Observing topotactic phase transformation and resistive switching behaviors in low power SrCoOx memristor
Graphical abstract
A complex oxide SrCoOx (SCO) used as the dielectric layer in resistive random access memory (RRAM) is demonstrated the excellent RRAM properties. This study provides not only the switching mechanism of the SCO RRAM devices, which resulted from the topotatic phase transformation between BM-SCO and P–SCO, but also the novel aspect for the RRAM dielectric materials.
Introduction
Recently, the electronic devices have booming evolution [[1], [2], [3]], especially in the memory devices. Resistive random access memory (RRAM) has been regarded as a candidate for next generation nonvolatile memory [4,5] (NVM) due to its low cost, low power consumption, high stability and high switching speed [[6], [7], [8], [9], [10]]. Furthermore, an RRAM device also has the advantage of having a simple structure, which is generally composed of a metal-insulator-metal (MIM) structure. According to previous literature [[11], [12], [13], [14]], conducting filament theory is the most widely accepted theory to explain the switching mechanism. The formation/rupture of the filament will result in different resistance states of the device. When the filaments connect two electrodes, the device is in a LRS; in contrast, when the filament ruptures, it is known as a HRS.
Based on the composition of the conducting filament, the switching behaviors can be distinguished as an electrochemical metallization mechanism (ECM) and a valence change mechanism (VCM). An ECM device is composed of an MIM in which one electrode is an inert metal (ex., Pt or Au) and the other electrode is an active metal (ex., Cu or Ag) [15,16]. On the other hand, the two electrodes of a VCM device are composed of an inert metal or an inactive conducting material [[17], [18], [19]]. The reliability of a VCM-based device is generally higher than that of an ECM-based device because the diffusion coefficient of an active metal ion is large in the dielectric layer, where Ag+ and Cu2+ are estimated to be ≈ 10−13 cm2 s−1 [20], which is likely to cause the hard breakdown of the device [8,12]. Therefore, the VCM-based device has attracted considerable interest as the next generation NVM.
In the last few years, many transition metal oxides used for VCM-based RRAM dielectric materials have been broadly studied, such as HfO2, TiO2, Ta2O5, etc. [[21], [22], [23]]. Because of the diffusion of the oxygen vacancies, the transition metal oxides will become oxygen nonstoichiometric, which leads to the setting of the devices. More recently, complex oxides, such as SrFeOx, SrTiO3, and La2/3Sr1/3MnO3, have also drawn great attention in RRAM applications [9,13,24]. When the oxygen ions diffuse, the structure of the complex oxides will transform between brownmillerite and perovskite [25,26], where brownmillerite is generally insulating and perovskite is conducting, and this transformation is the so-called topotactic phase transformation [27]. However, there is still a lack of direct evidence to connect this topotactic phase transformation to the switching behaviors in RRAM.
In a previous study, SrCoOx could be transformed between brownmillerite SrCoO2.5 (BM-SCO) and perovskite SrCoO3 (P–SCO) via electrical potential due to losing or receiving oxygen ions [25,28]. BM-SCO possesses higher resistance than P–SCO because of the atomic structural difference [29]. BM-SCO stacks with alternating octahedral CoO6 and tetrahedral CoO4 layers, which will result in the insulating property of BM-SCO; however, P–SCO stacks with octahedral CoO6 layers, which makes the electrical and magnetic properties of P–SCO and BM-SCO different (shown in Fig. S1) [30,31]. The detailed crystal information about SCO is also presented in Table S1 [32,33]. Based on this characteristic, we consider that SCO can be applied to the memory material. In this work, a complex oxide SrCoOx (SCO) was introduced as the dielectric layer, which was deposited on a niobium-doped SrTiO3 (Nb-STO) substrate with Au as the inert top electrode to produce Au/SCO/Nb-STO RRAM devices. The SCO devices possessed very uniform RRAM properties and high reliability. In addition, the topotactic phase transformation phenomenon and formation/rupture of the conducting filaments were investigated by high-resolution (HR) TEM/STEM images, an energy dispersive spectrometer (EDS) and electron energy loss spectroscopy (EELS) after the electrical measurements in the atmosphere. Furthermore, the behavior of the topotactic phase transformation from BM-SCO to P–SCO was directly observed by in situ TEM, which operated in a high vacuum environment. This work indicate that SCO exhibited great properties for RRAM applications and revealed the switching mechanism, which contributes to the phase transformation between BM-SCO and P–SCO.
