Pb Thin Films Research Articles

Imaging the Electron-Phonon Interaction at the Atomic Scale

Thin Pb films epitaxially grown on 7×7 reconstructed Si(111) represent an ideal model system for studying the electron-phonon interaction at the metal-insulator interface. For this system, using a combination of scanning tunneling microscopy and inelastic electron tunneling spectroscopy, we performed direct real-space imaging of the electron-phonon coupling parameter. We found that λ increases when the electron scattering at the Pb/Si(111) interface is diffuse and decreases when the electron scattering is specular. We show that the effect is driven by transverse redistribution of the electron density inside a quantum well.

Distribution of pyrochlore phase in Pb(Mg1/3Nb2/3)O3-PbTiO3 films and suppression with a Pb(Zr0.52Ti0.48)O3 interfacial layer

Thin films of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) on Pt/Ti/SiO2/Si (Pt/Si) substrates both with and without a Pb(Zr0.52Ti0.48)O3 (PZT) interfacial layer were investigated. Perovskite and pyrochlore coexistence was observed for PMN-PT thin films without a PZT interfacial layer. Interestingly, most of the pyrochlore phase was observed in single-coated films and in the first layer of multi-coated films. The pyrochlore phase exhibited grains with an average size of about 25nm, which is smaller than those of the perovskite phase (about 90nm). In contrast, for PMN-PT thin films grown on a PZT interfacial layer, the formation of a pyrochlore phase at the interface between PMN-PT layers and the substrate is completely suppressed. Moreover, small grains are not observed in the films with a PZT interfacial layer. The measured polarization-electric field (P-E) hysteresis loops of PMN-PT films with and without PZT layers indicate that enhanced electrical properties can be obtained when a PZT interfacial layer is used. These enhanced properties include an increase in the value of remanent polarization Pr from 2.7 to 5.8μC/cm2 and a decrease in the coercive field Ec from 60.5 to 28.0kV/cm.

Superconductivity of Nanostructured Pb<SUB>7</SUB>Bi<SUB>3</SUB> Films Doped by Ce

By means of electrochemical deposition from electrolytes, containing salts of Pb and Bi (0.03 mol/l and 0.02 mol/l respectively) thin films of intermetallic Pb7Bi3 have been fabricated. The superconducting transition temperature of the films was measured to be around 7.8 K. The deposition of the films with thickness of 50-100 nm was performed via passing rectangular current pulses with given amplitude and length. It was shown that adding salt of Ce into the electrolyte leads to a significant growth of the Tc for the deposited films reaching its maximum at the salt concentration of 0.06 mol/I. X-ray analysis data revealed the single phase of Pb7Bi3 films with hexagonal structure (SG) having a textures parallel to (101) plane. The morphology of the film surface is characterized by nanocluster structure with typical grain size around 70-80 nm. For the films, fabricated with adding salt of Ce, together with the intermetallic phase of Pb7Bi3, the second phase containing Bi is detected. At the same time, the typical grain size is reduced to 20-30 nm. Additionally, the suppression of the superconductivity in the grown films is investigated. The influence of the composition and structure on the superconducting critical temperature is discussed for both types of the fabricated films.

Modulation of Step Heights of Thin Pb Films by the Quantum Size Effect Observed by Non-Contact Atomic Force Microscopy

Using a home-made Q-plus sensor, simultaneous scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements were performed on the wedge-shaped Pb islands grown on Si(111)-7 × 7. Atomic resolved AFM images were observed. The contrast of AFM topography shows no dependence on the sample bias (tip is grounded), while the simultaneously obtained tunneling current image exhibits strong bias dependence due to quantum well states (QWS). Furthermore, In the AFM mode, neighboring Pb films with one monolayer (ML) thickness difference within the same Pb island show the same apparent height, which means that the apparent step heights of Pb films oscillate with a bilayer periodicity, being consistent with previous observations by helium atom scattering, x-ray diffraction, and STM. The possible reasons underlying the oscillation of apparent step heights in AFM topography are discussed.

