Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaO x interface
© Prakash et al.; licensee Springer. 2014
Received: 21 February 2014
Accepted: 4 March 2014
Published: 17 March 2014
Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >105 cycles, and good data retention of >104 s with a good resistance ratio of >102 at 85°C are obtained using a Ti nanolayer to form a W/TiO x /TaO x /W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaO x /W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm2 is observed by transmission electron microscopy. The presence of oxygen-deficient TaO x nanofilament in a W/TiO x /TaO x /W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaO x interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.2.
KeywordsResistive switching W/TaO x Ti nanolayer Oxygen ion migration Nanofilament
Resistive switching random access memories (RRAM) with simple metal-insulator-metal stacks are under intensive investigation owing to their great promise for use in next-generation memory applications [1–5]. However, their nonuniformity in switching, low yield, and unclear switching mechanism hinder their practical realization. RRAM devices with simple composition, easy fabrication process, and good 3D integration compatibility will be needed in the future. Methods such as doping, formation polarity control, bottom electrode modification, nanocrystal insertion, and interfacial engineering have recently been investigated to improve the characteristics of resistive switching memory [6–10]. Among other important switching materials such as TiO x [11, 12], NiO x [13–15], HfO x [10, 16–18], ZrO x [19–27], Na0.5Bi0.5TiO3, SrTiO3, ZnO [30, 31], GeO x , and SiO x , tantalum oxide (TaO x ) is one of the most promising choices for future RRAM applications. However, TaO x -based RRAM devices are infrequently reported [5, 34–39]. Terai et al.  used a TiO2 layer in a Ru/Ta2O5/TiO2/Ru stack with good thermal stability. Ninomiya et al.  reported an Ir/Ta2O5−δ/TaO x /TaN structure, and Lee et al.  reported a Pt/Ta2O5−x/TaO2−x/Pt crossbar structure with two layers of TaO x and at least one of the inert electrodes such as Ru, Ir, and Pt. Generally, many researchers use one inert electrode to improve the performance of resistive switching memory [5, 39]; however, tungsten (W) as both bottom and top electrodes in a W/TiO x /TaO x /W structure has not yet been reported. Furthermore, the RRAM devices with low current operation (<100 μA) is also a challenging issue. In this work, a resistive switching memory device using a Ti nanolayer at the W/TaO x interface and enhanced memory characteristics such as excellent 10,000 consecutive stable dc switching cycles, long read pulse endurance of >105 cycles, and good data retention of 104 s at 85°C with a large resistance ratio of >102 under a low compliance current (CC) of 80 μA are reported. Furthermore, the device can be operated with a small ‘RESET’ current of 23 μA. For comparison, the W/TaO x /W memory device is also fabricated. The device size of 150 × 150 nm2 is observed using a high-resolution transmission electron microscope (HRTEM). The thicknesses of TiO x and TaO x nanolayers are 3 and 7 nm, respectively. The presence of oxygen-deficient TaO x conducting filaments is investigated by Auger electron spectroscopy (AES) before and after switching of the memory devices. The switching mechanism of the oxygen ion migration owing to a lower barrier height of electrons is investigated, and a filament diameter of approximately equal to 3 nm is calculated using a new method also reported in this work. Considering a small filament diameter, a high memory density of approximately equal to 100 Tbit/in.2 could be designed in the future.
