Vibrations of the Interstitial Oxygen Pairs in Silicon

First-principles methods are used to calculate the structures and local vibrational modes of interstitial oxygen pairs in silicon. The staggered Oi−Si−Oi and skewed Oi−Si−Si−Oi structures are nearly degenerate in energy. The calculated local vibration frequencies and their pure and mixed 18O→16O isotopic shifts agree closely with experiments: the highest frequency is assigned to the skewed and the four lower ones to the staggered structure. This result may clear up the controversy of oxygen dimers in silicon, and also suggests a mechanism for fast oxygen diffusion.Peer reviewe

Computational study of interstitial oxygen and vacancy-oxygen complexes in silicon

The formation and binding energies, the ionization levels, the structures, and the local vibrations of Oi, O2i, O3i, VO, VO2, and V2O (V=vacancy) in silicon are calculated using a self-consistent total-energy pseudopotential method. The most important results are as follows: The ionization levels and associated structures are given for VO and V2O as well as the local vibration modes for the negative charge states of VO. The experimental frequency of Oi at 517 cm−1 is associated tentatively with an oxygen-induced silicon mode of weakly interacting Oi’s. We find two competing structures for O2i: the staggered configuration and the skewed Oi−Si−Si−Oi configuration with the binding energies of 0.2 and 0.1 eV, respectively. The experimental frequencies of O2i at 1060, 1012, 690, and 556 cm−1 are found to originate from the staggered O2i. The experimental frequency of O2i at 1105 cm−1 is found to originate from the skewed Oi−Si−Si−Oi configuration of O2i. The calculated effects of pressure on the structures and local vibration frequencies (Grüneisen parameters) of Oi and O2i are presented.Peer reviewe

Spin-density study of the silicon divacancy

The possible charge states of the silicon divacancy V2 are studied using the local spin-density pseudopotential method. The ionic coordinates are relaxed without any symmetry constraints. We obtain the formation and binding energies as well as the ionization levels from total-energy calculations and use them to discuss several experiments. We find using the 216-atom-site supercell that V02 and V−2 have a “mixed” structure that includes both pairing and resonant-bond characters, V02 being more of the pairing type and V−2 more of the resonant-bond type.Peer reviewe

Convergence of supercell calculations for point defects in semiconductors: Vacancy in silicon

The convergence of first-principles supercell calculations for defects in semiconductors is studied with the vacancy in bulk Si as a test case. The ionic relaxations, defect formation energies, and ionization levels are calculated for supercell sizes of up to 216 atomic sites using several k-point meshes in the Brillouin-zone integrations. The energy dispersion, inherent for the deep defect states in the supercell approximation, and the long range of the ionic relaxations are shown to postpone the convergence so that conclusive results for the physical properties cannot be obtained before the supercell size is of the order of 128–216 atomic sites.Peer reviewe

Local vibrations of thermal double donors in silicon

The local vibrational modes (LVM’s) of the oxygen chains assigned to thermal double donors (TDD’s) and other related oxygen defects in silicon are studied using accurate total-energy calculations. We find that the calculated LVM frequencies as well as their isotopic shifts and charge-state dependences (temperature dependences) for the oxygen chains agree closely with the corresponding experimental quantities, which supports our assignments of the O2i−O2r chain to TDD1 and the Oi−Onr−Oi chains to TDDn (n>1) (Oi is an interstitial oxygen and Or a threefold coordinated oxygen belonging to a ring).Peer reviewe

Vacancies in SiGe: Jahn–Teller distortion and spin effects

The electronic structure of a vacancy in silicon-germanium is studied using ab initio total-energy minimization methods. The calculations are based on density-functional theory in the local-spin-density approximation. We report ionic relaxations, defect formation energies and ionization levels of Si and Gevacancies in a zinc blende model structure (SiGe). The Gevacancy in SiGe is characterized by symmetry-lowering Jahn–Teller (JT) distortions and a negative-effective-U effect, in those respects resembling the vacancy in elemental silicon. For Si vacancy, the exchange-coupling energy is found to overcome the JT energy, and symmetric high-spin ground states are predicted.Peer reviewe

Aggregation Kinetics of Thermal Double Donors in Silicon

A general kinetic model based on accurate density-functional-theoretic total-energy calculations is introduced to describe the aggregation kinetics of oxygen-related thermal double donors (TDD's) in silicon. The calculated kinetics, which incorporates the reactions of associations, dissociations, and isomerizations of all relevant oxygen complexes, is in agreement with experimental annealing studies. The aggregation of TDD's takes place through parallel-consecutive reactions where both mobile oxygen dimers and fast migrating chainlike TDD's capture interstitial oxygen atoms.Peer reviewe

Comparison of oxygen-chain models for late thermal double donors in silicon

The electronic and atomic structures of the oxygen chains assigned to late thermal double donors (TDDs) in silicon are studied using accurate total-energy calculations. We find that the ring-type O-chain model is best suited for TDDs and better than the di-Y-lid-type O-chain model. The ring-type O chains have slightly alternating C2v–C1h symmetry consistent with the recent high-field electron paramagnetic resonance experiments. The spin densities of the double-donor states are located outside the region of the O atoms, which makes the hyperfineinteraction of an unpaired donor electron with the 17Onuclear spins very weak.Peer reviewe

First-principles study of migration, restructuring, and dissociation energies of oxygen complexes in silicon

Migration, restructuring, and dissociation energies of oxygen complexes in silicon are studied theoretically through density-functional total-energy calculations. We find that the stablest oxygen complexes are straight chains that also have the lowest migration energies. The calculated migration energies decrease from 2.3 eV for an interstitial oxygen atom (Oi) to low values of 0.4–1.6 eV for O2–O9 chains and 1.9–2.2 eV for longer chains. The oxygen chains (which are thermal double donors) are expected to grow so that the migrating oxygen chains capture less-mobile but abundant Oi’s: On+Oi→On+1. Restructuring energies of chains with a side Oi into straight oxygen chains are 1.9–2.5 eV. Restructuring gives an essential contribution to the fast diffusion. We find that the shorter O2–O9 chains dissociate primarily by ejecting one of the outermost oxygen atoms.Peer reviewe