Electroluminescence Of TiO₂/p⁺-Si Heterostructure

TiO2, as a semiconductor material, is primarily employed in photo-catalysis and dye solar cells at present. Although TiO2 is of indirect bandgap semiconductor thus leading to a low efficiency of band-edge emission, it possesses defect-related light emission centers, which generate visible emissions. In this context, electroluminescent devices based on TiO2 films are worthy to be pursued. In this thesis, the effects of formation conditions and hydrogen annealing of TiO2 films on the electroluminescence of TiO2/p+-Si heterojunctions have been investigated. The main results achieved in this thesis are listed as following.Titanium (Ti) films were prepared on heavily boron-doped silicon (p+-Si) substrates by magnetron sputtering under different conditions. They were then oxidized at 500℃under oxygen ambient to form TiO2 films. It was revealed that only the Ti films sputtered with relatively low powers, which were dense and have fine grains, could be oxidized into single-anastate-phase TiO2 films. In order to make the TiO2/p+-Si heterojunctions generate pronounced electroluminescence (EL), the TiO2 films should be of single-anastate-phase and possess appropriate thicknesses. It was found that EL from the TiO2/p+-Si heterojunctions was significantly weakened if the TiO2 films contained a small amount of rutile phase, which was believed to be due to the increase of non-radiative centers.The anastate TiO2 films formed by oxidation of Ti films on p+-Si substrates were annealed in hydrogen ambient to form hydrogen-annealed TiO2/p+-Si heterojunctions. It was shown that when the TiO2 films were annealed at 500℃for 1 h under hydrogen ambient with a working pressure of 2 Pa, the EL of TiO2/p+-Si heterojunctions was significantly enhanced with a factor of 1.5. It is believed that the hydrogen annealing of TiO2 films leads to increased amount of oxygen vacancies that are light-emission centers because hydrogen ambient is a strong reducing agent. Consequently, the EL of TiO2/p+-Si heterojunctions is enhanced.