Thin Film Diffusion Research Articles

Determination of thermal diffusivity of thin films by applying Fourier expansion analysis to thermo-reflectance signal after periodic pulse heating

The flash methods, which are the most popular transient methods for measuring the thermal diffusivity of solid materials, have evolved into ultrafast laser flash methods by using picosecond or nanosecond pulse lasers as a heating source and a thermo-reflectance technique such as high-speed thermometry. In conventional ultrafast laser flash methods, thermal diffusivity is determined by fitting an analytical equation after single pulse heating to observe thermo-reflectance signals, although actual thermo-reflectance signals are observed after periodic pulse heating. This paper presents an exact analytical solution of the temperature response expressed by Fourier series for one-dimensional heat diffusion after periodic pulse heating. These Fourier coefficients are directly related to the Laplace transformation of the temperature response after single pulse heating. The signal observed for a 100 nm thick platinum thin film on a fused quartz substrate was analyzed by this Fourier expansion analysis and fitted by analytical equations with three parameters: heat diffusion time across thin film, the ratio of heat effusion of the substrate to thin film, and the amplitude of the signal over the entire range of pulse interval in the time domain. Robustness in determining the thermal diffusivity of the thin film by the ultrafast laser flash method can be improved by this new analysis approach.

Combined use of high-resolution dialysis, diffusive gradient in thin films (DGT) technique, and conventional methods to assess trace metals in reservoir sediments.

Recently, reservoirs in southern China are witnessing incidents involving black water, which are harmful to the aquatic ecosystem. This study unravels the cause of the black water events by studying the occurrence and the ecological risks of contaminants (Pb, Cu, Cd, Zn, Ni, TFe, Mn, S, P, and DOC) in sediments of Tianbao reservoir. Due to the significantly high concentration of TFe, Mn, and P in the sediments, the study further used the thin film diffusion gradient (DGT) technology and high-resolution dialysis method to investigate the movement of Fe2+, Mn2+, S2-, and reactive P within the sediments. The ecological risk assessment (threshold effect level and probable effect level) showed that the sediments had a low concentration of Pb, Cu, Cd, Zn, and Ni. High organic matter from the Eucalyptus plantation surrounding the reservoir, as well as the intense thermal stratification of the reservoir, caused the hypolimnion to be hypoxic (DO < 2mg/L). The diffusion fluxes at the water-sediment boundary (WSB) demonstrated a significant movement of Fe2+, Mn2+, and PO43- from the sediments into the overlying water, while the movement of S2- was in both directions due to hypoxia. A high correlation Fe-DOC (r = 0.9), Fe-S (r = 0.8), and Mn-S (r = 0.7) and the redox interaction of Fe2+, Mn2+, S2-, P, and DOC at the hypoxic WSB caused the production of black substances in the hypolimnion contributing to the so-called black water reservoir.

Theoretical Analysis of Experimental Data of Sodium Diffusion in Oxidized Molybdenum Thin Films

In this work, the diffusion process of sodium (Na) in molybdenum (Mo) thin films while it was deposited on soda lime glass (SLG) was studied. A small amount of oxygen was present in the chamber while the direct-current (DC) magnetron sputtering was used for the deposition. The substrate temperatures were varied to observe its effect. Such molybdenum films, with or without oxidations, are often used in thin film solar cells, either as back contact or as hole transport layers. Secondary ion mass spectrometry (SIMS) was used to quantify the concentration of the species. A grain diffusion mechanistic model incorporating the effect of grain and grain boundary geometrical shape and size was developed. The model was used to provide an in-depth theoretical analysis of the sodium diffusion in molybdenum thin films that lead to the measured SIMS data. It was observed that not only diffusion coefficients should be considered when analyzing diffusion processes in thin films but also the ratio of grain boundary size to grain size. Both depend on substrate temperature and directly affect the amount of diffused species in the film. The data were analyzed under the light of the film growth speed versus diffusion front speed, the effect of oxygen content, and the effect of substrate temperature on the overall diffusion process. The temperature inversely affects the ratio of grain boundary size and grain size and directly affects the diffusion coefficient, which leads to a preferable temperature at which the highest amount of alkali can be found in the film.

