Z-contrast Imaging Research Articles

Nanofabrication and Growth Processes with Ultra-High Resolution Electron Beam Induced Deposition on Thin Films

Developing techniques for controlled fabrication of nanostructures is a topic of intense research and is critical to exploit the full potential of nanotechnology. An enormous variety of approaches utilizing self-assembly are being developed but most of these approaches result in either randomly positioned features of ill-defined size (e.g. island formation through Stranski-Krastanov type mechanism) or uniform particles and periodic arrays using highly controlled solution chemistry. Electron beam induced deposition (EBID) can be used to deposition and precisely position arbitrary shaped nanostructure onto substrates. Such an approach may provide an effective method for fabrication of masks and templates for nanofabrication projects. Several groups have noticed that linewidths of several nanometers are possible with this technique when high energy focused electron beams are employed [1,2,3]. However, many aspects of the deposition and fabrication processes are not well understood at the nanometer level. Here we present work on W nanostructures synthesized using the EBID approach with line widths approaching 1 nm. Nanostructure synthesis and characterization was conducted in a Tecnai environmental transmission electron microscope (ETEM) allowing the entire growth process to be observed and controlled in situ and in real time. The microscope is equipped with a field emission electron source operating at 200 kV and has a nominal beam spot size of ~ 0.2 nm with a current of approximately 20 pA. The microscope is fitted with an environmental cell and essentially operates as a cold-wall CVD reactor. Reactive gases are admitted into a sample chamber at low pressures and are allowed to react on resistively heated substrates. The W nanostructures were deposited onto a Si3N4 membrane utilizing W(CO)6 precursor. In a typical experiment, the precursor flux was admitted directly into the chamber via a precision leak valve, the electron beam was focused to a point on the substrate surface causing an immediate reaction that produced highly localized growth of W. Figure 1 shows a Z-contrast STEM image and intensity linescan from part of a periodic array of W dots grown at 100 o C and precursor pressure of ~ 10 -3 Torr. The full width half maxima of these dots is 1.2 – 1.5 nm proving that linewidths approaching 1 nm can be achieved with this method. Figure 2 shows another array of dots used to investigate the deposition process in more detail. Two sets of aligned dots are deposited on each side of the thin film when EBID is performed with high energy electrons. Tilting the sample reveals the two sets of W dots (Figure 2b) on the entrance and exit side of the film. The dot intensity alternates along the columns with the bright dots corresponding to the exit surface. Image intensity analysis shows that there is almost twice as much tungsten deposited on the exit surface compared to the entrance surface. Thus there is a strong anisotropy in the flux of those secondary electrons that are most efficient at precursor dissociation. This is a consequence of the momentum transfer direction of the primary fast electron scattering event which is correlated with the momentum distribution of the excited continuum electron. The secondary electrons responsible for precursor dissociation either come from this initial excitation event or are related to this event mainly through plural scattering (since these events preserve the directionality of the primary momentum transfer). This result agrees with recent Monte Carlo simulations [4].

HAADF Imaging: An Effective Technique for the Study of NonhomogeneousNanostructures

Atomic number contrast (Z-contrast) imaging using high-angle annular dark field (HAADF) detector, along with high resolution electron microscopy (HREM), is used to study the nanostructured metal, semiconductor, mixed oxide, and soft matter composites of inhomogeneous nature. A comparison between the HREM and HAADF images for the analysis of crystal structure, defects, and compositional inhomogenity in those nanostructures has been made. While the HREM technique is efficient in determining bulk crystallinity and defect structures, the HAADF imaging technique is superior in determining the surface inhomogenity, defect structures in the interior of the nanostructures, even at atomic resolution. The efficiency of the HAADF imaging technique in determining the surface inhomogenity and defect structures is demonstrated for the Au-Pt bimetallic clusters, CdSe nanofibers and nanowires, Nb16W18O94 mixed oxide, and polystyrene-mormorillonite clay nanocomposites.

