Dislocation mechanisms of epitaxial strain relaxation in Ta/V multilayer films

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The relaxation of epitaxial elastic strains ε hkl generated at the nucleation stage of tantalum growth on the (110) plane of a vanadium substrate was studied as a function of tantalum film thickness h Ta by the X-ray and electron microscopy methods. The pseudomorphic state of the tantalum film was found to be broken down along the [1 1 0] and [002] crystallographic directions of the Ta(110)/V interface at different values of h Ta: 0.45 nm and 1.8 nm respectively. The relaxation of ε hkl results from the introduction of vacancy loops in the tantalum film. This process has a multistage character owing to the large value of the lattice misfit (8.7%) and the low symmetry of the (110) interface. As in the other b.c.c. metals, the relaxation of compressive strains ε hkl in tantalum films starts with the nucleation of imperfect vacancy loops with the Burgers vector b= 1 2 〈110〉 in the Ta{110} crystallographic planes. By shear reaction at some critical size R cr 1 the imperfect loops are transformed into perfect loops with b= 1 2 〈111〉 or b=〈001〉 which subsequently change their loop planes at R cr 2> R cr 1. The type of the perfect loop depends on the value and the sign of the residual epitaxial strain of the tantalum film.