High-Stability Memristive Devices Based on Pd Conductive Filaments and Its Applications in Neuromorphic Computing.

Memristive devices with high-density and high-speed performance have considerable potential for neuromorphic computing applications in data storage and artificial synapses. However, current memristive devices that are based on conductive filaments, such as silver, are unstable owing to the high mobility and low thermodynamic stability of the filaments. A high-quality SnSe film was deposited using the pulsed laser deposition technology, and high-performance Pd/SnSe/NSTO devices were fabricated. High-stability memristive devices can not only implement simple arithmetic function but also exhibit the centralized distribution of SET/RESET voltage and cell-cell uniformity. The SET/RESET power can achieve approximately 4.1 and 61 μW power. The possibility of Pd filament formation and Pd2+ diffusion in SnSe thin films is first confirmed by combining high-resolution transmission electron microscopy, energy-dispersive spectrometer mapping, and first principle calculation. The formation and destruction process of Pd filaments can simulate the influx and extrusion kinetics of K+, Ca2+, or Na+ in biological synapses and implements considerable synaptic functions. This study thus provides a new idea for improving device performance using different filament materials, which can greatly facilitate the development of neuromorphic computing.

Room temperature oxygen exchange and diffusion in nanometer-thick ZrO2 and MoO3 films

The diffusion of oxygen in thin films of ZrO2 and MoO3 was investigated with atomic 18O as a tracer using low energy ion scattering sputter depth profiling. 3 nm amorphous and 20 nm polycrystalline films were prepared by reactive magnetron sputtering and exposed to atomic oxygen species at room temperature. Exposure results in a fast diffusion of oxygen to a limited depth of ~1 nm and ~2.5 nm for ZrO2 and MoO3, respectively, and surface exchange limited to a maximum of 65% to 75%. The influence of the crystalline structure of the films on exchange and diffusion was negligible. We propose that the transport of oxygen in oxides at room temperature is dominated by a field-induced drift, generated by the chemisorption of reactive oxygen species. The maximum penetration of oxygen is limited by the oxide space charge region, determined by the oxide electrical properties. We applied a drift–diffusion model to extract values of surface potential and kinetic parameters of oxygen exchange and diffusion. The developed experimental analysis and modelling suggest that the electric field and consequent distribution of charged species are the main factors governing exchange rates and species diffusion in an oxide thin film at room temperature.

Thermal diffusivity downscaling of molybdenum oxide thin film through annealing temperature-induced nano-lamelle formation: a photothermal beam deflection study

The present work proposes a method of downscaling the thermal diffusivity (α) of MoO3 thin films through annealing temperature-induced nano-lamelle formation. The thermal diffusivity modification of the MoO3 films, prepared by the doctor blade method, is investigated by the sensitive transverse photothermal beam deflection technique. The X-ray diffraction analysis confirms the structural phase transformation from monoclinic to orthorhombic in the films annealed from 300 to 450 °C. The thermal induced anisotropy of the film is evident from the variation of the morphology index and texture coefficient. The field emission scanning electron microscopic analysis unveils the morphology modifications from blocks to the nano-lamelle structure with layers of average thickness ~ 77 nm. The thermal diffusivity measurement reveals a 53% reduction upon annealing the film to 450 °C. The drastic reduction is achieved through the annealing temperature-induced nano-lamelle formation and the phase transformation from monoclinic to orthorhombic in the MoO3 films.

Iron, thermal stratification, Eucalyptus sp., and hypoxia: drivers to water blackening in southern China reservoirs.

