Cross-sectional High-resolution Electron Microscopy Research Articles

In-situ formation of Ag nanoparticles in the MAO coating during the processing of cp-Ti

Silver nanoparticle (Ag-NP) containing antibacterial micro-arc oxidation (MAO) coatings have already been synthesized over titanium-based materials via the MAO process employed in silver acetate (AgC2H3O2) containing electrolyte. However, the way of incorporation and in-situ formation of Ag-NPs within the MAO coating have not been documented yet. Present work was initiated to reveal the mechanism of Ag-NP formation within the MAO coatings. Thus, the structure of the MAO coating fabricated on commercial purity titanium in the AgC2H3O2-containing electrolyte was investigated by electron microscopy techniques. To this end, the cross-sectional high-resolution electron microscopy studies were carried out on lamella cut out with the focused ion beam technique, and these investigations were backed by X-ray photoelectron spectroscopy measurements of chemical composition on the surface of the MAO coating. These studies revealed that Ag is dispersed in the form of nanoparticles throughout the coating and that a higher density was confirmed closer to the micro-pores.

Adhesion of Pt Nanoparticles Supported on γ-Al2O3 Single Crystal

A model catalyst system of Pt nanoparticles (2–5 nm) dispersed on γ-Al2O3 (111) was prepared to study the support effects on nanoparticles shape and structure by cross-sectional high-resolution electron microscopy. A dewetting equilibrium shape of Pt nanoparticles was observed with a low-index plane orientation relation: Pt(111)[211]∥γ-Al2O3(111)[211] and Pt(100)[011]∥γ-Al2O3(111)[211]. Lattice-matching epitaxy of Pt(110)∥γ-Al2O3(110) was found across their interface to maintain a minimum interface strain during particle growth. To quantitatively interpret the interfacial energy and adhesion energy of the nanoparticles, the Wulff–Kaischew theorem was applied by taking into account the γ-Al2O3 support modifications to the equilibrium shape of nanoparticles. The present approach quantitatively solved the adhesion energy of the particle/support and could help to interpret nanoparticles equilibrium shape and stability.

Roles of interface roughness and internal stress in magnetic anisotropy of CoPt/AlN multilayer films

Magnetic anisotropy of CoPt/AlN multilayer films has been studied by systematically varying the nominal thickness of CoPt layers, tCoPt (1–10nm), and the annealing temperature, Ta (300–500°C). The as-deposited films show in-plane magnetic anisotropy in the full range of tCoPt, whereas the annealed films show perpendicular magnetic anisotropy (PMA) within small tCoPt but change to in-plane magnetic anisotropy when tCoPt is over a certain thickness. The critical thickness for such anisotropic transformations increases as the Ta increases. The maximum PMA obtained in this work is 1.13×107ergcm−3. The interface roughness was analyzed by cross-sectional high-resolution electron microscopy and X-ray reflectivity using an abrupt interface model with a Debye exponent shape. The internal stress was analyzed by X-ray diffraction using an equal biaxial stress model. The results show that the CoPt/AlN interface roughness decreases from 0.385nm to 0.158nm and the internal stress increases from −2.36GPa (compressive) to 1.73GPa (tensile), as the Ta increases to 500°C. The roles of the interface roughness and the internal stress in the magnetic anisotropy of CoPt/AlN multilayer films are studied.

Analysis by high-resolution electron microscopy of elastic strain in thick InAs layers embedded in Ga 0.47In 0.53As buffers on InP(0 0 1) substrate

Elastic strain has been investigated by transmission electron microscopy in nanometric InAs layers grown on Ga 0.47In 0.53As/InP(0 0 1) by molecular beam epitaxy using a residual Sb flux. Deposits of 10 and 15 monolayers of InAs (3 and 4.5 nm) remain elastically stressed with a two-dimensional growth mode. The out-of-plane strain in the layers is analyzed by cross-sectional high-resolution electron microscopy. A distortion of the substrate below and on top of the InAs layers is detected and is attributed to a significant surface relaxation effect due to thinning. Surface relaxation is modeled by three-dimensional finite element modeling. An additional relaxation effect is obtained when the sample is not infinite along the direction perpendicular to the thinning. This effect enhances the buffer distortion of the buffers below and on top of the strained layers. Taking into account thin foil effects, the experimental out-of-plane strain is in excellent agreement with the theoretical value calculated for a pure InAs layer (i.e. 0.035), demonstrating the high level of strain and stress in the layers.

