Crystalline Al2O3 Research Articles

Fabrication of ZnO–Al2O3 inverse opals with atomic layer deposited Amorphous-Al2O3 for enhanced photocatalysis

Zinc oxide inverse opal (IO) has excellent photocatalytic properties, while Al2O3 has high chemical stability and negligible photocatalytic activity. However, combining these materials creates a crystalline ZnO-amorphous Al2O3 composite IO photonic crystal material with improved photocatalytic performance. In this study, ZnO IO and ZnO–Al2O3 combined structures were grown on self-assembled polystyrene (PS) nanosphere templates. ZnO layer was coated by thermal atomic layer deposition (TALD) in the pores of the template, and the template was then removed by annealing to yield a self-standing ZnO IO nanostructure. An ultra-thin film of Al2O3 was coated on the top of ZnO IO by TALD or plasma-enhanced ALD (PEALD). SEM imaging, Raman spectroscopy, and XRD analysis confirmed the presence of a periodically arranged, and wurtzite ZnO IO structure, with an amorphous Al2O3 layer on top. The UV–Vis results demonstrated distinctive absorption bands in both regions, with a notable increase in visible light absorption attributed to the slow photon effect within the near-bandgap region of the photonic materials. The ZnO/Al2O3-TALD photocatalyst exhibited enhanced photocatalytic performance in degrading various pollutants including methylene blue, rhodamine 6G, and 4-nitrophenol under visible light illumination compared to its pristine IO and PEALD composite counterparts. This is attributed to its periodic arrangement of the IO structure that acts as a photonic crystal, precisely controlling light interaction through photonic band gap manipulation and the slow photon effect. This tailored light manipulation within the visible spectrum significantly enhances photodegradation efficiency.

Simulation Study of Crystalline Al2O3 Thin Films Prepared at Low Temperatures: Effect of Deposition Temperature and Biasing Voltage

In this paper, classical molecular dynamics simulations were used to explore the impact of deposition temperature and bias voltage on the growth of Al2O3 thin films through magnetron sputtering. Ion energy distributions were derived from plasma mass spectrometer measurements. The fluxes of deposited particles (Ar+, Al+, and O−) were categorized into low, medium, and high energies, and the results show that the films are dominated by amorphous Al2O3 at low incident energies without applying bias. As the deposition temperature increased, the crystallinity of the films also increased, with the crystals predominantly consisting of γ-Al2O3. The crystal content of the deposited films increased when biased with −20 V compared to when no bias was applied. Crystalline films were successfully obtained at a deposition temperature of 773 K with a −20 V bias. When biased with −40 V, crystals could be obtained at a lower deposition temperature of 573 K. Increasing the bias enables the particles to have higher energy to overcome the nucleation barrier of the crystallization process, leading to a greater degree of film crystallization. At this stage, the average bond length between Al-O is measured to be approximately 1.89 Å to 1.91 Å, closely resembling that of the crystal.

Two-step growth of alumina nanoparticle decorated graphene oxide surfaces: Effect on photocatalytic activity

As water pollution is increasing due to industrialization, there is a high demand for easy-processing photocatalytic materials to clean wastewater. Here, the improvement in the photocatalytic dye degradation effect of graphene oxide (GO) surfaces, made of ultra-sonication assisted modified Hummer's method, is demonstrated with increasing alumina (Al2O3) concentration in the range of 4–30 wt. %. Scanning electron microscopy and x-ray diffraction results suggest a gradual increment in crystalline Al2O3 nanoparticles (NPs) by reducing GO, in good agreement with the first-principles calculations. Moreover, x-ray photoelectron spectroscopy reveals the appearance of oxygenated functional groups with increasing Al2O3 concentration, leading to the formation of defect-rich GO as demonstrated by Raman spectroscopy. Ultraviolet-visible spectroscopy further reflects a maximum reduction in the optical bandgap from 1.88 to 1.56 eV up to a concentration of 15 wt. % Al2O3. Interestingly, the methylene blue degradation efficiency of GO under ultraviolet irradiation is also found to be improved from 45% to 64% with 15 wt. % Al2O3, whereas the formation of Al2O3 NPs in the range of 100–300 nm with 30 wt. % Al2O3 is found to be detrimental for photocatalytic activity.

