Cross-sectional Transmission Electron Microscopy Research Articles

Optical constants and thickness determination of La[formula omitted]Sr[formula omitted]MnO3 thin films on Nb:SrTiO3 substrates by spectro-ellipsometry: Combination of optical and X-ray techniques

40 nm thick La2/3Sr1/3MnO3 (LSMO) thin films were epitaxially grown by Pulsed Laser Deposition (PLD) onto niobium doped SrTiO3 (Nb:STO) substrates, with different Nb concentration from 0.01%wt to 0.5%wt. The optical characterization of the heterostructures by spectroscopic ellipsometry enables us to extract the optical constants of the manganite heteroepitaxial layer at room temperature. Performing spectrophotometry in the same wavelength range brings a useful cross-validation of the extracted results. In addition, the thickness evaluation of the LSMO layer by spectro-ellipsometry is further validated by both High Resolution X-ray diffraction and X-ray reflectivity, as well as a Transmission Electron Microscopy cross section, taken as a physical reference. This study validates quantitatively the spectro-ellipsometry as a suitable routine tool to measure accurately both thickness and complex refractive index of the LSMO thin film, picturing their peculiar electrical behaviour between both metallic and insulating phases. The relative error on the thickness measurement between X-ray and ellipsometry is less than 5%. The LSMO complex refractive index enabled also a simultaneous estimation of further material properties, such as the optical gap ωg or the mass density ρm, determined with less than 1.5% relative error compared to X-ray reflectivity results.

Co-deposited layers on gap surfaces of bulk tungsten divertor tiles in JET ITER-like wall: Directional effects and nanostructures

Co-deposited layers on surfaces of bulk tungsten divertor tiles (W lamellae) from the first campaign of JET with the ITER-Like Wall (JET-ILW, 2011–2012) were examined by means of a cross-sectional transmission electron microscope observation. The focus was on geometrical effects in impurity deposition, mainly beryllium (Be), on surfaces located in the poloidal gap separating adjacent lamellae. The study was carried out on the sides (gap surfaces) of two W lamellae from Stack C (located on the outboard part of the horizontal section of the lower divertor), from the region most exposed to the plasma (lamella C23) and in the magnetic shadow of the upstream divertor module (lamella C3). The tile manufacturing process (cold rolling) left shallow grooves, i.e. structures classified as convex (hill) and concave (valley) regions. These regions are decisive for the deposition structure. The main results were: (i) two kinds of impurity deposition features, “homogeneous” and “directional”, have been distinguished; (ii) the directional ones were characterized by nanoscale inclined vertical stripes in the deposition layer; (iii) homogeneous deposition without directional features were in the valley region. The results clearly indicate the impact of the surface finish, even in the tile gaps, on the qualitative and quantitative aspects of deposition.

Angle-resolved X-ray photoelectron spectroscopy intensity modeling of SiNx ultrathin layer grown on Si (100) and Si (111) substrates by N2 plasma treatment

In this work, we investigate the silicon nitridation process using a N2 plasma Glow Discharge Source (GDS) in an ultra-high vacuum chamber. In situ Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS) measurements were performed to determine various chemical environments of the surface species. The main goals of this study are determination of the film chemical composition, to estimate its thickness and identify various phenomena taking place during the nitridation process under different experimental conditions. To achieve these objectives, we developed an ARXPS intensity model for an ultrathin SiNx overlayer. Based on the ARXPS results, which indicate that the silicon nitride film is not homogenous, we adopted a bi-layer model composed of two SixNyOz layers with varying thickness and chemical composition. The model is built from analysis of photoelectron intensities, considering a comparison between experimental and theoretical data. This bi-layer model revealed that the silicon nitride layer consists of a surface layer composed of approximately equal amounts of silicon and nitrogen (50% each), and an interfacial layer containing varying amounts of oxygen depending on experimental parameters. The presence of oxygen is due to residual air contamination in the vacuum chamber and/or to Si native oxide. The estimated thickness of silicon nitride film is less than 9 nm, depending on the nitridation time and temperature.The model developed in this work enabled us to study the effects of initial surface state (as received or chemically cleaned), Si substrate crystallographic orientation (such as (100) and (111)), and the nitridation temperature (room temperature and 500°C). The diffusing species (N+, N2+, O+), which play a crucial role during nitridation, were found to be influenced by temperature, treatment time, and substrate orientation. The diffusion phenomenon competed with other processes such as NO desorption, leading to a significant impact on formation of the SiNx film. Furthermore, we compared the thickness obtained from the ARXPS model using cross-sectional High-Resolution Transmission Electron Microscopy (HR-TEM) images. Additional Low-Energy Electron Diffraction (LEED) measurements indicated that the SiNx film was amorphous.

