Thick Film Limit Research Articles

Determination of thermal conductivity of phase pure 10H-SiC thin films by non-destructive Raman thermometry

The 10 H SiC thin films are potential candidates for devices that can be used in high temperature and high radiation environment. Measurement of thermal conductivity of thin films by a non-invasive method is very useful for such device fabrication. Micro-Raman method serves as an important tool in this aspect and is known as Raman thermometry. It utilises a steady-state heat transfer model in a semi-infinite half space and provides for an effective technique to measure thermal conductivity of films as a function of film thickness and laser spot size. This method has two limiting conditions i.e. thick film limit and thin film limit. The limiting conditions of this model was explored by simulating the model for different film thicknesses at constant laser spot size. 10H SiC films of three different thicknesses i.e. 104, 135 and 156 nm were chosen to validate the thin film limiting condition. Thermal conductivity of these thin films varied from 0.60 – 4.80 (Wm−1K−1).

Symmetry, topology, and geometry: The many faces of the topological magnetoelectric effect

A delicate tension complicates the relationship between the topological magnetoelectric effect (TME) in three-dimensional (3D) Z2 topological insulators (TIs) and time-reversal symmetry (TRS). TRS underlies a particular Z2 topological classification of the electronic ground state of crystalline band insulators and the associated quantization of the magnetoelectric response coefficient calculated using bulk linear-response theory but, according to standard symmetry arguments, simultaneously forbids a nonzero magnetoelectric coefficient in any physical finite-size system. This contrast between theories of magnetoelectric response in formal bulk models and in real finite-sized materials originates from the distinct approaches required to introduce notions of (electronic) polarization and orbital magnetization in these fundamentally different environments. In this work we argue for a modified interpretation of the bulk linear-response calculations in nonmagnetic Z2 TIs that is more plainly consistent with TRS and use this interpretation to discuss the effect's observation—still absent over a decade after its prediction. Our analysis is reinforced by microscopic bulk and thin-film calculations carried out using a simplified but still realistic effective model for the well established V2VI3 [V=(Sb,Bi) and VI=(Se,Te)] family of nonmagnetic Z2 TIs. When a uniform dc magnetic field is included in this model, the anomalous n=0 Landau levels (LLs) play the central role, both in thin films and in bulk. In the former case, only the n=0 LL eigenfunctions can support a dipole moment, which vanishes if there are no magnetic surface dopants and is quantized in the thick-film limit if magnetic dopants at the top and bottom surfaces have opposite orientation. In the latter case, the Hamiltonian projected into the n=0 LL subspace is a one-dimensional Su-Schrieffer-Heeger model with ground-state polarization that is quantized in accordance with the bulk linear-response coefficient calculated for (a lattice regularization of) the starting 3D model. Motivated by analytical results, we conjecture a type of microscopic bulk-boundary correspondence: a bulk insulator with (generalized) TRS supports a magnetoelectric coefficient that is purely itinerant (which is generically related to the geometry of the ground state) if and only if magnetic surface dopants are required for the TME to manifest in finite samples thereof. We conclude that in nonmagnetic Z2 TIs the TME is activated by magnetic surface dopants, that the charge-density response to a uniform dc magnetic field is localized at the surface and specified by the configuration of those dopants, and that the TME is qualitatively less robust against disorder than the integer quantum Hall effect. Published by the American Physical Society 2024

Enhanced thermoelectricity at the ultra-thin film limit

At the ultra-thin film limit, quantum confinement strongly improves the thermoelectric figure of merit in materials such as Sb2Te3 and Bi2Te3. These high quality films have only been realized using well controlled techniques such as molecular beam epitaxy. We report a twofold increase in the Seebeck coefficient for both p-type Sb2Te3 and n-type Bi2Te3 using thermal co-evaporation, an affordable approach. At the thick film limit greater than 100 nm, their Seebeck coefficients are around 100 μV/K, similar to the results obtained in other works. When the films are thinner than 50 nm, the Seebeck coefficient increases to about 500 μV/K. With the Seebeck coefficient ∼1 mV/K and an estimate ZT ∼0.6, this pair of materials presents the first step toward a practical micro-cooler at room temperature.

