Theory of strain states in InAs quantum dots and dependence on their capping layers

The dependence of strain states in InAs self-assembled quantum dots (QDs) on their capping layers was investigated by valence-force field model calculations. An InAs QD on (001) GaAs and embedded in a GaNAs capping layer and the one with its dot surface terminated with nitrogen (N) and embedded in a GaAs capping layer show reduced compressive strain within the QDs in the (001) growth plane due to the lateral expansion of the QDs, while the one embedded in an InGaAs capping layer shows enhanced tensile strain along the [001] growth direction. The strain energies around the center of the InAs QDs with the GaNAs capping layer and with the N-surface termination are lowered compared with those for conventional GaAs capping layers. The burying conditions of InAs QDs also modify the sizes of QDs. The stress distributions obtained by strain energy mapping showed that In atoms around the top of QDs undergo inward stress. This inward stress prevents In segregation and explains the experimentally observed improved optical properties of GaNAs-embedded and N-terminated QDs.