Misfit Dislocations in Epitaxial Ni/Cu Bilayer and Cu/Ni/Cu Trilayer Thin Films
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Misfit Dislocations in Epitaxial Ni/Cu Bilayer and Cu/Ni/Cu Trilayer Thin Films Tadashi Yamamoto, Amit Misra, Richard G. Hoagland, Mike Nastasi, Harriet Kung, and John P. Hirth MST-8, Materials Science and Technology Division Los Alamos National Laboratory, Los Alamos, NM ABSTRACT Misfit dislocations at the interfaces of bilayer (Ni/Cu) and trilayer (Cu/Ni/Cu) thin films were examined by plan-view transmission electron microscopy (TEM). In the bilayers, the spacing of misfit dislocations was measured as a function of Ni layer thickness. The critical thickness, at which misfit dislocations start to appear with the loss of coherency, was found to be between 2 and 5 nm. The spacing of the misfit dislocations decreased with increasing Ni layer thickness and reached a plateau at the thickness of 30 nm. The minimum spacing is observed to be about 20 nm. A g·b analysis of the cross-grid of misfit dislocations revealed 90° Lomer dislocations of the {001} type lying in the (001) interface plane at a relatively large thickness of the Ni layer, but 60° glide dislocations of the {111} type at a relatively small thickness of the Ni layer. In the trilayers, misfit dislocations formed at both interfaces. The spacing of the misfit dislocation is in agreement with that of the bilayers with a similar Ni layer thickness. The misfit dislocation arrays at the two interfaces, having the same line directions, are 60° dislocations with edge components with opposite signs but are displaced with respect to each other in the two different interface planes. This suggests that interactions of the strain fields of the dislocations have a strong influence on their positions at the interface. INTRODUCTION Misfit dislocations are known to form at the interface to relieve the elastic strains due to lattice mismatch in heteroepitaxial films [1-7]. In the early stages of the overgrowth, the misfit is entirely accommodated by coherency strains, where two lattice planes are strained in one-to-one registry. As the overgrowth thickness increases, the misfit dislocations start forming at the critical thickness, where the introduction of misfit dislocations lowers the total energy of the system arising from elastic strain and dislocations [1, 2]. Analyses of critical thickness and strain field have been the major subject of both theoretical and experimental studies, where most detailed analyses have been done for semiconductor bilayer couples [2-7]. Once the dislocations form as the thickness exceeds the critical thickness, the dislocation spacing, or strain complementary to the spacing, decreases with either increasing thickness or increasing lattice mismatch [4, 5]. As a function of character, {111} type 60° dislocations are generally observed at relatively small thickness or small lattice mismatch. As the thickness or lattice mismatch increases, the fraction of 60° dislocations decreases and 90° dislocations of the {001} type predominantly form [4, 5]. The 90° dislocation is thought to form by the reaction of two 60° dislocations at the interfaces [6, 8]
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