Diamond is known to incorporate a range of impurities, either during growth or subsequently by diffusion or implantation. Transition metals, such as Co and Ni, are known to form point defects in high-temperature--high-pressure growth from the solvent-catalyst, but other transition metal species present in the environment during growth appear not to become incorporated in a way that can be detected as stable, grown-in centres, although there is some experimental evidence that Fe might be incorporated in defect complexes, and any impurity might be added by implantation.In this report, we present the results of first principles simulations of Fe-containing defect centres, including the complexes with native defects and other common impurities.We find that interstitial Fe is unstable, but substitional Fe along with its complexes with other defects provide a wide range of observable properties.In particular, complexes with vacancies show interesting electronic structures that might be suited to defect-based applications including single-photon sources and magnetometry.

Using density functional theory calculations, the interaction between oxygen vacancies and carbon in bi-axially strained SrTiO3 has been investigated. C-Ti-V-o binding energy and reorientation of C-Ti have been examined for +/- 1%, +/- 2%, +/- 3% and +/- 4% compressive and tensile strains. The results show that compressive strain is an effective route to restrict the diffusion of electrically active V-o in the presence of C-Ti dopants. According to our results the reorientation barrier volumetric and bi-axial strain dependences differ in the compression and tension regimes and is expected to affect dielectric characteristics of SrTiO3 under an alternating electric field.

As a consequence of its high atomic number density, diamond can incorporate a relatively limited range of impurities as distributed point-defects, chiefly N, B and H. A few other species can be grown-in, and other impurity species incorporated via implantation and annealing. For applications including electronic, electrical and quantum devices, the presence of states deep within the wide band-gap is of importance, and the list of potential colour centres available for exploitation continues to grow. Although B can be grown into diamond at high concentration, study of other group-13 elements is rather limited. In this paper we present the results of modelling of Al, Ga and In. We find all species readily form complexes with vacancies, and exhibit electronic structures that parallel those of the SiV complex. We report electronic structures, electrical levels, optical transitions and hyperfine interactions of the colour centres, as well as reflect upon the thermodynamics of the complexes. We suggest that co-implanting group-13 elements with nitrogen would give rise to the defect charge states with potential for quantum applications.

The ability to accurately and consistently determine the surface electronic properties of polar materials is of great importance for device applications. Polar surface modelling is fundamentally limited by the spontaneous polarisation of these materials in a periodic boundary condition scheme. Surface data are sensitive to supercell parameters, including slab and vacuum thicknesses, as well as the non-equivalence of surface adsorbates on opposite surfaces. Using 4H–SiC as a specific case, this study explores calculation of electron affinities (EAs) of (000 1 ̄ ) and (0001) surfaces varying chemical termination as a function of computational parameters. We report the impact in terms of band-gap, electric fields across the vacuum and slab for single and double cell slab models, where the latter is constructed with inversional symmetry to eliminate the electric field in the vacuum regions. We find that single cells are sensitive to both slab and vacuum thickness. The band-gap narrows with slab thickness, ultimately vanishing and inducing charge transfer between opposite surfaces. This has a consequence for predicted EAs. Adsorbate species are found to play a crucial role in the rate of narrowing. Back to back cells with inversional symmetry have larger electric fields present across the slab than the single slab cases, resulting in a greater band-gap narrowing effect, but the vacuum thickness dependence is completely removed. We discuss the relative merits of the two approaches.

Control over the chemical termination of SiC surfaces is of great importance for the material in high temperature and hostile environment applications, where 4H-SiC in particular enjoys frequent use in areas including sensors and MOSFETs. Despite this, a wealth of surface specific data for the 4H polytype is yet to be accounted for, even under the most basic of terminations. Detailed structural and surface electronic properties highlight areas in which the use of SiC can expand and excel. In this work, we report density functional calculations on the effect of electron affinity upon adsorption of a variety of elements and their stability on the non-polar (1 1 (2) over bar 0) surface. We find that monolayer hydrogen and lithium termination work to reduce the electron affinity to 1.86 eV and 0.08 eV respectively, whereas for fluorine and chlorine it increases to large positive values of 5.05 eV and 3.90 eV respectively. All reactions with the unterminated surface are exothermic. In addition, lithium termination generating a near zero electron affinity makes it an exciting potential application in the field of emission. Notably, the increased efficiency of cold cathode devices with negative electron affinity make this surface a candidate for further work into functionalised SiC.

