P. E. Batson

Sub-ångstrom resolution using aberration corrected electron optics

Following the invention of electron optics during the 1930s, lens aberrations have limited the achievable spatial resolution to about 50 times the wavelength of the imaging electrons. This situation is similar to that faced by Leeuwenhoek in the seventeenth century, whose work to improve the quality of glass lenses led directly to his discovery of the ubiquitous “animalcules” in canal water, the first hints of the cellular basis of life. The electron optical aberration problem was well understood from the start, but more than 60 years elapsed before a practical correction scheme for electron microscopy was demonstrated, and even then the remaining chromatic aberrations still limited the resolution. We report here the implementation of a computer-controlled aberration correction system in a scanning transmission electron microscope, which is less sensitive to chromatic aberration. Using this approach, we achieve an electron probe smaller than 1 Å. This performance, about 20 times the electron wavelength at 120 keV energy, allows dynamic imaging of single atoms, clusters of a few atoms, and single atomic layer ‘rafts’ of atoms coexisting with Au islands on a carbon substrate. This technique should also allow atomic column imaging of semiconductors, for detection of single dopant atoms, using an electron beam with energy below the damage threshold for silicon.

Electron microscope studies of an alloyed Au/Ni/Au‐Ge ohmic contact to GaAs

The interface structures resulting from the alloying reactions between a Au/Ni/Au‐Ge composite film and a (100) GaAs substrate were studied by transmission electron microscopy and scanning transmission electron microscopy. Electron microscope examinations of the cross‐sectional samples prepared in this study offered excellent lateral and depth resolution of local structures which are not available by other analytical techniques used previously in similar studies. The distributions and chemical compositions of various phases formed, and the morphologies of the interfaces between these phases were monitored and compared with the measured contact resistances at three different stages of alloying. A correlation between the interface structure and the contact resistance was found.

Examination of flatband and threshold voltage tuning of HfO2∕TiN field effect transistors by dielectric cap layers

The authors have examined the role of sub nanometer La2O3 and LaN cap layers interposed in Si∕HfO2∕TiN high-k gate dielectric stacks in tuning the flatband and threshold voltages of capacitors and transistors. High performance, band edge n metal oxide field effect transistors with channel lengths down to 60nm may be fabricated without significant compromise in mobility, electrical thickness, and threshold voltage. They have carried out a microstructural evaluation of these stacks and correlated these results with the electrical behavior of the devices.

Formation of nanopores in a SiN∕SiO2 membrane with an electron beam

An electron beam can drill nanopores in SiO2 or silicon nitride membranes and shrink a pore to a smaller diameter. Such nanopores are promising for single molecule detection. The pore formation in a 40?nm thick silicon nitride?SiO2 bilayer using an electron beam with a diameter of 8?nm (full width of half height) was investigated by electron energy loss spectroscopy with silicon nitride facing toward and away from the source. The O loss shows almost linear—independent of which layer faces the source, while N loss is quite complicated. After the formation of a pore, the membrane presents a wedge shape over a 70?nm radius around the nanopore.

Reactions of Pd on (100) and (110) GaAs surfaces

The reactions of Pd on atomically clean or air‐exposed (100) and (110) GaAs surfaces at temperatures between 20 to 500 °C in different ambients were investigated by transmission electron microscopy. Interfacial reactions quite different from previous x‐ray results were observed and two new Pd‐Ga‐As ternary phases were identified for the first time. At lower temperatures (T≲250 °C) the formation of a ternary phase PdGa∼0.3As∼0.2, which has a hexagonal structure very similar to that of Pd2Ge or Pd2Si with a0=b0=0.672 nm and c0=0.340 nm, was observed. This ternary phase is epitaxially oriented with (120)ternary∥(100)GaAs and [001]ternary∥[011]GaAs on (100) GaAs substrates, and with (110)ternary∥(110)GaAs and [001]ternary∥[110]GaAs on (110) GaAs substrates. At temperatures between 350 and 500 °C only one phase, PdGa, was observed to form in a high vacuum environment, whereas in a forming gas ambient, either a mixture of PdAs2 and another ternary phase PdGa∼0.6As∼0.4 (at 350 °C) or a mixture of PdAs2 and Pd...

