Analytical Methods for Nanostructures: Limits, Challenges and Prospects
Ideally, methods for the characterization of nanostructures would provide
intuitively interpretable atomic structure images, along with information
on bonding and electronic structure of a particular atomic arrangement.
An example in microelectronics where such comprehensive information is
necessary are ultrathin layers of alternative high dielectric constant
oxides currently investigated to replace SiO2 as a gate dielectric.
talk will discuss several recent developments in high-spatial resolution
analytical methods to characterize the internal atomic and electronic
structure, using alternative gate dielectrics as an example.
In recent years fundamentally new tools for understanding nanoscale
properties have become available through major advances in atomic-level
imaging and spectroscopy in scanning transmission electron microscopy
(STEM). In STEM, a lattice resolution high-angle annular dark-field, or
Z-contrast, image is acquired. The Z-contrast image is chemically
sensitive, in particular to heavy elements, and individual atoms can be
imaged. In contrast to conventional high-resolution transmission electron
microscopy, no preconceived atomic structure models are necessary to
interpret the image. The Z-contrast image is also used to position the
probe precisely at the region of interest, for example an internal
interface or defect, for electron energy-loss spectroscopy (EELS). EELS
can be used to profile the chemistry across the films. The near-edge
fine-structure of EELS core-loss edges represent the local density of
unoccupied states, and can be used to probe bonding changes with near
atomic spatial resolution. We discuss limitations of EELS fine-structure
analysis, in particular due to the energy-resolution available with
current instrumentation and challenges in theoretical methods to calculate
EELS fine-structures. However, in both instrumentation and theory,
significant progress has been made within the last two years.
The talk will also present several complementary methods. In particular,
medium energy-ion scattering (MEIS) can provide compositional analysis
with sub-nm depth resolution and has a greater sensitivity for heavy
elements than EELS. Inelastic electron tunneling spectroscopy (IETS) is a
vibrational spectroscopy technique that can provide a wealth of
information on bonding, impurities and defects in ultrathin dielectric
layers with an excellent energy resolution.