Metastable orthorhombic Ta3N5 thin films grown by magnetron sputter epitaxy

Sammanfattning: The semiconductor tritantalum pentanitride (Ta3N5) is a promising green-energy material for photoelectrolyzing water to produce oxygen and hydrogen owing to its proper bandgap of 2.0 ± 0.2 eV and band positions to redox potential of water. Compare with the conventional setup of water splitting, such as TiO2, Fe2O3, Cu2O, and WO3, the Ta3N5 shows a proper band gap, which leads to a theoretical efficiency as high as 15.9%. However, the complexity of the Ta-N system and the metastability of the Ta3N5 result in the limited research of the growth of high quality stoichiometric Ta3N5.Conventionally, the two-step growth of oxidation and nitridation of a metal Ta using thermal annealing in oxygen and ammonia environment is used to produce the Ta3N5. However, the amount of incorporated oxygen in the Ta3N5 samples and film’s thickness and interface are hardly to be controlled, and the use of ammonia as the nitridation gas is harmful to the environment. Hence, in this thesis work, the reactive magnetron sputtering is used to synthesis the Ta3N5, which demonstrates some advantages, such as possibility to grow on a substrate with nanostructure on the surface, a simplification of growth process, usage of environmental-friendly reactive gas, and even scaling up to the industrial application.The thesis presents a successful growth of orthorhombic Ta3N5-type Ta-O-N compound thin films on Si and sapphire substrates, specifically Ta3-xN5-yOy, using reactive magnetron sputtering with a gas mixture of Ar, N2, and O2. In the deposition process, the total working pressure was increasing from 5 to 40 mTorr, while keeping same partial pressure ratio (Ar: N2: O2 = 3: 2: 0.1). When the total pressure in the region between 5-30 mTorr, a low-degree fiber-textural Ta3-xN5-yOy films were grown. In addition, with the characterization of elastic recoil detection analysis (ERDA), the atomic fraction of O, N, and Ta of as-grown Ta3-xN5-yOy films were found varying from 0.02 to 0.15, 0.66 to 0.54, and 0.33 to 0.31, respectively, which leads to a b-lattice constant decrease around 1.3 %, shown in X-ray diffraction (XRD) results. For a total working pressure up to 40 mTorr, an amorphous O-rich Ta-O-N compound film was formed mixed with non-stoichiometric TaON and Ta2O5, which further raised the oxygen atomic fraction to ~0.48. The increasing total working pressure results in an increasing band gap from 2.22 to 2.66 eV of Ta3-xN5-yOy films, and further increasing to around 2.96 eV of O-rich Ta-O-N compound films. The mechanism of increasing oxygen atomic fraction in the film is founded correlated with the forming oxide on the Ta target surface during the deposition process due to the strong reactivity of O to Ta by the characterization of optical emission spectroscopy (OES). Moreover, the sputter yield was reduced due to the target poisoning, and which is evidenced by both plasma analysis and depth profile from ERDA.A further studies with the deposition parameters for nearly pure Ta3N5 films (oxygen atomic fraction ~2%) was performed using c-axis oriented Al2O3 substrate. In this research, it is found that a Ta2O5 seed layer and a small amount of oxygen were necessary for the growth of Ta3N5. Without the help of seed layer and oxygen, only metallic TaN phases, either mixture of ε- and δ- TaN or δ-TaN were grown, evidenced by X-ray photoelectron spectroscopy (XPS). Furthermore, the structure and phase purity of Ta3N5-phase dominated films was found highly correlated with the thickness of the Ta2O5 seed layer. With the increasing thickness of the seed layer from 5, 9, to 17 nm, the composition of grown films was changed from 111-oriented δ-TaN mixed with c-axis oriented Ta3N5, c-axis oriented Ta3N5, to polycrystalline Ta3N5. In addition, the azimuthal φ-scans in grazing incident geometry demonstrates that the c-axis oriented Ta3N5 contained epitaxially three-variant-orientation domains, in which the a and b planes parallel to the m and a planes of c-axis oriented Al2O3. With the simulation of density functional theory (DFT), the growth of thin seed layers of orthorhombic Ta2O5 (β-Ta2O5) was found promoting by introducing a small amount of oxygen, after calculating the interplay between the topological and energy selection criteria. By the co-action of the mentioned criteria, this already grown Ta2O5 seed layer favored the growth of the orthorhombic Ta3N5 phase. Hence, the mechanism of the domain epitaxial growth of c-axis oriented Ta3N5 on c-axis oriented Al2O3 is attributed to the similar atomic arrangement Ta3N5(001) and β-Ta2O5(201) with a small lattice mismatch around of 2.6% and 4.5%, for the interface of film/seed layer and seed layer/substrate, respectively, and a favorable energetic interaction between involved materials.