Structure, chemistry and bonding at grain boundaries in Ni3Al—I. The role of boron in ductilizing grain boundaries

Abstract Boron segregation is known to change the fracture mode from intergranular to transgranular in Ni-rich Ni3Al. This paper addresses the question of why boron segregation at grain boundaries improves their resistance to fracture. Grain boundaries in B-free and B-doped Ni-rich Ni3Al (76 at. % Ni) were examined using spatially resolved electron energy loss spectroscopy (EELS), energy dispersive X-ray spectroscopy (EDS) and annular dark field (ADF) imaging in an UHV scanning transmission electron microscope, as well as conventional electron microscopy techniques. Ni-enrichment was seen in a 0.5 – 1.0 nm wide region at large angle boundaries, both in the absence and presence of B. Using EELS, B segregation to the boundary was observed to vary along the interface. EELS of the Ni L2,3 edge showed that the B-rich regions have a bonding similar to that in bulk Ni3Al, while the B-free regions have a bonding similar to the more Ni-like character of undoped boundaries. These results demonstrate that boron segregation increases the cohesive strength of grain boundaries in Ni3Al by making the bonding at the boundary similar to that in the bulk and, in this manner, increases their fracture resistance.

A new surface plasmon resonance in clusters of small aluminum spheres

Abstract Inelastic scattering of fast electrons in clusters of small aluminum spheres reveals a new surface plasmon mode near 4 eV. Electron energy loss filtered images show that the mode occurs within 10–40 nm spheres which touch other single spheres or clusters of spheres. The energy and spatial variations of the mode are consistent with a surface plasmon resonance at the metal-oxide-metal interface which is present at the point of contact between neighboring spheres.

Effect of arsenic segregation on the electrical properties of grain boundaries in polycrystalline silicon

Equilibrium arsenic segregation to the grain boundaries of polycrystalline silicon was measured directly by x‐ray microanalysis in the temperature range 700–1000 °C. A direct link was observed between arsenic segregation and resistivity. Increasing arsenic segregation at the lower annealing temperatures is consistent with an observed increase in resistivity. Fitting the enhancement levels at various temperatures with the McLean segregation isotherm, a binding energy of 0.65 eV/atom and a boundary saturation limit of 12 at. % for arsenic was obtained. A model for the effect of the segregation of arsenic to silicon grain boundaries is proposed. Segregation to boundary defects that cause trapping states can remove these interfacial traps, and segregation to other boundary sites can create a degenerately‐doped interfacial layer. The electrical consequences of this segregation are considered, and by comparison of the measured resistivity changes with temperatures with those predicted from these simple models i...

Motion of Gold Atoms on Carbon in the Aberration-Corrected STEM

The movement of heavy atoms on a thin carbon substrate is readily observed using a sub-Angstrom electron probe. The observed movement is consistent with an electron beam activation mechanism whereby atoms are occasionally detached from bonding sites, allowing rapid diffusion to new sites that may be quite far from the original. The bonding sites are most often observed to lie at defects, steps, and other asperities in the substrate. Formation of three-dimensional clusters can occur during diffusion of several isolated atoms. Coalescence and dissolution of larger clusters and islands both occur under varying observation conditions, but island coalescence appears most probable for islands that are greater than 2 nm in size.

Advanced spatially resolved EELS in the STEM

Commercial availability of high spatial resolution STEM instruments is leading to widespread use of EELS and ADF imaging techniques. Future instruments will need to greatly improve levels of stability and accuracy to allow use of these techniques with atomic level precision. I review some experimental results which suggest an urgent need for a 0.1 nm diameter probe with a usable EELS spectral resolution of about 100 meV.