Theoretical study of structural, electronic, and optical properties of ternary metal sulfides MBiS2 (M = Ag, Na)

We study the structural, electronic, and optical properties of MBiS2 (M = Ag, Na) using the relativistic all-electron calculations. We find that for both M = Ag and Na, the and superstructures are the lowest-energy superstructures. and MBiS2 are indirect band-gap semiconductors with the differences between the indirect and direct band gaps, Δ, of about 0.5 and 0.1 eV for M = Ag and Na, respectively. and MBiS2 exhibit high absorption coefficients of the order of 105 cm−1. Employing the spectroscopically limited maximum efficiency as a metric for quantifying the photovoltaic performance, it is found that the power conversion efficiency in the thick-film limit for M = Na is 32% because of its small Δ while that for M = Ag is 15% because of its large Δ.

Transmission of high-energy electrons through metal-semiconductor Schottky junctions

Using the electron beam of a scanning electron microscope as an external current source with tunable energy, we investigate the transport properties of high-energy electrons injected from vacuum into the metal layer of Pt/Cu/Si Schottky junctions. When the injection energy is varied between 1 and 30 keV, the current transmitted into the semiconductor increases by several orders of magnitude and reaches values orders of magnitude larger than the current injected from vacuum. Inspecting the energy dependence of the transmitted current we identify two transport regimes. In the limit of low injection energies and thick metal films, the transport is dominated by the formation and propagation of a secondary electron distribution in the metal layer. However, when the injection energy is sufficiently large and the metal layer sufficiently thin, electrons are transmitted into the semiconductor with negligible energy loss, i.e., the metal layer becomes essentially transparent. The transmitted current is then dominated by impact ionization in the semiconductor. When the metal layer of the Schottky junction is relatively thick and the injection energy of a few keV typically, the transmitted current increases abruptly. The origin of this abrupt change is interpreted as a combined effect of a quasiballistic electron transport in the metal layer and a sudden variation of the density of states in the semiconductor substrate.

Microstructural control of the transport properties of β-FeSe films grown by sputtering

We have investigated the correlation between structural and transport properties in sputtered β-FeSe films grown onto SrTiO3 (100). The growth parameters, such as substrate temperature and thickness, have been varied in order to explore different regimes. In the limit of textured thick films, we found promising features like an enhanced Tc∼12 K, a relatively high Hc2, and a low anisotropy. By performing magnetoresistance and Hall coefficient measurements, we investigate the influence of the disorder associated with the textured morphology on some features attributed to subtle details of the multiband electronic structure of β-FeSe. Regarding the superconductor-insulator transition induced by reducing the thickness, we found a nontrivial evolution of the structural properties and morphology associated with a strained initial growth and the coalescence of grains. Finally, we discuss the origin of the insulating behavior in high-quality stressed epitaxial thin films. We found that a lattice distortion, described by Poisson’s coefficient associated with the lattice parameters a and c, may play a key role.

Statistical field theory with constraints: Application to critical Casimir forces in the canonical ensemble.

The effect of imposing a constraint on a fluctuating scalar order parameter field in a system of finite volume is studied within statistical field theory. The canonical ensemble, corresponding to a fixed total integrated order parameter (e.g., the total number of particles), is obtained as a special case of the theory. A perturbative expansion is developed which allows one to systematically determine the constraint-induced finite-volume corrections to the free energy and to correlation functions. In particular, we focus on the Landau-Ginzburg model in a film geometry (i.e., in a rectangular parallelepiped with a small aspect ratio) with periodic, Dirichlet, or Neumann boundary conditions in the transverse direction and periodic boundary conditions in the remaining, lateral directions. Within the expansion in terms of ε=4-d, where d is the spatial dimension of the bulk, the finite-size contribution to the free energy of the confined system and the associated critical Casimir force are calculated to leading order in ε and are compared to the corresponding expressions for an unconstrained (grand canonical) system. The constraint restricts the fluctuations within the system and it accordingly modifies the residual finite-size free energy. The resulting critical Casimir force is shown to depend on whether it is defined by assuming a fixed transverse area or a fixed total volume. In the former case, the constraint is typically found to significantly enhance the attractive character of the force as compared to the grand canonical case. In contrast to the grand canonical Casimir force, which, for supercritical temperatures, vanishes in the limit of thick films, in the canonical case with fixed transverse area the critical Casimir force attains for thick films a negative value for all boundary conditions studied here. Typically, the dependence of the critical Casimir force both on the temperaturelike and on the fieldlike scaling variables is different in the two ensembles.