Structure and morphology of laser-ablated Pb thin films

The growth of polycrystalline Pb thin films on Si substrates at room temperature by near infrared laser ablation of pure Pb target is presented. Several diagnostic techniques were used to deduce the structure, the morphology and the thickness of the deposited films. We investigated the above properties as a function of laser fluence (1–8J/cm2). Detailed morphological studies were also carried out on the irradiated target surface to deduce the laser ablation process. According to our results, the increase of laser fluence improves the crystallinity of the deposited films but not the deposition rate and the droplet density. The measured deposition rate was extremely low and varied between 0.04 and 0.09Å per pulse. The final goal of this research activity is the realization of photocathodes based on Pb-thin-film to be used in superconducting Nb radio-frequency cavity.

Optical and photoconductive properties of Pb 0.9Sn 0.1Se nano-structured thin films deposited by thermal vacuum evaporation and pulsed laser deposition

In this work, thin films of Pb 0.9Sn 0.1Se were deposited by thermal vacuum evaporation and pulsed laser deposition techniques. XRD measurements reveal a cubic crystalline structure for the prepared powder and deposited films. SEM micrographs show that the films consist of fine grains of size in the nanoscale. EDX analysis confirms the compositional analysis of both powder and films. Both the optical and photoconductive properties of those films were studied for comparison. Free carrier absorption was observed in the thermally evaporated films which were reduced by vacuum annealing at 773 K for 1 h under as a result of removing defects. The optical band gap of the thermally evaporated films was also reduced by annealing which was explained by removal of a Burstein–Moss shift. A strong increase of band gap of films deposited by pulsed laser technique was also abserved while it was explained by enegy gap confinment effect associated with nanostructures of semiconducting materials. Photoconductivity measurment shows that the photosensitivity of the pulsed laser deposited films is higher than those of the thermally deposited films by five order of magnitude. While the photoconduction process of thermally evaporated films is slower than that in pulsed laser deposited films.

Manipulating the Rashba-type spin splitting and spin texture of Pb quantum well states

Using spin- and angle-resolved photoemission spectroscopy we show that the spin splitting and the spin texture of quantum well states in thin Pb films can be manipulated through changes in the interface to the substrate. Compared with films grown on the Pb reconstructed Si(111) substrate the Bi interface reduces the spin splitting of the Pb states by a factor of 2 and the spin polarization vector is rotated by 32${}^{\ensuremath{\circ}}$ out of the sample plane. The spin splitting on an Ag reconstructed substrate is below the experimental resolution. Based on a model supported by ab initio calculations we interpret these changes as due to a modification of the charge density distribution in the quantum well states.

Atomic scale control of catalytic process in oxidation of Pb thin films

Using scanning tunneling microscopy (STM), we demonstrate that oxidation on Pb films is greatly enhanced by atomic Cs catalysts. With only a minute concentration of isolated Cs substitutional atoms in the surface layer (0.004ML coverage), surface oxidation rates are greatly enhanced. First-principles density functional theory (DFT) calculations reveal that a substitutional Cs atom strongly increases O2 binding on the surface. Then, with additional oxygen exposure this substitutional Cs-initiated oxidation process results in growth of PbO layers in an auto-catalytic manner. Furthermore, we investigate the role of temperature in the oxidation of the Pb films with and without Cs, and we explore the overall morphology of the resultant oxide layers.

Interface effects on the quantum well states of Pb thin films

Using scanning tunneling spectroscopy, we have studied the interface effect on quantum well states of Pb thin films grown on various metal-terminated (Pb, Ag, and Au) n-type Si(111) surfaces and on two different p-type Si(111) surfaces. The dispersion relation E(k) of the electrons of the Pb film and the phase shift at the substrate interface were determined by applying the quantization rule to the measured energy positions of the quantum well states. Characteristic features in the phase shift versus energy curves were identified and were correlated to the directional conduction band of the silicon substrate and to the Schottky barrier formed between the metal film and the semiconductor. A model involving the band structure of the substrate, the Schottky barrier, and the effective thickness of the interface was introduced to qualitatively but comprehensively explain all the observed features of the phase shift at the substrate interface. Our physical understanding of the phase shift is critically important for using interface modification to control the quantum well states.

