Oxide confined GaAs/AlGaAs vertical cavity surface emitting lasers
This thesis presents the design, fabrication, characterisation and theoretical modelling of oxide confined GaAs/AlGaAs VCSELs operating around 850nm wavelength. Three VCSEL structures utilising different oxide aperture thickness were designed. The oxide apertures contained in these VCSELs can be easily formed through the selective wet oxidation process. Detailed experimental investigations were performed on these devices with emphasis placed on the dependence of the resonant wavelength characteristics, the threshold and modal properties on the aperture size and oxide thickness. A feasibility study of polarisation control through using nonsymmertical apertures was also carried out. These experimental activities were supplemented by theoretical modelling which included calculation of resonant wavelengths and eigenmode sizes using a scalar variational method that assumed Hermite-Gaussian transverse field variations. In addition, the light-current characteristics of the first two eigenmodes were also simulated by solving the photon and carrier rate equations, after their modal reflectance and transmittance had been calculated. The theoretical predictions were then compared with the experimental findings and ways of improving the model have also been suggested. The oxide apertures in these VCSELs provides both current constriction and optical confinement. The resultant index guiding strength is dependent on the thickness as well as on the longitudinal position of the oxide. Devices with stronger guiding have smaller mode sizes which results in lower modal reflectance, larger blue shifts in resonant wavelengths from that of plane wave values, and larger wavelength separations between the fundamental mode and the higher order modes. Devices with smaller aperture sizes also have larger spectral separations and blue shift in resonant wavelengths. When the symmetry in aperture shape is broken, the wavelength degeneracy between the TEM, o and TEM0, mode is also broken. Among the three VCSEL structures, the one with the strongest guiding provides single mode operation at larger aperture size and is less affected by thermal lensing effects. However, lower threshold currents and higher efficiencies can be obtained from devices with weakest guiding. Despite being more susceptible to thermal lensing effects, single mode power up to 1.72mW and threshold current as low as 350μA can be obtained from devices with weakest guiding design.