Simultaneous dynamic characterization of charge and structural motion during ferroelectric switching

Monitoring structural changes in ferroelectric thin films during electric field induced polarization switching is important for a full microscopic understanding of the coupled motion of charges, atoms, and domain walls in ferroelectric nanostructures. We combine standard ferroelectric test sequences of switching and nonswitching electrical pulses with time-resolved x-ray diffraction to investigate the structural response of a nanoscale $\mathrm{Pb}\left({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}\right){\mathrm{O}}_3$ ferroelectric oxide capacitor upon charging, discharging, and polarization reversal. We observe that a nonlinear piezoelectric response of the ferroelectric layer develops on a much longer time scale than the $RC$ time constant of the device. The complex atomic motion during the ferroelectric polarization reversal starts with a contraction of the lattice, whereas the expansive piezoelectric response sets in after considerable charge flow due to the applied voltage pulses on the electrodes of the capacitor. Our simultaneous measurements on a working device elucidate and visualize the complex interplay of charge flow and structural motion and challenges theoretical modeling.

Figure 1

Device structure and characterization. (a) Sample structure showing the stacking sequence of the different layers, connections to the external bias, and direction of the x-ray beam. (b) Polarization $P$, electric field $E$ hysteresis loop, and switching current $j$ recorded at 4 kHz. (c) Typical RSM around the 002 Bragg reflections of PZT, ${\mathrm{SrRuO}}_3$, and ${\mathrm{SrTiO}}_3$ of the as-grown film and (d) line profile obtained from panel (c) at $q_{\parallel }=0$.

Figure 2

Device response to the applied rectangular PUND sequence. The red color represents positive bias and the blue color encodes negative bias. (a) Applied PUND voltage sequence $U\left(t\right)$. (b) Measured current response $j\left(t\right)$. (c) Corresponding charge $Q\left(t\right)$ obtained after integrating the current signal in (b) over time. (d) Out-of-plane lattice constant $c\left(t\right)$. (e) Integrated intensity $I\left(t\right)$. (f) Normalized FWHM of the out-of-plane Bragg peak ${\tilde{w}}_{\perp }\left(t\right)$. (g) Normalized FWHM of the corresponding scattering vector parallel to film surface ${\tilde{w}}_{\parallel }\left(t\right)$.

Figure 3

Switching dynamics of PZT due to a triangular voltage ramp. (a) First half of the applied voltage pulse pattern $U\left(t\right)$. (b) Out-of-plane lattice constant $c\left(t\right)$. (c) Normalized FWHM ${\tilde{w}}_{\perp }$ of the out-of-plane Bragg peak and (d) normalized FWHM ${\tilde{w}}_{\parallel }$ of the in-plane component.

Figure 4

Sequence of events during polarization reversal. (a) Current $j\left(t\right)$ (solid red line) as measured during the polarization reversal plotted together with the current $j_{\mathrm{FE}}\left(t\right)$ due to the FE switching. (b) The calculated charges $Q\left(t\right)$ (solid lines) and $Q_{\mathrm{FE}}\left(t\right)$ (symbols) associated to the FE switching. (c) The strain $S\left(t\right)$ (filled red circles) as measured compared to the strain due to the FE polarization reversal (open symbols). (d) Transient intensity reduction $\mathrm{\Delta }I\left(t\right)/I(t=0)$ and transient width increase $\mathrm{\Delta }{w}_{\parallel }\left(t\right)/w_{\parallel }(t=0)$. For the definition of the symbols, see main text. The dashed vertical line indicates ${\tau }_{1/2}$, the time after which 50% of the sample has been reversed. This time coincides with the observed maximum of $j_{\mathrm{FE}}$, the beginning of the rise of the expansive strain $S\left(t\right)$, the minimum of $S_{\mathrm{FE}}$, and the minimum of the transient intensity reduction. Note that the compensation time of the charges ${\tau }_{1/2}$ lags behind by ${\tau }_{RC}$.