Giant polarization in tetragonal-like BiFeO3 film via (Ba,Ca)TiO3 doping

BiFeO3 is a multiferroic material, its tetragonality (T) phase usually exhibits large tetragonality (c/a ratio) and strong ferroelectricity. Unlike the commonly used method of strain engineering via substrate, here we present a general and practical approach for obtaining T-like phase BiFeO3 films through (Ba,Ca)TiO3 doping (BF-BCT). The BF-BCT film coherently grows on the La0.7Sr0.3MnO3/(La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) substrate even at 180 nm thick, implying a very large critical thickness, and shows a large tetragonality of 1.12. Impressively, the films exhibit a giant remanent polarization (2Pr) of 320 μC/cm2, which is higher than the corresponding values of any other lead-free ferroelectric films reported to date. Cross-sectional scanning transmission electron microscopy measurements confirm the presence of the T-like phase in BF-BCT films, where the giant polarization might be attributed to the off-center B-site atom. The BF-BCT films are structurally homogeneous without any precipitation of the second phase. The giant polarization observed in the BF-BCT films indicate their great potential for application in next-generation storage and information devices.

Impact of B4C buffer layer on interface diffusion in Cr/Sc multilayers: combined study by x-ray reflectivity, scattering and fluorescence

In thin film multilayer based optical componentsof x-ray imaging system, diffusion of one material into the other degrades the reflectivity of the mirrors severely. Along with this thermodynamically driven diffusion, there are also growth generated interface roughness of different special frequencies and microstructures which can increase the diffused scattering from the multilayer and reduce the resolution of an image. Generally grazing incidence x-ray reflectivity in specular geometry (specular GIXR) and diffused x-ray scattering measurement in rocking scan geometry yield information regarding microstructure and overall diffusion at the interfaces of a multilayer. In this paper it is shown that grazing incidence x-ray fluorescence (GIXRF) measurement in standing wave condition alongwith the above measurements can give precise information regarding element-specific diffusion at the interfaces of a multilayer structure. Periodic multilayers made of 75 Cr/Sc bilayers with bilayer thickness ∼4 nm with and without B4C barrier layer of 0.2 nm thickness at the interfaces have been prepared using ion beam sputtering system and characterized by GIXR, diffused x-ray scattering and GIXRF measurements using synchrotron x-ray radiation just above the Cr K-edge. From the above measurements, drastic reduction in interface diffusion of Cr and improvement of interface morphology after addition of B4C barrier layer at the interfaces of Cr/Sc multilayers have been observed which is also corroborated by cross-sectional transmission electron microscopy of the multilayers. Finally, in the water window soft x-ray region of 2.3–4.4 nm performance of these multilayers have been tested and the Cr/B4C/Sc multilayer with improved interface quality has been found to yield ∼30.8% reflectivity at 3.11 nm wavelength which is comparable with the best reported reflectivities in the literature at this wavelength.

Characterization of Polish-Induced Gouges on Single Crystal Sapphire Substrates

Single-crystal sapphire is known to be among the hardest insulators. Its mechanical properties and chemical inertness make it a challenging material to polish for the atomic-level surface smoothness required for its applications. Mechanical polish with diamond abrasives renders high removal rates but creates unacceptable levels of polish-induced gouges. Chemical mechanical polish on the other hand results in atomic smoothness but is a slow process. Hence, a combination of the two is used in the industry. In this work, we have attempted to characterize gouging and subsurface damage using atomic force microscopy, X-ray diffraction, and cross-section transmission electron microscopy on C-plane and A-plane sapphire induced by diamond abrasive mechanical polish and chemical mechanical polish with colloidal silica.