Thermal stability of thick α- and γ-Al2O3 coatings deposited by high-speed PVD

α- and γ-Al2O3 coatings, are often used for high temperature applications T ≥ 800 °C. While stable α-Al2O3 is typically synthesized by chemical vapor deposition (CVD) due to its high formation temperature T ≥ 1000 °C, conventional physical vapor deposition (PVD) is limited to T ≈ 600 °C. Nevertheless, this method may result in metastable or amorphous Al2O3 phases, which limits their thermal stability and application at higher temperatures. Previous studies showed that High-Speed PVD can synthesize thick α- and γ-Al2O3 coatings at T ≈ 780 °C. This study investigates the thermal stability of these coatings deposited by HS-PVD. High-temperature X-ray diffraction (HT-XRD) measurements show no phase transformations of coatings up to T = 1200 °C in a vacuum or atmosphere. The coating morphology remained unchanged, and no cracks were detected by scanning electron microscopy (SEM) after annealing during HT-XRD. In situ, HT nanoindentation (HT-NI) revealed a reduction in indentation hardness of X ≈ 33 % and Y ≈ 40 % at THT-NI = 650 °C and THT-NI = 750 °C, respectively. However, hardness values remained similar to the as-deposited state after annealing in HT-XRD. These findings reveal a high thermal stability of crystalline Al2O3 coatings deposited by HS-PVD up to T = 1200 °C, which can extend the application of these coatings to higher temperatures.

Enhanced adhesion and thermal stability of thick (Al,Cr)2O3 coatings on hot work steel

Al2O3 coatings deposited by chemical vapor deposition (CVD) at temperatures T ˃ 1000 °C are commonly used for wear and oxidation protection in manufacturing processes. Physical vapor deposition (PVD) gained also great interest to synthesize crystalline Al2O3 at lower temperatures by modification using Cr. High Speed PVD has the potential to synthesize thick crystalline (Al,Cr)2O3 coatings. However, previous studies showed that increasing adhesion strength is necessary for corresponding applications. This study evaluated various interlayer systems to enhance the adhesion strength between (Al,Cr)2O3 functional layer and steel X40CrMoV5-1. AlCrN interlayers of varying thicknesses were deposited, as well as two graded AlCrNg interlayers at different bias voltages. Afterwards, AlCrN/(Al,Cr)2O3 coatings were produced. Hereby, the functional layer thickness is varied. The AlCrNg/(Al,Cr)2O3 compounds show no spallation at the edge of Rockwell imprints and scratches at higher normal forces. The thermal stability of the coatings was investigated by annealing in vacuum at T = 1000 °C and T = 1200 °C. Moreover, no diffusion processes leading to chemical and structural changes was obtained after annealing. Therefore, thick (Al,Cr)2O3 coatings, s ≈ 30 μm, with a graded AlCrNg interlayer provide high potential for protecting hot-work steel against thermal loads up to T = 1200 °C.

Nanostructured sustainable carbon derived from biomass as catalyst support for alumina in catalytic methanol conversion to DME as hydrogen carrier

A novel series of nanostructured catalysts composed of Al2O3 supported on a phosphorous (P) modified mesoporous carbon (ACP) derived from orange peel residues was formed. The formed, 1−7% (w/w) alumina/ACP, catalysts were investigated for conversion of methanol to dimethyl ether (DME) as a promising hydrogen carrier. The catalysts were characterized with a group of standard characterization techniques. Increasing of alumina loading caused remarkable structural and textural changes. Thus, new bands due to Al–O–P was detected by ATR−FTIR, which was confirmed by XPS as due to surface AlPO4 species. However, no crystalline Al2O3 and/or AlPO4 nanoparticles were detected by XRD or HRTEM. Meanwhile, the mesoporous surface texture was preserved. The loading of alumina led to increasing of Brønsted acidic sites population. The catalyst exhibited high conversion of methanol and complete selectivity for DME at low, DME formation rate of 17.88 and 19.85 mmol g−1 h−1 at 300 and 350 °C, respectively. The catalyst efficiency is among the most efficient traditional and recently reported catalysts. Thus, the work showed successful biomass valorization into efficient, selective, low price catalysts that facilitate hydrogen storage via conversion of methanol to DME. This conversion, which is advantageous in transportation and distribution as H2 carrier, will in fact support and advance the UN's sustainable development goals.

