Device and material characterisation of vertical cavity surface emitting lasers

This thesis presents the characterisation of oxide-confined vertical-cavity surface-emitting laser (VCSEL) devices, with circular and elliptical oxide apertures, for miniaturised atomic clock (MAC) applications. The use of elliptical oxide apertures, which doesn't require additional processing steps or electron-beam lithography, as an alternative to surface gratings for polarisation control was investigated. Additionally, the measured power-current, emission wavelength, and beam characteristics of the VCSEL devices were compared to the requirements of the MAC. 2.5 um diameter circular oxide aperture VCSEL devices met most of the requirements. However, with the epitaxial structure considered in this thesis, the small oxide aperture sizes required to achieve side-mode suppression ratios (SMSR) of 30 dB, resulted in a mean divergence angle of 51.6 ± 0.8°; more than twice the maximum allowed value of 25°. There was also a large variation in the measured orthogonal polarisation suppression ratio (OPSR). 1.5 \times 3.7 um elliptical oxide aperture VCSEL devices had measured SMSRs that were greater than 30 dB up to 0.5 mW. The mean maximum OPSR was 14.2 ± 0.3 dB, for optical powers between 0.2-0.5 mW, the minimum differed from the maximum by less than 0.4 dB for all the measured devices. The segmented contact stripe-length method was applied directly on VCSEL material to measure the transverse electric (TE) polarised modal gain spectra, as a function of current density and temperature. The cap thickness was varied to reduce the mirror reflectivity and suppress lasing in the vertical direction. The TE-polarised modal gain was converted into a material gain by calculating the confinement factor and effective index of the mode. For the structure considered, the threshold material gain was determined to be 1440 ± 140 cm-1 at 30 °C. To demonstrate the usefulness of this technique in the optimisation of VCSEL structures, the measured gain-peak wavelength, and its temperature dependence is compared to the lasing wavelength