Misfit relaxation of the AlN/Al2O3 (0001) interface - art. no. 195329

The epitaxial interface formed by a thin buffer layer of AlN deposited by m olecular beam epitaxy on (0001) Al2O3 is investigated using electron micros copy techniques. Plan-view observations display a two-dimensional translati onal moire pattern resulting from the difference in lattice parameters betw een the two crystals. The effective misfit of -0.1091 suggests the presence of a network of 60 degrees misfit interfacial dislocations. These are, mos t of the time, introduced every 8 atomic planes of the AlN lattice or 9 ato mic planes of the Al2O3 lattice, which is directly verified by cross-sectio n high-resolution electron microscopy (HREM). The density of threading disl ocations terminating at the buffer/substrate interface is in agreement with the density of threading dislocations near the surface of the GaN film. Th is in conjunction with terminating fringes observed in the moire patterns p rovides strong evidence that threading dislocations are connected with the interfacial misfit dislocations. Plan-view HREM images reveal that a thread ing 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 eq uals the sum of the Burgers vectors of the two misfit dislocations and ther efore is of the a type. Although misfit dislocations relax the larger fract ion of the misfit strain, the AlN buffer layer is still under a compressive residual strain (- 8.2 x 10(-3)), since the effective misfit is smaller th an the natural misfit between the two lattices.