Role of Quantum and Surface-State Effects in the Bulk Fermi-Level Position of Ultrathin Bi Films.

Department of Physics, Ochanomizu University, Tokyo 112-8610, Japan(Received 13 February 2015; revised manuscript received 2 April 2015; published 3 September 2015)We performed high-resolution photon-energy and polarization-dependent ARPES measurements onultrathin Bi(111) films [6–180 bilayers (BL), 2.5–70 nm thick] formed on Si(111). In addition to theextensively studied surface states (SSs), the edge of the bulk valence band was clearly measured by usingS-polarized light. We found direct evidence that this valence band edge, which forms a hole pocket in thebulk Bi crystal, does not cross the Fermi level for the 180 BL thick film. This is consistent with thepredicted semimetal-to-semiconductor transition due to the quantum-size effect [V.B. Sandomirskii, Sov.Phys. JETP 25, 101 (1967)]. However, it became metallic again when the film thickness was decreased(below 30 BL). A plausible explanation for this phenomenon is the modification of the charge neutralitycondition due to the size effect of the SSs.