Single Hf atoms inside the ultrathin SiO2 interlayer between a HfO2 dielectric film and the Si substrate: How do they modify the interface?

We show that individual Hf atoms may get incorporated into the SiO2 interlayer which is formed between the HfO2 dielectric film and the Si substrate during rapid thermal annealing. We report atomically-resolved Z-contrast images of a Si/SiO2/HfO2 structure together with first-principles calculations which demonstrate that single Hf atoms are in fact present in the interlayer. The location of individual Hf atoms within the interlayer oxide is closely related to the structure of the amorphous oxide near the Si/SiO2 interface. The Hf defects may affect channel mobility and leakage currents in the HfO2/Si electronic devices.

Growth Mechanisms and Oxidation Resistance of Gold-Coated Iron Nanoparticles

We report the chemical synthesis of Fe-core/Au-shell nanoparticles by a reverse micelle method, and the investigation of their growth mechanisms and oxidation-resistant characteristics. The core-shell structure and the presence of the Fe & Au phases have been confirmed by transmission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, Mossbauer spectroscopy, and inductively coupled plasma techniques. Additionally, atomic-resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) have been used to study details of the growth processes. The Au-shell grows by nucleating on the Fe-core surface before coalescing. The magnetic moments of such nanoparticles, in the loose powder form, decrease over time due to oxidation. The less than ideal oxidation-resistance of the Au shell may have been caused by the rough Au surfaces. However, in the pressed pellet form, electrical transport measurements show that the particles are fairly stable, as the resistance of the pellet does not change appreciably over time.

Nanoparticle Arrays Patterned by Electron-Beam Writing: Structure, Composition, and Electrical Properties

Direct electron beam writing in nanoparticle films is employed to create nanoscale wires between prepatterned gold electrodes on SiO(2)/Si wafers. Characterization of these nanowires using AFM, SEM, and EDX reveals a core/sheath morphology, where a gold-rich core is surrounded by a sheath which is mainly of carbon. Z-contrast STEM images indicate that the central core consists of a distribution of metal cores in a carbon network. The results suggest that the nanoparticle network is created through cross-linking of the ligands of adjacent particles. The high resistivities obtained in conductivity measurements are consistent with this picture. The work illustrates the ability to generate patterned nanoparticle arrays which can be addressed electrically.

Atomic scale characterization of the Pt/TiO 2 interface

We present results from an investigation of the Pt/TiO 2 catalyst system using a combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Evidence of a strong interaction between the Pt particles and the support is found to be dependent on the Pt cluster size, being manifested either as an encapsulation of the Pt particles by the support or a distortion of the structure of the Pt particles. In the case of clusters that are only a few atoms in size, we show direct evidence of an epitaxial nucleation relationship between Pt and Titania. The results also show unexpectedly that Pt particles exhibit a preferential nucleation on rutile rather than anatase.

Controlled Synthesis and Characterization of Co-Au Core-shell Nanoparticles

AbstractCo-Au Core-shell nanoparticles are synthesized by slowly reducing an organo-gold compound on pre-made cobalt seeds with a weak reducer at mild condition. For the first time, these coreshell nanoparticles are generated in non-polar solvent in a controlled manner. The formation theory of core-shell structure, especially the seed size effect, is addressed as well. These coreshell structures are confirmed with a wide range of transmission electron microscopy (TEM) methods, which includes routine TEM images, high resolution TEM, and z-contrast imaging.

The Effect of Substrates / Ligands on Metal Nanocatalysts Investigated By Quantitative Z-Contrast Imaging and High Resolution Electron Microscopy

