Atomic layer deposition of functional materials
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Atomic layer deposition (ALD) has emerged as an important technique for depositing thin films in both scientific research and industrial applications. The goal of this work is to integrate functional materials using ALD including high-κ dielectric, LaAlO₃, ferroelectric BaTiO₃, photocatalytic CoO, and room temperature ferromagnetic thin films of Co metal for spin-transfer torque random-access memory applications. The work is also to demonstrate the formation of a quasi-two-dimensional electron gas (2-DEG) at the γ-Al₂O₃/SrTiO₃ heterointerface enabling a method for all-oxide device manufacturing using ALD. High permittivity oxide thin films are needed to replace SiO₂ in complementary metal oxide semiconductor (CMOS) transistors. The replacement of SiO₂ by hafnium oxide-based high-κ materials in CMOS devices in 2007 was a revolutionary development in semiconductor front end of line. The continued device feature shrinking requires higher-κ dielectrics, compared to HfO₂-based materials. Crystalline perovskite oxides, such as SrTiO₃, LaAlO₃, and BaTiO₃, etc. have from high to very high dielectric constant and being proposed to replace HfO₂-based materials in CMOS devices if the leakage problem is resolved. The work explores the monolithic integration of crystalline perovskite oxide films with Si(001) using combined molecular beam epitaxy (MBE) and ALD techniques. Four unit cells of SrTiO₃ were grown directly on Si(001) by MBE and transferred in-situ into the ALD chamber for further depositions. The integration of oxide thin films on Si(001) using the MBE-ALD technique allows us to maintain clean oxide/Si(001) interfaces since low temperatures (180–250 °C) were maintained during the ALD deposition. The goal of my work is also to explore processes to enable area selective deposition of cobalt (II) oxide, CoO. The effectiveness of poly(trimethylsilylstyrene) in selectively inhibiting surface nucleation of CoO on SiO₂ and MgO substrates is demonstrated. Carbon-free cobalt thin films are formed by reducing CoO using Al and Sr metals to scavenge oxygen from CoO. The work explores the ability to control the structure and morphology of the resultant cobalt film by tuning the reduction conditions, allowing us to tune magnetic properties of the cobalt thin film. My work also focuses on the growth of γ-Al₂O₃ on the TiO₂-terminated SrTiO₃ substrate at temperatures higher than 300 °C. The formation of a quasi-2-DEG is found at the γ-Al₂O₃/TiO₂-terminated SrTiO₃ interface. In-situ x-ray photoelectron spectroscopy reveals the presence of Ti³⁺ feature at the heterointerface. Conductivity at the interface was found to be proportional to the amount of Ti³⁺ species. Oxide quasi-2-DEG might provide opportunities for new generations of all-oxide electronic devices using ALD.