Lithium Ion Intercalation-Induced Metal-Insulator Transition in Inclined-Standing Grown 2D Non-Layered Cr2S3 Nanosheets.

Gate-controlled ionic intercalation in the van der Waals gap of 2D layered materials can induce novel phases and unlock new properties. However, this strategy is often unsuitable for densely packed 2D non-layered materials. The non-layered rhombohedral Cr2S3 is an intrinsic heterodimensional superlattice with alternating layers of 2D CrS2 and 0D Cr1/3. Here an innovative chemical vapor deposition method is reported, utilizing strategically modified metal precursors to initiate entirely new seed layers, yields ultrathin inclined-standing grown 2D Cr2S3 nanosheets with edge instead of face contact with substrate surfaces, enabling rapid all-dry transfer to other substrates while ensuring high crystal quality. The unconventional ordered vacancy channels within the 0D Cr1/3 layers, as revealed by cross-sectional scanning transmission electron microscope, permitting the insertion of Li+ ions. An unprecedented metal-insulator transition, with a resistance modulation of up to six orders of magnitude at 300 K, is observed in Cr2S3-based ionic field-effect transistors. Theoretical calculations corroborate the metallization induced by Li-ion intercalation. This work sheds light on the understanding of growth mechanism, structure-property correlation and highlights the diverse potential applications of 2D non-layered Cr2S3 superlattice.

Microstructure Evolution and Fretting Wear Mechanisms of Steels Undergoing Oscillatory Sliding Contact in Dry Atmosphere.

Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time. During the accumulation of fretting cycles, the subsurface microstructure evolved into stable dislocation cells with cell walls aligned parallel to the surface and the sliding direction. The thickness of the damaged subsurface region increased with the load, consistent with the depth distribution of the maximum shear stress. The primary surface oxide evolved as Fe2O3 and Fe3O4 with increasing sliding time, leading to the formation of a uniform oxide scale at the sliding surface. It is possible that the development of the dislocation cell structure in the subsurface also enhanced oxidation by pipe diffusion along dislocation cores. The results of this study reveal complex phase changes affecting the wear resistance of steels undergoing fretting wear, which involve a synergy between oxidative wear, crack initiation, and crack growth along dislocation cell walls due to the high strains accumulating under high loads and/or prolonged surface sliding.

High-temperature annealing effects on epitaxial TiN films on 4H-SiC

To gain deeper insights into the impact of very high-temperature annealing on TiN film quality and thermal diffusion phenomena, the deposited TiN films underwent annealing in an induction heating furnace across a temperature range from 1150 °C to 1700 °C. Our findings indicate that TiN films remain stable, and the full-width at half-maximum (FWHM) of TiN (111) noticeably reduced by about 30 % when annealed at 1250 °C. However, as the annealing temperature rises to 1400 °C, the film composition gradually enriches with carbon as TiCxN1-x with x up to 0.5. After annealing at 1700 °C, the film is further enriched with x larger than 0.5. For a long time annealing at 1700 °C, the decomposition of SiC substrate with void formation is verified from observations by cross-sectional transmission electron microscopy.

The Incorporation of Graphene Nanoplatelets in Tung Oil-Urea Formaldehyde Microcapsules: A Paradigm Shift in Physicochemical Enhancement.

Tung oil (TO) microcapsules (MCs) with a poly(urea-formaldehyde) (PUF) shell were synthesized via one-step in situ polymerization, with the addition of graphene nanoplatelets (GNPs) (1-5 wt. %). The synergistic effects of emulsifiers between gelatin (gel) and Tween 80 were observed, with gel chosen to formulate the MCs due to its enhanced droplet stability. SEM images then displayed an increased shell roughness of the TO-GNP MCs in comparison to the pure TO MCs due to the GNP species on the shell. At the same time, high-resolution transmission electron microscopy (TEM) images also confirmed the presence of GNPs on the outer layer of the MCs, with the stacked graphene layers composed of 5-7 layers with an interlayer distance of ~0.37 nm. Cross-sectional TEM imaging of the MCs also confirmed the successful encapsulation of the GNPs in the core of the MCs. Micromanipulation measurements displayed that the 5% GNPs increased the toughness by 71% compared to the pure TO MCs, due to the reduction in the fractional free volume of the core material. When the MCs were dispersed in an epoxy coating and applied on a metallic substrate, excellent healing capacities of up to 93% were observed for the 5% GNP samples, and 87% for the pure TO MC coatings. The coatings also exhibited excellent corrosion resistance for all samples up to 7 days, with the GNP samples offering a more strenuous path for the corrosive agents.

