Study of Size, Shape, and Etch pit formation in InAs/InP Droplet Epitaxy Quantum Dots*

InAs/InP self-assembled quantum dots (QDs) emitting in the telecom range (~1550 nm) with a small FSS have been developed recently as a result of reduced lattice mismatch and improved growth techniques. We investigated metal-organic vapor phase epitaxy grown droplet epitaxy (DE) and Stranski-Krastanov (SK) InAs/InP QDs by cross-sectional scanning tunneling microscopy (X-STM) and present an atomic-scale comparison of structural characteristics of QDs grown by both growth techniques proving that the DE yields uniform and shape symmetric QDs. Both DE and SKQDs are found to be truncated pyramid-shaped with a large and sharp top facet. The presence of the etch pits underneath the DEQDs is explained by the phenomenon of local droplet etching. Finite element simulations are performed to fit the experimental outward relaxation and local lattice constant profiles of the cleaved DEQDs. The composition of the DEQDs is estimated to be pure InAs obtained by combining both FE simulations and X-STM results with a minor intermixing close to the QD edges. The formation of a discontinuous DE wetting layer from As to P surface exchange is compared with the standard SKQDs wetting layer. We found that the DEQDs have preferential {136} or {122} side facets. The structural characterization performed in this work provides detailed feedback to the growers to further optimize the growth of DEQDs.