Common brown colour in natural diamond forms by plastic deformation during storage in the subcontinental lithospheric mantle (SCLM). Dislocation movement generates vacancies, which aggregate into clusters of perhaps 30–60 vacancies. Positron annihilation lifetime spectroscopy (PALS) and electron energy loss spectroscopy (EELS) support such vacancy clusters as the cause of brown colour. Brief treatment in a high-pressure–high-temperature (HPHT) vessel at 1800–2700 °C can destroy the brown colour. There has been speculation that similar colour removal should occur continuously at depth in the SCLM. Diamonds are stored at 900–1400 °C in the SCLM, according to inclusion thermometry. The effect of temperature on the time required to destroy brown colour has been calculated from published data. The activation energy for the breakup of vacancy clusters is a critical component. The time required to destroy brown colour in the SCLM is significant at the scale of geological time. Brown diamonds should easily maintain their colour for millions of years during cooler mantle storage at or below about 1000 °C. Warmer temperatures toward the base of the lithosphere may be able to reduce or eliminate brown colour within thousands of years. The survival of brown colour in the lithospheric mantle does not require the colour to be formed late in the storage history nor does it require metastable storage in the graphite stability field. Crystal strain is preserved upon loss of brown colour during HPHT treatment. Inhomogeneous crystal strain was measured in 18 natural diamonds using micro-X-ray diffraction (μXRD) χ-dimension peak widths. There is a correlation between strain and depth of brown colour. None of the colourless diamonds examined have high strain, as should be expected for a diamond that has gained and lost brown colour. This suggests that removal of brown colour is not a common natural occurrence. Infrared spectroscopy was used to determine nitrogen concentration and aggregation state in 60 natural diamonds. A loose association was found between brown colour and lower total nitrogen content. Within single diamonds, regions with less nitrogen tend to exhibit more anomalous birefringence due to strain. Colour zoned diamonds tend to have less nitrogen in the darker brown regions. This supports the hypothesis that diamonds with less nitrogen are more susceptible to plastic deformation and the development of brown colour. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2009-09-17 17:10:11.078

Creation of semi-insulating layers in SiC is highly desirable for high voltage device fabrication. Specifically PiN diodes can be fabricated with a compensated semi-insulating layer that would be capable of blocking a large reverse voltage. Semi-insulating (SI) behavior in SiC has been traditionally achieved via passivation of shallow dopants with vanadium-related deep levels. Degraded electrical properties of SiC devices result from the use of vanadium compensated SiC because unintentional formation of additional defects due to vanadium segregation and stress generation in the material occur. In this work, the possibility of low doped or SI epilayers via engineering of the boron related defects in SiC is investigated. High temperature treatment (up to 2000°C) of boron doped samples is used to stimulate boron diffusion and formation of deep boron centers in concentration sufficient for compensation of shallow dopants, without simultaneous formation of undesirable shallow boron levels. High temperature annealing of both epitaxial layers in-situ doped with boron and boron implanted 4H-SiC is investigated. The possibility of diffusion from highly boron doped substrate is also investigated. The diffusion profiles are modeled and the diffusion coefficients extracted to give information about diffusion mechanisms. The boron D-center was observed using photoluminescence (PL) after high temperature annealing of the implanted samples. Clear temperature dependence of the creation of the D-center was observed. Compensated material was revealed after an Inductively Coupled Plasma (ICP) etch on an epi-over grown sample.