Two classes of stacked structures with (i) varying InAs nominal thickness, θ, but fixed GaAs spacer layer (d = 25nm), and ii) a varying d but fixed InAs nominal thickness (θ = 2.73 monolayers) were grown by solid-source molecular beam epitaxy. The formation of large so-called volcano-like defects with stacking faults and dislocations, penetrating several layers into the stack, was observed for a nominal InAs deposition of 2.73ML and 3.62ML in case (i). In case (ii), with, d = 50nm, a dramatic improvement in structural integrity was observed with the absence of the volcano-like defect. The photoluminescence intensity where, d = 50nm, was almost 132 times stronger for, θ = 3.64ML, indicating that the improvement in crystal structure resulted in better photoluminescence efficiencies. Remarkably, significant improvements in photoluminescence intensity were observed when the material was etched to selectively remove the topmost stacks thus demonstrating that the volcano-like defects were responsible for the severely suppressed photoluminescence efficiencies.

Defect Investigation of Stacked Self-Assembled InAs/GaAs Quantum Dot Lasers. J.Ng, U.Bangert, M.Missous: Physica Status Solidi C, 2007, 4[8], 2986-91