Aluminium and oxygen termination of diamond for thermionic applications

This thesis presents a joint computational and experimental approach to developing a novel diamond surface termination with aluminium that has a negative electron affinity (NEA). There is a need for a thermally stable NEA for thermionic applications, since hydrogen termination desorbs from the diamond surface at thermionic temperatures. The choice of aluminium is based on it being a light, electropositive element that bonds strongly to different diamond surfaces. Using density functional theory, the adsorption of Al onto bare, oxygenated and nitrogenated diamond surfaces was studied. The two key parameters considered were the adsorption energy and the electron affinity values, for different coverages and configurations of Al. NEA values of up to -1.47 eV and 1.88 eV were observed for Al addition to the (100) and (111) bare surfaces, respectively, at 1 monolayer (ML) coverage, and up to -1.36 eV and -2.17 eV for Al addition to the (100) and (111) oxygen-terminated surfaces, respectively, at 0.25 ML coverage. Al adsorbed more strongly on the ketone O-terminated surface than the ether surface. Adsorption energies for the AlO-terminations were up to -6.36 eV/atom and -8.19 eV/atom for the (100) and (111) surfaces, respectively, at 0.25 ML Al coverage, considerably larger than that of H termination (~-4 eV/atom). Three different oxidation procedures were investigated experimentally. It was determined that oxidation of diamond by UV/ozone treatment results in a large ketone component on the surface, whilst also having ~1 ML coverage. The surface structure and electronic behaviour were determined for both hydrogen and oxygen terminations. AlO-terminations were fabricated experimentally by depositing Al onto O terminated diamond. Three different methods were explored: (i) thick-film deposition of Al followed by an acid etch to remove excess metal, (ii) thin-film deposition of Al by atomic layer deposition, and (iii) thin-film deposition of Al by electron-beam evaporation. Low work function and NEA were observed for each method, but these values were spatially and temperature dependant. Annealing caused a change in surface structure, identified as the incorporation of O atoms into the Al layer. Tests of thermionic emission showed a small emission current density, but at a higher temperature than attainable for H termination. Thus, AlO-terminated diamond shows promise but further optimisation is required for use in devices.