Phase Field Modeling of Metal Oxidation Kinetics

The oxidation kinetics of metals/alloys at elevated temperatures is essential to the corrosion-life prediction of the structural materials in advanced energy systems. The growth of oxide film on a metal surface typically involves coupling of chemical reaction and diffusion of charged point defects. A phase-field model is developed to address the essential physics during metal oxidation over a continuous range of time and film thickness. The model is able to reproduce the classic Deal-Grove model with respect to the linear-parabolic transition. In particular, the model reproduces Wagner’s theory in the thick film limit and predicts a deviation from the Wagner-parabolic growth when the oxide film thickness is on the order of Debye length. The essential characteristics of electric field developed in an oxide film during high temperature metal oxidation and some other progresses in terms of realistic oxidation modeling will also be discussed.

Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure

We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100-500~\AA) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS). All films investigated are phase-pure and of high quality with a sharp interface between MnSi and Si. The investigated MnSi films are in a thickness regime where the magnetic transition temperature $T_\mathrm{c}$ assumes a thickness-independent enhanced value of $\geq$43~K as compared with that of bulk MnSi, where $T_\mathrm{c} \approx 29~{\rm K}$. A detailed refinement of the EXAFS data reveals that the Mn positions are unchanged, whereas the Si positions vary along the out-of-plane [111]-direction, alternating in orientation from unit cell to unit cell. Thus, for thick MnSi films, the unit cell volume is essentially that of bulk MnSi --- except in the vicinity of the interface with the Si substrate (thin film limit). In view of the enhanced magnetic transition temperature we conclude that the mere presence of the interface, and its specific characteristics, strongly affects the magnetic properties of the entire MnSi film, even far from the interface. Our analysis provides invaluable information about the local strain at the MnSi/Si(111) interface. The presented methodology of polarization dependent EXAFS can also be employed to investigate the local structure of other interesting interfaces.

Nonequilibrium superconducting thin films with sub-gap and pair-breaking photon illumination

We calculate nonequilibrium quasiparticle and phonon distributions for a number of widely-used low transition temperature thin-film superconductors under constant, uniform illumination by sub-gap probe and pair-breaking signal photons simultaneously. From these distributions we calculate material-characteristic parameters that allow rapid evaluation of an effective quasiparticle temperature using a simple analytical expression, for all materials studied (Mo, Al, Ta, Nb, and NbN) for all photon energies. We also explore the temperature and energy-dependence of the low-energy quasiparticle generation efficiency η by pair-breaking signal photons finding in the limit of thick films at low bath temperatures that is material-independent. Taking the energy distribution of excess quasiparticles into account, we find as the bath temperature approaches the transition temperature in agreement with the assumption of the two-temperature model of the nonequilibrium response that is appropriate in that regime. The behaviour of η with signal frequency scaled by the superconducting energy gap is also shown to be material-independent, and is in qualitative agreement with recent experimental results. An enhancement of η in the presence of sub-gap (probe) photons is shown to be most significant at signal frequencies near the superconducting gap frequency and arises due to multiple photon absorption events that increase the average energy of excess quasiparticles above that in the absence of the probe.

Thickness- and microstructure-dependent mechanical properties induced by surface layers in nanofilms

The material properties of silica nanofilms were numerically investigated by using molecular dynamics and were theoretically confirmed by a homogenization method. Stress–strain curves under tensile deformations were obtained for two different microstructure films (α-quartz and amorphous) with various bulk thicknesses. Surface layers for both microstructure films formed by energetic relaxation were determined by coordination numbers of surface atoms, and they were shown to affect the mechanical properties of the nanofilms in a systematic manner, as predicted using the homogenization theory. We suggested a generalized prediction rule for effective material properties of nanostructured materials with interfacial phases such as surface layers in low dimensions, which is generally ignorable in the thick film limit.

