Ultrathin Oxide Films Research Articles

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.

Atomic Layer Deposition of Ultrathin Tungsten Oxide Films from WH2(Cp)2 and Ozone

Atomic Layer Deposition of Ultrathin Tungsten Oxide Films from WH₂(Cp)₂ and Ozone

Ultrathin Aluminum Oxide Films Induced by Rapid Thermal Annealing for Effective Silicon Surface Passivation

A novel method of preparing ultrathin aluminum oxide films by rapid thermal annealing (RTA) treatment for effective silicon surface passivation is proposed. The high‐temperature RTA processing (750–825 °C) for tens of seconds in an oxygen atmosphere completely converts the thermally evaporated aluminum metal nanofilms (1–5 nm) on crystalline silicon to aluminum oxide (Al2O3) films, with a thin SiOx layer formed at the interface between the silicon and the Al2O3. The generated Al2O3 film can provide superior passivation quality for the silicon surface, even better than that obtained by the thermal atomic layer deposition technique. Moreover, the growth kinetics of the Al2O3 passivating film indicates that it is a diffusion‐controlled activation process, with an energy barrier of 3.3 eV for aluminum ions diffusing across the metal/oxide interface.

Zoology of spin and orbital fluctuations in ultrathin oxide films

Many metallic transition-metal oxides turn insulating when grown as films that are only a few unit-cells thick. The microscopic origins of these thickness induced metal-to-insulator transitions however remain under dispute. Here, we simulate the extreme case of a monolayer of an inconspicuous correlated metal -- the strontium vanadate SrVO$_3$ -- deposited on a SrTiO$_3$ substrate. Crucially, our system can have a termination to vacuum consisting of either a SrO or a VO$_2$ top layer. While we find that both lead to Mott insulating behavior at nominal stoichiometry, the phase diagram emerging upon doping -- chemically or through an applied gate voltage -- is qualitatively different. Indeed, our many-body calculations reveal a cornucopia of nonlocal fluctuations associated with (in)commensurate antiferromagnetic, ferromagnetic, as well as stripe and checkerboard orbital ordering instabilities. Identifying that the two geometries yield crystal-field splittings of opposite signs, we elucidate the ensuing phases through the lens of the orbital degrees of freedom. Quite generally, our work highlights that interface and surface reconstruction and the deformation or severing of coordination polyhedra in ultra-thin films drive rich properties that are radically different from the material's bulk physics.

Unveiling the Origin of Robust Ferroelectricity in Sub-2 nm Hafnium Zirconium Oxide Films.

HfO2-based ferroelectrics are highly expected to lead the new paradigm of nanoelectronic devices owing to their unexpected ability to enhance ferroelectricity in the ultimate thickness scaling limit (≤2 nm). However, an understanding of its physical origin remains uncertain because its direct microstructural and chemical characterization in such a thickness regime is extremely challenging. Herein, we solve the mystery for the continuous retention of high ferroelectricity in an ultrathin hafnium zirconium oxide (HZO) film (∼2 nm) by unveiling the evolution of microstructures and crystallographic orientations using a combination of state-of-the-art structural analysis techniques beyond analytical limits and theoretical approaches. We demonstrate that the enhancement of ferroelectricity in ultrathin HZO films originates from textured grains with a preferred orientation along an unusual out-of-plane direction of (112). In principle, (112)-oriented grains can exhibit 62% greater net polarization than the randomly oriented grains observed in thicker samples (>4 nm). Our first-principles calculations prove that the hydroxyl adsorption during the deposition process can significantly reduce the surface energy of (112)-oriented films, thereby stabilizing the high-index facet of (112). This work provides new insights into the ultimate scaling of HfO2-based ferroelectrics, which may facilitate the design of future extremely small-scale logic and memory devices.

Influence of Surface Adsorption on the Electrical Transport Properties of Ultrathin Indium Tin Oxide Films

We systematically investigated the temperature-dependent behaviors of the resistivities ρ and carrier concentrations n of a series of ultrathin indium tin oxide (ITO) films with thickness t ranging from ∼5.3 to ∼35.0 nm grown on yttrium-stabilized ZrO2 substrates. For the t ≳ 17.5 nm films, the resistivity versus temperature can be described by the Bloch–Grüneisen formula, and the carrier concentration almost retains constant over the whole measured temperature range. For the t ≃ 5.3 nm (7.0 nm) film, the carrier concentration increases with increasing temperature above ∼300 K, and the resistivity initially increases with increasing temperature and then decreases with further increasing temperature above ∼330 K (∼355 K). By comparison of the transport properties of ITO films depositing on different substrates and exposing the films on different atmospheres, it is found that the enhancement of carrier concentration and reduction of resistivity at high temperature regime originate from the dissociative adsorption of water vapor on the surface of the ITO films.

