Sanghamitra Mukhopadhyay Peter. V. Sushko and Alexander L. Shluger The Effect of Local and Medium Range Order On the Properties of Oxygen Vacancies in Amorphous Silica Sanghamitra Mukhopadhyay Peter. V. Sushko and Alexander L. Shluger Condensed Matter and Materials Physics Department of Physics & Astronomy University College London, Gower Street, London WC1E 6BT, UK Acknowledgements: EPSRC, EU Framework5 HIKE

Outline Preparation of amorphous silica by molecular dynamics. We have investigated the statistical distribution of formation and properties oxygen vacancies in amorphous SiO2 How to relate defect properties of amorphous SiO2 with local and medium range structure ? Preparation of amorphous silica by molecular dynamics. Use ab-initio quantum mechanical methods to study oxygen vacancies : embedded cluster method. Finding out statistical distribution of the different types of neutral and positively charged oxygen vacancy centres. Compare with optical and EPR experiments.

Defects in disordered materials Amorphous SiO2 Crystalline SiO2 (-quartz) Lattice sites are different  Defects have different properties Different types of defects All Lattice sites are equivalent  Defects have same properties

Preparation of amorphous structures Molecular Dynamics calculations Periodic boundary conditions (648 atom) supercells : DL_POLY Potential : Buckingham type pair potential – van Beest et al, PRL, 64, 1955, (1990) NPT Ensemble 3-stage process Melting rate 100K / 6 ps Equilibration at 7000 K for 1 ns Quenching rate 50K / 6 ps Equilibration at 0K for 1 ns Density  = 2.37 gm/cm3 equilibration Time Temperature melt quench

General scheme: embedded cluster approach Step 1. Finite nano-cluster calculation Step 2. Correction due to infinite crystal Cut out a finite nano-cluster Define spherical Region I Centre QM cluster at the centre of Region I Region I: shell model Region IIa: shell model (linear response Region IIb: continuum

Oxygen vacancy in amorphous silica Wide distribution in vacancy formation energies is a result of medium range relaxation

Vacancy formation energies and structural relaxations Small vacancy formation energies lead to small Si-Si distance Large medium range structural relaxation reduces formation energy for neutral vacancy

Relaxation and local structure Larger relaxation produce smaller Si-Si distance at vacancy position Relaxations are long ranged Important for MOS device having oxide width around 15Å Average Si-O distance is a good parameter to find out most suitable position for formation of Oxygen vacancy

Optical absorption σ-σ* + σ-π1 σ-π1 + σ-π2 σ-σ* CB π2 π1 σ* σ VB The width of the red tail in absorption spectrum is indicative about how many different types of neutral vacancies present in the sample σ-π1 + σ-π2 σ-σ* Experiment CB π2 π1 σ* σ VB

Si-Si distance at VO is a Different type of E' centres and Correlation with local structure Si-Si distance at VO is a good parameter to determine the type of a E' centre

Hyperfine constants and g tensor Type g Theory Exp. E'δ g1 g2 g3 2.0018 2.0034 2.0043 2.0021 E'γ 2.0008 2.0009 2.0003 2.0006 Hyperfine Constants Type Theory Experiment E'δ Si1 Si2 10.5 10.0 E'γ Si1 40.83 0.19 41.9

Conclusions The distribution of oxygen vacancy formation energy in amorphous silica is wide with FWHM 1.8eV. Thermally produced oxygen vacancy (low vacancy formation energy) induce large relaxation in the system. A long red tail in optical absorption spectrum is indicative of presence of different types of vacancies. The formation of neutral and charged vacancies are correlated with the local structure of silica. Hyperfine constants and g tensors are compared well with experiments.