Excellent resistive memory characteristics and switching mechanism using a Ti nanolayer at the Cu/TaOx interface
© Rahaman et al.; licensee BioMed Central Ltd. 2012
Received: 19 March 2012
Accepted: 26 June 2012
Published: 26 June 2012
Excellent resistive switching memory characteristics were demonstrated for an Al/Cu/Ti/TaOx/W structure with a Ti nanolayer at the Cu/TaOx interface under low voltage operation of ± 1.5 V and a range of current compliances (CCs) from 0.1 to 500 μA. Oxygen accumulation at the Ti nanolayer and formation of a defective high-κ TaOx film were confirmed by high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photo-electron spectroscopy. The resistive switching memory characteristics of the Al/Cu/Ti/TaOx/W structure, such as HRS/LRS (approximately 104), stable switching cycle stability (>106) and multi-level operation, were improved compared with those of Al/Cu/TaOx/W devices. These results were attributed to the control of Cu migration/dissolution by the insertion of a Ti nanolayer at the Cu/TaOx interface. In contrast, CuOx formation at the Cu/TaOx interface was observed in an Al/Cu/TaOx/W structure, which hindered dissolution of the Cu filament and resulted in a small resistance ratio of approximately 10 at a CC of 500 μA. A high charge-trapping density of 6.9 × 1016 /cm2 was observed in the Al/Cu/Ti/TaOx/W structure from capacitance-voltage hysteresis characteristics, indicating the migration of Cu ions through defect sites. The switching mechanism was successfully explained for structures with and without the Ti nanolayer. By using a new approach, the nanoscale diameter of Cu filament decreased from 10.4 to 0.17 nm as the CC decreased from 500 to 0.1 μA, resulting in a large memory size of 7.6 T to 28 Pbit/sq in. Extrapolated 10-year data retention of the Ti nanolayer device was also obtained. The findings of this study will not only improve resistive switching memory performance but also aid future design of nanoscale nonvolatile memory.
KeywordsTi nanolayer Nanoscale Resistive memory Nanofilament Charge-trapping.
Recently, many resistive switching random access memory (ReRAM) devices containing oxides such as SrTiO3[1–3], Al2O3, NiOx[5–7], Na0.5Bi 0.5TiO3, ZnO [9, 10], Ta2O5, ZrO2[12–15], GdOx[16, 17], HfOx[18, 19], and TiOx[21–23] have been reported for future nanoscale nonvolatile memory applications. However, the resistive switching mechanism of ReRAM devices is currently debated. On the other hand, conductive-bridge ReRAM devices with different solid-electrolytes, such as GeSex[24–27], GeS [28, 29], Ta2O5[30, 31], ZrO2, SiO2, Ag2S [34, 35], HfO2[36, 37], SrTiO3, and Cu-Te/Al2O3 have also been reported by several groups. In these cases, silver (Ag) or copper (Cu) metal can be used as one of the electrodes to mobilize Ag+ or Cu2+ ions. Under an external bias on Ag or Cu electrode, metallic filament can be formed (or dissolved) into solid-electrolyte films. In general, Cu is a more preferable material than Ag because it is used as an interconnection metal in computer motherboards. High-κ Ta2O5 is considered the most promising as a resistive switching material. [30, 31] Therefore, a resistive switching memory device with a Cu/TaOx/W structure could be desirable but also may have the drawback of copper oxidation (CuOx) at the Cu/TaOx interface, which can hinder resistive switching characteristics. Moreover, controlling Cu ion transportation and recovery under external bias is difficult. Tada et al.  reported resistive switching memory based on a dual layered TiOx/TaSiOx solid-electrolyte under a high current compliance (CC) of 800 μA and a large operation voltage of 2.5 V. To prevent CuOx formation as well as promote easier Cu filament formation/dissolution through the high-κ TaOx solid-electrolyte, a Ti nanolayer at the Cu/TaOx interface is a promising approach to design an Al/Cu/Ti/TaOx/W resistive switching memory device, which has not been reported to date. Furthermore, Ti has good adhesion behavior and provides a good Cu diffusion barrier. The Gibbs free energies of TiO2, Ta2O5, CuO, and Cu2O films at 300 K are −889.5, –760.75, –129.7, and −149.0 kJ/mole, respectively , suggesting that the Ti nanolayer can be easily oxidized compared with the other possible materials. Therefore, a Ti nanolayer will consume more oxygen from the TaOx layer and will form TiOx/TaOx. Consequently, the Cu electrode will not form a CuO layer at the Cu/TaOx interface because of this greater oxygen consumption at the Ti nanolayer. This also has the benefit of easily controlling Cu migration and collection through the resulting higher defective high-κ TaOx solid-electrolyte under external bias. In this study, excellent resistive switching memory characteristics were observed in the proposed Al/Cu/Ti/TaOx/W structure with a Ti nanolayer at the Cu/TaOx interface annealed at 350°C in ambient N2 compared with a similar structure without a Ti nanolayer. This configuration will be useful for complementary metal-oxide-semiconductor (CMOS) processing after back end of line. The fabricated Al/Cu/Ti/TaOx/W structure memory device with a small area of 150 × 150 nm2 was observed by high-resolution transmission electron microscopy (HRTEM), X-ray photo-electron spectroscopy (XPS), and energy dispersive X-ray (EDX). In addition, the migration and collection (or formation/dissolution) of Cu ions through the defective TaOx solid-electrolyte under external bias, as well as its switching mechanism, were determined from capacitance-voltage (C-V) hysteresis. Improved resistive switching memory performance of the Al/Cu/Ti/TaOx/W structure compared with that of the Al/Cu/TaOx/W structure was also obtained, such as repeatable switching cycles with maintenance of a high resistance ratio (approximately 104), long extrapolated program and erase endurance of > 106 cycles, multi-level capability, and extrapolated 10 y data retention. Furthermore, Cu nanofilament diameters were calculated under current compliances (CCs) of 0.1 to 500 μA using a new approach. A large memory size of 28 Pbit/sq in. was achieved with a small nanofilament diameter of 0.17 nm under a CC of 0.1 μA.