Relaxation in the photoelectric and photovoltaic responses of thin-film Pb(Zr0.25, Ti0.75)O3 ferroelectric

We study the relaxation of photoresponses in self-polarized Pb(Zr0.25, Ti0.75)O3 film structures with a semitransparent NiCr top electrode under focused laser illumination with a wavelength of 0.63 μm and a power of about 40 mW. Dependences of the relaxation times of the photoresponses on the field directed both along the self-polarization vector and opposite to it are experimentally obtained. The dependences are found to be qualitatively different. We assume that in the structure under study, a screening charge occurs due to the capturing of photoinduced carriers in deep traps localized in the contact region.

A simple chemical method for deposition of electrochromic Prussian blue thin films

This paper is about a recently developed new chemical method for deposition of Prussian blue thin films. The films are easily prepared by successive immersion of the substrates into an acidic aqueous solution of Fe 2(SO 4) 3 and K 4[Fe(CN) 6]. It is calculated of the results from AFM analysis that the growth in the film thickness by one immersion cycle corresponds to an average increase of 6 nm. The characterization of the films with X-ray diffraction, SEM–EDS analysis and FTIR spectroscopy shows that the deposited material is amorphous hydrated Fe 4[Fe(CN) 6] 3. The electrochromic properties are characterized by cyclic voltammetry and VIS spectrophotometry. The PB thin films exhibit stability and excellent reversibility, which make these films favorable for electrochromic devices.

AC susceptibility of thin Pb films in intermediate and mixed state

Thin films of type I superconductors of a thickness comparable or less than a flux penetration length behave like type II superconductors in a mixed state. With decreasing film thickness normal domains carrying a magnetic flux get smaller with smaller number of flux quanta per domain and finally transform into single quantum flux lines, i.e. quantum vortices similar to those found in type II superconductors. We give an evidence of this behavior from the measurements of the nonlinear response of a total magnetic moment to an applied AC magnetic field, directly from the temperature dependence of an AC susceptibility.

Nano-embossing technology on ferroelectric thin film Pb(Zr0.3,Ti0.7)O3 for multi-bit storage application

In this work, we apply nano-embossing technique to form a stagger structure in ferroelectric lead zirconate titanate [Pb(Zr0.3, Ti0.7)O3 (PZT)] films and investigate the ferroelectric and electrical characterizations of the embossed and un-embossed regions, respectively, of the same films by using piezoresponse force microscopy (PFM) and Radiant Technologies Precision Material Analyzer. Attributed to the different layer thickness of the patterned ferroelectric thin film, two distinctive coercive voltages have been obtained, thereby, allowing for a single ferroelectric memory cell to contain more than one bit of data.

Domain-wall and reverse-domain superconducting states of a Pb thin-film bridge on a ferromagnetic BaFe12O19single crystal

We report on imaging of the nonuniform superconducting states in a Pb thin film bridge on top of a ferromagnetic BaFe_{12}O_{19} single crystal with a single straight domain wall along the center of the bridge by low-temperature scanning laser microscopy. We have visualized domain wall superconductivity (DWS) close to the critical temperature of Pb, when the Pb film above the domain wall acts as a superconducting path for the current. The evolution of the DWS signal with temperature and the external-field-driven transition from DWS to reverse domain superconductivity was visualized.