AbstractOur direct density function-based simulations of Ru-, Pt- and mixed Ru-Pt clusters on carbon-based supports reveal that substrates can mediate the PtRu5 particles [1]. Oblate structure of PtRu5 on C has been found [2]. Nevertheless, the cluster-substrate interface interactions are still unknown. In this work, we present the applications of combinations of quantitative z-contrast imaging and high resolution electron microscopy in investigating the effect of different substrates and ligand shells on metal particles. Specifically, we developed a relatively new and powerful method to determine numbers of atoms in a nanoparticle as well as three-dimensional structures of particles including size and shape of particles on the substrates by very high angle (~96mrad) annular dark-field (HAADF) imaging [2-4] techniques. Recently, we successfully synthesize icosahedra Au13 clusters with mixed ligands and cuboctahedral Au13 cores with thiol ligands, which have been shown by TEM to be of sub-nanometer size (0.84nm) and highly monodisperse narrow distribution. X-ray absorption and UV-visible spectra indicate many differences between icosahedra and cuboctahedral Au13 cores. Particles with different ligands show different emissions and higher quantum efficiency has been found in Au11 (PPH3) SC12)2C12. We plan to deposit those ligands-protected gold clusters onto different substrates, such as, TiO2 and graphite, etc. Aforementioned analysis procedure will be performed for those particles on the substrates and results will be correlated with that of our simulations and activity properties. This approach will lead to an understanding of the cluster-substrates relationship for consideration in real applications.

Challenges in nanoelectronics - gate dielectrics and device modeling

Microelectronics has been tracking Moore's law for several decades with the same choices of materials: Silicon has been the semiconductor of choice; its native oxide, SiO{sub 2} has been used both as a gate dielectric (where a high dielectric constant is in principle desirable) and as an insulator for device isolation and interconnects (where low dielectric constant is in principle desirable); aluminum has been the metal of choice for interconnects. For device modeling, standard approximations to the Boltzmann equation, calculations of mobilities, and Fowler-Nordheim tunneling, have been very adequate. As feature dimensions are entering the 'nano' regime, however, revolutionary changes are becoming inevitable. Copper has already replaced aluminum as the metal of choice for interconnects. Silicon itself is facing changes, such as the adoption of strained layers for mobility enhancement, and is facing challenges from germanium as an alternative. The biggest change, however, is in the dominance of SiO{sub 2} as the gate dielectric. Despite decades of efforts, the Si-SiO{sub 2} system is the only one that works for a Metal-Oxide-Semiconductor field effect transistor. The key reason is that no other semiconductor has a stable native oxide that can serve as gate dielectric. Deposition of other insulators on semiconductors doesmore » not yield interfaces with low enough defect densities suitable for devices. Nevertheless, SiO{sub 2} has reached the end of its reign because scaling laws have pushed the gate-oxide thickness to about 1 nm, where tunneling currents become intolerable. Efforts to develop crystalline dielectrics deposited on Si have not been very promising. The main problem is the band offsets that one gets from materials that have a reasonable lattice match to Si. The most promising candidates are oxides that are 'piggy backed' on a very thin SiO{sub 2} layer. This talk covers new unpusblished results obtained by atomic-resolution Z-contrast microscopy on the Si-SiO{sub 2}-HfO{sub 2} system and first-principles theory. Z-contrast images show a very sharp Si-SiO{sub 2} interface, a very thin {approx}0.5 nm amorphous SiO{sub 2} layer, followed by HfO{sub 2}; they also show individual Hf atoms scattered in the SiO{sub 2} layer, but never getting closer than about 0.3 nm to the interface. First-principles calculations also find that approaching the interface closer than 0.3 nm is not energetically favored. The isolated Hf atoms have energy levels in the gap region and may contribute both to mobility degradation and to leakage currents. The new era of nanoelectronics also has raised new challenges in device modeling. Mobilities in MOSFETs fabricated with strained Si layers and in double-gated MOSFETs with ultrathin channels fabricated using SOI technology do not obey the 'universal mobility curve' that has served the industry very well for a long time. Attempts to model mobilities in these structures have run into difficulties because the standard approximations fail (effective approximation, infinite potential barrier at the Si-SiO{sub 2} interface, phenomenological interfaces roughness). This talk covers a new formulation for the calculation of mobilities using a first-principles approach. An atomic-scale model of an abrupt 'ideal' interface is used, the self-consistent Hamiltonian is calculated, and then the calculation is repeated for an interface containing elementary deviations from abruptness (Si-Si bonds on the oxide side, Si-O-Si protrusions in the Si side, single impurity, etc.). The difference represents a scattering potential that is used to calculate mobilities. The wave functions of Si electrons naturally penetrate into the oxide. Initial unpublished results are very illuminating and promising. Finally, modeling Fowler-Nordheim tunneling currents using the classical triangular potential barrier, even with modifications, is running into difficulties. Failure of the effective mass approximation is again the main culprit. This talk covers unpublished results of calculations of Fowler-Nordheim currents using atomic-scale models of the interface and the Lippmann-Schwinger method, recently demonstrated in calculations of current-voltage characteristics of single molecules. The results elucidate the role of atomic-scale detail in determining the Fowler-Nordheim tunneling current.« less

