diagnostica. 1966;12:139-157

Indium: A surfactant for the growth of ɛ/κ-Ga2O3 by molecular beam epitaxy

The influence of In on the growth of ɛ-Ga2O3 by plasma-assisted molecular beam epitaxy is investigated. We demonstrate alloying of ɛ-Ga2O3 with In and describe its incorporation limits and catalytic effect on the growth kinetics. A special focus lies on the metal-rich growth regime, where we show that In acts as a surfactant for the ɛ-Ga2O3 growth. Both the In-incorporation and the ɛ-(In,Ga)2O3 growth rate follow a non-monotonous trend with increasing In-supply. Whereas both entities show an increase for low In-fluxes, they decrease again for very high In-fluxes, combined with the formation of an atomically smooth surface for layers with thicknesses of several hundred nanometers in this regime. Based on these results, the influence of the growth temperature and the amount of Sn, supplied to initiate the ɛ-Ga2O3 phase formation, is discussed, revealing their impact on the surfactant ability and incorporation of In. Using In as a surfactant, we demonstrate the pseudomorphic growth of ɛ-(In,Ga)2O3/ɛ-(Al,In,Ga)2O3 heterostructures with sharp interfaces and surfaces

Single photon emission from InGaN/GaN quantum dots up to 50 K

We have investigated the optical properties of single InGaN quantum dots (QDs) by means of microphotoluminescence (mu PL) spectroscopy. The QDs were grown on sapphire substrate using metal organic vapor phase epitaxy. Sharp and isolated single exciton emission lines in the blue spectral range were observed. The QD luminescence shows a strong degree of linear polarization up to 96% perpendicular to the growth axis (c-axis) with no preferential alignment in the xy plane. Second order autocorrelation measurements were performed under pulsed excitation and single photon emission up to 50 K is demonstrated. (C) 2012 American Institute of Physics. [doi:10.1063/1.3683521]</p

Was uns angeht. Zur Einleitung - Introduction to the Theme of Focus Topic: Concerning Us

Bergermann U, Eschkötter D, Figge M, et al. Was uns angeht. Zur Einleitung - Introduction to the Theme of Focus Topic: Concerning Us. Zeitschrift für Medienwissenschaft. 2019;11(20):10–14.Im Jubiläumsheft Nr.20 beziehen wir Position zu Wissenschaft und Wissenschaftspolitik, diskutieren Fragen der Verantwortung und der Sorge, reflektieren das eigene Tun als Forscher_ innen, Schreiber_innen, Gestalter_innen – und als Redakteur_innen einer Zeitschrift für Medienwissenschaft. Es geht um situiertes Schreiben, um alternative Empirien auch und gerade in der Wissenschaft, um die Universität als sozialer wie politischer Raum und als Austragungsort von Begehren, von Wissenslust, von sexualisierter Hierarchie und von sexueller Gewalt. Es geht um analoges und digitales Publizieren und darum, wie die ZfM gemacht wird, um die Dekolonialisierung der Archive und darum, wie medienethnologische Forschung eurozentristische Perspektiven verschiebt. «From what distance are things clear?»In this anniversary issue No. 20, we take a position on science and science policy, discuss questions of responsibility and care, and reflect on our own actions as researchers, writers, designers—and editors of a journal devoted to media studies. The issue is about situated writing and alternative forms of empiricism – especially in science. It’s about the university as a social and political space and as a site for desire and the pleasure of knowledge, for sexualized hierarchies and sexual violence. It’s about analog and digital publishing and how the ZfM is made, the decolonization of archives and how ethnological media research is shifting Eurocentric perspectives. «From what distance are things clear?