Figge, Stephan
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Item-typ:Veröffentlichung, GaN-basierte Laserdioden : Epitaxie und Simulation(2003-07-15); ; ; 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.Dissertation362 612 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Controlled Laser-Thinning of MoS2 Nanolayers and Transformation to Amorphous MoOx for 2D Monolayer Fabrication(American chemical society, 2020-07-20); ; ; ; Laser-thinning of 2D materials such as MoS2 is a promising approach for a local reduction of the number of multilayers down to a monolayer. For a precise control of the thinning process real-time monitoring is required. In this work, short-wavelength lasers emitting at 325 or 406 nm respectively are used for laser-thinning and simultaneous Raman or photoluminescence spectroscopy of MoS2. The time evolution of the Raman and photoluminescence bands during the process shows a layer-by-layer thinning of MoS2 and a transformation into amorphous MoOx in an oxygen-containing atmosphere. In addition to the E2g1 and A1g Raman modes, the E1g, B2g1, and second-order modes are analyzed by using the 325 nm laser for excitation to achieve a more accurate determination of the number of layers. As a promising alternative, photoluminescence spectroscopy is used to monitor the thinning progress by analysis of the emission energy and intensity of the direct as well as the indirect band gap transition. Atomic force microscopy measurements show an increased total height of the laser-treated region after thinning of MoS2 due to the presence of transformed MoOx. Local micropatterning of a bilayer is also demonstrated by laser-thinning down to a monolayer at selected positions. The results show a new monitoring approach for controlled fabrication of 2D monolayers.Wissenschaftlicher ArtikelBand:3Heft:842 35