X-ray Reflectometry Data Research Articles

Investigation of the Solid Electrolyte Interphase of Silicon Wafers Using a Fluorine-Free Electrolyte

Lithium-ion (Li-ion) batteries have become an important solution for energy storage owing to their high gravimetric and volumetric energy density. The electrolyte solution of the battery has a large effect on the stability and performance of the cell. During the first cycles of a battery, the solid electrolyte interphase (SEI) layer is formed due to the degradation of the electrolyte. Hence the composition of this layer, and the stability, are dependent on the electrolyte composition.The most commonly used electrolyte for Li-ion batteries is the salt lithium hexafluorophosphate LiPF6 dissolved in organic solvents together with additives, both containing fluorinated compounds.1 However these fluorinated compounds are susceptible to release HF which is toxic and corrosive which could be detrimental for the cell performance but also hinder the recycling process. Therefore, fluorine-free electrolytes could be a promising alternative. While this type of electrolytes has been investigated with graphite anodes, when moving to higher energy densities, such as silicon, their performance and degradation mechanism is not yet clear.2 Previous research has shown increased stability for Li-ion batteries using a fluorine-free electrolyte based on the salt lithium bis(oxalato) borate (LiBOB).2 Furthermore, operando X-ray reflectometry (XRR) measurements on silicon wafers half-cells with the same electrolyte have shown the build-up of multiple layers on the anode, providing the thicknesses and densities of the different layers. This work presents the chemical composition of these different layers formed on the silicon wafers at different potentials when using a fluorine-free electrolyte based on LiBOB in ethylene carbonate and methyl ethyl carbonate. These results are also compared with the effect of adding vinylene carbonate VC as an additive. The different components in these layers are correlated to the previous XRR data to explain the SEI-formation mechanism with fluorine-free electrolytes. Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem Rev 104, (2004).Hernández, G. et al. Elimination of fluorination: The influence of fluorine-free electrolytes on the performance of LiNi1/3Mn1/3Co1/3O2/silicon-graphite li-ion battery cells. ACS Sustain Chem Eng 8, (2020).

GenX 3: the latest generation of an established tool.

Since its publication more than 15 years ago the GenX software has been continuously developed and has established itself as a standard package for analyzing X-ray and neutron reflectometry data. The evolution of the software during the last two major revisions is reported here. This includes a simplified model builder for beginners, simple samples, additional sample models, statistical error analysis and the use of just-in-time compilation modules for the reflectometry kernel to achieve higher performance. In addition, the influence of experimental errors on the reflectivity curve is discussed, and new features are described that allow the user to include these in the error statistics to improve the fitting and uncertainty estimation.

Application of X-ray diffraction and reflectometry methods for analysis of damaged layers on polar faces of ZnO after surface chemical-mechanical treatment

ZnO single crystals are used for the fabrication of laser targets for high-energy electron irradiated UV laser cathode-ray tubes and homoepitaxial substrates for lasers. The technology of ZnO based UV LEDs imposes strict requirements to surface quality. Chemical-mechanical polishing delivers good surface quality but it is known that polishing of ZnO polar faces may yield different results. Surface-sensitive high-resolution X-ray diffraction (HRXRD) and X-ray reflectometry (XRR) methods have been used for studying the structure of (0001) and (000–1) polar faces of ZnO after chemical-mechanical polishing. Two double-sided polished (0001) ZnO substrates have been cut out from different hydrothermally grown ingots. The damage and density depth profiles for the Zn and O faces of the specimens have been retrieved from the X-ray diffraction curves and the specular reflection curves, respectively. Intensity distributions in the vicinity of the [0002] and [0000] reciprocal lattice sites have been taken on a D8 Discover X-ray diffractometer (Bruker-AXS, Germany) in a triple-crystal setup. For separating the coherent and incoherent scattering components, the intensity profiles have been analyzed along sections perpendicular to the diffraction vector and located at different distances from the reciprocal lattice sites. The HRXRD and XRR data have been compared with atomic force microscopy (AFM) data. The HRXRD method has revealed damaged layers at both faces of the specimens, with the layer thicknesses differing for the Zn and O faces, i.e., 5–7 nm for the Zn face and 10–11 nm for the O face. The XRR method has shown that both faces are sufficiently smooth. These results have been confirmed by AFM (RMS roughness ~ 0.23 ± 0.07 nm). However, the concentration of electrons in the superficial layers has been found to change. The layer thickness proves to be greater for the O face. We have hypothesized that the phenomena observed are caused by the difference in the chemical interaction of the Zn and O faces with the polishing agents.

