Ultrathin Pb Films Research Articles

Conductivity of ultrathin Pb films during growth on Si(111) at low temperatures

The electronic properties of thin metallic films strongly depend on their structure. For the preparation of ultrathin films (0.8 to 12 ML) Pb was grown on a Si (111)-7×7 structure at temperatures below 25 K. Percolation as measured by DC conductance starts at 0.7 ML. Up to 4 ML the growing film is disordered. During continuation of the growth the film recrystallizes epitaxially and layer-by-layer growth connected with oscillations of conductivity and LEED intensities is observed even at 25 K. The defect structure as determined by SPA-LEED and the quantum size effect allow a quantitative description of the conductivity oscillations. The experiments show, that the electronic properties of ultrathin Pb films are clearly correlated with the structural properties of the film.

Investigation on morphological stability of ultrathin Pb films

Ultrathin Pb films supported by Al film were made by using cold-rolling and ion-beam thinning techniques. The morphological instability of the Pb film under electron-beam irradiation was investigated by means of in situ transmission electron microscopy observations. It has been found that under electron-beam irradiation, Pb films with an incoherent Pb/Al interface spheroidized into Pb particles, but those with a semicoherent Pb/Al interface were stable in morphology. The morphological stability of thin films depends on the microstructure and the thermodynamic property of the interphase boundary. A critical interfacial energy for the spheroidization of thin films was determined based on a thermodynamics analysis.

Order and disorder in ultrathin Pb films grown on Si(1 1 1) 7×7 substrates at low temperatures

The growth of Pb on Si is of special interest due to the formation of metallic monolayers on a substrate, which is insulating at low temperatures. Since the knowledge of structural details is decisive for the interpretation of electronic properties, many studies have been made, which unfortunately did not provide a consistent description. Especially studies at liquid He temperatures are needed for the interpretation of electronic studies in this temperature range. For that purpose Pb has been deposited onto Si(1 1 1)7×7 at 25 K. During deposition, annealing and after cooling the structure and especially the defect structure has been carefully studied using high resolution low-energy electron diffraction for spot profile analysis (SPA-LEED). Several unexpected features show the special situation of ultrathin Pb films. The first three monolayers are heavily disordered, so that no LEED pattern is observed in the full energy range of LEED. Nevertheless oscillations of the LEED intensities are observed for coverages θ>4 ML. Therefore, a recrystallization to an epitaxial structure occurs even at 25 K. Rotational disorder is derived from an azimuthal broadening of the Pb diffraction spots. Improved epitaxial quality films with larger domains and no azimuthal broadening are observed, when at first 2D islands are formed due to annealing at room temperature and further deposition is performed at low temperatures.

Electronic transport in ultrathin epitaxial Pb films on Si(111) surfaces

Epitaxial metallic films are candidates for two-dimensional (2D) electronic systems. In that respect, the DC conductivity is a good tool to check the electronic structure for quantum phenomena. The annealed Pb films on the Si(111) 7×7 and on the Si(111) 3 × 3 Pb surfaces show a very contrary behavior to that of classical metals. Whereas at 8 K and a coverage above 6 monolayers (ML) the conductivity is well described by the classical Drude model with the mean free path in the order of the film thickness, at low coverages, a metal–insulator transition (MIT) is observed. The conductivity shows different regimes pointing to strong localization, weak localization or classical behavior depending on film thickness, annealing and measuring temperature. Structural changes modify directly the conductivity. Superconductivity with the critical temperature close to the bulk value is observed in annealed films with a thicknesses down to 4 ML. Resistivity oscillations during the growth of the film on Si(111)- 3 × 3 Pb with 1 ML and 2 ML period are reported.

Infrared Studies of the Onset of Conductivity in Ultrathin Pb Films

In this paper we report the first experimental measurement of the infrared conductivity of ultra-thin quenched-condensed Pb films. For dc sheet resistances such that $\omega \tau \ll 1$ the ac conductance increases with frequency but is in disagreement with the predictions of weak localization. We attribute this behavior to the effects of an inhomogeneous granular structure of these films, which is manifested at the very small probing scale of infrared measurements. Our data are consistent with predictions of two-dimensional percolation theory.

