Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-015-3889-z Ó 2015 The Minerals, Metals & Materials Society Study of the Resistive Switching Effect in Chromium Oxide Thin Films by Use of Conductive Atomic Force Microscopy KIM NGOC PHAM,1 MINSU CHOI,2 CAO VINH TRAN,3 TRUNG DO NGUYEN,1 VAN HIEU LE,1 TAEKJIB CHOI,4 JAICHAN LEE,2 and BACH THANG PHAN1,3,5 1.—Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam 2.—School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Republic of Korea 3.—Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam 4.—Hybrid Materials Research Center and Faculty/Institute of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, Republic of Korea 5.—e-mail: pbthang@hcmus.edu.vn Reversible resistive switching of Cr2O3 films was studied by use of conductive atomic force microscopy Resistive switching in Cr2O3 films occurs as a result of Ag filament paths formed during electrochemical redox reactions A large memory density of 100 Tbit/sq inch was achieved with a small filament diameter of 2.9 nm under the action of a compliance current of 10 nA A fast switching speed of 10 ns, high scalability, and low set/reset currents suggest that Cr2O3-based resistive memory is suitable for nanoscale devices Key words: Chromium oxide, resistive switching, electrochemical redox reactions, C-AFM, Ag filament Recent research has shown that resistance switching random access memory (ReRAM) is a promising candidate for nanoscale nonvolatile memory applications Oxide-based ReRAM structures exploit the functionality of capacitor structures in which the oxide materials, for example perovskite (Cr-doped SrTiO3, Cr-doped SrZrO3, Pr0.7Ca0.3MnO3, etc.),1–8 chalcogenide materials (GeSbTe),9 transition metal oxides (TMOs), or binary oxides (NiO, TiO2, CuOx, HfO2, ZrOx, ZnO, Nb2O5, Al2O3, WOx, CrOx)10–18 are sandwiched between two metal electrodes Choosing a material compatible with CMOS processes is a crucial challenge in current research on ReRAM Among the different materials used, TMOs have the major advantages of simple fabrication and compatibility with CMOS processes.19–22 We have focused on correlation of the switching behavior of oxide films (SrTiO3, ZnO, TiO2, WO3 and CrOx) with crystallinity and electrode material.5–8,14–17 From the perspective of application, the basic requirement for next-generation non-volatile memory is high scalability Because it has recently been shown that switchable conducting nano-filaments may have (Received March 12, 2015; accepted June 4, 2015) potential for realizing high-density devices, filamentary switching in nanoscale devices has attracted much attention.20–23 Further physical insights into geometrical aspects of conducting filaments, for example their number, size, and location, can be obtained by use of conductive atomic force microscopy (C-AFM) Recently, we reported the switching behavior of CrOx thin films, and that the mechanism of switching was an electrochemical redox reaction.17 To complement previous work, in this paper we report the progressive appearance of conducting filaments in CrOx thin films during resistance switching, studied by use of C-AFM Silver and chromium oxide films were fabricated, by use of the DC sputtering technique at room temperature, from metallic Ag and Cr targets, on commercial Pt substrates Deposition of 100-nmthick chromium oxides was performed in a gaseous mixture of 6% oxygen in argon with the total pressure kept at 10À3 Torr During deposition of the Ag top electrode, in an argon environment at 10À3 Torr, a mask was used for top electrode patterning X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state of the films Current–voltage (I–V) measurements were obtained by use of a semiconductor-characterization system (Keithley 4200 SCS) and probe station The Pham, Choi, Tran, Nguyen, Hieu Le, Choi, Lee, and Phan Fig XPS spectra of the (a) Cr 2p3/2 and (b) O 1s core levels for chromium oxide film voltage profile for I–V measurements was V fi À (+)Vmax fi V fi + (À)Vmax fi V The Pt bottom electrode was biased and the top electrode was grounded For C-AFM measurement, 10-nmthick chromium oxide was deposited on the Ag bottom electrode C-AFM measurements were conducted under ambient conditions by use of a Veeco Dimension D3100 atomic force microscope with Pt conductive tips as the top electrode Figure shows the Cr 2p and O 1s core level XPS spectra of CrOx films prepared at room temperature As shown in Fig 1a, the Cr 2p3/2 core level spectrum was deconvolved into three peaks with binding energies of 576.1 eV, 577.5 eV, and 579.2 eV The 576.1 eV-peak was attributed to Cr3+ in Cr2O3 The two peaks at higher binding energies ($577.5 eV and 579.2 eV) were assigned to Cr3+ and Cr6+, corresponding toCrO(OH)/Cr(OH)3 and CrO3, respectively The relative amounts of Cr2O3, CrO(OH)/ Cr(OH)3, and CrO3, estimated by Gaussian–Lorentzian curve fitting, were 51.63%, 36.7%, and 11.5%, respectively It is clearly apparent that the Cr2O3 phase is predominant Deconvolution of the O 1s spectrum in Fig 1b resulted in three peaks centered at 530.2 eV, Fig (a) Typical bipolar current–voltage characteristics of Ag/ Cr2O3/Pt structures and (b) endurance of Ag/Cr2O3/Pt devices under the action of cycling pulses 10 ns wide 532 eV, and 533.6 eV The highest-intensity peak of 530.2 eV was assigned to lattice oxygen or a stoichiometric Cr2O3 phase The lower-intensity peak at higher binding energy was assigned to non-lattice oxygen or non-stoichiometric phases The binding energy of 532 eV corresponds to