Amorphous Alumina Thin Films Research Articles

Alumina coatings on Ni-based superalloys: The impact of annealing on heavy oil fouling

As the sweet crude oil reserves decline, refiners must treat sulfur-rich heavy oil, requiring harsher operating conditions, which are detrimental to process equipment. Application of coatings on critical components protects surfaces against sulfidation, corrosion, and fouling, extends the equipment's lifetime, and reduces the frequency of costly turnarounds. In the present work, we coated Inconel 625 and Inconel 718 substrates with amorphous alumina thin films at room temperature using reactive RF magnetron sputtering. Annealing of the deposited coatings at 800, 900, and 1000 °C increased hardness, improved adhesion, and generated crystalline polymorphs, predominantly γ-Al2O3 at lower temperatures, while α-Al2O3 was present at 1000 °C. The annealed substrates formed thermally grown oxides (TGOs), which interacted with the alumina coatings. The TGOs followed grain boundaries in the case of IN718 and a crater-like pattern on IN625. Annealed substrate precipitates generated columnar-like protrusions responsible for inducing crack propagation, which exhibited TGO formation. After 2 h exposure to heavy oil (containing 0.06 g g−1 sulfur) at 450 °C and 11.3 MPa the as-deposited amorphous alumina presented no clear sign of adherent fouling, while the 1000 °C annealed crystalline alumina surfaces presented evidence of fouling.

Amorphous Alumina Thin Films Deposited on Carbon Microfibers As Interface Layer for Thermal Oxidation Barriers

Amorphous Alumina Thin Films Deposited on Carbon Microfibers As Interface Layer for Thermal Oxidation Barriers

Reduced thermal resistance of amorphous Al2O3 thin films on β-Ga2O3 and amorphous SiO2 substrates via rapid thermal annealing

The impact of rapid thermal annealing (1000 °C for 1 min) on the thermal transport properties of amorphous alumina (a-Al2O3) thin films grown by atomic layer deposition on β−Ga2O3 and amorphous silica (a-SiO2) substrates is determined using frequency-domain thermoreflectance measurements. The annealing more than doubles the a-Al2O3 thermal conductivity for both substrates (1.54 ± 0.13 to 3.14 ± 0.27 W m−1 K−1 for β−Ga2O3 and 1.60 ± 0.14 to 3.87 ± 0.33 W m−1 K−1 for a-SiO2) while keeping the film amorphous. The thermal conductivity increase is attributed to partial recrystallization and off-gassing of embedded impurities. Annealing halves the thermal boundary resistance of the a-Al2O3/a-SiO2 interface (10.5 ± 1.0 to 4.47 ± 0.42 m2 K GW−1), which is attributed to compositional mixing and structural reorganization that are enabled by the elastic matching of these two materials. The thermal boundary resistance of the a-Al2O3/β−Ga2O3 interface is not affected by annealing due to the elastic mismatch. Reducing the thermal resistance of a-Al2O3 dielectric films and adjacent interfaces by annealing will promote lateral heat spreading adjacent to hot spots and improve device longevity.

Toward an increased reliability of chemical bonding assignment in insulating samples by x-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) spectra from solid samples are conventionally referenced to the spectrometer Fermi level (FL). While, in the case of metallic samples, alignment of the sample and the spectrometer FLs can be directly verified from the measured Fermi edge position, thus allowing to assess the surface electrical potential, this is not a workable option for insulators. When applied, it generates a large spread in reported binding energy values that often exceed involved chemical shifts. By depositing insulating amorphous alumina thin films on a variety of conducting substrates with different work functions, we show not only that FL referencing fails but also that the Al2O3 energy levels align instead to the vacuum level, as postulated in the early days of XPS. Based on these model experiments that can be repeated for all sorts of thin-film insulators, a solution to the binding energy reference problem is proposed for reliable assessment of chemical bonding.

Mechanical properties of crystalline and amorphous aluminum oxide thin films grown by atomic layer deposition

Mechanical properties of atomic-layer deposited alumina thin films containing amorphous, α-Al2O3, θ-Al2O3, δ-Al2O3 and γ-Al2O3 phases were characterized. To initiate the α-Al2O3 growth, α-Cr2O3 seed layers deposited on Si substrates were used. Application of AlCl3 and H2O as the most appropriate precursors together with sufficiently long precursor pulse durations appeared to be important to obtain α-Al2O3 at temperatures down to 350 °C. The highest hardness value measured by the nano-indentation method for as-grown films was 26 GPa. This value was recorded for films deposited on α-Cr2O3 at 450 °C. Post-growth annealing of these films at 750 °C caused a hardness increase up to 30 GPa, that is, close to the corresponding value of single crystal α-Al2O3. For comparison, the hardness values of amorphous films did not exceed 13 GPa while those of the films, containing θ-Al2O3, δ-Al2O3 and γ-Al2O3 phases grown on pristine Si substrates at 750 °C reached 18 GPa. The increase in hardness and elastic modulus was shown to be in correlation with the increase in crystallinity and density of the film material.

