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Au RRAM devices
Journal Pre-proof Multi-factors induced evolution of resistive switching properties for TiN/Gd2O3/Au RRAM devices C. Sun, S.M. Lu, F. Jin, W.Q. Mo, J.L. Song, K.F. Dong PII:
S0925-8388(19)33810-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.152564
Reference:
JALCOM 152564
To appear in:
Journal of Alloys and Compounds
Received Date: 9 August 2019 Revised Date:
1 October 2019
Accepted Date: 4 October 2019
Please cite this article as: C. Sun, S.M. Lu, F. Jin, W.Q. Mo, J.L. Song, K.F. Dong, Multi-factors induced evolution of resistive switching properties for TiN/Gd2O3/Au RRAM devices, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152564. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Multi-Factors Induced Evolution of Resistive Switching Properties for TiN/Gd2O3/Au RRAM Devices C. Sun, S. M. Lu, F. Jin, W. Q. Mo, J. L. Song, K. F. Dong* 1
2
School of Automation, China University of Geosciences, Wuhan 430074, China
Hubei key Laboratory of Advanced Control and Intelligent Automation for Complex Systems, Wuhan 430074, China
Abstract In order to increase the resistive switching performance of Gd2O3 based memory devices, it was desirable to find the impact of multi-factors including thickness, deposition temperature and doping method on RRAM devices. In the present paper, we reported that decreasing the thickness of Gd2O3 film can increase the endurance of RRAM devices and high temperature deposited Gd2O3 based devices showed better uniformity as well as endurance. The reason of improvement for high temperature deposited devices was due to larger O defects content in Gd2O3 film compared with room temperature deposited Gd2O3 film. Moreover, the combination of thin and high temperature deposited Gd2O3 film can further improve the properties of RRAM devices while doping method integrating thickness and deposition temperature performed no advantages in properties compared with undoped devices. The study in this paper clarified the multi-factors induced effect of Gd2O3 film on resistive switching process, which may be applied in RRAM devices area. Keywords: RRAM; multi-factors; uniformity and endurance
I. INTRODUCTION Resistive random access memories (RRAM) is one of candidates for next-generation non-volatile memory, and rapid progress has been achieved in resistive switching mechanism, properties as well as applications [1-4]. Till now, researches in RRAM area always focus on basic unit [5-6], unit integration [7-8] and coupling application in other area (magnetic coupling RRAM, optical coupling RRAM, synapse RRAM etc) [9-12]. The resistive switching materials used for the area above are various, and among them transition-metal-oxide-based RRAM attracts much attention in which the oxygen vacancies (VOs) play a crucial role in controlling the device performance [13]. It is reported that the electrode and the oxide layer can affect the VO-relevant switching process [16]. Among them, oxide layer attracted much attention and extensive studies have been carried out. It is reported that oxide film thickness, deposition temperature, doping metal particles into oxide layer are important factors for the devices. In details, oxide film thickness can change the high resistance state (HRS) and low resistance state (LRS) as well as switching voltage [14-15]; deposition temperature affects the stoichiometric and electrical characteristics of oxide [16]; doping metal particles into oxide layer can significantly suppress the distributions of switching parameters [17-18]. However, all the studies only focus on single factor effect and the influence of combining the thickness, temperature as well as doping with different structure is rarely reported. Since the interactions between those factors may produce a positive impact on the properties of RRAM devices, it is worthwhile to study the multi-factors induced variations in switching performance. For this purpose, we choose Gd2O3 film as basic switching layer due to its higher energy bandgap, dielectric constant, thermal stability [19] and systematic study based on multi-factors is carried out. It is found that the thinner and high temperature deposited Gd2O3 film improves the uniformity and endurance of the devices but FePt doping method can not significantly change the devices’ performance. The method in this paper may reveal the effect of those factors on switching mechanism and provide a way for RRAM devices with high performance in industry application. II. EXPERIMENTS Gd2O3-based RRAM devices were fabricated by magnetron sputtering at a base pressure of 2×10-8 Torr. TiN (deposited at 400°C)/ CrRu (deposited at 280°C)/ Glass was used as bottom electrode for all devices. To study the influence of thickness, deposition temperature, FePt
particles doping on RRAM devices, different thickness of Gd2O3 films in room temperature or 700
high temperature and FePt doped Gd2O3 films with different thickness, deposition
temperature, structures were prepared. For the doped Gd2O3:FePt film, it was formed from the accumulation of alternate FePt and Gd2O3 films layer by layer. After that, the top Au electrode was subjected to deposition by dc magnetron sputtering and to patterning by using a circular shadow mask with a diameter of 200 um. The microstructures of the samples were measured by transmission electron microscopy (TEM). The elemental compositions and chemical states of the Gd2O3 films were determined by X-ray photoelectron spectroscopy (XPS). Keithley 4200 semiconductor analyzer was used to measure the current-voltage (I-V) characteristics. III. RESULTS AND DISCUSSION Three different thickness Gd2O3 films were used as switching layers and RRAM devices based them were fabricated to study the thickness effect. Figure 1a showed the I-V curve of TiN/Gd2O3/Au device and the electric field was applied on the bottom electrode with the Au connected to the ground, as shown in the insert of Fig. 1a. It can be seen that the device performed bipolar switching mode and the voltage bias was scanned as 0 V→3 V→0 V→-4 V→0 V. By sweeping a positive voltage on TiN electrode from 0 V to 3 V, the device was switched from HRS to LRS at about 1.5 V due to the formation of VO conducting filaments (CFs), known as set process. Subsequently, the device switched back to HRS at about -3 V through a reset process by sweeping the negative voltage from 0 V to -4 V, in which the CFs were ruptured with external electric field effect. The formation and rupture of CFs were related to Gd-O states and the direction of external electric field. With applying a positive voltage on TiN electrode, dielectric soft breakdown occurred and oxygen vacancies generated due to the rupture of Gd-O bonds [20]. Then oxygen ions were absorbed by TiN electrode and CFs formed of oxygen vacancies were accumulated from TiN electrode side to Au electrode side [21]. Finally, conical conducting filaments switched the devices from HRS to LRS. When a negative bias was applied on the TiN electrode, opposite process happened. Oxygen ions would be extracted from TiN electrode and Gd-O bonds formed again, resulting in rupture of CFs at the interface between Au and Gd2O3. In this process, TiN electrode was used as good oxygen reservoir, which was verified in our previous study [22-23]. However, the switching process was not stable and the repeated I-V loops were