Passive Layer Formation at Ferroelectric PbTiO3/Pt Interfaces Studied by EELS

AbstractPolarization fatigue with repeated electric cycles in ferroelectric thin films is a major degradation problem in ferroelectric nonvolatile memories. However, the origin of this phenomenon is still not properly understood. The fatigue mechanism of a ferroelectric perovskite in a multilayer ferroelectric PbTiO3 thin film material has been investigated here using scanning transmission electron microscopy (STEM). Z-contrast images of the interfaces show that the ferroelectric PbTiO3 layer has partly decomposed into a single crystal PbTiO3 layer and an amorphous layer. Nanometer-sized precipitates are present near the Pt electrode. Electron energy-loss spectroscopy (EELS) analysis reveals that the amorphous layer is a Ti-rich phase between TiO2 and PbTiO3. The precipitates are determined to be a Pt-Pb rich crystalline phase. It is suggested that the formation of the structure-distorted intermediate layer and precipitates may be associated with the ferroelectric degradation process by acting as a passive layer in a ferroelectric capacitor. In addition, the formation of the Pt-Pb rich precipitates may cause an interruption of the consistent Pt electrode, which may result in failure of the device.

Fe-Core/Au-Shell Nanoparticles: Growth Mechanisms, Oxidation and Aging Effects

ABSTRACTWe report the chemical synthesis of Fe-core/Au-shell nanoparticles (Fe/Au) by a reverse micelle method, and the investigation of their growth mechanisms and oxidation-resistant characteristics. The core-shell structure and the presence of the Fe and Au phases have been confirmed by transmission electron microscopy, energy dispersive spectroscopy, x-ray diffraction, Mössbauer spectroscopy, and inductively coupled plasma techniques. Additionally, atomic-resolution Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope have been used to study details of the growth processes. The Au-shells grow by nucleating on the Fe-core surfaces before coalescing. First-order reversal curves, along with the major hysteresis loops of the Fe/Au nanoparticles have been measured as a function of time in order to investigate the evolution of their magnetic properties. The magnetic moments of such nanoparticles, in the loose powder form, decrease over time due to oxidation. The less than ideal oxidation-resistance of the Au shell may have been caused by the rough Au surfaces. In a small fraction of the particles, off-centered Fe cores have been observed, which are more susceptible to oxidation. However, in the pressed pellet form, electrical transport measurements show that the particles are fairly stable, as the resistance and magnetoresistance of the pellet do not change appreciably over time. Our results demonstrate the complexity involved in the synthesis and properties of these heterostructured nanoparticles.

