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Oxygen vacancy modulation and enhanced switching behavior in HfOx film induced by Al doping effect
Journal of Alloys and Compounds 686 (2016) 669e674
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Oxygen vacancy modulation and enhanced switching behavior in HfOx film induced by Al doping effect Tingting Guo*, 1, Tingting Tan**, 1, Zhengtang Liu, Bangjie Liu State Key Lab of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2016 Received in revised form 8 June 2016 Accepted 9 June 2016 Available online 11 June 2016
The effect of Al doping concentration on resistive switching characteristics of Pt/Al-doped HfOx/Cu structures was investigated. According to the X-ray photoelectron spectroscopy (XPS) analysis and the first-principle calculation, the Al doping introduced more oxygen vacancies in HfOx film and these oxygen vacancies can be easily generated along Al dopants. Significant enhancement in resistive switching behaviors was achieved for 6.8% Al-doped HfOx film in terms of ON/OFF ratio, switching voltages and uniformity, whereas the higher Al doping concentration of 9.4% caused excess oxygen vacancies in the film, resulting in the degeneration of switching performance. A schematic diagram based on the formation and rupture of oxygen vacancy filaments was proposed to illustrate the distinct switching behaviors of HfOx and Al-doped HfOx samples. The results indicated that the concentration and distribution of oxygen vacancies played an important role on resistive switching behavior. The reliability properties for the prepared HfOx samples were also investigated. This work might provide a guidance for the further optimization of HfOx-based memory. © 2016 Elsevier B.V. All rights reserved.
Keywords: Resistive switching Al doping concentration Oxygen vacancies Conductive filaments
1. Introduction Due to the theoretical and physical limitation of traditional flash memory as the device size scales down continuously, novel non-volatile memories with advanced device performance and high integration density are urgently required and being extensively studied [1]. Recently, the merits such as simple structure, low power consumption, and compatibility with complementary metal oxide semiconductor (CMOS) process give an advantage of resistive random access memory (RRAM) as a promising candidate for the next generation memory [2e5] and the increasing attention has been focused on RRAM. The resistive switching (RS) phenomenon can be realized in a large scale of materials, especially in binary metal oxides (ZrO2 [6], TaOx [7], TiO2 [8], HfO2 [9]) which are identified as the most promising material in RRAM due to their easily controlled composition, low consumption and good compatibility with CMOS technology. At present, good performance including large ON/OFF ratio and reliable data retention has
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Guo), [email protected] (T. Tan). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jallcom.2016.06.090 0925-8388/© 2016 Elsevier B.V. All rights reserved.
been demonstrated in HfOx-based devices [9,10], while there still exist some practical issues required to be solved urgently for its application, such as how to seek out an effective way to improve the uniformity of switching parameters and the reliability of the device [11,12]. Since it has been widely accepted that the RS behaviors for HfOx film are closely related to oxygen vacancies (Vos) in the film [13e15], the origin of performance variation can be mainly attributed to the random distribution of Vos in the film. To effectively improve the RS performance, it is necessary to find a method to modulate the distribution and concentration of Vos in the switching layer. According to the first-principle calculations and experimental results [16,17], trivalent dopant can greatly reduce the formation energy Vos along the doping sites in HfO2 film, improving the uniformity of RS behavior finally. In this work, the HfOx films with different Al doping concentrations were fabricated. The influence of Al doping concentration on chemical composition and electrical properties of HfOx sample was demonstrated. The improved RS performance was achieved by modulating the Al doping concentration across switching layer due to the appropriate introduction of Vos. The results confirmed the important role of Vos on RS behaviors. Moreover, the switching mechanisms of the prepared samples were discussed. This work might offer a method to optimize the RRAM device by controlling the dopants concentration.
