| Summary: | Self-assembled InAs quantum dot lasers have been characterised by measuring the modal absorption and gain along with radiative and non-radiative current densities as well as determining threshold current densities as a function of length. The number of dot layers stacked and the GaAs spacer layers used are both shown to influence the dot density and distribution. The peak ground state gain is found to saturate at around a third of the available absorption in intrinsic quantum dot samples (reaching a value of 1.2 0.2 cm"1 per layer). The 'average spontaneous lifetime of a single dot is determined from measurements of the optical cross section (yielding a value of 2.0 0.5 ns). From the lifetime measurements, the number of dots occupied at a given injection has been determined this has shown that gain saturation in quantum dot lasers is due to the incomplete filling of states. The occupancy is shown to increase with the inclusion of p- type modulation doping (from 31% for an intrinsic structure to 43 % and 51 % for doping levels of 15 and 50-p dopants per dot respectively), hence increasing the available ground state gain to 2.2 cm"1 per layer for 50 dopants per dot. In some of the samples studied the use of modulation doping has been shown to lead to an increase in the non-radiative current density. The temperature dependence of the threshold current density in InAs quantum dot lasers is also investigated. It is found that in the p-doped structures the threshold current shows an initial decrease in the threshold current, before increasing at higher temperatures (for example a decrease of lOOAcm" occurs between 180 and 290 K for a 2 mm long cavity with 15-p dopants per dot), this is shown to originate from the temperature dependence of the modal gain in these structures.
|
|---|
