Growth modes of multiferroic BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ -- Real time X-ray synchrotron study

Real time X-ray scattering measurements during heteroepitaxial film deposition provide details of initial nucleation and growth, thus giving insight into atomic-scale processes and growth mechanisms. In this work the growth of pulsed laser deposited (PLD), and RF-magnetron sputter deposited multiferroic BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ substrates are studied using \textit{in-situ} X-ray specular and diffuse scattering.~ Both the out-of-plane (height distribution) and lateral (surface mounds and correlations) information is obtained from these measurements. Specular X-ray intensity monitored at (00 $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ shows unit-cell growth oscillations during the first few monolayers, while both diffuse scattering width and intensity oscillate out-of-phase with specular, characteristic of 2D layer-by-layer growth for PLD deposited films. The coarsening process is consistent with growth and coalescence of islands - identified as key fundamental processes in epitaxial growth. Beyond several monolayers oscillations decay and the diffuse intensity increases indicating rapid growth of mounds.~ However, the mounds themselves subsequently merge after a few more deposited layers, leaving arrays of mesas with some holes due to incomplete coalescence. This behavior was corroborated with ex-situ Atomic Force Microscopy (AFM) at each stage. Results for sputter deposited films showed unusually strong specular and diffuse oscillations; implications for interface structure and formation will be discussed.