MEMS 공정을 이용한 마이크로 PZT 외팔보 에너지 수확소자의 제작 및 특성

In this work, we designed and fabricated a multilayer thin film Pb(Zr,Ti)<TEX>$O_3$</TEX> cantilever with a Si proof mass for low frequency vibration energy harvesting applications. A mathematical model of a mu lti-layer composite beam was derived and applied in a parametric analysis of the piezoelectric cantilever. Finally, the dimensions of the cantilever were determined for the resonant frequency of the cantilever. W e fabricated a device with beam dimensions of about 4,930 <TEX>${\mu}M$</TEX> <TEX>${\times}$</TEX> 450 <TEX>${\mu}M$</TEX> <TEX>${\times}$</TEX> 12 <TEX>${\mu}M$</TEX>, and an integrated Si proof mass with dimensions of about 1,410 <TEX>${\mu}M$</TEX> <TEX>${\times}$</TEX> 450 <TEX>${\mu}M$</TEX> <TEX>${\times}$</TEX> 450 <TEX>${\mu}M$</TEX>. The resonant frequency, maximum peak voltage, and highest average power of the cantilever device were 84.5 Hz, 88 mV, and 0.166 <TEX>${\mu}Wat$</TEX> 1.0 g and 23.7 <TEX>${\Omega}$</TEX>, respectively. The dimensions of the cantilever were determined for the resonance frequency of the cantilever.

Erratum: Strong Quantum Size Effects in Pb(111) Thin Films Mediated by Anomalous Friedel Oscillations [Phys. Rev. Lett.105, 066101 (2010)

Erratum: Strong Quantum Size Effects in Pb(111) Thin Films Mediated by Anomalous Friedel Oscillations [Phys. Rev. Lett.<b>105</b>, 066101 (2010)

Paramagnetic Meissner Transitions in Pb Films and the Vortex Compression Model

The magnetic flux exclusion known as Meissner effect resulting from a perfectly diamagnetic susceptibility χ=−1 is one of the defining features of type-I superconductivity. More recently some materials have been reported to undergo a transition to a paramagnetic state as they are cooled through their superconducting transition temperature (Svedlindh et al. in Physica C, pp. 162–164, 1989; Li in Phys. Rep. 376:133–223, 2003). This is known as the Paramagnetic Meissner Effect (PME). Here we report the observation of a transition from the Meissner state to a PME state in thin Pb films. We go on to show how simple modifications to the vortex compression model developed by Koshelev and Larkin (Phys. Rev. B 52:13559–13562, 1995) yield magnetization-vs-temperature curves in good qualitative agreement with the magnetization curves reported in the experiment.

Microscopic picture of the critical state in a superconductor with a periodic array of antidots

By using scanning Hall microscopy we visualize the progressive formation of the critical state with individual vortex resolution in a Pb thin film with a periodic pinning array. Slightly above the first penetration field, we directly observe a terraced critical state as proposed theoretically by Cooley and Grishin [Phys. Rev. Lett. 74, 2788 (1995)]. However, at higher fields, the flux front tends to disorder and the classical Bean profile is restored. This study allows us to establish a clear link between the widely used integrated response measurements in the superconducting state and the nanoscale landscape defined by individual vortices.

Controlling the effective mass of quantum well states in Pb/Si(111) by interface engineering

The in-plane effective mass of quantum well states in thin Pb films on a Bi reconstructed Si(111) surface is studied by angle-resolved photoemission spectroscopy. It is found that this effective mass is a factor of three lower than the unusually high values reported for Pb films grown on a Pb reconstructed Si(111) surface. Through a quantitative low-energy electron diffraction analysis the change in effective mass as a function of coverage and for the different interfaces is linked to a change of around 2% in the in-plane lattice constant. To corroborate this correlation, density functional theory calculations were performed on freestanding Pb slabs with different in-plane lattice constants. These calculations show an anomalous dependence of the effective mass on the lattice constant including a change of sign for values close to the lattice constant of Si(111). This unexpected relation is due to a combination of reduced orbital overlap of the 6p_z states and altered hybridization between the 6p_z and 6p_xy derived quantum well states. Furthermore it is shown by core level spectroscopy that the Pb films are structurally and temporally stable at temperatures below 100 K.

Extrinsic screening of ferroelectric domains in Pb(Zr0.48Ti0.52)O3

The variation in the surface potential as a function of the ferroelectric polarization of micron scale domains in a thin epitaxial film of Pb(Zr0.48Ti0.52)O3 is measured using mirror electron microscopy. Domains were written using piezoforce microscopy. The surface potential for each polarization was deduced from the mirror to low energy electron microscopy transition in the local reflectivity curve. The effect of extrinsic screening of the fixed polarization charge at the ferroelectric surface is demonstrated. The results are compared with density functional theory calculations.