Robert Hull

Metastable Structures in Al Thin Films Before the Onset of Corrosion Pitting as Observed using Liquid Cell Transmission Electron Microscopy

One of the fundamental challenges in understanding the early stages of corrosion pitting in metals protected with an oxide film is that there are relatively few techniques that can probe microstructure with sufficient resolution while maintaining a wet environment. Here, we demonstrate that microstructural changes in Al thin films caused by aqueous NaCl solutions of varying chloride concentrations can be directly observed using a liquid flow cell enclosed within a transmission electron microscope (TEM) holder. In the absence of chloride, Al thin films did not exhibit significant corrosion when immersed in de-ionized water for 2 days. However, introducing 0.01 M NaCl solutions led to extensive random formation of blisters over the sample surface, while 0.1 M NaCl solutions formed anomalous structures that were larger than the typical grain size. Immersion in 1.0 M NaCl solutions led to fractal corrosion consistent with previously reported studies of Al thin films using optical microscopy. These results show the potential of in situ liquid cell electron microscopy for probing the processes that take place before the onset of pitting and for correlating pit locations with the underlying microstructure of the material.

Simulation Study of Aberration-Corrected High-Resolution Transmission Electron Microscopy Imaging of Few-Layer-Graphene Stacking

Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. The high carrier mobility and mechanical robustness of single layer graphene make it an attractive material for beyond CMOS devices. The current work investigates through high-resolution transmission electron microscopy (HRTEM) image simulation the sensitivity of aberration-corrected HRTEM to the different graphene stacking configurations AAA/ABA/ABC as well as bilayers with rotational misorientations between the individual layers. High-angle annular dark field-scanning transmission electron microscopy simulation is also explored. Images calculated using the multislice approximation show discernable differences between the stacking sequences when simulated with realistic operating parameters in the presence of low random noise.

Coupled effects of ion beam chemistry and morphology on directed self-assembly of epitaxial semiconductor nanostructures

We study the coupled effects of ion beam chemistry and morphology on the assembly of templated epitaxial nanostructures. Using a focused ion beam (FIB) system equipped with a mass-selecting filter, we pattern Si substrates with local ion doses of Si, Ge and Ga to control subsequent Ge(x)Si(1 - x) epitaxial nanostructure assembly. This capability to employ different templating species allows us to study how different incorporated ion species in the near surface region affect the ability to localize nucleation during subsequent epitaxial growth. Our results indicate that FIB-directed self-assembly is a complex process, dependent on dose-induced morphology in addition to ion-specific chemical effects.

Surface roughness dependence of the electrical resistivity of W(001) layers

The resistivity ρ of epitaxial W(001) layers grown on MgO(001) at 900u2009°C increases from 5.63u2009±u20090.05 to 27.6u2009±u20090.6 μΩ-cm with decreasing thickness du2009=u2009390 to 4.5u2009nm. This increase is due to electron-surface scattering but is less pronounced after in situ annealing at 1050u2009°C, leading to a 7%–13% lower ρ for du2009<u200920u2009nm. The ρ(d) data from in situ and ex situ transport measurements at 295 and 77u2009K cannot be satisfactorily described using the existing Fuchs-Sondheimer (FS) model for surface scattering, as ρ for du2009<u20099u2009nm is larger than the FS prediction and the annealing effects are inconsistent with a change in either the bulk mean free path or the surface scattering specularity. In contrast, introducing an additive resistivity term ρmound which accounts for surface roughness resolves both shortcomings. The new term is due to electron reflection at surface mounds and is, therefore, proportional to the ballistic resistance times the average surface roughness slope, divided by the layer thickness. This is confir...

Quantification of electron–phonon scattering for determination of temperature variations at high spatial resolution in the transmission electron microscope

This work investigates the quantification of electron-phonon thermal diffuse scattering (TDS) for detection of temperature variations with nanometer spatial resolution in transmission electron microscopy (TEM). Observations of TDS intensity for (100) single crystal Si and Ge show interdependences of temperature and sample thickness which can be understood through the angular distributions of electron-phonon scattering as a function of temperature. The temperature sensitivity of the integrated TDS intensity can be of the order of 10(-3) K(-1) for Si and Ge. This shows that measurement of the TDS intensity in the TEM is a promising means for nanoscale temperature measurement; our measurements to date have demonstrated that temperature changes as small as 5 K are detectable.

