Biaxial texture development in the ion beam assisted deposition of magnesium oxide

Low-energy ion-beam irradiation (<1 keV) during the concurrent deposition of cubic oxide materials results in the growth of crystallographically textured thin films. A model system, magnesium oxide (MgO), has been successfully used as a biaxially textured template film for the heteroepitaxial deposition of many materials with texture dependent properties like high temperature superconductors, tunable microwave materials, and ferroelectrics. Here, we present data on the initial nucleation of biaxial crystallographic texture in this model system using an in situ quartz crystal microbalance (QCM) substrate combined with in situ reflected high-energy electron diffraction (RHEED). Correlation of mass uptake with the RHEED images of the growing surface shows that the development of crystallographic biaxial texture in this material system occurs suddenly as the initially polycrystalline MgO films reaches a critical film thickness of 2 nm. This texture continues to improve during subsequent growth. A simple model shows that the effect is not due simply to coverage effects. We use a combination of in situ RHEED and ex situ transmission electron microscopy to further elucidate the mechanism of this sudden texture formation. We present a physical model to describe this behavior and explain the role of ion-to-atom arrival ratio and underlying nucleation surface on texture development.