Optical recognition of structural and electronic transformation of Pb ultrathinfilms

Abstract

In this study, the transformation of ultrathin Pb film on aSi(111)-(7 × 7) surface from amorphous-like to crystalline state is examined at low temperaturesduring the film growth by measuring the differential reflectance (DR)ΔR/R, as a function of film thickness. The experiments were performed under ultrahigh vacuumconditions in the visible and infrared spectral range. The determined imaginary part of thePb dielectric function changes steeply at the critical thickness of the film of about5 monoatomic Pb(111) layers. High energy electron diffraction (RHEED) andscanning tunnelling microscopy (STM) studies revealed a film with fine granularmorphology before the structural transition and a well ordered epitaxial layer after thetransformation. The variation of the optical (AC) conductivity was compared with DCelectrical conductance data. We show that the DR technique can be efficientlyused to characterize structural and electronic changes in ultrathin metallic films.