The composite structures based on nickel rods in the matrix of silicon dioxide formation peculiarities study using synchrotron XANES in electrons and photons yield registration modes

Abstract

Purpose. The aim of this work was atomic and electronic structure and phase composition study of submicron Ni rods arrays formed by electrochemical deposition in a porous SiO₂ matrix on crystalline silicon depending on the conditions of their production by the synchrotron technique of X-ray Absorption Near Edge Structure (XANES) in the near-surface layers sensitive registration mode of total electron yield detection and bulk sensitive registration mode of fluorescence yield detection.

Methods and Methodology. Nickel rods arrays were obtained by electrochemical deposition of metal into the pores of the silicon dioxide matrix formed by the ion-tracking technique. Latent tracks were formed by irradiating the SiO₂ layer with heavy gold ions at the Han-Meitner Institute accelerator (Berlin, Germany). In the first case, ion fluence under irradiation was 1∙10⁸ cm^-2, and etching time was 40 minutes (group A), in the second case ion fluence was 5∙10⁸ cm^-2, etching time was 80 minutes (group B). The initial SiO₂/Si structures (from the formed oxide layer to the finished porous matrix) and Ni/SiO₂/Si composite structures were studied using the scanning electron microscopy (SEM) with the use of LEO1455-VP microscope. The surface morphology was studied in the mode of secondary electrons detection. The local atomic and electronic structure in Ni/SiO₂/Si composite structures was studied by XANES spectra. XANES were simultaneously recorded by the method of measuring total electron yield and by the fluorescent photons detection mode. X-ray spectroscopy data were obtained at the Russian German beamline of BESSY II synchrotron of the Helmholtz Zentrum Berlin (Germany). The vacuum in the spectrometers chambers was 10^-10 Torr, the instrument broadening was 0.1 eV. When registering TEY, the analysis depth was ~ 10 nm (O K absorption edge) and 15 nm (Ni L_2,3 absorption edge). At the same time the depth of analysis for XANES data recording in fluorescence yield mode exceeds hundreds of nanometers. A pure gold foil signal was used to calibrate and normalize the experimental spectra. The following objects were used as reference objects: SiO₂ matrix without nickel filling, thermal SiO₂ film with thickness of 100 nm, commercial: metallic nickel, nickel oxide NiO, nickel silicide Ni₂Si produced by Alfa Aesar.

Results. SEM studies have shown effective electrodeposition of nickel in the pores of the oxide layer on crystalline silicon. The direct dependence of the formation efficiency and density of nickel particles coating (resp. pores) of the composite structure Ni/SiO₂/Si is demonstrated from the fluence of ions as the main parameter of the formation tracks, and the time of electrochemical deposition. However, the metal/semiconductor interface (particle / substrate), the characteristics of which can have a significant effect on the properties of the composite structure, is generally hidden for SEM analysis from the surface at least by the layer thickness of the silicon oxide dielectric matrix.

When filling the pores of silicon dioxide with nickel and the formation of Ni/SiO₂/Si composite structures, it is possible to effectively use the synchrotron XANES method in the "surface-" and "volume-" sensitive modes of the total yield of electrons or fluorescence yield detection respectively. The obtained data allow us to characterize the specifics of the local environment of the atoms included in the nanolayers on the surface of the studied structures, not exceeding 15 nm in the depth of the information layer, and the volume of the same structures, up to micrometers, in a single synchrotron experiment.

Conclusions. Electrodeposition of metallic nickel in type a samples (within 40 minutes) into pores formed at fluence values of 10⁸ ion/cm² does not lead to a noticeable interatomic interaction on the surface and the "volume" boundaries of the "metal-semiconductor" (metal-substrate) or "metal-dielectric" (metal-film). The samples with fivefold increased pores density and coated with the particles of metallic nickel (with a twofold increase in time of SiO₂ films etching) have the considerably stimulated interatomic interactions of strongly etched matrix of silicon dioxide with the deposited nickel. This leads to the formation of Ni₂Si silicide in the bulk part of the Ni/SiO₂/Si composite structure, and slightly different silicide in the surface nanolayers, that also enables smooth, through the composition, control of the electronic structure of the composite Ni/SiO₂/Si. The formation of surface and volume silicides should be taken into account to optimize the transport properties of the structures studied.

SOURCE OF FINANCING

The study was supported by the RFBR (project No. 18-32-01046 mol_a) and in part by Ministry of Education and Science of Russia in the framework of State Tasks for Higher Education Organizations in Science for 2017–2019 (Project 16.8158.2017/8.9).

ACKNOWLEDGMENTS

The authors are grateful to the Director and administration of Helmholtz Zentrum Berlin, as well as to the Coordinators of the Russian-German laboratory and BESSY II beamlines.

CONFLICT OF INTEREST

The authors declare the absence of obvious and potential conflicts of interest related to the publication of this article.

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