Atomic structure of submonolayer grown InAs/GaAs quantum dots

Submonolayer quantum dots are formed by a cycled deposition of the dot material with a thickness well below one monolayer and several monolayers of matrix material. Structural correlation, both vertically as in plane is coupled to strain originating from the dot material. Here, cross-sectional scanning tunneling microscopy (XSTM) is the most powerful tool to determine the spatial structure as well as the stoichiometry with atomic resolution. XSTM measurements demonstrate clearly that there is an island formation instead of a layer-like structure. The InAs is not assembled within a single atomic plane, but segregated along growth direction. The lateral separation between the islands is only about 2 nm, resulting in a very high dot density in the 1012 cm-2 range. The height of the islands is about 4-5 ML, and the width is approximately 5 nm. The vertical segregation is determined in detail by the analysis of the lattice parameter. At each layer where 0.5 ML InAs was deposited, the measured InAs concentration jumps up to a lattice parameter corresponding to xIn=15-20%, followed by an exponential decrease. For both structures with differently thick GaAs spacer layers a segregation coefficient of about 0.73 was determined.