A. Q. Jiang

Nanosecond-range measurements of imprint effect for Pt∕IrO2∕Pb(Zr0.4Ti0.6)O3∕IrO2∕Pt thin-film capacitors

The determination of the coercive voltage from the height of domain-switching current plateau in ferroelectric thin films provides the liability to estimate the coercive-voltage shift with imprint time shortly on the order of polarization-reversal time. The voltage shift exhibits a linear time dependence in a logarithmic scale above an initial time (∼1μs), below which the voltage is nearly constant. The modeling of imprint on the basis of the interfacial charge injection at different stressing voltages strongly supports Schottky emission as a dominant mechanism, instead of Frenkel-Poole emission and Fowler-Nordheim tunneling.

Temporary formation of highly conducting domain walls for non-destructive read-out of ferroelectric domain-wall resistance switching memories

Erasable conductive domain walls in insulating ferroelectric thin films can be used for non-destructive electrical read-out of the polarization states in ferroelectric memories. Still, the domain-wall currents extracted by these devices have not yet reached the intensity and stability required to drive read-out circuits operating at high speeds. This study demonstrated non-destructive read-out of digital data stored using specific domain-wall configurations in epitaxial BiFeO3 thin films formed in mesa-geometry structures. Partially switched domains, which enable the formation of conductive walls during the read operation, spontaneously retract when the read voltage is removed, reducing the accumulation of mobile defects at the domain walls and potentially improving the device stability. Three-terminal memory devices produced 14 nA read currents at an operating voltage of 5 V, and operated up to T = 85 °C. The gap length can also be smaller than the film thickness, allowing the realization of ferroelectric memories with device dimensions far below 100 nm.

Accelerated domain switching speed in single-crystal LiNbO3 thin films

By using ionic implantation and wafer bonding technologies, we peeled off a single-crystal LiNbO3 thin film in the atomic-layer smoothness from the surface of a bulk Z-cut LiNbO3 single crystal. X-ray diffraction patterns showed only (00l) orientation of the film. From positive-up-negative–down pulse characterization, we measured domain switching current transients under various short-pulse voltages, where we observed domain switching currents to occur separately at time after initial capacitor charging currents. This is similar to early observations in bulk ferroelectric single crystals, where apparent positive/negative coercive fields of domain switching determined from polarization-electric field hysteresis loops always equal maximum/minimum applied fields. However, after pulse stressing of the film for more than 1000 cycles, the domain switching speed is accelerated, where domain switching current overlaps with the initial capacitor charging current with a well-defined coercive field independent of the applied-field strength. Finally, we simulated the whole domain switching current transients with the assumption of the resistance degradation across interfacial passive layers between the film and electrodes.

Nanosecond-range imprint and retention characterized from polarization-voltage hysteresis loops in insulating or leaky ferroelectric thin films

We transferred ferroelectric domain switching currents under pulses into polarization-voltage (P-V) hysteresis loops. With this transformation, it is possible to derive the remanent polarization and coercive voltage from domain switching currents after the shortest imprint and retention time of 35 ns. After the separation of film leakage current from domain switching current, we measured the P-V hysteresis loop in a semiconducting BiFeO3 leaky thin film, where the apparent coercive field highly reaches 320 kV/cm2, suggestive of a different domain switching mechanism from other insulators. This technique facilitates nanosecond-range measurements of both ferroelectric capacitive and resistive memories.

Reversible charge injection in antiferroelectric thin films

High-energy storage antiferroelectric capacitors operated in a high speed require the quick release of stored charges after the removal of the electrical field accompanying ferroelectric-to-antiferroelectric phase transition. However, the phase-transition time can vary from a few nanoseconds to milliseconds due to the reversible charge injection into the film to temporally stabilize the high-field ferroelectric phase. The consequent theoretical modeling discloses the nearly Ohmic contact of an antiferroelectric Au/Cr/Pb(Zr0.95Ti0.05)O3/Pt thin-film capacitor for the charge injection unlike the Schottky emission of a typical ferroelectric capacitor.

