Degradation of the switching speed of HfO₂-based ferroelectric memory during its lifetime

Commercialization of nonvolatile ferroelectric memory based on thin HfO₂ films is mainly hindered by its limited retention time. Physical mechanism behind this challenge is injection and accumulation of non-ferroelectric charge in the nearby-electrode passive layer of the ferroelectric. In this talk, we show that this parasitic effect is the root of the limitation of another important performance of ferroelectric HfO₂-based memory that has not been raised before. This performance is the switching time.

It is known that in a ferroelectric memory, the write and read procedures can take only a few nanosecond or tens of nanoseconds, and this time determines the operation frequency of the memory chip. We experimentally demonstrate that upon long-term storage of ferroelectric HfO₂-based memory, the switching time could increase by several orders of magnitude. This effect can cause a readout failure in a memory chip designed for a certain operating frequency. Physical origin of this challenge consists in the offset of the applied field by the field of the injected charge and the subsequent decrease in the real field in HfO₂ upon the readout. Due to the laws of polarization switching kinetics, smaller switching electric field causes slower switching.

In addition, we demonstrate that the switching speed depends on the entire pre-history of the memory cell, including the write procedure, information storage and operating temperatures. Furthermore, the switching speed can change right during the readout procedure. By theoretical simulations, we calculate the evolution of switching speed with storage time and prove that the effect of degradation of the switching speed originates from charge injection into functional layer, i.e., from the same physical phenomenon as that responsible for poor retention. Therefore, the effect of the deteriorated switching speed and limited retention time in the ferroelectric HfO₂ are two sides of the same phenomenon.