  Bulk Bi2Te3 is known to be a topological insulator. We investigate surface
states of Bi2Te3(111) thin films using density-functional theory including
spin-orbit coupling. We construct a method to unambiguously identify surface
states of thin film topological insulators. Applying this method for one to six
quintuple layers of Bi2Te3, we find that the topological nature of the surface
states remains robust with the film thickness and that the films of three or
more quintuple layers have topologically non-trivial or protected surface
states, in agreement with recent experiments.
