Metal Ions Implantation‐Induced Effects in GaN Thin Films

MOCVD-grown GaN n-type epilayers were implanted with 150keV Co+ and Cr+ ions at different fluences at room temperature. Co+ was implanted at 3x1016 and 5×1016 ions/cm2 and samples rapid-thermal-annealed at 700, 800 and 900°C for 5 minutes, while Cr+ was implanted at 3x1016 ions/cm2 and annealed at 800 and 900°C for 2 minutes. Diffraction patterns of implanted samples showed satellite peaks at the lower side of the main GaN (0002) reflection and these were assigned to implantation induced-damage and the formation of Ga1−xCoxN or Ga1−xCrxN phases. The coercivity (Hc) at 5K from SQUID for Co+ implanted GaN at 3x1016 ions/cm2 was 275 Oe and that at 5x1016 ions/cm2 was 600 Oe. For Cr+ implanted GaN at 3x1016 ions/cm2, Hc was 175 Oe. At the same dose of Cr+ and Co+ implanted ions, the saturation magnetization (Ms) values were almost similar. But after annealing at 900°C, the Ms value of Cr+ implanted GaN was higher than that of Co+ implanted at 5K. For Co+ implanted GaN, magnetization was retained up to 370K while in Cr+ implanted GaN, magnetization was retained above 380K. These findings are the highest reported Curie temperatures for Co+ and Cr+ implanted GaN diluted magnetic semiconductors

Muon-spin spectroscopy studies of hydrogen-related defects relevant to doping of diamond

This study focused on investigation of the behaviour of hydrogen-related defects in diamond. Diamond represents a material with potential in electronics, particularly for high power and high temperature electronic devices operating under radiation and in corrosive environments. However, the production and characterization of high-quality diamond for electronic devices is a great challenge. The main objective is to understand the origin of structural imperfections and impurities and to reduce or control these in order to improve the electronic and optical properties of diamond. The presence of hydrogen is known to influence these properties. Therefore, very significant experimental and computational e ort is expended in trying to understand and predict the behaviour of hydrogen in diamond and in other semiconducting materials, and the recent discovery of its ability to act as a shallow dopant to enhance conductivity in some materials has generated much interest. However, most of the experimental information on hydrogen in diamond has hitherto been obtained from studies of the light hydrogen pseudo-isotope, muonium. In this thesis, we employ transverse field muon spin rotation (TF- SR) and longitudinal field muon spin relaxation (LF- SR) to investigate two specific aspects of muonium (and hence hydrogen) in diamond: Firstly, the high temperature stability of bond-centred muonium (MuBC) is investigated in a search for its possible ionization. The MuBC state in diamond is easily observed and there is a very pleasing correlation between theoretical and experimental hyperfine parameters. Curiously, despite its predicted stability, the bond-centred hydrogen (HBC) state has not yet been observed in diamond. As one proceeds with LF- SR measurements above room temperature, one encounters firstly the expected increase in the MuBC population corresponding to the well known MuT ! MuBC transition, but observed here for the first time in diamond in longitudinal field. At still higher temperatures (setting in near 1 000 K), the MuBC population decreases, a result which is consistent with MuBC ionization. There is also an indication, from the TF- SR measurements, that this is correlated with the increase in the population of the diamagnetic ( + D) species. Secondly, the muonium states in high-purity type IIa diamond grown by high-pressure and high-temperature (HPHT) synthesis are investigated in a search for the origin of the missing fraction (MF) observed in many previous muonium studies in diamond. The pure synthetic diamond gave similar fractions of the muonium states as pure natural samples. There is still a small missing fraction

Solid state dewetting of a metal –semiconductor bi-layers deposited onto c-Si substrate

A bi-layers stack consisting of a semiconductor thin film of a varied thickness and a very thin Pd layer (SiC/Pd/c-Si).was deposited onto c-Si by e-beam evaporation at room temperature. The multi-layers structure was subjected to a thermal annealing process at near eutectic temperature of the Si – Pd phase. It is noticed, through top view SEM and cross-section STEM analyses, that the sandwiched Pd metal layer dewets from the interface with the c-Si substrate in well dispersed nanoparticles and it diffuses inward onto the top few monolayers of the substrate; at times it permeates shallowly through the SiC semiconductor top layer. The size distribution of the nanoparticles was found to be closely linked to the thickness of the top semiconductor layer

Modelling of Radiological Health Risks from Gold Mine Tailings in Wonderfonteinspruit Catchment Area, South Africa