Finite-temperature effects on the x-ray absorption spectra of crystalline alumina from first principles

By including phonon-assisted transitions within plane-wave density functional theory methods for calculating the x-ray absorption spectrum (XAS), we obtain the Al K-edge XAS at 300 K for two crystalline Al2O3 phases. The 300 K XAS reproduces the pre-edge peak for α-Al2O3, which is not visible at the static lattice level of approximation. Configurations from Monte Carlo sampling of the γ-Al2O3 phase space at the 300 K XAS correctly describe two out of the three experimental peaks. We show that the second peak arises from 1s to mixed s-p transitions and is absent in the 0 K XAS. This letter serves as an insight into the electronic origins of the characteristic peaks in the Al K-edge XAS for alumina crystals.

Epitaxial growth of high quality Mn3Sn thin films by pulsed laser deposition

Noncollinear antiferromagnet Weyl semimetal Mn3Sn has recently attracted great research interest. Although large anomalous Hall effect (AHE), anomalous Nernst effect (ANE), and magneto-optical effect have been observed in Mn3Sn, most studies are based on single crystals. So far, it is still challenging to grow high quality epitaxial Mn3Sn thin films with transport and optical properties comparable to their single crystal counterparts. Here, we report the structure and magneto-optical and transport properties of epitaxial Mn3Sn thin films fabricated by pulsed laser deposition (PLD). Highly oriented Mn3+xSn1−x (0001) and (112¯0) epitaxial films are growth on single crystalline Al2O3 and MgO substrates. Large anomalous Hall effect up to ΔρH=3.02 μΩ cm and longitudinal magneto-optical Kerr effect with |θK| = 38.1 mdeg at 633 nm wavelength are measured at 300 K, which are comparable to Mn3Sn single crystals. Our work demonstrates that high quality Mn3Sn epitaxial thin films can be fabricated by PLD, paving the way for future device applications.

In situ investigation of aluminum oxide under irradiation by electron beam at atomic-scale

It has yet to be experimentally proven whether the native aluminum oxide layer can still effectively protect pure aluminum from further degradation under an irradiation environment. Here, we directly investigate the microstructure evolution of amorphous Al2O3 in real time by using an electron beam as an irradiation source with in-situ transmission electron microscopy (TEM). At the atomic scale, crystalline Al2O3 grains were found to nucleate and grow along amorphous substrate. With continuous irradiation, both amorphous and newly formed crystalline Al2O3 were removed completely, and then the internal fresh aluminum was exposed. By regulating electrons, new oxides quickly formed on the fresh metal surface and merged with adjacent old oxides, finally leaving a uniform amorphous oxide protection layer. Our work provides experimental evidence for understanding the protection mechanism for the oxide layer on its internal metal under a special service environment.

Role of Nb in promoting protective scale formation on Al2O3-forming austenitic alloys

The high-temperature oxidation behavior of Fe–25Ni–17Cr–7.7Al–0.03Zr alloys with and without 0.5 or 1 at% Nb addition was investigated at 800 °C in air. Nb addition was found to promote the formation of an external Al2O3 scale. The Nb-free alloy formed a scale consisting of a thick Fe/Ni-rich oxide layer and a Cr2O3 layer, beneath which Al was internally oxidized. However, a continuous protective Al2O3 scale was developed below a thin Cr2O3 layer on the 1 Nb alloy, which significantly decreased the oxidation rate. Examination of the initial oxidation process during heating to 800 °C revealed that all of the alloys formed a Cr2O3 layer and an amorphous Al2O3 layer upon heating to 650 °C, but these layers became discontinuous upon further heating to 800 °C for the Nb-free alloy. The beneficial role of Nb was ascribed to its ability to maintain the initially formed thin Cr2O3 layer by the formation of a Nb-containing Cr-rich oxide layer at the interface between the Cr2O3 and amorphous Al2O3 layers. This Nb-containing Cr-rich layer prevented the breakdown of the transient amorphous Al2O3 layer until its transition to a protective crystalline Al2O3 scale.

Chemical Stability of Atomic Layer Deposited (ALD) Alumina Thin Films in Aqueous Solutions

