Controlling the electrical properties of oxide heterointerfaces through their interface chemistry

Rose, Marc-André; Taubner, Thomas (Thesis advisor); Dittmann, Regina (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022


Multiple routes are taken today to improve the processing power of modern computers. Besides software and electrical engineering approaches, high eort is taken in improvingthe most fundamental building block of computers, namely their electronic devices. To enhance the performance of electronic devices, the semiconductor industry appliedfurther and further optimization, driven by a down-scaling approach. The advent ofthe physical limitation of this approach has led to the search of alternative materials and concepts, which could allow even higher computational performance. Oxides and their interfaces are promising candidates due to their highly diverse and tunable properties. In this context, the LaAlO3/SrTiO3 (LAO/STO) interface sparked high interest, as it possesses an interfacial 2-dimensional electron gas (2DEG), which is a vital partof modern transistor technology (in particular for high-electron-mobility-transistors). However, even though the system has been researched for over a decade, the detailsof its electronic behavior are still not fully understood. This thesis investigates how the interface chemistry of LAO/STO heterostructures inuences the electronic properties of the 2DEG. A two fold approach is taken to elucidate this topic. For one, the lateral direction was analyzed (along the interface),where local inhomogeneities of the 2DEG due to the STO substrate termination appear. Second, the vertical direction was analyzed (across the interface), where local defect concentrations can be altered through thermodynamic annealing and trigger ionicmotion across the interface. In both cases, in-situ methods are used, which allow investigation of the pristine process at the interface. In the lateral direction, in-situ scanning probe techniques are applied. In this way, a mapping of the 2DEG is achieved, which allows to correlate the nanoscopic 2DEG distribution to macroscopic sample properties. It is shown how the 2DEG distribution can be manipulated, for one by the use of naturally occurring STO substrate termination variations and second, by the controlled growth of SrO sub-monolayers on single terminated STO. Depending on the arrangement of conducting and insulating regions, the electrical macroscopic behavior is changed down to the low-temperaturesrange. In the vertical direction, in-situ near-ambient pressure X-ray photoelectron spectroscopyis applied. This enables the monitoring of evolving secondary phases andchanging band alignment, while applying thermodynamic annealing. Upon oxidation,Sr ions precipitate out of the interfacial region, depleting the 2DEG. An unexpectedhigh mobility of Sr ions is observed, as the applied moderate temperatures (470 °C)would not allow signicant cation diusion in the STO bulk. The induced change ofinterfacial defect chemistry alters not only electronic, but also the magnetic properties of the interface, leading to a signicant increase of low-temperature magnetismin oxidized heterostructures. Based on the acquired knowledge from spectroscopic analysis, a route to control the magnetic behavior upon oxidation is established, using a combination of LAO stoichiometry and thermodynamic annealing. At the end, the gained results are summarized and a defect chemical model is proposed, which can consistently explain the observed behavior. Through these results, an important step towards the full understanding of the relationship between ionic structure and electronic properties is taken.