Z-contrast Imaging Research Articles

Atomic Scale Characterization of HgTe/CdTe Superlattices Using STEM Z-Contrast Imaging and VEELS

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Single atomic layer detection of Ca and defect characterization of Bi-2212 with EELS in HA-ADF STEM

By forming a small electron probe in a scanning transmission electron microscope equipped with a high-angle annular dark-field (HA-ADF) detector, the Bi–O atomic planes in Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) can be directly observed with the incoherent Z-contrast imaging technique. Using a combination of electron energy loss spectroscopy (EELS) and HA-ADF imaging, we were able to detect the Ca signals from individual Ca atomic planes in this structure, so that the Ca distribution could be probed within individual unit cells. This high-spatial-resolution EELS technique has been successfully applied to characterize planar defects such as the half-unit cell intergrowth of Bi-2201 on the nanometer scale. Present results show that EELS, in conjunction with high-resolution HA-ADF imaging, provides a powerful tool to study chemical and structural nature of this material on the atomic scale.

Search for ferromagnetism in conductive Nb:SrTiO3 with magnetic transition element (Cr, Co, Fe, Mn) dopants

Thin films of (0.5%, 1%) Nb:SrTiO3 dilutely doped with (2at.%) magnetic transition elements (Cr, Co, Fe, Mn) are examined for ferromagnetism. X-ray diffraction, Rutherford backscattering ion channeling, scanning transmission electron microscopy Z-contrast imaging, and electron energy loss spectroscopy techniques establish high crystalline quality of the films with no impurity phase(s) and highly uniform dopant distribution. Although the film conductivity improves dramatically by Nb doping, no ferromagnetism is found in any of our samples over the temperature range of 365 down to 5K. This is contrasted to the case of ferromagnetism reported in cobalt doped (La,Sr)TiO3.

Reducing the missing wedge: High-resolution dual axis tomography of inorganic materials

Electron tomography is a powerful technique that can probe the three-dimensional (3-D) structure of materials. Recently, this technique has been successfully applied to inorganic materials using Z-contrast imaging in a scanning transmission electron microscope to image nanomaterials in 3-D with a resolution of 1 nm in all three spatial dimensions. However, an artifact intrinsic to this technique limits the amount of information obtainable from any object, namely the missing wedge. One way to circumvent this problem is to acquire data from two perpendicular tilt axes, a technique called “dual axis tomography.” This paper presents the first dual axis data at high resolution for inorganic materials, and by studying a CdTe tetrapod sample, demonstrates the increase in information obtained using this technique.

Nature of domains in lanthanum calcium cobaltite perovskite revealed by atomic resolution Z-contrast and electron energy loss spectroscopy

Lamella-type planar features in La 0.6Ca 0.4CoO 3 perovskite were investigated in a dedicated scanning transmission electron microscope. Atomic resolution Z-contrast imaging and modelling, along with electron energy loss spectroscopy, gave evidence these features could be Ca-enriched domains of the rock salt structure in the perovskite lattice. Z-contrast of a second type of planar fault, occurring after repeated investigations in the electron microscope, suggests formation of ordered O-vacancy arrangements on Co–O planes.

Atomic-scale microstructures of Zr 2Al 3C 4 and Zr 3Al 3C 5 ceramics

The microstructures of bulk Zr 2Al 3C 4 and Zr 3Al 3C 5 ceramics have been investigated using transmission electron microscopy and scanning transmission electron microscopy. These two carbides were determined to have a point group 6/ mmm and a space group P6 3/ mmc using selected-area electron diffraction and convergent beam electron diffraction. The atomic-scale microstructures of Zr 2Al 3C 4 and Zr 3Al 3C 5 were investigated through high-resolution imaging and Z-contrast imaging. Furthermore, intergrowth between Zr 2Al 3C 4 and Zr 3Al 3C 5 was identified. Stacking faults in Zr 3Al 3C 5 were found to result from the insertion of an additional Zr–C layer. Cubic ZrC was occasionally identified to be incorporated in elongated Zr 3Al 3C 5 grains. In addition, Al may induce a twinned ZrC structure and lead to the formation of ternary zirconium aluminum carbides.