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2. Experimental details 2.1. Device fabrication and characterization The Si/SiO2/Ti/Pt substrates were used as bottom electrodes and first cleaned in deionized water, alcohol and acetone respectively. Subsequently, 20-nm-thick HfOx films doped with different Al doping concentrations were deposited on Pt substrates by radio frequency magnetron co-sputtering. The introduction of Al atoms was realized by loading the metal Al pieces on the metal Hf target, and the Al doping concentration was controlled by the area ratio of Al/Hf. During the deposition, the Ar:O2 was 12:3, the working pressure was 0.3 Pa and the sputtering power was 80 W. The chemical compositions of Al-doped HfOx films were analyzed by Xray photoelectron spectroscopy (XPS) and the Al doping concentrations were estimated to be 0%, 3.2%, 6.8%, 9.4% respectively. To measure the electrical properties of the prepared HfOx films, the 50-nm-thick Cu top electrodes with diameter of 2 mm were deposited by evaporation with a metal shadow mask to pattern the size. The electrical properties were characterized by an Agilent 4155C semiconductor parameter analyzer. During the test, the bias voltage was applied on Cu top electrode and the bottom electrode was grounded. 2.2. Theoretical calculation To investigate the variations of formation energy of Vos by Al doping, the first principles calculations based on density functional theory were carried out by using a generalized gradient approximation [18] for the exchange-correlation potential and Vanderbilt type ultrasoft pseudopotentials [19], as implemented in the CASTEP code [20]. In the calculations, the HfO2 structures doped with Al atom were constructed by 2 2 2 supercells of monoclinic HfO2, the doping effect was investigated by using Al atoms to replace Hf atoms and the Vo properties were simulated by removing one oxygen atom from the supercell. The formation energy of Vos was calculated as Ef(O) ¼ Etot(O)Etot(bulk) þ mo. 3. Results and discussion To explore the composition changes of HfOx films by Al doping, the XPS analyses were performed, as shown in Fig. 1. All peaks have been calibrated by C 1s (284.6 eV) peak. Fig. 1(a) shows the XPS spectra of Hf 4f core levels of HfOx and Al-doped HfOx films. The spectrum for HfOx film (0%) can be deconvoluted into a double peak of Hf 4f7/2 and Hf 4f5/2 at binding energies (BEs) of 16.9 eV and 18.6 eV respectively, which correspond to HfeO [21]. Apparently,
the Hf 4f peak of Al-doped HfOx film shifts toward the higher BE in comparison to the peak of HfOx film. Guittet’ study [22] showed that when mixing two oxides, the cation of the more ionic metal oxide is expected to become even more ionic after formation of the complex oxide. As a result, with the increasing Al doping concentration, Hf becomes more ionic and loses more charges. Therefore, the shift of Hf 4f peak to higher BE indicates that Al atoms bond to HfOx structure to form HfAlOx [23]. Fig. 1(b) shows the percentage of non-lattice oxygen in the prepared films. The inset of Fig. 1(b) shows the curve of O 1s spectrum of HfOx film. The O 1s peak can be fitted into two peaks which are attributed to the lattice oxygen at lower BE of 530.3 eV and non-lattice oxygen at higher BE of 532.1 eV. The fitting results for O 1s of Al-doped HfOx films are similar to that of HfOx film. Based on the reports [24,25] that nonlattice oxygen is related to the formation of Vos and the ratio of nonlattice oxygen is proportional to the amount of Vos, the concentration of Vos in HfOx film can be estimated by analyzing the ratio of non-lattice oxygen in the film. In Fig. 1(b), the ratio of non-lattice oxygen clearly shows a positively proportional dependence on Al doping concentration, indicating the increased Vos by Al doping. The formation energies of Vos in HfO2 and Al-doped HfO2 structures were calculated by first principles to further investigate the variation of Vos in HfO2 film by Al doping, as shown in Table 1. The results showed that the formation energy of Vos decreased by Al doping, which was consistent with Gao’ study [16]. Besides, the Vos can be more easily formed near Al atom than far from Al atom. Combining the calculation results with XPS analysis, it can be deduced that the doping of Al in HfOx film results in the increase of Vos. Fig. 2(a) shows the schematic diagram of Cu/Al-doped HfOx/Pt structures for I-V measurements. Fig. 2(b)e(e) show the RS behaviors of HfOx and Al-doped HfOx samples respectively. The forming processes are presented in the inset of the corresponding figures. The initial states for all prepared samples were in a high resistance state (HRS). For HfOx sample (0%), a forming voltage (FV) of ~1.2 V was required to activate the RS behavior, switching the sample into a low resistance state (LRS), as presented in the inset of Fig. 2(b). The same electrical measurement was applied to Al-doped HfOx samples. The Al-doped HfOx samples exhibited a lower FV than HfOx sample, which can be attributed to the increased Vos in Al-doped HfOx samples. After the forming process, typical bipolar RS characteristics of HfOx (0%) and Al-doped HfOx samples (3.2%, 6.8% and 9.4%) were achieved, as shown in Fig. 2(b), (c), (d), and (e) respectively. During the switching process, the current compliance was set to 10 mA to prevent the sample from damage and the voltage swept in a counterclockwise direction, as indicated by arrows in Fig. 2(b). Obviously, enlarged ON/OFF ratio can be observed
Fig. 1. (a) The XPS spectra of Hf 4f and (b) the percentage of non-lattice oxygen in HfOx and Al-doped HfOx films. The inset of (b) shows the O 1s spectrum in HfOx film.
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Table 1 The formation energy of Vos in HfOx and Al-doped HfOx. Location of Vos
Vos in HfOx
Vos in Al-doped HfOx nearest to Al atom
Vos in Al-doped HfOx far from Al atom
Formation energy (eV)
6.63/6.53 [16]
4.05
4.79
Fig. 2. (a) The schematic diagram of Pt/Al-doped HfOx/Cu structures for I-V measurements. The I-V curves of (b) HfOx and (c) (d) (e) Al-doped HfOx samples. The insets of (b) (c) (d) (e) show the corresponding forming processes. (f) The I-V curves of HRS in double-log scale and the fitted slope. The inset of (f) shows the I-V curves of LRS in double-log scale.
