Controlled growth of metastable Ta3N5 semiconducting films

Sammanfattning: The semiconductor tritantalum pentanitride (Ta3N5) is a promising material for green energy applications, specifically in the photoelectrolysis of water to produce oxygen and hydrogen. With a bandgap of approximately 2 eV, Ta3N5 is well-suited for efficient solar light absorption across a broad spectrum, and its band positions align favorably with the redox potential of water. Theoretically, this material could achieve a solar-to-hydrogen efficiency of up to 15.9%. However, the intricate nature of the Ta-N compounds and its metastability have limited research into the development of high-quality Ta3N5.   In this thesis, the metastable Ta3N5 films were grown using two types of reactive magnetron sputtering techniques, direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS). Several key parameters were found to stabilize the formation of Ta3N5 phase, including the amount of oxygen in a gas mixture of Ar and N2, total working pressure, the Ta2O5 seed layer, and Ar/N2 partial pressure ratio.   First, sputter growth of Ta-N film using a gas mixture of Ar and N2 without oxygen gas, only metallic -TaN and ε-TaN phase were formed. After introducing a small amount of oxygen in the process gas (~2% of total working pressure), the oxygen atoms, with higher electronegativity, replace nitrogen atoms to trigger and stabilize the formation of crystalline Ta3N5-type structure. In addition, with a suitable Ar/N2 partial pressure ratio for Ta3N5 formation, a low-degree fiber-textural orthorhombic Ta3N5 film was formed at the total working pressure range from 5 to 30 mTorr. At 40 mTorr total working pressure, the deposited film transforms to O-rich amorphous Ta-O-N compound. Second, the effect of Ta2O5 seed layer on the control of Ta-N phase was studied. The Ta3N5 phase can be grown only with a Ta2O5 seed layer assistance. Without the seed layer, only metallic TaN phases were formed no matter if the film was grown with or without oxygen assistance. Furthermore, domain epitaxial growth of Ta3N5 film on sapphire substrate was achieved through the control of seed layer’s thickness and crystallinity. While the film was grown on an amorphous TaOx seed layer, the Ta3N5 structure becomes polycrystalline. Third, the formation mechanism and epitaxial growth were studied through microstructural analysis in combination of first-principle density-functional theory calculations. Time-dependent growth evolution of Ta3N5 films combined with HRTEM and EDX measurement revealed that the nitridation of Ta2O5 seed layer and Ta-N film deposition occurs simultaneously at the beginning of the Ta3N5 deposition. Further deposition, the Ta3N5 layer was dominated by {00k} domain mixed with (113) domain with a thin TaN layer between Ta3N5 layer and substrate. Last, various Ta-N compounds were grown via controlling the Ar/N2 partial pressure ratio and total working pressure. When the reactive gas was changed from pure Ar to pure nitrogen, the deposited films transformed from Ta metal (mixed with TaOx), TaN, TaN mixed with Ta3N5 to polycrystalline Ta3N5 phase. To summarize the work conducted in this thesis, I have established a reproducible and precise method for cultivating metastable Ta3N5 through the magnetron sputter deposition technique. The elucidated growth mechanism holds promise for synthesizing Ta3N5 on diverse substrates using alternative techniques, ensuring a controlled and adaptable approach.