scattered for this device. By decreasing the Gd2O3 thickness to half of the former devices, the repeated I-V times increased a little as shown in Fig. 1b. Then, the thinner Gd2O3 film could further improved the endurance from a few times to dozens of times (Fig. 1c). By comparing I-V curves of those devices, HRS became larger while LRS had no noteworthy variation with decreasing the Gd2O3 thickness, indicating the higher ratio of HRS/LRS. The cycling endurance characteristic was measured in Fig. 1d and the degradation of HRS in the first ten times cycling was observed, which may be attributed to the unstable CFs. Figure 2 showed the microstructures corresponding to the devices in Fig. 1a and Fig. 1c. It can be seen that the thicknesses of Gd2O3 film were 45 nm and 10 nm for different devices, as shown in Fig. 2a,e. The HR-TEM showed that Gd2O3 film was crystallized and different phases were observed in Fig. 2c,f, which indicated that the Gd2O3 film was polycrystalline. The polycrystalline Gd2O3 film deposited by sputtering was also reported by Li et al [24] and it suggested this phenomenon was in nature for Gd2O3 film. Since the CFs preferring to grow along grain boundary [19], polycrystalline structure had weak bonds on the grain boundary sites leading to the repeated switching memory characteristics. Furthermore, the influence of high deposition temperature and thickness of Gd2O3 film on RRAM devices was investigated in Fig. 3. The three devices in Fig. 3 were corresponding to those in Fig. 1 due to the same sputtering conditions apart from the deposition temperature, respectively. Figure 3a showed the repeated I-V curves of TiN/Gd2O3/Au device with high deposition temperature and high coincidence could be observed. The endurance was much better than it in Fig. 1a but the distribution of HRS/LRS showed that the LRS was unstable while HRS changed a little in Fig. 3b. Interestingly, decreasing the thickness still could improve the uniformity and endurance as shown in Fig. 3c. The endurance increased from 30 times to 70 times than the former device and the fluctuation of LRS still existed in Fig. 3d. Continuing to decrease the thickness of Gd2O3 film the switching properties were improved again and the endurance increased to hundreds of times accompanying that LRS also became stable as shown in Fig. 3e and Fig. 3f. The cumulative probability distributions corresponding to the devices in Fig. 3b,e indicated that the thinner device had better uniformity and larger ratio of HRS/LRS as shown in Fig. 4. The larger ratio was attributed to the higher HRS and the trend that HRS got higher with decreasing the
thickness of Gd2O3 film was the same as the devices deposited at room temperature. Thus, it can be concluded that the thin and high temperature deposited Gd2O3 film can improve the performance of RRAM devices. The TEM images corresponding to the devices in Fig. 3a,e were shown in Fig. 5. The thickness of Gd2O3 film was 36 nm and 8 nm for different devices as shown in Fig. 5a,e, which was thinner than the devices at room temperature in Fig. 2a,e. Since the deposition rate and time were the same, the thinner Gd2O3 film was due to the lateral adatom mobility. With an increase in the temperature, the adatom mobility in the film plane increased. Besides, the high deposition temperature didn’t change the polycrystalline structure of Gd2O3 film as shown in Fig. 5c,f, which indicated that the reason of property improvement for high temperature deposition devices was not the crystalline state of Gd2O3 film. In order to clarify the deposition temperature effect on the chemical structure of Gd2O3 film, XPS of the different temperature deposited Gd2O3 film corresponding to the devices in Fig. 1a and Fig. 3a were measured, respectively. The binding energies of Gd 4d5/2 and 4d3/2 for Gd2O3 grown with high temperature would shift to low energy direction compared with that of Gd2O3 grown with room temperature as shown in Fig. 6a, indicating that the value state of Gd became low for high temperature deposited Gd2O3 film. The O 1s XPS spectrums of the two devices were also shown in Fig. 6b and it was found that the intensity of O defects was much higher while lattice O was lower for high temperature deposited film than that at room temperature. Subsequently, the detailed contents of chemical bonds showed that it contained three components: O-H, Gd-O, and O defects in Fig. 6c,d. For high temperature deposited Gd2O3 film, the molar ratios of O-H, Gd-O, and O defects were 16.6%, 42.2%, and 41.2% while the corresponding molar ratios of O-H, Gd-O, and O defects for room temperature deposited Gd2O3 film were 45.3%, 21.5%, and 33.2%, respectively. During resistive switching process, the VOs (O defects) would be drifted to form or rupture conducting filaments by the high electric field. If there was more molar ratio of O defects in the initial Gd2O3 film, the conducting filaments would be much easier formation or rupture. The results of XPS indicated that the O defects content in high temperature deposited Gd2O3 film was much larger than that at room temperature deposited Gd2O3 film, directly verifying the reason of properties improvement for RRAM devices with high temperature
deposition. The combination of deposited temperature and thickness had been discussed above and doping method integrating thickness, deposited temperature was studied in Fig. 7. The doping structure was TiN/Gd2O3/FePt:Gd2O3/Gd2O3/Au and the I-V curve was shown in Fig. 7a. The resistive layer was deposited at room temperature and the devices still showed bipolar mode. The endurance of the devices was poor and decreasing the thickness of both Gd2O3 and FePt:Gd2O3 film can increase the repeated times of the devices as shown in Fig. 7b. It should be noticed that the ratio of HRS/LRS became small, which was attributed to the stronger electric conductivity of doping Gd2O3 film. The high temperature deposited devices were also shown in Fig. 7c, whose fabrication conditions were the same as the devices in Fig. 7a expect for deposition temperature. The improvement of endurance was obvious as well as large ratio of HRS/LRS for high temperature
deposited
devices.
