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4D-S Systems - A Closer look, page-36

  1. higo
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    21/08/15
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    1. Introduction

    Resistive random access memory (ReRAM) is being studied as a promising candidate for the next generation nonvolatile memory due to its simple structure of metal/insulator/metal (MIM), fast read/write speed, low energy operation, and good retention properties. So far, a variety of resistive switching characteristics are observed in most transition metal oxides (TMO) with the MIM structures of asymmetric electrodes and various resistive switching mechanisms have been previously suggested [1], [2], [3],[4], [5], [6], [7] and [8]. Recently, among the candidate materials for ReRAM, Pr0.7Ca0.3MnO3 (PCMO) in perovskite manganite R1 − xAxMnO3 (R = rare-earth metal; A= divalent element) of a strongly correlated electron system has been of great value in industry despite its complex stoichiometry, because it shows bipolar switching without a ‘forming process’, in contrast to a binary TMO system. In TMO, extra holes/electrons are generally located at transition metals (TMs) or in oxygen depending on whether the metal is early transition or late transition [9] and [10]. In PCMO, it shows the intermediate state of charge transfer gap and Mott Hubbard gap. Therefore, intermediate members of Pr1 − xCaxMnO3, excess holes/electrons are primarily located in Mn 3d and O 2p by Ca2 +of divalent cation doping, which controls the Mn 3d and O 2p filling. The basic picture that emerges for the mixed compound Pr1 − xCaxMnO3 is that the Mn 3d e1g band (which is completely filled for x = 0) is progressively depleted with the increase of the Ca concentration x, with complete depletion for x = 1. At the x = 0, actually Mn 3d e1g band of CaMnO3 (CMO) is empty, thus can be considered to be a “band insulator”. However the other end composition, x = 1, has actually 1 electron per site in the 2-fold degenerate e1gband (1/4 filled). So the x = 1 case, it is not expected to be an insulator. However, the experimental fact shows that it is a Mott insulator. For the intermediate compounds, the polaron hopping in the e1g bands couples the t2g spins. And the conductivity is changed according to the bond angle and distance of MnOMn, Mn 3d filling, and spin structure due to its strongly correlated electron system [11], [12] and [13].
    The present, resistive switching characteristics are largely classified into unipolar switching and bipolar switching. The origin of the unipolar switching characteristic is explained with a filament model. As mentioned above, the PCMO is an attractive material for use in ReRAM applications. Nevertheless, there are some arguments in the bipolar switching mechanism due to various resistive switching properties [1] and [14]. Until now, various suggested candidate mechanism models for the bipolar switching characteristic of PCMO included a redox of top electrode (TE), change of Schottky like barrier height and/or width model with electrochemical migration, redox of MnOMn conduction chain, and metal–insulator transition etc. from various experimental results[1], [2], [3], [4], [5], [6], [7] and [8]. However, oxygen vacancy concentration at the interface is generally accepted in the suggested mechanisms. Consequently, a change of oxidation state at the interface is an important topic of this study. According to an oxygen vacancy model, almost TMs having the multi-valence can show the resistive switching property as a function of z in the TMOz. In the application to industry, TMOs have several barriers such as forming process of binary compound, current level, control of resistive switching property, and switching direction. Therefore, this study is progressed for control of resistive switching property and the mechanism using Ti TE with CMO, PCMO and PCMO/CMO bi-layer film. Also one method of the switching property control is demonstrated here.
    2. Experimental details

    CMO, PCMO, and PCMO/CMO thin films were deposited by off-axis rf magnetron sputter on a chemically cleaned Pt(111)/Ti/SiO2/Si(100) substrate to measure the electrical properties. The PCMO and CMO targets were prepared by a standard solid state reaction method using Pr2O3, CaCO3, and Mn2O3 powders as starting materials. The background pressure in the sputter chamber was 2.66 × 10− 4 Pa, and the deposition conditions of power, substrate temperature, working pressure, and Ar:O2 ratio were 100 W, 550 °C, 0.266 Pa, and 4:1, respectively. Deposition time for CMO was 10 min, PCMO 10 min, PCMO (5 min)/CMO (5 min) for equal thickness of the films. The PCMO/CMO bi-layer film was deposited by in-situ process. For current–voltage (I–V) characterization of the films, a 150-nm thick Ti TE with a 100-μm radius was deposited by e-beam evaporator. Fig. 1 shows the schematic diagram of CMO, PCMO, PCMO/CMO films and the scanning electron microscopy (SEM) image. Structures of the deposited three films were Ti (150 nm)/(CMO (36 nm) or PCMO (36 nm) or PCMO/CMO (36 nm))/Pt (150 nm). And thickness of the bi-layer film is expected to PCMO (18 nm)/CMO (18 nm) from equivalent deposition time.