W/Ti/TaO x /W-structured (device S1) and W/TaO x /W-structured (device S2) resistive switching memory stacks were fabricated. A small via size of 150 × 150 nm2 was etched into the SiO2 on W bottom electrode (BE), which was about 100 nm in thickness. Standard photolithography and dry etching processes were used to open the via holes for the RRAM devices. The photoresist (PR) was coated and opened on active and top electrode (TE) regions for lift-off process. Then, a high-κ Ta2O5 film with a thickness () of approximately equal to 7 nm was deposited by an e-beam evaporator, followed by the sequential deposition of a thin (approximately equal to 3 nm) interfacial layer of titanium (Ti) and approximately equal to 200-nm-thick W layer as a TE by radio-frequency (rf) sputtering. The W and Ti targets were used. Initial vacuum was approximately 10−5 Torr. Argon gas (Ar) with a flow rate of 25 sccm and deposition power of 100 W was used to deposit W. The W deposition rate was 10 nm/min. For Ti deposition, Ar with a flow rate of 15 sccm and deposition power of 150 W were used. The Ti deposition rate was approximately 6.5 nm/min. For device S2, no Ti layer was deposited. The final devices were obtained after a lift-off process. Memory device structure and thicknesses of all layers were observed by transmission electron microscopy (TEM) with an energy of 200 keV. The TaO x material was also confirmed by quadrupole secondary ion mass spectroscopy (SIMS; ATOMIKA SIMS 4500, MA-Tek, Hsinchu, Taiwan) which had a high-depth resolution. Primary beam was O2+ with an energy of 0.5 keV and analysis area of 37.5 × 37.5 μm2. A bias was applied to the TE, and the BE was electrically grounded. Pristine S1 and S2 devices were electroformed by applying positive voltage to the TE before consecutive resistive switching cycle measurements.
Results and discussion
Improvement in resistive switching performance, particularly 10,000 consecutive switching cycles with tight distribution in HRS/LRS of >102, long read pulse endurance of >105, and good data retention of 104 s at 85°C, have been achieved under a low CC of 80 μA by exploiting the oxygen-getter nature of a Ti nanolayer in a W/TiO x /TaO x /W structure. A small device of 150 × 150 nm2 and a defective TaO x film are confirmed by TEM. O2− ion migration because of lower barrier height for electrons leads to a switching mechanism based on filament formation/rupture. The presence of controllable oxygen-deficient TaO x nanofilament after switching has been investigated by AES. Furthermore, the device could be operated with a small RESET current of 23 μA. A small nanofilament diameter of 3 nm under a low CC of 80 μA has been calculated using a new method, which has a high memory density of ≈ 100 Tbit/in.2, expected to be very useful for future sub-10-nm applications.
This work was supported by the National Science Council (NSC), Taiwan, under contract numbers NSC-98-2221-E-182-052-MY3, NSC-101-2221-E-182-061, and NSC-102-2221-E-182-057-MY2. The authors are grateful to the Electronic and Optoelectronic Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan for their support on W bottom electrode pattern.
- Waser R, Dittmann R, Staikov G, Szot K: Redox-based resistive switching memories - nanoionic mechanisms, prospects, and challenges. Adv Mater 2009, 21: 2632. 10.1002/adma.200900375View Article
- Sawa A: Resistive switching in transition metal oxides. Mater Today 2008, 11: 28.View Article
- Liu Q, Sun J, Lv H, Long S, Yin K, Wan N, Li Y, Sun L, Liu M: Real-time observation on dynamic growth/dissolution of conductive filaments in oxide-electrolyte-based ReRAM. Adv Mater 1844, 2012: 24.