Diffuse-Interface Modeling and Multiscale-Relay Simulation of Metal Oxidation Kinetics—With Revisit on Wagner’s Theory

Oxidation of metals generally involves coupling between chemical reactions, mass transport, and electrostatic interaction, and oxidation kinetics is usually a multiscale problem. Existing theories mostly work for either a very thin oxide film or a thick one, leaving a length scale gap for oxidation kinetics. An electrochemistry based diffuse-interface model plus a multiscale-relay scheme are developed to study oxidation kinetics in a gas–oxide–metal environment. The multiscale-relay scheme allows the model to coherently cover a wide range of lengths and times and study the transition stage oxidation kinetics. The coupling between interfacial reactions and ionic transport with the moving boundary problem is solved, without using assumptions such as steady state, coupled currents, local charge neutrality, or local chemical equilibrium. For the model oxidation system, in the thick film limit perfect parabolic growth law is obtained with the rate constant in agreement with Wagner’s theory. Nevertheless, the Wagner-parabolic law is violated either when the oxide film thickness is on the order of the Debye length or when the interfacial reaction is rate-limiting. In addition, computer simulations reveal two space charge related effects in different situations and their linkage to experimental observations is discussed.

Strain stabilization and thickness dependence of magnetism in epitaxial transition metal monosilicide thin films on Si(111)

We present a comprehensive study of different $3d$ transition metal monosilicides in their ground state crystal structure (B20), ranging from equilibrium bulk over biaxially strained bulk to epitaxial thin films on Si(111), by means of density functional theory. The magnetic properties of MnSi and FeSi films are found to be considerably modified due to the epitaxial strain induced by the substrate. In MnSi bulk material, which can be seen as a limit of thick films, we find a strain-induced volume expansion, an increase of the magnetic moments, and a significant rise of the energy difference between different spin configurations. The latter can be associated with an increase of the Curie temperature, which is in accordance with recent experimental results. While a ferromagnetic spin alignment is found to be the ground state also for ultrathin films, we show that for films of intermediate thickness a partially compensating magnetic ordering is more favorable; however, the films retain a net magnetic moment. Furthermore, we analyze the orbital structure in FeSi around the band gap, which can be located somewhere in the density of states for all studied B20 transition metal monosilicides, and find that FeSi becomes metallic and ferromagnetic under epitaxial strain. Finally, the influence of on-site electronic correlation and the reliability of ab initio calculations for $3d$ transition metal monosilicides are discussed.

A Model for Drying Control Cosolvent Selection for Spin-Coating Uniformity: The Thin Film Limit

Striation defects in spin-coated thin films are a result of unfavorable capillary forces that develop due to the physical processes commonly involved in the spin-coating technique. Solvent evaporation during spinning causes slight compositional changes in the coating during drying, and these changes lead to instability in the surface tension, which causes lateral motions of the drying fluid up to the point where it gels and freezes in the thickness variations. In an earlier publication, we looked at the case where evaporation happens fast enough that the compositional depletion is mostly a surface effect. In terms of the mass transport rate competition within the coating solution, that work covered the thick film limit of this instability problem. However, in many cases, the coatings are thin enough or diffusion of solvent within the coating is fast enough to require a different solvent mixing strategy, which is developed here. A simple perturbation analysis of surface roughness is developed, and evaporation is allowed in the thin film limit. The perturbation analysis allows for a simple rubric to be laid out for cosolvent additions that can reduce the Marangoni effect during the later stages of coating deposition and drying when the thin film limit applies.

Exact surface plasmon dispersion relations in a linear-metal-nonlinear dielectric structure of arbitrary nonlinearity

Exact analytic expression for obtaining the surface plasmon (SP) dispersion relations in a system consisting of a thin metallic film sandwiched between a linear dielectric and a nonlinear dielectric of arbitrary nonlinearity is derived, based on a generalized first integral approach. The changes in SP dispersion relations on film thicknesses and nonlinearity are studied by model calculations. The thick-film limit generalizes previous results on metal/Kerr-type nonlinear interface to arbitrary nonlinearity. An analytic expression is obtained for the limiting SP frequency at large wave vectors with previous result recovered as a special case.

Complex Dynamics during Metal Dissolution: From Intrinsic to Faceted Anomalous Scaling

The kinetic roughening of dissolving polycrystalline pure iron has been studied. A depth analysis of surface images has shown two consecutive growth regimes characterized by different scaling anomalous properties: an initial intrinsic anomalous scaling evolving in the thick film limit towards the theoretically conjectured faceted anomalous scaling. This represents the first experimental evidence of such scaling as well as of such transition. The dynamics presented here may account for the striped surface pattern observed during the evolution of metals or alloys in a large number of processes.

