Ferroelectric Field Effect Induced Asymmetric Resistive Switching Effect in BaTiO3/Nb:SrTiO3 Epitaxial Heterojunctions
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Abstract
Asymmetric resistive switching processes were observed in BaTiO3/Nb:SrTiO3 epitaxial heterojunctions. The SET switching time from the high-resistance state to low-resistance state is in the range of 10 ns under + 8 V bias, while the RESET switching time from the low-resistance state to high-resistance state is in the range of 105 ns under − 8 V bias. The ferroelectric polarization screening controlled by electrons and oxygen vacancies at the BaTiO3/Nb:SrTiO3 heterointerface is proposed to understand this switching time difference. This switch with fast SET and slow RESET transition may have potential applications in some special regions.
Keywords
Ferroelectric Asymmetric resistive switching Ferroelectric/semiconductor heterojunctionsAbbreviations
- BTO
BaTiO3
- HRS
High-resistance state
- LRS
Low-resistance state
- NSTO
Nb:SrTiO3
- PFM
Piezoresponse force microscopy
- SKPM
Scanning Kelvin probe microscopy
Background
Ferroelectric resistive switching effects have attracted lots of research interests, since the polarization reversal is based on purely electronic mechanism, which does not induce a chemical alteration and is an intrinsically fast phenomenon [1, 2]. Ferroelectric resistive switching effects have been observed in ferroelectric heterojunctions sandwiched by two metal or semiconductor electrodes [3, 4, 5]. Lots of interesting behaviors have been observed in ferroelectric/semiconductor heterojunctions. For example, a greatly enhanced tunneling electroresistance is observed in BaTiO3 (BTO)/(001)Nb:SrTiO3 (NSTO) [4, 5] and MoS2/BaTiO3/SrRuO3 [6] heterojunctions since both the barrier height and width can be modulated by ferroelectric field effect. A coexistence of the bipolar resistive switching and negative differential resistance has been found in BaTiO3/(111)Nb:SrTiO3 heterojunctions [7]. The optically controlled electroresistance and electrically controlled photovoltage were observed in Sm0.1Bi0.9FeO3/(001)Nb:SrTiO3 heterojunctions [8]. A ferroelectric polarization-modulated band bending was observed in the BiFeO3/(100)NbSrTiO3 heterointerface by scanning tunneling microscopy and spectroscopy [9]. A transition from the rectification effect to the bipolar resistive switching effect was observed in BaTiO3/ZnO heterojunctions [10].
Here we observe an asymmetric resistive switching effect in the BaTiO3/Nb:SrTiO3 Schottky junction, which has not been reported yet. Furthermore, we propose a ferroelectric field effect to understand this asymmetric resistive switching effect. Specifically, the SET transition from the high- to low-resistance state is in 10 ns under + 8 V bias, while the RESET transition from the low- to high-resistance state is in the range of 105 ns under − 8 V. This can be understood by the ferroelectric polarization screening by electrons and oxygen vacancies at the BaTiO3/Nb:SrTiO3 interface. This switch with fast SET and slow RESET transitions may have potential applications in some special regions.
Methods
The commercial (100) 0.7 wt% NSTO substrates were successively cleaned in 15 min with ethanol, acetone, and de-ionized water and then blown with air before deposition. The BTO film was grown on NSTO substrates by pulsed laser deposition (PLD) using a KrF excimer laser (248 nm, 25 ns pulse duration, COMPexPro201, Coherent) at an energy of 300 mJ and frequency of 5 Hz, with the base vacuum of 2 × 10−4 Pa. During growth, the substrate temperature was kept at 700 °C, and the target-substrate distance was 6.5 cm. The oxygen partial pressure was 1 Pa, and the growth time was 15 min. After growth, the sample was kept under the oxygen partial pressure of 1 Pa for 10 min, and then, the temperature was reduced to room temperature at 10 °C/min within a vacuum environment. The thickness of BTO thin films is around 100 nm. Au top electrodes (0.04 mm2) were sputtered on BTO thin films through a shadow mask by DC magnetron sputtering, and the bottom electrode was indium (In) pressed on NSTO substrate. Keithley 2400 sourcemeter was used to conduct transport measurements. Voltage pulses were supplied by an arbitrary waveform generator (Agilent 33250A) with a pulse duration ranging from 10 ns to 1 s. The atomic force microscopy (AFM), piezoresponse force microscopy (PFM), and scanning Kelvin probe microscopy (SKPM) results were carried out to characterize the morphology, ferroelectricity, and electrostatic potential of the BTO film surface by an Oxford AR instrument. The PFM out-of-plane phase, PFM out-of-plane amplitude, current, and SKPM images were recorded with a biased conductive tip of 0.5 V over the same area after writing an area of 2 × 2 μm2 with − 8 V and then the central 1.25 × 1.25 μm2 square with + 8 V. In all measurements, the bottom electrodes were grounded and voltages were applied onto the top electrodes or the tip. All measurements were performed at room temperature.