Structural Changes of Bimetallic PdX/Cu (1-X) Nanocatalysts Developed for Nitrate Reduction of Drinking Water

AbstractReductive removal by hydrogeneration using supported Pd/M (M= Cu, Pt, Ag, Co, Fe, Mo, Ni, Rh, Ir, Mn and Cr) bimetallic catalysts has emerged as a promising alternative for nitrate removal in drinking water [1]. Fundamental understanding how the atomic arrangement of Pd and a second element, such as Cu, affect the activity nitrite reduction and selectivity of dinitrogen will be accomplished by coordinated synthesis (Shapley), activity/selectivity/efficiency measurements (Werth) and nanostructure determination (Yang & Xu). In this paper, we report a systematic study of novel polyvinylpyrrolidone (PVP) stabilized nanoscale Pd-Cu colloids, with homogeneous and narrow size distribution, with Pd: Cu ratios varying from 50:50 to 90:10. Initial measurements on catalytic activity for nitrate reduction demonstrated a dependence on the relative composition. Electron microscopy studies, including Z-contrast imaging [2], energy-dispersive X-ray emission (EDX), electron diffraction and high-resolution electron microscopy (HREM), revealed a surprising change in structure at the 80:20 Pd-Cu composition, where, with less than 80% Pd,the nanoparticle forms a core-shell structure but for nanoparticles containing 80% or more Pd, it is homogeneous. We are at the pivotal point of directly correlating these nano-structures with the catalytic activity. Such an understanding is essential for the efficient development of catalysts for the purification of drinking water.

Investigating the Structure-Property Relationships at Grain Boundaries in MgO Using Bond-Valence Pair Potentials and Multiple Scattering Analysis

We describe the use of bond–valence analysis to investigate the segregation of calcium atoms at an MgO [001] tilt grain boundary. For small deviations away from the equilibrium metal-oxide bond length, the bond–valence parameter approximates well to a Born–Mayer-type pair potential. Therefore, by starting with a structural model determined from an atomic resolution Z-contrast image, compositional and cation valence state information obtained from electron energy loss spectroscopy (EELS) can be incorporated into a comprehensive model for the grain boundary. For the boundary under investigation here, it is found that specific sites for preferential segregation can be identified, resulting in 0.3–0.4 monolayers of calcium in the boundary plane.

Atomic resolution electron energy-loss spectroscopy

Electron energy-loss spectroscopy (EELS) has been successfully used to measure the electronic structure of materials with atomic (i.e. sub-nanometer) spatial resolution. Furthermore, the combination of incoherent Z-contrast imaging and EELS allows us to correlate structural features, such as defects or interfaces directly with the changes in the local electronic structure. In this review, we will discuss the theoretical aspects and experimental procedures for achieving atomic-resolution EELS. In particular, we will describe the practicalities of the combination of Z-contrast imaging and EELS, used in the scanning transmission electron microscopy mode and also describe some of our recent results where column-by-column EELS has helped solve important material science problems.

Direct evidence of substituent clustering in lower valent K+ modified PbZrO3 by high-resolution Z-contrast imaging

The distribution of substituents in lead zirconate (PZ) modified by K+ (PKZ) has been studied by high-resolution Z-contrast imaging. Pronounced intensity variations were observed between various Pb columns, giving direct evidence that a number of columns had higher concentrations of (K+) substituents. High-resolution Z-contrast imaging was also performed in La+3-modified lead zirconate titanate (PLZT) for comparison.

Si 0.85Ge 0.15 oxynitridation in wet-nitric oxide ambient

Nitric oxide (NO) aided Si 0.85Ge 0.15 wet-oxynitridation has been performed at 400–700 °C, while the wet-NO feed gas was preheated to higher temperatures before entering the reaction zone. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy data suggests that both nitrogen and oxygen incorporation increases within the dielectric bulk with increasing wet-oxynitridation temperature, while there is no apparent germanium segregation towards the dielectric/substrate interface at all temperatures studied. Moreover, angle-resolved XPS analysis suggests that wet-oxynitridation at temperatures higher than 600 °C volatilizes some germanium oxide from the surface region, while silicon monoxide is outgassed from the dielectric bulk and accumulates near the surface. Nitrogen incorporation is found to hinder germanium segregation. Z-contrast imaging with scanning transmission electron microscopy shows that oxynitrides grown in wet-NO have sharp interfaces with bulk Si 0.85Ge 0.15, while the roughness of the dielectric/Si 0.85Ge 0.15 substrate interface is less than 2 Å. These results are discussed in the context of an overall mechanism of SiGe wet-oxynitridation.

