Nanoscale modification of silicon and germanium surfaces exposed to low-energy helium plasma

Abstract

Complex surface nanostructures were observed in germanium and silicon samples exposed to low energy (24 or 36 eV ion kinetic energy) helium plasma. Pyramidal growth is observed in germanium across the temperature range studied (185 °C to 336 °C), while significant modification in silicon was only observed at 630 °C. Nano-wire growth was observed in both germanium and silicon, and appears to be linked to the strength of the electric field, which in turn determines the implantation energy of the helium ions. Nanostructure formation is proposed to be driven by surface adatom migration which is strongly influenced by an Ehrlich-Schwoebel-type surface instability. The role of helium in this model is to drive germanium interstitial formation by ejecting germanium atoms from lattice sites, leading to germanium interstitial diffusion towards the sample surface and subsequent adatom and surface nanostructure formation.

The dependence of Mo ratio on the formation of uniform black silicon by helium plasma irradiation

“…Recently, to fabricate black silicon, plasma irradiation has been employed with some distinct advantages, such as simple process, economical, and soft for the Si substrate [15,16]. Nonetheless, the morphology and the uniformity of the Si surface are still difficult to control [17][18][19]. Because the reflectance depends on many features of the surface morphology, such as density, size, and height of the nanostructure, it is desired to tailor the morphology with uniformity in a larger area by plasma irradiation.…”

Effect of temperature and incident ion energy on nanostructure formation on silicon exposed to helium plasma

“…This temperature range was selected, as it is known from past work to produce well‐formed nanostructures. [ 23 ] Temperature measurements were taken early in the exposure before any significant nanostructure formation. Due to the very high heat fluxes in NAGDIS‐II sample, heating was achieved solely through plasma heating.…”

“…Here, we focus our attention on a process which utilises a high-flux helium plasma to induce the growth of fine nanoscale surface features in a wide range of materials, including tungsten, [18,19] platinum, [20] silicon [21,22] and germanium. [23] This process is attractive, as it eliminates the need for complex chemical precursors or catalytic seeds that are widely used in other bottom-up fabrication processes.…”

“…The helium-plasma process is interesting, as a limited chemical interaction occurs between the helium plasma and target material, suggesting that the nanostructures are produced through some combination of sputtering, atomic (re) deposition, surface adatom kinetics and the dynamics of vacancy-helium clusters. [19,23] Although the nanostructure formation regime has been widely studied in tungsten in the context of nuclear fusion research, where nanoscale structures can lead to enhanced degradation of reactor walls, [24][25][26] the specifics of the nanostructure growth process are not fully understood.…”

Germanium nanostructures by helium plasma irradiation

“…It was found that He plasma irradiation leads to the formation of black Si with nanocones on the surface. [29][30][31][32][33] Unlike metals, on Si, whose bandgap is 1.1 eV, He bubbles may not play a major role in the morphological change. Rather, in addition to adatom diffusion, 30) a small amount of impurity deposition formed clusters on the surface and started to form protrusion, leading to the nanocones.…”