Application of X-ray diffraction and reflectometry methods for analysis of damaged layers on polar faces of ZnO after surface chemical-mechanical treatment

ZnO single crystals are used for the fabrication of laser targets for high-energy electron irradiated UV laser cathode-ray tubes and homoepitaxial substrates for lasers. The technology of ZnO based UV LEDs imposes strict requirements to surface quality. Chemical-mechanical polishing delivers good surface quality but it is known that polishing of ZnO polar faces may yield different results. Surface-sensitive high-resolution X-ray diffraction (HRXRD) and X-ray reflectometry (XRR) methods have been used for studying the structure of (0001) and (000–1) polar faces of ZnO after chemical-mechanical polishing. Two double-sided polished (0001) ZnO substrates have been cut out from different hydrothermally grown ingots. The damage and density depth profiles for the Zn and O faces of the specimens have been retrieved from the X-ray diffraction curves and the specular reflection curves, respectively. Intensity distributions in the vicinity of the [0002] and [0000] reciprocal lattice sites have been taken on a D8 Discover X-ray diffractometer (Bruker-AXS, Germany) in a triple-crystal setup. For separating the coherent and incoherent scattering components, the intensity profiles have been analyzed along sections perpendicular to the diffraction vector and located at different distances from the reciprocal lattice sites. The HRXRD and XRR data have been compared with atomic force microscopy (AFM) data. The HRXRD method has revealed damaged layers at both faces of the specimens, with the layer thicknesses differing for the Zn and O faces, i.e., 5–7 nm for the Zn face and 10–11 nm for the O face. The XRR method has shown that both faces are sufficiently smooth. These results have been confirmed by AFM (RMS roughness ~ 0.23 ± 0.07 nm). However, the concentration of electrons in the superficial layers has been found to change. The layer thickness proves to be greater for the O face. We have hypothesized that the phenomena observed are caused by the difference in the chemical interaction of the Zn and O faces with the polishing agents.

Review on the thickness measurement of ultrathin oxide films by mutual calibration method

Mutual calibration was suggested as a method to determine the absolute thickness of ultrathin oxide films. It was motivated from the large offset values in the reported thicknesses in the Consultative Committee for Amount of Substance (CCQM) pilot study P‐38 for the thickness measurement of SiO2 films on Si(100) and Si(111) substrates in 2004. Large offset values from 0.5 to 1.0 nm were reported in the thicknesses by ellipsometry, X‐ray reflectometry (XRR), medium‐energy ion scattering spectrometry (MEIS), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), and transmission electron microscopy (TEM). However, the offset value for the thicknesses by X‐ray photoelectron spectroscopy (XPS) was close to zero (−0.013 nm). From these results, the mutual calibration method was reported for the thickness measurement of SiO2 films on Si(100) by combination of TEM and XPS. The mutual calibration method has been applied for the thickness measurements of hetero oxide films such as Al2O3 and HfO2. Recently, the effect of surface contamination was reported to be critical to the thickness measurement of HfO2 films by XPS. On the other hand, MEIS was proved to be a powerful zero offset method which is not affected by the surface contamination. As a result, the reference thicknesses in the CCQM pilot study P‐190 for the thickness measurement of HfO2 films on Si(100) substrate were determined by mutual calibration method from the average XRR data and MEIS analysis. Conclusively, the thicknesses of ultrathin oxide films can be traceably certified by mutual calibration method and most thickness measurement methods can be calibrated from the certified thicknesses.

Anaklasis: a compact software package for model-based analysis of specular neutron and X-ray reflectometry data sets.

anaklasis constitutes a set of open-source Python scripts that facilitate a range of specular neutron and X-ray reflectivity calculations, involving the generation of theoretical curves and the comparison/fitting of interfacial model reflectivity against experimental data sets. The primary focus of the software is twofold: on one hand to offer a more natural framework for model definition, requiring minimum coding literacy, and on the other hand to include advanced analysis methods that have been proposed in recent work. Particular attention is given to the ability to co-refine reflectivity data and to the estimation of model-parameter uncertainty and covariance using bootstrap analysis and Markov chain Monte Carlo sampling. The compactness and simplicity of model definition together with the streamlined analysis do not present a steep learning curve for the user, an aspect that may accelerate the generation of reproducible, easily readable and statistically accurate reports in future neutron and X-ray reflectivity related literature.

