Structural and Electrical Properties of Epitaxial Functional Oxides on Silicon

Abstract

Functional oxides hold immense potential for revolutionizing electronics due to their diverse applications, from energy storage and actuation to electro-optics, sensors, and memory. However, their integration with Si CMOS technology has been hindered by growth complexities. Silicon offers the ideal platform due to its mature fabrication techniques. A key challenge is the formation of an amorphous SiO2 layer when depositing oxides on Si, which hinders epitaxial growth. Domain-matched epitaxial TiN buffer layers can circumvent this problem. While other buffer layers like Yttria-stabilized Zirconia (YSZ) have been used, TiN is a preferred choice. It’s a CMOS-compatible conductor suitable as the bottom electrode, eliminating the need for additional epitaxial conducting layers. This thesis addresses the challenge of epitaxially integrating functional oxide thin films, mainly VO2 and BaTiO3, onto Si using a TiN buffer layer deposited by pulsed laser deposition (PLD). We further investigate the structural and electrical properties of these epitaxial functional oxides. Here, we report the development of an in-house PLD system with an eclipsed off-axis configuration for achieving epitaxial growth. We successfully optimized growth conditions for the epitaxial TiN buffer layer on Si, resulting in smooth, pit-free TiN films with (100) crystallographic orientation and low resistivity. The epitaxial relationship of TiN with Si was established. Furthermore, the growth of epitaxial yttria-doped HfO2 on the developed TiN/Si platform is demonstrated, suggesting a possible orthorhombic crystal structure. Finally, the feasibility of alternative non-conducting buffer layer platforms, like integrating BaTiO3 on SrTiO3/MgO/Si, is explored for future devices. Vanadium oxide (VO2) exhibits a metal-insulator transition (MIT) with unique properties desirable for various electronic applications. We optimized growth parameters to achieve a stoichiometric VO2 phase and epitaxially integrated VO2 thin films on the TiN/Si(100) platform via PLD. The orientation relationship of all the layers, including an unintentionally grown TiO2 layer at the VO2-TiN interface, was determined. The texture properties of VO2 films were studied using pole figures, reciprocal space mapping (RSM), and transmission electron microscopy (TEM). Importantly, we establish a correlation between the texture of VO2 films and the thermal hysteresis of the MIT transition, with a reduction in hysteresis width observed for more oriented films. Epitaxial BaTiO3 (BTO) thin films were grown on TiN/Si using PLD. We optimized growth parameters (pressure, temperature, fluence) to achieve ferroelectric BTO, which XRDandelectrical characterization techniques confirmed. The influence of defect dipoles on BTOproperties was investigated through temperature-dependent capacitance-voltage (C-V) analysis, revealing pinning/de-pinning mechanisms and defect migration. The microscopic distribution of ferroelectric domains was shown by polarization mapping of high-resolution STEM images. The effect of growth conditions on the surface and bulk chemical composition of the BTO films was investigated using X-ray photoelectron spectroscopy. Finally, we explore the possibility of influencing the BTO film characteristics via an in-situ electric field applied during PLD. This work investigates the effect of an electric field on the crystal structure, out-of-plane lattice parameters, and ferroelectric properties of BTO films. Our findings reveal significant differences in film properties between samples grown with and without an electric field. "Field on" samples exhibit smaller hysteresis loops, lower remnant polarization, and lower coercive voltage than "field off" samples. The work suggests a possible influence of the electric field on domain-switching behavior and defect dipole distribution within the films. In conclusion, this thesis successfully demonstrates the epitaxial integration of VO2 and ferroelectric BaTiO3 thin films on silicon using a TiN buffer layer deposited by pulsed laser deposition (PLD). We established a correlation between film texture and the metal-insulator transition (MIT) behavior in VO2, paving the way for potential device applications. Furthermore, for BaTiO3, we delved into the influence of defect dipoles on its ferroelectric characteristics. Additionally, we explored the intriguing possibility of manipulating BTO film properties through an in-situ electric field applied during PLD. This work paves the way for developing novel functional oxide devices integrated onto silicon platform.