Chapter 8 - MOx in ferroelectric memories

Abstract

Ferroelectricity is a material property that can be used for binary information storage and as such is promising for adoption in non-volatile memories. Ferroelectric materials feature two non-zero spontaneous polarization states that can be reversed by application of an external electrical field. Already in 1952 first realizations of ferroelectric memories based on barium titanate crystals were investigated. However, the difficulty of fabricating ferroelectric layers based on the complex materials and their incompatibility with complementary metal oxide semiconductor (CMOS) manufacturing processes has hindered rapid scaling. The recent discovery of ferroelectricity in thin hafnium oxide films in 2011 has led to a resurgence of interest in ferroelectric memory devices. Hafnium oxide is a standard material available in CMOS processes. It can be deposited with matured atomic layer deposition (ALD) techniques or physical vapor deposition (PVD) and as such is compatible with three-dimensional capacitors: Hence is has the potential to solve the scaling limitations in 1-transistor-1-capacitor (1T-1C) ferroelectric random-access memories (FeRAMs). Moreover, the low permittivity and high coercive field enables the adoption of the material for the realization of ferroelectric field-effect-transistors (FeFETs) that appear as an alternative for embedded charge trapping flash devices in particular. Moreover, the negative differential capacitance effects associated with ferroelectric polarization switching elevate the negative-capacitance-transistors (NCFETs) as potential candidates to solve the so-called Boltzmann-tyranny. This section summarizes the current status of ferroelectric memories and focusses on the recent findings on ferroelectricity in hafnia.