Re-examination of the Aqueous Stability of Atomic Layer Deposited (ALD) Amorphous Alumina (Al2O3) Thin Films and the Use of a Postdeposition Air Plasma Anneal to Enhance Stability.

Amorphous aluminum oxide (alumina) thin films are of interest as inert chemical barriers for various applications. However, the existing literature on the aqueous stability of atomic layer deposited (ALD) amorphous alumina thin films remains incomplete and, in some cases, inconsistent. Because these films have a metastable amorphous structure─which is likely partially hydrated in the as-deposited state─hydration and degradation behavior likely deviate from what is expected for the equilibrium, crystalline Al2O3 phase. Deposition conditions and the aqueous solution composition (ion content) appear to influence the reactivity and stability of amorphous ALD alumina films, but a full understanding of why these alumina films hydrate, solvate, and/or dissolve in near-neutral pH = 7 conditions, for which crystalline Al2O3 is expected to be stable, remains unsolved. In this work, we conduct an extensive X-ray photoelectron spectroscopy investigation of the surface chemistry as a function of water immersion time to reveal the formation of oxyhydroxide (AlOOH), hydroxide (Al(OH)3), and possible carbonate species. We further show that brief postdeposition exposures of these ALD alumina films to an air plasma anneal can significantly enhance the film's stability in near-neutral pH aqueous conditions. The simplicity and effectiveness of this plasma treatment may provide a new alternative to thermal annealing and capping treatments typically used to promote aqueous stability of low-temperature ALD metal oxide barrier layers.

Cryo-ePDF: Overcoming Electron Beam Damage to Study the Local Atomic Structure of Amorphous ALD Aluminum Oxide Thin Films within a TEM.

Atomic layer deposition (ALD) provides uniform and conformal thin films that are of interest for a range of applications. To better understand the properties of amorphous ALD films, we need an improved understanding of their local atomic structure. Previous work demonstrated measurement of how the local atomic structure of ALD-grown aluminum oxide (AlOx) evolves in operando during growth by employing synchrotron high-energy X-ray diffraction (HE-XRD). In this work, we report on efforts to employ electron diffraction pair distribution function (ePDF) measurements using more broadly available transmission electron microscope (TEM) instrumentation to study the atomic structure of amorphous ALD-AlOx. We observe electron beam damage in the ALD-coated samples during ePDF at ambient temperature and successfully mitigate this beam damage using ePDF at cryogenic temperatures (cryo-ePDF). We employ cryo-ePDF and reverse Monte Carlo (RMC) modeling to obtain structural models of ALD-AlOx coatings formed at a range of deposition temperatures from 150 to 332 °C. From these model structures, we derive structural metrics including stoichiometry, pair distances, and coordination environments in the ALD-AlOx films as a function of deposition temperature. The structural variations we observe with growth temperature are consistent with temperature-dependent changes in the surface hydroxyl density on the growth surface. The sample preparation and cryo-ePDF procedures we report here can be used for the routine measurement of ALD-grown amorphous thin films to improve our understanding of the atomic structure of these materials, establish structure–property relationships, and help accelerate the timescale for the application of ALD to address technological needs.

Ba‐based complex perovskite ceramics with superior energy storage characteristics

AbstractLinear dielectrics are widely used to create high power capacitors, where it is a big challenge to achieve high energy storage density in such dielectrics. Here, Ba‐based complex perovskite ceramics with high dielectric strength, medium dielectric constant, and ultra‐low dielectric loss are proposed as the candidates for high energy storage density dielectric materials, and the significant effects of 1:2 B‐site ordering and ordering domain structure are systematically investigated. In Ba(Mg1/3Nb2/3)O3 ceramics, high dielectric strength of 1452 kV cm−1 combined with high energy storage density of 3.31 J cm−3 are achieved in the samples after post‐densification annealing, and they are 28% and 57%, respectively, higher than those in the as‐sintered samples. The significant enhancement of energy storage performance could be attributed to the increased B‐site ordering degree, and the uniform ordering domain structure. Furthermore, amorphous alumina thin films are introduced as the charge blocking layers, which significantly enhance the energy storage density to 5.09 J cm−3. The present work provides a new approach to develop the dielectric ceramics with high energy storage density.