Aluminum oxide (alumina) thin films deposited through atomic layer deposition (ALD) are of great interest in chemical barrier and corrosion protection applications. However, the stability of ALD alumina in aqueous solutions is still not fully understood. Due to its metastable amorphous phase, the hydration and degradation behavior of ALD alumina films behaves differently from its crystalline Al2O3 counterpart. A full understanding of why these films hydrate and/or dissolve requires the exploration of different deposition conditions and ion content in solutions used. This talk will discuss efforts to further elucidate the hydration and dissolution behavior of ALD alumina films. For this study, alumina thin films were ALD deposited onto silicon substrate at 150 °C using trimethylaluminum (TMA) and H2O. These films were then studied in Type 1 DI water, 2M NaCl, .5M NaCl, and .1M NaCl solutions at room temperature. Films were gently dried using a nitrogen gun and thickness was measured using a Cauchy ellipsometry model. After 15 days of immersion in Type 1 DI water, significant thickness growth is observed at twice (27 days) and 2.5 times (33 days) the normalized thickness. Similar hydration is not observed in salt-containing aqueous solutions nor upon exposure to air. We will discuss the possible effects of CO2 dissolution and carbonate formation as well as ionic species on the hydration and dissolution processes of these alumina films. Finally, we will show how a post-ALD plasma treatment to the film’s surface alters the hydration and dissolution behavior, improving stability in several aqueous conditions.

Role of vibrational properties and electron–phonon coupling on thermal transport across metal-dielectric interfaces with ultrathin metallic interlayers

We systematically analyze the influence of 5 nm thick metal interlayers inserted at the interface of several sets of different metal-dielectric systems to determine the parameters that most influence interface transport. Our results show that despite the similar Debye temperatures of Al2O3 and AlN substrates, the thermal boundary conductance measured for the Au/Al2O3 system with Ni and Cr interlayers is ∼2× and >3× higher than the corresponding Au/AlN system, respectively. We also show that for crystalline SiO2 (quartz) and Al2O3 substrates having highly dissimilar Debye temperature, the measured thermal boundary conductance between Al/Al2O3 and Al/SiO2 are similar in the presence of Ni and Cr interlayers. We suggest that comparing the maximum phonon frequency of the acoustic branches is a better parameter than the Debye temperature to predict the change in the thermal boundary conductance. We show that the electron–phonon coupling of the metallic interlayers also alters the heat transport pathways in a metal-dielectric system in a nontrivial way. Typically, interlayers with large electron–phonon coupling strength can increase the thermal boundary conductance by dragging electrons and phonons into equilibrium quickly. However, our results show that a Ta interlayer, having a high electron–phonon coupling, shows a low thermal boundary conductance due to the poor phonon frequency overlap with the top Al layer. Our experimental work can be interpreted in the context of diffuse mismatch theory and can guide the selection of materials for thermal interface engineering.

Dissolution Behavior of Different Alumina Phases within Plasma Electrolytic Oxidation Coatings

The influence of chemical redissolution in the PEO layer-growth equilibrium on aluminum is evaluated differently in literature. In order to estimate whether and to what extent various alumina modifications could be affected by redissolution processes during PEO, immersion experiments were carried out on PEO coatings in model electrolytes of different alkalinity and silicate concentration. Their composition was determined spectroscopically before and after the experiments. The layers were characterized by XRD before and after the tests and examined at affected and unaffected zones using SEM, EDX, and EBSD. The results show that chemical oxide dissolution can only be observed at the layer/substrate interface and that primarily amorphous alumina is affected. This process is intensified by higher alkalinity and inhibited by silicate additives. The crystalline Al2O3 modifications show no significant attack by the electrolytes used. Transferring these observations to plasma electrolytic oxidation, they allow the conclusion that the electrochemically active zone in the pore ground of discharge channels interacts with the electrolytic and electrical process parameters throughout the entire PEO procedure. Influences of bath alkalinity and silicate content on layer growth rates are to be understood as impact on the passivation processes at the layer/substrate interface rather than chemical redissolution.

Resolving the Heat of Trimethylaluminum and Water Atomic Layer Deposition Half-Reactions.

Atomic layer deposition (ALD) is a surface synthesis technique that is characterized by self-limiting reactions between gas-phase precursors and a solid substrate. Although ALD processes have been demonstrated that span the periodic table, a greater understanding of the surface chemistry that affords ALD is necessary to enable greater precision, including area- and site-selective growth. We offer new insight into the thermodynamics and kinetics of the trimethylaluminum (TMA) and H2O ALD half-reactions with calibrated and time-resolved in situ pyroelectric calorimetry. The half-reactions produce 3.46 and 2.76 eV/Al heat, respectively, which is greater than the heat predicted by computational models based on crystalline Al2O3 substrates and closely aligned with the heat predicted by standard heats of formation. The pyroelectric thin-film calorimeter offers submillisecond temporal resolution that uniquely and clearly resolves precursor delivery and reaction kinetics. Both half-reactions are observed to exhibit multiple kinetic rates, with average TMA half-reaction rates at least 2 orders of magnitude faster than the H2O half-reaction kinetics. Comparing the experimental heat with published computational literature and additional first-principles modeling highlights the need to refine our models and mechanistic understanding of even the most ubiquitous ALD reactions.

