Novel information processing techniques are being actively explored to overcome fundamental limitations associated with CMOS scaling. A new paradigm of adaptive electronic devices is emerging that may reshape the frontiers of electronics and enable new modalities. Creating systems that can learn and adapt to various inputs has generally been a complex algorithm problem in information science, albeit with wide-ranging and powerful applications from medical diagnosis to control systems. Recent work in oxide electronics suggests that it may be plausible to implement such systems at the device level, thereby drastically increasing computational density and power efficiency and expanding the potential for electronics beyond Boolean computation. Intriguing possibilities of adaptive electronics include fabrication of devices that mimic human brain functionality: the strengthening and weakening of synapses emulated by electrically, magnetically, thermally, or optically tunable properties of materials.In this review, we detail materials and device physics studies on functional metal oxides that may be utilized for adaptive electronics. It has been shown that properties, such as resistivity, polarization, and magnetization, of many oxides can be modified electrically in a non-volatile manner, suggesting that these materials respond to electrical stimulus similarly as a neural synapse. We discuss what device characteristics will likely be relevant for integration into adaptive platforms and then survey a variety of oxides with respect to these properties, such as, but not limited to, TaOx, SrTiO3, and Bi4-xLaxTi3O12. The physical mechanisms in each case are detailed and analyzed within the framework of adaptive electronics. We then review theoretically formulated and current experimentally realized adaptive devices with functional oxides, such as self-programmable logic and neuromorphic circuits. Finally, we speculate on what advances in materials physics and engineering may be needed to realize the full potential of adaptive oxide electronics.

Adaptive oxide electronics: A review

Neural interfaces connect signal processing electronics to the nervous system via implanted microelectrode arrays such as the Utah electrode array (UEA). The active sites of the UEA are coated with thin films of either platinum (Pt) or iridium oxide (IrOx). Pt and IrOx have attracted attention as a stimulating or recording material due to their ability to transfer between ionic and electronic current and to resist corrosion. The physical, mechanical, chemical, electrical and optical properties of thin films depend on the method and deposition parameters used to deposit the films. In this work, surface morphology, impedance and charge capacity of Pt and sputtered iridium oxide film (SIROF) were investigated and compared with each other. UEAs with similar electrode area and shape were employed in this study. DC sputtering was used to deposit Pt films and pulsed-dc reactive sputtering was used to deposit SIROF. The electrodes coated with SIROF and Pt were characterized by scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy and potential transient measurements to measure charge injection capacity (CIC). SIROF and Pt selectively deposited on the electrode tip had dendrite and granular microstructure, respectively. The CIC of unbiased SIROF and Pt was 2 and 0.3 mC cm(-2), respectively. The average impedance at 1 kHz, of SIROF and Pt electrodes, was 6 kOmega and 125 kOmega, respectively. Low impedance and high CIC make SIROF promising stimulation/recording material for neural prosthetic applications.

https://iopscience.iop.org/article/10.1088/1748-6041/5/1/015007/pdf

In vitro comparison of sputtered iridium oxide and platinum-coated neural implantable microelectrode arrays.

Ferroelectric materials are well-suited for a variety of applications because they can offer a combination of high performance and scaled integration. Examples of note include piezoelectrics to transform between electrical and mechanical energies, capacitors used to store charge, electro-optic devices, and nonvolatile memory storage. Accordingly, they are widely used as sensors, actuators, energy storage, and memory components, ultrasonic devices, and in consumer electronics products. Because these functional properties arise from a noncentrosymmetric crystal structure with spontaneous strain and a permanent electric dipole, the properties depend upon physical and electrical boundary conditions, and consequently, physical dimension. The change in properties with decreasing physical dimension is commonly referred to as a size effect. In thin films, size effects are widely observed, whereas in bulk ceramics, changes in properties from the values of large-grained specimens is most notable in samples with grain sizes below several micrometers. It is important to note that ferroelectricity typically persists to length scales of about 10 nm, but below this point is often absent. Despite the stability of ferroelectricity for dimensions greater than ~10 nm, the dielectric and piezoelectric coefficients of scaled ferroelectrics are suppressed relative to their bulk counterparts, in some cases by changes up to 80%. The loss of extrinsic contributions (domain and phase boundary motion) to the electromechanical response accounts for much of this suppression. In this article, the current understanding of the underlying mechanisms for this behavior in perovskite ferroelectrics is reviewed. We focus on the intrinsic limits of ferroelectric response, the roles of electrical and mechanical boundary conditions, grain size and thickness effects, and extraneous effects related to processing. In many cases, multiple mechanisms combine to produce the observed scaling effects.

/pdf/scaling-effects-in-perovskite-ferroelectrics-fundamental-qcglgkpx94.pdf

Scaling Effects in Perovskite Ferroelectrics: Fundamental Limits and Process‐Structure‐Property Relations

A comprehensive study of: leakage conduction in Pb(ZrxTi1-x)O-3 films with Pt electrodes is carried out. The conduction properties of films prepared in different ways (sol-gel coating, metalorganic chemical vapor deposition, sputtering) were studied by using different experimental techniques including variation of the applied voltage profile, photoassisted measurements, measurements at elevated temperatures, and variation of the prehistory of the samples. Based on the collective data, it is concluded that two different regimes of carrier injection (with the critical electric field of the crossover between these regimes being independent of the measuring technique) are responsible for true leakage conduction in Pt-Pb(ZrxTi1-x)O-3-Pt films. A space-charge influenced-injection model is proposed for the interpretation of the experimental results obtained. This model describes well the main features of the observed current-voltage characteristics and provides a reasonable fit for the current-voltage curves measured at elevated temperatures, the values of the fitting parameters being in good agreement with the results of other studies. The true leakage current in the Pt-Pb(ZrxTi1-x)O-3-Pt system is shown to be time dependent because of the influence of the injected charge entrapment during measurement. According to the present results, an activation energy of about 0.9 eV describes the temperature dependence of conduction in the range of 70-200 degrees C. It is shown that a possible origin of the crossover in the activation energy at the temperature range 120-140 degrees C widely reported in the Literature can be a consequence of the measuring procedure. (C) 1998 American Institute of Physics.

