RETRACTED ARTICLE: Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaOx interface
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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/TiOx/TaOx/W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaOx/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 TaOx nanofilament in a W/TiOx/TaOx/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/TaOx 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/TaOx 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, 2, 3, 4, 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, 7, 8, 9, 10]. Among other important switching materials such as TiOx[11, 12], NiOx[13, 14, 15], HfOx[10, 16, 17, 18], ZrOx[19, 20, 21, 22, 23, 24, 25, 26, 27], Na0.5Bi0.5TiO3, SrTiO3, ZnO [30, 31], GeOx, and SiOx, tantalum oxide (TaOx) is one of the most promising choices for future RRAM applications. However, TaOx-based RRAM devices are infrequently reported [5, 34, 35, 36, 37, 38, 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/TaOx/TaN structure, and Lee et al.  reported a Pt/Ta2O5−x/TaO2−x/Pt crossbar structure with two layers of TaOx 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/TiOx/TaOx/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/TaOx 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/TaOx/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 TiOx and TaOx nanolayers are 3 and 7 nm, respectively. The presence of oxygen-deficient TaOx 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/TaOx/W-structured (device S1) and W/TaOx/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 photo-lithography 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 (tTa2O5) of approximately equal to 7 nm was then 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. 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 TaOx 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 LRS/HRS of >102, long read pulse endurance of >105, and good data retention of 104 s at 85°C, has been achieved under a low CC of 80 μA by exploiting the oxygen-getter nature of a Ti nanolayer in a W/TiOx/TaOx/W structure. A small device of 150 × 150 nm2 and a defective TaOx 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 TaOx 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.
- 3.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.Google Scholar
- 8.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 IrOx nano-dots. Nanoscale Res Lett 2012, 7: 194. 10.1186/1556-276X-7-194CrossRefGoogle Scholar
- 39.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 TaOx ReRAM and direct evidence of redox reaction mechanism. Tech Dig - Int Electron Devices Meet 2008, 293: 293.Google Scholar
- 40.The interactive Ellingham diagram http://www.doitpoms.ac.uk/tlplib/ellingham_diagrams/interactive.php
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