Laser MBE growth of cobalt-platinum multilayers with perpendicular magnetic anisotropy

Periodic multilayers with perpendicular magnetic anisotropy (PMA) attract a lot of attention nowadays being fundamentally interesting from the point of view of interface magnetism. They also provide a suitable test bench for the studies of spatially resolved magnetization dynamics at picosecond time scale conducted at x-ray free electron laser (XFEL) facilities. In the present paper we have developed a novel laser molecular beam epitaxy (LMBE) approach to grow Co/Pt multilayers on crystalline Al2O3 (0001) substrates. By applying Kerr-effect magnetometry to the samples fabricated at different conditions, we have found it essential for the appearance of PMA, to perform metal deposition in 0.03 mbar of argon and to keep Co layer thickness below 4 Å. Along with magnetic studies, we have investigated crystal structure and depth resolved layer composition of the fabricated Co/Pt multilayers by applying electron diffraction, x-ray diffraction and x-ray reflectometry. The combined atomic-force/magnetic-force microscopy studies have revealed a very low surface roughness of down to 1 Å (for the samples grown at properly optimized growth conditions) and a pronounced labyrinth pattern of magnetic domains with a periodicity of 250 nm appropriate for XFEL studies of ultrafast optical magnetization. The presented LMBE approach to fabricate magnetically ordered heterostructures with a pronounced PMA is supposed to be an important milestone on the way of facilitating future design of nanomaterials for applications in magnetic memories and for fundamental studies of ultrafast magnetization dynamics.

Congruent dissolution of Al2O3 in a K-disilicate melt, with constraints on the reaction mechanism

High resolution O 1s X-ray Photoelectron Spectroscopy (XPS) was employed to determine oxygen speciation in 31 mol% K2O silicate glass containing 0.0, 1.0 and 3.0 mol% dissolved Al2O3. Highly resolved bridging oxygen (BO) and non-bridging oxygen (NBO) peaks were observed. The NBO peak intensity decreased and the BO increased with dissolution of Al2O3. The enhanced BO intensity is localized at two binding energies (BE), one characteristic of a Si-O-Si moiety (e.g., quartz, or v-SiO2), the other of a Si-O-Al moiety (e.g., kyanite, Al2SiO5). The dissolution reaction proceeds in two steps; NBO− of a Q3 species attacks and bonds to an under-coordinated surface Al atom of crystalline Al2O3. The reaction produces a Si-O-Al surface species thus converting the attacking NBO to a BO and altering the reactant Q3 species to a Q4 product species:[Q3]−melt + [AlO1.5]surface → [Q4]0melt + [AlO2]−meltThe second step involves reaction of [AlO2]−melt with K+converting it from a modifier cation to a charge compensator, resulting in the polymerization reaction:[K-Q3]0melt + [AlO2]−melt → [Q4]0 + [KAlO2]0melt + 1/2O2−The stoichiometric reaction is the sum of the two:[Q3]−melt + [K-Q3]0melt + [AlO1.5]surface → 2[Q4]0 + [KAlO2]0melt + 1/2O2−Al species detached from the solid surface initially must be bonded to some NBO. Within the melt, nucleophilic substitution attack of the Al-bearing species by Q3 species converts NBO to BO, resulting in [KAlO2]melt being bonded four BOs. This reaction may not go to completion in all melts, depending on dissolved Al content and melt composition.

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.

In-depth analysis of anisotropic magnetoconductance in Bi2Se3 thin films with electron–electron interaction corrections

A combination of out-of-plane (OOP) and in-plane (IP) magnetoconductance (MC) study in topological insulators (TI) is often used as an experimental technique to probe weak anti-localization (WAL) response of the topological surface states (TSSs). However, in addition to the above WAL response, weak localization (WL) contribution from conducting bulk states are also known to coexist and contribute to the overall MC; a study that has so far received limited attention. In this article, we accurately extract the above WL contribution by systematically analyzing the temperature and magnetic field dependency of conductivity in Bi2Se3 films. For accurate analysis, we quantify the contribution of electron–electron interactions to the measured MC which is often ignored in the WAL studies. Moreover, we show that the WAL effect arising from the TSSs with finite penetration depth, for OOP and IP magnetic field can together explain the anisotropic magnetoconductance (AMC) and, thus, the investigated AMC study can serve as a useful technique to probe the parameters like phase coherence length and penetration depth that characterise the TSSs in 3D TIs. We also demonstrate that increase in bulk-disorder, achieved by growing the films on amorphous SiO2 substrate rather than on crystalline Al2O3(0001), can lead to stronger decoupling between the top and bottom surface states of the film.

