APPLIED PHYSICS LETTERS 90, 183507 共2007兲
Reproducible unipolar resistance switching in stoichiometric ZrO2 films X. Wu Department of Optical Science and Engineering, Fudan University, Shanghai 200433, People’s Republic of China
P. Zhoua兲 School of Microelectronics, Fudan University, Shanghai 200433, People’s Republic of China
J. Lib兲 and L. Y. Chen State Key Laboratory for Advanced Photonic Materials and Devices, Fudan University, Shanghai 200433, People’s Republic of China
H. B. Lv, Y. Y. Lin, and T. A. Tang ASIC and System State Key Laboratory, Fudan University, Shanghai 200433, People’s Republic of China
共Received 11 March 2007; accepted 5 April 2007; published online 1 May 2007兲 The resistance switching characteristics of stoichiometric ZrO2 film were investigated for nonvolatile memory. The Al/ ZrO2 / Al device presents reliable and reproducible switching behaviors. The on/off ratio of two stable states is larger than 2 ⫻ 103. It is suggested that the current-voltage characteristics are governed by the Schottky conduction mechanism in high voltage region, while the filament conduction is suggested in low voltage region. The switching process is explained in terms of the spontaneous reversible reaction between electrode and ZrO2 films with the contribution of Joule heating effect by the external current. It provides a possible solution for low device yield of nonstoichiometric oxides. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2734900兴 Resistance switching random access memory 共RRAM兲 has attracted great interest for the application in next generation nonvolatile memory.1 The current candidate materials for RRAM devices include doped perovskite SrZrO3,2 ferroelectric Pb共Zr0.52Ti0.48兲O3,3 ferromagnetic materials such as 共Pr, Ca兲MnO3,4 and binary transition metal oxides such as NiOx,5 TiOx,6 AlOx,7 and CuxO.8 Compared with ternary or quaternary oxide semiconductor films such as doped SrZrO3 or 共Pr,Ca兲 MnO3, binary metal oxides have the advantage of a simple fabrication process and are more compatible with complementary metal-oxide semiconductor processing. Recently, Lee et al.9 found that the nonstoichiometric zirconium oxide 共ZrOx兲 showed resistance switching behavior and good retention characteristics for both high and low resistance states. The resistance switching behavior was explained by electron trapping and detrapping at Zr+ trap in ZrO2 matrix. The zirconium oxide has obviously excellent characteristics compared with other binary transition metal oxides particularly in retention and on/off ratio. However, the dominant parameter that has to be considered for the application of binary metal oxides is that the nonstoichiometric oxides such as ZrOx, NiOx, and Nb2O5−x show low device yield.10 Therefore, stoichiometric ZrO2 is supposed to be a promising candidate for nonvolatile memory device because it has several merits such as simple constituent, high breakdown field, and superior thermal stability.11,12 Especially it gives a possible way to solve the problem of low device yield of nonstoichiometric oxides. In this letter, we investigated the resistance switching characteristics of stoichiometric ZrO2 thin film prepared by radio frequency 共rf兲 magnetron sputtering for nonvolatile a兲
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memory application. It is presented that the stoichiometric ZrO2 thin film shows reliable and reproducible switching phenomenon. The resistance switching mechanism is discussed. Samples were prepared with the LAB600SP high vacuum multitarget magnetron sputtering system. After thermal oxidation of a 2-in.-diameter Si 共100兲 substrate wafer, Al was deposited as bottom electrode by direct current magnetron sputtering. Then, a 60-nm-thick ZrO2 thin film was deposited on Al/ SiO2 / Si substrates by rf magnetron sputtering with an Ar working pressure of 8.0⫻ 10−3 mbar. After ZrO2 deposition, Al top electrodes with a diameter of 80 m were fabricated by a lift-off photolithography process. The electrical characteristics of Al/ ZrO2 / Al structure were measured by Keithley 4200-SCS semiconductor parameter analyzer with biased top and grounded bottom electrodes. The x-ray photoelectron spectroscopy 共XPS兲 was employed to determine the chemical bonding state of thin oxide films. A scanning electron microscope 共SEM兲 was used to observe microstructure of RRAM cell and the cross-sectional image is presented in Fig. 1. Two black areas can be clearly seen in Fig. 1 which are the top and bottom electrodes. The bright area between these two electrodes is the ZrO2 layer. Beneath the bottom electrode, a 200-nm-thick SiO2 layer as an insulting layer can be clearly observed. Figure 2 shows typical I-V characteristics of the RRAM device cell based on a 60-nm-thick ZrO2 film. After the initial forming process at a voltage of 8.0 V 共for most of the cells兲, the device reached a low resistance state 共LRS兲 共on state兲. By sweeping the applied positive voltage to a certain voltage of about 0.4 V with a current compliance of 0.1 A, a sudden drop of current appeared and the film returned to a high resistance state 共HRS兲 共off state兲. Then, by continuing to sweep the voltage to a higher voltage of about 2.5 V with a current compliance of 0.01 A, an abrupt increase of current
0003-6951/2007/90共18兲/183507/3/$23.00 90, 183507-1 © 2007 American Institute of Physics Downloaded 08 May 2007 to 202.120.224.18. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 1. Cross-sectional structure SEM image of the Al/ ZrO2 / Al RRAM device.
