Physics and performance of InGaAs quantum dot lasers.

Quantum dot lasers are becoming increasingly technologically important. It is therefore essential to understand the factors affecting their current performance and be able to predict future performance. The gain and unamplified spontaneous emission spectra have been measured for a selection of quantum dot devices and a quantum well device. The quotient of the gain and spontaneous emission spectra were used to calculate the PF spectra and investigate the carrier distribution within the devices. Whilst the quantum well device and devices with one or three layers of dots exhibited characteristics consistent with Fermi-Dirac statistics, devices with more layers dots produced an unusual set of spectra, determined to be due to a non-thermal distribution of carriers in the ground state by looking at the unamplified spontaneous emission spectra. A model was developed to investigate the effects of non-thermal carrier distributions on the calculated PF spectra. From this it was deduced that it was possible to use a fit of a thermal PF to the excited state PF to calibrate the measured unamplified spontaneous emission spectra. The resultant PF, gain and spontaneous emission spectra are sensitive to the exact balance between the homogeneous and inhomogeneous broadenings. This calibration was used to calculate the radiative current densities and compare the radiative efficiencies of different structures, including both Dots-in-Well (DWELL) and standard dot structures. There was no large difference in efficiency found due to improved carrier injection in the DWELL structures. Calculated gain-radiative current density curves were used to predict the minimum transparency and threshold current densities that may be possible in the future. It is clear that the limits of quantum dot device performance have not yet been reached and that a factor of 1.7 improvement in threshold current density over state of the art devices could be achieved, even without reduced inhomogeneous broadening