  Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique
in which transition metal cations are delivered using a metal-organic
precursor, has emerged as the state-of-the-art approach to the synthesis of
electronic-grade complex oxide films with a stoichiometric growth window.
However, numerous questions remain regarding the chemical mechanisms of the
growth process and the surface properties of the resulting films. To examine
these properties, thin film SrTiO$_{3}$ (STO) was prepared by hMBE using a
titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr
source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux
ratios to examine the conditions for the reported stoichiometric growth window.
The films were transferred in vacuo to an x-ray photoelectron spectroscopy
system to study the surface elemental composition. Samples were examined using
x-ray diffraction to compare our surface sensitive results with previously
reported measurements of the bulk of the films in the literature. Ex situ
studies by atomic force microscopy, scanning tunneling microscopy and low
energy electron microscopy confirmed the presence of surface reconstructions
and an Ehrlich-Schwoebel barrier consistent with an A-site SrO termination. We
find that a surface exhibiting a mixture of SrO and TiO$_{2}$ termination, or a
full SrO termination is necessary to obtain stoichiometric
adsorption-controlled growth. These results indicate that surface Sr is
necessary to maintain chemical equilibrium for stoichiometric growth during the
hMBE process, which is important for the design of future interfacial systems
using this technique.
