Browsing by Author "Dumitru, Viorel"

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Open Access
Physics and technology of nitride lasers
(2004) Dumitru, Viorel; Schweizer, Heinz (Prof. Dr.)
This work approaches the realization of the group-III nitride laser diodes using SiC as substrate from the device processing and characterization point of view. The work is structured in 5 parts. After a short introduction in chapter 1, in chapter 2 there are presented some basic properties of the III-nitride laser diodes. Chapter 3 presents the investigations done for establishing a process technology for fabrication of nitride laser diodes on SiC. Together with some basic contacts theory and literature considerations, there are presented the experiments done for realization of the electrical contacts as well as regarding the Mg (p-type dopant in GaN) activation procedure. Once established a reliable procedure for obtaining the electrical contacts it was possible to further process and characterize complete devices. A first approach (presented in chapter 4) was to realize by a simple processing a fast feedback tool to epitaxy. Using a shadow-mask approach it was obtained an electroluminiscent diode that was used for studying and optimisation of different growth parameters as regard their influences on the electrical and optical device characteristics. From electrical point of view there were investigated and compared different approaches related to the p-contact and n-side buffer layers growth, as well as optimised an InGaN cap layer with respect to the gas flows, growth temperature and layer thickness. The epitaxial growth parameters adjusted in this manner, together with the improvements obtained by the electrical contacts process development were finally leading to a significant voltage drop reduction on the fabricated laser diodes and the obtaining of a very low differential resistivity. The shadow-mask devices were also used for optical characterizations of the epitaxially grown structures. Using a multi-segment device approach there were done both electroluminiscence spectra and optical loss measurements, being also investigated their dependence of Mg doping profile in the region close to the active zone. Chapter 5 presents the results obtained as regard the multiple quantum well nitride laser diodes processing and characterization. The first part of this chapter deals with the Fabry-Perot lasers. After describing the fabrication procedure and measurements done for the devices characterization, there are then presented and discussed the obtained results regarding the threshold current value and its characteristic temperature, as well as the characteristic temperature of lasing wavelength. There are evidenced and discussed the effects of the incompletely screened internal electric fields which are present in these structures, leading to increased wavelength stability against temperature variations. On the obtained laser diodes there were done also current spreading investigations. By comparing diodes with different contact geometry as well as different ridge waveguide height, it was concluded that while a significant current spreading effect it is present in the n-side of the epitaxial structure, such a effect is negligible in the p-one –down to the active zone – therefore not affecting the threshold current of the fabricated diodes. The processed laser diodes were also investigated with respect to their thermal resistance properties. Because the found values of this parameter were relatively high ones, there also were tested, with promising results, some possible improvements. The chapter 5.2 presents the realization and characterization of a laterally coupled DFB nitride laser diode with second order grating. This approach, a new one for this material system, allows a simple fabrication procedure, avoiding the need of the epitaxial overgrowth step, typically employed in case of such lasers. The obtained DFB diodes were allowing the measurement of the structure's refractive index and, using different grating periods, also of its dispersion relation. Furthermore, it was confirmed the previously evidenced presence of the incompletely screened internal electric fields in case of the electrically injected nitride lasers, which were leading to the obtaining in case of this device of an extremely small wavelength shift with temperature. Finally, in chapter 5.3 it is presented a successfully test for realization of a nitride surface emitting laser diode. Employing a second order grating incorporated in a DBR laser structure it was obtained a vertical emitting laser diode, avoiding altogether the mirror realization step.