Section snippets
Results and discussion
The SCO thin film was deposited on the Nb-STO(001) substrate. Fig. 1(a) shows the cross-sectional TEM image of the Au/SCO/Nb-STO devices, which demonstrates the good contact of the SCO/Nb-STO interface, and Fig. S3 is the EDS analysis of the initial device. Furthermore, the thickness of the dielectric layer SCO is approximately 25 nm, and according to the fast-Fourier transform diffraction pattern (FFT-DP), the structure of SCO is amorphous. The schematic diagram of the device is shown in Fig. 1
Conclusion
In summary, the asymmetric Au/SCO/Nb-STO RRAM devices with an atomically uniform thickness exhibited bipolar switching characteristics and also possessed highly endurable and excellent stability. The electrical measurement results of the polycrystalline-SCO demonstrated that the Set and Reset voltages had narrow distributions, the retention time reached more than 104 s, and the ON/OFF ratio was as high as 103. We further investigated and analyzed the switching behavior and the composition of
Fabrication of SCO RRAM devices
The 25 nm polycrystalline brownmillerite SrCoO2.5 thin films were grown with pulsed laser deposition (PLD) using a KrF excimer laser (λ = 248 nm) on a 10 × 10 mm2 0.5 wt% Nb-doped SrTiO3 substrate at a temperature of 650 °C and a pressure of 100 mTorr, and the target is bulk SrCoO2.5. Then, a 130-nm-thick Au top electrode was deposited on the SCO via an electron beam evaporation system (E-gun). The devices were defined as having a top electrode 200 μm in diameter through a shadow mask.
Characterization of SCO RRAM properties
The ex
CRediT authorship contribution statement
Hung-Yang Lo: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing - original draft. Chih-Yu Yang: Methodology. Guan-Ming Huang: Methodology. Chih-Yang Huang: Formal analysis. Jui-Yuan Chen: Investigation. Chun-Wei Huang: Formal analysis. Ying-Hao Chu: Writing - review & editing. Wen-Wei Wu: Conceptualization, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare no competing financial interest.
Acknowledgements
The author W.-W.W. acknowledges the support from Ministry of Science and Technology (MOST) in Taiwan (MOST 103-2221-E-009-222-MY3, MOST 106-2628-E-009-002-MY3, MOST-107-2119-M-009-019 and MOST 106-2119-M-009 -008). This work was financially supported by the “Center for Semiconductor Technology Research of National Chiao Tung University” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education in Taiwan. Also
Hung-Yang Lo is a Ph.D. candidate in Materials Science and Engineering at National Chiao Tung University. His main research interests are preparation and applications of metal-oxide RRAM devices, in situ TEM investigation of dynamical changes in nanostructured materials.
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Hung-Yang Lo is a Ph.D. candidate in Materials Science and Engineering at National Chiao Tung University. His main research interests are preparation and applications of metal-oxide RRAM devices, in situ TEM investigation of dynamical changes in nanostructured materials.
Chih-Yu Yang is a Master candidate in Materials Science and Engineering at National Chiao Tung University. Her main research interests are preparation and applications of complex oxides, laser molecular beam epitaxy/pulse laser deposition, flexible and transparent oxide electronics.
Dr. Guan-Ming Huang received his Ph.D. degree in Materials Science and Engineering from National Chiao Tung University in 2019. His main research interests are preparation and applications of CNT/Graphene/ metal-oxide, in situ TEM investigation the nanostructures for Supercapacitor Devices.
Chih-Yang Huang is a Ph.D. candidate in Materials Science and Engineering at National Chiao Tung University. His main research interests are preparation and applications of metal-oxide nanodevices, in situ TEM investigation of dynamical changes in nanostructured materials.
Prof. Jui-Yuan Chen received his Ph.D. degree in Materials Science and Engineering from National Chiao Tung University, 2015. Then he worked as Postdoctoral Fellow (2015–2019) at Materials Science and Engineering, National Chiao Tung University. After his Postdoctoral Fellowship, he joined in Department of Materials Science and Engineering, National United University from 2019. His main research interests are preparation and applications of metal-oxide nanodevices, in situ TEM investigation the nanostructures for micro/nano-memory systems.
Dr. Chun-Wei Huang received his M.S. degree in Materials Science and Engineering from National Taiwan Ocean University, Taiwan in 2009 and Ph.D. degree in Materials Science and Engineering from National Chiao Tung University in 2014. Then he worked as Postdoctoral Fellow (2014–2017) at Materials Science and Engineering, National Chiao Tung University. He joined Material and Chemical Research Laboratories, Nanotechnology Research Center, Industrial Technology Research Institute from 2017. His main research interests are preparation and applications of ZnO nanodevices, in-situ TEM investigation for nanodevices.
Prof. Ying-Hao Chu received his Ph.D. degree in Materials Science and Engineering from National Tsing Hua University, 2004. Then he worked as Postdoctoral Fellow (2004–2008) at University of California, Berkeley. After his Postdoctoral Fellowship, he joined in Department of Materials Science and Engineering, National Chiao Tung University from 2008. His main research interests are complex oxide thin film and nanostructures, oxide thin heterostructure and interface phenomenon, Nanoscale characterization of multiferroic materials.
Prof. Wen-Wei Wu received his Ph.D. degree in Materials Science and Engineering from National Tsing Hua University, 2003. Then he worked as Postdoctoral Fellow (2003–2008) at Materials Science and Engineering, National Tsing Hua University. He joined in Materials Science and Engineering, National Chiao Tung University from 2008. His main research interests are in situ TEM investigation of dynamical changes in nanostructured materials, synthesis metal silicide thin films and nanostructures, and metallization on Si and Si–Ge alloy.