for Al-doped HfOx samples, especially for 6.8% Al-doped HfOx sample which shows ON/OFF ratio larger than 103 and good reproducibility. To explore the conductive mechanisms of HfOx and Al-doped HfOx samples, the I-V curves were replotted in double-log scale, as shown in Fig. 2(f). The fitting curves in LRS for all samples showed a linear line with slope of approximately 1, as presented in the inset of Fig. 2(f), indicating that the conduction behavior in LRS was ohmic conduction. However, the curves for HRS were a little complicated, which exhibited ohmic conduction at low-voltage region (I ~ V) and Child’s law at high-voltage region (I ~ V2). This
conduction behavior can be well explained by the trap controlled space-charge-limited-current effect, which was commonly observed in insulators [10,26,27]. Metal ions or Vos which can migrate to form filaments in the film are believed to play a crucial role on switching behavior. To explore the composition of the filaments, the temperature dependence of resistance for HfOx and Al-doped HfOx samples was performed, as shown in Fig. 3(a). The resistance in HRS for all samples decreased with the increase of temperature, indicating the oxide or semiconducting characteristics. For LRS, the resistance also slightly decreased as the temperature increased, exhibiting the
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Fig. 3. (a) The dependence of resistance on temperature and (b) the forming processes under positive and negative voltage for HfOx and Al-doped HfOx samples.
semiconducting conduction mechanism. This semiconductor behavior of LRS was related to the filament formation by Vos in the film. Fig. 3(b) compares the positive and negative forming processes of HfOx and Al-doped HfOx samples. All samples showed the similar behavior. Under the positive voltage, the current abruptly increased as the voltage reached about 1 V, switching the sample from HRS to LRS. However, there was no obvious change in current by applying a negative voltage even up to 2 V. The change of current only under positive voltage can be attributed to electrochemically active electrode of Cu, indicating the indispensable role of Cu on switching behavior. Based on above analyses, the migration of Vos and the redox of Cu electrode were both responsible for the switching behavior, while the former may play a dominant role due to Al doping according to experimental results. Besides, since the same electrodes of Cu were used for all samples and we focused more on the variation of Vos by Al doping, the effect of Cu on switching behavior was not discussed further. As a result, in this work, the Vos can be regarded as carrier trapping centers and the dominant role in assisting the electron transport was attributed to the formation of Vos filaments. The distribution of switching voltages and resistances for HfOx and Al-doped HfOx samples were shown in Fig. 4(a) and (b) respectively. In Fig. 4(a), the Vset and Vreset for 3.2% Al-doped HfOx sample showed nearly no decrease compared with HfOx sample, which may be due to the low doping concentration. With the increase of Al doping concentration, the voltages decreased obviously. Fig. 4(b) shows the distribution of resistances in HRS and LRS of the prepared samples. The increased resistance in HRS was
observed for 3.2% and 6.8% Al-doped HfOx samples which may be due to the ionized impurity scattering caused by interstitial Al atoms, leading to the improved ON/OFF ratio. The decreased resistance in HRS of 9.4% Al-doped HfOx sample would be discussed later. The uniformity of the switching voltages and resistances was both improved by Al doping. Our calculation results showed that doping Al atoms into HfOx film decreased the formation energy of Vos around Al doping sites, which was consistent with Yu’ study [28]. Thus, the decreased switching voltages in Al-doped HfOx samples were due to the easy aggregation of Vos and the creation of Vos along Al atoms led to the formation of fixed conductive paths which suppressed the randomness of Vos filaments and improved the uniformity of switching parameters. To get further information on switching stability of HfOx and Aldoped HfOx samples, more than 100 switching cycles were performed in each sample and the endurance properties were presented in Fig. 5. In Fig. 5(a), the current in LRS and HRS for HfOx sample showed large fluctuations because the Vos in HfOx sample were randomly formed during deposition. Due to the easy formation of Vos along the Al doping sites, the improved uniformity of RS performance should be achieved by Al doping. Actually, only HfOx samples doped with lower Al doping concentration (3.2% and 6.8%) showed improved uniformity, as shown in Fig. 5 (b) and (c). For 9.4% Al-doped HfOx sample (Fig. 5(d)), the current in HRS exhibited somewhat fluctuations and an increase of current in HRS can be observed. The same phenomenon was found in other studies [29,30] that a high doping concentration resulted in a reduced ON/ OFF ratio. This can be attributed to the creation of abundant Vos by
Fig. 4. (a) The distribution of switching voltages (Vset and Vreset) and (b) the statistical distribution of the resistances in HRS and LRS of HfOx and Al-doped HfOx samples. The data are collected from 50 switching cycles.