Crystallization of InSb Phase Near the Bonding Interface of Silicon-on-Insulator Structure

The behavior of Sb and In atoms embedded into silicon-on-insulator structure (SOI) near the bonding interface was investigated as a function of annealing temperature. Two kinds of the ionimplanted SOI structures were prepared. First kind of the structures contained the buried SiO2 layer implanted with In+ and Sb+ ions near the top Si/SiO2 interface. In second kind, the ion-implanted regions were placed on each side of the bonding interface: Sb+ ions were implanted into Si film; In+ ions were implanted into SiO2 layer. Rutherford backscattering spectrometry (RBS) and crosssectional high-resolution electron microscopy (XTEM) were employed to study the properties of the prepared structures. The formation of InSb nanocrystals was observed within the SiO2 bulk from first kind of the SOI structures as annealing temperature increased to 1100o C. In the case of the double side implanted SOI structures, an increase in annealing temperature to 1100o C was accompanied by the up-hill diffusion of In atoms from the SiO2 bulk toward the bonding interface and by the endotaxial growth of InSb nanocrystals on the top Si/SiO2 interface. It was concluded from the experimental results that Sb atoms were the nucleation centers of InSb phase.

Growth and structure of epitaxial CeO 2 films on yttria-stabilized ZrO 2

Thirty to a hundred-nm thick epitaxial CeO 2 layers are grown on YSZ (100), (110) and (111) surfaces of yttria-stabilized ZrO 2 (YSZ) by electron beam evaporation of Ce in oxygen at reduced pressure. Their growth, structure and thermal stability are studied with several bulk and surface sensitive techniques including Rutherford backscattering spectrometry, cross-sectional high resolution electron microscopy, low energy electron diffraction and low energy reflection electron microscopy. Excellent epitaxy is obtained on all YSZ surfaces at a growth temperature of 750 K. The surfaces of films grown on (111)-oriented substrates are flat, whereas those on the other substrates are faceted into small (111) planes. The grain sizes in the films are in the 10 nm range and smaller.

Formation of epitaxial SiC nanocrystals

Epitaxial 3C–SiC grains are formed at 1190 °C in the top region of silicon, when Si wafers coated by SiO 2 are annealed in CO atmosphere. The formed SiC grains are 40–50 nm high and 100 nm wide in cross-section and contain only few defects. Main advantage of the method is that the final structure is free of voids. The above method is further developed for the generation of SiC nanocrystals, embedded in SiO 2 on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO 2 close to the Si/SiO 2 interface by a two-step annealing of oxide covered Si: first in a CO, than in a pure O 2 atmosphere. The first (carbonization) step created epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moved the interface beyond them. Conventional and high resolution cross-sectional electron microscopy showed pyramidal Si protrusions at the Si/SiO 2 interface under the grains. The size of the grains, as well as their distance from the Si/SiO 2 interface (peak of pyramids) can be controlled by the annealing process parameters. The process can be repeated and SiC nanocrystals (oriented in the same way) can be produced in a multilevel structure.

Mechanism for coarsening of P-mediated Ge quantum dots during in-situ annealing

The coarsening of phosphorus-mediated Ge quantum dots (QDs) on Si(0 0 1) during in-situ annealing at 550 °C is studied. In-situ annealing makes the as-grown sample morphology be remarkably changed: the larger dots are formed and the dot density is greatly reduced. The results of chemical etching and Raman spectra reveal that the incorporation of Ge atoms which originate from the diminishing dots, rather than substrate Si atom incorporation is responsible for the dot coarsening at the incipient stage of in-situ annealing. Besides, Raman spectra suggest that the larger dots formed during in-situ annealing are dislocated, which was confirmed by cross-sectional high-resolution electron microscopy observation. Through the generation of dislocations, the strain in the dots is relaxed by about 50%.

