2025 Theses Doctoral

Stability and performance in metal oxide thin film transistors

Alshanbari, Reem

Amorphous indium-gallium-zinc oxide (𝛼-IGZO) is one of the most promising materials for next-generation displays, sensors, and flexible electronics owing to its high carrier mobility, transparency within the visible wavelength range, and low-temperature processing. Despite these advantages, the performance of IGZO devices is sensitive to interface instabilities between IGZO and the gate insulator and metal contacts, which affect carrier density, trap formation, and long-term reliability. My dissertation will discuss the fundamental chemical and structural mechanisms governing these interfaces, establishing experimental guidelines for designing oxide-semiconductor devices that operate stably.

We combined different sets of techniques, such as atomic layer deposition (ALD), transmission electron microscopy/scanning transmission electron microscopy (TEM/STEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to investigate the interface between IGZO, the gate insulator, and the contact. The experimental evidence in this dissertation establishes material-dependent interfacial reaction between IGZO and ALD high-𝜅 insulators. We observed that the deposition of aluminum oxide (Al₂O₃) by thermal ALD on IGZO exhibits significant intermixing with IGZO, driven by the reduction of IGZO cations and/or the creation of oxygen vacancies, which is thermodynamically favorable due to TMA-induced reduction reactions. In comparison, the hafnium oxide (HfO₂) forms a sharp and chemically stable interface with IGZO with almost negligible diffusion. These results clearly demonstrate that dielectric selection and ALD surface chemistry are crucial in determining the stability of IGZO.

The second part of this dissertation focuses on understanding the interface between metal/IGZO interfaces, revealing strong redox reactions between titanium (Ti) and IGZO. Ti pulls oxygen out from IGZO to form a TiOx interfacial layer at the interface, generates oxygen vacancies within IGZO, and promotes elemental diffusion, especially under thermal annealing. In contrast, titanium nitride (TiN) suppresses oxygen exchange, leading to a chemically inert and diffusion-barrier interface. Electrical measurement correlates these reactions with TFT behavior: while vacancy-enhanced conductivity is introduced into the channel by Ti contacts, stability is expected to be degraded. Meanwhile, TiN maintains consistent device characteristics. In general, this dissertation shows experimentally how interfacial chemistry dictates the performance of oxide semiconductor devices. From atomic-scale processes to electronic behavior, the critical insight this work provides enables interface engineering in IGZO electronics, with even broader relevance to emerging quantum and oxide-based device technologies.