Tracks and Voids in Amorphous Ge Induced by Swift Heavy-Ion Irradiation

Abstract

Ion tracks formed in amorphous Ge by swift heavy-ion irradiation have been identified with experiment and modeling to yield unambiguous evidence of tracks in an amorphous semiconductor. Their underdense core and overdense shell result from quenched-in radially outward material flow. Following a solid-toliquid phase transformation, the volume contraction necessary to accommodate the high-density molten phase produces voids, potentially the precursors to porosity, along the ion direction. Their bow-tie shape, reproduced by simulation, results from radially inward resolidification. DOI: 10.1103/PhysRevLett.110.245502 PACS numbers: 61.80.Jh, 61.43.Dq, 61.43.Bn, 61.05.cf Swift heavy-ion irradiation (SHII) has many applications, spanning geochronological dating [1] to nanostructure fabrication [2]. Though this approach has found industrial application [3], the fundamental nature of ionsolid interactions at very high ion energies remains poorly understood. Such interactions are dominated by inelastic processes (electronic stopping) resulting in the excitation and ionization of substrate atoms while, in contrast, the elastic processes (nuclear stopping) that lead to ballistic atomic displacements at much lower energies are negligible in the SHII regime. The efficiency with which energy deposited in the electronic subsystem is subsequently transferred to the lattice is governed by the electronphonon coupling parameter g where typically g amorphous > g crystalline due to a reduced electron mean free path in the former. When the lattice temperature exceeds that required for melting, a narrow cylinder of molten material is formed along the ion path. The ensuing rapid resolidification of this transient liquid phase can yield remnant structural modifications within the substrate in the form of an ion track.Crystalline Ge (c-Ge) is relatively insensitive to SHII such that ion-track production necessitates very high electronic stopping S e values. Discontinuous tracks follow single-ion irradiation (S e ¼ 35 keV=nm) [4,5] while cluster-ion irradiation (S e ¼ 37-51 keV=nm) yields tracks of diameter 5-15 nm [5]. In contrast, amorphous Ge (a-Ge) is rendered porous under SHII with S e > $10 keV=nm [6] while ion hammering results for S e > $12 keV=nm [6], the latter manifested as a nonzero deformation yield [7]. These observations are consistent with g amorphous > g crystalline and ion-track formation has been suggested as the origin of these two phenomena [6,7]. A recent molecular dynamics (MD) study of irradiated a-Ge [8] suggested voids originate from outgoing shock waves resulting from rapid heating and expansion of the ion-track core. The sole report of ion tracks in a-Ge is that of Furuno et al. [9] who reported recrystallization of tracks in a 5-nm a-Ge layer following SHII in a grazing-incidence orientation, a geometry that can lead to significant reductions in threshold S e values for ion-track formation [10]. The proximity of the surface could also perturb resolidification and enable recrystallization given the molten i...

Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse

“…38,47,48 Assuming a Fermi distribution for these electrons, one can determine their corresponding chemical potential and temperature. 38 The evolution of these two quantities determines the evolution of the electron distribution and contributes to the potential energy surface (see Sec.…”

“…11,33,[44][45][46] The high-energy-electron and the low-energy-electron domains are interconnected, as electrons can gain or lose energy and go from one domain to another. This forms the source/sink terms for the temperature equation, 47,48 as the changing number and energy of low-energy electrons directly affect their temperature. Additionally, atomic motion and the evolution of the electronic band structure also influence the electron temperature.…”

Latent ion tracks in amorphous silicon

“…We suggest the overdense ion track core could conceivably contain frozen-in structural remnants of the molten HDL Si phase, potentially in the form of the pressureinduced high-density amorphous (HDA) Si phase reported by McMillan et al 27 The rapid cooling rate associated with resolidification of an ion track may well be sufficient to quench in metastable HDA Si. Alternatively, SAXS determinations of core-shell structures for ion tracks in a-SiO 2 12 or a-Ge 13 were consistent with an underdense core and overdense shell, which can be attributed to radially outward material flow. 13,28 While the SAXS experiment cannot unambiguously establish the sign of the density of core and shell, the MD simulations support the notion of ion tracks in a-Si consisting of an overdense core and underdense shell.…”

“…Alternatively, SAXS determinations of core-shell structures for ion tracks in a-SiO 2 12 or a-Ge 13 were consistent with an underdense core and overdense shell, which can be attributed to radially outward material flow. 13,28 While the SAXS experiment cannot unambiguously establish the sign of the density of core and shell, the MD simulations support the notion of ion tracks in a-Si consisting of an overdense core and underdense shell.…”

“…We have recently identified and characterized ion tracks in amorphous metals, 10 silica (a-SiO 2 ), 11,12 and Ge (a-Ge) 13 by utilizing synchrotron-based small-angle x-ray scattering (SAXS). In the present study we provide direct and definitive experimental evidence for ion tracks in a-Si by combining SAXS with molecular dynamics (MD) simulations.…”