Comproportionation Reaction Synthesis to Realize High‐Performance Water‐Induced Metal‐Oxide Thin‐Film Transistors

AbstractSolution‐processed metal‐oxide thin films have been widely studied in low‐power and flexible electronics. However, the high temperature required to form a condensed and uniform film limits their applications in flexible and low‐cost electronics. Here, a novel and environmental‐friendly comproportionation reaction synthesis (CRS) is presented to obtain amorphous aluminum oxide (AlOx) thin films for solution‐processed thin‐film transistors (TFTs) employing water as the precursor solvent. The thermal decomposition of CRS‐AlOx precursor is completed at ≈300 °C, which is 100 °C lower than that of the conventional water‐induced AlOx. The morphological, optical, compositional, and electrical properties of CRS‐AlOx dielectric films are studied systematically. Meanwhile, TFTs based on water‐induced In2O3 metal oxide semiconductor layers deposited on these dielectrics at low temperatures are formed and characterized. Compared with TFTs based on conventional AlOx showing low mobility and low clockwise hysteresis, In2O3 TFTs based on CRS‐AlOx exhibit improved electrical performance and counterclockwise hysteresis in the transfer curves. Water‐induced TFTs fabricated on CRS‐AlOx formed at a low temperature of 250 °C have average mobility of 98 cm2 V−1 s−1. Through chemical composition characterization and electrical characterization, the high mobilities of TFTs based on CRS‐AlOx dielectrics are correlated to trap states, which resulted in counterclockwise hysteresis in the transfer curves.

Synthesis, dielectric properties and application in a thin film transistor device of amorphous aluminum oxide AlxOy using a molecular based precursor route

Generation of dielectric amorphous aluminum oxide using a novel chimie douce molecular precursor route is reported.

Optically pumped rare-earth-doped Al2O3 distributed-feedback lasers on silicon [Invited

This paper reviews recent progress in the field of optically pumped rare-earth-doped channel waveguide lasers, with a focus on operation utilizing distributed-feedback resonators on silicon wafers. Rare-earth-doped amorphous aluminum oxide thin films have been deposited onto silicon wafers by RF reactive co-sputtering from metallic Al and rare-earth targets, the spectroscopy and optical gain of Er3+, Yb3+, Nd3+, and Tm3+ ions has been investigated, and the near-infrared laser transitions near 1 μm in Yb3+, 1.5 μm in Er3+, and 2 μm in Tm3+ and Ho3+ have been demonstrated. Output power between a few μW and hundreds of mW have been achieved in different waveguide geometries, and ultranarrow-linewidth laser operation has been demonstrated.

Dielectric property and self-repairing capability of silicon and titanium co-doped amorphous alumina thin films prepared by sol–gel technology

In this article, (Al1−0.02−xSi0.02Tix)2Oy (x = 0.2, 0.9 and 2%) thin films were prepared on Pt/Ti/SiO2/Si substrates using sol–gel technique. The dielectric properties of undoped and Si–Ti co-doped Al2O3 thin films were investigated. The leakage current of (Al0.971Si0.02Ti0.009)2Oy thin film is reduced by 2 orders of magnitude compared with Al2O3 film. Meanwhile, the modified sample exhibits the ultrahigh energy density of 14.01 J/cm3 under the breakdown strength of 647 MV/m, which is an enhancement of 11.26 J/cm3 over that of the undoped Al2O3 film. The improvement of dielectric properties is ascribed to the forming of Al–O–Si, Al–O–Ti bonds and the anodic oxidation of Ti3+, which could strengthen the stability of Al2O3 structure and self-repair the defects of the films under applied electric field. Another reason is that cation vacancies generated by Si–Ti co-doping could effectively prevent the formation of oxygen vacancies and decrease the breakdown probability of the films. This work provides a promising route to dielectric thin films materials for electrical energy storage applications.

Roughness scaling extraction method for fractal dimension evaluation based on a single morphological image

Fractal dimension (D) is an effective parameter to represent the irregularity and fragmental property of a self-affine surface, which is common in physical vapor deposited thin films. D could be evaluated through the scaling performance of surface roughness by using atomic force microscopy (AFM) measurement, but lots of AFM images with different scales (L) are needed. In this study, a roughness scaling extraction (RSE) method was proposed to evaluate D values of a single AFM image, in which the roughness at smaller L was estimated by image segmentation with flatten modification. Firstly, a series of artificial fractal surfaces with ideal dimension (Di) values ranging from 2.1 to 2.9 were generated through Weierstrass-Mandelbrot (W-M) function, in order to compare RSE method with traditional methods such as box counting method and power spectral density method. The calculated dimension (Dc) by RSE method was much closer to Di than the other methods, with a mean relative error of only 0.64%. Secondly, RSE method was utilized to deal with real surfaces, which were AFM images of amorphous alumina thin films with L of 1–70 μm. Dc obtained by RSE method based on a single AFM image was also close to the result in our previous study by multi-image analysis at L above 10 μm, while the larger Dc at smaller L was consisted with the actual surface feature. The validity of RSE method and the physics nature of real surfaces were discussed, which might be helpful to obtain more understandings of fractal geometry.

