Ferroelectric HfO2 On Germanium Tunnel Junctions Towards Sub-Femto Joule Switching
The rapid increase in data generated by mobile devices and computers necessitates research and development of advanced solid-state data storage technologies. The ferroelectric effect, a switchable polarization charge, which occurs in certain materials combined together with an advanced memory device concept provides a promising approach to achieve a transformative improvement in non-volatile memory technologies. The discovery of ferroelectricity in lead-free hafnium dioxide thin films in 2011 offers a sustainable route for further scaling of the next generation of manufacturable, low-power, and high performance nonvolatile memory ferroelectric devices. In this project, an advanced device concept known as a ferroelectric tunnel junction (FTJ) will be investigated using the ferroelectric hafnium dioxide thin films. In a FTJ, the tunneling electroresistance is switched between a low and a high value by switching the polarization direction (internal electric field). Numerical studies indicate that the hafnium dioxide-based FTJ can yield substantial reductions in the amount of power required to store data. In a two-pronged approach, experiments and numerical modeling will be conducted to test the numerical models and to advance the fundamental understanding and state-of-the-art in nonvolatile FTJ memory technology using hafnium dioxide thin films.
The overriding goal of the project is to understand and tailor the atomic structure of the ferroelectric phase of hafnium dioxide in thin film capacitors, thereby enabling complementary metal-oxide-semiconductor (CMOS)-compatible FTJs. A comprehensive and methodological study evaluating the deposition and processing conditions of hafnium dioxide-based thin film ferroelectrics will provide critical insights about the effect of impurities, dopants, film growth and annealing temperatures, and interfacial stability that influence the ferroelectric properties. These studies will pave the way toward investigating the physical limits of ferroelectric hafnium dioxide-based films and the realization of CMOS-compatible and scalable FTJs. Recent studies on macroscopic variation of the dopants concentration as well as the atomic layering of mixed mono-layers of dopants in hafnium dioxide suggest extreme possibilities in fine-tuning ferroelectric properties of doped hafnium dioxide thin films. The discovery of CMOS compatible and scalable ferroelectric hafnium dioxide offers a unique opportunity to investigate the performance potential of CMOS compatible hafnium dioxide FTJ through carefully coupled theoretical and experimental investigation. Numerical studies indicate that ferroelectric hafnium dioxide FTJ on germanium substrate can approach sub-femto Joule/bit for a feature size of F=20nm. This ability to experimentally tune the characteristics of the ferroelectric hafnium dioxide thin films of varying thickness coupled with advanced ab initio material and device simulation enable this investigation of the performance limits of FTJ devices employing ferroelectric hafnium dioxide on germanium.