Considering
the
doping
structure
effect,
the
TiN/FePt:Gd2O3/Gd2O3/Au devices were prepared and repeated I-V curves were shown in Fig. 7d. It can be seen that the I-V curves were scattered and the endurance was no more than 20 times for this devices. Usually, the doping method can significantly improve the properties of RRAM devices but in this study the doping devices showed no advantages compared with the undoped devices. This can be proved by the poor endurance characteristics of doping devices as shown in Fig. 7e,f. Besides, for doped devices high temperature deposited and thinner switching layer can improve the properties of the RRAM devices, which was the same as undoped devices. To observe the distribution state of doping particles, the corresponding TEM images were shown in Fig. 8. For room deposited switching layer, the thicknesses of Gd2O3 and FePt:Gd2O3 were 10nm, 30nm in Fig. 8b while the corresponding thicknesses were 8nm, 30nm for high temperature deposited devices in Fig. 8d,e, respectively. For the TiN/FePt:Gd2O3/Gd2O3/Au devices, the thicknesses of Gd2O3 and FePt:Gd2O3 were 24nm, 30nm in Fig. 8g. From high resolution TEM of the three devices in Fig. 8e, Fig. 8f and Fig. 8i, the FePt particles evenly distributed in Gd2O3 film for both devices and the polycrystalline structure of Gd2O3 film can be observed. It is reported that the uniformly dispersed FePt particles were served as the optimum growing positions for conducting filaments due to stronger local electric field [17]. However, the FePt particles would affect the polycrystalline state of Gd2O3 film in this study. Since CFs
preferred to grow along grain boundary, the reason of the doped devices showing no significantly improvement of properties may be due to the change of grain boundary with the effect of FePt particles. IV. CONCLUSION The effect of thickness, deposition temperature and doping method for Gd2O3 film on the resistive switching behaviors in RRAM devices was studied. It was found that the endurance can be improved by decreasing the thickness of Gd2O3 film or adopting high temperature deposited Gd2O3 film. The XPS results showed that O defects content was much more in high temperature deposited Gd2O3 than room temperature deposited Gd2O3, which was the main reason of improved properties for high temperature deposited Gd2O3 devices. Subsequently, the combination of thin thickness and high temperature deposition can further improve the properties of RRAM devices while doping method integrating thickness, deposited temperature showed no significantly advantages on the contrary. Since the high resolution TEM indicated Gd2O3 film performed polycrystalline structure, conducting filaments would grow along grain boundary. However, the HR-TEM of FePt particles dispersed devices showed that the FePt particles affected the polycrystalline state of Gd2O3 film, which caused the failure of improving properties for doped devices. ACKNOWLEDGEMENT This work is partially supported by the National Natural Science Foundation of China (Grant No.51501168, 41574175, 41204083), and the Fundamental Research Funds for the Central Universities, China University of Geosciences（Wuhan） (No.CUG150632 and CUGL160414)
REFERENCES [1] R. Waser, and M. Aono, Nat. Mater. 6 (2007)833. [2] S. Song, B. Cho, T.W. Kim, Y. Ji, M. Jo, G. Wang, M. Choe, Y.H. Kahng, H. Hwang, and T. Lee, Adv. Mater. 22 (2010)5048. [3] H.- S. P. Wong, H. Y. Lee, S. Yu, Y. S. Chen, Y. Wu, P. S. Chen, B. Lee, F. T. Chen, and M. T. Tsai, Proc. IEEE. 100 (2012)1951.
[4] T. Tsuruoka, K. Terabe, T. Hasegawa, and M. Aono, Nanotechnology. 21 (2010)425205. [5] W. Shen, R. Dittmann, U. Breuer, and R. Waser, Appl. Phys. Lett. 93 (2008)222102. [6] S. Gao, C. Chen, Z. Zhai and H.Y. Liu, Appl. Phys. Lett. 6 (2014)105. [7] J. J. Huang, Y. M. Tseng, C. W. Hsu, and T. H. Hou, IEEE Electron Device Lett. 10 (2011)1427. [8] Y. Zhang, Z. Duan. R. Li, C. J. Ku, R. I. Reyes, A. Ashrafi, J. Zhong, and Y. Lu, J. Phys. D: Appl. Phys. 46 (2013)145101. [9] S. X. Ren, G. W. Sun, J. Zhao, J. Y. Dong, Y. Wei, Z. C. Ma, X. Zhao, and W. Chen, Appl. Phys. Lett. 104 (2014)232406. [10] C. He, J. Li, X. Wu, P. Chen, J. Zhao, K. Yin, M. Cheng, W. Yang, G. Xie, D. Wang, D. Liu, R. Yang, D. Shi, Z. Li, L. Sun, and G. Zhang, Adv. Mater. 25 (2013)5593. [11] Z. Q. Wang, H. Y. Xu, X. H. Li, H. Yu, Y. C. Liu, and X. J. Zhu, Adv. Funct. Mater. 22 (2012)2759. [12] T. Ohno, T. Hasegawa, T. Tsuruoka, K. Terable, J. K. Gimzewski, and M. Aono, Nat. Mater. 10 (2011)591. [13] M. Fujimoto, and H. Koyama, Appl. Phys. Lett. 89 (2006)223509. [14] S. Lee, T. Kim, B. Jang, W. Y. Lee, K. C. Song, H. S. Kim, G. Y. Do, S. B. Hwang, S. Chung, and J. Jang, IEEE Electron Device Lett. 39 (2018)668. [15] D. Jana, M. Dutta, S. Samanta, and S. Maikap, Nanoscale Research Lett. 9 (2014)680. [16] K. A. Morgan, R. Huang, S. Pearce, C. H. Groot, IEEE International Symposium on Circuits and Systems, 2014, pp. 432-435. [17] B. J. Choi, A. C. Torrezan, K. J. Norris, F. Miao, J. P. Strachan, M. X. Zhang, D. A. A. Ohlberg, N. P. Kobayashi, J. J. Yang, and R. S. Williams, Nano. lett. 13 (2013)3213. [18] Q. Liu, S. Long, W. Wang, S. Tanachutiwat, Y. Li, Q. Wang, M. Zhang, Z. Huo, J. Chen, and M. Liu, IEEE Eletron Device Lett. 31 (2010)1299.
