Thickness and growth-condition dependence of \emph{in-situ} mobility and carrier density of epitaxial thin-film Bi${}_2$Se${}_3$

Bismuth selenide Bi${}_2$Se${}_3$ was grown by molecular beam epitaxy while carrier density and mobility were measured directly \emph{in situ} as a function of film thickness. Carrier density shows high interface n-doping (1.5 x 10${}^{13}$ cm${}^{-2}$) at the onset of film conduction, and bulk dopant density of $\sim$5 x 10${}^{18}$ cm${}^{-3}$, roughly independent of growth temperature profile. Mobility depends more strongly on the growth temperature and is related to the crystalline quality of the samples quantified by \emph{ex-situ} AFM measurements. These results indicate that Bi${}_2$Se${}_3$ as prepared by widely employed parameters is \emph{n}-doped before exposure to atmosphere, the doping is largely interfacial in origin, and dopants are not the limiting disorder in present Bi${}_2$Se${}_3$ films.