Ferroelectric Field Effect Induced Asymmetric Resistive Switching Effect in BaTiO3/Nb:SrTiO3 Epitaxial Heterojunctions
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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.
KeywordsFerroelectric Asymmetric resistive switching Ferroelectric/semiconductor heterojunctions
Piezoresponse force microscopy
Scanning Kelvin probe microscopy
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  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 . The optically controlled electroresistance and electrically controlled photovoltage were observed in Sm0.1Bi0.9FeO3/(001)Nb:SrTiO3 heterojunctions . A ferroelectric polarization-modulated band bending was observed in the BiFeO3/(100)NbSrTiO3 heterointerface by scanning tunneling microscopy and spectroscopy . A transition from the rectification effect to the bipolar resistive switching effect was observed in BaTiO3/ZnO heterojunctions .
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.
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 apparent asymmetry in switching time has also been observed in Al/W:AlOx/WOy/W , La2/3Sr1/3MnO3/Pb(Zr0.2Ti0.8)O3/La2/3Sr1/3MnO3 , and Pt/LaAlO3/SrTiO3  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 . 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  and ZnO/NSTO Schottky junctions . 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.
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.
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.
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.
The authors declare that they have no competing interests.
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