Latent tracks formation in silicon single crystals irradiated with fullerenes in the electronic regime

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Abstract

Silicon targets of (1 0 0) orientation were prepared for transmission electron microscopy observations and then irradiated with either 30 MeV C602+ or 40 MeV C603+ at normal incidence. All the irradiations were performed at room temperature, up to fluences of a few 109 clusters cm−2. The incident electronic stopping powers were 48 and 57 keV nm−1 for C602+ and C603+ projectiles, respectively. High resolution observations at normal incidence evidenced amorphous zones of circular shape at each projectile impact. The track diameters near the target surface were 8.4 and 10.5 nm for irradiations at 30 and 40 MeV, respectively. This effect, which was never observed in silicon single crystals bombarded with swift heavy ions, was ascribed to the high density of electronic energy associated with the correlated electronic stopping of the cluster components. Observations at conventional resolution of samples tilted in the microscope allowed to follow the depth evolution of the entire tracks. The damage extends from the surface to a maximum depth L which depends on the incident energy of the clusters (L=160 nm and L=190 nm for irradiations at 30 and 40 MeV, respectively). This progressive extinction of the radiation-induced disorder was linked to the decorrelation process of the C60 ions during their slowing-down in the target.

Introduction

During the last decade, many experiments showed that energy deposition through high electronic excitations can induce stable defects in many radiolysis-resistant materials. By using heavy ions (up to 238U) accelerated up to a few tens of GeV, the response of a given target can be studied in a large electronic stopping power range (1 keV nm−1 < Se < 100 keV nm−1). This allowed systematical investigations of the Se-induced effects. For instance: (i) Latent tracks formation in insulating [1] and conducting [2] oxides, (ii) Anisotropic plastic deformation in amorphous metallic alloys [3], (iii) Defect creation in pure metals [4]. Despite the increasing number of materials found to be sensitive to collective electronic excitations induced by high energy heavy ion irradiation, an intriguing exception remains as regards semiconductors. In amorphous silicon and germanium prepared by vacuum evaporation and subsequently irradiated with swift ions (from 100 MeV 16O to 207 MeV 197Au), Izui et al. [5] evidenced tracks which consist of small recrystallized particles. This effect occurs above a Se threshold dependent on the target (Seth  5 keV nm−1 in Ge and Seth  15 keV nm−1 in Si) and was interpreted in the framework of the thermal spike model [6]. Besides, silicon and germanium single crystals are up to now considered as insensitive to collective electronic excitations. The irradiations of these materials at the highest electronic stopping powers available on the high energy ion accelerators [7], [8], [9], [10] did not evidence any defect creation via inelastic processes. However, recent works devoted to the irradiation effects of energetic clusters require to revisit the insensitivity of single crystalline semiconductors. As a matter of fact, the use as projectiles of clusters (like Aun and Cn) accelerated to a total energy of a few tens of MeV allows to reach energy densities via electronic processes which exceed those obtained with ions [11]. In such irradiation conditions, new experimental results have been evidenced. For instance: (i) Non-linear sputtering effects in organic and inorganic films [12]; (ii) Dissolution of alkaline precipitates embedded in MgO matrix [13]; (iii) Latent tracks registration in pure metals [14] and in sapphire [15].

In this paper, we present preliminary results concerning the lattice disorder induced in silicon single crystals by fullerene irradiations in the MeV range. Our purpose can be summarized by the following question: Is it possible to create amorphous latent tracks in silicon by means of very high confinements of electronic energy?

Section snippets

Experimental procedure

The starting material was a p-type boron-doped silicon wafer of (1 0 0) orientation purchased from Siltronix. The irradiated targets consist of small pieces (≈2 mm diameter), cut out from the original wafer, and prepared for TEM observations in plane view configuration. These specimen were mechanically thinned down to about 50 μm and then ion milled using 5 keV argon bombardment of 1 mA beam current and a milling angle of 15°. The surface amorphization due to the milling does not extend above a

Results and discussion

The TEM micrographs presented in Fig. 1a and b exhibit arrays of contrasted zones which are normal to the sample surface (upper part of the figures). The mean distance Δ between two adjacent dark areas is about 100 nm, a value consistent with the irradiation fluence (Φ=1/Δ2  109–1010 cm−2). These features allow to conclude to the formation of a latent track at each projectile impact. The track diameters d remain constant in the first 50 nm below the target surface and are of 8.4 and 10.5 nm for

Conclusion

In opposition to what has been usually believed, we have shown that single crystalline silicon is sensitive to collective electronic excitations. By using C60 clusters accelerated in the 10 MeV range, the formation of amorphous latent tracks was evidenced for the first time in this material. This was ascribed to the very high density of electronic energy deposited by the correlated components of the projectile. The limited depth of the observed tracks is mainly due to the nuclear scattering of

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