The management of black water depends primarily on the knowledge of the dynamics of organic matter (OM), iron (Fe), sulfide (S), and manganese (Mn), at the water-sediment boundary (WSB). However, the mechanistic path of these substances leading to black water remains unsettled. In this study, a 35-day field study was conducted using the thin-film diffusion gradient technology (DGT) and the planar optrode to address the unknown combined effects of Fe, Mn, OM, S, and tannins from Eucalyptus species on Tianbao reservoir.Our results indicated that the hypolimnion was hypoxic due to thermal stratification, which caused the reduction of insoluble Fe and Mn from sediments to bottom water. Correlation analysis (Fe:S (r:0.5-0.9); Mn:S (r:0.2-0.8)) and elevated fluxes (Fe2+, Mn2+, S2-) connoted that these parameters interacted chemically to give black matter. The content of OM, Fe2+, and tannic acid in the benthic region diminished remarkably (p < 0.05) from day 1 (strong stratification) to day 35 (weak stratification), connoting that these parameters also interacted chemically to give black matter. The turbidity (clarity of the water) increased from day 1 to 35 with a significant difference (p < 0.05) recorded on day 14 confirming that black water was formed on this day when the thermal structure of the reservoir was annihilated. Correlation analysis supported the assertion that the variability in oxygen and redox conditions caused changes in Fe, Mn, and OM content at the WSB.The finding from the field research provides useful information to stakeholders on how to improve the quality of freshwater management designs.

Organometallic gold nanoparticles and thin films from cis- and trans-tetrazonium gold(III) salts for electrochemical and photothermal mirror properties

Tetrazonium gold(III) salts of syn-(cis) and anti-(trans) geometry of 4,4′-(1,3-phenylenediisopropylidene)bisaniline and 4,4′-(1,4-phenylenediisopropylidene)bisaniline were synthesized. HAuCl4 was used as a metal-based counter anion precursor. The gold-carbon nanoparticles (AuNPs) were constructed using the mild reducing agent organoborane 9-borabicyclo[3.3.1]nonane (9-BBN). The nanoparticles dispersed in acetonitrile showed a plasmon peak at 540 nm. Dynamic light scattering measurements showed the size of cis-AuNPs is smaller than the trans-AuNPs. Transmission electron microscopy images showed the formation of spherical shaped AuNPs. X-ray diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy results supported the presence of metallic gold in both the cis and trans AuNPs. Electrochemical studies of the tetrazonium salts showed the facile reduction and the blocking effect of the films formed on glassy carbon electrodes. The photothermal mirror method was used to study the thermal diffusivity of gold thin films deposited over a glassy carbon substrate. The values of the thermal diffusivity coefficient were determined for the cis and trans isomers of the film.

Grain boundary diffusion and its relation to segregation of multiple elements in yttrium aluminum garnet

Abstract. We studied grain boundary diffusion and segregation of La, Fe, Mg, and Ti in a crystallographically defined grain boundary in yttrium aluminum garnet (YAG). Bi-crystals were synthesized by wafer bonding. Perpendicular to the grain boundary, a thin-film diffusion source of a La3.60Al4.40O12 was deposited by pulsed laser deposition. Diffusion anneals were performed at 1000 and 1450 ∘C. Via a gas phase small amounts of elements were added during the experiment. The element concentration distributions in our bi-crystals were mapped using analytical transmission electron microscopy (ATEM). Our results show strong segregation of La and Ti at the grain boundary. However, in the presence of Ti, the La concentrations dropped below the detection limit. Quantitative element distribution profiles along and across the grain boundary were fitted by a numerical diffusion model for our bi-crystal geometry that considers the segregation of elements into the grain boundary. The shape of the diffusion profiles of Fe requires the presence of two diffusion modes, e.g., the co-diffusion of Fe2+ as well as Fe3+. The absence of a detectable concentration gradient along the grain boundary in many experiments allows a minimum value to be determined for the product of sDgb. The resulting sDgb are a minimum of 7 orders of magnitude larger than their respective volume diffusion coefficient, specifically for La = 10−14 m2 s−1, Fe = 10−11 m2 s−1, Mg = Si = 10−12 m2 s−1, and Ti = 10−14 m2 s−1 at 1450 ∘C. Additionally, we model the effect of convolution arising from the given spatial resolution of our analysis with the resolution of our modeled system. Such convolution effects result in a non-unique solution for the segregation coefficient, e.g., for example for Mg between 2–3. Based on our data we predict that bulk diffusion of impurities in a mono-phase polycrystalline aggregate of YAG is effectively always dominated by grain boundary diffusion.