Dependence of power trench metal-oxide-semiconductor field-effect transistor processes on wafer thickness

The dependence of trench metal-oxide-semiconductor field-effect transistor processes on wafer thickness has been studied. In the photolithography process, it was found that the photoresist thickness decreased with the decrease of wafer thicknesses. This is due to the fact that thinner wafer has less thermal mass and hence less dissipation time. Unless compensated for, this change in resist thickness adds to critical dimension and trench depth variations. After the rapid thermal processing, the thinner wafers exhibited a lower resistivity and higher silicide stress (i.e., 3.23E+10dyn∕cm2 for 508μm wafers and 4.60E+09dyn∕cm2 for 675μm wafers). Also, the stress is more uniform across the thinner wafers, possibly due to uniform Si-rich TixSiy (x<y) phase. Both Z-contrast transmission electron microscopy imaging and energy dispersive x-ray profiling on the silicide layers of the thicker wafers revealed a Ti-rich layer (possibly TiSi) on the top of TixSiy (x<y) layer and at the Si-silicide interface; whereas only scattered regions of Ti-enriched regions were observed in thinner wafers. Higher temperature in thinner wafers seems to assist in the removal of this Ti-rich layer.

In situ synthesis and characterization of Ru promoted Co/Al 2O 3 Fischer–Tropsch catalysts

We have conducted a detailed investigation of the initial stages of the formation of metal nanoparticles during the incipient wetness synthesis process for both Co on γ-Al 2O 3 and Ru-promoted Co on γ-Al 2O 3 catalysts. The synthesis was performed in the reaction cell of an environmental transmission electron microscope so that the processes can be followed using in situ atomic resolution imaging, Z-contrast imaging and nanospectroscopy. The metal precursors were found to have a highly non-uniform distribution on the alumina support, which leads to a non-uniform distribution of metal particles in the final catalysts. In situ atomic resolution images and electron energy-loss nano-analyses showed that during the reduction process, the metal precursor material transformed into an intermediate phase of cubic CoO. With the addition of Ru, however, in addition to the presence of large CoO particles, small particles of either CoRu or pure Ru were also formed. Our experimental results directly show that the addition of Ru promotes the formation of individual CoRu bimetallic nanoparticles and enhanced the reducibility of small Co particles in the Co(Ru)/γ-Al 2O 3 systems.

Atomic scale defect analysis in the scanning transmission electron microscope

Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope provide the ability to investigate the structure-composition-property relationship at individual defects on the atomic scale. In this article, the main principles behind the techniques will be described. The application of these methods to the analysis of individual dislocations in GaN will also be discussed. In this case, the atomic scale methods indicate that many of the structural and electronic properties of dislocations are modified by the presence of impurities, such as oxygen.

Atomic structure and electronic properties of c-Si∕a-Si:H heterointerfaces

The atomic structure and electronic properties of crystalline-amorphous interfaces in silicon heterojunction solar cells are investigated by high-resolution transmission electron microscopy, atomic-resolution Z-contrast imaging, and electron energy-loss spectroscopy. With these combined techniques, we directly observe abrupt and flat transition from crystalline Si to hydrogenated amorphous Si at the interface of Si heterojunction solar cells. We find that high-quality hydrogenated amorphous Si layers can be grown abruptly by hot-wire chemical vapor deposition on 200°C (100) Si substrates after a two-step pretreatment of the substrate, comprised of exposure to hot-wire decomposed H2-diluted NH3 followed by atomic H etching.

Ultra-high resolution EEL studies of domains in Perovskite

Lamella-type planar features in La0.6Ca0.4CoO3 were investigated in the NorthWest STEM and the Daresbury SuperSTEM. Structural and chemical information was obtained by combining EEL spectrum- and atomic resolution HAADF Z-contrast imaging. The domains appear to be ½-unit-cell-wide Ca-enriched lamellae of the rock salt structure, faultlessly embedded into the Perovskite lattice.