Results and Discussion
The current-voltage curves of the Au/BTO/NSTO/In system at small biases between − 0.5 and 0.5 V after applying a pulse in width of 100 ms with various amplitudes (a, b). The current-voltage curves of the Au/BTO/NSTO/In system at small biases after applying a pulse with an amplitude of + 8 V (c) and − 8 V (d) with various pulse widths. The junction resistance of the Au/BTO/NSTO/In system recorded at − 0.3 V after the application of voltage pulses with various amplitudes and widths starting from the HRS (e, g) or LRS (f, h), where the pulse widths in e and f are 100 ms and the different curves in e and f correspond to different consecutive measurements, with varying positive or negative maximum voltages. The inset of a shows the schematic drawing of the device structure
a Surface morphology of the BTO films on NSTO substrates. b Local PFM out-of-plane hysteresis loops: blue, phase signal; black, amplitude signal. c PFM out-of-plane phase, d PFM out-of-plane amplitude, e current, and f SKPM images recorded over the same area (a) after writing an area of 2 × 2 μm2 with − 8 V and then the central 1.25 × 1.25 μm2 square with + 8 V using a biased conductive tip. The scale bar is 500 nm for the images of a and c–f. The labels in c–e are corresponding to the value of out-of-plane current, PFM phase, and PFM amplitude, respectively
The apparent asymmetry in switching time has also been observed in Al/W:AlOx/WOy/W [14], La2/3Sr1/3MnO3/Pb(Zr0.2Ti0.8)O3/La2/3Sr1/3MnO3 [15], and Pt/LaAlO3/SrTiO3 [16] devices. Wu et al. proposed an asymmetric redox reaction in W:AlOx/WOy bilayer devices and attributed the switching time difference to the different Gibbs free energy in AlOx and WOy layers [14]. However, in the present BTO/NSTO heterojunction, the voltage can only be applied to BTO film since NSTO is a heavily doped semiconductor. Thus, the asymmetric redox reaction can be ruled out in the present work. Qin et al. and Wu et al. attribute the asymmetry in switching time to the different internal electric field that drives oxygen vacancy migration across the LSMO/Pb(Zr0.2Ti0.8)O3 and LaAlO3/SrTiO3 interfaces [15, 16]. According to this model of oxygen vacancies across interface, the oxygen vacancy will migrate from BTO to NSTO under a positive bias, and the resistance in BTO will increase due to the decrease of oxygen vacancy concentration in BTO, while the resistance in NSTO will not change much since it already has high concentration of Nb donors; thus, the resistance of the whole system will increase under positive bias, which is opposite to our observation in Fig. 1. Furthermore, the ionic process is supposed to be much slower than the electron process, so a pure ion process cannot account for the fast SET process of 10 ns, as shown in Fig. 2g. Therefore, it is hard to understand the asymmetric resistive switching speed by only considering the physical process of polarization reversal or chemical process of drifted oxygen vacancies. Actually, an asymmetric switching speed has also been observed in Au/NSTO [17] and ZnO/NSTO Schottky junctions [18]. An asymmetric Schottky barrier can also lead to an asymmetric resistive switching speed. However, based on the PFM and SKPM results, the resistive switching in the BTO/NSTO heterojunction in the present work is observed to be caused by ferroelectric field effect. Therefore, we propose a model of ferroelectric polarization reversal coupled with the migration of oxygen vacancy across the BTO/NSTO interface to understand this asymmetric behavior.
The schematic drawings (a, b) and corresponding potential energy profiles (c, d) of the Au/BTO/NSTO structures for the low- and high-resistance states. In BTO, the red arrows denote the polarization directions, and the “plus” and “minus” symbols represent positive and negative ferroelectric bound charges, respectively. The “circled plus” symbols represent the ionized oxygen vacancies. The blue arrows show the direction of oxygen vacancies drifting across the BTO/NSTO interface
Conclusions
In conclusion, asymmetric resistive switching time is observed in BTO/NSTO heterojunctions. The pulse duration required for RESET operation is four orders longer than that for the SET process. The positive and negative ferroelectric bound charges screened by electrons and oxygen vacancies at the BTO/NSTO interface play an important role at a positive and negative bias, respectively. The process of electron screening is much faster than that of oxygen vacancies, so the SET transition (HRS to LRS) induced by positive bias is much faster than the RESET transition (LRS to HRS) induced by negative bias. Furthermore, this switch exhibits fast SET and slow RESET transition, which may have potential applications in some special regions.
Notes
Funding
This work was supported by the National Natural Science Foundation of China (51202057), Natural Science Foundation of Henan Province (162300410016), and Key Scientific Research Projects of Henan Province (17A140004).
Availability of Data and Materials
All the data and materials are available by contacting the corresponding author.
Authors’ Contributions
CJ carried out the experimental design. JL and GY carried out the growth and measurement. YC participated in the experimental analysis. WZ supervised the research. All authors read and approved the final manuscript.
Competing Interests
The authors declare that they have no competing interests.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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