List of Figures

• Schematic diagram showing Czochralski Si growth. (1) SiO₂ crucible, (2) Carbon susceptor, (3) Graphite Heater, (4) Single crystal Si. Note that the crystal and crucible are rotated in opposite directions. More than 99% of dissolving oxygen is lost as SiO.
• Schematic diagram showing the principle of the local density approximation and also Thomas Fermi theory, namely that for a given radial slab, dr, the local charge density can be considered to be n(r), the density of an equivalent uniform homogeneous electron gas.
• The 4s (full) and pseudo- (dashed) radial wavefunction (in atomic units) for the Ni atom.
• Schematic diagram showing terms included in the Musgrave Pople potential
• Clusters used in the InP work. (a) 134 atom $\rm In_{30}P_{40}H_{64}$ containing $\rm V_{In}H_4^+$, (b) 88 atom cluster $\rm BeIn_{21}P_{22}H_{43}$ containing H passivated Be
• Calculated Phonon Dispersion Curve for InP. $\times$=Koteles et al [65], $\circ$=Hilsum et al [64], $\triangle$=Borcherds et al [63]
• Schematic diagrams showing $\rm V_{In}H_n, n=4,0$. The tetrahedral symmetry is shaded in $\rm V_{In}H_4$.
• Top Kohn-Sham eigenvalues of the hydrogenated vacancies, (a) $\rm V_{In}H_4^+$, (b) $\rm V_{In}H_3$, (c) $\rm V_{In}H_2^-$, (d) $\rm V_{In}H^{2-}$, (e) $\rm V_{In}^{3-}$. Filled boxes indicate electrons and empty boxes indicate holes. The eigenvalues have been arbitrarily shifted to align the highest filled level below the t₂-like state with zero.
• Calculated structure for H passivated Be in InP (bond lengths in Å). Black dots mark ideal lattice positions.
• The OV centre in Si. The box indicates the [100] directions.
• The O₂V centre in Si. The box indicates the [100] directions.
• The OV₂ centre in Si. The box indicates the [100] directions. The two central dots mark the vacant lattice Si sites.
• The O₃V centre in Si. The box indicates the [100] directions.
• The OV-O_i centre in Si, pre-cursor to the O₂V. The box indicates the [100] directions.
• Schematic of interstitial oxygen in silicon. For explanation of the labelling, see text.
• Structure of O_i, bond lengths in Å. Dots mark the ideal lattice sites.
• Atomic motion associated with each vibrational mode for the oxygen interstitial (modes in cm^-1) (see Table 6.3).
• The stable assymmetric dimer in silicon (lengths in Å).
• Atomic motion associated with each vibrational mode for the assymetric dimer (modes in cm^-1). The top two modes roughly correspond to the assymetric stretch for the inner and outer O atoms respectively, the lower modes are wag modes. The 593 cm^-1 mode is largely localised on the core Si atoms and hence shows little shift with oxygen isotope, in agreement with experiment.
• Schematic diagram of a possible alternative dimer model that should show no mixed isotope vibrational mode coupling. The atoms sit bond centred, on opposite sides of the hexagonal interstitial site.
• The symmetric dimer in silicon (lengths in Å). This is 0.259 eV higher in energy than the assymetric dimer. Black dots mark the ideal lattice sites.
• The split dimer configuration, i.e. two O_i atoms with an empty Si-Si bond between them. All bond lengths in Å. This structure is less stable than the puckered dimer structure.
• A saddle point for oxygen dimer diffusion. This structure is 1.36-1.76 eV higher in energy than the stable puckered dimer structure. Black dots mark ideal lattice sites
• Time taken for a single O_i to hop from one bond centred site to another at varying temperature. Calculated using $x=\sqrt{\rm Dt}$where the distance between sites, x, comes from AIMPRO cluster calculations and is set to 3.639Å. $\rm D= 0.13 e^{-2.53 / kT}$.
• The initial changes of the bands 975-1012 cm^-1 and [O_i] for annealing at (a) 370$^\circ$C (top) and (b) 450$^\circ$C (bottom). Taken from Reference [239]. In the 450$^\circ$C run, the 975 cm^-1 signal was too weak for inclusion, and has a maximum that appears and disappears too quickly to appear on this scale.
