Different Directions of Switching of Chromium Oxide Thin Films
Abstract
Introduction
Experimental
Results and Discussion
Conclusions
Acknowledgements
References
Nội dung
Journal of ELECTRONIC MATERIALS, Vol 43, No 7, 2014 DOI: 10.1007/s11664-014-3193-3 Ó 2014 TMS Different Directions of Switching of Chromium Oxide Thin Films NGOC KIM PHAM,1 DO TRUNG NGUYEN,1 BANG TAM THI DAO,1 KIEU HANH THI TA,1 VINH CAO TRAN,2 VAN HIEU NGUYEN,3 SANG SUB KIM,4 SHINYA MAENOSONO,5 and THANG BACH PHAN1,2,6 1.—Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh, Vietnam 2.—Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh, Vietnam 3.—Faculty of Physics and Physics Engineering, University of Science, Vietnam National University, Ho Chi Minh, Vietnam 4.—Department of Materials Science and Engineering, Inha University, Incheon, Republic of Korea 5.—Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan 6.—e-mail: pbthang@hcmus.edu.vn We have investigated the switching behavior of as-deposited CrOx and postannealed CrOy films by use of a variety of electrodes (top electrode Ag, Ti; bottom electrode Pt, fluorine tin oxide (FTO)) Resistance switching is highly dependent on electrode material and post-annealing treatment Among Pt devices, I–V hysteresis was observed for the Ag/CrOx/Pt device only; no resistance switching was observed for Ag/CrOy/Pt, Ti/CrOx/Pt, and Ti/CrOy/Pt devices Among FTO devices, I–V hysteresis was observed for the Ag/CrOx/ FTO device whereas I–V hysteresis with the opposite switching direction was observed for Ag/CrOy/FTO, Ti/CrOx/FTO, and Ti/CrOy/FTO devices The direction of switching depends not only on electrode material but also on postannealing treatment, which affects the density of grain boundaries Thus, the density of grain boundaries determines the type of charge carrier involved in the switching process For as-deposited CrOx films with a high density of grain boundaries Ag filament paths mediated by electrochemical redox reaction were observed, irrespective of bottom electrode material (Pt or FTO) Postannealed CrOy films with a low density of grain boundaries suppressed electrochemical redox reaction in the Ag/CrOy/Pt device but promoted short-range movement of O2À ions through the bottom interface, resulting in resistance switching in the Ag/CrOy/FTO device Electrochemical redox reaction-controlled resistance switching occurred solely in oxides with a high density of grain boundaries or dislocations Key words: Chromium oxide, resistance switching, grain boundaries, electrochemical redox reactions, oxygen ion migration INTRODUCTION Recent research has demonstrated that non-volatile resistance switching resistive random access memory (ReRAM) is a promising alternative to floating-gate technology beyond the 32 nm technology node Oxide-based ReRAM structures exploit the functionality of capacitor structures in which oxide materials, for example perovskite (Cr-doped SrTiO3, Cr-doped SrZrO3, Pr0.7Ca0.3MnO3, etc.),1–8 (Received October 28, 2013; accepted April 16, 2014; published online May 20, 2014) chalcogenide materials (GeSbTe),9 transition metal oxides (TMOs), or binary oxides (NiO, TiO2, CuOx, HfO2, ZrOx, ZnO, Nb2O5, Al2O3, WOx)10–15 are sandwiched between two metal electrodes Choosing a material that is compatible with CMOS processes is currently a crucial challenge in ReRAM research Among the various materials used, TMOs have the major advantages of simple fabrication and compatibility with CMOS processes.16 Chromium oxide thin films are widely applicable in catalysis, solar thermal energy collectors, and as black matrix films in liquid-crystal displays.17,18 Although the effect of Cr dopant on resistance switching of SrTiO3 and 2747 2748 Pham, D.T Nguyen, Dao, Ta, Tran, V.H Nguyen, Kim, Maenosono, and Phan SrZrO3 has been reported,1,3,5–8 there are few published papers on the switching behavior of chromium oxide thin films Chen et al.19,20, studied the Pt/Cr2O3/Pt and Pt/Cr2O3/TiN structures and reported switching behavior for the Pt/Cr2O3/TiN structure only, and that the mechanism of switching is controlled by oxygen migration through the interface between Cr2O3 and TiN In addition to the effects of the top and bottom electrodes in the switching behavior of many oxide systems,21,22 we believe the density of extended defects, for example