Experimental determination of the band offsets at the UWBG p-LiGa5O8/Ga2O3 interface

LiGa5O8, a recently discovered ultrawide bandgap semiconductor exhibiting p-type conductivity at room temperature, is grown on (010) β-Ga2O3 substrate. Utilizing a mist chemical vapor deposition method, LiGa5O8 thin film grown on insulating Ga2O3 substrate exhibits hole concentration ∼2.31 × 1018 cm−3 and hole mobility ∼2.07 cm2/V s. The resulting heterostructure is characterized comprehensively. A smooth, uniform film growth was observed by scanning electron microscopy and atomic force microscopy imaging. Cross-sectional scanning transmission electron microscopy and x-ray diffraction confirm the growth of spinel cubic crystal structure of LiGa5O8 along the [110] direction. Moreover, the valence and conduction band offsets at p-LiGa5O8/β-Ga2O3 heterointerface were determined via x-ray photoelectron spectroscopy. The determined band alignments at LiGa5O8/β-Ga2O3 interface reveal the formation of a type-II (staggered) heterojunction. The experimental measurements provide valuable parameters of the band offsets at the heterointerface, addressing a crucial aspect for future power electronic device design and fabrication based on β-Ga2O3 ultrawide bandgap semiconductors.

Vertically Stacked Amorphous Ir/Ru/Ir Oxide Nanosheets for Boosted Acidic Water Splitting.

Integrating multiple functional components into vertically stacked heterostructures offers a prospective approach to manipulating the physicochemical properties of materials. The synthesis of vertically stacked heterogeneous noble metal oxides remains a challenge. Herein, we report a surface segregation approach to create vertically stacked amorphous Ir/Ru/Ir oxide nanosheets (NSs). Cross-sectional high-angle annular darkfield scanning transmission electron microscopy images demonstrate a three-layer heterostructure in the amorphous Ir/Ru/Ir oxide NSs, with IrOx layers located on the upper and lower surfaces, and a layer of RuOx sandwiched between the two IrOx layers. The vertically stacked heterostructure is a result of the diffusion of Ir atoms from the amorphous IrRuOx solid solution to the surface. The obtained A-Ir/Ru/Ir oxide NSs display an ultralow overpotential of 191 mV at 10 mA cm-2 toward acid oxygen evolution reaction and demonstrate excellent performance in a proton exchange membrane water electrolyzer, which requires only 1.63 V to achieve 1 A cm-2 at 60 °C, with virtually no activity decay observed after a 1300 h test.

Thermal annealing induced graphite/diamond structure processed by high-voltage hydroxide ion treatments

Ohmic contacts with low specific contact resistance are necessary for fabrication of diamond electronic devices. This work reports that graphite/diamond ohmic contacts can be obtained by thermal annealing of diamond, patterned by hydroxide ion treatment. Surface graphitization of diamond is achieved through rapid thermal annealing at 1050 °C in vacuum. The thickness of graphite layer is found to be dependent on the duration of annealing with an approximate rate of 3.3 ∼ 3.6 nm/minute. The alteration in surface structure is analyzed using Raman spectra, showing a continuously enhanced G peak with increasing annealing time. Early stages of graphitization are discovered by X-ray photoelectron spectroscopy, after annealing diamond samples for merely 0.5 ∼ 2 min. Additionally, a distinct atomic interface between graphite and diamond is observed via cross-sectional transmission electron microscopy. Further, the surface graphite is selectively etched by hydroxide ion treatments, protected by gold patterns. In the end, the specific contact resistance of the graphite/diamond contact is determined to be 2.53 × 10-4 Ω cm2 employing the transmission line model.