• The initial changes of the 1006 and 1012 bands during the adjustments to their respective equilibrium levels and the growth of TD+1 (1s-3p$\pm$) for an as-grown sample and a dispersed sample (2h at 1100$^\circ$C). Note that the TD data applies to the r.h.axis, and the 1006/1012 data to the left, hence the initial dispersed 1006 level is almost zero. Taken from  Reference [239].
• The `manx' oxygen trimer in silicon. The top diagram shows the view along the $\langle$111$\rangle$ , C<sub>3</sub> symmetry axis.
• The linear chain trimer in silicon; dots mark ideal Si lattice sites.
• The N-pair defect in Si. Vertical direction is $\langle$001$\rangle$ , horizontal is $\langle$110$\rangle$ 
• Absorbance spectrum observed after annealing at 650$^\circ$C for 1 hour, of samples with various combinations of <sup>14</sup>N, <sup>15</sup>N, <sup>16</sup>O and <sup>17</sup>O. In all cases the total N and O doses are equal. Data obtained by Berg Rasmussen et al [103]
• The alternative `Humble Ring' model of NNO. This was discarded since it did not match experimental data.
• Structure of the NNO defect. All bond lengths in Å.
• The N_iO_i defect in the neutral charge state. Lengths in Å, angles in degrees.
• The N_iO_i defect in the +1, neutral, and -1 charge state. The core Si atom has a p-type orbital in the plane of the defect that is empty in the +1 charge state. This is partially and fully occupied in the neutral and -1 charge states respectively, which is reflected in the change in bonding character.
• Kohn-Sham wavefunction for the partially occupied level of N_iO_i in the neutral charge state. Note that the diagram is reversed from the ball and stick diagrams, i.e. the O atom is on the right of the plot. The wavefunction lies primarily on the core Si atom, and is localised on the side away from the O_i atom.
• The NNOO defect.
• The core structure of the +1 N_i-O_2i defect. All bond lengths are in Å. N atoms are dark grey, O atoms light grey, black dots mark the ideal lattice sites.
• Top Kohn-Sham eigenvalues for neutral N_i, N_iO_i, and N_iO_2i. Filled (empty) boxes denote electrons (holes). These have been scaled to the experimental 1.16 eV band gap.
• The pseudo-wavefunction ( $\times$ 100 a. u. ) of the highest occupied orbital of neutral N_i-O_2i. Note that it has little amplitude on N and is localised on the Si radical. There are nodal surfaces lying between this atom and the O atoms demonstrating anti-bonding behaviour.
• Structure of the proposed VO_sN_s defect. This does not have the required bonding or gap states to behave as a shallow donor defect. The cube marks the (100) directions, corners on ideal lattice sites. The N-O distance in Å.
• Structure of the proposed bistable NON(+) defect core. Arrows show approximate movement of N_i and O_i atoms from the electrically inactive NNO defect structure.
• The (C-H)_i-O_2i `C<sub>2v</sub>' defect core in the +1 charge state. Numbered atoms are referred to in the text. Symmetry with H included is C<sub>1h</sub> (see text). Black dots mark the ideal lattice sites. This is less stable than the assymetric structure in Figure 8.7. The cross-hatched H atom lies perpendicular to the defect plane.
• The (C-H)<sub>i</sub>-O<sub>2i</sub> defect with O<sub>2i</sub> neighbouring (CH)<sub>i</sub>, in the neutral charge state. Numbered atoms are referred to in the text. This structure is 1.36 eV more stable than its equivalent with one O atom on either side of the (CH)<sub>i</sub> unit. Black dots mark the ideal lattice sites.
• The neutral (CH)<sub>i</sub>-O<sub>4i</sub> defect (bond lengths in Å). Black dots mark ideal lattice sites, black is carbon, light grey oxygen, with the C-H bond perpendicular to the plane of the diagram.
• Structure of (CH)<sub>i</sub>-O<sub>4i</sub> in the +1, 0 and -1 charge states. Black dots mark ideal lattice sites. The neutral and +1 charge state structures are almost identical and so the neutral diagram has not been included seperately. In the -1 charge state the oxygen pair on one side bow out of the defect core, removing the elecrostatic compression on the core Si atom; this shifts off site to localise the deep gap state on its p-type orbital.
