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.

Polarization Switching Kinetics in Thin Ferroelectric HZO Films

“…The authors also concluded that at low external field E position of Lorentzian distribution satisfies the following relation: where α is the activation field, i.e., the electric field that would cause immediate polarization switching. In previous work [ 4 ], we indeed obtained a linear dependence of t Lorentz on voltage in the range of small voltages, which is consistent with the results obtained in [ 22 ]. However, as can be seen from the plot in Figure 2 d, the dependence obtained in this work for wider range of voltage is more similar to that obtained using the classical distribution function (2) for the NLS model [ 8 ].…”

“…An alternative explanation of retardation behavior may come from charge injection and charge accumulation in the nearby-electrode passive layer of the ferroelectric film. During the long set pulse, due to field-induced charge injection, the built-in field emerges and increases with time, which causes the offset of applied voltage V appl and decrease in the real voltage V f right during the time of switching [ 4 ]. To calculate the instant real voltage V f , we numerically solved the following system of Equations (2)–(4): …”

“…The amplitude and duration of the read pulses were chosen to ensure the complete switching of the polarization (3 V/10 µs). To avoid emergence of the built-in field, delays between all pulses were kept equal to 50 ns [ 4 ].…”

“…For a part of domains, the real field occurs smaller than the coercive voltage; these domains do not switch during the readout procedure after long-term information storage, which means polarization loss and could cause readout failure. Another undesirable consequence of the offset of the applied field is a decrease in the switching speed [ 4 ]. Indeed, as it is predicted by laws of polarization switching kinetics, the polarization switching speed depends on the electric field in the ferroelectric [ 5 , 6 , 7 , 8 , 9 ], and this is a fundamental property of any ferroelectric film.…”

“…On the other hand, the impact of several physical phenomena in HfO 2 on time dependence of polarization switching has been examined in the frame of the NLS model. In particular, it has been shown that it depends on local field inhomogeneity [ 17 ] and charge injection into the functional HZO layer [ 4 ]. For other (non-hafnia) polycrystalline ferroelectric films, by means of the statistical model of switching kinetics, it has been reported that the depolarization field also contributes to polarization switching [ 18 ].…”