**1. Introduction**

Material modification induced by electronic excitation under high-energy (> 0.1 MeV/u) ion impact has been observed for many non-metallic solids since the late 1950's; for example, the formation of tracks (each track is characterized by a long cylindrical disordered region or amorphous phase in crystalline solids) in LiF crystal (photographic observation after chemical etching) by Young [1], in mica (a direct observation using transmission electron microscopy, TEM, without chemical etching, and often called a latent track) by Silk et al. [2], in SiO2-quartz, crystalline mica, amorphous P-doped V2O5, etc. (TEM) by Fleischer et al. [3,4], in oxides (SiO2-quartz, Al2O3, ZrSi2O4, Y3Fe5O12, high-Tc superconducting copper oxides, etc.) (TEM) by Meftah et al. [5] and Toulemonde et al. [6], in Al2O<sup>3</sup> crystal (atomic force microscopy, AFM) by Ramos et al. [7], in Al2O<sup>3</sup> and MgO crystals (TEM and AFM) by Skuratov et al. [8], in Al2O<sup>3</sup> crystal (AFM) by Khalfaoui et al. [9], in Al2O<sup>3</sup> crystal (high resolution TEM) by O'Connell et al. [10], in amorphous SiO<sup>2</sup> (small angle X-ray scattering (SAXS)) by Kluth et al. [11], in amorphous SiO<sup>2</sup> (TEM) by Benyagoub et al. [12], in polycrystalline Si3N<sup>4</sup> (TEM) by Zinkle et al. [13] and by Vuuren et al. [14], in amorphous Si3.55N<sup>4</sup> (TEM) by Kitayama et al. [15], in amorphous SiN0.95:H and SiO1.85:H (SAXS) by Mota-Santiago et al. [16], in epilayer GaN (TEM) by Kucheyev et al. [17], in epilayer GaN (AFM) by Mansouri et al. [18], in epilayer GaN and InP (TEM) by Sall et al. [19], in epilayer GaN (TEM) by Moisy et al. [20], in InN single crystal (TEM) by Kamarou et al. [21], in SiC crystal (AFM) by Ochedowski et al. [22] and in crystalline mica (AFM) by Alencar et al. [23]. Amorphization has been observed for crystalline SiO<sup>2</sup> [5] and the Al2O<sup>3</sup> surface at a high ion fluence (though the XRD peak remains) by Ohkubo et al. [24] and Grygiel et al. [25]. The counter process, i.e., the recrystallization of the amorphous or disordered regions, has been reported for SiO<sup>2</sup> by Dhar et al. [26], Al2O<sup>3</sup> by Rymzhanov [27] and InP, etc., by Williams [28]. Density

**Citation:** Matsunami, N.; Sataka, M.; Okayasu, S.; Tsuchiya, B. Modification of SiO2, ZnO, Fe2O<sup>3</sup> and TiN Films by Electronic Excitation under High Energy Ion Impact. *Quantum Beam Sci.* **2021**, *5*, 30. https://doi.org/10.3390/qubs5040030

Academic Editors: Akihiro Iwase and Rozaliya Barabash

Received: 30 August 2021 Accepted: 17 October 2021 Published: 27 October 2021

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modification, i.e., a lower density in the track core surrounded by a shell with a higher density, has been observed for Al2O<sup>3</sup> [10], amorphous SiO<sup>2</sup> [11], Si3N<sup>4</sup> [14] and amorphous SiN0.95:H and SiO1.85:H [16]. Interestingly, an electrically conducting track formation in tetrahedralamorphous carbon (sp<sup>3</sup> into sp<sup>2</sup> bond transformation) has been observed by Gupta et al. [29]. The track radius, hillock height and diameter characterizing the surface morphology modification associated with the track are well described in terms of the electronic stopping power S<sup>e</sup> (defined as the energy loss due to electronic excitation and ionization per unit path length), and the velocity effect has been noticed [12]. The threshold of S<sup>e</sup> for the track formation has been reported [3,6,8,9,12,13,30] and the data appear to scatter, and it seems that the threshold S<sup>e</sup> depends on the observation method of the track [12]. No track formation by monatomic ions has been observed in AlN [19].

Moreover, electronic sputtering (the erosion of solid materials caused by electronic energy deposition) has been observed for various compound solids: UO<sup>2</sup> by thermal-neutroninduced <sup>235</sup>U fission fragments by Rogers [31,32] and by Nilsson [33], UO<sup>2</sup> by energetic ions by Meins et al. [34], Bouffard et al. [35] and Schlutig [36], H2O ice by Brown et al. [37,38], Bottiger et al. [39], Baragiola et al. [40], Dartois et al. [41] and Galli et al. [42], frozen gas films of Xe, CO<sup>2</sup> and SF<sup>6</sup> [39], those of CO, Ar and N<sup>2</sup> by Brown et al. [43], CO<sup>2</sup> ice by Mejia et al. [44], SiO<sup>2</sup> by Qui et al. [45], Sugden et. al. [46], Matsunami et al. [47,48], Arnoldbik et al. [49] and Toulemonde et al. [50,51], MgAl2O<sup>4</sup> [48], UF<sup>4</sup> ([34], by Griffith et al. [52] and Toulemonde et al. [53]), LiNbO<sup>3</sup> [45], Al2O<sup>3</sup> ([45] and by Matsunami et al. [54]), various oxides by Matsunami et al. (SrTiO<sup>3</sup> and SrCeO<sup>3</sup> [47,54], CeO2, MgO, TiO<sup>2</sup> and ZnO [54], Y2O<sup>3</sup> and ZrO<sup>2</sup> [55], Cu2O [56,57], WO<sup>3</sup> [58], CuO [59], Fe2O<sup>3</sup> [60]), Si3N<sup>4</sup> [45], Si3N<sup>4</sup> and AlN by Matsunami et al. [55], Cu3N by Matsunami et al. [56,61], LiF ([50], by Assmann et al. [62] and Toulemonde et al. [63]), KBr [56], NaCl [63], CaF<sup>2</sup> [53] and SiC [56]. The sputtering of frozen Xe films has been observed for low energy electron impact, against the anticipation of no atomic displacement [39], and the result confirms that the sputtering is caused by electronic excitation. Mechanisms of electronic excitation leading to atomic displacement will be discussed in Section 4.