Ionic Distribution at the Ionic Liquid/Water Interface: An x-Ray Reflectometry Study

Electrochemistry at ITIES, which has been extensively studied for the oil (O)/water(W) interface, has been extended to the liquid/liquid interfaces of ionic liquids (ILs), namely, to the IL/W interface [1] and even to the IL/O interface [2,3]. One of several characteristics of ILs, compared with conventional molecular liquids like W and O, is the formation of ionic multilayers [4-6] at the interfaces due to the excluded volume and local electrostatic interactions between ions. Although such a multilayering behavior of ILs had been expected to appear in the electric double layer (EDL) at the IL/W interface, it had been challenging to detect the ionic distribution at the IL/W interface.In the present paper, we will introduce our recent study on the ionic distribution at the IL/W interface, revealed by x-ray reflectometry (XR) using the liquid surface reflectometer at NSF’s ChemMatCARS Sector 15-ID at Advanced Photon Source in Chicago [7]. We designed an ionic liquid that has both high hydrophobicity and high electron-density contrast between the cation and anion, which enabled us to widely polarize the IL/W interface and sensitively probe the cation-rich and anion-rich layers by XR, respectively. We observed the growth of alternately charged cation-rich and anion-rich layers along with a polarity reversal of the layers as the potential changed sign. We quantitatively compared the XR results with the BSK model [8], a phenomenological EDL model in ILs that takes into account both excluded volume and overscreening effects, by combining it with the Gouy-Chapman-Stern model for EDL on the W side of the IL/W interface. Our XR data reveal that the excess charge beyond the first ionic layer decays more rapidly than the model prediction, suggesting that the BSK model needs an improvement with which the layering periodicity and decay length are separately evaluated.References T. Kakiuchi and N. Nishi, Electrochemistry, 74 (2006) 942.N.E.R. Cousens and A.R. Kucernak, Electrochem. Commun., 31 (2013) 63.Y. Kuroyama, N. Nishi, T. Sakka, submitted. N. Nishi, Y. Yasui, T. Uruga, H. Tanida, T. Yamada, S. Nakayama, H. Matsuoka, T. Kakiuchi, J. Chem. Phys., 132 (2010) 164705.N. Nishi, T. Uruga, H. Tanida, T. Kakiuchi, Langmuir, 27 (2011) 7531.N. Nishi, T. Uruga, H. Tanida, J. Electroanal. Chem., 759 (2015) 129.S. Katakura, K. Amano, T. Sakka, W. Bu, B. Lin, M.L. Schlossman, N. Nishi, J. Phys. Chem. B, 124 (2020) 6412.M.Z. Bazant, B.D. Storey, A.A. Kornyshev, Phys. Rev. Lett., 106 (2011) 046102.

Towards a neutron and X-ray reflectometry environment for the study of solid\u2013liquid interfaces under shear

A novel neutron and X-ray reflectometry sample environment is presented for the study of surface-active molecules at solid–liquid interfaces under shear. Neutron reflectometry was successfully used to characterise the iron oxide–dodecane interface at a shear rate of 7.0times {}10^{2}hbox {s}^{-1} using a combination of conventional reflectometry theory coupled with the summation of reflected intensities to describe reflectivity from thicker films. Additionally, the structure adopted by glycerol monooleate (GMO), an Organic Friction Modifier, when adsorbed at the iron oxide–dodecane interface at a shear rate of 7.0times {}10^{2}hbox {s}^{-1} was studied. It was found that GMO forms a surface layer that appears unaltered by the effect of shear, where the thickness of the GMO layer was found to be 24.3^{+9.9}_{-10.2} Å under direct shear at 7.0times {}10^{2}hbox {s}^{-1} and 25.8^{+4.4}_{-5.2} Å when not directly under shear. Finally, a model to analyse X-ray reflectometry data collected with the sample environment is also described and applied to data collected at 3.0times {}10^{3}hbox {s}^{-1}.

