Structural investigation of the 2D to 3D transition in stacked submonolayer InAs nanostructures

A direct comparison of the structures of 2D and 3D types of capped stacked submonolayer (SML) InAs nanostructures is evaluated by transmission electron microscopy (TEM). Results of the TEM observation of SML samples with three stacks of InAs unambiguously show a stark contrast between the structures of 2D and 3D SML nanostructures, where the 2D SML nanostructures exhibit a planar structure with thickness that is consistent with the deposited stack height, whereas the 3D SML nanostructures exhibit several-nm-high structures that exceed the height of the deposited stack. In addition, structural evolution at the 2D to 3D transition in uncapped SML nanostructures is investigated by atomic force microscopy (AFM). The AFM results clearly reveal that the 2D to 3D transition occurred during the deposition of the third (and last) InAs SML stack in the present samples, where the density of 3D structures increases in orders of magnitude with the deposited amount of InAs on the order of a tenth of a monolayer at the onset. This effectively bridges the gap between the 2D and 3D nanostructures elucidating the abrupt nature of the transition.