Atomic-scale structural analyses of epitaxial Co∕Re superlattices

High-resolution transmission electron microscopy and scanning transmission electron microscopy (STEM) have been used to investigate atomic-scale structural properties of Co∕Re trilayers and superlattices grown via magnetron sputtering. The sample growth was epitaxial with the (101¯0) plane of Co and Re parallel to the (112¯0) plane of Al2O3, and the [001] direction of Re and Co coinciding with that of the Al2O3. Both low-angle and high-angle Z-contrast STEM images show a very uniform layer thickness. However, the interface roughness between the Re and Co layers monotonically increases with interface distance from the substrate. These results strongly imply that, in the epitaxial Re∕Co superlattice system, interface roughness plays a more important role in the giant magnetoresistance effect than thickness fluctuations of the spacer layer. Previous anisotropic magnetoresistance measurements can be explained in terms of the observed atomic-scale structure.

FIB Lift-Out and Milling of Cylindrical Specimens for Electron Tomography (or Atom Probe Field Ion Microscopy)

Abstract Electron tomography using transmission electron microscopy (TEM) and related techniques (e.g., scanning transmission electron microscopy (STEM) or energy filtered TEM (EFTEM)) allow for 3-D microstructural and elemental mapping of specimens, and has been used successfully in the biological sciences where mass-thickness contrast dominates these mostly amorphous materials. Z-contrast STEM imaging via high angle annular dark field (HAADF) tomography has also been used successfully in the physical sciences. STEM, EFTEM, and holography tomography are more useful techniques for crystalline materials, since diffraction contrast in conventional TEM images can hinder image reconstruction. Typical tomography routines utilize conventional electron transparent foils, whereby the dimensions of the specimen perpendicular to the electron beam may be orders of magnitude greater than the specimen thickness parallel to the electron beam. Using this conventional specimen geometry, the effective specimen thickness increases as the specimen is tilted through the ± 70 degrees necessary for the tomographic acquisition process.

Microstructural characterization of HgTe/HgCdTe superlattices

The effects of annealing on the microstructure of HgTe/Hg0.05Cd0.95Te/CdZnTe(211) superlattices (SLs) grown by molecular beam epitaxy have been investigated by high-resolution electron microscopy, and Z-contrast imaging was utilized to study the chemical abruptness of the HgTe/HgCdTe interfaces. Low-temperature annealing at 225 or 250°C induced interdiffusion, leading to changes in the well/barrier widths. The SLs became more defective even after 30min of annealing. The widths of the HgTe well layers increased and the widths of the HgCdTe barrier layers decreased dramatically. Thus, activation of p-type dopants such as arsenic through annealing will be difficult to achieve without degradation of electrical and optical properties.

Atomic layer deposition and characterization of hafnium oxide grown on silicon from tetrakis(diethylamino)hafnium and water vapor

In this work thin films of hafnium oxide are deposited on Si(100) substrates by means of atomic layer deposition (ALD) using tetrakis(diethylamino)hafnium and water vapor at substrate temperatures of 250–350ºC. Our system capabilities include fast transient delivery of reactive fluids, real-time vapor phase detection (in situ tunable diode laser hygrometer), precursor thermochemical capabilities, and ppt level elemental analysis by inductive coupling plasma mass spectrometry. The composition, purity, and other properties of the films and resulting interfaces are determined using x-ray and Fourier transform infrared spectroscopies, Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope with Å scale resolution, and spectroscopic ellipsometry. The observed ALD rate is ∼1.4Å per cycle. The nonuniformity across the film is less than 4%. Negligible carbon contamination is found in the resulting stoichiometric films under all conditions studied. The pulse sequence was optimized to prevent disastrous particulate problems while still minimizing purge times. The film deposition is investigated as a function of substrate temperature and reagent pulsing characteristics. A mild inverse temperature dependence of the ALD rate is observed. The initial stage of the HfO2 growth is investigated in detail.