Optical recognition of structural and electronic transformation of Pb ultrathin films

In this study, the transformation of ultrathin Pb film on a Si(111)-(7 × 7) surface from amorphous-like to crystalline state is examined at low temperatures during the film growth by measuring the differential reflectance (DR) DgrR/R, as a function of film thickness. The experiments were performed under ultrahigh vacuum conditions in the visible and infrared spectral range. The determined imaginary part of the Pb dielectric function changes steeply at the critical thickness of the film of about 5 monoatomic Pb(111) layers. High energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM) studies revealed a film with fine granular morphology before the structural transition and a well ordered epitaxial layer after the transformation. The variation of the optical (AC) conductivity was compared with DC electrical conductance data. We show that the DR technique can be efficiently used to characterize structural and electronic changes in ultrathin metallic films.