EFFECT OF BISMUTH DOPING ON THERMAL-EXPANSION AND MISFIT DISLOCATIONS IN EPITAXIAL IRON GARNETS

The growth of high quality, bismuth-substituted, rare earth iron garnet epitaxial films on rare earth gallium garnet substrates is limited by the effects of stress between the film and substrate. Thick films in particular have a low tolerance for lattice mismatch at room temperature, so the film and substrate must be closely matched to prevent cracking. However, there is a difference in thermal expansion between rare earth iron garnets and rare earth gallium garnets that is increased by bismuth substitution in the epitaxial film. This extrapolates to a thermal expansion for pure bismuth iron garnet of 12.9 x 10(-6)-degrees-C-1, which is 37% greater than that of rare earth gallium garnets and 26% greater than that of pure rare earth iron garnets. Consequently the thermal expansion mismatch at the growth temperature is increased, which requires that, to maintain a lattice match at room temperature, the films must be grown highly in compression. The critical misfit strain required to form misfit dislocations during film growth also decreases with increasing bismuth content and extrapolates to zero at a bismuth concentration of 2.6 atoms per formula unit. These effects combine at the film growth temperature of 800-degrees-C to create a limiting value of the bismuth content, beyond which it is difficult or impossible to grow high quality dislocation-free epitaxial films.