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Fig. 5. The endurance properties of HfOx and Al-doped HfOx samples.
the higher Al doping concentration. These Vos filaments can not be entirely ruptured and part of them still remained during the reset process, which led to the increased HRS current and reduced ON/ OFF ratio [31]. On the basis of above results, the RS behaviors of HfOx and Aldoped HfOx samples can be attributed to the formation and rupture of Vos filaments and the schematic diagram is shown in Fig. 6. The initial states for all samples were in a insulating state. During the forming process, the Vos migrated from the top to bottom electrode under voltage stress to form Vos filaments, switching the sample into LRS. For HfOx sample, the Vos were few and randomly distributed, requiring larger forming voltage to drive the migration of Vos, whereas the chains of Vos in Al-doped HfOx sample were easier to be generated due to the increased Vos by Al doping, leading to the lower forming voltage. By applying the opposite voltage, the Vos filaments ruptured and the sample was
Fig. 6. The schematic diagram of switching mechanisms for HfOx and Al-doped HfOx samples.
switched back to HRS. As mentioned above, for HfOx sample doped with higher Al concentration of 9.4%, multiple Vos filaments were formed and part of Vos filaments were left inside the film during reset process, resulting in the increase of HRS current and the degeneration of RS behaviors. In the successive set process, since the distribution of Vos were stochastic in HfOx sample, it was difficult to control the same formation path. The wide dispersion of the switching parameters in the HfOx sample could be attributed to the random formation of Vos filaments during switching process. On the contrary, due to the easier formation of conductive filaments by Vos localized near Al atoms, the fixed conductive paths in Aldoped HfOx samples were expected to be formed. The Vos filaments were reconnected along the almost same path during the set process, as shown in Fig. 6, which could account for the
Fig. 7. The retention properties of HfOx and Al-doped HfOx samples.
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experimentally observed remarkable uniformity improvement of switching parameters, such as switching voltages and resistances. The retention properties of HfOx and Al-doped HfOx samples are also measured, as shown in Fig. 7. The currents in LRS and HRS for all samples were stable and can maintain for 104 s without degeneration. The two resistance states for all samples can be distinguished clearly with the increasing time. The results indicate that Al-doped HfOx-based RRAM is potentially suitable for nonvolatile memory application. 4. Conclusion In summary, the RS behaviors of HfOx and Al doped HfOx samples were investigated and the switching mechanism based on Vos filaments model was demonstrated. The doping Al into HfOx film resulted in the increase of Vos which has lower formation energy near Al atoms. Compared with HfOx sample, the increased ON/OFF ratio, decreased switching voltages, and improved uniformity were achieved for 6.8% Al-doped HfOx sample due to the formation of fixed conductive filaments by the Vos near Al dopants. While the higher Al doping concentration of 9.4% caused excess Vos in HfOx film which led to the increased currents in HRS and reduced ON/ OFF ratio. The prepared HfOx samples all exhibited good retention properties. Our results indicate that the control of Al doping concentration can modulate the Vos in the film and optimize the switching performance of HfOx-based RRAM. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51202196), the Fundamental Research Funds for the Central Universities (Grant No. 3102014JCQ01032), the 111 Project (Grant No. B08040), the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (Grant No. CX201612), and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 155-QP-2016). References [1] Z. Ji, Q. Mao, W. Ke, Solid State Commun. 150 (2010) 1919e1922.
[2] W. Lee, J. Park, S. Kim, J. Woo, J. Shin, D. Lee, Appl. Phys. Lett. 100 (2012) 142106. [3] L. Zhang, Y.-Y. Hsu, F.T. Chen, H.-Y. Lee, Y.-S. Chen, W.-S. Chen, Pei-Yi Gu, W.H. Liu, S.-M. Wang, C.-H. Tsai, R. Huang, M.-J. Tsai, Nanotechnology 22 (2011) 254016. ~e , E. Miranda, Appl. Phys. Lett. 107 (2015) [4] P. Lorenzi, R. Rao, F. Irrera, J. Suu 113507. [5] F. Pan, S. Gao, C. Chen, C. Song, F. Zeng, Mater. Sci. Eng. R. 83 (2014) 1e59. [6] J. Kim, K. Lee, Y. Kim, H. Na, D.-H. Ko, H. Sohn, S. Lee, Mater. Chem. Phys. 142 (2013) 608e613. [7] S. Gao, F. Zeng, M. Wang, G. Wang, C. Song, F. Pan, Phys. Chem. Chem. Phys. 17 (2015) 12849e12856. [8] H.Y. Jeong, Y.I. Kim, J.Y. Lee, S.-Y. Choi, Nanotechnology 21 (2010) 115203. [9] H.Y. Lee, P.-S. Chen, T.-Y. Wu, Y.S. Chen, F. Chen, C.-C. Wang, P.-J. Tzeng, C.H. Lin, M.-J. Tsai, C. Lien, IEEE Electron Device Lett. 30 (2009) 703e705. [10] K.-L. Lin, T.-H. Hou, J. Shieh, J.