Development of telecom wavelength InAs Quantum Dot lasers by MOCVD

The subject of this thesis is to develop quantum dot lasers around the telecom wavelengths of 1300nm and 1550nm for optical fibre communications. Quantum dots (QD) were grown by Metal-Organic Vapour Deposition (MOCVD) utilising both the conventional Stranski-Krastanov (S.K) method and a novel droplet epitaxy (DE) approach. The first section compares 1.1 µm InAs/GaAs QD lasers grown on the on-axis GaAs(100) substrates and substrates offcut 3° towards (110). QD lasers on the off-axis substrates had a lower threshold current density (Jth) and higher gain. An ~20% increase in the QD density for the 3-degree off-axis sample was found compared to the on-axis samples. The higher QD density is related to the change of morphology of the GaAs spacer layer, with more steps formed on the surface of the off-axis GaAs, providing a favourable nucleation site for QDs. These 1.1µm QDs are the first step towards the future realisation of 1.3µm QD lasers by MOCVD in comparison to MBE literature of 1310nm QDs. The longer term aim of the research is to incorporate these QD lasers on Silicon substrate for photonic integration. The second section presents the design, growth and characterisation of 1550nm InAs QD lasers grown by Droplet Epitaxy. The DE approach has several potential advantages, including removing the influence of the well-known wetting layer seen in the SK growth of QDs. The QD structure with the InP waveguide layer blue-shifts the QDs' wavelength to 1530nm, close to the target wavelength of ~1550nm. An approximately five times increase in emission intensity from QD samples was achieved when the Zn doping was reduced. The waveguide material was changed to InP, which significantly improved the carrier injection into the QDs. A high tail of diffused arsenic and a high oxygen concentration observed on the QD & QW samples may be preventing lasing. A new Quantum Well structure incorporating AlInGaAs as waveguide layer material was also introduced and formed the basis of further optimisation in future work to achieve room temperature lasing of these DE QD lasers.