Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation

Abstract

Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independentlyfrom the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscopeallows for local implantation and patterning down to the nanometer resolution, which is of interest for deviceapplications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally byhelium implantation. Our combined Raman, x-ray diffraction, and transmission electron microscopy (TEM)study shows that the implantation causes an elongation of the BiFeO3unit cell and ultimately a transition towardsthe so-called supertetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset ofamorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phasetransition from the R-like to T-like BiFeO3appears as first-order in character, with regions of phase coexistenceand abrupt changes in lattice parameters.