[19] X. Cao, X. Li, X. Gao, W. Yu, X. Liu, Y. Zhang, L. Chen, and X. Cheng, J. Appl. Phys. 106 (2009)073723. [20] N. Xu, B. Gao, L. F. Liu, B. Sun, X. Y. Liu, R. Q. Han, J. F. Kang, and B. Yu, Symposium on VLSI Technology, 2008, pp. 100–101. [21] Sun Young Choi, Min Kyu Yang, Sangsig Kim, and Jeon-Kook Lee, Phys. Status Solidi RRL 4 (2010)233. [22] C. Sun, S. M. Lu, F. Jin, W. Q. Mo, J. L. Song, and K. F. Dong, J. Alloys. Compd. 749 (2018)481. [23] C. Sun, S. M. Lu, F. Jin, W. Q. Mo, J. L. Song, and K. F. Dong, J. Electron. Mater. 48(2019)2992. [24] Y. L. Li, N. F. Chen, J. P. Zhou, S. L. Song, L. F. Liu, Z. G. Yin, and C. L. Cai, J. Cryst. Growth. 265 (2004)548.
Captions Figure 1 (a) the I-V curve of TiN/Gd2O3(45nm deposited at room temperature)/Au RRAM devices, the insert was the corresponding structure; (b) the repeated I-V curves of TiN/Gd2O3(23nm deposited at room temperature)/Au RRAM devices; (c) the repeated I-V curves of TiN/Gd2O3(10nm deposited at room temperature)/Au RRAM devices; (d) the endurance of TiN/Gd2O3(10nm deposited at room temperature)/Au RRAM devices Figure 2 (a), (b) and (c) the TEM and high resolution TEM of TiN/Gd2O3(45nm deposited at room temperature)/Au RRAM devices; (d), (e) and (f) the TEM and high resolution TEM of
TiN/Gd2O3(10nm deposited at room temperature)/Au RRAM devices Figure 3 (a), (b) the repeated I-V curves of TiN/Gd2O3(36nm deposited at high temperature)/Au RRAM devices and the corresponding endurance characteristics; (c), (d) the repeated I-V curves of TiN/Gd2O3(18nm deposited at high temperature)/Au RRAM devices and the corresponding endurance characteristics; (e), (f) the repeated I-V curves of TiN/Gd2O3(8nm deposited at high temperature)/Au RRAM devices and the corresponding endurance characteristics Figure 4 (a) the distribution of LRS and HRS of TiN/Gd2O3(18nm deposited at high temperature)/Au RRAM devices; (b) the distribution of LRS and HRS of TiN/Gd2O3(8nm deposited at high temperature)/Au RRAM devices Figure 5 (a), (b) and (c) the TEM and high resolution TEM of TiN/Gd2O3(36nm deposited at high temperature)/Au RRAM devices; (d), (e) and (f) the TEM and high resolution TEM of TiN/Gd2O3(8nm deposited at high temperature)/Au RRAM devices Figure 6 (a) XPS Gd 4d and (b) O 1s spectrum for both room temperature deposited and high temperature deposited Gd2O3 film; (c) detailed O 1s spectrum of room temperature deposited Gd2O3 film; (d) detailed O 1s spectrum of high temperature deposited Gd2O3 film Figure 7 (a) the I-V curve of TiN/Gd2O3(10nm)/FePt:Gd2O3(30nm)/Gd2O3(10nm)/Au RRAM devices with room temperature deposited switching layer, the insert was the corresponding
structure;
(b)
the
repeated
I-V
curves
of
TiN/Gd2O3(1.5nm)/FePt:Gd2O3(13nm)/Gd2O3(1.5nm)/Au RRAM devices with room temperature
deposited
switching
layer;
(c)
the
repeated
I-V
curves
of
TiN/Gd2O3(8nm)/FePt:Gd2O3(30nm)/Gd2O3(8nm)/Au RRAM devices with high temperature
deposited
switching
layer;
(d)
the
repeated
I-V
curves
of
TiN/Gd2O3(24nm)/FePt:Gd2O3(30nm)/Au RRAM devices with high temperature deposited switching layer; (e) and (f) the endurance characteristics corresponding to the devices in Fig. 7b and Fig. 7c Figure
8
(a),
(b)
and
(c)
the
TEM
and
high
resolution
TEM
of
TiN/Gd2O3(10nm)/FePt:Gd2O3(30nm)/Gd2O3(10nm)/Au RRAM devices with room temperature deposited switching layer; (d), (e) and (f) the TEM and high resolution
TEM of TiN/Gd2O3(8nm)/FePt:Gd2O3(30nm)/Gd2O3(8nm)/Au RRAM devices with high temperature deposited switching layer; (g), (h) and (i) the TEM and high resolution TEM of TiN/Gd2O3(24nm)/FePt:Gd2O3(30nm)/Au RRAM devices with high temperature deposited switching layer
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Highlights: The impact of multi-factors including thickness, deposition temperature and doping method on TiN/Gd2O3/Au RRAM devices was studied. Decreasing the thickness of Gd2O3 film can increase the endurance of RRAM devices and high temperature deposited Gd2O3 based devices showed better uniformity as well as endurance the combination of thin and high temperature deposited Gd2O3 film can further improve the properties of RRAM devices while doping method integrating thickness and deposition temperature performed no advantages in properties compared with undoped devices.
