• Christensen, Dennis Valbjørn

Abstract

The increasing impact of electronic devices on our daily lives has caused the strong market pull that has empowered the tremendous development in realizing faster, smaller and more energy efficient devices. Two routes are used to satisfy this market pull: (i) Improving existing devices or (ii) designing devices with new functionalities. The functionalities that can be achieved in devices are determined by the constituent materials. Appealing functionalities may thus be realized by using materials beyond the semiconducting materials that currently constitute the backbone of state-of-theart electronic devices. An example is the 3D-Xpoint memory technology introduced in Intel/Micron’s next generation of memory devices, which are using new memristive functionalities in chalcogenides rather than the traditional semiconducting floating gate transistors used in solid state drives (SSD).<br/><br/>In 2004, a new material platform was discovered, which in the following decade remarkably turned out to exhibit a plethora of functionalities. The material platform was formed by depositing a thin film of LaAlO3 (LAO) epitaxially on SrTiO3 (STO). Despite both oxides were considered non-magnetic and insulating, conductivity and magnetism emerged at the interface. Numerous other functionalities were also discovered including gate-tunable superconductivity, non-volatile resistive switching, and a giant Seebeck coefficient. <br/><br/>In 2013, LAO was replaced with γ-Al2O3 (GAO) resulting in an improved epitaxial growth and electron mobility. Open questions remained, however, regarding the origin of the electron gas confined at the interface and whether GAO/STO would exhibit appealing functionalities similar or perhaps superior to LAO/STO. In this thesis, I first describe the non-isomorphic epitaxial growth of the spinel GAO on perovskite STO and how it leads a useful symmetry breaking at the interface. Second, I present how oxygen vacancies lead to the emergence of an electron gas at the GAO/STO interface. The electron gas is highly tunable by deposition control, postannealing inoxygen, electrostatic gating and light exposure. At room temperature the electron mobility is limited to 12 cm²/Vs by phonon scattering. At 2 K the mobility exceeds 100,000 cm²/Vs, which I propose is due to an electron-donor separation. The electron gas exhibits a colossal positive magnetoresistance of 80,000% at 2 K and 15 T with a great potential for realizing extraordinary magnetoresistance. In addition, a straintunable magnetic state is observed in GAO/STO. The thesis ends with my view on how the understanding and number of functionalities can be improved further.

Topics

• Deposition
• perovskite
• impedance spectroscopy
• mobility
• thin film
• Oxygen
• superconductivity
• superconductivity