Perovskite Oxides Tunable by Electromechanical and Electrothermal Couplings
Authors: Gopalakrishnan Pratheek , Sebastian Ann Rose , Ahn Ethan C 
Published: December 2020

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