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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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Pseudolocal vibrational modes (pLVMs) are defect-related vibrational modes which are localized despite being below the phonon maximum. Such modes are sometimes seen as phonon replicas in photoluminescence spectra. The pLVMs associated with two copper-related defects are calculated from first-principles density-functional theory in periodic supercells. The localization of the pLVMs is quantified using the magnitude of the eigenvectors of the dynamical matrix.

The properties of epitaxial graphene on SiC substrates can be modified by intercalation of different atomic species. In this work, mechanisms of hydrogen intercalation into the graphene-SiC(0001) interface, and properties of hydrogen and fluorine intercalated structures have been studied with the use of density functional theory. Our calculations show that the intercalation of hydrogen and fluorine into the interface is energetically favorable. Energy barriers for diffusion of atomic and molecular hydrogen through the interface graphene layer with no defects and graphene layers containing Stone-Wales defect or two- and four-vacancy clusters have been calculated. It is argued that diffusion of hydrogen towards the SiC surface occurs through the hollow defects in the interface graphene layer. It is further shown that hydrogen easily migrates between the graphene layer and the SiC substrate and passivates the surface Si bonds, thus causing the graphene layer decoupling. According to the band structure calculations the graphene layer decoupled from the SiC(0001) surface by hydrogen intercalation is undoped, while that obtained by the fluorine intercalation is p-type doped. © 2012 American Physical Society