Third sound and stability of thinHe3−He4films

We study third sound in thin $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ mixture films from first-principles, microscopic theory and compare these results to the usual film-averaged, hydrodynamic approach. The hydrodynamic approach yields third-sound speeds that depend only on the thickness of the superfluid film and the distribution of impurities---i.e., $^{3}\mathrm{He}$. In very thin films, this result clearly must be modified to account for the effects of nonuniform $^{4}\mathrm{He}$ film density. Utilizing the variational, hypernetted-chain--Euler-Lagrange theory as applied to inhomogeneous boson systems, we calculate accurate chemical potentials for both the $^{4}\mathrm{He}$ superfluid film and the physisorbed $^{3}\mathrm{He}$. Numerical density derivatives of the chemical potentials lead to the sought-after third-sound speeds that clearly reflect a layered structure of at least seven oscillations. We are thus able to gauge the range of applicability of the film-averaged hydrodynamic results as applied to thin quantum liquid films. We study third sound on two model substrates: Nuclepore and glass. We compute the change in third-sound speed as a function of $^{3}\mathrm{He}$ coverage in the linear (low-concentration) regime, which is then studied for the two substrates as a function of $^{4}\mathrm{He}$ film thickness and compared to existing experiments.$^{3}\mathrm{He}$ density profiles are calculated as a function of $^{4}\mathrm{He}$ film thickness, and we show explicitly the smooth transition from Andreev states in the thick-film limit to lateral mixtures in the submonolayer limit. This effect was first seen by Noiray et al. [Phys. Rev. Lett. 53, 2421 (1984)]. Our results predict that the addition of a small amount of $^{3}\mathrm{He}$ can increase, as well as decrease, the third-sound speed relative to that of the pure $^{4}\mathrm{He}$ film. Further, we show that the addition of a small amount of $^{3}\mathrm{He}$ can destabilize the film and drive a phase separation into lateral regions of $^{3}\mathrm{He}$-rich and $^{3}\mathrm{He}$-poor patches. This latter result may help explain the phase transitions reported by Bhattacharyya and Gasparini [Phys. Rev. Lett. 49, 919 (1982)] and Cs\'athy, Kim, and Chan [Phys. Rev. Lett. 88, 045301 (2002)] in thin mixture films.

Modification of Surface States in Ultrathin Films via Hybridization with the Substrate: A Study of Ag on Ge

The Shockley surface state of Ag(111) develops unusual band dispersion relations for Ag films of decreasing thicknesses on Ge(111), as observed by angle-resolved photoemission. Its parabolic dispersion in the thick-film limit shifts toward higher binding energies and splits into multiple bands with dispersions that reflect the valence band structure of Ge including the heavy-hole, light-hole, and split-off bands. The results are explained in terms of a hybridization interaction between the Ag surface state and the Ge substrate states.

Equilibrium helium film in the thick-film limit

There are still some open questions about how the thickness of a liquid or solid quantum film, such as liquid helium or solid hydrogen, develops in certain limits. One of these is the thick-film limit, i.e., the crossover from the thick film to the bulk. We have performed measurements in this range using the surface plasmon resonance technique and an evaporated Ag film deposited on a glass substrate. The thickness of the adsorbed helium film is varied by changing the distance h of the bulk reservoir to the surface of the substrate. In the limiting case when h→0 the film thickness approaches about 100 nm, following the van der Waals law in the retarded regime. The film thickness and its dependence on h is determined precisely and modeled theoretically. The behavior of the equilibrium film thickness is discussed in detail. The agreement between theory and experiment is very good.

On the Possibility of Observing Third Sound on 3He

This paper focuses on the hydrodynamics of third sound on a superfluid 3 He film. We solve the hydrodynamical equations in the limit of thick films with weak interaction with the substrate. The surface tension at the free interface is shown to have a large effect on the third sound velocity and on the attenuation for frequencies larger than 1Hz. In the case of a diffusely scattering substrate a ripplon-like dispersion relation is found for this frequency range.