Thickness Independent Surface Transport of Bi$_{2}$Se$_{3}$ on Al$_{2}$O$_{3}$(0001) Substrates

The key requirement for exploiting the newly emerging three-dimensional (3D) topological insulators (TI) as a novel platform for coherent spin-polarized electronics is TI thin films with dominant surface transport properties. So far, while researchers have been able to observe the existence of surface states locally \textit{in situ}, verification over a wide thickness range outside the growth chamber has not yet been reported. Here, we report large signature of surface transport in TI Bi$_{2}$Se$_{3}$ thin films. The Bi$_{2}$Se$_{3}$ films used for this study were grown on c-axis Al$_{2}$O$_{3}$ substrates with MBE. Hall-effect measurements in the standard Van der Pauw geometry provided clear evidence of two conducting channels for 4QL-2750QL thick samples, with the transport properties for one of the channel being thickness independent and the other varying with thickness. This thickness independent carrier density of $\sim $1.5 $\times $ 10$^{13}$ cm$^{-2}$ has been observed over the entire thickness range down to 2 QL, clearly suggesting that this is due to surface states. Furthermore, another surface transport property directly related to the topological protection mechanism, the weak-antilocalization (WAL) effect, exhibited similar thickness- and bulk-independent characteristics.