Recent progress in the development of 1.3 mm InAs/InGaAs/GaAs dots-in-a-well (DWELL) laser structures has led to efficient CW room temperature laser operation with low current thresholds. However, present devices suffer from non-ideal temperature characteristics due to gain saturation, consequence of the finite dot density and carrier escape due to the small energy separation between the quantum dot (QD) ground and first-excited states. In order to improve device performance, we have examined methods to increase the QD quality and density. In these studies, we have examined the effect of different growth parameters which strongly modify the InAs QDs structure such as temperature and thickness of barrier layers and thickness and composition of the well. Analysis by Transmission Electron Microscopy (TEM), Photoluminescence (PL) and atomic force microscopy (AFM) have identified the presence of defects arising from the complex interaction of QDs, which propagate through the structure into the upper regions being the primary cause of the poor electronic device characteristics. The use of optimized growth has allowed, however, the fabrication of a defect free five layer-stacked structure with record low threshold 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 1.3μm 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.