S0925-8388(19)33810-1
DOI:
https://doi.org/10.1016/j.jallcom.2019.152564
Reference:
JALCOM 152564
To appear in:
Journal of Alloys and Compounds
Received Date: 9 August 2019 Revised Date:
1 October 2019
Accepted Date: 4 October 2019
Please cite this article as: C. Sun, S.M. Lu, F. Jin, W.Q. Mo, J.L. Song, K.F. Dong, Multi-factors induced evolution of resistive switching properties for TiN/Gd2O3/Au RRAM devices, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152564. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Multi-Factors Induced Evolution of Resistive Switching Properties for TiN/Gd2O3/Au RRAM Devices C. Sun, S. M. Lu, F. Jin, W. Q. Mo, J. L. Song, K. F. Dong* 1
2
School of Automation, China University of Geosciences, Wuhan 430074, China
Hubei key Laboratory of Advanced Control and Intelligent Automation for Complex Systems, Wuhan 430074, China
Abstract In order to increase the resistive switching performance of Gd2O3 based memory devices, it was desirable to find the impact of multi-factors including thickness, deposition temperature and doping method on RRAM devices. In the present paper, we reported that decreasing the thickness of Gd2O3 film can increase the endurance of RRAM devices and high temperature deposited Gd2O3 based devices showed better uniformity as well as endurance. The reason of improvement for high temperature deposited devices was due to larger O defects content in Gd2O3 film compared with room temperature deposited Gd2O3 film. Moreover, the combination of thin and high temperature deposited Gd2O3 film can further improve the properties of RRAM devices while doping method integrating thickness and deposition temperature performed no advantages in properties compared with undoped devices. The study in this paper clarified the multi-factors induced effect of Gd2O3 film on resistive switching process, which may be applied in RRAM devices area. Keywords: RRAM; multi-factors; uniformity and endurance
I. INTRODUCTION Resistive random access memories (RRAM) is one of candidates for next-generation non-volatile memory, and rapid progress has been achieved in resistive switching mechanism, properties as well as applications [1-4]. Till now, researches in RRAM area always focus on basic unit [5-6], unit integration [7-8] and coupling application in other area (magnetic coupling RRAM, optical coupling RRAM, synapse RRAM etc) [9-12]. The resistive switching materials used for the area above are various, and among them transition-metal-oxide-based RRAM attracts much attention in which the oxygen vacancies (VOs) play a crucial role in controlling the device performance [13]. It is reported that the electrode and the oxide layer can affect the VO-relevant switching process [16]. Among them, oxide layer attracted much attention and extensive studies have been carried out. It is reported that oxide film thickness, deposition temperature, doping metal particles into oxide layer are important factors for the devices. In details, oxide film thickness can change the high resistance state (HRS) and low resistance state (LRS) as well as switching voltage [14-15]; deposition temperature affects the stoichiometric and electrical characteristics of oxide [16]; doping metal particles into oxide layer can significantly suppress the distributions of switching parameters [17-18]. However, all the studies only focus on single factor effect and the influence of combining the thickness, temperature as well as doping with different structure is rarely reported. Since the interactions between those factors may produce a positive impact on the properties of RRAM devices, it is worthwhile to study the multi-factors induced variations in switching performance. For this purpose, we choose Gd2O3 film as basic switching layer due to its higher energy bandgap, dielectric constant, thermal stability [19] and systematic study based on multi-factors is carried out. It is found that the thinner and high temperature deposited Gd2O3 film improves the uniformity and endurance of the devices but FePt doping method can not significantly change the devices’ performance. The method in this paper may reveal the effect of those factors on switching mechanism and provide a way for RRAM devices with high performance in industry application. II. EXPERIMENTS Gd2O3-based RRAM devices were fabricated by magnetron sputtering at a base pressure of 2×10-8 Torr. TiN (deposited at 400°C)/ CrRu (deposited at 280°C)/ Glass was used as bottom electrode for all devices. To study the influence of thickness, deposition temperature, FePt
particles doping on RRAM devices, different thickness of Gd2O3 films in room temperature or 700
high temperature and FePt doped Gd2O3 films with different thickness, deposition
temperature, structures were prepared. For the doped Gd2O3:FePt film, it was formed from the accumulation of alternate FePt and Gd2O3 films layer by layer. After that, the top Au electrode was subjected to deposition by dc magnetron sputtering and to patterning by using a circular shadow mask with a diameter of 200 um. The microstructures of the samples were measured by transmission electron microscopy (TEM). The elemental compositions and chemical states of the Gd2O3 films were determined by X-ray photoelectron spectroscopy (XPS). Keithley 4200 semiconductor analyzer was used to measure the current-voltage (I-V) characteristics. III. RESULTS AND DISCUSSION Three different thickness Gd2O3 films were used as switching layers and RRAM devices based them were fabricated to study the thickness effect. Figure 1a showed the I-V curve of TiN/Gd2O3/Au device and the electric field was applied on the bottom electrode with the Au connected to the ground, as shown in the insert of Fig. 1a. It can be seen that the device performed bipolar switching mode and the voltage bias was scanned as 0 V→3 V→0 V→-4 V→0 V. By sweeping a positive voltage on TiN electrode from 0 V to 3 V, the device was switched from HRS to LRS at about 1.5 V due to the formation of VO conducting filaments (CFs), known as set process. Subsequently, the device switched back to HRS at about -3 V through a reset process by sweeping the negative voltage from 0 V to -4 V, in which the CFs were ruptured with external electric field effect. The formation and rupture of CFs were related to Gd-O states and the direction of external electric field. With applying a positive voltage on TiN electrode, dielectric soft breakdown occurred and oxygen vacancies generated due to the rupture of Gd-O bonds [20]. Then oxygen ions were absorbed by TiN electrode and CFs formed of oxygen vacancies were accumulated from TiN electrode side to Au electrode side [21]. Finally, conical conducting filaments switched the devices from HRS to LRS. When a negative bias was applied on the TiN electrode, opposite process happened. Oxygen ions would be extracted from TiN electrode and Gd-O bonds formed again, resulting in rupture of CFs at the interface between Au and Gd2O3. In this process, TiN electrode was used as good oxygen reservoir, which was verified in our previous study [22-23]. However, the switching process was not stable and the repeated I-V loops were