Mining is one of the major causes of elevation of naturally-occurring radionuclide material (NORM) concentrations on the Earth’s surface. The aim of this study was to evaluate the human risk associated with exposure to NORMs in soils from mine tailings around a gold mine. A broad-energy germanium detector was used to measure activity concentrations of these NORMs in 66 soil samples (56 from five mine tailings and 10 from the control area). The RESidual RADioactivity (RESRAD) OFFSITE modeling program (version 3.1) was then used to estimate the radiation doses and the cancer morbidity risk of uranium-238 (238U), thorium-232 (232Th), and potassium-40 (40K) for a hypothetical resident scenario. According to RESRAD prediction, the maximum total effective dose equivalent (TEDE) during 100 years was found to be 0.0315 mSv/year at year 30, while the maximum total excess cancer morbidity risk for all the pathways was 3.04 × 10−5 at year 15. The US Environmental Protection Agency considers acceptable for regulatory purposes a cancer risk in the range of 10−6 to 10−4. Therefore, results obtained from RESRAD OFFSITE code has shown that the health risk from gold mine tailings is within acceptable levels according to international standards

Health Risk Assessment of Heavy Metals in Soils from Witwatersrand Gold Mining Basin, South Africa

The study evaluates the health risk caused by heavy metals to the inhabitants of a gold mining area. In this study, 56 soil samples from five mine tailings and 17 from two mine villages were collected and analyzed for Asernic (As), Lead (Pb), Mercury (Hg), Cadmium (Cd), Chromium (Cr), Cobalt (Co), Nickel (Ni), Copper (Cu) and Zinc (Zn) using ICP-MS. Measured concentrations of these heavy metals were then used to calculate the health risk for adults and children. Their concentrations were such that Cr > Ni > As > Zn > Cu > Co > Pb > Hg > Cd, with As, Cr and Ni higher than permissible levels. For the adult population, the Hazard Index value for all pathways was found to be 2.13, making non-carcinogenic effects significant to the adult population. For children, the Hazard Index value was 43.80, a value >>1, which poses serious non-carcinogenic effect to children living in the gold mining area. The carcinogenic risk was found to be 1.7 × 10−4 implying that 1 person in every 5882 adults may be affected. In addition, for children, in every 2725 individuals, 1 child may be affected (3.67 × 10−4). These carcinogenic risk values were both higher than acceptable values

An Assessment of Radiological Hazards from Gold Mine Tailings in the Province of Gauteng in South Africa

Radiological hazards associated with exposure to Naturally Occurring Radionuclides Materials from gold mine tailings in the province of Gauteng in South Africa were evaluated. A comparison was made with soil samples from a control area. In this study, gamma spectroscopy was used to measure the activity concentrations of these radionuclides in 56 soil samples from the mine tailings and 10 soil samples from the control area. The average activity concentrations in Bq∙kg−1 for Uranium-238, Thorium-232, and Potassium-40 from the mine tailings were found to be 785.3 ± 13.7, 43.9 ± 1.0 and 427.0 ± 13.1, respectively. On the other hand, the average activity concentrations in Bq∙kg−1 for Uranium-238, Thorium-232, and Potassium-40 from the control area were found to be 17.0.1 ± 0.4, 22.2 ± 0.5 and 496.8 ± 15.2, respectively. Radiological hazard parameters calculated from these activity concentrations were higher than recommended safe limits. In particular, calculated average values for the external hazard (Hex) and the internal hazard (Hin) from the mine tailings were found to be 2.4 and 4.5. Both these values were higher than unity, posing a significant health risk to the population in the area

Structural and photoluminescence analysis on the implantation of carbon and proton for the creation of damage-assisted emission in silicon

We study the induced defects in the depth profiling of the silicon structure after being implanted with carbon and followed by high energy proton irradiation. It has been reported before that the formation of the optically active pointdefect, specifically the G-centre is due to the implantation and irradiation of carbon and proton, respectively. It is crucial to quantify the diffusional broadening of the implanted ion profile especially for proton irradiation process so that the radiation damage evolution can be maximized at the point-defect formation region. Profiling analysis was carried out using computational Stopping and Range of Ions in Matter (SRIM) and Surrey University Sputter Profile Resolution from Energy Deposition (SUSPRE) simulation. The energies of carbon ions adopted for this investigation are 10, 20, 30, and 50 keV, while proton irradiation energy was kept at 2 MeV. Photoluminescence measurements on silicon implanted with carbon at different energies were carried out to study the interrelation between the numbers of vacancies produced during the damage event and the peak emission intensities