Growth and characterisation of multiple layer quantum dot lasers

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.