Space-charge influenced-injection model for conduction in Pb(ZrxTi1−x)O3 thin films

Photoferroelectrics offer unique opportunities to explore light energy conversion based on their polarization-driven carrier separation and above-bandgap voltages. The problem associated with the wide bandgap of ferroelectric oxides, i.e., the vanishingly small photoresponse under visible light, has been overcome partly by bandgap tuning, but the narrowing of the bandgap is, in principle, accompanied by a substantial loss of ferroelectric polarization. In this article, we report an approach, ‘gap-state’ engineering, to produce photoferroelectrics, in which defect states within the bandgap act as a scaffold for photogeneration. Our first-principles calculations and single-domain thin-film experiments of BiFeO3 demonstrate that gap states half-filled with electrons can enhance not only photocurrents but also photovoltages over a broad photon-energy range that is different from intermediate bands in present semiconductor-based solar cells. Our approach opens a promising route to the material design of visible-light-active ferroelectrics without sacrificing spontaneous polarization. Overcoming the optical transparency of wide bandgap of ferroelectric oxides by narrowing its bandgap tends to result in a loss of polarization. By utilizing defect states within the bandgap, Matsuo et al. report visible-light-active ferroelectrics without sacrificing polarization.

Gap-state engineering of visible-light-active ferroelectrics for photovoltaic applications.

Optical properties of RF sputtered strontium substituted barium titanate thin films

Polycrystalline Ba1−xSrxTiO3 (BST) thin films with three different compositions have been deposited by radio-frequency magnetron sputtering technique on platinum coated silicon substrates. Samples with buffer and barrier layers for different film thicknesses and processing temperatures have been studied. Crystallite size of BST films has been found to increase with increasing substrate temperature. Thickness dependent dielectric constant has been studied and discussed in the light of an interfacial dead layer and the finite screening length of the electrode. Ferroelectric properties of the films have also been studied for various deposition conditions. The electrical resistivity of the films measured at different temperatures shows a positive temperature coefficient of resistance under a constant bias voltage.

Thickness and temperature dependent electrical characteristics of crystalline BaxSr1−xTiO3 thin films

Radio frequency magnetron sputtered Ba0.8Sr0.2TiO3 thin films have been deposited on silicon and Si/SiO2/SiN/Pt substrates. The analysis of plasma discharge has been carried out using the Langmuir probe technique. Both the pressure and power have been found to influence the ion density and self-bias of the target. Introduction of oxygen into the discharge effectively decreases the ion density. The structural and electrical properties have been investigated using x-ray diffraction, atomic force microscopy of deposited films and capacitance–voltage, conductance–voltage, and current density–electric field characteristics of fabricated capacitors. The growth and orientation of the films have been found to depend upon the type of substrates and deposition temperatures. The 〈100〉 texture in the film is promoted at a pressure 0.25 Torr with a moderately high value of ion density and low ion bombardment energy. Films deposited on Si/SiO2/SiN/Pt substrate have shown higher dielectric constant (191) and lower leaka...

Relationship between plasma parameters and film microstructure in radio frequency magnetron sputter deposition of barium strontium titanate

A simple single‐source electron‐beam evaporation technique has been used for the deposition of lead‐lanthanum‐zirconate‐titanate (PLZT) thin films for silicon‐based device applications. An optimized annealing condition has been established for the formation of crystalline perovskite phases. The effect of the bottom electrodes and the barrier layer on the growth of the films has been studied. Films with good dielectric and optical properties have been obtained under opt‐ imized conditions. Electrical properties of the films have been evaluated using metal–insulator–semiconductor and metal–insulator–metal structures. A moderately low interface trap density and very low leakage current density demonstrate the potential of the deposited films for device applications.

Electron beam deposited lead‐lanthanum‐zirconate‐titanate thin films for silicon based device applications

Lead–lanthanum–zirconate–titanate (PLZT) multilayer capacitors involving platinum bottom electrodes have been fabricated on a silicon nitride/silicon (SiN/Si) substrate system. Leakage current characteristics show strong dependence on the processing temperature of the bottom electrode. A drop in leakage current by five orders of magnitude has been observed for capacitor with platinum electrode deposited at room temperature. Scanning tunneling microscopy (STM) studies reveal significant differences in the microstructure of platinum films deposited at different substrate temperatures. Based on STM results, a correlation between the microstructure of the bottom electrode, space layer at PLZT/Pt interface, and the nucleation of PLZT has been suggested to explain this new observation.

B. Panda

Papers

Optical properties of RF sputtered strontium substituted barium titanate thin films

Thickness and temperature dependent electrical characteristics of crystalline BaxSr1−xTiO3 thin films

Relationship between plasma parameters and film microstructure in radio frequency magnetron sputter deposition of barium strontium titanate

Electron beam deposited lead‐lanthanum‐zirconate‐titanate thin films for silicon based device applications

Effect of electrode microstructure on leakage current in lead-lanthanum-zirconate-titanate multilayer capacitors