Nanodiamonds containing negatively charged nitrogen vacancy (NV−) centres are highly promising biolabels due to NV− photostability and spectral range. For effective cell tracking we require NV− to be stable in this charge state, but it is known that nanodiamond surfaces may alter the NV charge state; intermittent fluorescing and conversion to NV0 are frequently observed. Different models have been proposed linking surface termination type to the resultant NV charge state, but a full understanding has not yet been reached. For the work presented in this paper, we use density functional theory to examine how the NV electronic structure in nanodiamond clusters changes with different surfaces. We move beyond examining fully terminated surfaces and focus on the role of surface radicals, based on recent research showing pH also affects NV charge. Our work shows that surface radicals can explain the intermittent fluorescence observed for hydrogenated surfaces, and that different absorbates on the surface influence the resultant NV charge for a specified termination type. We have found that both the termination type, and the absorbates on the surface, play important roles in determining NV charge and should be considered together when predicting surface coverage that will stably produce NV−.

The neutral silicon-vacancy complex in diamond is of interest for quantum applications due to its favourable optical properties relative to both its negative charge state and the nitrogen-vacancy centre. To establish an uncharged form, co-doping with electrically active impurities has been suggested, and although complexes with hydrogen or nitrogen have been identified, complexes with boron are largely unstudied. This report presents results from density-functional modelling of SiB, SiVB and SiVBH complexes, some of which are expected to produce highly characteristic magnetic signatures. Critically for the neutral silicon-vacancy complex, we find that boron binds less strongly than nitrogen or hydrogen.

Publisher's copyright statement: This is the accepted version of the following article: Alsnani, Hind, Goss, J. P., Briddon, P. R., Rayson, M. J. Horsfall, A. B. (2019). First Principles Study of the Stability and Di usion Mechanism of a Carbon Vacancy in the Vicinity of a SiO2/4HSiC Interface. physica status solidi (a) 216(17): 1900328 which has been published in nal form at https://doi.org/10.1002/pssa.201900328. This article may be used for non-commercial purposes in accordance With Wiley Terms and Conditions for self-archiving.

We present a first principles density functional theory study of microscopic properties of hydrogen defect centres in diamond. Several configurations, involving interstitial hydrogen impurities, have been considered either forming with other defects, such as hydrogen defects and vacancies. The atomic structures, and hyperfine parameters of hydrogen result compared with the experimental data on electrically active centres in synthetic diamond. Based on Local density functional theory our calculations are in excellent agreement with one interpretation of electron paramagnetic resonance of hydrogen in diamond.

B G Breeze,1 C J Meara,2, 3 X X Wu,1 C P Michaels,1 R Gupta,1 P L Diggle,4 M W Dale,4 B L Cann,4 T Ardon,5 U F S D’Haenens-Johansson,5 I Friel,6 M J Rayson,2 P R Briddon,2 J P Goss,2 M E Newton,1, 3 and B L Green1, ∗ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK School of Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK EPSRC Centre for Doctoral Training in Diamond Science and Technology, UK De Beers Group Technology, Maidenhead, Berkshire, SL6 6JW, UK Gemological Institute of America, 50 W 47th Street, New York, New York 10036, USA Element Six, Global Innovation Centre, Fermi Avenue, Didcot OX11 0QR, UK We report the first experimental observation of a doubly-charged defect in diamond, SiV2−, in silicon and nitrogen co-doped samples. We measure spectroscopic signatures we attribute to substitutional silicon in diamond, and identify a silicon-vacancy complex decorated with a nearestneighbor nitrogen, SiVN, supported by theoretical calculations. Samples containing silicon and nitrogen are shown to be heavily photochromic, with the dominant visible changes due to the loss of SiV0/− and gain in the optically-inactive SiV2−.

We report the first experimental observation of a doubly-charged defect in diamond, SiV2-, in silicon and nitrogen co-doped samples. We measure spectroscopic signatures we attribute to substitutional silicon in diamond, and identify a silicon-vacancy complex decorated with a nearest-neighbor nitrogen, SiVN, supported by theoretical calculations. Samples containing silicon and nitrogen are shown to be heavily photochromic, with the dominant visible changes due to the loss of SiV0/- and gain in the optically-inactive SiV2-.

A quantum-chemical study of the positive charge-state of the nitrogen-vacancy center in diamond is presented. Charge control of this promising qubit candidate is a focus of diamond quantum technology research, as currently charge stability relating to surfaces and nearby defects causes some difficulties for quantum applications. To demonstrate full charge control over the nitrogen vacancy, all three charge states should be identified and the processes that lead to charge state changes understood. However, experimental markers for the positive state remain elusive compared to the readily detectable zero-phonon lines of the neutral and negative. In this work we present predicted hyperfine and zero-field splitting tensors as clear signatures of the normally spinless NV+ ((1)A(1) ground state) by probing a long-lived spin-triplet excited-state similar to 0.7 eV above the NV+ ground state. We find a relatively narrow excitation energy range of approximately 0.7-1.1 eV between excitation into an E-3 state and conversion into the neutral charge state. To provide insight into the thermal stability of the positive charge state, we have calculated binding and diffusion energies for both charged and uncharged systems. We predict, given that the activation energies are only weakly charge state dependent, all three charge states would diffuse in the 1600-1900 degrees C range, but the positive state has a significantly lower binding energy, suggesting that it will dissociate at temperatures around 1000 degrees C rather than migrate.