scattered for this device. By decreasing the Gd2O3 thickness to half of the former devices, the repeated I-V times increased a little as shown in Fig. 1b. Then, the thinner Gd2O3 film could further improved the endurance from a few times to dozens of times (Fig. 1c). By comparing I-V curves of those devices, HRS became larger while LRS had no noteworthy variation with decreasing the Gd2O3 thickness, indicating the higher ratio of HRS/LRS. The cycling endurance characteristic was measured in Fig. 1d and the degradation of HRS in the first ten times cycling was observed, which may be attributed to the unstable CFs. Figure 2 showed the microstructures corresponding to the devices in Fig. 1a and Fig. 1c. It can be seen that the thicknesses of Gd2O3 film were 45 nm and 10 nm for different devices, as shown in Fig. 2a,e. The HR-TEM showed that Gd2O3 film was crystallized and different phases were observed in Fig. 2c,f, which indicated that the Gd2O3 film was polycrystalline. The polycrystalline Gd2O3 film deposited by sputtering was also reported by Li et al [24] and it suggested this phenomenon was in nature for Gd2O3 film. Since the CFs preferring to grow along grain boundary [19], polycrystalline structure had weak bonds on the grain boundary sites leading to the repeated switching memory characteristics. Furthermore, the influence of high deposition temperature and thickness of Gd2O3 film on RRAM devices was investigated in Fig. 3. The three devices in Fig. 3 were corresponding to those in Fig. 1 due to the same sputtering conditions apart from the deposition temperature, respectively. Figure 3a showed the repeated I-V curves of TiN/Gd2O3/Au device with high deposition temperature and high coincidence could be observed. The endurance was much better than it in Fig. 1a but the distribution of HRS/LRS showed that the LRS was unstable while HRS changed a little in Fig. 3b. Interestingly, decreasing the thickness still could improve the uniformity and endurance as shown in Fig. 3c. The endurance increased from 30 times to 70 times than the former device and the fluctuation of LRS still existed in Fig. 3d. Continuing to decrease the thickness of Gd2O3 film the switching properties were improved again and the endurance increased to hundreds of times accompanying that LRS also became stable as shown in Fig. 3e and Fig. 3f. The cumulative probability distributions corresponding to the devices in Fig. 3b,e indicated that the thinner device had better uniformity and larger ratio of HRS/LRS as shown in Fig. 4. The larger ratio was attributed to the higher HRS and the trend that HRS got higher with decreasing the
thickness of Gd2O3 film was the same as the devices deposited at room temperature. Thus, it can be concluded that the thin and high temperature deposited Gd2O3 film can improve the performance of RRAM devices. The TEM images corresponding to the devices in Fig. 3a,e were shown in Fig. 5. The thickness of Gd2O3 film was 36 nm and 8 nm for different devices as shown in Fig. 5a,e, which was thinner than the devices at room temperature in Fig. 2a,e. Since the deposition rate and time were the same, the thinner Gd2O3 film was due to the lateral adatom mobility. With an increase in the temperature, the adatom mobility in the film plane increased. Besides, the high deposition temperature didn’t change the polycrystalline structure of Gd2O3 film as shown in Fig. 5c,f, which indicated that the reason of property improvement for high temperature deposition devices was not the crystalline state of Gd2O3 film. In order to clarify the deposition temperature effect on the chemical structure of Gd2O3 film, XPS of the different temperature deposited Gd2O3 film corresponding to the devices in Fig. 1a and Fig. 3a were measured, respectively. The binding energies of Gd 4d5/2 and 4d3/2 for Gd2O3 grown with high temperature would shift to low energy direction compared with that of Gd2O3 grown with room temperature as shown in Fig. 6a, indicating that the value state of Gd became low for high temperature deposited Gd2O3 film. The O 1s XPS spectrums of the two devices were also shown in Fig. 6b and it was found that the intensity of O defects was much higher while lattice O was lower for high temperature deposited film than that at room temperature. Subsequently, the detailed contents of chemical bonds showed that it contained three components: O-H, Gd-O, and O defects in Fig. 6c,d. For high temperature deposited Gd2O3 film, the molar ratios of O-H, Gd-O, and O defects were 16.6%, 42.2%, and 41.2% while the corresponding molar ratios of O-H, Gd-O, and O defects for room temperature deposited Gd2O3 film were 45.3%, 21.5%, and 33.2%, respectively. During resistive switching process, the VOs (O defects) would be drifted to form or rupture conducting filaments by the high electric field. If there was more molar ratio of O defects in the initial Gd2O3 film, the conducting filaments would be much easier formation or rupture. The results of XPS indicated that the O defects content in high temperature deposited Gd2O3 film was much larger than that at room temperature deposited Gd2O3 film, directly verifying the reason of properties improvement for RRAM devices with high temperature
deposition. The combination of deposited temperature and thickness had been discussed above and doping method integrating thickness, deposited temperature was studied in Fig. 7. The doping structure was TiN/Gd2O3/FePt:Gd2O3/Gd2O3/Au and the I-V curve was shown in Fig. 7a. The resistive layer was deposited at room temperature and the devices still showed bipolar mode. The endurance of the devices was poor and decreasing the thickness of both Gd2O3 and FePt:Gd2O3 film can increase the repeated times of the devices as shown in Fig. 7b. It should be noticed that the ratio of HRS/LRS became small, which was attributed to the stronger electric conductivity of doping Gd2O3 film. The high temperature deposited devices were also shown in Fig. 7c, whose fabrication conditions were the same as the devices in Fig. 7a expect for deposition temperature. The improvement of endurance was obvious as well as large ratio of HRS/LRS for high temperature
deposited
devices.