The insertion of the ALD diffusion barriers: An approach to improve the quality of the GaN deposited on Kapton by PEALD

The influence of growth temperature (200–300 ℃) on the properties of gallium nitride (GaN) directly deposited on Kapton by plasma-enhanced atomic layer deposition (PEALD) has been systematically investigated. With increasing growth temperatures, the O content and surface roughness of the GaN thin films increase. Besides, the diffusion behavior of GaN in Kapton was found when the growth temperature exceeds 250 ℃. In this study, ultrathin ALD aluminum oxide (Al2O3) and aluminum nitride (AlN) films were proposed to see if they could alleviate and/or even suppress the negative effects of interface diffusion during high-temperature (≥250 ℃) GaN growth. It was found that oxygen impurities were significantly decreased within the as-deposited GaN. Furthermore, we observed a complete suppression of GaN diffusion into the Kapton throughout the entire growth process. Additionally, an introduction of either the Al2O3 or AlN before the GaN deposition could improve the surface morphology of the GaN deposited on Kapton.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces.

Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnOxHy films on Pd(111) under varying gas-phase conditions [ultrahigh vacuum (UHV), 5 × 10-7 mbar of O2, and a D2/O2 mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D2/O2 mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnOxHy films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.

Two-step model for reduction reaction of ultrathin nickel oxide by hydrogen

Nickel (Ni) is used as a catalyst for nitric oxide decomposition and ammonia production but it is easily oxidized and deactivated. Clarification of the reduction process of oxidized Ni is essential to promote more efficient use of Ni catalysts. In this study, the reduction processes of ultrathin oxide films formed on Ni(111) surfaces by thermal oxidation under vacuum and a hydrogen atmosphere were investigated by in situ time-resolved photoelectron spectroscopy. On the basis of these results, we propose a reaction model for the reduction of Ni oxide films. Our results show that the reduction of Ni oxide films on heating under vacuum does not yield a clean Ni(111) surface owing to formation of a residual stable suboxide structure on the Ni(111) surface. Conversely, in a hydrogen atmosphere of 1 × 10−5 Pa, the Ni oxide was completely reduced and a clean Ni(111) surface was obtained, even when heating below 300 °C. The reduction in a hydrogen atmosphere was best described by a two-step reaction model. The rate of the first step depends on the reduction temperature, and the rate of the second step depends on the H2 pressure. The rate-limiting process for the first step is surface precipitation of O atoms and that of the second step is dissociation of H2 molecules.

Solution processable transition metal oxide ultra-thin films as alternative electron transport and hole blocking layers in dye sensitized solar cells

An electron transporting and hole-blocking layer (ETL) between the fluorine-doped tin oxide (FTO) and mesoporous titanium dioxide interface of photoanode is essential in dye-sensitized solar cells (DSCs) for improving their photovoltaic performance. In the present work, we have examined the potential for further improving device performance by using alternative materials for ETLs. Accordingly, the effect of replacing TiO2 based ETLs with spin coated ultra-thin transition metal oxides (TMOs), molybdenum trioxide (MoO3) and tungsten trioxide (WO3) films has been studied. TMO films of 10 nm thickness have been found to be optimal for obtaining good transparency and charge transport properties, without compromising device performance. Further, devices fabricated with surfactant assisted (SA-) MoO3 ETLs have shown efficiency ∼4%, which is comparable with devices based on conventionally prepared TiO2 ETLs. In addition, a significant improvement in efficiency (15 %) has been observed for DSCs prepared using SA-WO3 films. The overall improvement in device efficiency is attributed to the increase in short-circuit current density without loss of open circuit potential and fill factor. Detailed analyses of results suggest that in addition to surface morphology, transparency and charge transport properties; oxygen vacancies in sub-stoichiometric TMOs also plays an important role in determining the device performance. The findings reveal that TMOs have significant potential to replace TiO2 ETL in DSCs and other third generation solar cell, viz. perovskite and polymer solar cells.

