Reflecting properties of narrowband Si/Al/Sc multilayer mirrors at 58.4 nm

New method for the determination of photoabsorption from transmittance measurements in the extreme ultraviolet

We have developed a new method for the determination of photoabsorption at extreme ultraviolet wavelengths longer than 20 nm, where reliable refractive index values are sparse or non-existent. Our method overcomes the obstacle of multiple reflections that occur inside thin films in this spectral range, which up until now has prevented the accurate determination of photoabsorption from transmittance measurements. We have derived a mathematical expression that is independent of internal reflection amplitudes, while taking advantage of the transmittance oscillations stemming from such reflections. The method is validated on measurements of aluminum thin films. This advance will enable accurate refractive index values for many important materials for optical instrumentation, thus facilitating high-priority research on topics including coherent light sources, planetary and solar physics, and semiconductor manufacturing.

Phonon, plasmon and electronic properties of surfaces and interfaces of periodic W/Si and Si/W multilayers

The phonon and plasmon excitations and electronic properties of interfaces of periodic W/Si and Si/W multilayer structures were investigated. The Boson band originated from quasilocal surface acoustic phonons for ultrathin Si layers, excited by Raman scattering. In confined Si layers, a small fraction of crystalline Si nanoclusters were embedded within a large volume fraction of amorphous Si (a-Si) nanoclusters. The size of the a-Si nanoclusters was smaller for the thinner Si layer in the periodic layers. The plasmon energy in the Si layer was blueshifted with a decrease in the thickness of this layer. This was explained by the size-dependent quantization of plasmon shift. The valence band spectra comprised a substantial fine structure, which is associated with the interaction of valence orbitals of the W and Si atoms at the interface boundaries. For thinner Si layers, the binding interaction of W5d and Si3p states leads to the splitting of the density of states near the Fermi level in the energy range of 1.5-5 eV. However, the energy splitting with two maxima was observed at 0.7 and 2.4 eV for thicker layers. Thus, the results of X-ray photoelectron spectroscopy have indicated that the interface of W/Si multilayers consists of metal-enriched tungsten silicide. Both the atomic structure and the elemental composition of the silicide were modified with a change in the thickness of the Si layers. This novel investigation could be essential for designing nanomirrors with higher reflectivity.