J. J. Jia

Characterization of buried thin films with resonant soft x-ray fluorescence

The geometric and electronic structure of a buried monolayer of boron nitride (BN) has been probed using resonant soft x‐ray fluorescence (SXF). By using the strong π* resonance feature in the resonant fluorescence spectrum near the B (1s) threshold, we were able to detect the BN thin film and examine changes in its electronic structure when the monolayer is placed between different materials. Our results demonstrate the capability of the resonant SXF technique for probing the element‐specific electronic structure of a buried thin film nondestructively.

The structural homogeneity of boron carbide thin films fabricated using plasma‐enhanced chemical vapor deposition from B5H9+CH4

Boron carbide thin films of several B/C ratios have been deposited on Si(111) using plasma‐enhanced chemical vapor deposition from nido‐pentaborane(9) (B5H9) and methane (CH4). X‐ray diffraction studies of boron carbide thin films on Si(111) exhibited characteristic microcrystalline diffraction lines. Soft x‐ray emission spectroscopy was used to verify that the local electronic structure and composition of each sample corresponded to a homogeneous solid solution boron carbide phase.

Crystal-momentum-resolved electronic structure of solids using resonant soft-X-ray fluorescence spectroscopy

Abstract Resonant inelastic X-ray scattering (RIXS) has been observed in graphite and hexagonal boron nitride (hBN) above and below their K edges. Below the core threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation goes above the core binding energy, nonlinear dispersive effects are observed in graphite but not in hBN. We show that these two effects, which have previously been thought of as separate processes, i.e. resonant X-ray Raman scattering (below threshold) and RIXS (above threshold), are in fact described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. The role core-excitons play in the RIXS process is examined in finer detail, by using narrow-band excitation. Our results indicate that core-hole effects play a minor role in the RIXS observed from graphite but are more pronounced in hBN.

A simple variable line space grating monochromator for synchrotron light source beamlines

Abstract In this paper, we present and analyze an improved spectrometer design that has evolved from studies of the focal properties of transmission gratings in converging ligid beams. This design uses a variable line space grating at a fixed angle of incidence to focus the diffracted light in one dimension, producing resolutions in excess of 1000 over a large wavelength region. Attractive features of this concept include high throughout, simple wavelenght scanning, simple optical elements (plane gratings and plane and toroidal mirrors), and relatively inexpensive realizations. After presenting a mathematical analysis of the primary design features, we discuss implementations of this design both as a spectrometer and as two versions of beamline monochromators. Design parameters for the two monochromators will be given.

Soft-x-ray fluorescence study of buried silicides in antiferromagnetically coupled Fe/Si multilayers

Multilayer films made by alternate deposition of two materials play an important role in electronic and optical devices such as quantum-well lasers and x-ray mirrors. In addition, novel phenomena like giant magnetoresistance and dimensional crossover in superconductors have emerged from studies of multilayers. While sophisticated x-ray techniques are widely used to study the morphology of multilayer films, progress in studying the electronic structure has been slower. The short mean-free path of low-energy electrons severely limits the usefulness of photoemission and related electron free path of low-energy electrons severely limit spectroscopies for multilayer studies. Soft x-ray fluorescence (SXF) is a bulk-sensitive photon-in, photon-out method to study valence band electronic states. Near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) measured with partial photon yield can give complementary bulk-sensitive information about unoccupied states. Both these methods are element-specific since the incident x-ray photons excite electrons from core levels. By combining NEXAFS and SXF measurements on buried layers in multilayers and comparing these spectra to data on appropriate reference compounds, it is possible to obtain a detailed picture of the electronic structure. Results are presented for a study of a Fe/Si multilayer system.

Soft-x-ray fluorescence studies of solids

Abstract Resonant inelastic X-ray scattering (RIXS) has been observed in many systems above and below their core threshold. Below threshold, inelastic-loss features are observed, which disperse linearly with excitation energy, but as the excitation increases above the core binding energy, nonlinear dispersive effects are observed. These two effects are described by the same physics of coherent fluorescence. Very good agreement between experiment and simulated RIXS is achieved using a simple one-electron framework. However, significant questions have arisen concerning core-hole effects and their influence on RIXS. In this work, the role core-excitons play in the RIXS process is examined in finer detail in graphite, by using narrow-band excitation, and through comparison between experiment and simulated spectra which include the core-hole effects explicitly in the modeling. Based on these findings, we conclude that core-hole effects play a minor but detectable role in the RIXS observed from graphite.

A soft‐x‐ray‐emission investigation of cobalt implanted silicon crystals

The Si L2,3 emission spectra of silicon crystals implanted with Co at doses of (1–8) × 1017 Co/cm2 have been examined using soft‐x‐ray‐emission (SXE) spectroscopy. At the lowest dose, the spectra are little modified from that of crystalline Si, indicating that only a small fraction of Si is in the form of silicides within the probe depth of SXE spectroscopy. For higher doses and implant profiles with Co extending to the surface, there is clear evidence for ordered CoSi2 combined with richer Co phases, but little evidence for pure Si or for ordered regions of CoSi.

Study of buried interfaces by soft x‐ray fluorescence spectroscopy excited by synchrotron radiation

This article is a summary of four different aspects of soft x‐ray spectroscopy that are being used or developed by our group to probe the interface. The first is to study the change in valence band fluorescence emission by mimicking a high density of interface atoms through the use of a multilayer. The second method uses an intense localized excited state to characterize the buried interface. The third uses Raman scattering to probe changes in band structure produced by stoichiometric changes in the valence band density of states for atoms at the interface. Finally, a method to study interface atoms by using valence emission from interface atoms that are excited by x‐ray standing waves is being developed.

Resonant inelastic scattering in localized solid system by soft X-ray fluorescence spectroscopy

An insertion device beamline, BL 8.0 at the ALS, utilizes the radiation from a 5.0 cm period undulator to cover the 100 – 1500 eV photon energy range. In a number of solid systems with localized conduction band states and excitonic states by tuning the energy of the monochromatic x-rays near core level absorption edges of cation atoms, a resonant inelastic scattering feature is observed. We have carried out careful studies in various systems. As an example, we present resonant X-ray inelastic scattering spectra from CaF2, CaSi and CaSi2 with distinctive localized empty 3d states, to give a general overview of resonant inelastics scattering observations in these systems. When a corelectrtron is excited into a localized state, it stays there long enough eventuall to recombine with the core hole, resulting in resonant elastic peaks. For the inelastic process, the energy loss observed indicates an electronic transition of the valence electron to the localized states in the conduction band accompanying the excitation.

Research Opportunities in Fluorescence with Third-Generation Synchrotron Radiation Sources

Synchrotron radiation sources have opened a new window on the century-old use of x rays as a scientific tool. X-ray fluorescence, excited by the photoabsorption process, has been a part of this research picture almost since the day that x rays were first discovered. However, the investigation of multi-photon processes in gases and solids had to wait until the second half of the 20th century. The advent of intense synchrotron radiation sources based on the use of specialized insertion devices will provide many new scientific opportunities for the 21st century. This presentation will outline some of the recent exciting discoveries in soft x-ray fluorescence spectroscopy and discuss a new type of laser-synchrotron hybrid experimental technique based on the time structure of the synchrotron radiation.