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Scanning Microscopy

Abstract

The Ge/Si heteroepitaxial system grows in the Stranski-Krastanov (layer + island) growth mode with an equilibrium intermediate layer thickness of 3 monolayers. Initially, coherent (dislocation free) islands form above the intermediate layer. These coherent islands relieve part of their lattice mismatch by elastic deformation of the layer + island + substrate system. After cluster growth, these islands may reach a critical radius above which it becomes energetically favorable for strain relief through the introduction of misfit dislocations. Particle size distributions generated from digitally acquired secondary electron images of Ge/Si(100) films grown in situ in an ultra-high scanning transmission electron microscope (UHV - STEM) have been used to study particle coarsening processes. By exploiting the large magnification range available it is possible to obtain reliable statistics for islands with radii ranging from ~2nm to over 500nm. These size distributions show that coherent islands significantly impact coarsening processes in this system. In all cases studied, the coherent islands compete less effectively for the diffusing adatoms and consequently grow much more slowly than the dislocated islands. This difference in the growth rate between coherent and dislocated islands is due to the extra energy required for increasing the strain field of growing coherent islands. The reduction of interfacial strain via the introduction of misfit dislocations is shown to relieve a substantial fraction of the elastic energy and lowers the energy cost per additional adatom in the growing cluster. Preliminary results of a continuum elasticity calculation which approximates coherent island growth confirm that elastic deformation of the substrate is a viable means of strain relief for these elastic systems.

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