Surface-state dependent optical properties of OH-, F-, and H-terminated 4H-SiC quantum dots.

Density functional calculations are performed for OH-, F- and H-terminated 4H-SiC 10-20 Å diameter clusters to investigate the effect of surface species upon the optical absorption properties. H-termination results in a pronounced size-dependent quantum-confinement in the absorption, whereas F- and OH-terminations exhibit much reduced size dependent absorption due to surface states. Our findings are in good agreement with recent experimental studies, and are able to explain the little explored dual-feature photoluminescence spectra of SiC quantum dots. We propose that along with controlling the size, suitable surface termination is the key for optimizing optical properties of 4H-SiC quantum structures, such as might be exploited in optoelectronics, photovoltaics and biological applications.

Extended point defects in crystalline materials: Ge and Si.

B diffusion measurements are used to probe the basic nature of self-interstitial point defects in Ge. We find two distinct self-interstitial forms--a simple one with low entropy and a complex one with entropy ∼30 k at the migration saddle point. The latter dominates diffusion at high temperature. We propose that its structure is similar to that of an amorphous pocket--we name it a morph. Computational modeling suggests that morphs exist in both self-interstitial and vacancylike forms, and are crucial for diffusion and defect dynamics in Ge, Si, and probably many other crystalline solids.

Ab initio studies of fluorine passivation on the electronic structure of the NV- defect in nanodiamond.

We have investigated using density functional theory the effect of fluorine termination of a (001) diamond surface on the electronic energy levels of an NV- centre buried beneath the surface. We find that, like OH termination, fluorine passivates the surface and reduces the influence of the surface on the electronic properties of the NV- centre. The results have significance for the optical properties of NV- defects in nanodiamonds.

Structure and electron affinity of the 4H-SiC (0001) surfaces: a methodological approach for polar systems.

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 (0001̄) 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.

Silicon and germanium terminated (0 0 1)-(2 [Formula: see text] 1) diamond surface.

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.

Identification of the structure of the 3107 cm(-1) H-related defect in diamond.

A prominent hydrogen-related infrared absorption peak seen in many types of diamonds at 3107 cm(-1) has been the subject of investigation for many years. It is present in natural type-Ia material and can be introduced by heat-treating synthetic or CVD diamond. Based upon the most recent experimental data, it is thought that the defect giving rise to this vibrational mode is vacancy-related and is likely to contain nitrogen. Using first-principles simulations we present a VN3H model for the originating centre that simultaneously satisfies the different experimental observations including the strain response.

Hyperfine interactions at nitrogen interstitial defects in diamond.

Diamond has many extreme physical properties and it can be used in a wide range of applications. In particular it is a highly effective particle detection material, where radiation damage is an important consideration. The WAR9 and WAR10 are electron paramagnetic resonance centres seen in irradiated, nitrogen-containing diamond. These S = 1/2 defects have C(2v) and C(1h) symmetry, respectively, and the experimental spectra have been interpreted as arising from nitrogen split-interstitial centres. Based upon the experimental and theoretical understanding of interstitial nitrogen defect structures, the AIMPRO density functional code has been used to assess the assignments for the structures of WAR9 and WAR10. Although the calculated hyperfine interaction tensors are consistent with the measured values for WAR9, the thermal stability renders the assignment problematic. The model for the WAR10 centre yields principal directions of the hyperfine tensor at variance with observation. Alternative models for both centres are discussed in this paper, but no convincing structures have been found.

A density functional theory study of models for the N3 and OK1 EPR centres in diamond.

The defects observed in natural and synthetic diamonds provide a fingerprint of their differing growth conditions, as well as the thermal and mechanical processes they have experienced. Of the first row elements it is perhaps surprising that little evidence exists for oxygen in the form of distributed point-defects. Paramagnetic centres labelled N3 and OK1 have been assigned to two structural arrangements of pairs of substitutional nitrogen and oxygen, but there is no direct evidence for the involvement of the oxygen. In this paper we present the results of density functional simulations of N-O pairs in diamond, and review them in light of the experimental evidence. We also present analysis for other structures proposed in the literature (Ti-N, Ti-V-N, NV(2) and NVO), and show that none are particularly plausible.

Density functional studies of muonium in nitrogen aggregate containing diamond: the Mu(X) centre.