Enhancing the stability of polymer nanostructures via ultrathin oxide coatings for nano-optical device applications

Polymer nanostructures have drawn tremendous attention due to their wide applications in nanotechnology. However, the morphology of the polymer nanostructures is fragile under harsh conditions such as high-power irradiation and organic-solution environments during the fabrication or the measurement processes, significantly limiting their potential applications. In this work, we propose and demonstrate a simple approach to improve the stability of polymer nanostructures by coating a conformal ultrathin oxide film via atomic-layer deposition. Due to the refractory and dense coating of the oxide layer, the stability of polymer structures is enhanced by the prohibition of deformation occurrences from thermally induced reflow and organic solution. As a proof of concept, poly(methyl methacrylate) (PMMA) nanostructures coated with a sub-10-nm TiO2 layer are demonstrated, and the structures exhibit high temperature stability at 180 °C and good resistance to soluble damage from organic solutions. Subsequently, the mechanism of the improved thermal stability is analyzed via mechanical simulations. Such an effective approach is proposed to significantly broaden the application of polymer nanostructures as functional elements for optical structures/devices that require excellent thermal and chemical stability.

Temperature dependency of structure and order evolution in 2D confined oxide films grown on Al substrates using reactive molecular dynamics

We performed molecular dynamics simulations by using Reactive Force Field method to study the impact of temperature on the structure, topology and order given in terms of configurational entropy of 2D-confined oxide film formed upon thermal oxidation of bare Al surfaces. The presence of O–O connections in 2D-confined oxide films has little impact on Al–O bond length and more pronounced one for coordination of atomic sites. The pentacoordinated Al sites and a random network of oxygen ions in the growing ultrathin oxide film promote the structural disorder. Amorphous structure forms with violated stoichiometry and reduced molar density of the 2D-confined films.

Uncertainty in the mutual calibration method for the traceable thickness measurement of ultra-thin oxide films

Mutual calibration by a combination of a zero-offset method and a length-unit traceable method has been suggested as a promising approach to determine the traceable thickness of ultra-thin oxide films. However, the measurement uncertainty is somewhat complicated to calculate because the standard uncertainties from the two measurement methods and the linear regression process should be combined. In this study, the mutual calibration method to evaluate the film thickness and uncertainty of ultra-thin oxide films was investigated. The algorithm of the linear regression equation in the mutual calibration method was studied and the uncertainty calculation program for the thickness measurement by mutual calibration was developed. The result will provide an effective and useful guideline to certify the thickness of the ultra-thin oxide film on Si(100) substrate which is used as the gate oxide in the semiconductor devices. The magnitude of the relative expanded uncertainty in the thickness measurement by mutual calibration is in the range from 8.6% to 9.3%.

Inversion-Symmetry Engineering in Layered Oxide Thin Films.

Inversion-symmetry breaking is a ubiquitous concept in condensed-matter science: It is a prerequisite for technologically relevant effects such as piezoelectricity, nonlinear optical properties, and spin-transport phenomena. It also determines abstract properties, like the electronic topology in quantum materials. Therefore, the creation of materials where inversion symmetry can be turned on or off by design may be a versatile approach for controlling parity-related functionalities. Here, we engineer inversion symmetry on a sub-unit-cell level in ultrathin hexagonal manganite films. Although an odd number of half-unit-cell layers breaks inversion symmetry, an even number of such layers remains centrosymmetric. Optical second harmonic generation as an inversion-symmetry-sensitive functionality is thus activated and deactivated on demand and at the same time used for in situ tracking of the symmetry state of our films. Symmetry engineering on the sub-unit-cell level thus suggests a new platform for controlled activation and deactivation of symmetry-governed functionalities in oxide-electronic epitaxial thin films.

Study on Porosity in Zinc Oxide Ultrathin Films from Three-Step MLD Zn-Hybrid Polymers.

Deriving mesoporous ZnO from calcinated, molecular layer deposited (MLD) metal-organic hybrid thin films offers various advantages, e.g., tunable crystallinity and porosity, as well as great film conformality and thickness control. However, such methods have barely been investigated. In this contribution, zinc-organic hybrid layers were for the first time formed via a three-step MLD sequence, using diethylzinc, ethanolamine, and maleic anhydride. These zinc-organic hybrid films were then calcinated with the aim of enhancing the porosity of the obtained ZnO films. The saturation curves for the three-step MLD process were measured, showing a growth rate of 4.4 ± 0.2 Å/cycle. After initial degradation, the zinc-organic layers were found to be stable in ambient air. The transformation behavior of the zinc-organic layers, i.e., the evolution of the film thickness and refractive index as well as the pore formation upon heating to 400, 500, and 600 °C were investigated with the help of spectroscopic ellipsometry and ellipsometric porosimetry. The calculated pore size distribution showed open porosity values of 25%, for the sample calcinated at 400 °C. The corresponding expectation value for the pore radius obtained from this distribution was 2.8 nm.