Origin of the modified orientation relationship for S(S″)-phase in Al–Mg–Cu alloys

The formation of S-phase with a modified orientation relationship (OR) has been previously observed in several Al–Cu–Mg alloys. In this paper, high-resolution transmission electron microscopy and Z-contrast imaging have been used to study the origin of the modified OR in an alloy with low Cu/Mg ratio and small Si addition. Based on the observations, and supported by ab initio simulations, the formation is governed by coherency at the (021)S//(014)Al S-phase/matrix interface, which is shown to coexist with the more commonly reported (001)S//(021)Al interface. This new (021)S//(014)Al S-phase/matrix interface explanation is compared with previously published explanations based on the invariant line concept and establishment of a different S-phase/matrix interface. Energy dispersive X-ray spectroscopy and atom probe tomography indicate that the S-phase is slightly enriched in Si. The role of Si as well as the overall alloy composition is discussed. Because of the similarities between our results and the early work of Bagaryatsky, the S″-phase notation is adopted for this early-forming, strained version of the S-phase.

Characterization of hafnium oxide grown on silicon by atomic layer deposition: Interface structure

Ultra-thin films of hafnium oxide deposited on Si(1 0 0) substrates by means of atomic layer deposition using tetrakis(diethylamino)hafnium as the hafnium precursor are characterized. These films and interface structures are probed using Fourier transform infrared spectroscopy along with Z-contrast imaging and electron energy loss spectroscopy (EELS) of a scanning transmission electron microscope. The interface structure of HfO 2/Si(1 0 0) is further investigated using angle resolved X-ray photoelectron spectroscopy to probe the core level orbitals (Hf 4f, Si 2p, O 1s) at high resolution. The interfacial differences are also examined by probing the Hf 4f bonding with normal incidence XPS in thin and thick films. The XPS studies show that the binding energies remain unchanged with film depth and that there is no apparent signature of silicate structure in the as-deposited films. EELS spectra taken at the interface and XPS measurements suggest the interface is mainly silicon oxide. Two different cleaning methods used show difference only in the thickness of the silicon oxide interlayer.

Characterization of MOVPE-grown GaInNAs quantum well with multi-barriers by Z-contrast imaging and SIMS

Structural and optical properties of MOVPE-grown GaInNAs quantum wells (QW) with multiple barriers (InGaAs/GaNAs) have been investigated. Significant improvement of optical performance in the 1.3 μm range is demonstrated with the proposed structure. PLE studies show that the density of N-related localized states can be reduced by the introduction of a thin InGaAs layer due to the improved interface quality. Based on the structural analysis, it is concluded that the InGaAs layer supports the nitrogen redistribution within the QW, resulting in a sharp interface. The blue-shift trend of the GaInNAs QW as a function of the GaNAs thickness can be explained by the reduction of the N content in the GaInNAs QW.

Carbon-nanopillar tubulization caused by liquidlike iron catalyst nanoparticles

Amorphous carbon nanopillars that we fabricated with electron-beam induced chemical vapor deposition were tublized by heating at 600°C-650°C in the presence of iron nanoparticles. The tublization process of the amorphous carbon nanopillar was observed in situ by using transmission electron microscopy and scanning transmission electron microscopy. A molten catalyst nanoparticle penetrated the amorphous carbon nanopillar, dissolving it and leaving a graphite track behind. The nanoparticle moved with its shape changing like the movement of an earthworm. Z-contrast images of the molten catalyst nanoparticle revealed that dissolved carbon atoms were diffused through its outer layer. The tubulization mechanism is a solid-quasiliquid-solid mechanism in which the carbon phase transformation is a kind of liquid phase graphitization of amorphous carbon catalyzed by liquefied metal-carbon alloy nanoparticles. [DOI: 10.1380/ejssnt.2006.401]

