Memcomputing and Nondestructive Reading in Functional Ferroelectric Heterostructures

Georgia A. Boni, Lucian D. Filip, Cristina Chirila, Alin Iuga, Iuliana Pasuk, Luminita Hrib, Lucian Trupina, Ioana Pintilie, and Lucian Pintilie

Phys. Rev. Applied 12, 024053 – Published 26 August 2019

Abstract

Multiple nonvolatile and well-separated capacitive states can be obtained in a two-terminal ferroelectric capacitor setup by fine tuning the polarization switching process. This approach allows for the implementation of memcomputing (same platform for storage and computing) capable ferroelectric structures. Digital and analog storage modes are exemplified in this work together with an algorithm for simple binary computation functions such as or/nor and and/nand for data processing on the same device. Results are obtained by controlling the polarization switching process in ferroelectric multilayers such as $Pb ({Zr}_{0.2} {Ti}_{0.8}) O_{3} / {Sr Ti O}_{3} / Pb ({Zr}_{0.2} {Ti}_{0.8}) O_{3}$ and $Pb ({Zr}_{0.2} {Ti}_{0.8}) O_{3} / {Ba Ti O}_{3} / Pb ({Zr}_{0.2} {Ti}_{0.8}) O_{3}$ . Besides memcomputing, these results can be used for nondestructive capacitive reading of information in simple ferroelectric capacitors or can open the way toward applications such as neuromorphic and chaotic circuits.

Received 27 March 2019
Revised 24 July 2019

DOI:https://doi.org/10.1103/PhysRevApplied.12.024053

Physics Subject Headings (PhySH)

Dielectric properties Electric polarization

Artificial neural networks Capacitors Ferroelectrics Multilayer thin films

Landau-Ginzburg theory Piezoresponse force microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Georgia A. Boni, Lucian D. Filip ^*, Cristina Chirila, Alin Iuga, Iuliana Pasuk, Luminita Hrib, Lucian Trupina, Ioana Pintilie, and Lucian Pintilie

National Institute of Materials Physics, Atomistilor str. 405A, PO Box MG7, Magurele, Ilfov 077125, Romania

^*lucian.filip@infim.ro

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Vol. 12, Iss. 2 — August 2019

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Images

Figure 1
The theoretical description of stable ferroelectric states and the switching between them based on the LGD model. (a) The free-energy landscape (energy vs polarization of each ferroelectric layer) of an $F$ - $I$ - $F$ structure in equilibrium. (b) The positive polarization hysteresis branch of an $F$ - $I$ - $F$ heterostructure and the schematic representation of the polarization orientation in each ferroelectric layer for different states.
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Figure 2
Two different capacitance states. (a) The hysteresis loops obtained for a complete switching cycle for the case of a BTO interlayer exhibiting four switching peaks in current-voltage characteristics accompanied by a steplike increase in the polarization loop; the extreme peaks situated at higher voltages correspond to a totally reversed polarization, where the ferroelectric polarization is either oriented up, toward the top interface in both ferroelectric layers, for negative voltages, or down, toward the bottom electrode in both ferroelectric layers, for positive voltages; the intermediate peaks are obtained when the applied voltage changes its polarity and indicates a partial reversal of polarization in only one ferroelectric layer, generating either a head-to-head or a tail-to-tail configuration between the polarizations of the two ferroelectric layers. (b) The capacitance vs frequency, using $0.2$ - $V$ ac signal, measured after setting HCS and LCS states, respectively. (c) Voltage pulse sequence, combining high-amplitude and low-amplitude pulses with $0.1 s$ duration, used to repeatedly change the capacitance of the system between LCS and HCS; the capacitance values are measured at 1 kHz frequency and with 0.5-V ac signal. (d) RPC ratios for the BTO and STO interlayer cases, respectively, for different frequencies and ac signal amplitudes. (e) evolution of the two distinct capacitance states, during the 104 s time period, measured at 1 kHz frequency and 0.2 V amplitude of the ac signal. (f) Distribution of capacitance values for HCS and LCS, measured at 1 kHz and 0.5-V ac signal, and distribution of the voltage threshold defined as the voltage necessary for switching from LCS to HCS for 15 different contacts.
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Figure 3
Schematic representation of the switching possibilities between the four polarization states with the two different capacitive values using corresponding signals. The red and blue triangular pulses are the low- and high-amplitude voltage pulses, respectively.
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Figure 4
Logic operation using an $F$ - $I$ - $F$ capacitor. The representation of the polarization states in an $F$ - $I$ - $F$ structure during different stages (initialization and computation) of a logic operation for the or/nor case (a) and for the and/nand case (b), together with the corresponding simulations of the logic operations obtained by changing the capacitance state (HCS or LCS) of the system using different combinations of pulses. The HCS and LCS states can have 0 or 1 values associated for logic operations and the results are memorized on the computation cell and can be accessed at any time.
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Figure 5
Multiple stable states with continuous capacitive values. (a) A continuous spectrum of capacitance values, with stable intermediate states measured for the STO interlayer case at 1 kHz frequency with 0.5-V ac signal; insets show schematic illustrations of polarization configurations associated with distinct capacitive states. (b) An example of a voltage sequence combining pulses with different amplitudes and polarities used to access different capacitive states. (c) The piezoresponse phase signal obtained using the poling map: the upper PZT layer present totally reverses polarization toward the surface for negative applied bias (bright central rectangular zone) while for positive bias the polarization remains partially reversed, forming with $180^{\circ}$ domain structure. (d) The piezoresponse phase signal obtained by applying the poling map with a voltage gradient on the totally reversed polarization area from (c): the switching of polarization takes place gradually with increasing the amplitude of voltage; different degrees of partial switching of polarization are obtained in the 8–37 V range.
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