A view of the implanted SiC damage by Rutherford backscattering spectroscopy, spectroscopic ellipsometry, and transmission electron microscopy

4 H - Si C single crystalline substrates were implanted at room temperature with 150keV Al+ ions using fluences of 4×1014, 1×1015, and 2×1015cm−2 with current density of 2.5μAcm−2. The samples were subsequently annealed at 1100°C in N2 for 1h in order to analyze their structural recovery. The disorder induced in both sublattices by the Al+ ions was studied by backscattering spectrometry in channeling geometry with a 3.5MeV He2+ beam. The results were compared with the optical properties of the samples measured by spectroscopic ellipsometry. In a previous work, we concluded that during the postimplantation annealing of a highly damaged SiC crystalline material the short distance order can be recovered, while the long distance disorder remains. We also presented the possibility to have grains of different polytypes oriented faraway from the original direction. Now, this alternative is confirmed by the cross-sectional transmission and high resolution electron microscopy studies, carried out to obtain information about the crystal structure.

248 nm cathodoluminescence in Al1−xInxN(0001) thin films grown on lattice-matched Ti1−yZryN(111) seed layers by low temperature magnetron sputter epitaxy

Single-crystal Al0.8In0.2N(0001) thin films were grown epitaxially onto lattice-matched Ti0.2Zr0.8N(111) seed layers on MgO(111) substrates at 300°C by magnetron sputter epitaxy. Low-energy ion-assisted epitaxial growth conditions were achieved by applying a substrate potential of −15V. Cross-sectional high-resolution electron microscopy verified the epitaxy and high-resolution x-ray diffraction ω-rocking scans of the Al0.8In0.2N 0002 peak (full width at half maximum ∼2400arcsec) indicated a high structural quality of the films. Cathodoluminescence measurements performed in a scanning electron microscope at 5K revealed Al0.8In0.2N luminescence at 248nm, or equivalently 5.0eV, showing that Al0.8In0.2N is a promising material for deep-ultraviolet optoelectronic devices.

The formation mechanism of aluminum oxide tunnel barriers: Three-dimensional atom probe analysis

Magnetic tunnel junctions have applications in a range of spin-electronic devices. The functional properties of such devices are critically dependant on the nanoscale morphology of the insulating barrier (usually only a few atomic layers thick) that separates two ferromagnetic layers. Here we report atomic-resolution three-dimensional visualization of magnetic tunnel junctions with an aluminum oxide barrier, using three-dimensional atom probe analysis and cross-sectional high resolution electron microscopy. Our results on barriers with a range of degrees of oxidation have enabled a mechanism for the oxidation to be proposed. Low oxidation times result in discrete oxide islands and further oxidation leads to a more continuous, but nonstoichiometric, barrier. Post-deposition annealing leads to an increase in the barrier area and near stoichiometric chemistry with evidence that oxidation proceeds along the top of grain boundaries in the underlying CoFe. Our three-dimensional visualization enables us to show that even perforated barriers (formed at shorter oxidation times) can have reasonable transport properties.

Isolated SiC nanocrystals in SiO2

A simple method, compatible with conventional silicon technology, is introduced to generate SiC nanocrystals, embedded in SiO2 on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO2 close to the Si∕SiO2 interface by a two-step annealing of oxide covered Si: first in a CO, then in a pure O2 atmosphere. The first (carbonization) step creates epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moves the interface beyond them. Conventional and high-resolution cross-sectional electron microscopy shows pyramidal Si protrusions at the Si∕SiO2 interface under the grains. The size of the grains, as well as their distance from the Si∕SiO2 interface (peak of pyramids) can be controlled by the annealing process parameters.

Characterization of nanotextured AlN thin films by x-ray absorption near-edge structures

AlN thin films have been grown on c-cut sapphire substrates by pulsed-laser deposition. The film epitaxially grown at 1073 K under vacuum of 5×10−4Pa was used to examine the crystallographic orientation dependence of Al K-edge x-ray absorption near-edge structures (XANES), which satisfactorily agrees with theoretical spectra obtained by first-principles calculations. The film grown at 1073 K with N2 backfill of 7×10−2Pa shows nanotextured structure with its c plane parallel to the substrate. Although the nanotexture is not evident by x-ray diffraction, XANES can unambiguously indicate the texturing. Cross-sectional high-resolution electron microscopy provides the evidence of the nanostructure.