Structural phase transitions and their effects on oxygen diffusion in Y2(Zr1-xTix)2O7 thin films

Rare earth zirconate pyrochlore compounds with their unique crystal structure have been extensively used as thermal barrier coatings, ionic conductors and buffer layer for high-temperature coated conductors. In this work, Y2(Zr1-xTix)2O7 (YZTO) thin films, as one kind of rare earth zirconates doped with Ti4+ ions, were employed to investigate the structural phase transition and its effect on oxygen diffusion in YZTO thin films. YZTO thin films were prepared on single-crystal silicon substrates by sol–gel method with adding a certain amount of Ti4+ ions into Y2Zr2O7 precursor solution. It was found that the crystal structure of YZTO thin films started gradually to transform from defective fluorite structure with disordered oxygen vacancies to pyrochlore structure with highly ordered oxygen vacancies as the doping ratio of Ti4+ ions was increased. Remarkably, in YZTO thin films, the higher the doping ratios of Ti4+ ions are, the stronger the ability to block oxygen diffusion will become.

Electrical and structural properties of conductive nitride films grown by plasma enhanced atomic layer deposition with significant ion bombardment effect

Conductive metal nitrides are widely used in the microelectronics industry as interconnects, thin film resistors, electrodes, and diffusion barriers. These films are commonly prepared by sputtering and chemical vapor deposition, which are suitable for planar geometries. However, conformal deposition onto 3D and complex structures requires the use of atomic layer deposition (ALD). In this work, we compare the electrical and structural properties of various metallic nitrides (namely, TiNx, ZrNx, HfNx, and TaNx) prepared by ALD from metalorganic precursor and H2/Ar plasma. Despite similar bulk resistivity values of these films, we find significant differences in their measured resistivity for the thin film (by ALD). TiNx and ZrNx show metallic behavior with a positive temperature coefficient of resistance (TCR), whereas HfNx and TaNx show semiconducting behavior with negative TCR values. Microstructure and film chemistry of deposited films are investigated by x-ray photoelectron spectroscopy and transmission electron microscopy, and the correlation between the electrical and structural parameters of the deposited films is discussed. It is shown that a high concentration of carbon contamination is related to smaller grain size and higher electrical resistivity. TiNx exhibits the lowest carbon contamination, largest degree of crystallinity and lowest resistivity (∼60 μΩ cm) highlighting its potential as ALD-grown metal. Other nitrides and their combinations can be used to tailor specific resistivity and TCR values for thin film resistor applications in 3D and complex geometries such as deep trenches. Overall, this study provides useful guidelines toward the development of ALD nitrides for use in the microelectronics industry.

Study of Li diffusion in thin film of Re annealed at high temperatures

Depth profiles of 6Li implanted into high-melting point rhenium refractory metal has been measured for as-implanted, as well as thermally annealed samples using a non-destructive Thermal Neutron Depth Profiling technique at the LWR-15 research reactor in Řež. The Gaussian-like shape, typical for as-implanted Li-depth profile, is preserved after the thermal annealing at high temperature demonstrating the activation of the Li atom diffusion process at the Re-target. The diffusion coefficient at 1830̂C has been evaluated by the comparison of the Li profiles before and after the thermal annealing. This study aims to investigate the applicability of Re metal in ISOL related applications.

Comparative analysis of metal diffusion effects in polymer films coated with spin coating and floating film transfer techniques

In this work a comparative analysis between the extent of gold diffusion of the top coated gold layer into the conducting polymer matrix of spin coated polymer films and polymer films coated via floating film transfer method (FTM) has been investigated. For the sake of comparison two different device configurations i.e. ITO/rr-P3HT (spin coated)/Au and ITO/rr-P3HT (spin coated)/rr-P3HT (FTM coated)/Au have been compared. The J-V characteristics of these devices revealed a non-Ohmic injection occurring near the top gold electrode/polymer interface. Furthermore, the current flowing through the device consisting of FTM layer/Au interface was observed to be lower as compared to the one consisting of spin coated layer/Au interface, indicating higher gold diffusion in well-ordered FTM thin films. These observations have been validated by optical and structural characterization methods viz. absorption, photoluminescence, Raman spectroscopy and X-Ray Diffraction (XRD) measurement. Finally Fowler-Nordheim (FN) tunneling currents were studied under high electric field applications for the two devices which confirmed presence of higher barrier at the interface of gold and FTM coated polymer film.

