High resolution transmission electron microscopy (HRTEM) investigations of defect clusters produced in silicon by electron and neutron irradiations Leona Nistor and Ioana Pintilie National Institute of Materials Physics, Bucharest 28th RD Workshop (Turin)

Cross section specimen preparation for TEM/HRTEM: Aim: To compare, by TEM/HRTEM studies, the irradiation damage effects produced by electrons of different energies and fluence values and by neutrons in FZ silicon. Studied samples: 1. DOFZ Si irradiated at RT with electrons: energy 3.5 Mev, fluence 1x1017 cm-2 2. STFZ Si irradiated at RT with electrons: energy 15 MeV, fluence 1x1016 cm-2 3. DOFZ Si irradiated at RT with electrons: energy 27 MeV, fluence 2x1016 cm-2 4. 75 µm epitaxial (EPI) Si layer deposited on CZ Si substrate irradiated with neutrons: energy 1 MeV, fluence 1x1016 cm-2 Instrument: High resolution analytical JEOL ARM 200F operating at 200 kV, with a resolution in the HRTEM mode of 0.19 nm. TEM/HRTEM imaging was performed at low current densities, to reduce possible radiation effects produced by the electron beam. All Si specimens were oriented along the [110] zone axis, where most defects (clusters, extended) are visible. Cross section specimen preparation for TEM/HRTEM: - sawing strips (2x1) mm; - gluing the strips face to face with a glue which cured at room temperature; - mechanical thinning to a thickness of ~20 μm followed by ion milling to thicknesses <100 nm.

TEM results The 3.5 MeV, 1x1017 cm-2 (low energy, high fluence) electron irradiated sample (sample 1) Small clusters of point defects, vacancies and interstitials, visible as dark dots (some larger ones are marked with white arrows). Most have dimensions < 2 nm. The largest ones do not exceed 4 nm.

The 15 Mev, 1x1016 cm-2 electron irradiated sample (sample 2) As in sample 1, clusters of point defects with dimensions of 1 - 4 nm are revealed in black contrast. Unless sample 1, on sample 2 the clusters of larger dimensions (3 - 4 nm) prevail.

The 27 MeV, 2x1016 cm-2 (high energy, higher fluence) electron irradiated sample (sample 3) Compared with sample 2, the density of the clusters of defects is much higher. Clusters of small dimensions (<1- 2 nm) prevail. They are very uniformly distributed in the whole sample. Clusters of larger dimensions appear less frequently.

The neutron irradiated EPI Si sample: 1 MeV, 1x1016 cm-2 (sample 4) Compared with the electron irradiated samples 1 - 3, the sample irradiated with neutrons shows a very high density of small clusters of defects of 1 – 2 nm, most of them agglomerated in damaged regions with an average dimension of 4 nm. Sample 4 is heavily damaged.

HRTEM results The 3.5 MeV, 1x1017 cm-2 (low energy, high fluence) electron irradiated sample (sample 1) Small clusters of defects evidenced as a single dark dots or agglomerates of point defect clusters along <111> and <110> crystallographic directions forming star-like extended defects. Black patches (indicated by white arrows) results from a random accumulation of small clusters. All these extended defects have dimensions around 3 nm.

The 15 MeV, 1x1016 cm-2 electron irradiated sample (sample 2) As in sample 1, small clusters of defects are revealed. They agglomerate into star-like or black patches-like extended defects of slightly larger dimensions (3 - 4 nm). Unlike sample 1, in sample 2 the density of small clusters (single dark dots) is smaller.

The 27 MeV, 2x1016 cm-2 (highest energy, higher fluence) electron irradiated sample (sample 3) Compared with samples 1 and 2, in sample 3 the clusters of defects are relatively uniformly distributed in a high density and agglomerated. The damage in sample 3 is higher than in samples 1 and 2.

The neutron irradiated EPI Si sample: 1 MeV, 1016 cm-2 (sample 4) As in the electron irradiated Si samples, clusters of point defects, vacancies and interstitials are formed, but in a very high density. They agglomerate rather randomly, forming large distored regions of ~4 nm ( black patches), marked by red arrows. The damage of the neutron irradiated sample is high.

The neutron irradiated EPI Si sample: 1 MeV, 1016 cm-2 (sample 4) HRTEM images at higher magnifications reveal the structure of a complex extended defect (black patch), marked by red arrows, which seems to occur quite frequently in the neutron irradiated sample. The defect lies in three successive {110} planes, where the Si lattice is very disturbed. It might be a precursor of the {113} defect formed by the agglomeration of self interstitials which, in the first stage of the defect formation, accumulate along the <110> directions and then nucleate in the {113} planes [S. Takeda, T Kamio, Phys. Rev. B, 51, 2148, (1995-II). This kind of extended defect was not observed in the FZ Si samples after irradiation with electrons.

Conclusions The HRTEM studies revealed that irradiation with neutrons or electrons of different energy and fluence values introduces in FZ Si clusters of point defects showing a similar dark contrast. These clusters of defects agglomerate, either along specific crystallographic directions, or randomly, resulting in larger distorted regions with dimensions of 3 - 4 nm, formed by the accumulation of excess vacancies and self interstitials, the end products of the collision cascade. The density of the defect clusters, is much larger in the neutron irradiated sample than in the in 3.5 MeV or 15 MeV electron irradiated ones. The 27 MeV irradiated sample also showed a high density of clusters of point defects, but of smaller dimensions compared with the neutron irradiated sample (~2 nm vs. ~4 nm, respectively). Both in electron and neutron irradiated samples, the structure of the damaged zones is highly disturbed, but not completely amorphous, as in the case of the latent tracks induced by high energy heavy ion irradiation of crystalline materials (for example, U or Au in GeS and mica).

Conclusions 4. The amount of damage produced by electrons depend on their energy and fluence values. Low energy (3.5 MeV) but high fluence (1x1017 cm-2) electrons produce quite similar damaging effects as much higher energy (15 MeV) but one order of magnitude lower fluence (1x1016 cm-2) electrons. 5. Neutrons of 1 MeV energy and 1x1016 cm-2 fluence produce significantly higher damaging effects compared with electrons of 15 MeV energy and the same 1x1016 cm-2 fluence.