appeared and the on state was achieved. The reliable switching cycles including “set” 共from HRS to LRS兲 and “reset” 共from LRS to HRS兲 processes were obtained, as presented in Fig. 2. There was a slight variation in the switching voltages 共Vreset and Vset兲 like other group’s work on TiOx.6 The film shows good retention characteristics for both high and low resistance states. Moreover, the resistance of HRS is about 90 k⍀ while the resistance of LRS is approximately 40 ⍀. The average on/off ratio between HRS and LRS is larger than 2 ⫻ 103. The giant on/off ratio certainly suggests a possibility to achieve high density memory by means of multibit or multilevel storage in future applications. It also guarantees the device reliability and excellent fault tolerance because of its large margin. According to the I-V characteristics discussed above, it is clear that the device shows a unipolar switching behavior. To confirm the composition and Zr ion chemical states in ZrO2 film, XPS depth profile was performed, as shown in Fig. 3. It can be seen that at the region from surface to 10 nm in depth, the Zr 3d3/2 and Zr 3d5/2 peaks have been observed with binding energies of 183.9 and 181.6 eV, respectively. The energy interval of 2.3 eV between these two peaks represents the fully oxidized value of Zr 共Zr4+兲. The two peak positions of Zr 3d3/2 and Zr 3d5/2 remain the same as depth regions from 20 to 30 nm and from 50 nm to bottom, as shown by the corresponding curves in Fig. 3. Thus, the XPS results illustrate that there was no metallic Zr in the sample
FIG. 3. 共Color online兲 XPS depth profile spectra of Zr 3d at three different depth regions: 共a兲 from surface to 10 nm in depth, 共b兲 from 20 to 30 nm in depth, and 共c兲 from 50 nm to bottom in depth.
