the possible use of electrostrictive materials for information processing devices has been widely discussed because it could allow low-power logic operation by overcoming the fundamental limit of subthreshold swing greater than 60 mV/decade in conventional MOSFETs. However, existing proposals for electrostrictive fet applications typically adopt approaches that are entirely theoretical and simulative, thus lacking practical insights into how an electrostrictive material can be best interfaced with a channel material. Here we propose an electrostrictive FET device, involving the epitaxial oxide heterostructure as an ideal material platform for maximum strain transfer. The ON/ off switching occurs due to a stress-induced concentration change of oxygen vacancies in the memristive oxide channel layer. Based on finite-element simulations, we show that the application of a minimal gate voltage bias can induce stress in the channel layer as high as 10 8 N/m 2 owing to the epitaxial interface between the electrostrictive and memristive oxide layers. conductive AfM experiments further support the feasibility of the proposed device by demonstrating the stressinduced conductivity modulation of a perovskite oxide thin film, SrTiO 3 , that is well known to serve as the substrate for epitaxial growth of other functional oxide layers.

Perovskite oxide thin films is an attractive material group due to their unique multi-functionalities. To broaden the impact of perovskite oxides to practical electronic device applications, their electrical properties will need to be tunable by external means. In this work, we show that electrical conductivity of perovskite STO (SrTiO3) thin films can be largely tuned by mechanical stress (electromechanical coupling) and thermal annealing (electrothermal coupling). The conductive atomic force microscope setup was carefully calibrated to enable precise measurements of electrical response upon application of varying forces, resulting in a high electromechanical coupling sensitivity of up to ~1,000 Sm-1MPa-1. The thermal annealing study also suggests that electrical conductivity of STO is strongly affected by the ambient temperature due to the varying amount of oxygen vacancies.

In this work, a novel interface engineering method is proposed to address the relatively large cycle-to-cycle variability of the emerging metal-oxide resistive random access memory (RRAM) device technology. This is achieved by synthesizing the solution-processable graphitic nanosheet (reduced graphene oxide, rGO) with defects of a controllable amount and further integrating it into RRAM as an oxygen exchange layer (OEL). It is demonstrated that rGO-inserted RRAM exhibits reduced cycle-to-cycle variability in the SET switching as compared with one that has a conventional transition metal thin film as OEL. This is best attributed to the fact that our rGO thin film provides nearly the same amount of oxidation-prone atomic sites for each programming cycle. This study is expected to greatly advance the RRAM-based neuromorphic computing by paving a practically viable route to enhance the accuracy of the deep learning model.

Objectives: Oxides hold the great promise for emerging electronic applications (other than conventional CMOS components such as gate oxides) because they can significantly increase the operational range of temperature and bias voltage due to the much higher bandgap energy than semiconductors. However, using the oxides for such novel purposes requires their electronic properties to be properly tuned and tailored to specific device applications. Therefore, this work seeks to develop the way to address this challenge by focusing on the tunable characteristics of perovskite oxides driven by their unique electromechanical and electrothermal couplings. Among various types of oxides, crystalline perovskite titanate structures such as STO (SrTiO3) have been chosen as the representative material platform in this study, due to their multifunctional properties and well-established growth techniques. New Results: Previously, researchers have demonstrated the heat-induced tunable electrical characteristics of STO doped with various impurities [1-2]. Due to the complex chemistry where both the thermal and doping effects contribute to the observed change in electrical properties, accurate physical origin of the STO’s tunable behavior has not been clearly identified. In this work, we focused on an undoped, 1 mm-thick STO crystal to quantitatively measure its response when solely exposed to an external stimulus such as mechanical pressure. Fig. 1 depicts the C-AFM (conductive atomic force microscope) experimental setup to study the tunable behavior of STO driven by an electromechanical coupling. The commercially available conductive tip (Asyelec.01-R2) was used to apply a bias voltage (Vbias = 2V) while in contact with the top surface of the STO sample. This enables simultaneous measurement of electrical response during the experiment. The force was applied and varied by changing the set-point voltage (Vsetpoint, kept below 3V to prevent any possible damage to the measurement system) that is used to make the cantilever tip bent with the varying height. Conversion from the control parameter (Vsetpoint) to the mechanical pressure (N/m2) was conducted through precise calibration of the C-AFM measurement setup, including spring constant (0.92 N/m) and deflection sensitivity (0.0576 mm/V) (Table 1). Fig. 2 shows the measured electrical conductivity vs. applied pressure plot for the undoped STO sample used in the experiment. To ensure statistically meaningful data, each experiment was performed repetitively (at least 10 times) and both the mean (data points) and standard deviation (error bars) values were presented in the figure. A clear trend of increase in electrical conductivity with increasing pressure was observed, which is best attributed to creation of oxygen vacancies by mechanical stress in STO [3]. This will surely help researchers study the fundamental electromechanical coupling-driven behavior of perovskite oxide thin films. Fig. 3 further presents the C-AFM image when pressure was applied to the STO surface (the white particles are believed to be due to pile-up of electronic charges upon the application of localized electrical bias). We next studied the electrothermal coupling-driven conductance change in STO by performing a systematic study of thermal annealing (at temperatures ranging from 200°C to 800°C and in dry air). For this study, a sufficient amount of oxygen vacancies was purposely introduced when the STO thin film was prepared (see Fig. 4 for the device schematic). Because subsequent annealing annihilated oxygen vacancies, decrease in electrical conductivity was observed in STO samples annealed at high temperatures (Fig. 5). The reasoning of relating the observed trend in Fig. 5 to the change in the number of oxygen vacancies upon thermal annealing is well supported by measuring the hysteretic behavior of the STO thin film. Fig. 6 shows a measured butterfly curve that is typical for memristor devices where removal and restoration of oxygen vacancies play a key role as the switching mechanism. Significance: This work is of significant importance for the development of future electronic devices with significantly enhanced reliability features. By demonstrating that electrical characteristics of multifunctional oxide thin films can be largely tuned by external stimuli such as mechanical pressure and thermal annealing, it will spark various research initiatives in using the oxide for novel, more versatile applications. References: Kawada, T., et al., High temperature transport properties at metal/SrTiO3 interfaces. Journal of the European Ceramic Society, 1999. 19: p. 687. Ohly, C., et al., Electrical conductivity and segregation effects of doped SrTiO3 thin films. Journal of the European Ceramic Society, 2001. 21: p. 1673. Kalabukhov, A., et al., Effect of oxygen vacancies in the SrTiO 3 substrate on the electrical properties of the LaAlO3∕ SrTiO3 interface. Physical Review B, 2007. 75: p. 121404. Figure 1