Diamond has potential as a wide band-gap semiconductor with high intrinsic carrier mobility, thermal conductivity and hardness. Hydrogen is involved in electrically active defects in chemical vapour deposited diamond, and muonium, via muon spin spectroscopy, can provide useful characterization for the configurations adopted by H atoms in a crystalline material. We present the results of a computational investigation into the structure of the Mu(X) centre proposed to be associated with nitrogen aggregates. We find that the propensity of hydrogen or muonium to chemically react with the lattice makes the correlation of Mu(X) with nitrogen aggregates problematic, and suggest alternative structures.

p-type doping of graphene with F4-TCNQ.

We use local density function theory to study the electronic properties of tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) deposited on a graphene surface. We show that charge transfer of 0.3 holes/molecule between graphene and F4-TCNQ occurs, which makes graphene p-type doped. These results are in agreement with experimental findings on F4-TCNQ.

Stability of singly hydrated silanone on silicon quantum dot surfaces: density functional simulations.

A key to understanding the optical characteristics of silicon quantum dots is the role of surface bonded species that introduce states to the band-gap. In particular, oxygen bonded in a silanone configuration is thought to be a source of shifts in emission during oxidation. We report the results of density-functional calculations examining the properties of such surface structures. We find single hydration of a simple, neutral silanone molecule leads to a barrierless conversion into a di-hydroxyl structure, and that similar processes are weakly activated on larger systems. However, we show that charging has a significant impact upon stability, with the attachment of an electron greatly increasing the barrier for converting silanone to di-hydroxyl termination. The relatively stable, negatively-charged silanone structures are predicted to lead to large red-shifts in the onset of optical absorption.

Self-interstitial in germanium.

Low-temperature radiation damage in n- and p-type Ge is strikingly different, reflecting the charge-dependent properties of vacancies and self-interstitials. We find, using density functional theory, that in Ge the interstitial is bistable, preferring a split configuration when neutral and an open cage configuration when positively charged. The split configuration is inert while the cage configuration acts as a double donor. We evaluate the migration energies of the defects and show that the theory is able to explain the principal results of low-temperature electron-irradiation experiments.

First-principles study of the diffusion of hydrogen in ZnO.

Zinc oxide, a wide-gap semiconductor, typically exhibits n-type conductivity even when nominally undoped. The nature of the donor is contentious, but hydrogen is a prime candidate. We present ab initio calculations of the migration barrier for H, yielding a barrier of less than approximately 0.5 eV. This indicates isolated hydrogen is mobile at low temperature and that thermally stable H-related donors must logically be trapped at other defects. We argue this is also true for other oxides where H is a shallow donor.

Theory of Jahn-Teller distortions of the P donor in diamond.

Phosphorus, the current standard n-type dopant in diamond, has been correlated with isotropic, trigonal and tetragonal paramagnetic centres, suggesting that it may undergo a symmetry lowering distortion, perhaps of a Jahn-Teller type. We present first-principles calculations for examining the energetics of various sub-group symmetries of the on-site, tetrahedral donor, and show that C2v, C3v and D2d conformations reduce the total energy and conform to the Jahn-Teller theorem. We also present a qualitative explanation of the resulting quantum-mechanical states. The small amount of energy saved by the distortion may indicate a dynamic Jahn-Teller effect.

Importance of quantum tunneling in vacancy-hydrogen complexes in diamond.

Our ab initio calculations of the hyperfine parameters for negatively charged vacancy-hydrogen and nitrogen-vacancy-hydrogen complexes in diamond compare static defect models and models which account for the quantum tunneling behavior of hydrogen. The static models give rise to hyperfine splittings that are inconsistent with the experimental electron paramagnetic resonance data. In contrast, the hyperfine parameters for the quantum dynamical models are in agreement with the experimental observations. We show that the quantum motion of the proton is crucial to the prediction of symmetry and hyperfine constants for two simple defect centers in diamond. Static a priori methods fail for these systems.

Shallow donors in diamond: chalcogens, pnictogens, and their hydrogen complexes.

The utility of diamond as an electronic material is compromised by the lack of a suitable shallow donor. Here, ab initio theory is used to investigate the donor levels of substitutional pnictogen (N, P, As, and Sb) and chalcogen (S, Se, and Te) impurities and chalcogen-hydrogen defects in diamond. Substitutional S is found to be a deep donor, while As and Sb possess donor levels significantly shallower than P, which so far is the most effective shallow donor found by experiment.