and the thin functional film was composed of stoichiometric ZrO2 totally. It is easy to understand that stoichiometric oxides have overwhelming merits compared with nonstoichiometric oxides in composition control and compatibility with conventional semiconductor process in practical application. Obviously, different oxygen concentrations in functional thin oxide film induce unstable technical performance of RRAM device since it is hard to control exact oxygen vacancies. Therefore, we provide a possible solution here for the low device yield due to the uncontrollability of the oxygen concentration of nonstoichiometric oxide devices. The plot of log I vs log V of device was shown in Fig. 4共a兲. In the low voltage region 共V ⬍ Vreset兲, both the LRS and HRS curves present Ohmic behaviors 共slopes of 1兲. Moreover, taking into account the fact that forming process is necessary to develop resistance switching, the filament conduction is suggested in the low voltage region for both the stable LRS and HRS. It is natural to suggest that the Al/ ZrO2 / Al structure has a Schottky nature because the Schottky equation is widely used in the leakage current analysis of the metal/semiconductor structures including metal/ZrO2.11,12 The I-V relation can be expressed as follows: I ⬀ T2 exp共e冑共eV兲 / 共4i0d兲 / kT兲, where I is the current, e is the electronic charge, 0 is the permittivity of free space, i is the optical dielectric constant, V is the applied voltage, d is the thickness of a film, k is the Boltzmann’s constant, and T is temperature. If the Schottky conduction is obeyed in HRS, a linear relationship between log I vs V1/2 should be obtained and the slope should give the refractive index n 共n = 1/2 i 兲. Figure 4共b兲 shows the curves of log I vs V1/2 in the high voltage region, where linear behaviors 共slope of 2.56兲 are observed. The refractive index calculated from the slope is about 2.42 共with the temperature T of 290 K and thickness d of 60 nm兲, which is very close to the value of 2.5 for ZrO2.13,14 Hence, it appears that the I-V characteristics of the ZrO2 film are governed by the Schottky conduction mechanism in the high voltage region 共Vreset ⬍ V ⬍ Vset兲. According to the analysis results given above, the switching of the initial ZrO2 film to LRS is due to the forming process, which is accompanied by the soft breakdown. For the reset process, the resistance increases suddenly
FIG. 2. 共Color online兲 I-V characteristics of the RRAM device cell based on ZrO2 thin film with 80-m-diameter Al electrode. Downloaded 08 May 2007 to 202.120.224.18. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 4. 共Color online兲 共a兲 Plots of log I vs log V at low voltage region and 共b兲 plots of log I vs V1/2 at high voltage region.
which indicates that the filamentary conducting paths might be ruptured. The Joule heating effect by the external current is considered for the rupture of filaments.15 By increasing the voltage to Vreset, the high current flow through many filaments heats up the film which induces a simultaneous rupture of the filaments and the HRS is achieved. For the set process, because the sample is composed of stoichiometric ZrO2, it is impossible to form conducting filaments by excess Zr or oxygen vacancies. This suggests that some reactions might take place in the sample during the set process. It is reasonable to suppose that aluminum electrode plays an important role in switching cycles. There might be reversible reactions occurring between aluminum electrode and stoichiometric ZrO2 thin film, otherwise it is not possible to switch on or off for pure stoichiometric ZrO2 thin film without any mobile carriers. No stable switching phenomena but only hard breakdown of thin film or very few switching cycles were observed when high work function metal such as Pt was used as electrode in the same device structure. It also proved our suggestion about switching mechanism of this RRAM device based on stoichiometric ZrO2 thin film. Due to the strong reducing capacity of Al and external current, a spontaneous reaction between the Al electrode and O2− ions in the ZrO2 might take place. This spontaneous reaction consumes O2− ions and leads to local conducting filaments, thus the LRS is obtained. However, this spontaneous reaction is reversible. When the external current density is large enough in low resistance state, sufficient electrons are injected in the film to fill excess Zr4+ demand and the Joule heating effect might break the reaction between Al and O2− ions and cause a simultaneous rupture of the filaments. Therefore, it seems that the reset and set processes are resulted from the Joule heating effect with external electron injection and the reaction between Al and ZrO2 thin film, respectively. In summary, the resistance switching behavior of stoichiometric ZrO2 thin film in RRAM device fabricated by general semiconductor process has been investigated for nonvolatile memory device application. A typical Al/ ZrO2 / Al device cell showed stable and reproducible conductivity switching behaviors. The I-V characteristics of the ZrO2 film are governed by the Schottky conduction mechanism in the high voltage region 共Vreset ⬍ V ⬍ Vset兲, while the filament conduction is suggested in the low voltage region
共V ⬍ Vreset兲. A possible mechanism of the formation and rupture of conducting filament is proposed based on the Joule heating effect with external electron injection and spontaneous reaction between Al and stoichiometric ZrO2 thin film. The authors would like to thank all of their collaborators. This work was partly supported by National Natural Science Foundation of China with Contract Nos. 60578047 and 60676007. One of the authors 共P.Z.兲 was supported by the Korea Foundation for Advanced Studies 共KFAS兲 International Scholar Exchange Fellowship for the academic year of 2006–2007. 1
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