We directly measure the modal gain and spontaneous emission spectra in three quantum dot structures that are nominally identical except for the level of p doping to ascertain the effect that p doping has on quantum dot lasers. The maximum modal gain increases at fixed quasi-Fermi level separation as the level of p doping increases from 0 to 15 to 50 acceptors per dot. The internal optical mode loss is similar for all three samples but the measured nonradiative current is larger for the p-doped structures.

We measure, in real units, the radiative and total current density in high performance 1.3-/spl mu/m InAs quantum-dot-laser structures. Despite very low threshold current densities, significant nonradiative recombination (/spl sim/80% of the total recombination) occurs at 300 K with an increasing fraction at higher current density and higher temperature. Two nonradiative processes are identified; the first increases approximately linearly with the radiative recombination while the second increases at a faster rate and is associated with the loss of carriers to either excited dot states or the wetting layer.

We assess three methods to increase the gain in quantum-dot-lasers based on increasing the number of dots states and increasing the population of the available states by reducing the effect of the wetting layer

The segmented contact method is used to study the performance of intrinsic and p-doped quantum dot structures emitting at 1.3 mu m. From measurements of the absorption, it is shown that despite being doped to a level of 18 acceptor atoms per dot, only 10% of the quantum dot states are filled by excess holes, illustrating the importance of the continuum states in the wetting layer. We directly measure the modal gain and non-radiative recombination and show that the modal gain is increased as a function of transparency point when p-dopants are introduced without a significant increase in non-radiative recombination. These results explain the 65% reduction in threshold current observed for uncoated 1500 mu m long devices at 300 K.

Greatly improved threshold current and modal gain performance of 1.3 mum quantum dot lasers is achieved by engineering the GaAs spacer layers between dot layers to improve dot homogeneity and enable closer dot layers.

We have measured modal gain and absorption data for doped and undoped quantum dot devices. We show that p doping results in an increase in the amount of gain available at a fixed current.

We investigate the mechanisms by which high growth temperature spacer layers (HGTSLs) reduce the threshold current of 1.3-mu m emitting multilayer quantum-dot lasers. Measured optical loss and gain spectra are used to characterize samples that are nominally identical except for the HGTSL. We find that the use of the HGTSL leads to the internal optical mode loss being reduced from 15 +/- 2 to 3.5 +/- 2 cm(-1), better defined absorption features, and more absorption at the ground state resulting from reduced inhomogenous broadening and a greater dot density. These characteristics, together with a reduced defect density, lead to greater modal gain at a given current density.

Quantum dots have demonstrated improved performance relative to quantum wells in lasers and amplifiers for structures where the total optical loss, and hence the gain required from the dot active material, has been kept low. In many applications higher gain and/or high differential gain are required and high gain structures must be routinely produced if quantum dots are to replace quantum wells in more than a few niche applications. The obvious approach is to use multiple layers of quantum dots in the active region of the laser or amplifier. However, stacking multiple quantum dot layers modifies the growth of subsequent layers and in the extreme case leads to defect formation.In this work we study an approach where the negative effects caused by the introduction of multiple layers of quantum dots are minimised using a high growth temperature spacer layer (HGTSL) to planarize the surface before deposition of the subsequent layer of dots. We show that this has a dramatic affect on the threshold current of our 1.3 mu m emitting lasers and by use of detailed characterisation show that this is due to 4 physical effects. Samples containing the HGTSL exhibit less inhomogenous broadening, have an increased dot density, a lower internal optical mode loss and contain fewer defects than samples containing a conventional spacer layer. Our results demonstrate the importance of going beyond an approach based on defect reduction alone.

This work examines the radiative and remaining nonradiative recombination processes in quantum dot lasers with low levels of defect related nonradiative recombination. The segmented contact method is used to obtain the spontaneous emission rate spectra of a 5-layer InAs/In0.15Ga0.85As DWELL structure at drive current densities between 22 and 490 Acm-2 taken at 300 K.

We demonstrate improved gain and loss performance of 1.3 /spl mu/m In(Ga)As quantum dot laser material using high growth temperature spacer layers to reduce, the internal loss and inhomogeneous broadening, and improve the efficiency.