Trajectory-dependent neutralization of 1 keV He + ions scattered from Pb(111) and Pb films on Cu(100)

We observe trajectory-dependent neutralization of 1 keV He + ions scattered from the Pb(111) surface. Varying the azimuthal angle of the ion trajectories we observe neutralization by charge densities of the next and second next neighbors of the target atom occurring at polar angles between 40 and 15°. This corresponds to ion–neighbor distances between 2.3 and 1 Å. We adjust a three-parameter neutralization model for Pb to fit the trajectory dependence of the survival probability for the well known Pb(111) surface geometry. Good agreement with experimental results is found only if a shell-like neutralization region is centered at a radius of 1.46 Å. In regard to the spatial distribution of the neutralization rate we consider the Pb 6sp electrons to be the source of the long distance neutralization that leads to the trajectory dependence. The same neutralization model is then applied for a structural analysis of ultrathin Pb films evaporated on Cu(100), where we confirm the existence of a Pb–Cu surface alloy on the c(4×4) Pb/Cu(100) and the existence of a Pb overlayer on the c(2×2) Pb/Cu(100). It turns out that the neutralization model developed for Pb atoms embedded in the (111) surface holds also for adsorbed Pb atoms.

Formation of polycrystalline structure in metallic films in the early stages of Zone I growth

The morphology of ultrathin Au and Pb films vapor deposited onto cryogenically cooled substrates has been studied by in situ Scanning Tunneling Microscopy (STM). At low substrate temperatures, 4< T S<77 K, where thermally activated grain growth processes are negligible, i.e. in the Zone I regime, films with equivalent bulk mass deposited thicknesses, d≥0.9 nm uniformly cover the substrate and exhibit no discernible crystalline structure for d≤1.6 nm for T S=77 K. Thicker films exist in a polycrystalline phase with a uniform grain size. It is argued that the observed polycrystalline structures do not form through normal thin film nucleation and growth processes. Instead, they form from a transient amorphous phase through athermal crystallization processes after a critical film thickness is reached. Results arealso presented from annealing studies that support this view.

Photoemission study of the transition from the insulating to metallic state in ultrathin layers

The transition from the insulating to metallic state in ultrathin Pb films was studied using high-resolution photoemission. The states near ${\mathrm{E}}_{\mathrm{F}}$ were probed as Pb was deposited onto a thin insulating layer, thin enough to reduce macroscopic charging. The results, which experimentally show a vanishing of states at ${\mathrm{E}}_{\mathrm{F}}$, are consistent with the arguments of Efros and Shklovskii based on electron-electron interactions, and as far as we know are the first in situ study of the electronic properties as one goes from the insulating to metallic state in a two-dimensional system.

Quantized Hall effect in ultrathin metallic films.

The Hall coefficient ${R}_{H}$ of ultrathin epitaxial Pb films is determined experimentally. A comparison with electrical conductivity data leads to the conclusion that the investigated Pb layers behave like a size-quantized metal. Pronounced variations of ${R}_{H}$ with the film thickness were found. The observed reversal of the sign of ${R}_{H}$ is discussed within the available theory of the quantum size effect describing the galvanomagnetic properties of metals. We find that the observed phenomenon cannot be explained by the free electron model of a quantized layer.

Superconductor-insulator transition in ultrathin Pb films: Localization and superconducting coherence.

We have measured temperature-dependent resistivities of a set of Pb ultrathin films by repeating in situ deposition at liquid-He temperature. Observed conductance in the fermion region can be described by a single-parameter scaling function ($\ensuremath{\beta}$ function) with a self-consistent theory for localization in two dimensions. Based on estimated values of electrical localization length (${\ensuremath{\xi}}_{\mathrm{loc}}$) and superconducting coherence length (${\ensuremath{\xi}}_{\mathrm{super}}$), the superconductor-insulator transition is confirmed to occur at the point where ${\ensuremath{\xi}}_{\mathrm{loc}}\ensuremath{\sim}2{\ensuremath{\xi}}_{\mathrm{super}}$. Moreover, only samples exhibiting $\mathrm{ln}T$ dependence in the fermion region become global coherent superconductors at low temperature.

Bose glass phase in ultrathin Pb films

We have observed the superconductor-insulator transition in two dimensional Pb films. The films are made by repeatingin-situ deposition at liquid He temperature. We have measured temperature-dependent resistivities and confirmed observed conductance in the fermion region can be described by a single parameter scaling function (β-function) with a self-consistent theory for localization in two dimensions. We estimated values of electrical localization length (ξsuper), and superconducting coherence length (ξloc), and discovered the superconductor-insulator transition occur at the point where ξloc ∼ 2ξsuper. We confirmed that the films in the region of ξsuper < ξloc < 2ξsuper are not global coherent superconductors. We suggested that the films in this region areBose glass at the low temperatures.

Pb layer-by-layer growth at very low temperatures.

