Influence of two different adatom mobilities on the initial heteroepitaxial growth kinetics

In the heteroepitaxy deposited adatoms in the first layer migrate on a substrate with a heterodiffusion coefficient, while those in all the next layers – with a self-diffusion coefficient. In this paper the effect of two different adatom surface mobilities on the layer heteroepitaxial growth in the presence of an Ehrlich–Schwoebel barrier is studied quantitatively using kinetic model based on rate equations for adatom and 2D island density and coverage in successive layers combined with a feeding zone taking into account interlayer mass transport. It is shown that this effect is essential in the first few monolayers and strongly depends on a ratio hetero-to-self-diffusion coefficient and Ehrlich–Schwoebel barrier. A faster heterodiffusion leads to a more rough initial growth whereas a slower heterodiffusion – to a smoother initial growth. In a transient (homoepitaxial) growth regime from smooth layer-by-layer – to rough growth a slower heterodiffusion retards this transition, while a faster heterodiffusion promotes it, especially at higher Ehrlich–Schwoebel barrier. It is shown that a critical coverage for the next layer nucleation is more appropriate parameter for characterization growth mode transition than a widely used critical island radius because it does not depend on island density and uniquely determines growth mode transition and its dependence on hetero- and self-diffusion coefficients and a critical nucleus size is obtained. The results suggest an opportunity for improving ultrathin (a few monolayers) film surface morphology by manipulating growth mode during deposition by initial (first layer) deposition either at low substrate temperature (low diffusivity) or with high deposition rate and consecutive one either at higher temperature (high diffusivity) or with a lower deposition rate.