Theoretical Study On Hafnia-based Ferroelectric Memory

As nodes of semiconductor process continue to shrink,existing memory technologies will face its physical limits.There is an urgent need to develop new memory technologies which will drive memory to higher density,lower energy consumption,and higher speed.Ferroelectric memory has high potential as the next-generation nonvolatile memory.Traditional ferroelectric materials for memory applications are Pb-and Bi-based perovskites and layered perovskites.Commercial Ferroelectric memory dates back to the1990s,but Ferroelectric memory with 65 nm silicon-based technology or even smaller dimensions are still absent on the market.Several problems persist during the scaling-down of traditional Ferroelectric memory,such as the high crystallization temperature and degradation of ferroelectric properties with ultrathin films.The discovery of ferroelectricity of doped or undoped hafnia thin films has attracted the attention of researchers in the academic and industrial fields quickly.The hafnia-based memory is not only perfectly compatible with existing semiconductor processes,but also reaches its maximum spontaneous polarization at¹0 nm thickness,which will break the bottleneck of traditional ferroelectric memory.However,hafnia-based ferroelectric memories are still unclear in terms of stability in ferroelectric phase and breakdown in ferroelectric films.This paper will theoretically study the hafnia-based ferroelectric memory through the first principle calculations.In the third chapter of this paper,the stability of Hf_xZr_1-xO₂ ferroelectric memory is studied systematically.The optimal atomic structures of the materials are obtained by the analysis of the total energies.Then the thermodynamic calculations show that the Hf_x Zr_1-xO₂ ferroelectric film has a competitive relationship in monoclinic phase,tetragonal phase and ferroelectric phase.The ferroelectric phase is easy to form when Zr content is 50%in Hf_xZr_1-xO₂ ferroelectric film.After the existence of oxygen vacancies,the competition between ferroelectric phase and tetragonal phase becomes more intense.The ferroelectric phase will be easy to form when Zr content is less than 30%.The chapter provides a powerful explanation for the optimal composition of the Hf_xZr_1-xO₂ferroelectric memory.In the fourth chapter,the dielectric breakdown model of hafnia-based ferroelectric memory is proposed to explain the serious breakdown problem.A direct application of the model is to explain the distinct cycling endurance characteristics in TaN/HfO₂/TaN and TiN/HfO₂/TiN ferroelectric memory.The Schottky barriers of TiN/HfO₂/TiN and TaN/HfO₂/TaN devices are calculated by GGA-1/2 new algorithm.Therefore,using TiN rather TaN,the cycing endurance of hafnia-based ferroelectric memoey is improved.The chapter provides ways to solve dielectric breakdown problem in hafnia-based ferroelectric memory and will be ussful for guiding the design of hafnia-based ferroelectric memory.