A detailed characterization study was made, of stacked InGaAs quantum-dot and InAs/InGaAs dot-in-well structures, using cross sectional transmission electron microscopy. For In.5Ga.5As/GaAs quantum-dot structures, optimized stacking was observed by using a barrier thickness of about 12nm. Studies were also made of stacking in dot-in-well laser structures. It was noted that performance was limited by gain saturation and thermal excitation effects. Solutions to these problems were explored by stacking multiple dot-in-well layers of 3, 5 and 10 repeats. Initial attempts at stacked multilayer structures, especially samples with a large number of repeats, produced variable results; with some devices being characterized by poor emission and electrical characteristics. Analysis by transmission electron microscopy identified the presence of large defective regions arising from the complex interaction of dots on several planes and propagating threading dislocations into the cladding layers. The origin of this defect was traced to the coalescence of quantum-dots at very high density, and the resultant dislocation propagating to higher dot planes.

Strain Interactions and Defect Formation in Stacked InGaAs Quantum Dot and Dot-in-Well Structures. M.Gutiérrez, M.Hopkinson, H.Y.Liu, J.S.Ng, M.Herrera, D.González, R.Garcia, R.Beanland: Physica E, 2005, 26[1-4], 245-51