Al2O3 Thin Layer Formed inside Porous Membrane Using Spray Synthesis Method and Its Application

Aluminum oxide (Al2O3) films have been investigated for use in various applications, and numerous deposition techniques have been reported. The spray synthesis method has the advantage of forming a thin layer of crystal at low temperatures using the appropriate precursors. A precursor prepared by diluting Methylaluminoxane with N-methyl pyrrolidone was sprayed onto a porous membrane while varying conditions such as the substrate temperature, feeding speed, and spray amount. The solution penetrated the film during spray application, and the ultra-thin layers deposited on the side wall of the internal pores were observed using a cross-sectional transmission electron microscope (XTEM). The lattice image obtained using the TEM and the composition analysis conducted using a scanning TEM and an energy-dispersive X-ray spectroscope suggest that this thin layer is a layer of Al2O3. The formation of Al2O3 occurred at lower temperatures than in previous reports. This is a major advantage for applications with low-melting-point materials. The most suitable spraying conditions were determined based on the state of deposition on the surface and inside the membrane. These conditions were applied to a three-layer separator for lithium-ion batteries and their effect on thermal stability was investigated. Through heating experiments and XRD analysis, it was confirmed that the shrinkage and melting of the separator are suppressed by spraying. This process can be expected to have wide applications in low-melting-point materials such as polyolefin.

Atomic layer deposition of ZnO thin films using a liquid cyclopentadienyl-based precursor

A new compound, bis(n-propyltetramethylcyclopentadienyl)zinc [Zn(Cppm)2], was synthesized as a Zn precursor for atomic layer deposition (ALD) of ZnO films. The compound is a liquid at room temperature and is thermally stable below ∼230 °C. Although the vapor pressure of this precursor is low (1 Torr at 99 °C), it is sufficient for the material to be used in ALD. ZnO thin films were deposited on p-type Si(100) wafers at 200 °C by ALD using the precursor. Three types of ZnO films were grown using different oxidants: (a) O2 plasma, (b) H2O, and (c) H2O followed by O2 plasma. The growth rates expressed in the growth per cycle were (a) 0.13 nm/cycle, (b) 0.07 nm/cycle, and (c) 0.10 nm/cycle, respectively, and all of the resultant films had a hexagonal wurtzite structure. Cross-sectional transmission electron microscopy revealed that, although all of the ZnO films were crystalline. X-ray diffraction analysis determined that the (002) orientation was dominant when the oxidant was H2O, and that the (103) orientation was observed when O2 plasma was used. All of the obtained films were nearly stoichiometric. Using H2O as an oxidant, very small amounts of impurities remain in the film. However, high-purity ZnO films can be deposited using H2O followed by O2 plasma as oxidants.

Monitoring the Electrochemical Failure of Indium Tin Oxide Electrodes via Operando Ellipsometry Complemented by Electron Microscopy and Spectroscopy.

Transparent conductive oxides such as indium tin oxide (ITO) are standards for thin film electrodes, providing a synergy of high optical transparency and electrical conductivity. In an electrolytic environment, the determination of an inert electrochemical potential window is crucial to maintain a stable material performance during device operation. We introduce operando ellipsometry, combining cyclic voltammetry (CV) with spectroscopic ellipsometry, as a versatile tool to monitor the evolution of both complete optical (i.e., complex refractive index) and electrical properties under wet electrochemical operational conditions. In particular, we trace the degradation of ITO electrodes caused by electrochemical reduction in a pH-neutral, water-based electrolyte environment during electrochemical cycling. With the onset of hydrogen evolution at negative bias voltages, indium and tin are irreversibly reduced to the metallic state, causing an advancing darkening, i.e., a gradual loss of transparency, with every CV cycle, while the conductivity is mostly conserved over multiple CV cycles. Post-operando analysis reveals the reductive (loss of oxygen) formation of metallic nanodroplets on the surface. The reductive disruption of the ITO electrode happens at the solid-liquid interface and proceeds gradually from the surface to the bottom of the layer, which is evidenced by cross-sectional transmission electron microscopy imaging and complemented by energy-dispersive X-ray spectroscopy mapping. As long as a continuous part of the ITO layer remains at the bottom, the conductivity is largely retained, allowing repeated CV cycling. We consider operando ellipsometry a sensitive and nondestructive tool to monitor early stage material and property changes, either by tracing failure points, controlling intentional processes, or for sensing purposes, making it suitable for various research fields involving solid-liquid interfaces and electrochemical activity.

