Alison Mainwood

Recent developments of diamond detectors for particles and UV radiation

Diamonds many extreme properties, such as its radiation and corrosion resistance, large bandgap, high electron and hole mobility, make it an attractive semiconductor for UV and particle detectors. This paper reviews the properties that are exploited by the latest generation of detectors, such as the radiation hardness and detection mechanisms, and those which are less well understood, such as grain boundaries and priming effects. The UV and particle detectors and dosimeters that have been reported in the last few years are described and briefly assessed.

Radiation Damage of Diamond by Electron and Gamma Irradiation

Diamond is an exceptionally radiation-hard material, but the main mechanisms by which lattice damage results from irradiation of high-energy particles and photons are not well understood. Models of radiation damage in diamond have been built up for both electron and gamma irradiation using Monte Carlo computer simulations. The energies investigated ranged from 0.25 to 10 MeV for electron irradiation and 1 to 15 MeV for gamma irradiation. Electrons have a low collision cross-section with carbon atoms, and therefore much of their energy is dissipated in ionisation before the electron displaces an atom. Gamma radiation causes damage by the indirect process of generating electrons (by Compton scattering and pair production) which then displace atoms (and ionise the material). The knock-on atom may cause further damage by displacing further atoms. However, both electron and gamma irradiation form predominantly isolated vacancies and interstitial pairs (Frenkel pairs). The range of 1 MeV electrons in diamond is about 1.3 mm with a nearly constant damage profile up to this cut-off. The range of gamma photons is much greater, with about 85% of 1 MeV photons passing through a 5 mm diamond without causing any damage. The total damage rates were calculated to vary between 0.01 and 5.15 vacancies per incident electron and between 0.02 and 6.10 vacancies per photon over the energy ranges investigated.

The effect of nickel and the kinetics of the aggregation of nitrogen in diamond

Abstract Using infra-red absorption topographic spectroscopy the aggregation of nitrogen in synthetic diamonds grown with a nickel catalyst was investigated. It was found: (a) that the rate of aggregation is a function of the nickel concentration, showing conclusively that the presence of nickel is responsible the enhancement of the nitrogen aggregation in diamond: and (b) that the aggregation process in the specimens containing nickel, does not follow second order kinetics. Two possible mechanisms for the involvement of nickel in the nitrogen aggregation process are presented.

Lattice damage caused by the irradiation of diamond

Abstract Diamond is perceived to be radiation-hard, but the damage caused to the diamond is not well understood. The intrinsic defects (vacancies and interstitials) which are created by radiation damage are immobile at room temperature in diamond, unlike in silicon. Therefore, once the mechanisms of damage are understood for one type and energy of the particle, the dose and energy dependence of irradiation by other particles at a range of energies can be extrapolated. When a crystal is irradiated, the generation rates of vacancies and self-interstitials are generally determined by optical or electron paramagnetic resonance (EPR) spectroscopy experiments carried out after the irradiation has stopped. However, as the irradiation proceeds some of the carbon atoms displaced from their lattice sites may relax back into the vacant site, and the damage event will not be observed in the later measurement. In this paper, the mechanisms for radiation damage by charged particles in particular electrons and photons are investigated. The kinetics of damage creation and the subsequent recombination of closely paired vacancies and self-interstitials are studied by a combination of theoretical modelling and optical and EPR spectroscopy to indicate the eventual lattice damage caused to the diamond.

Stability of electronic states of the vacancy in diamond

The vacancy in diamond is a fundamental defect which has been studied theoretically and experimentally for forty years. However, although early theories (Coulson C A and Kearsley M J 1957 Proc. R. Soc. A 241 433) were extremely successful in explaining the nature of the ground state of the neutral defect and the Jahn - Teller distortion expected (Lannoo M and Stoneham A M 1968 J. Phys. Chem. Solids 29 1987), there are still several questions which have not been answered satisfactorily. In particular, the many-electron effects and configuration interaction are vital. They determine not only the order of electronic levels in the vacancy, but also the best-known optical transition, GR1, which cannot be expressed in terms of one-electron levels alone. We bring together much of the detailed recent experimental data on the different charge states and excited states of the vacancy to build up a simple empirical model of the defect. We show that the stability of the states and their photoconductivity, or lack of it, can be reproduced. We can predict that other states of the neutral vacancy, observable by EPR, lie very close above the ground state, and another high-energy optical transition might be detectable.

