Abstract
This dissertation explores how process-related defects present within an oxide dielectric film will influence the electrical performance of the assembled device in two different types of devices. The first topic shows a novel method of epitaxially growing magnesium oxide (MgO) via radio frequency (RF) reactive magnetron sputtering on gallium nitride (GaN). Gallium nitride is an attractive next-generation semiconductor due to its compatibility with operating at elevated temperatures and in high frequency switches; however, not many gate dielectrics are compatible with GaN because of its wide bandgap. The use of RF reactive sputtering—a scalable, cost-efficient, and widely employed deposition technique —demonstrated stable electrical performance up to 500 °C for over an hour, despite the presence of an interface layer in all grown films. The presence of an interface layer was found to originate from ion implantation during the initial stages of the MgO film growth. MgO films grown at 700 °C showed improved reliability compared to films grown at lower temperatures. It was deduced that fewer fixed oxide charges are present in MgO films grown at a higher deposition temperature, resulting in a better-matching interface with GaN and a lower defect density.
The second part of this dissertation aimed to investigate the mechanisms of oxygen diffusion in hafnium zirconium oxide (HZO) films and their application in computer memory devices. It is well known that oxygen vacancies (among other parameters) stabilize the ferroelectric phase in hafnia-based oxides; however, oxygen vacancies may, in part, cause phenomena such as retention loss, fatigue, and dielectric breakdown when accumulated near interfaces. Thus, it is crucial to understand the mechanisms controlling the motion of oxygen vacancies in hafnia-based films and to better tailor the position of oxygen vacancies throughout the film to improve electrical performance and reliability. First, the oxygen diffusion coefficients in the ferroelectric polar orthorhombic phase of HZO were calculated using an 18O tracer diffusion study, where a two-step diffusion model was developed and employed. The metal electrode used in conjunction with the HZO layer was found to affect the oxygen diffusion coefficients. For example, a higher oxygen diffusion coefficient was calculated when titanium nitride (TiN) electrodes were used compared to tungsten (W). One possible explanation is the different reactivities between the metal electrode and the HZO. This results in scavenging more oxygen and forming a higher concentration of oxygen vacancies within the HZO (when used with TiN electrodes), leading to increased diffusion. Furthermore, it was observed that oxygen vacancies are not the primary cause of imprint in poled devices, due to the extremely low oxygen diffusion coefficients. It is more likely to be caused by electronic carriers. Still, oxygen vacancies appear to contribute to the detrimental phenomenon of fatigue to some extent. Finally, a co-doping model was investigated as a possible route to overcome imprint and fatigue. Acceptor (Ta5+) and donor (Al3+) doping were simultaneously introduced within the HZO layer in order to inhibit oxygen migration from the bulk to the electrode interfaces. Applying this approach resulted in improved endurance for co-doped HZO films, at the expense of polarization magnitude.