-H. Lin, C.-T. Chou, Y.-J. Lee, J. Appl. Phys. 109 (2011) 084104. [11] G.I. Meijer, Science 319 (2008) 1625e1626. [12] A. Sawa, Mater. Today 11 (2008) 28e36. [13] F. De Stefano, M. Houssa, J.A. Kittl, M. Jurczak, V.V. Afanas’ev, A. Stesmans, Appl. Phys. Lett. 100 (2012) 142102. [14] B. Gao, B. Sun, H. Zhang, L. Liu, X. Liu, R. Han, J. Kang, B. Yu, IEEE Electron Device Lett. 30 (2009) 1326e1328. [15] R. Jiang, X. Du, Z. Han, W. Sun, Appl. Phys. Lett. 106 (2015) 173509. [16] B. Gao, H.W. Zhang, S. Yu, B. Sun, L.F. Liu, X.Y. Liu, Y. Wang, R.Q. Han, J.F. Kang, B. Yu, Y.Y. Wang, Symp. VLSI Technol. (2009) 30e31. [17] Y.S. Chen, B. Chen, B. Gao, L.F. Liu, X.Y. Liu, J.F. Kang, J. Appl. Phys. 113 (2013) 164507. [18] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865e3868. [19] D. Vanderbilt, Phys. Rev. B 41 (1990) 7892e7895. [20] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, J. Phys. Condens. Matter 14 (2002) 2717e2744. [21] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 87 (2000) 484e492. [22] M.J. Guittet, J.P. Crocombette, M. Gautier-Soyer, Phys. Rev. B 63 (2001) 125117. [23] Y.-K. Chiou, C.-H. Chang, C.-C. Wang, K.-Y. Lee, T.-B. Wu, R. Kwo, M. Hong, J. Electrochem. Soc. 154 (2007) G99eG102. [24] Q.N. Mao, Z.G. Ji, J.H. Xi, J. Phys. D. Appl. Phys. 43 (2010) 395104. [25] J. Shin, I. Kim, K.P. Biju, M. Jo, J. Park, J. Lee, S. Jung, W. Lee, S. Kim, S. Park, H. Hwang, J. Appl. Phys. 109 (2011) 033712. [26] S. Jou, C.-L. Chao, Surf. Coat. Tech. 231 (2013) 311e315. [27] Y.-S. Fan, P.-T. Liu, L.-F. Teng, C.-H. Hsu, Appl. Phys. Lett. 101 (2012) 052901. [28] S. Yu, B. Gao, H. Dai, B. Sun, L. Liu, X. Liu, R. Han, J. Kang, B. Yu, Electrochem. Solid-State Lett. 13 (2010) H36eH38. [29] C.-S. Peng, W.-Y. Chang, Y.-H. Lee, M.-H. Lin, F. Chen, M.-J. Tsai, Electrochem. Solid-State Lett. 15 (2012) H88eH90. [30] J. Frascaroli, F.G. Volpe, S. Brivio, S. Spiga, Microelectron. Eng. 147 (2015) 104e107. [31] M.S. Lee, S. Choi, C.-H. An, H. Kim, Appl. Phys. Lett. 100 (2012) 143504.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Oxygen vacancy modulation and enhanced switching behavior in HfOx film induced by Al doping effect Tingting Guo*, 1, Tingting Tan**, 1, Zhengtang Liu, Bangjie Liu State Key Lab of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an, 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2016 Received in revised form 8 June 2016 Accepted 9 June 2016 Available online 11 June 2016
The effect of Al doping concentration on resistive switching characteristics of Pt/Al-doped HfOx/Cu structures was investigated. According to the X-ray photoelectron spectroscopy (XPS) analysis and the first-principle calculation, the Al doping introduced more oxygen vacancies in HfOx film and these oxygen vacancies can be easily generated along Al dopants. Significant enhancement in resistive switching behaviors was achieved for 6.8% Al-doped HfOx film in terms of ON/OFF ratio, switching voltages and uniformity, whereas the higher Al doping concentration of 9.4% caused excess oxygen vacancies in the film, resulting in the degeneration of switching performance. A schematic diagram based on the formation and rupture of oxygen vacancy filaments was proposed to illustrate the distinct switching behaviors of HfOx and Al-doped HfOx samples. The results indicated that the concentration and distribution of oxygen vacancies played an important role on resistive switching behavior. The reliability properties for the prepared HfOx samples were also investigated. This work might provide a guidance for the further optimization of HfOx-based memory. © 2016 Elsevier B.V. All rights reserved.
Keywords: Resistive switching Al doping concentration Oxygen vacancies Conductive filaments
1. Introduction Due to the theoretical and physical limitation of traditional flash memory as the device size scales down continuously, novel non-volatile memories with advanced device performance and high integration density are urgently required and being extensively studied [1]. Recently, the merits such as simple structure, low power consumption, and compatibility with complementary metal oxide semiconductor (CMOS) process give an advantage of resistive random access memory (RRAM) as a promising candidate for the next generation memory [2e5] and the increasing attention has been focused on RRAM. The resistive switching (RS) phenomenon can be realized in a large scale of materials, especially in binary metal oxides (ZrO2 [6], TaOx [7], TiO2 [8], HfO2 [9]) which are identified as the most promising material in RRAM due to their easily controlled composition, low consumption and good compatibility with CMOS technology. At present, good performance including large ON/OFF ratio and reliable data retention has
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Guo), [email protected] (T. Tan). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jallcom.2016.06.090 0925-8388/© 2016 Elsevier B.V. All rights reserved.