Thickness measurement of nm HfO2 films

Main textA pilot study for the thickness measurement of HfO2 films was performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee for Amount of Substance (CCQM). The aim of this pilot study was to ensure the equivalency in the measurement capability of national metrology institutes for the thickness measurement of HfO2 films. In this pilot study, the thicknesses of six HfO2 films with nominal thickness from 1 nm to 4 nm were measured by X-ray Photoelectron Spectroscopy (XPS), X-ray Reflectometry(XRR), X-ray Fluorescence Analysis (XRF), Transmission Electron Spectroscopy (TEM), Spectroscopic Ellipsometry (SE) and Rutherford Backscattering Spectrometry (RBS). The reference thicknesses were determined by mutual calibration of a zero-offset method (Medium Energy Ion Scattering Spectroscopy (MEIS) of KRISS) and a method traceable to the length unit (the average thicknesses of three XRR data except the thinnest film). These reference thicknesses are traceable to the length unit because they are based on the traceability of XRR. For the thickness measurement by XPS, the effective attenuation length of Hf 4f electrons was determined. In the cases of XRR and TEM, the offset values were determined from a linear fitting between the reference thicknesses and the individual data by XRR and TEM. The amount of substance of HfO2, expressed as thickness of HfO2 films (in both linear and areal density units), was found to be a good subject for a CCQM key comparison.To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCQM.

Interface between Water-Solvent Mixtures and a Hydrophobic Surface.

The mechanism behind the stability of organic nanoparticles prepared by liquid antisolvent (LAS) precipitation without a specific stabilizing agent is poorly understood. In this work, we propose that the organic solvent used in the LAS process rapidly forms a molecular stabilizing layer at the interface of the nanoparticles with the aqueous dispersion medium. To confirm this hypothesis, n-octadecyltrichlorosilane (OTS)-functionalized silicon wafers in contact with water-solvent mixtures were used as a flat model system mimicking the solid-liquid interface of the organic nanoparticles. We studied the equilibrium structure of the interface by X-ray reflectometry (XRR) for water-solvent mixtures (methanol, ethanol, 1-propanol, 2-propanol, acetone, and tetrahydrofuran). The formation of an organic solvent-rich layer at the solid-liquid interface was observed. The layer thickness increases with the organic solvent concentration and correlates with the polar and hydrogen bond fraction of Hansen solubility parameters. We developed a self-consistent adsorption model via complementing adsorption isotherms obtained from XRR data with molecular dynamics simulations.

Ultrafast modification of oxide glass surface hardness

Application of femtosecond lasers is widely utilized in micromachining transparent materials. We have successfully altered the surface hardness of various commercial silicate glasses using a high-intensity femtosecond pulse laser. The femtosecond laser generates pulse energy of 500 nJ with a central wavelength of 800 nm. Using a peak power of 2.2 W and a repetition rate of 5.1 MHz, we observed an 18–20% increase surface hardness in glasses with low-modifier content and 16.6% decrease in glasses with high-modifier content. All laser exposed glasses show no detectable induced-crystallization or surface ablation. X-ray photoelectron spectroscopy results of our samples confirmed that the laser irradiation had no detectable effect on surface chemistry. X-ray reflectometry data showed the change in hardness was attributed to a thin layer with modified density. Experimental results suggest the strengthening mechanism derives from local structural transformation of interatomic bond distances and angles.

Reference-free grazing incidence x-ray fluorescence and reflectometry as a methodology for independent validation of x-ray reflectometry on ultrathin layer stacks and a depth-dependent characterization

Nanolayer stacks are technologically very relevant for current and future applications in many fields of research. A nondestructive characterization of such systems is often performed using x-ray reflectometry (XRR). For complex stacks of multiple layers, low electron density contrast materials, or very thin layers without any pronounced angular minima, this requires a full modeling of the XRR data. As such a modeling is using the thicknesses, the densities, and the roughnesses of each layer as parameters, this approach quickly results in a large number of free parameters. In consequence, cross correlation effects or interparameter dependencies can falsify the modeling results. Here, the authors present a route for validation of such modeling results which is based on the reference-free grazing incidence x-ray fluorescence (GIXRF) methodology. In conjunction with the radiometrically calibrated instrumentation of the Physikalisch-Technische Bundesanstalt, the method allows for reference-free quantification of the elemental mass depositions. In addition, a modeling approach of reference-free GIXRF-XRR data is presented, which takes advantage of the quantifiable elemental mass depositions by distributing them depth dependently. This approach allows for a reduction of the free model parameters. Both the validation capabilities and the combined reference-free GIXRF-XRR modeling are demonstrated using several nanoscale layer stacks consisting of HfO2 and Al2O3 layers.