Effect of asymmetric strain relaxation on dislocation relaxation processes in heteroepitaxial semiconductors

The effect of asymmetric interfacial strain configurations upon the generation of misfit dislocation arrays in lattice mismatched epitaxy is considered. For example, elastic strain relaxation for Si1−xGex/Si(110) films is uniaxial, assuming glide on {111} planes as expected for the diamond cubic system, which leads to asymmetric strain relief. Here, we extend our previously developed relaxation model for generation of dislocation arrays in SiGe/Si, by accounting for how the different energetics of asymmetrically strained films affect the kinetics of the relaxation process. Similarly, non-polar III-nitride epitaxial films have asymmetric strain from the outset of growth due to the different c/a lattice parameter ratios. In both systems, the asymmetric strain is represented by an additional term in the misfit dislocation applied stress equation. In SiGe/Si(110), a simple elasticity analysis of the strain produced by the uniaxial array of dislocations predicts that the relaxation orthogonal to the dislocatio...

Materials genomics of thin film strain relaxation by misfit dislocations

We summarize the development and implementation of a “process simulator” for modeling thin film strain relaxation by injection of misfit dislocations. The process simulator, initially developed for GexSi1−x/Si(100) lattice-mismatched epitaxy, integrates elasticity and dislocation theory with experimental measurements of kinetic parameters describing dislocation nucleation, propagation, and interactions. This enables predictive simulation of the development of misfit dislocation arrays during growth and thermal annealing sequences. Further, in the spirit of the materials genome initiative, we show how once a relatively complete description is built for one materials system, extension to a related system may be implemented using a greatly reduced data set. We illustrate this concept by translation of the simulator for GexSi1−x/Si(100) epitaxy into predictive simulation for the GexSi1−x/Si(110) system (which has quite different dislocation microstructure and kinetics) using greatly reduced data sets for the ...

Bridging the length scales between lithographic patterning and self assembly mechanisms in fabrication of semiconductor nanostructure arrays

We employ focused ion beam patterning of single crystal Si(100) surfaces to template the assembly of Ge(Si) nanostructure arrays. The evolution and final structures of the templated arrays are determined by combinations of transmission electron, low energy electron microscope, focused ion beam and scanning probe microscopies. It is shown how the positions of individual nanostructures may be controlled to the order of 10 nm. However, to achieve controlled spacings between elements that are in the 10 nm range requires careful matching of the characteristic lengths scales of self assembly mechanisms to the length scales of the external lithographic forcing functions.

Microstructural changes in silicon induced by patterning with focused ion beams of Ga, Si and Au.

We use focused beams of Ga(+), Au(+) and Si(++) ions to induce local microstructural changes in single crystal silicon. The ions were delivered as single spot pulses into thin Si membranes that could subsequently be imaged and annealed in situ in a transmission electron microscope. For each ion, the focused ion beam implantation created an array of amorphous regions in the crystalline membrane. Annealing causes solid phase epitaxial regrowth to take place, but we show that the resulting microstructure depends on the ion species. For Ga(+) and Au(+), precipitates remain after recrystallization, while for Si(++), dislocation loops form around the periphery of each implanted spot. We attribute these loops to defects formed during solid phase epitaxial regrowth, with controlled placement of the loops possible.

Simulation Study Of Transmission Electron Microscopy Imaging Of Graphene Stacking

Graphene is the subject of intense study due to its high mobility and mechanical integrity. These properties make it an attractive material for the “beyond CMOS” technology that will replace today’s transistor. Acceleration of process and device technology requires considerable advances in the imaging and characterization of graphene. The physical dimensions of available single and multi‐layer samples are not large enough for many metrology methods. For example, the spot size of ellipsometry is typically larger than available samples. Electron microscopy of graphene is also challenging. Carbon is a difficult element to image with electron microscopy because of its low atomic number. The high mobility of single layer and misoriented two and three layer graphene make it attractive for nanoelectronics. The current investigation explores HRTEM simulations of graphene stacking configurations AAA/ABA/ABC as well as bilayers with misorientations between the individual layers. HAADF (High Angle Annular Dark Field...