Quantum well states in ultrathin Bi films: Angle-resolved photoemission spectroscopy and first-principles calculations study

Quantum well states (QWSs) in ultrathin Bi(001) films grown on Si(111)-7 x 7 with thicknesses up to several tens of nanometers were studied by angle-resolved photoemission spectroscopy and first-principles calculations. We observed QWSs at various points in k-space; those located near (Gamma) over bar are very difficult to distinguish while the QWS peaks at off-normal emission (M) over bar are clearly resolved and show highly anisotropic features due to the saddle-point-like band dispersion near the Fermi level of bulk Bi along the L-X direction. The features of the QWSs are well-reproduced by ab initio calculations for free-standing Bi slabs. The standard method of the phase-shift accumulation model is applied to the QWSs and the bulk band dispersion perpendicular to the surface at finite parallel momentum is experimentally obtained for the first time. The phase shifts at the film interfaces are discussed in detail. The QWSs have little contribution to the electronic structure near the Fermi level and this suggests that the macroscopic physical properties of the films in the thickness of several atomic layers are likely determined by the highly metallic surface states.