Effect of Charge Injection on the Switching Speed of Ferroelectric Memory Based on HfO₂

Abstract

Ferroelectric hafnium oxide films are attracting interest as a promising functional material for non-volatile ferroelectric memory due to a number of excellent advantages including perfect compatibility with Si technology, full scalability, low power consumption, high endurance, and a nanosecond switching speed. A high switching speed is inherent for all inorganic ferroelectrics, and it originates from the thermodynamics of polarization reversal. Another fundamental property of ferroelectric films is that the polarization switching speed depends on the electric field in the ferroelectric, as it is predicted by laws of polarization switching kinetics. Meanwhile, during the lifetime of a memory cell, the internal electric field changes due to the emergence of the built-in field associated with charge injection and charge accumulation in the nearby-electrode passive layer of the ferroelectric film. In this work, we demonstrate that the switching speed strongly depends on the entire prehistory of memory cells and may vary by several orders of magnitude. For this purpose, we study the switching kinetics in Hf0.5Zr0.5O2 thin films as a function of time, applied voltage, and temperature. Our experiment shows that the elevated operating temperature, as well as any time delays, and even write and read pulses themselves cause an apparent slowing down of the polarization switching in the ferroelectric film and a real decrease in the switching speed of ferroelectric memory. In particular, after long-term information storage, this effect can cause a readout failure in a memory chip designed for certain operating frequency. By means of theoretical simulations, we prove that the effect of degradation of the switching speed is caused by the charge injection induced by both the field of spontaneous polarization and the external electric field.