Hydrogen diffusion through Ru thin films

In this paper, an experimental measurement of the diffusion constant of hydrogen in ruthenium is presented. By using a hydrogen indicative Y layer, placed under the Ru layer, the hydrogen flux through Ru was obtained by measuring the optical changes in the Y layer. We use optical transmission measurements to obtain the hydrogenation rate of Y in a temperature range from room temperature to 100 °C. We show that the measured hydrogenation rate is limited mainly by the hydrogen diffusion in Ru. These measurements were used to estimate the diffusion coefficient, D, and activation energy of hydrogen diffusion in Ru thin films to be D = 5.9 × 10−14 m2/s ∙ exp (-0.33 eV/kBτ), with kB the Boltzmann constant and τ the temperature.

A study of synchronous measurement of liable phosphorous and iron based on ZrO-Chelex (DGT) in the sediment of the Chaiwopu Lake, Xinjiang, Northwest China.

The water-sediment interface of lakes is an important and unique area of the water environment; the geochemical behavior of nutrients in this area has a significant impact on the quality of the water environment and ecosystems, especially in shallow lakes. However, most studies do not provide direct in situ evidence for this in shallow lakes in arid regions; in order to explore the coupling relationship between phosphorus (P) and iron (Fe) in a sediment profile, we conducted a high-resolution analysis of liable Fe and P in sediments taken from the Chaiwopu Lake using ZrO-Chelex thin film diffusion gradient technology (ZrO-Chelex DGT). The results show that (1) the vertical spatial distribution trend of the liable P and Fe in the sediments from each sampling site is essentially similar. The contents of the liable P and Fe ranged from 0.004-0.125mg/L and 0.050-0.190mg/L, respectively, and the synchronous distribution of the micro-interface concentration reflects the coupling relationship between them. (2) The correlation analysis of the liable P and Fe concentrations showed that there were significant linear correlations between them (P < 0.05, bilateral). (3) The diffusion fluxes of P and Fe were - 51.76~65.12μg(m2d)-1 and - 451.27~457.06μg(m2d)-1, respectively, and were shown to be negative at the sediment-water interface for most of the samples, which showed that P and Fe were released from the overlying water into the sediments. (4) This research showed that the diffusive fluxes at the different sites are quite different, which indicates that the phosphorus and iron pollution in the sediments of the Chaiwopu Lake is affected by exogenous inputs. There was no significant correlation between P release flux and pH, ORP, conductivity (EC), the TDS of the overlying water, or the pH, salinity (Ca2+, Mg2+), and nutrient (organic matter) content of the sediment. The release flux of Fe is affected by the pH of the sediment. The results of this study provide references for the research of elements in the water-sediment interface of shallow lakes in arid regions, as well as other areas.

Electrical fluctuation spectra due to characteristic thermal diffusion in Ta thin films deposited by RF-magnetron sputtering

In this study, we investigate electrical resistivity and resistance fluctuation of Ta thin films deposited by RF-magnetron sputtering, and discuss the relation between the crystal structure and resistance fluctuation. The resistance fluctuation in the films shows exponential behavior in power spectral densities (PSDs), and the exponents of the PSDs are dependent on the film thickness. While the PSDs and exponents of thin films less than 10 nm or more than 60 nm are lower, the intermediate thickness samples exhibit higher PSDs. The intermediate thickness samples have strained α- and β-phases, and a positive correlation is presumed between the power exponent and the crystalline strain. Additionally, the thermal diffusion path, which depends on the coexistence of the two strained phases, has a characteristic fractal-like dimension with less-than-2 dimensions in 2D thin films. Numerical simulation shows that the thermal diffusion path, which is derived from the coexistence of α- and β-Ta grains, is related to the exponent in the PSDs. The higher exponents originate from the strained phases, and measurement of the PSDs in these thin films enables the detection of crystalline strain.

