Improved characteristics of indium gallium nitride-based laser diodes

Many applications exist for short wavelength laser diodes (λ ∼ 400–450 nm), such as optical storage, laser printing, displays, and lighting, as well as medical applications. The operating characteristics and lifetimes of InGaN-based laser diodes need be improved to provide the reliability required for these applications. These improvements require extensive work in the growth and design of the epitaxial layer structure, as well as the device design and fabrication.; The epitaxial structure of the laser was investigated to enhance the operating characteristics. Optimization in the epitaxial design and growth conditions of the laser active region, claddings, and p-type layers was performed. The design of the quantum wells (QWs) and QW capping reduced the threshold current density of the laser and doubled the internal efficiency to 35%. The implementation of AlGaN/GaN strained-layer superlattices improved the structural properties of the cladding layers. Optimizing the Mg-doping in the laser reduces the resistivity of the p-type layers and the threshold voltage by 30%, thus reducing the heating of the lasers. The increased hole concentration with a reduced Mg-doping decreases the threshold current and increases the internal efficiency. The device design and fabrication was also addressed. Facet formation improvements were investigated to increase the facet reflectivity. Heat sinking was also investigated to provide heat extraction from the device. The devices were able to operate under a 10% duty cycle when the substrate was thinned and mounted on a copper heat sink, compared to 2% with no heat sinking.; The threading dislocation (TD) density in the laser structure has a large impact on the threshold current density of laser diodes. The TDs act as nonradiative recombination centers, which will reduce the internal efficiency of the laser diode. Removing the dislocations by the lateral epitaxial overgrowth (LEO) technique led to extraordinary improvements in device performance and is the enabling step for the demonstration of long-lifetime blue laser diodes. The threshold current density of lasers on LEO GaN was as low as 3.7 kA/cm 2, which is a factor of 2 lower than lasers on conventional GaN. The internal efficiency is increased from 3% to 22% when reducing the dislocation density. The TD distribution also had an impact on the growth mode and resulting surface morphology of laser diodes. Spirals are formed around the terminations of TD with a screw component and the changing in TD density on LEO GaN will drastically impact the spiral size in the laser structures.; The growth of laser diodes on SiC was investigated to take advantage of the thermal and conductive properties of the substrate over the typically employed sapphire substrates. Buffer layers need to be optimized to provide a low resistance at the interface and a low threading dislocation density, in order to surpass the performance of lasers on sapphire.