- Sato Y, Kinoshita K, Aoki M, Sugiyama Y: Reduction in the reset current in a resistive random access memory consisting of NiO x brought about by reducing a parasitic capacitance. Appl Phys Lett 2007, 90: 033503. 10.1063/1.2431792View Article
- Lee MJ, Lee CB, Lee D, Lee SR, Chang M, Hur JH, Kim YB, Kim CJ, Seo DH, Seo S, Chung UI, Yoo IK, Kim K: A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta2O(5−x)/TaO(2−x) bilayer structures. Nat Mater 2011, 10: 625. 10.1038/nmat3070View Article
- Yoon J, Choi H, Lee D, Park JB, Lee J, Seong DJ, Ju Y, Chang M, Jung S, Hwang H: Excellent switching uniformity of Cu-doped MoO x /GdO x bilayer for nonvolatile memory applications. IEEE Electron Device Lett 2009, 30: 457.View Article
- Prakash A, Maikap S, Lai CS, Lee HY, Chen WS, Chen F, Kao MJ, Tsai MJ: Improvement uniformity of resistive switching parameters by selecting the electroformation polarity in IrO x /TaO x /WO x /W structure. Jpn J Appl Phys 2012, 51: 04DD06. 10.7567/JJAP.51.04DD06View Article
- Banerjee W, Maikap S, Lai CS, Chen YY, Tien TC, Lee HY, Chen WS, Chen FT, Kao MJ, Tsai MJ, Yang JR: Formation polarity dependent improved resistive switching memory characteristics using nanoscale (1.3 nm) core-shell IrO x nano-dots. Nanoscale Res Lett 2012, 7: 194. 10.1186/1556-276X-7-194View Article
- Yoon JH, Kim KM, Lee MH, Kim SK, Kim GH, Song SJ, Seok JY, Hwang CS: Role of Ru nano-dots embedded in TiO2 thin films for improving the resistive switching behavior. Appl Phys Lett 2010, 97: 232904. 10.1063/1.3525801View Article
- Lee HY, Chen PS, Liu WH, Wang SM, Gu PY, Hsu YY, Tsai CH, Chen WS, Chen F, Tsai MJ, Lien C: Robust high-resistance state and improved endurance of HfO x resistive memory by suppression of current overshoot. IEEE Electron Device Lett 2011, 32: 1585.View Article
- Choi BJ, Jeong DS, Kim SK, Rohde C, Choi S, Oh JH, Kim HJ, Hwang CS, Szot K, Waser R, Reichenberg B, Tiedke S: Resistive switching mechanism of TiO2 thin films grown by atomic-layer deposition. J Appl Phys 2005, 98: 033715. 10.1063/1.2001146View Article
- Kwon DH, Kim KM, Jang JH, Jeon JM, Lee MH, Kim GH, Li XS, Park GS, Lee B, Han S, Kim M, Hwang CS: Atomic structure of conducting nanofilaments in TiO2 resistive switching memory. Nat Nanotechnol 2010, 5: 148. 10.1038/nnano.2009.456View Article
- Lee SR, Char K, Kim DC, Jung R, Seo S, Li XS, Park GS, Yoo IK: Resistive memory switching in epitaxially grown NiO. Appl Phys Lett 2007, 91: 202115. 10.1063/1.2815658View Article
- Long S, Cagli C, Ielmini D, Liu M, Sune J: Reset statistics of NiO-based resistive switching memories. IEEE Electron Device Lett 2011, 32: 1570.View Article
- Long S, Cagli C, Ielmini D, Liu M, Sune J: Analysis and modeling of resistive switching statistics. J Appl Phys 2012, 111: 074508. 10.1063/1.3699369View Article
- Yu S, Guan X, Wong HSP: Conduction mechanism of TiN/HfO x /Pt resistive switching memory: a trap-assisted-tunneling model. ApplPhys Lett 2011, 99: 063507. 10.1063/1.3624472View Article
- Chen YY, Goux L, Clima S, Govoreanu B, Degraeve R, Kar GS, Fantini A, Groeseneken G, Wouters DJ, Jurczak M: Endurance/retention trade-off on HfO2/ metal cap 1T1R bipolar RRAM. IEEE Trans Electron Devices 2013, 60: 1114.View Article
- Long S, Lian X, Ye T, Cagli C, Perniola L, Miranda E, Liu M, Sune J: Cycle-to-cycle intrinsic RESET statistics in HfO2-based unipolar RRAM devices. IEEE Electron Device Lett 2013, 34: 623.View Article