All-polymer Planar Photonic Crystals as an Innovative Tool for the Analysis of Air

The possibility to evaluate the molecular diffusivity in polymer thin films used for packaging and device encapsulation directly in-situ would represent a paradigm changer in the assesment of barrier properties and of air quality. Indeed, employing the packaging itself as a smart sensor could lead to waste reduction and mitigate food poisoning effects. In this work, we demonstrate a new technique that exploits simple UV-Vis reflectance spectroscopy to identify the kinetic of diffusion of small molecules in the vapor phase through polymer thin films and polymer multilayered structures. The new method allows then to assess the presence of the analyte in air and its diffusion coefficient in agreement with gravimetric data reported in literature.

Monitoring Charge Carrier Diffusion across a Perovskite Film with Transient Absorption Spectroscopy.

We have developed a new noninvasive optical method for monitoring charge carrier diffusion and mobility in semiconductor thin films in the direction perpendicular to the surface which is most relevant for devices. The method is based on standard transient absorption measurements carried out in reflectance and transmittance modes at wavelengths below the band gap where the transient response is mainly determined by the change in refractive index, which in turn depends on the distribution of photogenerated carriers across the film. This distribution is initially inhomogeneous because of absorption at the excitation wavelength and becomes uniform over time via diffusion. By modeling these phenomena we can determine the diffusion constant and respective mobility. Applying the method to a 500 nm thick triple cation FAMACs perovskite film revealed that homogeneous carrier distribution is established in few hundred picoseconds, which is consistent with mobility of 66 cm2 (V s)−1.

Influence of nitrogen content on the thermal diffusivity of TiN films prepared by magnetron sputtering

ABSTRACTTiN films with various nitrogen content were deposited using reactive magnetron sputtering system and the influence of TiN film’s microstructure on thermal diffusivity was investigated. The results showed that the titanium nitride film with 40.6–64.0at.% N exhibited a single face-centered cubic (fcc) structure. The density of states calculations of TiNx showed that excess N atoms in TiN matrix weaken the Ti-N bonds. The thermal diffusivity of TiN thin films decreased gradually and reached a stable range with an increase of N content. The minimum thermal diffusivity value was 2.291×10−6m2/s at 60.2at.% N. This reduction of thermal diffusivity could be attribute to the films’ microstructure, including the increased grain boundaries due to the decrease of grain size, the change of preferential orientation and the increased lattice distortion.

Limiting Current Measurements of Oxygen Transport Resistance in Platinum Group Metal-Free Fuel Cell Cathodes

Polymer electrolyte fuel cells (PEFCs) currently have a significant commercialization cost due to the price of the platinum or platinum-alloy catalysts they use on the cathode. Platinum group-metal (PGM) free catalysts have the potential to reduce this cost, but have lower volumetric activity, requiring the use of thicker catalyst layers. The increased catalyst layer thickness results in an increase in through-plane oxygen-transport resistance. In addition to that resistance, there are additional non-resistance due to diffusion through water filled pores and across ionomer films. Here, we elucidated the relative magnitude of the various oxygen-transport resistances from limiting current density measurements. To extract the various resistances, we measured the limiting current density for three PGM-free fuel cells with varied catalyst loading, testing each with varied total pressure and carrier gas (Helox, dry air, and 5% O2). Measuring limiting current density at various pressure separates pressure-dependent resistances from pressure-independent resistances. Pressure-dependent resistances include intermolecular gas diffusion while pressure-independent resistances include both Knudsen diffusion and diffusion through thin ionomer films that cover the catalyst. For increased loading and reduced local fluxes at the active site, local microstructure and ionomer thin-film diffusion resistances are less significant. Also, because of the lower volumetric activity and local oxygen flux, those resistances are generally lower within PGM-free cathodes. Varying the carrier gas influences the bulk gas diffusion. From this, and comparison to literature, the component of oxygen-transport-resistance due to the catalyst layer was determined apart from the channel and gas diffusion media of the cathode. Such information can be used to advance the morphology and materials of PGM-free cathodes for enhanced O2 transport and fuel cell power density. DOE ACKNOWLEDGEMENT This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008076. The authors gratefully acknowledge research support from the the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under theU.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract Number DE-EE0008076.