Results and discussion
Figure 1a shows the XPS spectra of Ta 4f core-level electrons. Peaks were fitted using Gaussian functions. The peak binding energies of Ta2O5 4f7/2 and Ta2O5 4f5/2 electrons in the sample without (w/o) Ti were centered at 26.7 and 28.6 eV, respectively. For the device with (w/) Ti, the peak binding energies of Ta2O5 4f7/2 and Ta2O5 4f5/2 electrons were centered at 26.48 and 28.39 eV, respectively. The w/ Ti sample showed slightly lower binding energies than the w/o Ti sample, attributed to oxygen accumulation in the Ti layer from the Ta2O5 layer. Chang et al.  previously reported a larger binding energy shift of approximately 4.5 eV after heat treatment of the Ti/Ta2O5 layers due to oxygen accumulation in the Ti nanolayer (i.e., formation of TiO2 or TiOx film). The binding energies of the Ta 4f7/2 and Ta 4f5/2 electrons for the w/o Ti sample were centered at 21.77 and 23.74 eV, respectively, while those of the w/ Ti sample were centered at 21.99 and 23.39 eV, respectively. These results suggest that the high-κ Ta2O5 film in both structures contained Ta metal (i.e.,TaOx where x < 2.5). The peak height ratios of the Ta 4f7/2, Ta 4f5/2, and Ta2O5 4f5/2 core levels for the w/o Ti samples with respect to the Ta2O5 4f7/2 peak height were 0.03, 0.03, and 0.77, respectively, while those for the w/ Ti samples were 0.27, 0.16, and 0.77, respectively. This also suggests that the Ta content was higher in the w/ Ti samples than in the w/o Ti samples. Furthermore, the TiO2 2p3/2 binding energy in the w/ Ti samples was 459.57 eV (Figure 1b), which is close to that reported in previous studies [43, 44]. These results indicate that a higher Ta metal content was present in the w/ Ti sample because of oxygen migration from the TaOx film to the Ti film, resulting in TiOx/TaOx bilayers. Because of this oxygen accumulation property of the Ti nanolayer, the Cu layer was expected to be protected from oxidation at the Cu/TaOx interface. In addition, the high-κ TaOx solid-electrolyte is expected to be more defective, which will lead to improve resistive switching memory characteristics in the Al/Cu/Ti/TaOx/W structure compared with the Al/Cu/TaOx/W structure. The resulting Cu oxidation and memory characteristics are explained below.
where L is the length (thickness of high-κ TaOx solid-electrolyte, approximately 18 nm), ρfilament is the resistivity (approximately 200 μΩ.cm ), Ф is the cross-sectional area, and D is the diameter of the Cu filament. Using RLRS and CC as obtained from equations 2 and 5, respectively, the filament diameter was found to decrease linearly with CC (Figure 10). The nanoscale filament diameter was 10.4 to 0.17 nm as the CC decreased from 500 to 0.1 μA. These calculated filament diameters were generally consistent with those in some reports [26, 32, 50] but were slightly different than others in the literature [33, 51], which may be due to different solid-electrolytes and structures used. The observed small diameter of 1.7 Å at a small CC of 0.1 μA indicates that this device will be scalable beyond the atomic scale in the future. Under a CC of 500 μA, a memory size of 7.6 Tbit/sq in will be obtained, higher than that previously reported (60 Gbit/sq in. ). If a low current operation of 0.1 μA is achieved, then an even larger memory size of 28 Pbit/sq in. will be obtained, a significant benefit to future high-density nonvolatile memory applications.
In summary, superior resistive switching memory performance of an Al/Cu/Ti/TaOx/W structure in a CC range of 0.1 to 500 μA was demonstrated compared with that of an Al/Cu/TaOx/W structure. The resistive switching mechanism was elucidated and explained by a schematic model, correlated with charge-trapping phenomena observed from C-V hysteresis characteristics. These findings suggest that the formation/dissolution of Cu nanofilament can be controlled by insertion of a Ti nanolayer at the Cu/TaOx interface or using a defective high-κ TaOx solid-electrolyte. The high-κ TiOx and TaOx films were amorphous as observed by HRTEM. The oxygen accumulation properties of the Ti nanolayer at the Cu/TaOx interface were confirmed by EDX and XPS analyses. Good uniformity with large resistance ratio of approximately 104, long P/E endurance of > 106 cycles, MLC operation, and 10 y data retention were observed for the devices with Ti nanolayer at the Cu/TaOx interface. The LRS increased linearly with decreasing CC in the range 500 to 0.1 μA, which resulted in a decrease of the Cu nanofilament diameter from 10.4 to 0.17 nm. These results suggest that a large memory size of 28 Pbit/sq in. at a small CC of 0.1 μA will be possible for the present devices for future nanoscale (0.17 nm) nonvolatile memory applications.
This work was supported by the National Science Council (NSC), Taiwan, under contract number: NSC-98-2221-E-182-052-MY3. We are grateful to NSC, Taiwan for their support. The authors are also grateful to MSSCORPS Co., Ltd., and MA-tek, Hsinchu for their HRTEM support and discussion of our resistive switching memory device. The authors are grateful to Prof. M. N. Kozicki, Arizona State University, USA for his insightful suggestions in this study.
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