- Lin CY, Wu CY, Wu CY, Lee TC, Yang FL, Hu C, Tseng TY: Effect of top electrode material on resistive switching properties of ZrO2 film memory devices. IEEE Electron Device Lett 2007, 28: 366.View Article
- Guan W, Long S, Jia R, Liu M: Nonvolatile resistive switching memory utilizing gold nanocrystals embedded in zirconium oxide. Appl Phys Lett 2007, 91: 062111. 10.1063/1.2760156View Article
- Liu Q, Guan W, Long S, Liu M, Zhang S, Wang Q, Chen J: Resistance switching of Au-implanted-ZrO2 film for nonvolatile memory application. J Appl Phys 2008, 104: 114514. 10.1063/1.3033561View Article
- Liu Q, Guan W, Long S, Jia R, Liu M, Chen J: Resistive switching memory effect of ZrO2 films with Zr+ implanted. Appl Phys Lett 2008, 92: 012117. 10.1063/1.2832660View Article
- Guan W, Long S, Liu Q, Liu M, Wang W: Nonpolar nonvolatile resistive switching in Cu doped ZrO2. IEEE Electron Device Lett 2008, 29: 434.View Article
- Wang SY, Lee DY, Tseng TY, Lin CY: Effects of Ti top electrode thickness on the resistive switching behaviors of rf-sputtered ZrO2 memory films. Appl Phys Lett 2009, 95: 112904. 10.1063/1.3231872View Article
- Wang SY, Lee DY, Huang TY, Wu JW, Tseng TY: Controllable oxygen vacancies to enhance resistive switching performance in a ZrO2-based RRAM with embedded Mo layer. Nanotechnology 2010, 21: 495201. 10.1088/0957-4484/21/49/495201View Article
- Li Y, Long S, Lv H, Liu Q, Wang Y, Zhang S, Lian W, Wang M, Zhang K, Xie H, Liu S, Liu M: Improvement of resistive switching characteristics in ZrO2 film by embedding a thin TiO x layer. Nanotechnology 2011, 22: 254028. 10.1088/0957-4484/22/25/254028View Article
- Lin CC, Chang YP, Lin HB, Lin CH: Effect of non-lattice oxygen on ZrO2-based resistive switching memory. Nanoscale Res Lett 2012, 7: 187. 10.1186/1556-276X-7-187View Article
- Zhang T, Zhang X, Ding L, Zhang W: Study on resistance switching properties of Na0.5Bi0.5TiO3 thin films using impedance spectroscopy. Nanoscale Res Lett 2009, 4: 1309. 10.1007/s11671-009-9397-4View Article
- Sun X, Li G, Chen L, Shi Z, Zhang W: Bipolar resistance switching characteristics with opposite polarity of Au/SrTiO3/Ti memory cells. Nanoscale Res Lett 2011, 6: 599. 10.1186/1556-276X-6-599View Article
- Chiu FC, Li PW, Chang WY: Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films. Nanoscale Res Lett 2012, 7: 178. 10.1186/1556-276X-7-178View Article
- Peng CN, Wang CW, Chan TC, Chang WY, Wang YC, Tsai HW, Wu WW, Chen LJ, Chueh YL: Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation. Nanoscale Res Lett 2012, 7: 559. 10.1186/1556-276X-7-559View Article
- Rahaman SZ, Maikap S, Chen WS, Lee HY, Chen FT, Kao MJ, Tsai MJ: Repeatable unipolar/bipolar resistive memory characteristics and switching mechanism using a Cu nanofilament in a GeO x film. Appl Phys Lett 2012, 101: 073106. 10.1063/1.4745783View Article
- Syu YE, Chang TC, Tsai TM, Chang GW, Chang KC, Lou JH, Tai YH, Tsai MJ, Wang YL, Sze SM: Asymmetric carrier conduction mechanism by tip electric field in WSiO x resistance switching device. IEEE Electron Device Lett 2012, 33: 342.View Article
- Rahaman SZ, Maikap S, Tien TC, Lee HY, Chen WS, Chen F, Kao MJ, Tsai MJ: Excellent resistive memory characteristics and switching mechanism using a Ti nanolayer at the Cu/TaO x interface. Nanoscale Res Lett 2012, 7: 345. 10.1186/1556-276X-7-345View Article