Electronic phase derived impedance spectroscopic behavior of La0.5Nd0.2A0.3MnO3 manganites

In the present communication, structural, microstructural and electrical properties of La0.5Nd0.2Ca0.3MnO3 (LNCMO) and La0.5Nd0.2Sr0.3MnO3 (LNSMO) mixed valent manganite films, grown on single crystalline (0001) Al2O3 (ALO) substrates using pulsed laser deposition (PLD) technique, have been studied. After deposition, one set of LNCMO and LNSMO films were annealed in oxygen environment at 500 °C (hereafter referred as LNC–A & LNS–A, respectively) and compared their properties with other set of PLD as–grown LNCMO and LNSMO films (hereafter referred as LNC and LNS, respectively). Single crystalline growth of LNCMO films and polycrystalline growth of LNSMO films have been identified through X–ray diffraction (XRD) measurement. Atomic force microscopy (AFM) measurement for morphological studies indicates that divalent dopant and oxygen annealing process play an important role in governing the grain growth and surface nature of the studied films. A decrease in the value of resistance (R) and impedance (Z) with increase in frequency under the applied magnetic field (H = 1 T) has been observed for all the studied films. For LNC & LNC–A films, reactance is found to decrease monotonically with increase in frequency throughout its range studied while for LNS and LNS–A films, reactance initially increases followed by reduction in its value with increase in frequency under 0 and 1 T magnetic field has been observed. The variation in MR for LNS and LNS–A films has been discussed in the context of charge transport mechanism and Lorentz force. Further, to discuss the role of grain and grain boundaries, nyquist plots and their suitable fits under zero and 1 T external magnetic field have been investigated for all the studied mixed valent manganite films. Important role of phase separation scenario in studied manganite films has been discussed in detail based on magnetization measurement. Double magnetic phase transition and fluctuations in derivative of magnetization have been ascribed to the phase separation scenario of LNCMO / ALO and LNSMO / ALO mixed valent manganite thin films.

Thermal atomic layer etching of amorphous and crystalline Al2O3 films

Thermal atomic layer etching (ALE) can be achieved with sequential, self-limiting surface reactions. One mechanism for thermal ALE is based on fluorination and ligand-exchange reactions. For metal oxide ALE, fluorination converts the metal oxide to a metal fluoride. The ligand-exchange reaction then removes the metal fluoride by forming volatile products. Previous studies have demonstrated the thermal ALE of amorphous Al2O3 films. However, no previous investigations have explored the differences between the thermal ALE of amorphous and crystalline Al2O3 films. This study explored the thermal ALE of amorphous and crystalline Al2O3 films. HF, SF4, or XeF2 were used as the fluorination reactants. Trimethylaluminum (TMA) or dimethylaluminum chloride (DMAC) were used as the metal precursors for ligand-exchange. Spectroscopic ellipsometry measurements revealed that the amorphous Al2O3 films had much higher etch rates than the crystalline Al2O3 films. When using HF and TMA at 300 °C, the amorphous Al2O3 film was removed at an etch rate of 0.78 Å/cycle. For the crystalline Al2O3 film, an etch rate of 0.06 Å/cycle was initially observed prior to the stoppage of etching after removing about 10 Å of the film. Thermal ALE with HF and DMAC resulted in similar results. Etch rates of 0.60 and 0.03 Å/cycle were measured for amorphous and crystalline Al2O3 films at 300 °C, respectively. Other fluorination agents, such as SF4 or XeF2, were also used together with TMA or DMAC for Al2O3 ALE. These reactants for fluorination and ligand-exchange were able to etch amorphous Al2O3 films at 300 °C. However, they were unable to etch crystalline Al2O3 film at 300 °C beyond the initial 10–20 Å surface layer. The investigations also examined the effect of annealing temperature on the etch rate per cycle using HF and TMA as the reactants at 300 °C. Amorphous Al2O3 films were etched at approximately the same etch rate of 0.78 Å/cycle until the crystallization of amorphous Al2O3 films at ≥ 880 °C. The differences between amorphous and crystalline Al2O3 thermal ALE could be used to obtain selective thermal ALE of amorphous Al2O3 in the presence of crystalline Al2O3.