In situ measurement of thickness dependent electrical resistance of ultrathin Co films on $SiO2/Si(111)$ substrate

Ultrathin Co films of thicknesses from 1 to 19 monolayers (ML) were grown on native oxide covered Si(111) surfaces near room temperature by thermal evaporations in ultrahigh vacuum. The thickness dependent total electrical sheet resistance $R (Ω/□)$ at room temperature was measured in situ by a four-point probe. The total sheet resistance as a function of thickness first increased and then decreased suggesting that the Co film morphology changed from discontinuous islands of 1 or 2 ML thick to continuous films for thickness $d>3$ ML. The sheet resistance $RCo$ of Co was extracted from the total sheet resistance assuming that the Co and $SiO2/Si$ are two parallel resistors. The value of $RCo$ gradually levels off to 31.5 (Ω/□) which corresponds to resistivity of 10.5 μΩ cm at ∼ 19 ML. The thickness dependent resistivity was fitted by models including surface, interface, grain boundary scatterings, and surface roughness. The surface roughness obtained from atomic force microscopy is consistent with that used in Namba's model to improve the fit of resistivity in a few ML thick regime.