Damage creation in porous silicon irradiated by swift heavy ions

B Canut,M Massoud,P Newby,V Lysenko,L Frechette,J.M Bluet,I Monnet

doi:10.1016/j.nimb.2013.09.047

Abstract

Mesoporous silicon (PS) samples were processed by anodising p+ Si wafers in (1:1) HF–ethanol solution. Different current densities were used to obtain three different porosities (41%, 56% and 75%). In all cases the morphology of the PS layer is columnar with a mean crystallite size between 12nm (75% porosity) and 19nm (41% porosity). These targets were irradiated at the GANIL accelerator, using different projectiles (130Xe ions of 91MeV and 29MeV, 238U ions of 110MeV and 850MeV) in order to vary the incident electronic stopping power Se. The fluences ranged between 1011 and 7×1013cm−2. Raman spectroscopy and cross sectional SEM observations evidenced damage creation in the irradiated nanocrystallites, without any degradation of the PS layer morphology at fluences below 3×1012cm−2. For higher doses, the columnar morphology transforms into a spongy-like structure. The damage cross sections, extracted from Raman results, increase with the electronic stopping power and with the sample porosity. At the highest Se (>10keVnm−1) and the highest porosity (75%), the track diameter coincides with the crystallite diameter, indicating that a single projectile impact induces the crystallite amorphization along the major part of the ion path. These results were interpreted in the framework of the thermal spike model, taking into account the low thermal conductivity of the PS samples in comparison with that of bulk silicon.

Highlights

It is well established that single crystalline silicon irradiated with monoatomic heavy ions accelerated in the GeV range cannot be damaged via electronic processes
The morphology of the samples was studied by scanning electron microscopy (SEM) imaging performed on the target cross-section
As the irradiation fluence increases, one evidences an additional large peak centered at 480 cmÀ1 which can be related to the transverse optical branch of a-Si [15]

Summary

Introduction

It is well established that single crystalline silicon irradiated with monoatomic heavy ions accelerated in the GeV range cannot be damaged via electronic processes. According to the thermal spike model [5,6], the prompt energy received by the target electrons after the passage of the projectile (10À17 s per unit cell) gives rise to a local thermalization of the electron gas within a period of %10À15 s. This step is followed by an electron–phonon coupling, causing a local temperature increase (thermal spike). For a few 10À12 s, a molten phase develops around the ion path and produces the so-called track after ultrafast quenching Such a scenario does not occur in materials having a high thermal conductivity, like metals and

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