Electrostatic and Structural Effects at the Adsorption of Polylysine on the Surface of the DMPS Monolayer

The effect of the adsorption of a polypeptide on the lateral interaction of dimyristoylphosphatidylserine molecules in different phase states on the surface of a 10 mM KCl aqueous solution has been studied. Changes in the surface pressure and Volta potential induced by the adsorption of large poly-D-lysine molecules (about 200 links in a chain) have been determined at different areas per lipid molecule in a monolayer. The adsorption of macromolecules noticeably increases the elasticity of the monolayer under lateral compression in the liquid expanded state of lipid and reduces the effective dipole moment from 0.48 to 0.38 D. These properties are in qualitative agreement with X-ray reflectometry data for the lipid monolayer obtained with synchrotron radiation with a photon energy of ≈70 keV. The electron density profiles perpendicular to the surface of the aqueous subphase have been reconstructed from reflectometry data within a model approach to the structure of an interface with two and three layers. These profiles indicate the existence of a wide diffuse polymer layer (150 ± 40) A in width at the interface of the monolayer in both the liquid expanded and liquid condensed states. A decrease in the area per molecule in the monolayer by a factor of 2 results in the doubling of the surface density of the macromolecule film. The adsorption of the polymer also affects the integral density of the layer of polar phospholipid groups, which decreases by a factor of ≈2 in the liquid expanded phase and by ∼30% in the liquid condensed phase.

Refnx: neutron and X-ray reflectometry analysis in Python.

refnx is a model-based neutron and X-ray reflectometry data analysis package written in Python. It is cross platform and has been tested on Linux, macOS and Windows. Its graphical user interface is browser based, through a Jupyter notebook. Model construction is modular, being composed from a series of components that each describe a subset of the interface, parameterized in terms of physically relevant parameters (volume fraction of a polymer, lipid area per molecule etc.). The model and data are used to create an objective, which is used to calculate the residuals, log-likelihood and log-prior probabilities of the system. Objectives are combined to perform co-refinement of multiple data sets and mixed-area models. Prior knowledge of parameter values is encoded as probability distribution functions or bounds on all parameters in the system. Additional prior probability terms can be defined for sets of components, over and above those available from the parameters alone. Algebraic parameter constraints are available. The software offers a choice of fitting approaches, including least-squares (global and gradient-based optimizers) and a Bayesian approach using a Markov-chain Monte Carlo algorithm to investigate the posterior distribution of the model parameters. The Bayesian approach is useful for examining parameter covariances, model selection and variability in the resulting scattering length density profiles. The package is designed to facilitate reproducible research; its use in Jupyter notebooks, and subsequent distribution of those notebooks as supporting information, permits straightforward reproduction of analyses.

Asymmetric alloy formation at the Fe-on-Ti and Ti-on-Fe interfaces

The Fe-on-Ti and Ti-on-Fe interfaces were studied experimentally by Mössbauer spectroscopy (MS), transmission electron microscopy (TEM) and x-ray reflectometry (XRR) on Ti/Fe/Ti trilayers grown on Si(1 1 1) substrates by vacuum evaporation. The nanoscale structure and composition were explored in cross sections using TEM, the layer structure and the interface widths by specular x-ray reflectometry. MS was applied to identify the interface alloy phases and to determine the pure and alloyed Fe layer fractions. The experimental results were compared with molecular dynamics (MD) simulations of layer growth on Fe or Ti underlayers of different orientations. The concentration distributions provided by MD simulations show an asymmetry at the interfaces in the layer growth direction. The transition is atomically sharp at the Ti-on-Fe interface for the (0 0 1) and (1 1 0) crystallographic orientations of the Fe underlayer, while it spreads over a few atomic layers for Fe(1 1 1) underlayer and for all studied Ti underlayer orientations at the Fe-on-Ti interface. MS and XRR data on Ti/Fe/Ti trilayers confirm the asymmetry between the bottom and top Fe interface, but the inferred interface widths considerable exceed those deduced from the MD simulations.