• Kohn-Sham eigenvalues for (CH)<sub>i</sub>O<sub>4i</sub> in the +1, neutral, and -1 charge states, showing its shift from a shallow to a deep level defect. The eigenvalues have been aligned and scaled to the experimental Si band gap of 1.16 eV.
• Schematic diagram of the `di-oxygen square' model for the thermal donor, as proposed in [#snyder_89b##1###]. This structure was found to be lower energy than the di-y-lid structure.
• Schematic diagram of the OSB model for the thermal donor [#ourmazd_84##1###]. The diagrams show the same defect along perpendicular directions. The bottom Si atom is a lattice atom displaced a long way from its original site.
• Schematic diagram of the Deák / Snyder / Corbett thermal donor model containing two trivalent O_i atoms and a Si_i [253].
• The core of the di-y-lid thermal donor. Bond lengths are in Å, bond angles are in degrees. Si atoms are white, O atoms are grey. Black dots mark the ideal Si lattice sites.
• The top Kohn-Sham eigenvalues for the 4O_i di-y-lid model for the thermal donor. Charge state of the defect is given beneath each column. Black boxes denote filled state, white boxes for empty states. The eigenvalues have been arbitrarily scaled to the experimental band gap of 1.16eV.
• Cross-sections through the Kohn-Sham wavefunction for the donor state of the di-y-lid thermal donor model in the +2 charge state. Vertical direction is $\langle$001$\rangle$ , horizontal direction is (a) $\langle$110$\rangle$ , (b) $\langle$110$\rangle$ .
• Side view of the 4O_i atom di-y-lid TD. This shows the complete cluster except for the surface hydrogen atoms. The oxygen atoms are shown in black.
• Overhead view of the 4O_i atom di-y-lid TD. This shows the complete cluster without surface H atoms. The tensile stress in the $\langle$110$\rangle$  and $\langle$110$\rangle$ is visible in the distortion to the lattice.
• Displacement of the core atoms associated with each calculated LVM (given in cm^-1). Vector length has been exaggerated for clarity. Overhead views of the 751 and 745 cm^-1 modes have been displaced slightly off axis so that it is possible to separate the vectors due to the O and Si atoms in the core. Stronger modes are given in bold.
• The top Kohn-Sham eigenvalues for various alternative thermal donor models, all in the +2 charge state. Black boxes denote filled state, white boxes for empty states. The eigenvalues have been arbitrarily scaled to the experimental band gap of 1.16eV. Structures are 3O_i species (a) Snyder/Stavola, (b) Partially dissociated trimer, 4O_i species (c) `Flanked square' model, (d) Di-square model, (e) 5O<sub>i</sub> STD analogue, and (f) 6O<sub>i</sub> di-y-lid with dimer in central Si-Si backbonds.
• The core of the 5O thermal donor model. Bond lengths in Å, angles in degrees. Symmetrically equivilent bonds have not been marked with lengths or angles. Si atoms are white, O atoms are grey. The difference between the central y-lid and its two neighbours can be seen in the bond lengths.
• The core of the alternative 4O `flanked square' thermal donor model. Bond lengths are in Å, bond angles are in degrees. Si atoms are white, O atoms are grey. Symmetrically equivilent bond lengths and angles have only been marked once.
• The core of the Snyder/Stavola 3O thermal donor model (lengths in Å). Black dots mark ideal Si lattice sites, showing the [001] compression and [110] lattice tension. The deviation from C_2v symmetry saves 0.063 eV over the relaxed C_2v symmetric structure.
• An alternative 3O thermal donor model similar to the di-y-lid but missing an end O_i; the trivalent O in the pair is stabilised by the presence of the neighbouring O_i.
• The top Kohn-Sham eigenvalues for the NL10(Al) Al/di-y-lid model, Al_sO_4i. Defect is in the +1 charge state. Black boxes denote filled state, white boxes for empty states. The eigenvalues have been arbitrarily scaled to the experimental band gap of 1.16eV.
• The proposed NL10(Al) structure, consisting of the di-y-lid structure with the core Si atom replaced by Al (cross-hatched atom). All bond lengths in Å, black dots mark ideal Si lattice sites.