Figure 1(left): Chemical depth profiling of a semiconductor gate stack by variable-kinetic energy x-ray photoelectron spectroscopy. As the photon energy is increased, the Si 1s photoelectrons probe more deeply into the buried structure. An interface effect is observed that reveals the Si oxide sub-stochiometry at the HfO2/SiO2 interface. Figure 2(right): Diffuse x-ray scattering patterns from misfit dislocations in a strained, 100 nm thick, 10 % Ge, GeSi thin film grown on Si(001). The patterns were recorded around the SiGe(004) and SiGe(044) diffractions at constant L. The x and y axis are H and K in Si reciprocal lattice units.
Figure 3: Ti-K and Ti-L near-edge x-ray absorption fine structure spectra from bulk SrTiO3 and a 2 nm thick SrTiO3 thin film grown coherently on Si(001). The spectra are compared to first-principles density functional and Bethe-Salpeter calculations. The presence of the large pre-edge feature in the K edge spectra of the film demonstrates that strain has driven a room temperature ferroelectric distortion in the SrTiO3 thin film, a material that is not typically ferroelectric at any temperature.
We have developed synchrotron based variable kinetic energy x-ray photoelectron spectroscopy to address the development and optimization of materials for microelectronics, catalysis, homeland security, and energy. Materials that can be investigated include monolayers to bulk materials of all classes. Depth selectivity is attained by utilizing different photon energies and core lines over a wide energy range. Examples of ongoing measurements include (1) chemistry and bonding at semiconductor oxide interfaces, (2) oxidation of metal electrodes on semiconductors, (3) ferroelectric oxides, (4) high-k gate dielectrics; and (5) organic electronics. Such measurement studies offer the opportunity to develop and utilize world class x-ray instrumentation and detectors. Similar materials are also characterized by x-ray diffraction and x-ray absorption fine structure spectroscopy to determine both their local and long range atomic structure in addition to the chemical information obtained from x-ray photoelectron spectroscopy.
Awards and Honors
Synchrotron Methods Group
1991-present: Physicist, Ceramics Division, NIST
Ph.D., Applied Physics, Stanford University, 1989