Misfit relaxation of the AlN/Al2O3 (0001) interface
The epitaxial interface formed by a thin buffer layer of AlN deposited by molecular beam epitaxy on (0001)Al2O3 is investigated using electron microscopy techniques. Plan-view observations display a two-dimensional translational moire pattern resulting from the difference in lattice parameters between the two crystals. The effective misfit of -0.1091 suggests the presence of a network of 60° misfit interfacial dislocations. These are,most of the time, introduced every 8 atomic planes of the AlN lattice or 9 atomic planes of the Al2O3 lattice,which is directly verified by cross-section high-resolution electron microscopy (HRTEM). The density of threading dislocations terminating at the buffer/substrate interface is in agreement with the density of threading dislocations near the surface of the GaN film. This in conjunction with terminating fringes observed in the moire patterns provides strong evidence that threading dislocations are connected with the interfacial misfit dislocations. Plan-view HRTEM images reveal that a threading dislocation is directly related to two extra prismatic AlN half-planes, the missing parts of which are associated with two misfit dislocations in the interfacial network. The Burgers vector of the threading dislocation equals the sum of the Burgers vectors of the two misfit dislocations and therefore is of the a type. Although misfit dislocations relax the larger fraction of the misfit strain, the AlN buffer layer is still under a compressive residual strain (-8.2X10-3), since the effective misfit is smaller than the natural misfit between the two lattices
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