absorbed oxygen species (OÀ,O2À ) on the surface of the film The lowest-intensity peak centered at 533.6 eV was attributed to the presence of CrO(OH)/Cr(OH)3 phases in the CrOx film Figure shows typical current–voltage characteristics of the Ag/100 nm-Cr2O3/Pt structure investigated by use of a dc sweeping voltage with electric pulses applied to Pt bottom electrode As is apparent from Fig 2a, the pristine Ag/Cr2O3/Pt structure has a high-resistance state (HRS) A negative bias voltage applied to the Pt bottom electrode switched the structure to the low-resistance state (LRS) Subsequently, on sweeping the positive voltage up to +2 V, the structure was converted back to the HRS The hysteresis I–V curve follows bipolar resistance switching The resistance switching described above can also realized by applying electric pulses with a width of 10 ns, as Study of the Resistive Switching Effect in Chromium Oxide Thin Films by Use of Conductive Atomic Force Microscopy Fig (a) Schematic diagram of the device used for C-AFM measurements (b) Local I–V hysteresis curve obtained by C-AFM at a compliance current of 10 nA (c–f) Current mapping images in 2D and 3D for the writing and erasing processes for Cr2O3 thin films (g) Statistical distribution of the size of silver conductive filaments during the writing process at a compliance current of 10 nA shown in Fig 2b The switching was a relatively fast process Therefore, RRAMs as universal memories should match DRAMs in terms of switching speed (DRAM write/erase time $10 ns/10 ns) The resistance ratio of the HRS and LRS is >30 and both the HRS and LRS are quite stable after 103 cycles, indicative of good endurance of the Ag/Cr2O3/ Pt structure Because the I–V curve of the LRS on the log–log scale is indicative of a linear relationship between current and voltage (not shown here), in addition to the nature of the electrodes, a reactive Ag electrode Pham, Choi, Tran, Nguyen, Hieu Le, Choi, Lee, and Phan Fig continued and an inert Pt electrode, and the switching direction, it is suggested that the mechanism of switching of Cr2O3 thin films involves electrochemical redox reactions, which are explained as follows On application of a negative voltage to the Pt bottom electrode (positive voltage to the Ag top electrode), an electrochemical reaction occurs at the anode (Ag), which oxidizes the Ag metal atoms to Ag ions These Ag+ ions start from the top interface and drift through the Cr2O3 films to connect with the bottom electrode At the Pt cathode, electrochemical reduction and electro-crystallization of Ag occur This electro-crystallization process results in the formation of an Ag filament, which grows toward the Ag electrode As a result, the Ag filaments grow and connect the Ag top electrode, leading to HRS to LRS switching To reset the cell, a positive voltage is applied to the Pt bottom electrode (negative switching voltage to the Ag top electrode), which leads to dissolution of the Ag filament, and LRS to HRS switching occurs XPS analysis shows the presence of oxygen vacancies V2+ o in the Cr2O3 thin films These oxygen vacancies can affect resistive switching of the Cr2O3 thin films To check the effect of these oxygen vacancies and of the Ag filaments, we replaced Ag by Ti as top electrode The Ti/Cr2O3/Pt structure had no resistive switching behavior Therefore, the oxygen vacancies not make a major contribution, if any, to the switching mechanisms It can again be concluded that Ag filament paths mediated by electrochemical redox reactions are responsible for resistive switching in the Cr2O3 thin films Local I–V hysteresis measurements for the Ag/ 10 nm-Cr2O3/Pt structure were conducted by use of C-AFM A conductive Pt-coated C-AFM tip was used as the top electrode, as shown schematically in Fig 3a The voltage was applied to the bottom electrode during the C-AFM scan The sweeping voltage followed the sequence fi + V fi fi À2 V fi 0, repeatedly A compliance current was used during measurements, to protect the C-AFM probe and the structure Hysteresis in the I–V curve at a compliance current Ic = 10 nA is clearly observed in Fig 3b In process 1, the initial HRS was switched to the LRS at an applied voltage (Vset) of approximately +1.7 V In the subsequent voltage sweep, process 3, a negatively applied voltage (Vreset) of À1.5 V resulted in reversion of the structure back to the HRS It is noted that the large set and reset currents hinder the application of TMOs to integrated RRAM devices However, our Ag/Cr2O3/Pt structure can switch repeatedly with low set and reset currents of 10 nA, leading to very low power consumption Conductivity mapping results for the writing and erasing processes with Cr2O3 films are shown in 2D and 3D images in Fig 3c–f In the writing process, a positive voltage of +0.5 V was applied to the Ag bottom electrode leading to the random presence of bright spots on a dark background; these represent conducting spots or multiple filaments In the erasing process, application of a negative voltage of À0.5 V to the Ag bottom electrode, deletes the current spots completely, resulting in a uniform dark background The presence of the conducting spots qualitatively confirms the filament model of resistive switching Figure 3g shows the statistical distribution of the size of conductive filaments obtained from the writing process at a compliance current of 10 nA The lateral size of the bright spots ranged from 2.9 nm to 30 nm The spot shape also indicates the spots contain multiple filaments The predominant size is