Considering
the
doping
structure
effect,
the
TiN/FePt:Gd2O3/Gd2O3/Au devices were prepared and repeated I-V curves were shown in Fig. 7d. It can be seen that the I-V curves were scattered and the endurance was no more than 20 times for this devices. Usually, the doping method can significantly improve the properties of RRAM devices but in this study the doping devices showed no advantages compared with the undoped devices. This can be proved by the poor endurance characteristics of doping devices as shown in Fig. 7e,f. Besides, for doped devices high temperature deposited and thinner switching layer can improve the properties of the RRAM devices, which was the same as undoped devices. To observe the distribution state of doping particles, the corresponding TEM images were shown in Fig. 8. For room deposited switching layer, the thicknesses of Gd2O3 and FePt:Gd2O3 were 10nm, 30nm in Fig. 8b while the corresponding thicknesses were 8nm, 30nm for high temperature deposited devices in Fig. 8d,e, respectively. For the TiN/FePt:Gd2O3/Gd2O3/Au devices, the thicknesses of Gd2O3 and FePt:Gd2O3 were 24nm, 30nm in Fig. 8g. From high resolution TEM of the three devices in Fig. 8e, Fig. 8f and Fig. 8i, the FePt particles evenly distributed in Gd2O3 film for both devices and the polycrystalline structure of Gd2O3 film can be observed. It is reported that the uniformly dispersed FePt particles were served as the optimum growing positions for conducting filaments due to stronger local electric field [17]. However, the FePt particles would affect the polycrystalline state of Gd2O3 film in this study. Since CFs
preferred to grow along grain boundary, the reason of the doped devices showing no significantly improvement of properties may be due to the change of grain boundary with the effect of FePt particles. IV. CONCLUSION The effect of thickness, deposition temperature and doping method for Gd2O3 film on the resistive switching behaviors in RRAM devices was studied. It was found that the endurance can be improved by decreasing the thickness of Gd2O3 film or adopting high temperature deposited Gd2O3 film. The XPS results showed that O defects content was much more in high temperature deposited Gd2O3 than room temperature deposited Gd2O3, which was the main reason of improved properties for high temperature deposited Gd2O3 devices. Subsequently, the combination of thin thickness and high temperature deposition can further improve the properties of RRAM devices while doping method integrating thickness, deposited temperature showed no significantly advantages on the contrary. Since the high resolution TEM indicated Gd2O3 film performed polycrystalline structure, conducting filaments would grow along grain boundary. However, the HR-TEM of FePt particles dispersed devices showed that the FePt particles affected the polycrystalline state of Gd2O3 film, which caused the failure of improving properties for doped devices. ACKNOWLEDGEMENT This work is partially supported by the National Natural Science Foundation of China (Grant No.51501168, 41574175, 41204083), and the Fundamental Research Funds for the Central Universities, China University of Geosciences（Wuhan） (No.CUG150632 and CUGL160414)
REFERENCES [1] R. Waser, and M. Aono, Nat. Mater. 6 (2007)833. [2] S. Song, B. Cho, T.W. Kim, Y. Ji, M. Jo, G. Wang, M. Choe, Y.H. Kahng, H. Hwang, and T. Lee, Adv. Mater. 22 (2010)5048. [3] H.- S. P. Wong, H. Y. Lee, S. Yu, Y. S. Chen, Y. Wu, P. S. Chen, B. Lee, F. T. Chen, and M. T. Tsai, Proc. IEEE. 100 (2012)1951.
[4] T. Tsuruoka, K. Terabe, T. Hasegawa, and M. Aono, Nanotechnology. 21 (2010)425205. [5] W. Shen, R. Dittmann, U. Breuer, and R. Waser, Appl. Phys. Lett. 93 (2008)222102. [6] S. Gao, C. Chen, Z. Zhai and H.Y. Liu, Appl. Phys. Lett. 6 (2014)105. [7] J. J. Huang, Y. M. Tseng, C. W. Hsu, and T. H. Hou, IEEE Electron Device Lett. 10 (2011)1427. [8] Y. Zhang, Z. Duan. R. Li, C. J. Ku, R. I. Reyes, A. Ashrafi, J. Zhong, and Y. Lu, J. Phys. D: Appl. Phys. 46 (2013)145101. [9] S. X. Ren, G. W. Sun, J. Zhao, J. Y. Dong, Y. Wei, Z. C. Ma, X. Zhao, and W. Chen, Appl. Phys. Lett. 104 (2014)232406. [10] C. He, J. Li, X. Wu, P. Chen, J. Zhao, K. Yin, M. Cheng, W. Yang, G. Xie, D. Wang, D. Liu, R. Yang, D. Shi, Z. Li, L. Sun, and G. Zhang, Adv. Mater. 25 (2013)5593. [11] Z. Q. Wang, H. Y. Xu, X. H. Li, H. Yu, Y. C. Liu, and X. J. Zhu, Adv. Funct. Mater. 22 (2012)2759. [12] T. Ohno, T. Hasegawa, T. Tsuruoka, K. Terable, J. K. Gimzewski, and M. Aono, Nat. Mater. 10 (2011)591. [13] M. Fujimoto, and H. Koyama, Appl. Phys. Lett. 89 (2006)223509. [14] S. Lee, T. Kim, B. Jang, W. Y. Lee, K. C. Song, H. S. Kim, G. Y. Do, S. B. Hwang, S. Chung, and J. Jang, IEEE Electron Device Lett. 39 (2018)668. [15] D. Jana, M. Dutta, S. Samanta, and S. Maikap, Nanoscale Research Lett. 9 (2014)680. [16] K. A. Morgan, R. Huang, S. Pearce, C. H. Groot, IEEE International Symposium on Circuits and Systems, 2014, pp. 432-435. [17] B. J. Choi, A. C. Torrezan, K. J. Norris, F. Miao, J. P. Strachan, M. X. Zhang, D. A. A. Ohlberg, N. P. Kobayashi, J. J. Yang, and R. S. Williams, Nano. lett. 13 (2013)3213. [18] Q. Liu, S. Long, W. Wang, S. Tanachutiwat, Y. Li, Q. Wang, M. Zhang, Z. Huo, J. Chen, and M. Liu, IEEE Eletron Device Lett. 31 (2010)1299.
