Abstract
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.
Highlights
Depositing GaAs while keeping the temperature at 500 ○C
In comparison with our results, we have found that the samples described in some works in the literature,[11,12] despite being assumed to have been grown in the 2D regime, are likely to have been grown in the 3D regime, which explains why these SML nanostructures resemble the properties of 3D SK quantum dots (QDs)
The image is consistent with transmission electron microscopy (TEM) reports of 2D SML nanostructures in the literature.[13,14,15,16]
Summary
Depositing GaAs while keeping the temperature at 500 ○C. Shown in Fig. 1(b) are the 4 K (PL) spectra of the two samples excited by a Ti:sapphire laser at 740 nm, 1.25 W cm−2. It was commonly assumed that, by keeping each SML InAs deposition per cycle well below the SK critical thickness of ∼1.7 ML, the growth of SML nanostructures occurs strictly in the 2D regime.[3,4,5,6,7,8] our recent reports have revealed that a transition from the 2D to 3D growth regime exists in SML growth,[9] allowing us to prepare both 2D islands and 3D structures by SML growth.[10] In comparison with our results, we have found that the samples described in some works in the literature,[11,12] despite being assumed to have been grown in the 2D regime (as this was the common assumption), are likely to have been grown in the 3D regime, which explains why these SML nanostructures resemble the properties of 3D SK quantum dots (QDs).
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