90 ° a − b domains in epitaxial ferroelectric Bi3.25La0.75Ti3O12 films

A cross-sectional high-resolution electron microscopy study was performed on (010)∕(100)-oriented ferroelectric Bi3.25La0.75Ti3O12 (BLT) films. Using Bragg filtering, curved, irregular, and nanoscale 90° domain configurations were observed. The weak ferroelastic strain of 0.024% suggests a negligible polarization clamping of 90°a–b domains under bipolar electric field and an easy 90° switching of the polar a axis in the ab plane. In addition, antiphase boundaries were characterized. A negligible contribution of 90°a–b domains to the fatigue of BLT is proposed.

Local structural change of amorphous Ge–Sb–Te thin film on annealing

Structures of amorphous Ge–Sb–Te film included in phase change optical disk have been examined using transmission electron microscopy. Cross-sectional high-resolution electron microscopy (HREM) observation indicated that locally ordered regions as small as ∼2nm exist in the amorphous Ge–Sb–Te thin film. These ordered regions grew in size after annealing at 160°C for 120min. On the basis of the HREM and nano-beam electron diffraction, it was concluded that the atomic structure of the locally ordered regions is similar to that of crystalline Sb.

Local structure analysis of Ge-Sb-Te phase change materials using high-resolution electron microscopy and nanobeam diffraction

As-sputtered and melt-quenched amorphous structures together with the laser-induced crystallized structure of Ge-Sb-Te thin films were investigated using high-resolution electron microscopy (HREM) and nanobeam electron diffraction (NBED). Each of the Ge-Sb-Te thin films was embedded in a four-layered stack, which is the same as the layered structure of phase-change optical disks. Cross-sectional HREM revealed crystalline atomic clusters in the melt-quenched amorphous layer at a greater frequency than in the as-sputtered amorphous layer. Autocorrelation function analysis of the HREM images revealed similarity between the structures of atomic ordered regions in the amorphous phase and that of crystalline Sb. Atomic pair-distribution functions derived from halo NBED intensity analysis indicated that the atomic neighbor correlations developed more in the melt-quenched amorphous phase than in the as-sputtered phase. The development of locally ordered regions is considered to be closely related to the differences in optical properties and crystallization behaviors between these two amorphous phases.

Growth and photoluminescence of InAlGaN films

Two quaternary InAlGaN films were grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire (0001) substrates with and without high-temperature GaN interlayer, respectively. The structural and optical properties of the quaternary films were investigated by high-resolution X-ray diffraction (HRXRD), high-resolution electron microscopy (HREM), temperature-dependent photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy. According to the HRXRD and PL results, it is demonstrated that two samples have the same crystal quality. The TRPL signals of both samples were fitted well as a stretched exponential decay from 14 K to 250 K, indicating significant disorder in the materials, which is attributed to recombination of excitons localized in disorder quantum nanostructures such as quantum dots or quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-section HREM measurement further proves that there exist disorder quantum nanostructures in the quaternary. By investigating the temperature dependence of the dispersive exponent beta, it is shown that the stretched exponential decays of the two samples originate from different mechanisms. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Comparison of interface structure of thin miscible films by grazing incidence x-ray scattering and high-resolution electron microscopy

Grazing incidence x-ray scattering and cross-sectional high-resolution electron microscopy (HREM) have been applied to the study of thin trilayers of the miscible system Co–Cr–Co grown by ultra-high vacuum evaporation on silicon. Good agreement was found between numerical values for layer thickness deduced by the two techniques. The Co–Cr interface roughness was found to be low with substantial interdiffusion, significantly greater than the roughness amplitude. HREM images confirmed the diffuseness of the interfaces. The roughness had a lateral correlation length of typically 150 Å and a very low fractal parameter h of 0.15. The correlation length was always found to correspond to the lateral grain size observed in the HREM images.

Growth Model of Epitaxial Pb(Zr0.52Ti0.48)O3 Nanoislands

ABSTRACTSingle-crystalline, single-c-domain Pb(Zr0.52Ti0.48)O3 nanoislands (truncated-pyramid in shape) with an average height of ∼9 nm and a base length of ∼50 nm were grown on compressive niobium-doped SrTiO3(001) substrates using chemical solution deposition. Cross-sectional highresolution electron microscopy investigations allowed to propose a growth model of the islands, and they proved the existence of edge-type misfit dislocations at the interface.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0 0 0 1) sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[−(t τ) β] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter β on the temperature, it is shown that the multiple trapping–detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime τ r increases linearly with increasing temperature.