Significantly enhanced energy density of amorphous alumina thin films via silicon and magnesium co-doping

The different Si-Mg co-doping content was explored to improve the dielectric properties of amorphous Al2O3 thin film. According to the analysis of DSC, FT-IR, and XPS spectra, it can be confirmed that a novel structure of glass network is formed in the co-doped Al2O3 thin film. More importantly, compared to Al2O3 thin film, the leakage current of (Al.97Si.02Mg.01)2Oy thin film is reduced by 2 orders of magnitude and the breakdown strength is improved from 276 MV/m to 544 MV/m. The corresponding energy density of the modified sample is up to 9.2 J/cm3, which is an enhancement of 6.2 J/cm3 over that of the undoped Al2O3 thin film. Based on finite element analysis, the simulation results show that the applied electric field is mainly focused on the glass network, which could strengthen the stability of Al2O3 structure and decrease the breakdown probability of the films. From the viewpoint of defect chemistry, another reason for the enhancement of the dielectric properties is that Si-Mg co-doping results in the generation of cation vacancies and thus the formation of oxygen vacancies could be effectively prevented. This work could provide a new design strategy for high-performance dielectric capacitor devices.

Transition-Metal-Incorporated Aluminum Oxide Thin Films: Toward Electronic Structure Design in Amorphous Mixed-Metal Oxides

Amorphous metal oxides have numerous applications spanning from electronics to catalysis. The ability to rationally design the electronic structure of such materials would thus have significant impact. Previous work has shown that the amorphous structure allows for uniformly incorporating a large number of different metal cations into ultrasmooth thin films. Here we investigate how the atomic structure, electronic structure, and optical absorption properties of amorphous aluminum oxide (a-Al2O3) thin films are modulated when metal cations of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Ga are added to form MyAl1–yOx where y varies from 0.1 to 0.7. We use X-ray absorption spectroscopy (XAS) to analyze oxidation state and local bonding around the incorporated cations (e.g., coordination number), valence-band X-ray photoelectron spectroscopy (VB-XPS) to assess the impact of the cations on the valence band electronic structure, and optical absorption spectroscopy to assess the presence and energies of new unoccupied electronic states. Cations with partially filled d-orbitals (e.g., Fe3+, Cr3+, Co2+, Ni2+, Cu2+, etc.) add filled 3d electronic states at the valence band edge of a-Al2O3 as well as unfilled 3d electronic states within the a-Al2O3 band gap. Cations with the d10 electronic configuration, such as Zn2+ and Ga3+, introduce filled 3d electronic states at energies well below the valence band edge in addition to new unfilled electronic states in the a-Al2O3 band gap. Cations with unfilled d-orbitals (e.g., those with a d0 configuration such as V5+ and Cr6+) do not modify the valence band of a-Al2O3, but affect the optical properties by inserting unfilled 3d electronic states within the band gap. These results provide guidance for the rational design of the electronic structure of amorphous mixed-metal oxides.

Sol-gel processed high-k aluminum oxide dielectric films for fully solution-processed low-voltage thin-film transistors

High-k oxide dielectric films have attracted intense interest for thin-film transistors (TFTs). However, high-quality oxide dielectrics were traditionally prepared by vacuum routes. Here, amorphous high-k alumina (Al2O3) thin films were prepared by the simple sol-gel spin-coating and post-annealing process. The microstructure and dielectric properties of Al2O3 dielectric films were systematically investigated. All the Al2O3 thin films annealed at 300–600 °C are in amorphous state with ultrasmooth surface (RMS ~ 0.2 nm) and high transparency (above 95%) in the visible range. The leakage current of Al2O3 films gradually decreases with the increase of annealing temperature. Al2O3 thin films annealed at 600 °C showed the low leakage current density down to 3.9 × 10−7 A/cm2 at 3 MV/cm. With the increase of annealing temperature, the capacitance first decreases then increases to 101.1 nF/cm2 (at 600 °C). The obtained k values of Al2O3 films are up to 8.2. The achieved dielectric properties of Al2O3 thin films are highly comparable with that by vapor and solution methods. Moreover, the fully solution-processed InZnO TFTs with Al2O3 dielectric layer exhibit high mobility of 7.23 cm2 V−1 s−1 at the low operating voltage of 3 V, which is much superior to that on SiO2 dielectrics with mobility of 1.22 cm2/V−1 s−1 at the operating voltage of 40 V. These results demonstrate that solution-processed Al2O3 thin films are promising for low-power and high-performance oxide devices.

