Direct observation of fine structure in ion tracks in amorphous Si3N4 by TEM

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Abstract

Thin films of amorphous Si3N4 (thickness 20 nm) were irradiated with 120–720 keV C60+,2+ ions and observed using transmission electron microscopy (TEM). The ion track produced in an amorphous material was directly observed by TEM. For quantitative analysis, the ion tracks were also observed using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The observed ion track consists of a low density core (radius ∼2.5 nm) and a high density shell (width ∼2.5 nm), which is very similar to the ion tracks in amorphous SiO2 irradiated with high energy heavy ions observed by small angle X-ray scattering (SAXS). Although the observed ion tracks may be affected by surface effects, the present result indicates that TEM and HAADF-STEM have potential to observe directly the fine structures of ion tracks in amorphous materials.

Introduction

The discovery of ion tracks dates back to 1959 when the tracks produced by single fission fragments from 235U in mica were observed by transmission electron microscopy (TEM) [1]. Since then, ion tracks have been observed in various materials irradiated with swift heavy ions, including insulators [2], [3], semiconductors [4], [5] and metals [6], [7] when the electronic stopping power Se is larger than a material dependent threshold value [8], [9]. The radius of the ion track increases with the electronic stopping power, indicating that the inelastic process is responsible for the ion track formation. There are several models proposed to explain the observed results, i.e. inelastic thermal spike model, Coulomb explosion model, bond weakening model and exciton self-trapping model. Among these models, the inelastic thermal spike model seems most promising, which quantitatively explains the evolution of the track radius with the electronic stopping power and the threshold stopping power for the track formation. However, because of the lack of information on non-equilibrium thermodynamical properties the applicability of the thermal spike concept is still under debate [10], [11].

In case of crystalline materials, the structure of the ion track can be easily observed by TEM. The track interior is amorphized or comprised of defect clusters depending on the material. In case of amorphous materials, direct TEM observation of ion tracks is difficult due to a lack of sufficient contrast. Indirect methods, such as Fourier transform infrared spectroscopy (FTIR) [12], [13] and etching [13], [14], have been almost exclusively used to study the ion tracks in amorphous materials. In the data analysis of FTIR, a simple cylindrical structure is always assumed. This does not allow to deduce detailed structures of ion tracks unlike TEM observation.

Recently, small angle X-ray scattering (SAXS) is employed to study a fine structure of ion tracks in amorphous SiO2 (a-SiO2) [15], [16]. A clear peak was observed in the SAXS spectra of a-SiO2 irradiated with 27–1430 MeV Au and Xe ions. The observed SAXS spectra were analyzed using a simple model structure of the ion track, i.e. a step-function-like radial density distribution. It was shown that a low density cylindrical core surrounded by a high density shell reproduces the observed SAXS spectra. The core–shell structure is qualitatively in agreement with the results of molecular dynamics (MD) simulations [16], [17], although the simulated density distribution is not so simple as was assumed in the SAXS analysis. The origin of the core–shell structure was suggested to be the density anomaly existent in a-SiO2, i.e. densification of a-SiO2 above 1800 K [16], [18]. It is noteworthy that the opposite density change, i.e. a high density core surrounded by a low density shell, cannot be excluded by the SAXS measurement because such a density distribution gives the same SAXS spectrum. In addition, SAXS as well as FTIR provide only average structural properties of the ion tracks. Thus, direct observation of the track structure by TEM is still highly desired to determine the detailed track structures without ambiguity and to understand the mechanism of the track formation.

The SAXS result showed that the density of the track core is as low as 40% of the bulk density of SiO2 for 185 MeV Au ion impact. The MD simulation predicted even larger density reduction at the track center (95% reduction) when the electronic stopping is 18 keV/nm. Such large density reduction should be easily observed by TEM. Actually, there were several studies on the ion tracks produced in metallic glasses by high energy ions using TEM [19], [20]. However, as we will discuss below, quantitative estimate of the density change is difficult by TEM. In the present paper, we report on direct observation of the ion tracks in amorphous Si3N4 irradiated with 120–720 keV C60+,2+ ions using TEM and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The radial density distribution of the ion track can be easily derived from the observed HAADF-STEM images.

Section snippets

Experimental

Self-supporting amorphous Si3N4 (a-Si3N4) films of thickness 20 nm were purchased from Silson Ltd., which were prepared by chemical vapor deposition on Si wafers. The a-Si3N4 films were irradiated with 120, 240, 360, 540 and 720 keV C60+,2+ ions to fluences 1–5 × 1011 ions/cm2. Neglecting the so-called cluster effect, the projected ranges of the C60+ ions in a-Si3N4 were estimated to be 5.3, 9.4, 13.6, 19.8 and 26 nm at 120, 240, 360, 540 and 720 keV, respectively, using the SRIM code [21]. For

Results and discussion

Fig. 1(a) shows an example of the observed plan-view TEM images of the a-Si3N4 film irradiated with 720 keV C602+ ions. There are circular structures of almost uniform diameter of ∼4 nm. Each structure has a bright core which is surrounded by a dark shell. The number of these structures agrees with the fluence of the C602+ ions, indicating that single C602+ impacts create individual circular structures. Similar structures were also observed for a-Si3N4 irradiated with C60 ions at different

Conclusion

Ion tracks produced by 120–720 keV C60+,2+ impacts on amorphous Si3N4 thin films were directly observed using TEM and HAADF-STEM. The observed ion tracks are continuous and the density is reduced by 20% at the track center. The low density region extends up to ∼2.5 nm from the center. This low density core is surrounded by a slightly high density shell. The density of the shell is 1–2% higher than the bulk density and the width of the shell is ∼2.5 nm. Although we cannot exclude possible surface

Acknowledgement

The authors are grateful to the crew of the 400-kV ion implanter at JAEA/Takasaki, which was used for the irradiation of C60+. This work was supported by Grant-in-Aid for Exploratory Research from JSPS (Grant Number 24651114).

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