Quantum Dot Growth on Strained MEMS Structures

As we have established over the years, Ge grows on Si(001) in the Stranski-Krastanov mode, producing crystallographically pseudomorphic nanocrystals that act as quantum dots (QDs), because the lattice constant of Ge is larger than that of Si.  Presumably if we modify the Si lattice constant by externally applied stress, we will change the nucleation, growth, and coarsening behavior of the Ge QDs, and possibly also the diffusion of Ge.  We have approached this problem by constructing MEMS devices that can serve as nano-tensile bars.  An example is shown in Fig. 1a below.  The tensile force is produced by a deposited film of Si3N4.  We have discovered a very interesting stress-temperature relationship in the nitride that also depends on how the nitride is formed.  We therefore have a range of accessible stresses, up to breaking stress for Si (corresponding to about 4% extension).  As the lattice constant of Ge is ~4% larger than that of Si, we ought to be able nearly to lattice match Ge.  As Fig. 1a shows, we fabricate (among many other shapes) a notched bar to concentrate stresses.  We then analyze the density of the Ge QDs that form after deposition of Ge.  The QD density is least where the tensile stress is greatest.  We are analyzing these results in terms of diffusion and nucleation in a stress field.  We have filed a patent disclosure on the MEMS devices and their application for these purposes. We expect that such devices will in the future serve as nanomechanical test instruments in, e.g., TEMs or x-ray microscopes.  We are planning to use them in a LEEM to investigate growth on stressed membranes in real time.
 

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