Surface roughness of aluminum oxide thin films deposited by DC and RF reactive magnetron sputtering

Aluminum oxide thin films produced by reactive sputtering processes are widely used in optical and microelectronic applications. This work investigates the dependency of the films’ surface roughness (Ra) on the operating pressure, the sputtering power and the deposition technique. Amorphous aluminum oxide thin films were grown on silicon (100) wafers by the DC and RF reactive magnetron sputtering deposition processes. In this experiment, high purity Al target (99.999%) was used. The operating pressures were set to 3, 5 and 10 mTorr. The sputtering powers were adjusted to 150 W and 300 W. Oxygen was injected into the chamber to react with sputtered Al atoms and formed aluminum oxide. For DC reactive magnetron sputtering, it was found that the surface roughness increased with increasing operating pressure. For the sputtering power of 300 W, the surface roughness increased from 1.03 nm to 2.17 nm as the operating pressure changed from 3 mTorr to 10 mTorr, respectively. It was also found that lower sputtering power resulted in smoother films. At 150 W sputtering power and 3 mTorr operating pressure, the Ra value was measured to be 0.81 nm. Higher operating pressure and sputtering power resulted in higher rates of particle collisions and scatterings. This reduced energies of sputtered atoms, preventing them to form dense and smooth films. The film deposited by the RF reactive magnetron sputtering yielded the lowest Ra value. At 150 W sputtering power and 3 mTorr operating pressure, the Ra value of the film was found to be 0.49 nm.

The linear and nonlinear refractive index of amorphous Al2O3 deduced from aluminosilicate optical fibers

AbstractThe Sellmeier coefficients for the linear refractive index of amorphous alumina (a‐Al2O3) are deduced from measurements on binary aluminosilicate glass optical fibers. This data is then used to approximate the nonlinear refractive index, n2, for a‐Al2O3, which was found to be 1.89 × 10−13 esu (4.77 × 10−20 m2/W at a wavelength of 958 nm) and found to increase slightly sublinearly with increasing alumina concentration in silica glass. Several commonalities with some amorphous alumina thin films are suggested. In addition, a proposal for the extension of a simple additive materials model for the calculation of n2 as a function of composition is presented. Alumina is an important dopant into silica for high energy optical fiber‐based lasers and so the determination and modeling of its optical properties is of significant technological value.

CaTiO3 linear dielectric ceramics with greatly enhanced dielectric strength and energy storage density

AbstractCaTiO3 is a typical linear dielectric material with high dielectric constant, low dielectric loss, and high resistivity, which is expected as a promising candidate for the high energy storage density applications. In the previous work, an energy density of 1.5 J/cm3 was obtained in CaTiO3 ceramics, where the dielectric strength was only 435 kV/cm. In fact, the intrinsic dielectric strength of CaTiO3 is predicted as high as 4.2 MV/cm. Therefore, it should be a challenge issue to enhance the dielectric strength and energy storage density of CaTiO3 ceramics by optimizing the microstructures. In the present work, dense CaTiO3 ceramics with fine and uniform microstructures are prepared by spark plasma sintering, and the greatly enhanced dielectric strength (910 kV/cm) and energy storage density (6.9 J/cm3) are obtained. This can be ascribed to the improved resistivity and thermal conductivity, associated with the fine and uniform microstructures. The different post‐breakdown features of CaTiO3 ceramics prepared by different process well interpret why the enhanced dielectric strength is achieved in the SPS sample. The energy storage density can be further improved to 11.8 J/cm3 by introducing the amorphous alumina thin films as the charge blocking layer, where the dielectric strength is 1188 kV/cm.

Refractive-index variation with rare-earth incorporation in amorphous Al2O3 thin films

Rare-earth-doped amorphous aluminum oxide (Al2O3:RE3+) thin films are attractive materials for near-IR amplifiers and lasers that can be integrated with silicon-on-insulator waveguides or deposited onto CMOS-fabricated integrated optical structures. We characterize reactively co-sputtered Al2O3:Tm3+ films on silicon by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The refractive index is systematically studied for different Tm3+ concentrations. The resulting increase of refractive index is explained by analyzing the mechanism of incorporating Tm3+ ions into the amorphous Al2O3 network. Sellmeier dispersion formulas are presented.