Stranski-Krastanov model

In addition to the strain caused by lattice mismatch, there exists another contribution, which gains in significance when the sample is cooled to very low temperatures. This thermally induced strain is caused by the different thermal expansion coefficients. At room temperature, the amount of the thermal strain is about one order smaller than the lattice-mismatch-induced strain, whereas this strain can become dominant for low temperatures [213]. The rather small strains for very thin layers can be explained by the growth of a kind of island structures, which can be used for the MBE fabrication of quantum dots (Stranski-Krastanov model see Refs. 71-75). In contrast to the lattice for a complete layer, the lattice at the edges of these islands is relaxed, which leads to a reduced strain shift. 

Figure 3.7 Three models of film growth (a) Frank-van der Merwe, layer by layer (b) Volmer-Weber, islands (c) Stranski-Krastanov, combination of layer by layer and islands.

Since SK-growth has previously been reported for this system [130, 131], we tried also a model structure that consisted of a thin layer with a single island type on top. The best fits we obtained for this Stranski-Krastanov structure from sample... 

Figure 2.4 Models of deposition showing layer-by-layer deposition (Frank—van der Merwe), island growth (Volmer—Weber) and mixed (Stranski-Krastanov).

The model predicts that the critical thickness for island formation depends inversely on the square of the misfit. For the growth of strained InGaAs layers on a GaAs substrate Wessels estimates that the transition to the Stranski-Krastanov mechanism occurs when hcrit x/ > 1.8 x 10 [cm]. 

Stranski-Krastanov (SK) model (film growth) Nucleation on a surface that changes structure during deposition (layer-plus-island growth). See Volmer-Weber growth Frank van der Merwe growth. 

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