Unravelling and controlling hidden imprint fields in ferroelectric capacitors

Magnetron-Sputtered Lead Titanate Thin Films for Pyroelectric Applications: Part 2-Electrical Characteristics and Characterization Methods

Pyroelectric materials are naturally electrically polarized and exhibits a built-in spontaneous polarization in their unit cell structure even in the absence of any externally applied electric field. These materials are regarded as one of the ideal detector elements for infrared applications because they have a fast response time and uniform sensitivity at room temperature across all wavelengths. Crystals of the perovskite lead titanate (PbTiO3) family show pyroelectric characteristics and undergo structural phase transitions. They have a high Curie temperature (the temperature at which the material changes from the ferroelectric (polar) to the paraelectric (nonpolar) phase), high pyroelectric coefficient, high spontaneous polarization, low dielectric constant, and constitute important component materials not only useful for infrared detection, but also with vast applications in electronic, optic, and MEMS devices. However, the preparation of large perfect and pure single crystals PbTiO3 is challenging. Additionally, difficulties arise in the application of such bulk crystals in terms of connection to processing circuits, large size, and high voltages required for their operation. In this part of the review paper, we explain the electrical behavior and characterization techniques commonly utilized to unravel the pyroelectric properties of lead titanate and its derivatives. Further, it explains how the material preparation techniques affect the electrical characteristics of resulting thin films. It also provides an in-depth discussion of the measurement of pyroelectric coefficients using different techniques.

Temperature Stable Piezoelectric Imprint of Epitaxial Grown PZT for Zero-Bias Driving MEMS Actuator Operation

In piezoelectric transducer applications, it is common to use a unipolar operation signal to avoid switching of the polarisation and the resulting nonlinearities of micro-electromechanical systems. However, semi-bipolar or bipolar operation signals have the advantages of less leakage current, lower power consumption and no additional need of a DC-DC converter for low AC driving voltages. This study investigates the potential of using piezoelectric layers with an imprint for stable bipolar operation on the basis of epitaxially grown lead zirconate titanate cantilevers with electrodes made of a metal and metal oxide stack. Due to the manufacturing process, the samples exhibit high crystallinity, rectangular shaped hysteresis and a high piezoelectric response. Furthermore, the piezoelectric layers have an imprint, indicating a strong built-in field, which shifts the polarisation versus electric field hysteresis. To obtain the stability of the imprint, laser doppler vibrometry and switching current measurements were performed at different temperatures, yielding a stable imprinted electric field of -1.83 MV/m up to at least 100 °C. The deflection of the cantilevers was measured with a constant AC driving voltage while varying the DC bias voltage to examine the influence of the imprint under operation, revealing that the same high deflection and low nonlinearities, quantified by the total harmonic distortion, can be maintained down to low bias voltages compared to unipolar operation. These findings demonstrate that a piezoelectric layer with a strong imprint makes it possible to operate with low DC or even zero DC bias, while still providing strong piezoelectric response and linear behaviour.

Non-volatile optical switch of resistance in photoferroelectric tunnel junctions

In the quest for energy efficient and fast memory elements, optically controlled ferroelectric memories are promising candidates. Here, we show that, by taking advantage of the imprint electric field existing in the nanometric BaTiO3 films and their photovoltaic response at visible light, the polarization of suitably written domains can be reversed under illumination. We exploit this effect to trigger and measure the associate change of resistance in tunnel devices. We show that engineering the device structure by inserting an auxiliary dielectric layer, the electroresistance increases by a factor near 2 × 103%, and a robust electric and optic cycling of the device can be obtained mimicking the operation of a memory device under dual control of light and electric fields.

Complementary Resistive Switching Using Metal-Ferroelectric-Metal Tunnel Junctions

Complementary resistive switching (CRS) devices are receiving attention because they can potentially solve the current-sneak and current-leakage problems of memory arrays based on resistive switching (RS) elements. It is shown here that a simple anti-serial connection of two ferroelectric tunnel junctions, based on BaTiO₃ , with symmetric top metallic electrodes and a common, floating bottom nanometric film electrode, constitute a CRS memory element. It allows nonvolatile storage of binary states ("1" = "HRS+LRS" and "0" = "LRS+HRS"), where HRS (LRS) indicate the high (low) resistance state of each ferroelectric tunnel junction. Remarkably, these states have an identical and large resistance in the remanent state, characteristic of CRS. Here, protocols for writing information are reported and it is shown that non-destructive or destructive reading schemes can be chosen by selecting the appropriate reading voltage amplitude. Moreover, this dual-tunnel device has a significantly lower power consumption than a single ferroelectric tunnel junction to perform writing/reading functions, as is experimentally demonstrated. These findings illustrate that the recent impressive development of ferroelectric tunnel junctions can be further exploited to contribute to solving critical bottlenecks in data storage and logic functions implemented using RS elements.

Control of the Polarization of Ferroelectric Capacitors by the Concurrent Action of Light and Adsorbates

Ferroelectric perovskites hold promise of enhanced photovoltaic efficiency and photocatalytic activity. Consequently, the photoresponse of oxide ferroelectric thin films is an active field of research. In electrode/ferroelectric/electrode devices, internal charge in the ferroelectric, free charge in the electrodes, and buried adsorbates at interfaces combine to screen the ferroelectric polarization and to stabilize the polar state. Under illumination, photoinduced carriers and photodissociated adsorbates may disrupt the screening equilibrium, modifying the switchable polarization and altering its expected benefits. Here, we explore the photoresponse of BaTiO₃ thin films in a capacitor geometry, focusing on the effects of visible illumination on the remanent polarization. By combining ferroelectric and X-ray photoelectron spectroscopy, we discover that photoreaction of charge-screening H₂O-derived adsorbates at the buried metal-ferroelectric Pt/BaTiO₃ interface plays an unexpected pivotal role, enabling a substantial modulation (up to 75%) of the switchable remanent polarization by light. These findings illustrate that the synergy between photochemistry and photovoltaic activity at the surface of a ferroelectric material can be exploited to tune photoferroelectric activity.

Tailoring Lattice Strain and Ferroelectric Polarization of Epitaxial BaTiO3 Thin Films on Si(001)

Ferroelectric BaTiO₃ films with large polarization have been integrated with Si(001) by pulsed laser deposition. High quality c-oriented epitaxial films are obtained in a substrate temperature range of about 300 °C wide. The deposition temperature critically affects the growth kinetics and thermodynamics balance, resulting on a high impact in the strain of the BaTiO₃ polar axis, which can exceed 2% in films thicker than 100 nm. The ferroelectric polarization scales with the strain and therefore deposition temperature can be used as an efficient tool to tailor ferroelectric polarization. The developed strategy overcomes the main limitations of the conventional strain engineering methodologies based on substrate selection: it can be applied to films on specific substrates including Si(001) and perovskites, and it is not restricted to ultrathin films.