In a condensed matter system, phonons and plasmons are well-known quasiparticles that represent unusual dispersion behavior of energy and momentum at nanoscales. In a nanoscale Mo/Si multilayer structure, phonon modes in Raman scattering indicated the coexistence of crystalline Si (c-Si) nanoclusters within an amorphous silicon (a-Si) matrix. The TO mode was red-shifted with a decrease in the nanocluster size of Si in nanolayer films. This was associated with the momentum of phonons and it is fundamentally correlated to phonon confinement. The correlation length of the Si network was significantly smaller in a-Si and the TO mode broadened asymmetrically and redshifted due to localized phonon density of state. Consequently, with a decrease in the thickness of the Si layer, blue shifts of plasmon energy for Mo 3d, Mo 4p, and Si 2p spectra were observed in X-ray photoelectron spectroscopy. Plasmon energy of the c-Si nanocluster was related to the forbidden gap, which increased with a decrease in cluster size. The concept of quantum confinement of phonon and electron states was used to determine the size of the c-Si nanoclusters in the a-Si matrix.

In the present work, formation of interfaces in the multilayer periodic Mo/Be mirror was studied using X-ray photoelectron spectroscopy. Chemical composition and significance of the interfaces depending on the number of periods were investigated by means of the XPS spectra decomposition technique. Formation of beryllide compounds at the interfaces was revealed. It was shown that two types of beryllide form at the interfaces depending on the film order: MoBe α , 4.0 < α < 5.0, at the Be-on-Mo (BOM) interface and MoBe β , 1.0 < β < 1.6, at the Mo-on-Be (MOB) interface. The increase in the number of periods from 1 to 3 leads to suppression of the MoBe α formation at the BOM interface, while quality of the MOB interface remains unchanged. In order to rationalize the observed phenomena, an assumption on the interface formation mechanism was made. According to this hypothesis, the chemical composition asymmetry of the interfaces arises from the difference in the diffusion mechanism of Be atoms: surface diffusion prevails during the MOB interface formation, while bulk diffusion is favorable during the BOM interface formation. In this regard, suppression of the beryllide formation at the BOM interface with increasing number of periods indicates reduction of bulk defects in the Mo film.

Broadband Mo/Be multilayer structures were designed for maximum uniform normal-incidence reflectivity in a broad range of 111–138 Å, which lies near and beyond the L2,3 absorption edge of Si. A comparison was made of the capabilities of two classes of aperiodic structures and of so-called “stack” structures, which are composed of several periodic structures with different periods stacked one over the other. Six-stack Mo/Be 80-layer structures were synthesized on concave (R = 1 m) superpolished fused silica substrates. Their absolute reflectivity was measured at 13% – 14% in the 111–138 Å optimization range using a laboratory reflectometer with a laser-plasma radiation source. The normal-incidence reflection spectra of the mirrors were recorded in the configuration of a transmission-grating spectrograph using the slowly varying quasicontinuum of a laser-driven tungsten plasma. Comparing the reflectivity data with the reflection spectra recorded with a CCD permitted estimating a decrease in the detector responsivity beyond the Si L-edge. The broadband normal-incidence multilayer mirrors facilitate the development of a high-resolution imaging spectrograph covering a usable range about the Si L-edge to characterize, for instance, the L-edge fine structures and chemical states. These mirrors will also find use in imaging solar instruments with a high spectral resolution operating aboard a spacecraft and in laboratory instruments for plasma diagnostics.

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