GaN based laser diodes : epitaxy and simulation

Abstract

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