As mentioned above, electronic sputtering has been observed for a variety of nonmetallic materials, indicating that it seems to be a general phenomenon for non-metallic solids by high-energy ion impact. In many cases, ions with an equilibrium charge have been employed, which is usually attained by inserting thin foils, such as carbon, metals, etc., before impact on samples, and sputtered atoms are collected in carbon, metals, etc., followed by neutron activation and ion beam analysis to obtain sputtering yields. This article concerns the equilibrium charge incidence, though charge-state effects for nonequilibrium charge incidence have been observed and discussed ([23,29,34,49,52,58,62,64]). The electronic energy deposition or electronic stopping power (Se) at the equilibrium charge can be calculated using a TRIM or SRIM code by Ziegler et al. [65,66], a CasP code by Grande et al. [67] and the nuclear stopping power (Sn, defined as the energy loss due to elastic collisions per unit path length) [65,66]. Characteristic features of the electronic sputtering by high-energy ions are as follows:


Gd3Ga5O<sup>12</sup> and Y3Fe5O<sup>12</sup> [69] and CaF<sup>2</sup> and UF<sup>4</sup> [53]. Only the heavy element of U [31–36] and the light element of O [49] have been detected.

Besides track formation and electronic sputtering, lattice disordering (the degradation of X-ray diffraction (XRD) intensity) with lattice expansion (an increase in the lattice parameter) by high-energy ions has been observed for the polycrystalline films of SiO<sup>2</sup> [70] and WO<sup>3</sup> [58], and lattice disordering with lattice compaction for those of Cu2O [57],

CuO [59], Fe2O<sup>3</sup> [60], Cu3N [61] and Mn-doped ZnO [71]. Only lattice disordering has been observed for the ultra-thin films of WO<sup>3</sup> [72]. It should be noted that a comparison between high-energy and low-energy ion impact effects is important. Lattice expansion has been observed for a few keV D ion irradiation on Fe2O<sup>3</sup> [73], and this can be understood by the incorporation of D into non-substitutional sites (incorporation or implantation effect). Thus, lattice expansion by medium-energy ions (100 keV Ne) on Fe2O<sup>3</sup> [60] could be understood by Ne incorporation and/or interstitial-type defects generated by ion impact, with a possible stabilization by incorporating Ne in the film, whereas lattice compaction has been observed for a 100 MeV Xe ion impact on Fe2O<sup>3</sup> [60]. It should be noted that the incorporation of ions in thin films does not take place for high-energy ions, since the projected range of ions (Rp) is much larger than the film thickness (e.g., R<sup>p</sup> of 14 µm for 100 MeV Xe in SiO2), unless the thickness is too large. The lattice expansion due to the incorporation effect has been observed for a few keV H and D irradiation at a low fluence on WNO<sup>x</sup> with x ≈ 0.4, whereas lattice compaction has been observed at a high fluence of D [74]. Peculiarly, lattice expansion at a low ion fluence and compaction at a high fluence, as well as disordering, have been reported for medium-energy (100 keV Ne and N) and high-energy (100 MeV Xe and 90 MeV Ni) ion impact on WNO<sup>x</sup> [75,76]. One speculation is that the lattice compaction is due to vacancy-type defects generated by ion impact, which is to be investigated. Furthermore, a drastic increase in electrical conductivity has been observed for Cu3N [61], Mn-doped ZnO [71] and WNO<sup>x</sup> with x ≈ 0.4 [75,76]. The conductivity increase is ascribed to the increase in the carrier concentration and mobility.

There are a few reports on the S<sup>e</sup> dependence of the XRD intensity degradation per unit fluence (YXD) for SiC and KBr [56] and WO<sup>3</sup> [72]. YXD is found to follow the power-law fit: YXD = (BXDSe) N XD, BXD being a material-dependent constant and the exponent NXD being comparable with the Nsp of the electronic sputtering. The results imply that similar mechanisms operate for lattice disordering and electronic sputtering. It is of interest to compare the S<sup>e</sup> dependence of lattice disordering with that of electronic sputtering for materials other than those mentioned above. In this paper, we have measured the lattice disordering of SiO2, ZnO, Fe2O<sup>3</sup> and TiN films, and the sputtering of TiN. The XRD results are compared with the sputtering. The exciton model is examined and scaling parameters are explored for representing electronic excitation effects.
