1. 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

2. 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.

3. 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

4. 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

5. 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

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.