- Prakash A, Maikap S, Lai CS, Tien TC, Chen WS, Lee HY, Chen FT, Kao MJ, Tsai MJ: Bipolar resistive switching memory using bilayer TaO x /WO x films. Solid State Electron 2012, 77: 35.View Article
- Chen C, Song C, Yang J, Zeng F, Pan F: Oxygen migration induced resistive switching effect and its thermal stability in W/TaO x /Pt structure. Appl Phys Lett 2012, 100: 253509. 10.1063/1.4730601View Article
- Terai M, Sakotsubo Y, Kotsuji S, Hada H: Resistance controllability of Ta2O5/TiO2 stack ReRAM for low-voltage and multilevel operation. IEEE Electron Device Lett 2010, 31: 204.View Article
- Ninomiya T, Wei Z, Muraoka S, Yasuhara R, Katayama K, Takagi T: Conductive filament scaling of TaO x bipolar ReRAM for improving data retention under low operation current. IEEE Trans Electron Devices 2013, 60: 1384.View Article
- Wei Z, Kanzawa Y, Arita K, Katoh Y, Kawai K, Muraoka S, Mitani S, Fujii S, Katayama K, Iijima M, Mikawa T, Ninomiya T, Miyanaga R, Kawashima Y, Tsuji K, Himeno A, Okada T, Azuma R, Shimakawa K, Sugaya H, Takagi T, Yasuhara R, Horiba K, Kumigashira H, Oshima M: Highly reliable TaO x ReRAM and direct evidence of redox reaction mechanism. Tech Dig - Int Electron Devices Meet 2008, 293: 293.
- The interactive Ellingham diagram .] [http://www.doitpoms.ac.uk/tlplib/ellingham_diagrams/interactive.php [.]
- Michaelson HB: The work function of the elements and its periodicity. J Appl Phys 1977, 48: 4729. 10.1063/1.323539View Article
- Stille S, Lenser C, Dittmann R, Koehl A, Krug I, Muenstermann R, Perlich J, Schneider CM, Klemradt U, Waser R: Detection of filament formation in forming-free resistive switching SrTiO3 devices with Ti top electrodes. Appl Phys Lett 2012, 100: 223503. 10.1063/1.4724108View Article
- Aarik J, Aidla A, Kiisler AA, Uustare T, Sammelselg V: Effect of crystal-structure on optical-properties of TiO2 films grown by atomic layer deposition. Thin Solid Films 1997, 305: 270. 10.1016/S0040-6090(97)00135-1View Article
- Gu D, Li J, Dey SK, Waard HD, Marcus S: Nanochemistry, nanostructure, and electrical properties of Ta2O5 film deposited by atomic layer deposition and plasma-enhanced atomic layer deposition. J Vac Sci Technol B 2006, 24: 2230. 10.1116/1.2335432View Article
- Maikap S, Wang TY, Tzeng PJ, Lin CH, Tien TC, Lee LS, Yang JR, Tsai MJ: Band offsets and charge storage characteristics of atomic layer deposited high- k HfO2/TiO2 multilayers. Appl Phys Lett 2007, 90: 262901. 10.1063/1.2751579View Article
- Yang JJ, Zhang MX, Strachan JP, Miao F, Pickett MD, Kelley RD, Medeiros-Ribeiro G, Williams RS: High switching endurance in TaO x memristive devices. Appl Phys Lett 2010, 97: 232102. 10.1063/1.3524521View Article
- Prakash A, Maikap S, Rahaman SZ, Majumdar S, Manna S, Ray SK: Resistive switching memory characteristics of Ge/GeO x nanowires and evidence of oxygen ion migration. Nanoscale Res Lett 2013, 8: 220. 10.1186/1556-276X-8-220View Article
- Yasuhara R, Fujiwara K, Horiba K, Kumigashira H, Kotsugi M, Oshima M, Takagi H: Inhomogeneous chemical states in resistance-switching devices with a planar-type Pt/CuO/Pt structure. Appl Phys Lett 2009, 95: 012110. 10.1063/1.3175720View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.