Shape And Size

It is not surprising that many different shapes have been reported for different (or even within the same) QD systems because both thermodynamic and kinetic effects play an important role in determining the shape and size of the QDs during the growth of strained heterostructures. The nature (including the material [71], surface anisotropy [72], and orientation [73-75]) and temperature [76-84] of the substrate, the deposition rate [80, 82, 83, 85-87], the chemical form of the epitaxial material, and the ambient conditions all affect the QD growth and lead to dramatically different morphologies even for the same combination of materials [88]. For example, for InAs QDs, increasing the substrate temperature results in larger QDs, a narrower size distribution, a lower QD density, and an increased aspect ratio [76]. This sensitivity to growth conditions explains why, for example, the shapes of the Ge QDs on Si (001), grown by different groups, are often quite different [24, 25].

In this section, we discuss the shapes of QDs in the most important and, therefore, the most widely investigated systems GexSi1-x/Si(001) (Section 2.1) and InxGa1-xAs/ GaAs(001) (Section 2.2). The shapes of QDs on (001) surfaces in other systems, including the InP and II-VI systems, are discussed together in Subsection 2.3. Because QDs grown on higher index surfaces are quite different from QDs grown on (001) surface, they are also discussed in a separate subsection (2.4).

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