Nucleation and stochiometry dependence of rutile-TiO$_{2}$ thin films grown by plasma-assisted molecular beam epitaxy

Considerable interest has been shown of late in transition-metal oxides. One case is the titanium dioxide system, which can have applications as a high-k dielectric gate insulator for Si-based devices\footnote{Z. J. Luo \textit{et al.}, Appl. Phys. Lett. \textbf{79}, 2803}. In this study, rutile-TiO$_{2}$ thin films were grown on GaN(0001) substrates by oxygen plasma-assisted molecular beam epitaxy. Two sets of films were grown, one in which the initial GaN surface is prepared WITH the pseudo 1$\times $1 Ga-rich surface reconstruction, and the other set, WITHOUT the pseudo 1$\times $1. On top of these two type of surfaces, the rutile-TiO$_{2}$ thin films were grown at T$_{s}\sim $ 600 $^{o}$C, and with a thickness $\sim $ 40 - 50 nm.~During growth, reflection high-energy electron diffraction indicated a reversible stoichiometry transition from O-rich to Ti-rich growth. Post-growth x-ray diffraction measurements performed on the samples WITHOUT the GaN pseudo 1$\times $1, show the presence of additional peaks at 2$\theta $ = 52.9$^{o}$, which implies the existence of additional phases. In addition, the high-resolution transmission electron microscopy performed on these samples show a high degree of disorder, as compared to the samples prepared WITH the pseudo 1$\times $1. Work supported by ONR.