Biomedical subjects

P R Briddon

Publications and source records attributed to P R Briddon.

9 recordsLinked to original sources

PubMed2006

E center in silicon has a donor level in the band gap.

It has been an accepted fact for more than 40 years that the E center in Si (the group-V impurity--vacancy pair)--one of the most studied defects in semiconductors--has only one energy level in the band gap: namely, the acceptor level at about 0.45 eV below the conduction band. We now demonstrate that it has a second level, situated in the lower half of the band gap at 0.27 eV above the valence band. The existence of this level, having a donor character, is disclosed by a combination of different transient-capacitance techniques and electronic-structure calculations. The finding seriously questions some diffusion-modeling approaches performed in the past.

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PubMed2006

First-principles study of the diffusion of hydrogen in ZnO.

Zinc oxide, a wide-gap semiconductor, typically exhibits n-type conductivity even when nominally undoped. The nature of the donor is contentious, but hydrogen is a prime candidate. We present ab initio calculations of the migration barrier for H, yielding a barrier of less than approximately 0.5 eV. This indicates isolated hydrogen is mobile at low temperature and that thermally stable H-related donors must logically be trapped at other defects. We argue this is also true for other oxides where H is a shallow donor.

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PubMed2005

Importance of quantum tunneling in vacancy-hydrogen complexes in diamond.

Our ab initio calculations of the hyperfine parameters for negatively charged vacancy-hydrogen and nitrogen-vacancy-hydrogen complexes in diamond compare static defect models and models which account for the quantum tunneling behavior of hydrogen. The static models give rise to hyperfine splittings that are inconsistent with the experimental electron paramagnetic resonance data. In contrast, the hyperfine parameters for the quantum dynamical models are in agreement with the experimental observations. We show that the quantum motion of the proton is crucial to the prediction of symmetry and hyperfine constants for two simple defect centers in diamond. Static a priori methods fail for these systems.

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PubMed2004

Degradation of boron-doped Czochralski-grown silicon solar cells.

The formation mechanism and properties of the boron-oxygen center responsible for the degradation of Czochralski-grown Si(B) solar cells during operation is investigated using density functional calculations. We find that boron traps an oxygen dimer to form a bistable defect with a donor level in the upper half of the band gap. The activation energy for its dissociation is found to be 1.2 eV. The formation of the defect from mobile oxygen dimers, which are shown to migrate by a Bourgoin mechanism under minority carrier injection, has a calculated activation energy of 0.3 eV. These energies and the dependence of the generation rate of the recombination center on boron concentration are in good agreement with observations.

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PubMed2004

Shallow donors in diamond: chalcogens, pnictogens, and their hydrogen complexes.

The utility of diamond as an electronic material is compromised by the lack of a suitable shallow donor. Here, ab initio theory is used to investigate the donor levels of substitutional pnictogen (N, P, As, and Sb) and chalcogen (S, Se, and Te) impurities and chalcogen-hydrogen defects in diamond. Substitutional S is found to be a deep donor, while As and Sb possess donor levels significantly shallower than P, which so far is the most effective shallow donor found by experiment.

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PubMed2003

Quantum Monte Carlo study of the optical and diffusive properties of the vacancy defect in diamond.

Fixed-node diffusion quantum Monte Carlo (DMC) calculations of the ground and excited state energetics of the neutral vacancy defect in diamond are reported. The multiplet structure of the defect is modeled using guiding wave functions of the Slater-Jastrow type with symmetrized multideterminant Slater parts. For the ground state we obtain the 1E state in agreement with experiment. The calculated energy of the lowest dipole allowed transition is consistent with the experimentally observed GR1 band, which has long been identified with the neutral vacancy in diamond, although no previous first-principles ab initio calculation of this transition exists. The calculated multiplet splitting of over 2 eV indicates the importance of a proper treatment of electron exchange and correlation in this system. DMC calculations of the formation and migration energy of the vacancy defect are presented.

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PubMed2003

Metastable Frenkel pair defect in graphite: source of Wigner energy?

The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitial-vacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation.

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PubMed2003

Z1/Z2 defects in 4H-SiC.

First-principles calculations are carried out on models for the Z(1)/Z(2) defects in 4H-SiC which are found in as-grown and irradiated n-type material. We show that an interstitial-nitrogen-interstitial-carbon defect is exceptionally thermally stable, bistable, and has negative-U character with donor and acceptor levels close to those attributed to the defect.

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PubMed2001

Thermal double donors and quantum dots.

Combined local mode spectroscopy and ab initio modeling are used to demonstrate for the first time that oxygen atoms in thermal double donors (TDD) in Si are in close proximity. The observed vibrational modes in (16)O, (18)O, and mixed isotopic samples are consistent with a model involving [110] aligned oxygen chains made up of an insulating core lying between electrically active ends. The model also explains the minute spin density observed on oxygen in TDD(+) as well as the piezospectroscopic tensors of the donors. The analogy between the thermal donors and quantum dots is emphasized.

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