Optoelectronic properties of InAs GaAs columnar quantum dot laser diodes

In this thesis results are described with the aim of examining the optoelectronic properties of InAs/GaAs columnar quantum-dots and comparing them with those of more conventional self-assembled quantum-dots. The polarisation properties of a set of columnar quantum-dot samples — of varied aspect ratio and In compositional contrast between the rod-shaped dot and the surrounding 2-D layer — are studied. For this investigation a new method to obtain the ratio of the fundamental TE/TM optical response using edge photo-absorption spectroscopy is proposed, which corrects for the polarisation-dependent features of the experimental set-up. The method is verified by application to compressive and tensile strained InGaP quantum well structures, where the results are in agreement with known ratios of the band-edge matrix elements. When applied to columnar quantum-dot samples it is shown that the TE/TM optical response depends on the dot aspect ratio and the In compositional contrast. A polarisation-independent photo-absorption is illustrated for a columnar quantum-dot of an aspect ratio (dot's height over diameter) 3.51:1, which is desired for use in semiconductor optical amplifiers. For the columnar dot of an extremely high aspect ratio, 7.5:1, a room temperature TM-dominant polarisation lasing emission is observed. By studying the Quantum-Confined Stark Effect, a dramatic enhancement of the Stark shift amplitude is shown for columnar quantum-dot samples of an increased dot aspect ratio from 0.63:1 up to 1.12:1, which may have application in optical modulation/switching. For a higher aspect ratio columnar quantum-dot the shift of the band edge of the photo-absorption spectra is reduced dramatically and this has been attributed to an overall effect where the observable shift becomes the result of higher energy transitions, where their oscillator strength change very rapidly within the studied range of field. For the highest aspect ratio dot, i.e. of aspect ratio 10:1, there is no any observable shift