CVD diamond particle detectors

Abstract CVD diamonds advantage as a particle detector over other materials is its radiation-hardness, its low atomic number, its high atom density and its robustness in hostile environments. It is possible to impose a very high electric field across a diamond, under which the electrons and holes created by an incident particle separate and are collected by electrodes on the surface of the film. The radiation-hardness is one of the supreme advantages of diamond over more conventional detector materials. We have investigated the radiation effects of high doses of neutrons, electrons, protons and alpha particles in diamond and compared the results to those in silicon, which is used for most conventional detectors of this type. The computer simulation packages GEANT and TRIM were used to plot the interactions of the incident particles with the diamond and the damage that they and the displaced carbon atoms cause within the solid. However, most of this damage anneals away while the irradiation continues, so we take into account the processes whereby interstitials recombine with vacancies. We can then show the damage profiles caused by these particles incident at energies of 1 MeV or other energies. The results confirm that approximately twice the number of vacancies are created in silicon compared to diamond. At room temperature, vacancies in silicon migrate to form complex defects with dopants, whereas in diamond, the vacancies are immobile, and the detectors are not doped. Therefore, the effect of the radiation damage in diamond on the operation of the detector is much less severe than equivalent damage would be on a silicon device.

Point Defects in Natural and Synthetic Diamond: What They Can Tell Us about CVD Diamond

More than 30 years of research on point defects in natural and high pressure, high temperature (HPHT) synthetic diamond helps us to characterise diamond produced by chemical vapour deposition (CVD). Nitrogen is the most common impurity in natural diamond, but hydrogen and boron are also detected. In HPHT diamond, nickel or cobalt complexes are seen as well. Irradiation produced interstitials and vacancies, which can help to identify some native defects. CVD diamond also may contain nitrogen and boron. A prominent defect, not normally seen in natural diamonds, is due to a silicon-vacancy complex. There are some hydrogen-related lines in the EPR spectra, which are peculiar to polycrystalline diamond and appear to arise near grain boundaries. Vacancies are seen close to the growth surfaces of particularly good quality CVD diamonds, although they are not present in the bulk of the diamond. Experience of vacancy migration in natural diamond allows us to explain this phenomenon.

Surface vacancies in CVD diamond

Abstract Investigation of unpolished, high-quality chemical vapour deposition diamond films has revealed a significant concentration of vacancies within a few microns of the surface, but not in the bulk. Modelling of the growth, diffusion and trapping of vacancies shows that this arises from the growth of vacancies into the surface of the diamond and their subsequent migration and trapping as growth proceeds. Because the temperature at which the vacancies migrate is comparable with that of diamond growth, a unique situation arises: the lattice defects created during growth persist in the crystals.

Ab initio study of the passivation and interaction of substitutional impurities with hydrogen in diamond

Abstract The production of n-type doped diamond has proved very difficult. Since boron, a shallow acceptor, can be passivated by hydrogen, it is possible that some of the difficulties in producing electrically active donors could be due to passivation by hydrogen which is present during the CVD growth of diamond. We report ab initio modelling of the interaction of hydrogen with boron, phosphorus and sulfur in diamond, and show that it is energetically favourable for hydrogen to bind to these centres and passivate them. We further investigated the possibility of the trapping of a second hydrogen atom at these centres, where we found that the complexes consisting of a dopant and two hydrogen atoms are energetically favourable for sulfur, with a binding energy similar to that found for H 2 * in diamond.

Neutron damage of CVD diamond

Abstract CVD diamond is being evaluated for the vertex detectors in the Large Hadron Collider. At the positions where diamond detectors would be located, the dose of high energy particles is expected to be equivalent to 1015 cm−2 1-MeV neutrons year−1. Three diamond films have been irradiated with 6 × 1015 neutrons cm−2. Before and after the full dose of irradiation, their cathodoluminescence and absorption spectra were recorded. The resistivity and the signal detected due to minimum ionising β particles was measured before and after irradiation and at five intermediate doses. We estimated the concentration of vacancies from the absorption spectra. The production rate for single neutral vacancies was about 0.5 per neutron cm−1. The signal detected when a minimum ionising β passes through the diamonds increases with dose by about 40% after 1015 neutrons cm−2. It decreases to below its initial value after the final dose. These results show definitively that CVD diamond detectors will operate with little degradation for 10 years in the high radiation zones of the Large Hadron Collider.