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First-principles computational methods are employed to investigate the structural, vibrational, optical and electronic properties of the self-interstitial aggregate, I4 in silicon. We find the defect to be electrically active and identify it with the B3 EPR center. We also show that its properties are consistent with DLTS and optical spectra observed following implantation of silicon. A crucial element is missing from our understanding of the behavior of silicon self-interstitials under annealing. Although substantial experimental [1] and theoretical [2] activity has provided an accepted model for the structure of the extended, interstitial related {311} defects, little is known about the processes by which they form or degrade via small multi-interstitials (I n). This is important as these processes are related to the transient enhanced diffusion (TED) of dopants [3], which leads to device degradation in heavy radiation environments. One important clue to understanding interstitial condensation lies with the B3 electron paramagnetic resonance (EPR) center which is classified as a simple interstitial aggregate [4]. In this letter we show that the tetra-interstitial (I 4) is responsible for the B3 EPR center. This assignment provides a vital link between small interstitial aggregates and extended defects. Recently, evidence has emerged that the structure of small self-interstitial aggregates is markedly different from that of {311} extended defects. The transient supersaturation of a system undergoing Ostwald ripening has been exploited to estimate the formation energies of small interstitial aggregates [5]. These experiments demonstrated that magic numbers exist for interstitial aggregates in the early annealing stage. I 4 and I 8 are found to be particularly stable with a transition at n 10 to a broad range of defects with the characteristic energy of {311} condensates. Furthermore, optical studies [6] confirm this picture, indicating that a structural transformation from I n clusters to {311} defects occurs at ∼ 600 • C. Deep level transient spectroscopy (DLTS) studies of Si ion implanted silicon has provided further information on the early stages of the ripening process [7]. Two donor (0/+) levels at E v + 0.29 eV and E v + 0.48 eV associated with small interstitial clusters are found to dominate the DLTS spectrum before the emergence of a different signal at E v + 0.50 eV. The latter level exhibits carrier capture kinetics typical of extended defects and is associated with {311} condensates. The E v + 0.29 eV level has been observed previously in carbon implanted silicon and is correlated with the B3 EPR center [8]. B3 is a prominent S = 1/2 center observed in boron doped, neutron irradiated and heat-treated silicon [4,9]. It is first observed upon annealing at ∼ 200 • C and completely anneals out at 500 • C [9,4]. B3 is one of only eight defect centers observed in irradiated silicon which have been reported to possess D 2d symmetry and its stability to high temperatures suggests a simple secondary irradiati...

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Low-temperature radiation damage in n- and p-type Ge is strikingly different, reflecting the charge-dependent properties of vacancies and self-interstitials. We find, using density functional theory, that in Ge the interstitial is bistable, preferring a split configuration when neutral and an open cage configuration when positively charged. The split configuration is inert while the cage configuration acts as a double donor. We evaluate the migration energies of the defects and show that the theory is able to explain the principal results of low-temperature electron-irradiation experiments.

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