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.