grain boundaries or dislocations, is also important as the dimensions of switching cells continue to shrink and approach the size of individual grains in oxide films Post-annealing treatment can determine grain sizes and the density of grain boundaries in oxides, both of which substantially affect the movement of charge carriers through the oxides In this study, therefore, we investigated the effect of electrode material and post-annealing treatment on the switching behavior of chromium oxide thin films EXPERIMENTAL Ag, Ti, and chromium oxide films 100 nm thick were fabricated on fluorine tin oxide (FTO) and Pt substrates by use of the DC sputtering technique, at room temperature, from metallic Ag, Ti, and Cr targets Deposition of 100-nm-thick chromium oxide films was executed under a total pressure PArỵO2 of 10À3 Torr The ratio of oxygen to argon gas, PO2 =PArỵO2 , was fixed at 6% Before deposition of the top electrode, as-deposited CrOx films were annealed at 500°C in air for h The post-annealed films are denoted CrOy films During deposition of the top electrode (Ag, Ti) in an argon environment at 10À3 Torr, a mask was used for top electrode patterning The device area was approximately 1.5 cm 1.5 cm The crystalline phases of the thin films were characterized in h–2h mode by use of an x-ray diffractometer (XRD) with Cu Ka radiation (k = 0.154 nm) The surface morphology of the films was studied by scanning electron microscopy (SEM) Current–voltage (I–V) measurements were obtained by use of a semiconductor characterization system and probe station The voltage profile for the I–V measurement was V fi À (+) Vmax fi V fi + (À)Vmax fi V The bottom electrode was biased and the top electrode was grounded RESULTS AND DISCUSSION Figure shows the I–V characteristics of asdeposited TE/CrOx/Pt devices and of post-annealed TE/CrOy/Pt devices, where ‘‘TE’’ denotes Ag and Ti electrodes I–V hysteresis after the V fi ÀVmax fi V fi +Vmax fi V sweeping profile was observed for the as-deposited Ag/CrOx/Pt device only (Fig 1a); no I–V hysteresis was observed for the other devices (Fig 1b–d) On the basis of the I–V hysteresis, the initial high-resistance state (HRS) was changed to a low-resistance state (LRS) as a negative bias (0 fi ÀVmax) was applied to the Pt bottom electrode The device remained in the LRS as the negative bias was increased and progressively changed to the HRS only on voltage sweeping in the positive voltage range (0 fi +Vmax) To examine the effect of the bottom electrode on the resistance switching characteristics, we replaced the Pt metallic electrode by an FTO electrode Figure shows the I–V characteristics of as-deposited TE/ CrOx/FTO devices and of post-annealed TE/CrOy/ FTO devices, where ‘‘TE’’ again denotes Ag and Ti electrodes Bipolar resistance switching characteristics were observed for all four devices However, the direction of switching depended both on top electrode material and post-annealing treatment Among these devices, only the Ag/CrOx/FTO device (Fig 2a) had the same I–V hysteresis as the as-deposited Ag/CrOx/ Pt device with the same voltage profile V fi ÀVmax fi V fi +Vmax fi V (Fig 1a) For the other devices, Ag/CrOy/FTO, Ti/CrOx/FTO, and Ti/ CrOy/FTO, I–V hysteresis was observed for the voltage profile V fi +Vmax fi V fi ÀVmax fi V (Fig 2b–d) For the switching characteristics, as shown in Fig 2b–d, the initial high-resistance state (HRS) was changed to a low-resistance state (LRS) by application of a positive bias (0 fi +Vmax) to the FTO bottom electrode The device remained in the LRS as the negative bias was increased and progressively changed to the HRS only on voltage sweeping in the negative voltage range (0 fi ÀVmax) It was observed that for as-deposited CrOx devices with the FTO bottom electrode, replacing the Ag top electrode with a Ti electrode caused a change of switching direction In addition, the post-annealing treatment only induced a change of switching direction for Ag devices Opposite switching directions were observed for the as-deposited Ag/CrOx/ FTO device and the post-annealed Ag/CrOy/FTO device In contrast, the switching direction for Ti devices was not affected by post-annealing treatment For the switching devices mentioned above, it was not necessary to apply an electroforming process to activate resistance switching An