[19] X. Cao, X. Li, X. Gao, W. Yu, X. Liu, Y. Zhang, L. Chen, and X. Cheng, J. Appl. Phys. 106 (2009)073723. [20] N. Xu, B. Gao, L. F. Liu, B. Sun, X. Y. Liu, R. Q. Han, J. F. Kang, and B. Yu, Symposium on VLSI Technology, 2008, pp. 100–101. [21] Sun Young Choi, Min Kyu Yang, Sangsig Kim, and Jeon-Kook Lee, Phys. Status Solidi RRL 4 (2010)233. [22] C. Sun, S. M. Lu, F. Jin, W. Q. Mo, J. L. Song, and K. F. Dong, J. Alloys. Compd. 749 (2018)481. [23] C. Sun, S. M. Lu, F. Jin, W. Q. Mo, J. L. Song, and K. F. Dong, J. Electron. Mater. 48(2019)2992. [24] Y. L. Li, N. F. Chen, J. P. Zhou, S. L. Song, L. F. Liu, Z. G. Yin, and C. L. Cai, J. Cryst. Growth. 265 (2004)548.
Captions Figure 1 (a) the I-V curve of TiN/Gd2O3(45nm deposited at room temperature)/Au RRAM devices, the insert was the corresponding structure; (b) the repeated I-V curves of TiN/Gd2O3(23nm deposited at room temperature)/Au RRAM devices; (c) the repeated I-V curves of TiN/Gd2O3(10nm deposited at room temperature)/Au RRAM devices; (d) the endurance of TiN/Gd2O3(10nm deposited at room temperature)/Au RRAM devices Figure 2 (a), (b) and (c) the TEM and high resolution TEM of TiN/Gd2O3(45nm deposited at room temperature)/Au RRAM devices; (d), (e) and (f) the TEM and high resolution TEM of
TiN/Gd2O3(10nm deposited at room temperature)/Au RRAM devices Figure 3 (a), (b) the repeated I-V curves of TiN/Gd2O3(36nm deposited at high temperature)/Au RRAM devices and the corresponding endurance characteristics; (c), (d) the repeated I-V curves of TiN/Gd2O3(18nm deposited at high temperature)/Au RRAM devices and the corresponding endurance characteristics; (e), (f) the repeated I-V curves of TiN/Gd2O3(8nm deposited at high temperature)/Au RRAM devices and the corresponding endurance characteristics Figure 4 (a) the distribution of LRS and HRS of TiN/Gd2O3(18nm deposited at high temperature)/Au RRAM devices; (b) the distribution of LRS and HRS of TiN/Gd2O3(8nm deposited at high temperature)/Au RRAM devices Figure 5 (a), (b) and (c) the TEM and high resolution TEM of TiN/Gd2O3(36nm deposited at high temperature)/Au RRAM devices; (d), (e) and (f) the TEM and high resolution TEM of TiN/Gd2O3(8nm deposited at high temperature)/Au RRAM devices Figure 6 (a) XPS Gd 4d and (b) O 1s spectrum for both room temperature deposited and high temperature deposited Gd2O3 film; (c) detailed O 1s spectrum of room temperature deposited Gd2O3 film; (d) detailed O 1s spectrum of high temperature deposited Gd2O3 film Figure 7 (a) the I-V curve of TiN/Gd2O3(10nm)/FePt:Gd2O3(30nm)/Gd2O3(10nm)/Au RRAM devices with room temperature deposited switching layer, the insert was the corresponding
structure;
(b)
the
repeated
I-V
curves
of
TiN/Gd2O3(1.5nm)/FePt:Gd2O3(13nm)/Gd2O3(1.5nm)/Au RRAM devices with room temperature
deposited
switching
layer;
(c)
the
repeated
I-V
curves
of
TiN/Gd2O3(8nm)/FePt:Gd2O3(30nm)/Gd2O3(8nm)/Au RRAM devices with high temperature
deposited
switching
layer;
(d)
the
repeated
I-V
curves
of
TiN/Gd2O3(24nm)/FePt:Gd2O3(30nm)/Au RRAM devices with high temperature deposited switching layer; (e) and (f) the endurance characteristics corresponding to the devices in Fig. 7b and Fig. 7c Figure
8
(a),
(b)
and
(c)
the
TEM
and
high
resolution
TEM
of
TiN/Gd2O3(10nm)/FePt:Gd2O3(30nm)/Gd2O3(10nm)/Au RRAM devices with room temperature deposited switching layer; (d), (e) and (f) the TEM and high resolution
TEM of TiN/Gd2O3(8nm)/FePt:Gd2O3(30nm)/Gd2O3(8nm)/Au RRAM devices with high temperature deposited switching layer; (g), (h) and (i) the TEM and high resolution TEM of TiN/Gd2O3(24nm)/FePt:Gd2O3(30nm)/Au RRAM devices with high temperature deposited switching layer
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Highlights: The impact of multi-factors including thickness, deposition temperature and doping method on TiN/Gd2O3/Au RRAM devices was studied. Decreasing the thickness of Gd2O3 film can increase the endurance of RRAM devices and high temperature deposited Gd2O3 based devices showed better uniformity as well as endurance the combination of thin and high temperature deposited Gd2O3 film can further improve the properties of RRAM devices while doping method integrating thickness and deposition temperature performed no advantages in properties compared with undoped devices.