Using density functional theory with van der Waals (vdW) corrections, we study the collapse of free-standing single-walled carbon nanotubes (also called "dogbone" nanotubes). Their thermodynamic stability is strongly influenced by the initial stacking sequence, with lateral shear allowing registry change with turbostratic stacking predominant. The electronic structure of collapsed zigzag and armchair carbon nanotubes is investigated, demonstrating sensitivity to the lattice registry. The opening of small (meV) band gaps is shown for both armchair and zigzag collapsed nanotubes, arising from quantum confinement and charge transfer between the bilayer graphenelike central region and nanotubelike edges. Different scaling rules for the band gaps of collapsed carbon nanotubes are obtained as a function of their widths taking stacking and chirality into account. We reconcile a complete understanding of electronic properties in these deformed tubes with literature theoretical and experimental results, suggesting collapsed nanotubes can be promising candidates as conductive nanoribbons in electronic and spintronic device applications.

The carbon vacancy in bulk 4H‐SiC and in the vicinity of an SiO2/(0001)‐4H‐SiC interface using density‐functional theory is studied. It is found that the migration is hindered in the immediate vicinity of the interface, with the energy barrier for diffusion being ≈15% greater than the same defect in bulk 4H‐SiC. Herein, it is shown that the increased barrier is a consequence of the stabilization of the vacancy in the immediate interface due to a combination of strengthened reconstructions and interfacial relaxation, coupled with the destabilization of the transition‐state structure.

Control over the chemical termination of diamond surfaces has shown great promise in the realization of field-emission applications, the selection of charge states of near-surface colour-centres such as NV, and the realisation of surface-conductive channels for electronic device applications. Experimental investigations of ultra-thin Si and Ge layers yield surface states both within the band-gap and resonant with the underlying diamond valence band. In this report, we report the results of density-functional simulations of a range of coverages of Si and Ge on diamond (0 0 1) surfaces. We have found that surface coverage with crystallogen:carbon ratios of 67% and 75% are more stable than both higher and lower coverages on the (0 0 1)-diamond surface, and that they can explain the observation of an occupied band around 1.7 eV below the valence band top. We also report geometries, adsorption energies and electron affinities of these surface structures, and show that the resonant state is made up from conventional spd-covalent [Formula: see text]-bonding orbitals between the surface adsorbates.

Experimental data indicate that carbon vacancies incorporated in active regions of SiC devices are important electrical defects, responsible for device limiting effects such as carrier lifetime reduction. For field-effect transistors that include a 4H-SiC/SiO2 interface, such as at the gate, the oxidation pro- cess is understood to introduce native defects to the SiC, including injection of carbon self-interstitials and vacancies, that diffuse into the active layer and interact with other defects and impurities. It is therefore important to understand the migration behaviour of primary native defects such as VC in the vicinity of 4H-SiC/SiO2 interfaces. We report here the results of a density-functional theory investi- gation into the diffusion of the carbon vacancy in such a region. We conclude that the migration of VC is significantly hindered in the immediate vicinity of the interface, with the energy of diffusion barrier being approximately 15% greater than the corresponding diffusion in bulk 4H-SiC.

Control over the chemical termination of diamond surfaces has shown great promise in the realization of field-emission applications, the selection of charge states of near-surface colour-centres such as NV, and the realisation of surface-conductive channels for electronic device applications. Experimental investigations of ultra-thin Si and Ge layers yield surface states both within the band-gap and resonant with the underlying diamond valence band. In this report, we report the results of density-functional simulations of a range of coverages of Si and Ge on diamond (0 0 1) surfaces. We have found that surface coverage with crystallogen:carbon ratios of 67% and 75% are more stable than both higher and lower coverages on the (0 0 1)-diamond surface, and that they can explain the observation of an occupied band around 1.7 eV below the valence band top. We also report geometries, adsorption energies and electron affinities of these surface structures, and show that the resonant state is made up from conventional spd-covalent [Formula: see text]-bonding orbitals between the surface adsorbates.

Control over the chemical termination of diamond surfaces has shown great promise in the realization of field-emission applications, the selection of charge states of near-surface color-centers such as NV, and the realization of surface-conductive channels for electronic device applications. Efficient electron-emission exploiting the negative electron-affinity requires a stable surface treatment that can operate over a useful range of temperatures, for which H-termination fails. For compounds involving oxygen, surface geometry and layer stoichiometry are thought to be critical parameters in engineering the desired affinity. In this study, we show that large negative electron-affinities (-2 to -3 eV) are found for surfaces with an AlO3 stoichiometry, and the formation is exothermic. Higher proportions of Al increases the likelihood of metallic bonding between Al atoms, decreasing the bond-polarity and increasing the electron affinity (making it less negative). AlO2 and Al2O3 present unfavorable electron affinities, and have a lower thermal stability.