Pulsed Current-Voltage Protocol to Reveal Polarization-Continuation in Ferroelectric Memory: Implications for Partial State Storage

Analog weight storage in polycrystalline hafnium zirconate, HfZrO4, (HZO) ferroelectric capacitors has been widely explored for use in two-terminal ferroelectric (Fe) memory [e.g. 1–3]. Read-out in commercial Fe random access memory (FRAM) is destructive; the polarization switching current is sensed to read the charge state, followed by a data restore [4]. It is desirable to realize a nondestructive resistive read, and in this paper we utilize pulsed measurements to obtain the transient current-voltage (I-V) characteristic at switching speeds commensurate with the memory. While HZO has been shown to switch at sub-ns speeds this short switching speed is achieved using pulse voltages more than 4 times the coercive voltage (8.7 V, 10ns, 10 nm HZO), i.e. with enough field strength and duration to fully polarize the Fe [5]. In contrast, at the lower voltages used to set the polarization to an intermediate value, the settling time becomes a function of the pulse amplitude and sequence. It has been shown that current transients associated with setting the partially polarized state in HZO can be surprisingly long, even up to 1 s, depending on the pulse sequence [6]. In this paper we present a new pulse measurement protocol applied to the characterization of the transient I-V at speeds specific to the application. This data reveals the current transient associated with polarization continuation after the slew interval and can be used to validate SPICE models at pulse speeds of interest to the design.