We have studied the growth of ultrathin Pb films on Si(111)-(6\ifmmode\times\else\texttimes\fi{}6)Au surfaces at very low temperatures by reflection high-energy electron diffraction (RHEED) and resistivity measurements. Pronounced RHEED specular beam intensity oscillations were recorded at temperatures as low as 16 K. The domain size on the surface of the growing Pb film was determined from the width of RHEED streaks. The electrical resistivity data were analyzed using the quantum size effect theory of Trivedi and Ashcroft. The mean free path of the Pb conduction electrons was determined and compared with the size of the Pb surface domains and with the size of the (6\ifmmode\times\else\texttimes\fi{}6) Au substrate domains. The role of morphology and electronic structure of the Si(111)-(6\ifmmode\times\else\texttimes\fi{}6)Au surface and of transient mobility during the nucleation of the growing Pb film is discussed.

Perpendicular upper critical field of granular Pb films near the superconductor-to-insulator transition.

We have measured the perpendicular upper critical field, ${\mathit{H}}_{\mathit{c}2}$(T), in ultrathin granular Pb films with normal-state sheet resistances, ${\mathit{R}}_{\mathit{N}}$, that range from 10 \ensuremath{\Omega} to 11 k\ensuremath{\Omega}. The films with the highest ${\mathit{R}}_{\mathit{N}}$ are near the threshold for the superconductor-to-insulator (S-I) transition. We compare our results to calculations for granular superconductors and obtain values for the grain size and coherence length characteristic of a single grain. In addition, we present a measurement of the ratio of the intergrain Josephson-coupling energy to the single-grain condensation energy near the S-I transition in these films. The small value of this ratio, 0.05, at ${\mathit{R}}_{\mathit{N}}$=11 k\ensuremath{\Omega} is consistent with the picture that fluctuations in the phase of the order parameter dominate the S-I transition in these films.

Photoemission from ultrathin metallic films: Quantum size effect, electron scattering, and film structure.

The quantum-size-effect structure of ultrathin Pb and Pb-In alloy films on Si(111) surfaces is studied by photoemission spectroscopy in the thickness range from 0 to about 30 monolayers and analyzed, taking into account electron scattering and film structure, which is characterized by high-energy-reflection-electron diffraction.

Experimental evidence for quantum-size-effect fine structures in the resistivity of ultrathin Pb and Pb-In films.

The resistivity of ultrathin single-crystalline Pb and Pb-In layers with thicknesses d smaller than the bulk mean free path l, is measured during deposition onto Si(111)-(6\ifmmode\times\else\texttimes\fi{}6)Au surfaces at about 110 K. The structure of the layers is monitored by reflection high-energy electron diffraction (RHEED). Oscillations of the RHEED specular beam intensity are highly correlated with fine structures of the resistivity. The quantum-size-effect theory is used for a quantitative analysis of the data. The fine structure, volume impurities, small-scale roughness, and large-scale thickness fluctuations are taken into account. The impact of the layer-by-layer growth mode of ultrathin metal films on the thickness dependence of the resistivity is discussed.

Superconductivity and the electronic density of states in disordered two-dimensional metals

We have measured the superconducting and normal-state properties of ultrathin quench-condensed Pb and Sn films by electron tunneling and transport. The normal-state measurements illustrate a substantial reduction in the electronic density of states about the Fermi surface. Good agreement with theory is obtained for this reduction. The collapse of {ital T}{sub {ital c}} can be accounted for by the reduced density of states but some problems stand in the way of a full description of the superconducting behavior.

Superconducting Transition of Quench-Condensed Ultrathin Films

The condition of the onset of superconductivity in highly homogeneous ultrathin Bi and Pb films has been investigated by measuring the temperature dependence of the sheet resistance R□. The experimental results suggest that highly homogeneous metal films with R□ lower than ∼6 k Ω could be superconducting, while those with R□ greater than ∼6 k Ω are insulating at zero temperature, in agreement with the condition of the onset of global superconductivity in granular films. The superconducting transition temperature Tc shows a remarkable decrease with increasing the normal-state sheet resistance, unlike the results of granular films.

Superconductivity in ultrathin Pb films deposited on silicon

Saperconducting Pb films 11 to 200 \AA{} thick have been deposited in ultrahigh vacuum on the clean (111) surface of crystalline silicon. Low-energy-electron diffraction indicated that the Pb preferentially grows with the Pb (111) plane parallel to the substrate surface. Measurements of normal-state film resistivity and of the superconducting transition temperature ${T}_{c}$ were taken in situ. The dependence of ${T}_{c}$ on film thickness was essentially identical to that previously measured for Pb films grown epitaxially on crystalline PbTe. This result is discussed in view of the dissimilarities between the two substrate materials.