Optoelectronic properties of InP AlGaInP quantum dot laser diodes

The aim of this thesis is to understand and optimise the optoelectronic properties of InP quantum dot laser diodes which operate in the range around 730nm required for various application such as the photodynamic therapy. The properties of wafers with two barrier widths, 8 and 6nm, each grown at different temperatures, 690, 710, 730 and 750T, and consisting of 5 layers of dots forms from different quantity of deposited material, 2, 2.5 and 3ML, are described and investigated. The laser and multisection devices of these structures are used to determine threshold current density, lasing wavelength, modal absorption, modal gain and spontaneous emission spectra. The modal absorption spectra show three different dot size distributions, small, large and very large dots. Their variation with growth temperature results in a blue shift accompanied by an increasing number of states while the variation with quantity of deposited material shows only an increase to the number of states. The lasing wavelength variation with growth temperature covers a range between 715–745nm. The threshold current density as a function of temperature for 2000/m long laser devices grown at temperature of 750°C exhibits a distinctive dependence on the operating temperature and becomes less pronounced when the growth temperature reduces. This is explained in terms of the carrier distributions in the quantum dot and quantum well states without invoking an effect from Auger recombination. The optimisation of threshold current density can be reached by using structures with higher barrier width grown at low temperature and deposited with high quantity of quantum dot material to minimise both the affect of the very large dot, which contain a number of defects associated with them, and carrier leakage from quantum dot to quantum well states. This reduces the room temperature threshold current density to ISO A/cm 2 for 2mm long lasers with uncoated facets