been demonstrated in HfOx-based devices [9,10], while there still exist some practical issues required to be solved urgently for its application, such as how to seek out an effective way to improve the uniformity of switching parameters and the reliability of the device [11,12]. Since it has been widely accepted that the RS behaviors for HfOx film are closely related to oxygen vacancies (Vos) in the film [13e15], the origin of performance variation can be mainly attributed to the random distribution of Vos in the film. To effectively improve the RS performance, it is necessary to find a method to modulate the distribution and concentration of Vos in the switching layer. According to the first-principle calculations and experimental results [16,17], trivalent dopant can greatly reduce the formation energy Vos along the doping sites in HfO2 film, improving the uniformity of RS behavior finally. In this work, the HfOx films with different Al doping concentrations were fabricated. The influence of Al doping concentration on chemical composition and electrical properties of HfOx sample was demonstrated. The improved RS performance was achieved by modulating the Al doping concentration across switching layer due to the appropriate introduction of Vos. The results confirmed the important role of Vos on RS behaviors. Moreover, the switching mechanisms of the prepared samples were discussed. This work might offer a method to optimize the RRAM device by controlling the dopants concentration.
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2. Experimental details 2.1. Device fabrication and characterization The Si/SiO2/Ti/Pt substrates were used as bottom electrodes and first cleaned in deionized water, alcohol and acetone respectively. Subsequently, 20-nm-thick HfOx films doped with different Al doping concentrations were deposited on Pt substrates by radio frequency magnetron co-sputtering. The introduction of Al atoms was realized by loading the metal Al pieces on the metal Hf target, and the Al doping concentration was controlled by the area ratio of Al/Hf. During the deposition, the Ar:O2 was 12:3, the working pressure was 0.3 Pa and the sputtering power was 80 W. The chemical compositions of Al-doped HfOx films were analyzed by Xray photoelectron spectroscopy (XPS) and the Al doping concentrations were estimated to be 0%, 3.2%, 6.8%, 9.4% respectively. To measure the electrical properties of the prepared HfOx films, the 50-nm-thick Cu top electrodes with diameter of 2 mm were deposited by evaporation with a metal shadow mask to pattern the size. The electrical properties were characterized by an Agilent 4155C semiconductor parameter analyzer. During the test, the bias voltage was applied on Cu top electrode and the bottom electrode was grounded. 2.2. Theoretical calculation To investigate the variations of formation energy of Vos by Al doping, the first principles calculations based on density functional theory were carried out by using a generalized gradient approximation [18] for the exchange-correlation potential and Vanderbilt type ultrasoft pseudopotentials [19], as implemented in the CASTEP code [20]. In the calculations, the HfO2 structures doped with Al atom were constructed by 2 2 2 supercells of monoclinic HfO2, the doping effect was investigated by using Al atoms to replace Hf atoms and the Vo properties were simulated by removing one oxygen atom from the supercell. The formation energy of Vos was calculated as Ef(O) ¼ Etot(O)Etot(bulk) þ mo. 3. Results and discussion To explore the composition changes of HfOx films by Al doping, the XPS analyses were performed, as shown in Fig. 1. All peaks have been calibrated by C 1s (284.6 eV) peak. Fig. 1(a) shows the XPS spectra of Hf 4f core levels of HfOx and Al-doped HfOx films. The spectrum for HfOx film (0%) can be deconvoluted into a double peak of Hf 4f7/2 and Hf 4f5/2 at binding energies (BEs) of 16.9 eV and 18.6 eV respectively, which correspond to HfeO [21]. Apparently,
the Hf 4f peak of Al-doped HfOx film shifts toward the higher BE in comparison to the peak of HfOx film. Guittet’ study [22] showed that when mixing two oxides, the cation of the more ionic metal oxide is expected to become even more ionic after formation of the complex oxide. As a result, with the increasing Al doping concentration, Hf becomes more ionic and loses more charges. Therefore, the shift of Hf 4f peak to higher BE indicates that Al atoms bond to HfOx structure to form HfAlOx [23]. Fig. 1(b) shows the percentage of non-lattice oxygen in the prepared films. The inset of Fig. 1(b) shows the curve of O 1s spectrum of HfOx film. The O 1s peak can be fitted into two peaks which are attributed to the lattice oxygen at lower BE of 530.3 eV and non-lattice oxygen at higher BE of 532.1 eV. The fitting results for O 1s of Al-doped HfOx films are similar to that of HfOx film. Based on the reports [24,25] that nonlattice oxygen is related to the formation of Vos and the ratio of nonlattice oxygen is proportional to the amount of Vos, the concentration of Vos in HfOx film can be estimated by analyzing the ratio of non-lattice oxygen in the film. In Fig. 1(b), the ratio of non-lattice oxygen clearly shows a positively proportional dependence on Al doping concentration, indicating the increased Vos by Al doping. The formation energies of Vos in HfO2 and Al-doped HfO2 structures were calculated by first principles to further investigate the variation of Vos in HfO2 film by Al doping, as shown in Table 1. The results showed that the formation energy of Vos decreased by Al doping, which was consistent with Gao’ study [16]. Besides, the Vos can be more easily formed near Al atom than far from Al atom. Combining the calculation results with XPS analysis, it can be deduced that the doping of Al in HfOx film results in the increase of Vos. Fig. 2(a) shows the schematic diagram of Cu/Al-doped HfOx/Pt structures for I-V measurements. Fig. 2(b)e(e) show the RS behaviors of HfOx and Al-doped HfOx samples respectively. The forming processes are presented in the inset of the corresponding figures. The initial states for all prepared samples were in a high resistance state (HRS). For HfOx sample (0%), a forming voltage (FV) of ~1.2 V was required to activate the RS behavior, switching the sample into a low resistance state (LRS), as presented in the inset of Fig. 2(b). The same electrical measurement was applied to Al-doped HfOx samples. The Al-doped HfOx samples exhibited a lower FV than HfOx sample, which can be attributed to the increased Vos in Al-doped HfOx samples. After the forming process, typical bipolar RS characteristics of HfOx (0%) and Al-doped HfOx samples (3.2%, 6.8% and 9.4%) were achieved, as shown in Fig. 2(b), (c), (d), and (e) respectively. During the switching process, the current compliance was set to 10 mA to prevent the sample from damage and the voltage swept in a counterclockwise direction, as indicated by arrows in Fig. 2(b). Obviously, enlarged ON/OFF ratio can be observed
Fig. 1. (a) The XPS spectra of Hf 4f and (b) the percentage of non-lattice oxygen in HfOx and Al-doped HfOx films. The inset of (b) shows the O 1s spectrum in HfOx film.