An analysis of imprinted hysteresis loops for a ferroelectric Pb(Zr,Ti)O3 thin film capacitor using the switching transient current measurements

This study examined the imprint mechanism of a ferroelectric Pt/Pb(Zr,Ti)O3(150-nm-thick)/Pt capacitor using pulse switching transient current measurements. The progression of the imprint was well explained by the propagation of a localized charge injection area, where there was an increase in coercive voltage and interfacial capacitance over the entire capacitor area. The as-received samples exhibited uniform interfacial capacitance over the total area. Charge injection resulted in a more rectified remanent polarization–applied voltage relationship compared with the as-received sample. Analytic functional forms for the switching charge and local switching area were also derived.

Remanent polarization reduction with enhanced temperature in ferroelectric thin films

The remanent polarization reduces monotonically with the rising temperature from 295to430K in Pt∕IrO2∕Pb(Zr0.4Ti0.6)O3∕IrO2∕Pt thin-film capacitors, although the saturation polarization of the films is nearly constant in the temperature range. Fast-pulse measurements of polarization retention shortened on the order of domain switching time indicate a rapid increase in the remanent polarization, which suggests backswitching of partial domains after the applied field with a long relaxation time. The qualitative modeling of domain switching currents predicts the presence of interfacial passive layers near top and bottom electrodes with damaged ferroelectricity, and the density of backswitched domains enhances with the elevated temperature along with reductions in both coercive voltage and interfacial capacitance, whereas the whole dielectric capacitance of the films increases abnormally, in agreement with the argument of the reversible domain contribution to the large dielectric permittivity in ferroelectric...

Congruent charge-injection spectrum from independent measurements of fatigue and imprint in ferroelectric thin films

The fundamental physics of fatigue and imprint in ferroelectric thin films is pertinent to the by-electrode charge injection. The detectable charge injection depends on the applied voltage and time. However, a broad-time description of the charge injection is still lacking. It is not clear if the charge-injection spectra are derivable and comparable from two independent measurements. In our works, we obtained two charge-injection spectra from discrete fatigue and imprint measurements with a time span of more than eight orders of magnitude. The charge-injection spectra are in agreement with the direct measurements of charge-injection current transients under/after different biasing voltages. All results are congruently described by a series of analytical equations derived from the assumption of interfacial Schottky emission for the films with the presence of interfacial passive layers. The initial time below which the charge injection is minor is found to be of the order of 2.5 μs under −6.0 V on the top e...

Giant Dielectric Permittivity in Ferroelectric Thin Films: Domain Wall Ping Pong.

The dielectric permittivity in ferroelectric thin films is generally orders of magnitude smaller than in their bulk. Here, we discover a way of increasing dielectric constants in ferroelectric thin films by ca. 500% by synchronizing the pulsed switching fields with the intrinsic switching time (nucleation of domain plus forward growth from cathode to anode). In a 170-nm lead zirconate titanate thin film with an average grain size of 850 nm this produces a dielectric constant of 8200 with the maximum nucleus density of 3.8 μm−2, which is one to three orders of magnitude higher than in other dielectric thin films. This permits smaller capacitors in memory devices and is a step forward in making ferroelectric domain-engineered nano-electronics.

Polarization switching and discharging behaviors in serially connected ferroelectric Pt/Pb(Zr,Ti)O3/Pt and paraelectric capacitors

The polarization switching behavior in serially connected ferroelectric Pt/Pb(Zr,Ti)O3/Pt and paraelectric capacitors with various capacitances was examined using transient switching current measurements. Provided that a high enough voltage to switch the ferroelectric capacitor was applied to the series capacitor, stable polarization switching was maintained before the applied voltage approached zero, irrespective of the capacitance of the paraelectric capacitor. For the series capacitor containing a small paraelectric capacitance, the voltage across the paraelectric capacitor was much higher than the coercive voltage of the PZT capacitor, which acted as a field depolarizing PZT capacitor resulting in back-switching when the applied voltage was removed. In contrast, no back-switching occurred in the series capacitor containing a sufficiently large paraelectric capacitance because the voltage across the paraelectric capacitor was smaller than the coercive voltage of the ferroelectric capacitor. The effects...