Fabrication and Characterization of Integration Compatible Semiconductor Lasers
Doctoral thesis, 1994
This thesis deals with the fabrication and characterization of integration compatible semiconductor lasers. An extensive investigation of electron-beam lithography (EBL) and chemically assisted ion beam etching (CAIBE) for the fabrication of vertical ultra- high quality facets in GaAs, as well as a systematic characterization of grating coupled surface emitting lasers is presented.
In the first part of the thesis, techniques for the fabrication of vertical ultra-high quality facets in GaAs are developed. Due to the high resolution requirements for optical elements, EBL was chosen for the pattern definition. For elimination of intermediate resolution deteriorating pattern transfers, electron-beam resist was used directly as etch mask. An exposure simulation program for calculation of the spatial distribution of the resist-mask thickness was developed and a simple pattern correction technique, for improving the resist-mask profile, was then used for producing ultra-high quality facets. The CAIBE technique combines the highly directional etching of an inert ion beam with the high etch rate from a reactive atmosphere and was chosen for the facet formation. An extensive theoretical and experimental investigation of the argon and molecular chlorine CAIBE process is presented. The dependence of etch rate and verticality of the etched profile on chlorine flow, ion flux, and ion energy was studied and analytical expressions for the etch rate are presented. Using a new surface-reaction model together with ion-trajectory simulations, computer programs for simulation of etch-rates and etch-profiles were developed.
The developed microfabrication techniques were used for the fabrication of integration- compatible grating-coupled surface-emitting (GSE) semiconductor lasers. A novel epitaxial structure, including a thin etch-stop layer, was used for precise positioning of the surface grating relative the waveguide. Various laser configurations were fabricated for systematic characterization of reflectors, gratings, and gain sections. The dependence of threshold current and light-current linearity on the lasing wavelength was studied for distributed Bragg-reflector (DBR) GSE lasers and record low threshold current densities (below 100 A/cm2) were achieved. By integrating a corner reflector as one feedback element, improved lateral mode control in broad area lasers was obtained, resulting in narrow far-fields (1.3o x 1.0o). The possibility of improving the surface emission efficiency (SEE) by the use of blazed gratings was also demonstrated. In the last part of the thesis, first order grating DBR lasers integrated with second order grating (SOGs) as surface output couplers are presented and investigated. Using this configuration, SEEs higher than 60% were achieved.