Quantitative investigation of polarization-dependent photocurrent in ferroelectric thin films

Abstract

Ferroelectric thin films are investigated for their potential in photovoltaic (PV) applications, owing to their high open-circuit voltage and switchable photovoltaic effect. The direction of the ferroelectric polarization can control the sign of the photocurrent through the ferroelectric layer, theoretically allowing for 100 percent switchability of the photocurrent with the polarization, which is particularly interesting for photo-ferroelectric memories. However, the quantitative relationship between photocurrent and polarization remains little studied. In this work, a careful investigation of the polarization-dependent photocurrent of epitaxial Pb(Zr,Ti)O3 thin films has been carried out, and has provided a quantitative determination of the unswitchable part of ferroelectric polarization. These results represent a systematic approach to study and optimize the switchability of photocurrent, and more broadly to get important insights on the ferroelectric behavior in all types of ferroelectric layers in which pinned polarization is difficult to investigate.

Optical reading of multistate nonvolatile oxide memories based on the switchable ferroelectric photovoltaic effect

“…The direction of the ferroelectric polarization can control the sign of both short-circuit photocurrent (Isc) and open-circuit voltage (Voc), theoretically allowing for 100% switchability of the PV characteristics. This polarizationdependence of the PV properties has been reported in many studies, from bulk ferroelectrics [11,12] to thin films [13][14][15][16][17][18][19][20]. As the signs of both Isc and Voc depend on the polarization direction, they can serve as read-out signals in a memory device.…”

“…By considering the depolarizing field Edep as the driving field for the photocurrent, following Ref. [20,28] Jsc can be expressed as Jsc = −(α σpv / ε) Pr with α the fraction of unscreened polarization, σpv the photoconductivity and ε the dielectric permittivity of the ferroelectric layer (ε = ε0 εr with ε0 the vacuum permittivity and εr the dielectric constant). Using this equation, a fraction of unscreened polarization α=3.5% is estimated from the slope of the Jsc-Pr dependence and the values of σpv (5.52 10 -8 S/m) and εr (211) measured for the device under illumination.…”

“…The ferroelectric response of the devices was studied using a TF Analyzer 1000 AixACCT in Positive-Up Negative-Down (PUND) mode, using triangular pulses with 2 ms width. Photovoltaic measurements (I(V), Isc(t), and Voc(t)) were performed using Keithley 2635 under UV illumination from the top, at 365 nm wavelength, corresponding to photon energy above the bandgap of PZT [20]. The UV illumination was provided by a diode with an incident fluence of 35 mW/cm².…”

“…In the initial state, the capacitors present an imprinted ferroelectric loop, probably caused by charged defects trapped at the FE/electrode top interface. Indeed, capacitors of SRO/PZT/Pt fabricated from epitaxial PZT films grown on STO/SRO with the same conditions do not show such imprint [20]. To reach optimal operation of the devices, 10 8 cycles of bipolar switching were applied with an amplitude of 2.2V.…”

A Van Der Waals Photo‐Ferroelectric Synapse

“…Through programming the ferroelectric polarization switching using electric writing and detecting I sc , we demonstrate the proposed synaptic plasticity photo‐ferroelectric, which is featured by a reasonable dynamic range of nonvolatile states, and large on/off ratio. [ 53 ] By implementing the experimentally measured synaptic characteristics into a crossbar circuit under a two‐layer artificial neural network architecture, we show the feasibility of training and inference of the model's hand‐written digital patterns.…”