Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Surface effect on ion track formation in amorphous Si3N4 films
Introduction
When an energetic ion passes through a material an ion track may be produced along the ion path if the electronic stopping power Se is larger than a material dependent threshold value [1], [2]. In case of crystalline materials, the ion tracks can be easily observed by transmission electron microscopy (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 generally difficult due to a lack of sufficient contrast. The ion tracks produced in amorphous materials were studied using mainly indirect measurement methods, such as Fourier transform infrared spectroscopy (FTIR) [3], [4] and etching [4], [5]. There were only a small number of studies on the ion tracks produced in amorphous materials by direct TEM observation. Dunlop and coworkers demonstrated that ion tracks produced by MeV C60 ions and GeV heavy ions in metallic glasses can be observed using TEM [6], [7]. The ion track appears as a bright contrast in TEM images, which means reduced thickness and/or reduced density in the ion track. They employed a topographical contrast imaging technique and found that crater-like structures are formed on the irradiated surfaces. Recently, a fine structure of ion tracks in amorphous SiO2 (a-SiO2) was observed by small angle X-ray scattering (SAXS) [8], [9]. Analyzing the observed SAXS spectra it was found that the ion track consists of a low density cylindrical core surrounded by a high density shell showing a qualitative agreement with the results of molecular dynamics (MD) simulations [9], [10].
Very recently, we have demonstrated that ion tracks in amorphous Si3N4 (a-Si3N4) thin films (20 nm) produced by sub-MeV C60 ion impact can be clearly observed using TEM and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) [11]. Because the contrast of the HAADF-STEM image is proportional to the integrated density along the electron beam, the density profile of the ion track can be derived from the HAADF-STEM image. We found that the core of the ion track is amorphous and the density is reduced by ∼20% at the track center. The core is surrounded by a shell whose density is slightly enhanced. However, possible surface effects on the track formation, e.g. cratering, could not be excluded. In this paper, we extend our study and address the surface effect on the track formation. For this purpose, we irradiate a-Si3N4 films of various thicknesses ranging from 5 to 30 nm with 360–720 keV ions. We found that the length of the track produced in a thicker film is shorter than that in a thinner film. We also found that the density reduction in the track is pronounced for the thinner film.
Section snippets
Experimental
Irradiation of sub-MeV C60 ions was performed using 400-kV ion implanter at JAEA/Takasaki. Self-supporting amorphous Si3N4 (a-Si3N4) films of thickness T ranging from 5 to 30 nm were irradiated with 360, 540, 720 keV ions to fluences 1–2 × 1011 ions/cm2. After the ion irradiation, TEM and HAADF-STEM observations were performed using a JEOL JEM-2200FS equipped with a field emission gun operating at 200 kV. The samples were held at the specimen tilting holder with the tilt angle from −30 to 30°.
Results and discussion
Fig. 1(a) shows an example of the observed plan-view TEM images of the a-Si3N4 film (20 nm) irradiated with 720 keV 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 ions, indicating that single impacts create individual circular structures. In order to estimate the length L of the ion tracks, the sample was observed at a tilt
Conclusion
Amorphous Si3N4 thin films of thickness ranging from 5 to 30 nm were irradiated with 360–720 keV ions. Cylindrical ion tracks of a diameter of ∼4 nm were observed using TEM and HAADF-STEM. It was found that the length of the ion tracks produced by 720 keV in 30-nm films is shorter than that produced in 20-nm films. This can be qualitatively explained in terms of the energy dissipation process. From the HAADF-STEM images the radial density profiles of the ion tracks were derived. The
Acknowledgements
The authors are grateful to the crew of the 400-kV ion implanter at JAEA/Takasaki for irradiation of C60 ions. This work was supported by Grant-in-Aid for Exploratory Research from JSPS (Grant Number 24651114).
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