Exploration of p-type complex oxides and their semiconducting properties

Recently, there has been a drive to add functionality in the back-end-of-line (BEOL) to maximise the use of space on semiconductor chips where the efficient use of real estate is becoming ever more important. The ultimate goal for this functionality is to enable logic in the BEOL which requires both n- and p-type semiconductor materials. While there are a range of candidate n-type semiconductor materials such as InGaZnO compounds, p-type materials with sufficiently high hole mobility are challenging to discover. Furthermore, transistor OFF-current is becoming more important for low power operation. Electron mobility in n-type semiconductors relies on the overlap of spherical s-orbitals enabling conduction paths even in amorphous materials. On the contrary, the mechanism for hole-mobility in p-type materials requires alignment of the directional p-orbitals, and thus, high crystallinity is a requirement. OFF-current is determined by the density of secondary charge carriers, thus further restricting material candidates by necessitating a band structure which inhibits electron mobility. Oxides offer a strong potential materials class for BEOL logic due to their robust structure with tolerance to the thermal budgets involved in the processes required. While complex oxides offer greater tunability in terms of required properties.

This project has a focus on materials exploration and understanding using imec’s state-of-the-art Materials and Interfaces lab. Through the investigation of p-type complex oxides, the mechanisms for hole mobility will be understood. The roles of crystallinity, microstructure, composition and defects will all be explored in terms of the effect on the mobility as well as the materials tolerance to degradation of these factors. Thin films will be deposited by PVD sputtering or pulsed laser deposition (PLD) and characterized through a variety of structural and electrical measurement techniques. Process parameters will be explored in order to understand the effects of growth kinetics, and to work towards engineering the structure and defect structure for maximised performance. The fundamental understanding of this key material type gained here will not only provide scientific insights but will also pave the way for enabling exciting new technologies.