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Table 1 The formation energy of Vos in HfOx and Al-doped HfOx. Location of Vos
Vos in HfOx
Vos in Al-doped HfOx nearest to Al atom
Vos in Al-doped HfOx far from Al atom
Formation energy (eV)
6.63/6.53 [16]
4.05
4.79
Fig. 2. (a) The schematic diagram of Pt/Al-doped HfOx/Cu structures for I-V measurements. The I-V curves of (b) HfOx and (c) (d) (e) Al-doped HfOx samples. The insets of (b) (c) (d) (e) show the corresponding forming processes. (f) The I-V curves of HRS in double-log scale and the fitted slope. The inset of (f) shows the I-V curves of LRS in double-log scale.
for Al-doped HfOx samples, especially for 6.8% Al-doped HfOx sample which shows ON/OFF ratio larger than 103 and good reproducibility. To explore the conductive mechanisms of HfOx and Al-doped HfOx samples, the I-V curves were replotted in double-log scale, as shown in Fig. 2(f). The fitting curves in LRS for all samples showed a linear line with slope of approximately 1, as presented in the inset of Fig. 2(f), indicating that the conduction behavior in LRS was ohmic conduction. However, the curves for HRS were a little complicated, which exhibited ohmic conduction at low-voltage region (I ~ V) and Child’s law at high-voltage region (I ~ V2). This
conduction behavior can be well explained by the trap controlled space-charge-limited-current effect, which was commonly observed in insulators [10,26,27]. Metal ions or Vos which can migrate to form filaments in the film are believed to play a crucial role on switching behavior. To explore the composition of the filaments, the temperature dependence of resistance for HfOx and Al-doped HfOx samples was performed, as shown in Fig. 3(a). The resistance in HRS for all samples decreased with the increase of temperature, indicating the oxide or semiconducting characteristics. For LRS, the resistance also slightly decreased as the temperature increased, exhibiting the
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Fig. 3. (a) The dependence of resistance on temperature and (b) the forming processes under positive and negative voltage for HfOx and Al-doped HfOx samples.
semiconducting conduction mechanism. This semiconductor behavior of LRS was related to the filament formation by Vos in the film. Fig. 3(b) compares the positive and negative forming processes of HfOx and Al-doped HfOx samples. All samples showed the similar behavior. Under the positive voltage, the current abruptly increased as the voltage reached about 1 V, switching the sample from HRS to LRS. However, there was no obvious change in current by applying a negative voltage even up to 2 V. The change of current only under positive voltage can be attributed to electrochemically active electrode of Cu, indicating the indispensable role of Cu on switching behavior. Based on above analyses, the migration of Vos and the redox of Cu electrode were both responsible for the switching behavior, while the former may play a dominant role due to Al doping according to experimental results. Besides, since the same electrodes of Cu were used for all samples and we focused more on the variation of Vos by Al doping, the effect of Cu on switching behavior was not discussed further. As a result, in this work, the Vos can be regarded as carrier trapping centers and the dominant role in assisting the electron transport was attributed to the formation of Vos filaments. The distribution of switching voltages and resistances for HfOx and Al-doped HfOx samples were shown in Fig. 4(a) and (b) respectively. In Fig. 4(a), the Vset and Vreset for 3.2% Al-doped HfOx sample showed nearly no decrease compared with HfOx sample, which may be due to the low doping concentration. With the increase of Al doping concentration, the voltages decreased obviously. Fig. 4(b) shows the distribution of resistances in HRS and LRS of the prepared samples. The increased resistance in HRS was
observed for 3.2% and 6.8% Al-doped HfOx samples which may be due to the ionized impurity scattering caused by interstitial Al atoms, leading to the improved ON/OFF ratio. The decreased resistance in HRS of 9.4% Al-doped HfOx sample would be discussed later. The uniformity of the switching voltages and resistances was both improved by Al doping. Our calculation results showed that doping Al atoms into HfOx film decreased the formation energy of Vos around Al doping sites, which was consistent with Yu’ study [28]. Thus, the decreased switching voltages in Al-doped HfOx samples were due to the easy aggregation of Vos and the creation of Vos along Al atoms led to the formation of fixed conductive paths which suppressed the randomness of Vos filaments and improved the uniformity of switching parameters. To get further information on switching stability of HfOx and Aldoped HfOx samples, more than 100 switching cycles were performed in each sample and the endurance properties were presented in Fig. 5. In Fig. 5(a), the current in LRS and HRS for HfOx sample showed large fluctuations because the Vos in HfOx sample were randomly formed during deposition. Due to the easy formation of Vos along the Al doping sites, the improved uniformity of RS performance should be achieved by Al doping. Actually, only HfOx samples doped with lower Al doping concentration (3.2% and 6.8%) showed improved uniformity, as shown in Fig. 5 (b) and (c). For 9.4% Al-doped HfOx sample (Fig. 5(d)), the current in HRS exhibited somewhat fluctuations and an increase of current in HRS can be observed. The same phenomenon was found in other studies [29,30] that a high doping concentration resulted in a reduced ON/ OFF ratio. This can be attributed to the creation of abundant Vos by
