Nadiia I. Semeniuk

Surfaces of some metals (copper and aluminum) and alloys (black and chromium–nickel stainless steels) were irradiated with argon ion-plasma flows under the trampoline mode conditions, i.e., at high ion current densities j≥10mAcm−2 and relatively low ion kinetic energies E=(50−200)eV, which brought about a formation of two-scale (micro-nano) hierarchical rough structures on the sample surfaces. In other experiments, black steel was preliminarily processed in the trampoline regime, and nano-clusters of stainless steel were deposited from an additional target mounted inside the same chamber. The deposited coatings reproduced the composition of the multi-component sputtered target. Strong adhesion between the coating and the sample was obtained, which was proved by dynamic micro-hardness measurements. A high quality of surface patterns created in the trampoline regime was ensured by the release of ion energy near the sample surface and the formation in the quasi-liquid state of a micro-nano-scale structure of the metal upper near-surface layers. Wettability measurements demonstrated that the freshly prepared textured surfaces were hydrophilic, whereas after tens-of-minutes exposures in the ambient atmosphere, they became hydrophobic. The dependence of the textures on the incidence angle θ was measured. It was shown that the increase of θ induces the expansion of micro-nano-scale formations on the surface. Theoretical calculations using the kinetic Monte Carlo approach and simulating the driving force of surface structuring (in our case, this is ion irradiation) were applied to analyze numerically the formation of hierarchical micro-nano-structures at the initial stage of surface modification when the seed inhomogeneities appear and evolve. The results qualitatively correlate with the experimental observations.

By irradiating the (111) surface of single-crystalline silicon wafers with Ar+ ions in the trampoline mode (high-density flows of low-energy ions) and with the assistance of metal initiator atoms, a transition of those wafers into the “black silicon” optical state was stimulated. This state is characterized by low values of light reflectance (less than 2%) from the irradiated wafer side in a spectral interval from 400 to 1000 nm. Scanning electron microscopy studies showed that this is a result of the emergence of a hierarchical submicron/nanoscale structure at the treated surface of the wafers. This transition can be performed only with the assistance of metal initiator atoms simultaneously evaporated from an auxiliary target in the operating chamber. At the same time, the application of metals initiators affects the effective lifetime of the minority charge carriers in the black state silicon, which is crucial for the efficiency of black silicon application in solar cells. Among the examined metals initiators (Al, Cr, Fe, Ni, Mo, and Cu), only the application of copper made it possible to preserve or even increase the value of this parameter. The electrostatic interaction between free electric charges and dipoles (playing the role of excitons in this case) near the interface in a two-layer system was considered theoretically. It was shown how the system parameters affect the preferable location of excitons and their orientation, which can be applied, e.g., to govern the solar cell efficiency. The formation of black silicon in the trampoline mode is environmentally friendly at all production stages.

The collective trampoline ion-plasma effect was used to modify surface properties of metals, ceramics, silicon and carbon in a desired manner. The rough surfaces created by low-energy dense ion flows were demonstrated to include double-level hierarchical structures. Namely, the submicron and nanoscale peaks and valleys were found. The coating films were deposited by the nanoclusters emitted in the trampoline mode, as well as by plasma-enhanced chemical vapor deposition (PECVD) and physical vapor deposition (PVD) methods. The substrates were prepared by prior ion processing in the trampoline mode. Preliminary nanoscale texturing of surfaces substantially modified the interaction of ion-plasma flows with surfaces during the subsequent film formation by PVD and PECVD techniques. The so created rough surface showed the strong film adhesion and a 25%–50% faster film formation during ion-plasma deposition as compared to the case of a non-textured surface. The transfer of target material in the form of nanoclusters allowed obtaining dense defect-free micron-scale films. The submicron structure leads to the infrared and visible light trapping. The results were analyzed on the basis of the previously suggested qualitative model of the trampoline sputtering.

Results of the newly found anomalous accelerated plasma ion sputtering of solid-state targets are presented. The rate of sputtered particles comprises from one to several tens of atoms per one impinging ion, ion energies being in the range of tens to five hundreds electron volts. The anomalous sputtering is realized for ion current density in the plasma flow being not less than 10 mA cm ⁻² and threshold specific power of the ion flow from 0.7 to 2 W cm ⁻² depending on the target material. The qualitative explanation proposed dubbed ‘collective trampoline accelerated sputtering’. The collective nature of this sputtering regime differs from the conventional pair collision cascade one. The trampoline sputtering leads to the intense texturing of the materials with submicron characteristic structure sizes. The new sputtering method could be applied (i) in the technology of plasma ion deposition of the functional coatings to replace the widespread magnetron sputtering sources of lower productivity; (ii) in the photovoltaics to solve the problem of the so-called ‘black’ multi-crystalline silicon in order to reduce the light reflectance coefficient; (iii) in the production of silicon composite anodes for lithium ion batteries to enhance their lifetimes regarding charge/discharge cycles, for which the final positive result is largely determined by the surface structure of the silicon wafer for photovoltaics and the surface structure of the anode current collector for lithium-ion batteries.