High-quality Ge epitaxial film based on dislocation trapping mechanism in patterned Si substrate

Threading dislocation density (TDD) in a Ge heteroepitaxial film on Si is reduced based on a dislocation trapping mechanism in a patterned substrate. An array of V-shaped grooves on the submicron scale is patterned in the [110] direction on a (001) Si substrate. Epitaxial growth of Ge with a thickness of 1 μm by CVD realizes a reasonable flat surface despite the non-flat starting surface. The TDD in Ge for a V-shaped groove pattern of 0.5 μm in width with an inter-groove distance of 0.3 μm is obtained as low as 4 × 107 cm–2, which is lower than about 6 × 107 cm–2 for the rectangular one with the same groove width and inter-groove distance and about 22 × 107 cm–2 for the unpatterned one. The reduction is attributed to the dislocation trapping at the groove regions, as observed by cross-sectional transmission electron microscopy.

Local strain distribution analysis in strained SiGe spintronics devices

This paper reports nanobeam X-ray diffraction (nanoXRD) measurement results for strained SiGe spintronics devices grown by MBE. A quantitative nanoXRD analysis verifies that in-plane strain is properly exerted on the SiGe spin channel layer in the device showing enhanced spin diffusion length, whereas overall strain relaxation and local change in crystallinity occur in the sample with unclear spin signals. Crystal defects such as dislocations and stacking faults found in cross-sectional transmission electron microscopy observations are correlated with the results of nanoXRD analysis and their influence on spin transport properties are discussed. The present results demonstrate the capability of the nanoXRD based method for quantitative nondestructive analysis of strain distribution and crystallinity in real device structures.

Boosted hydrophobic properties of leather surfaces by atmospheric pressure plasma brush reactor

In this article, we demonstrate the application of an atmospheric pressure plasma brush (APPB) reactor for improving the hydrophobic properties of leather surfaces treated with organo-silane precursors for the first time. A comprehensive investigation was conducted to analyze the influence of various process parameters, including plasma access current, treatment duration, nozzle height, and gas flow rate, on the observed outcomes effects. The dispersion of active particles in the APPB was scrutinized using an Optical Emission Spectrometer (OES), while the underlying reaction mechanism was elucidated through the integration of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), cross-sectional Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) analysis. The findings unveiled that the resultant polysiloxane polymers bestowed a remarkable degree of hydrophobicity upon the leather, persisting steadfastly even following a 90-day aging period (WCA >140°). This APPB treatment combined with organosilane strategy may open a new design avenue for the development of surface treatments for intricate and temperature-sensitive substrates materials.

Influence of Ge to the formation of defects in epitaxial Mg2Sn1−x Ge x thermoelectric thin films

Defect formation in epitaxial Mg2Sn1–x Ge x thermoelectric (TE) thin films grown via MBE was studied. We examined the defect formations and structures using cross-sectional transmission electron microscopy and positron annihilation spectroscopy. The defect formation tends to be influenced by Ge incorporation into the Mg2Sn matrix phase of epitaxial thin films. Mg vacancies (V Mg) were identified as point defects, primarily concentrated in the film’s mid-layer. In films with higher Ge composition, stacking faults were observed. The concentration of vacancy-type point defects decreased as the Ge concentration increased. This implies that the vacancy atoms, which would have otherwise been created by increasing chemical pressure due to the higher Ge content, might have played a role in the formation of stacking faults. The high concentration of vacancy-type defects resulted in the lowest thermal conductivity, demonstrating their significance as effective phonon scattering centers in epitaxial TE films.