Fig. 4. (a) The distribution of switching voltages (Vset and Vreset) and (b) the statistical distribution of the resistances in HRS and LRS of HfOx and Al-doped HfOx samples. The data are collected from 50 switching cycles.
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Fig. 5. The endurance properties of HfOx and Al-doped HfOx samples.
the higher Al doping concentration. These Vos filaments can not be entirely ruptured and part of them still remained during the reset process, which led to the increased HRS current and reduced ON/ OFF ratio [31]. On the basis of above results, the RS behaviors of HfOx and Aldoped HfOx samples can be attributed to the formation and rupture of Vos filaments and the schematic diagram is shown in Fig. 6. The initial states for all samples were in a insulating state. During the forming process, the Vos migrated from the top to bottom electrode under voltage stress to form Vos filaments, switching the sample into LRS. For HfOx sample, the Vos were few and randomly distributed, requiring larger forming voltage to drive the migration of Vos, whereas the chains of Vos in Al-doped HfOx sample were easier to be generated due to the increased Vos by Al doping, leading to the lower forming voltage. By applying the opposite voltage, the Vos filaments ruptured and the sample was
Fig. 6. The schematic diagram of switching mechanisms for HfOx and Al-doped HfOx samples.
switched back to HRS. As mentioned above, for HfOx sample doped with higher Al concentration of 9.4%, multiple Vos filaments were formed and part of Vos filaments were left inside the film during reset process, resulting in the increase of HRS current and the degeneration of RS behaviors. In the successive set process, since the distribution of Vos were stochastic in HfOx sample, it was difficult to control the same formation path. The wide dispersion of the switching parameters in the HfOx sample could be attributed to the random formation of Vos filaments during switching process. On the contrary, due to the easier formation of conductive filaments by Vos localized near Al atoms, the fixed conductive paths in Aldoped HfOx samples were expected to be formed. The Vos filaments were reconnected along the almost same path during the set process, as shown in Fig. 6, which could account for the
Fig. 7. The retention properties of HfOx and Al-doped HfOx samples.
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experimentally observed remarkable uniformity improvement of switching parameters, such as switching voltages and resistances. The retention properties of HfOx and Al-doped HfOx samples are also measured, as shown in Fig. 7. The currents in LRS and HRS for all samples were stable and can maintain for 104 s without degeneration. The two resistance states for all samples can be distinguished clearly with the increasing time. The results indicate that Al-doped HfOx-based RRAM is potentially suitable for nonvolatile memory application. 4. Conclusion In summary, the RS behaviors of HfOx and Al doped HfOx samples were investigated and the switching mechanism based on Vos filaments model was demonstrated. The doping Al into HfOx film resulted in the increase of Vos which has lower formation energy near Al atoms. Compared with HfOx sample, the increased ON/OFF ratio, decreased switching voltages, and improved uniformity were achieved for 6.8% Al-doped HfOx sample due to the formation of fixed conductive filaments by the Vos near Al dopants. While the higher Al doping concentration of 9.4% caused excess Vos in HfOx film which led to the increased currents in HRS and reduced ON/ OFF ratio. The prepared HfOx samples all exhibited good retention properties. Our results indicate that the control of Al doping concentration can modulate the Vos in the film and optimize the switching performance of HfOx-based RRAM. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51202196), the Fundamental Research Funds for the Central Universities (Grant No. 3102014JCQ01032), the 111 Project (Grant No. B08040), the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University (Grant No. CX201612), and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 155-QP-2016). References [1] Z. Ji, Q. Mao, W. Ke, Solid State Commun. 150 (2010) 1919e1922.
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