Surface-modified ultra-thin indium zinc oxide films with tunable work function for efficient hole transport in flexible indoor organic photovoltaics

The stability of the electrical and optical properties of electrodes subjected to physical strain need to be ensured to enhance the performance of indoor organic photovoltaics (OPVs). In this study, we demonstrate the stable performances of flexible OPVs by producing an ultra-thin (20 nm) indium zinc oxide (IZO) electrode by co-depositing its surface with Ni metal, which improves the electrical conductivity and energy-level alignment owing to a hole-transport layer. As an anode, the resulting ultra-thin IZO electrode exhibits a relative sheet resistance of 250 Ω sq−1, high transmittance of 91.5% at 450 nm, and high work function of 5.05 eV. More importantly, the proposed electrode shows an enhanced bending performance, which is attributable to its amorphous structure formed as a result of co-deposition. Therefore, flexible OPVs with the proposed electrode show much higher performances (42% power conversion efficiency under indoor illumination) than those with a reference IZO anode. Furthermore, they exhibit outstanding flexural endurance properties while maintaining 84% of their original power conversion efficiency after 1500 cycles of bending at a bending radius of 8.1–4.2 mm on polyimide substrates. This study demonstrates an effective strategy for improving the performance of optoelectronic devices requiring electrical and mechanical stability.

On the Origin of the OER Activity of Ultrathin Manganese Oxide Films

There is an urgent need for cheap, stable, and abundant catalyst materials for photoelectrochemical water splitting. Manganese oxide is an interesting candidate as an oxygen evolution reaction (OER) catalyst, but the minimum thickness above which MnOx thin films become OER-active has not yet been established. In this work, ultrathin (<10 nm) manganese oxide films are grown on silicon by atomic layer deposition to study the origin of OER activity under alkaline conditions. We found that MnOx films thinner than 1.5 nm are not OER-active. X-ray photoelectron spectroscopy shows that this is due to electrostatic catalyst-support interactions that prevent the electrochemical oxidation of the manganese ions close to the interface with the support, while in thicker films, MnIII and MnIV oxide layers appear as OER-active catalysts after oxidation and electrochemical treatment. From our investigations, it can be concluded that one MnIII,IV-O monolayer is sufficient to establish oxygen evolution under alkaline conditions. The results of this study provide important new design criteria for ultrathin manganese oxide oxygen evolution catalysts.

Effect of the surface contamination layer on the thickness measurement of ultra-thin HfO2 films

Mutual calibration by a combination of a zero-offset method (ZOM) and a length-unit traceable method (LTM) was suggested as a method to determine the true thickness of ultra-thin oxide films. Although, medium energy ion scattering spectroscopy (MEIS) and x-ray photoelectron spectroscopy (XPS) were suggested as the ZOMs, the offset value of thickness is affected by the surface contamination layer.In this study, we compared the effect of the surface contamination layer on the thickness measurement of a series of HfO2 films by MEIS and XPS. The surface of the HfO2 films were contaminated by thin carbon layers with the thicknesses from 0.25 nm to 1.30 nm. In MEIS, the variation of the offset value by the surface carbon layer was negligibly small in the range of 0.020 nm. However, it was large in XPS in the range of 0.131 nm. In conclusion, MEIS can be a reliable ZOM to calibrate the offset values in the thickness measurement of ultra-thin oxide films with negligibly small effect of surface contamination.

Spectroscopic analysis of ultrathin amorphous ZnO films grown by atomic layer deposition

Ultrathin amorphous Zinc Oxide films were prepared on glass substrates by homemade atomic layer deposition system using Diethyl Zinc and Deionized water as precursors and high pure Argon gas as purging gas. The substrate temperature was set as 125℃ and cycle number was varied. Different spectroscopic techniques were employed to unveil the structural and optical properties of films. The effect of quantum confinement on band gap energy values reduces with increase in thickness. Resonant Raman scattering was observed for all the samples when excited with laser wavelength of 325 nm.

Optoelectronic Properties of Ultrathin Indium Tin Oxide Films: A First-Principle Study

First-principle density functional theory simulations have been performed to predict the electronic structures and optoelectronic properties of ultrathin indium tin oxide (ITO) films, having different thicknesses and temperatures. Our results and analysis led us to predict that the physical properties of ultrathin films of ITO have a direct relation with film thickness rather than temperature. Moreover, we found that a thin film of ITO (1 nm thickness) has a larger absorption coefficient, lower reflectivity, and higher transmittance in the visible light region compared with that of 2 and 3 nm thick ITO films. We suggest that this might be due to the stronger surface strain effect in 1 nm thick ITO film. On the other hand, all three thin films produce similar optical spectra. Finally, excellent agreement was found between the calculated electrical resistivities of the ultrathin film of ITO and that of its experimental data. It is concluded that the electrical resistivities reduce along with the increase in film thickness of ITO because of the short strain length and limited bandgap distributions.