    Fig. 1.
    The schematic diagram of Ti (150 nm)/CMO (36 nm), PCMO (36 nm), PCMO (18 nm)/CMO (18 nm), /Pt (150 nm) films and the scanning electron microscopy image.
    Figure options

    The film thickness was confirmed by SEM (JSM-600F, JEOL). The resistance switching behavior of the PCMO film was measured using a two-probe measurement system with a Keithley 2635A source meter. A positive bias was defined as flow of current from the top of PCMO, CMO, PCMO/CMO films to the bottom electrode. Current–voltage characteristics were measured by voltage bias sweep as 0 → + Vmax → 0 → − Vmax → 0 V. All I–V measurements were taken at room temperature and a compliance of 10 mA was applied to prevent breakdown of the MIM structure.
    3. Results and discussion

    It has been found that the switching effect in PCMO junctions is highly dependent on the physical properties of the TE materials [15], [16], [17] and [18]. In this study, all the curves exhibit nonlinear behavior, suggesting the existence of the strong interface effect in the junctions as shown in Fig. 2. According to Schottky theory, a barrier would form at the interface between PCMO and TE, if the work function (WF) of the TE is less than 5.2 eV. The lower the WF value, the higher the Schottky barrier and hence the junction resistance would be expected [19]. However, a simple Schottky contact cannot well explain these observed resistance switching sequences in many reports due to a redox in interface and the change of PCMO band gap (Eg) according to Mn 3d filling. The oxygenated PCMO is believed to be p-type semiconductor of Mn3 + and Mn4 + mixed state with WF ~ 4.89 eV and Eg ~ 0.2 eV. And oxygenated CaMnO3 is classified as band insulator with WF ~ 5.4 eV and Eg ~ 1 eV as mentioned above [20]. However, native oxygen vacancy defect in CMO provides electrons to Mn 3d e1g conduction band as n-type dopant, and it offers conductivity to CMO. Therefore, the CaMnO3 − δ could be classified to n-type semiconductor. And Eg of the Pr1 − xCaxMnO3 family is changed by its oxidation state due to strong correlation energy [21]. First, the films show clockwise switching with positive applied voltage as shown in Fig. 2(a) and (b). In electrochemical migration model of oxygen vacancy, a Schottky-like barrier height and/or width model described the two types of switching (clockwise and counterclockwise) from p-type and n-type oxide semiconductors. The model explains the resistive switching direction with a change of oxygen vacancy concentration at the interface which was induced by an applied voltage with electrochemical migration. In p-type oxide semiconductors, oxygen vacancies are considered to be acceptor scavengers. Therefore, under positive voltage, a decrease of oxygen vacancies at the interfaces may cause the depletion layer to narrow in PCMO, resulting in a decrease in the contact resistance. On the other hand, since oxygen vacancy acts as an effective donor in n-type oxide semiconductors, the reduction in the number of oxygen vacancies may cause the depletion layer to widen at the interface, thus increasing the contact resistance [2]. In the case of reverse direction switching, some reports assert the redox of TE for an explanation of clockwise switching. In the reports, oxidation of TE takes oxygen ions from PCMO under a positive voltage. And, under the opposite voltage, the electrode gives oxygen ion to PCMO (clockwise). However, it has to be explained that observed reverse switching direction in spite of a same metal TE with a PCMO, and not following the Schottky barrier width or height model of p-type and n-type according to x in Pr1 − xCaxMnO3[2]. As shown in Fig. 2(a) and (b), the branch 1, positive-biased high resistance state (HRS) of all films can be well fitted by Poole–Frenkel emission (PFE) mechanism. The PFE mechanism strongly indicates an existence of defective TiOy layer since it is due to emission of trapped electrons from insulator layer [22]. Therefore, branch 1 shows existence of TiOy in Ti/PCMO, CMO interface initially. On the other side, PFE was not observed in the branch 2 of all films after the HRS switching. In the HRS switching, branches 1 and 2 show inducting of oxidation of TiOy and reduction of PCMO and CMO. And the redox of Ti/TiOy/CMO interface was a little compared with redox of Ti/TiOy/PCMO interface. Furthermore, in CMO low resistance state (LRS) switching of Fig. 2(a) negative voltage, the RH/RL ratio was larger than RH/RL ratio of HRS switching. On the contrary, RH/RL ratio in PCMO LRS switching of Fig. 2(b) was smaller than RH/RL ratio of HRS switching. As can be seen in the PEF expression Eq. (1)[22],
    equation(1)