Atomic structure of epitaxial SrTiO3–GaAs(001) heterojunctions

We have examined the atomic and electronic structures of epitaxial SrTiO3 thin films on GaAs (001) deposited under different growth conditions in order to understand the interfacial structure-property relationships. High-resolution Z-contrast images show an atomically sharp heterointerface with SrTiO3[110] in perfect registry with GaAs [100] and the interfacial structure remains unchanged if a submonolayer of Ti was deposited prior to the SrTiO3 film growth. X-ray photoelectron spectroscopy shows that the Fermi level was pinned during the initial stage of growth when a submonolayer of Ti was deposited on As-terminated GaAs(001); subsequent SrTiO3 growth alleviated this pinning. These results indicate a self-driven interfacial atomic structure formation, independent of the initial stage of growth.

The effect of Ni in Pd–Ni/(Ce,Zr)O x/Al 2O 3 catalysts used for stoichiometric CO and NO elimination. Part 1: Nanoscopic characterization of the catalysts

Pd–Ni catalysts supported on Al 2O 3, (Ce,Zr)O x /Al 2O 3, and (Ce,Zr)O x are characterized at a nanoscopic level using mainly electron microscopy-related techniques (HREM images, XEDS, and Z-contrast images/EELS spectra done with TEM and STEM instruments). The presence of rather homogeneous Ce–Zr mixed oxide nanostructures is revealed from analysis of HREM pictures. Analysis of the Pd–Ni/(Ce,Zr)O x /Al 2O 3 system shows the existence of preferential interactions of Pd and Ni with the (Ce,Zr)O x and Al 2O 3 components, respectively, as evidenced mainly by XEDS and confirmed by FMR. Dispersion states of the metals and particle size distributions of palladium are inferred from analysis of Z-contrast images and EELS spectra. The results are complemented by XPS and XAFS results used to analyse the chemical state of the metallic components in the catalysts and their dispersion over the different supports.

Nanostructure Functionality through Aberration-Corrected STEM

The 300 kV VG Microscopes’ HB603U STEM at Oak Ridge National Laboratory with a Nion aberration corrector has achieved the first direct image of a crystal at sub-Angstrom resolution, using the incoherent Z-contrast or high-angle annular dark field (HAADF) mode, as shown in Fig. 1a,b. [1] To validate use of the Fourier transform to measure a resolution limit of 0.61A, Fig. 1c compares Fourier transforms of a simulated image and the probe used for the simulation. Excellent agreement is seen for a thin crystal where ideal incoherent imaging applies. Thicker crystals show reduced high frequency transfer, but no spurious sum or difference frequencies [2]. The sub-Angstrom probe allows Z-contrast imaging of oxygen columns next to heavy columns (Fig. 1d). Furthermore, an efficient, simultaneous, aberration corrected, phase contrast image is available using a small axial detector giving improved oxygen visibility, although spurious features are seen between the Sr columns (Fig. 1e). The STEM has become a viable means of acquiring aberration-corrected phase contrast images, with the advantage of simultaneous Z-contrast imaging and EELS.

High Spatial and Energy Resolution EELS using a Monochromated STEM

The ability to directly correlate electron energy loss spectroscopy (EELS) with Z-contrast images in the scanning transmission electron microscope (STEM) provides a powerful tool to characterize nanoscale materials. The incorporation of a monochromator into the FEI Tecnai F20 UT at the National Center for Electron Microscopy (NCEM) extends the power of these techniques by increasing the sensitivity of core-loss spectra to small changes in fine-structure and by allowing the low-loss region of the spectrum to be analyzed in detail. Here, the configuration of the monochromated Tecnai will be discussed and examples of spectra obtained utilizing the highest spatial and energy resolution will be presented [1].

Z-contrast STEM Imaging of Rare Earths Segregation at Crystal/Intergranular Film Interfaces in Si3N4 CeramicI

Z-contrast STEM Imaging of Rare Earths Segregation at Crystal/Intergranular Film Interfaces in Si3N4 CeramicI