Features of Wavelet Analysis in X-Ray Reflectometry of Thin Films

Specific features of the use of wavelet transform for estimating the thickness of layers and their order in a film density profile based on X-ray and synchrotron reflectometry data are considered. Some ways are proposed to reveal the characteristic features of Langmuir film packing by constructing a wavelet transform for the corresponding reflectograms. Dependences of the X-ray attenuation length on the grazing incidence angle are plotted by an example of multilayered box model of film profile; these dependences demonstrate possibilities of mapping the spatial signal delay (which occurs when rays are reflected from layers of different density) in a waveletgram.

Grazing incident X-ray fluorescence combined with X-ray reflectometry metrology protocol of telluride-based films using in-lab and synchrotron instruments

Telluride films are widely applied in data storage devices (advanced resistive memories, DVDs and Blue-rays disks), photovoltaic cells and infrared detectors. The properties of thin telluride alloys are deeply influenced by their chemical composition and compositional depth profile, whereas surface/interface effects may become preponderant in ultrathin films. The combination of X-ray reflectometry (XRR) and grazing-incidence X-ray fluorescence (GIXRF) is particularly adequate to probe these complex thin layered materials. In this paper, we evaluate the performances of Lab-based and synchrotron-based XRR/GIXRF strategies to characterize ultrathin (<10 nm) amorphous titanium‑tellurium films elaborated by Physical Vapor Deposition (PVD), and capped in situ with 5 nm‑tantalum passivation layer. We highlighted the impact of the instrumental setup on the qualitative XRR and GIXRF data and on the quantitative information deduced from the combined analysis. Both synchrotron-based and Lab-based strategies were sensitive enough to track the impact of small PVD process changes on the chemical depth-profiles, and to unambiguously reveal undesired tantalum‑tellurium inter-diffusion that was confirmed by X-Ray Photoelectron Spectroscopy.

Influence of the Chemical Activity of Implanted Ions on the Structure of the Damaged Si Layer in SIMOX Substrates

High‐resolution X‐ray diffraction (HRXRD), X‐ray reflectometry (XRR), X‐ray photoelectron spectroscopy (XPS), and Atomic‐force microscopy (AFM) were used to study the influence of the chemical activity of implanted ions (N+, O+, F+ or Ne+) on the structure of the damaged Si layer in SIMOX substrates. The implantation conditions were chosen so that the number of primary defects and the projected range were approximately the same for the implanted ions. HRXRD showed that the depth distribution of the residual damage depends on the chemical activity of the impurity. This may result from the quasi‐chemical interaction between the radiation‐induced point defects (VSi and Sii) and the impurity atoms. The shape of the density profiles and the thickness of the transition surface layer obtained from the XRR data are different for the implanted ions. The [100] faceted surface features of the samples prepared by SIMOX technology are preserved after implantation with the ions. AFM revealed a reduction of the lateral correlation length. The magnitude of this effect depends on the chemical activity of the implanted ions. Silicon oxidation at a depth of 10–15 nm is shown by XPS to change depending on the implanted ion.

Kinetics of the formation of a phospholipid multilayer on a silica sol surface

The ordering of a multilayer consisting of DSPC bilayers on a silica sol substrate is studied within the model-independent approach to the reconstruction of profiles of the electron density from X-ray reflectometry data. It is found that the electroporation of bilayers in the field of anion silica nanoparticles significantly accelerates the process of their saturation with Na+ and H2O, which explains both a relatively small time of formation of the structure of the multilayer of 10^5 - 7x10^5 s and ~13 % excess of the electron density in it.

Corrosion Resistance of Atomic Layer Deposition-Generated Amorphous Thin Films.

Atomic layer deposition (ALD) was used to prepare amorphous thin films of Al2O3, Nb2O5, and Ta2O5 on both silicon substrates and aluminum blocks. Etch rates in 10 M NH4OH were determined from X-ray reflectometry data collected as a function of time. Amorphous Al2O3 thin films were found to have an etch rate of 0.5 nm min-1 and an increase in roughness of ∼0.01 nm min-1. Electron microscopy data showed etch pits, consistent with the increase in roughness. Amorphous Nb2O5 and Ta2O5 films showed no appreciable etching or roughening over the course of a ∼500 h continuous immersion. An Nb2O5-coated aluminum block showed no corrosion after immersion in 10 M NH4OH for over 200 h, suggesting that the coatings were pinhole-free. These results suggest that amorphous ALD thin films of Nb2O5 and Ta2O5 are candidates as barrier layers for aluminum in caustic environments.