    Turn MathJaxon

    where J is the current density, V is the applied voltage, di is the insulator thickness, εiis the permittivity, фB is the barrier height, k is the Boltzmann constant, and T is the temperature. If the oxidized TiOx in HRS switching works on insulating layer, current density J has to depend on di in LRS switching as shown in Eq. (1). However, RH/RLratio of LRS switching was a little compared with HRS switching in spite of large reduction. This difference of RH/RL ratio by polarity means that the insulating effect of oxidized TiOy does not strongly affect the resistive switching as insulating layer [23]. However the redox of Ti electrode serves to change in the oxygen vacancy concentration at the interface region of PCMO. In the case of LRS switching of Ti/CMO interface with negative applied voltage, it shows large RH/RL ratio in spite of a little redox. Fig. 2(c) and (d) shows I–V characteristics of negative voltage in log scale. As shown in Fig. 2(c) branch 3, current density of injecting electron shows trap-free space charge limited condition (TF-SCLC) (I ∝ V2) up to − 2 V. Above − 2 V, the current was increased largely. After the LRS switching (branch 4), trap-controlled space charge limited condition (TC-SCLC) (I ∝ V3.8) was observed above − 1.9 V. TF-SCLC of branch 3 means a presence of CaMnO3 high insulating material at the interfacial zone with Ti TE. Above − 2 V, a large increase of current corresponds to an incorporation of oxygen vacancy in CaMnO3 at the interfacial zone with Ti TE. Finally this incorporated oxygen vacancies induced TC-SCLC of branch 4. It means that oxygen ions and oxygen vacancies migrated to the interfacial CaMnO3 with applied positive and negative voltages, respectively. Consequently, Ti/CMO/Pt film follows the electrochemical migration model and on the contrary, Ti/PCMO/Pt film shows resistive switching mechanism by the redox of interface. The schematic conduction diagrams of TiOy/CMO and TiOy/PCMO junctions were given in Fig. 3. Represented HRS junction diagram of Ti/CMO film assumes that there is no electron diffusion by potential difference. The positive voltage induced an accumulation of oxygen ions to the interface in HRS switching. The LRS with TC-SCLC as shown as the branch 4 inFig. 2(c) shows that oxygen vacancy exists in Eg as trap site [24]. In TiOy/PCMO interface, an exchange of oxygen ion by redox is dominant for the resistive switching. Mn e1g band of the HRS PCMO is filled due to the reduction of Mn from + 4 to + 3, i.e. reduction of PCMO (a decrease of carrier concentration and an increase of WF). Furthermore, one thing to note in this experimental result is the difference of RH/RLratio in positive and negative voltages. It means that potential change affects more importantly on the resistive change than depletion layer width change at the interface.


    Fig. 2.
    Current–voltage curves of (a) Ti/CMO/Pt and (b) Ti/PCMO/Pt and current–voltage fits of log scale in negative voltage for (c) Ti/CMO and (d) Ti/PCMO.
    Figure options



    Fig. 3.
    The schematic conduction diagrams for (a) Ti/TiOx/CMO and (b) Ti/TiOx/PCMO junctions.
    Figure options

    For the control of resistive switching property, a Ti/TiOy/PCMO/CMO/Pt bi-layer film was fabricated as shown in Fig. 4(a). The film shows both resistive switching characteristics of PCMO and CMO. As in Fig. 4(b), it shows a redox of Ti/TiOy/PCMO which is less than the PCMO single layer film in HRS switching. Likewise, LRS switching shows intermediate RH/RL ratio of CMO and PCMO. Therefore, we could expect that both a redox and electrochemical migration might affect the resistive switching in the bi-layer film. From the bi-layer, this study could get the resistive switching property in a higher resistance than single layer films.

    Fig. 4.
    (a) The bi-layer structure and (b) current–voltage curve for Ti/PCMO/CMO/Pt film.
    Figure options

    4. Conclusions

    For controlling the resistive switching property and the current level, the resistive switching properties of Ti TE with CMO, PCMO, and PCMO/CMO films were studied. From the results, it can be concluded that the difference of a generation mechanism of oxygen vacancy depends on film resistance. Dominant factor is determined by the resistance of film and interface among the redox at the interface and electrochemical migration. It means that film thickness, oxidation state of interface, and thickness of oxidized TE can affect the resistive switching properties. And the generation mechanism of oxygen vacancy changes the switching direction. Therefore, the resistive switching property could be controlled by an insertion of layer with different resistance.
    Acknowledgment

    This work was supported by SK Hynix Inc. of Korea and by the second stage of the Brain Korea 21 Project in 2010. The experiments at the PLS were supported in part by MOST and POSTECH.

    References

      • [4]
      • A. Odagawa, H. Sato, I.H. Inoue, H. Akoh, M. Kawasaki, Y. Tokura, T. Kanno, H. Adachi
      • Phys. Rev. B, 70 (2004), p. 224403
      • Full Text via CrossRef
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