Investigating the Properties of Transition Metal-Based Thin Films

Abstract

Transition metal-based thin films, including oxides and nitrides, are widely studied due to their diverse properties and applications. Among these materials, transition metal oxides (TMOs) are known for their wide-band gaps, high refractive indices, and high transparency in the visible spectrum. Due to their low cost, low toxicity, and high natural abundance, efforts are being made to modify these materials for broader application range, such as visible light absorption. Developing optimal methods for producing and characterizing transition metal-based films remains an ongoing challenge. This research investigates the growth and properties of transition metal-based films using complementary thin film deposition approaches: atomic layer deposition (ALD), and physical vapor deposition (PVD). In this work, ALD thin films were grown by varying parameters such as deposition temperature and purge time. The resulting properties were analyzed using X-ray photoelectron spectroscopy (XPS) for chemical states and composition, Fourier transform infrared spectroscopy (FTIR) for bonding and crystal structure, spectroscopic ellipsometry (SE), and ultraviolet-visible spectroscopy (UV-Vis) foroptical properties, and four-point probe (4PP) for electrical conductivity. This project used a new method of adjusting the ALD deposition process to determine the optical functions, providing a complete picture of the observed properties. ALD-grown hafnium oxide (HfO2) and titanium dioxide (TiO2), grown from similar amine precursors, (X[((CH3)2N)4])X=Hf, Ti with comparable deposition conditions exhibit completely different properties. The properties of ALD TiO2 films are shown to be highly dependent on the processing conditions, particularly in the temperature range within the ‘ALD window’. These films incorporate metallic, carbon, and water-based impurities, resulting in variable optical constants at 633 nm (n = 1.7 – 2.4, k = 0.2 – 1.2) and a wide conductivity range (1000 – 30000 S/m). The properties of the ALD TiO2 films are benchmarked between PVD-grown TiOxNy and PVD TiO2 films, providing further insight into the observed behavior. Conversely, the ALD HfO2 films only incorporate carbon and water-based impurities. The resulting optical constants at 633 nm (n = 1.9 – 2.1, k = 0) and conductivity (400 – 1000 S/m) do not strongly depend on the processing conditions. The results highlight that the underlying ALD chemistry and reaction pathways are significantly more complex than is currently understood and cannot be generalized, even for similar materials. Future investigations require combined experimental, theoretical, and computational approaches. By systematically studying thin film growth using this approach, this research establishes pathways for applications in precursor design, photovoltaics, transparent conductive oxides and optoelectronics.