Investigation of Early Nucleation Events in Magnesium Oxide During Ion Beam Assisted Deposition

Abstract

Ion beam assisted deposition (IBAD) is used to biaxially texture magnesium oxide (MgO), which is useful as a template for the heteroepitaxial growth of various thin film devices and most notably as a template layer for high temperature superconductors. Improvements in the quality of IBAD MgO films have been largely empirical and there is uncertainty as to the exact mechanism by which this biaxial texture is developed. Using a specially built quartz crystal microbalance (QCM) as both a substrate and monitor in conjunction with reflected high-energy electron diffraction (RHEED) acting on the same surface, we have probed the initial stages of IBAD MgO growth in-situ. We have correlated corresponding RHEED images with real-time mass accumulation QCM data during the film growth. During IBAD growth, the mass accumulation exhibits a sharp change in slope corresponding to a sudden decrease in growth rate. Corresponding RHEED images show an abrupt onset of crystallographic texture at this point. A simple model incorporating differential etch rates of the MgO film and silicon nitride substrate can be used to fit the data but is inconsistent with the behavior during ion etching with no growth. It is, therefore, postulated that a more complex mechanism is responsible for the observed behavior.

IBAD-MgO technology for coated conductors

“…Simple cubic MgO, of which the channeling angle is 45 degree, has risen as a substitute candidate for YSZ, since its growth rate is much faster and only a few nanometer of layer is enough to get 10 or less in-plane azimuthal spread in the IBAD process. When Hammond et al of Stanford University developed this elegant technology [3,7,8], the underlying texturing mechanism was still lacking. However, the method immediately attracted attention of the whole CC community.…”

“…One of the striking results in IBAD-MgO is that as the thickness of the layer increases, the in-plane alignment is tilted, which is not true in the case of IBAD-YSZ. It is not yet clear whether the initial growth of MgO ultra-thin grains allows texturing, or if the amorphous MgO layers form during early nucleation [7,8]. In addition, it should be determined if globally-occurring biaxial texturing is a function of the thickness of IBAD-MgO.…”