endurance test was conducted; the results are shown in Fig Clear reversible switching for more than 100 cycles was observed for all devices The electroforming process was applied to all the nonswitching devices (Ag/CrOy/Pt, Ti/CrOx/Pt, Ti/CrOy/ Pt) but no resistance switching was observed On the basis of the nature of the electrode, reactive Ag electrode or inert Pt electrode, and the direction of switching, it seems that the mechanism of switching for as-deposited the Ag/CrOx/Pt device is controlled by electrochemical redox reactions.21,22 Because both the as-deposited Ag/CrOx/Pt device (Fig 1a) and the as-deposited Ag/CrOx/FTO device (Fig 2a) have the same switching behavior, the mechanism of switching of the as-deposited Ag/ CrOx/FTO structure must also be determined by Different Directions of Switching of Chromium Oxide Thin Films 2749 Fig I–V characteristics of as-deposited TE/CrOx/Pt devices and post-annealed TE/CrOy/Pt devices, where ‘‘TE’’ denotes Ag and Ti electrodes Fig I–V characteristics of as-deposited TE/CrOx/FTO devices and post-annealed TE/CrOy/FTO devices, where ‘‘TE’’ denotes Ag and Ti electrodes 2750 Pham, D.T Nguyen, Dao, Ta, Tran, V.H Nguyen, Kim, Maenosono, and Phan Fig Endurance of the switching devices: (a) Ag/CrOx/FTO, (b) Ag/CrOy/FTO, (c) Ti/CrOx/FTO, (d) Ti/CrOy/FTO, (e) Ag/CrOx/Pt electrochemical redox reactions Recently, Waser et al.21, reviewed the resistance switching phenomena of metal oxides on the basis of electrochemical metallization, in which a combination of an inert electrode and a reactive electrode is the prerequisite condition for electrochemical redox reactions However, no resistance switching was observed for the post-annealed Ag/CrOy/Pt device, even though both the Ag top and the Pt bottom electrodes are the reactive and the inert electrodes, respectively This abnormal behavior will be discussed below In the comparison of the as-deposited Ag/CrOx/Pt device (Fig 1a) with the as-deposited Ti/ CrOx/Pt device (Fig 1c), because Ag+ and V2+ o are positively charged and resistance switching does not occur in the as-deposited Ti/CrOx/Pt device, it can be concluded that oxygen vacancies not contribute into the switching mechanism Among the four post-annealed devices (Figs 1b and d, 2b and d), the same I–V hysteresis was observed for the two devices with the FTO bottom electrode only, Ag/CrOy/FTO and Ti/CrOy/FTO, which also occurs for the as-deposited Ti/CrOx/FTO device (Fig 2c) Therefore, the resistance switching mechanism for the as-deposited Ti/CrOx/FTO device and both the post-annealed Ag/CrOy/FTO and Ti/ CrOy/FTO devices is probably controlled by O2À ions To explain why the post-annealed Ag/CrOy/FTO device and the as-deposited Ag/CrOx/FTO device have opposite switching directions, we took FESEM images of the as-deposited CrOx (Fig 4a) and postannealed CrOy (Fig 4b) films From study of the three positions indicated by ‘‘1’’, ‘‘2’’, and ‘‘3’’ in both images it is clear that the surface morphology seen in Fig 4a is indicative of a high density of irregularly shaped small particles with clear grain boundaries whereas that in Fig 4b is indicative of larger particles with unclear grain boundaries On the basis of the switching direction and FESEM images of the as-deposited film and the post-annealed film, the resistance switching mechanism for these five switching devices can be explained as follows The resistance switching mechanism of both the as-deposited Ag/CrOx/FTO device and the as-deposited Ag/CrOx/Pt device, is modeled in Fig The Ag+ ions easily drift through the high density of grain boundaries in the as-deposited CrOx films to connect the bottom electrode As the Ag metallic path Different Directions of Switching of Chromium Oxide Thin Films 2751 Fig SEM images of (a) as-deposited CrOx film and (b) post-annealed CrOy film Fig Switching model for clockwise I–V hysteresis devices: Ag filament paths mediated by electrochemical redox reactions forms, resistance switching is controlled by electrochemical redox reactions As mentioned above, electrochemical redox reaction-controlled resistance switching occurs solely in oxides with an inert electrode and a reactive electrode.21,22 If this is true, further detailed study of the effect of FTO electrodes in electrochemical redox reactions is needed For the three other devices (the as-deposited Ti/ CrOx/FTO device and both the post-annealed Ag/ CrOy/FTO and Ti/CrOy/FTO devices), the resistance switching mechanism is modeled in Fig Resistance switching involves back and forth drift of O2À ions through the bottom interface It is suggested this occurs because the polycrystalline phase of the FTO electrode acts as a reservoir of defects such as 2À ions The FTO V2+ o sites and, simultaneously, O electrode contains enough vacancy sites near the interface to readily provide resting sites for migrating O2À ions This implies that the FTO electrode can be treated as an O2À ion source The O2À ions move a short distance from the bulk oxide films, through the bottom interface, into the FTO bottom electrode during the positive polarity process This movement of O2À ions is associated with formation of oxygen vacancies V2+ o in the bulk oxide films near the bottom interface; consequently, the set process (HRS to LRS) occurs During the negative polarity process, the O2À ions move back from the FTO bottom electrode to the bulk oxide films, eliminating oxygen vacancies V2+ o at the interface, resulting in the reset process (LRS to HRS) On the basis of electrochemical redox reactioncontrolled the resistance switching, Ag+ metal ions start from the top interface and drift through grain boundaries in the bulk oxide layer to connect the Pt bottom electrode In the post-annealed Ag/CrOy/Pt device, the low density of grain boundaries in the CrOy films suppresses the long-range migration of Ag+ ions to the bottom Pt interface, i.e the Ag metallic path cannot form For the as-deposited Ti/ CrOx/Pt device and the post-annealed Ti/CrOy/Pt device, Ti cannot be oxidized to Ti ions In addition, the bottom Pt interface is not a reservoir of defects 2À ions in such as V2+ o sites and, simultaneously, O 2752 Pham, D.T Nguyen, Dao, Ta, Tran, V.H Nguyen, Kim, Maenosono, and Phan Fig Switching model for anticlockwise I–V hysteresis devices: negative O2À ions migrate under polarity biases the same way as the FTO electrode Therefore, the resistance switching does not occur in these devices Many ReRAM publications mentioned about the role of grain boundaries21,22 but it is hard to see the argument on the density of grain boundaries Reduction of grain boundaries in the oxide because of post-annealing treatment can limit the migration of ions It is suggested that electrochemical redox reaction-controlled resistance switching, only, occurs in oxides with a high density of grain boundaries or dislocations CONCLUSIONS Resistance switching in chromium oxide thin films is highly dependent on electrode material (Ag or Ti as top electrode, FTO or Pt as bottom electrode) and post-annealing treatment The same I–V hysteresis is observed for as-deposited CrOx films in the Ag/CrOx/FTO and Ag/CrOx/Pt devices, in which Ag filament paths mediated by electrochemical redox reactions control the resistance switching mechanism Although the same I–V hysteresis is observed for the as-deposited Ti/CrOx/FTO device and both the post-annealed Ag/CrOy/FTO and Ti/ CrOy/FTO devices, the switching direction is opposite to that of the as-deposited Ag/CrOx/FTO and Ag/ CrOx/Pt devices The corresponding switching mechanism is the back and forth drift of O2À ions through the bottom interface under polarity biases The as-deposited Ti/CrOx/Pt device and the postannealed Ag/CrOy/Pt and Ti/CrOy/Pt devices have no resistance switching characteristics The change of switching direction is controlled not only by the electrode material but also by the density of grain boundaries in chromium oxide films, which is affected by the post-annealing treatment The density of grain boundaries controls the movement of the charge carriers involved in the resistance switching mechanism—long-range migration of Ag+ ions in the as-deposited Ag/CrOx/FTO and Ag/CrOx/ Pt devices and short-range movement of O2À ions in the as-deposited Ti/CrOx/Pt device and the postannealed Ag/CrOy/Pt and Ti/CrOy/Pt devices In this study, the crucial 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Films 2751 Fig SEM images of (a) as-deposited CrOx... carriers through the oxides In this study, therefore, we investigated the effect of electrode material and post-annealing treatment on the switching behavior of chromium oxide thin films EXPERIMENTAL