Strained Layer Growth: How Do 3D Islands Relax Strains?
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C. PRIESTER, I. LEFEBVRE, G.ALLAN and M. LANNOO IEMN, Dept ISEN, LILLE,41 Bvd Vauban 59046 Lille Cedex FRANCE
ABSTRACT We study the elastic strain relaxation in highly strained layers deposited on a [001] substrate induced by coherent 3D islanding. We first calculate the elastic strain distribution within several pyramidal 3D islands with different faces (the observed ones being usually [114] or [014]). For this, we use a valence force field (VFF) description. Our calculation includes interactions between islands, which appear to be a key parameter. We study how the shape of the island modifies the strain relaxation which is proved to vary within the island. Very simple considerations on surface tension indicate why pyramidal islands are the most stable. I. INTRODUCTION The growth of epitaxial films with large mismatch has already been shown to be characterized by a transition from two-dimensional (2D) to three-dimensional (3D) growth1"4. Whereas 3D islanding was first considered as degrading the performance of strained-layer based electronic and photonic devices, it recently appeared that it also provides regularly-spaced quantum size dots, in the early stages of growth 5 .A deeper understanding of this 3D growth mechanisms and an investigation of the physical parameters (such as misfit, coverage, or surface reconstruction as far as it can be controlled) which can be used as tools for tailoring the resulting quantum dots might thus lead to a better control of the technological processing steps. The work reported in this paper is an attempt at describing the different mechanisms which govern the early stages of the coherent growth of a material tensilely strained to the [0011 substrate on which it is deposited. Such leading parameters are of three types: i) modification of the relaxation within the island as it grows (which would favor small and sharp islands) ii) the interaction between different islands (which would favor, for a given coverage, widely spaced thus large islands). iii) modification of the surface tension as the island shape and size changes (which would favor, as we shall see, very large and rather flat islands) The stable and metastable configurations result from the competition between the three corresponding driving forces. Previous work 5 ' 6 has shown that, for high enough misfits, pyramidal islands appear before the film reaches the 2D critical thickness. These islands are rather flat (the observed faces were [ 114] for InAs on InP and [014] for InAs on GaAs) and present a square 5 or rectangular 6 basis. For InAs/GaAs, islands seem to be "calibrated" in shape and size, which suggests that for large misfits one has a well defined metastable situation for the islands. It has also been observed that coherent islanding only partially relaxes the elastic strain in the epilayer, since the mean in-plane lattice parameter of the islands lies between bulk material and epilayer lattice parameters 57 . In the following section we focus on mechanisms i) and ii) mentioned above. So, for a given coverage
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