Engineering of forming-free resistive switching characteristics in ZrO2 films

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Engineering of forming-free resistive switching characteristics in ZrO2 films

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2015 J. Phys. D: Appl. Phys. 48 225301

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1. Introduction

Resistive random access memory (RRAM) has been consid-ered as one of the most promising next-generation nonvolatile memories (NVM) owing to its advantages in terms of high-den-sity, high-speed, low-power consumption, excellent scalability and CM OS compatibility [1–3]. A variety of metal oxides, including TiO 2, NiO, ZnO, Ta 2O 5, HfO 2, and ZrO 2, have been explored for RRAM device applications [4–10]. Generally, a forming process, defined as applying a higher voltage than the operation voltage to the devices, is needed to activate the resistive switching (RS) behaviors for the subsequent stable cycles [2, 11]. This high-voltage forming process is not desir-able for periphery circuit design and may result in unpredict-able resistance states. Obviously, reducing or even eliminating the forming step is valuable for practical applications.

Significant efforts have been made to realize the so-called forming-free resistive switching [12–19]. In fact, two different

forming-free characteristics have been reported in the litera-ture. In the first case, the pristine state resistance is similar to the high resistance state (HRS) resistance after reset, and the forming voltage is also comparable with the subsequent set voltage [15, 17]. The other is conducting filaments (CFs) which already exist in the as-fabricated RRAM cells with an initially ON state [12, 14, 16, 19]. So the first reset process is performed to initialize the devices by rupturing the CFs. Recently, forming-free operation has been realized by var-ious approaches, including film thickness optimization [14, 17, 20], ion doping [15, 16], thermal treatment [12, 21] and fabrication modulation [15, 19

]. All these methods are aimed at elaborately controlling the defect profiles of the device. However, the coexistence of these two forming-free modes in the same material and the transition between two modes are rarely investigated.

It is well known that modulating the post-annealing con-ditions (annealing temperature, ambient and dwell time) is

Engineering of forming-free resistive switching characteristics in ZrO 2 films

Gang Du, Tao Li, Chao Wang, Bin Fang, Baoshun Zhang and Zhongming Zeng

Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-bionics, Chinese Academy of Sciences, Ruoshui Road 398, Suzhou 215123, People ’s Republic of China E-mail: zmzeng2012@https://www.360docs.net/doc/1618666555.html,

Received 14 January 2015, revised 21 March 2015Accepted for publication 30 March 2015Published 6 May 2015Abstract

The variation of forming-free resistive switching (RS) characteristics modulated by thermal annealing was investigated in ZrO 2 films fabricated by electron beam evaporation. A typical forming-free behavior with pristine resistance comparable to that of the high resistance state (HRS) was observed in the as-deposited devices. Whereas, the resistance was further reduced to a state with initial resistance even lower than the low resistance state (LRS) after annealing in N 2 ambient and a larger resistance ratio was verified in the annealed devices. Both devices exhibited stable bipolar RS without any forming step, which has great potential for memory applications. A possible RS mechanism associated with adjusting of oxygen vacancy formation and migration was proposed to explain these distinct forming-free behaviors. It is expected that controllable forming-free characteristics and improved device performance may be obtained by further optimizing the distribution and density of oxygen vacancies via post-annealing.

Keywords: resistive random access memory (RRAM), forming-free, oxygen vacancy, thermal annealing

(Some figures may appear in colour only in the online journal)

doi:10.1088/0022-3727/48/22/225301

J. Phys. D: Appl. Phys. 48 (2015) 225301 (6pp)

an effective and simple approach to adjust the stoichiometry and the defect distribution of oxide materials [12, 21, 22]. In this study, we demonstrated that two forming-free schemes coexisted in a Ti/ZrO 2/Pt device implemented by N 2 ambient annealing. The results showed that the initial resistance of the devices could be altered by thermal annealing. Both devices exhibited stable bipolar RS without any forming step, which was highly desirable from a device perspective. Based on the investigation of device characteristics, a possible mecha-nism was suggested to explain the distinct initial states and forming-free behaviors.2. Experimental

The RS memory devices in this study are fabricated as fol-lows. A 20 nm thick Ti adhesion layer and an 80 nm thick Pt bottom electrode (BE) were successively deposited on a SiO 2/Si substrate. After that, a 50 nm thick ZrO 2 film was grown at an ambient temperature on the BE layer under a base pressure of 9 × 10?4Pa via electron beam evaporation. Some of the samples were post-annealed at 600 °C for 20 s in N 2 ambient. Finally, 150 nm thick Ti top electrodes (TE) with a diameter of 200 μm were deposited by electron beam evaporation and patterned by lift-off technology to complete the Ti/ZrO 2/Pt device. The structure and morphology of the as-deposited and annealed thin films were examined by transmission elec-tron microscopy (TEM) and atom force microscope (AFM),

respectively. Electrical measurements were carried out using Agilent B1500A semiconductor characterization system. During all the measurements, the bias voltage was applied on the Ti TE with the Pt BE grounded. The schematic diagram of the device structure and the measurement configuration is depicted in figure 1(a).3. Results and discussion

Figure 1(b) shows the typical current –voltage (I –V ) curves of the as-deposited ZrO 2 RRAM devices. Bipolar RS behavior is clearly observed. After sweeping the applied positive voltage to a critical voltage (set voltage) of about 1 V with a current compliance of 5 mA, a sudden increase in the current appeared and the device switched to the low resistance state (LRS). The device remained at the LRS until a reverse bias of about ?0.7 V was applied. After that, a switching of the LRS to the HRS occurred and the voltage where the current begins to drop is defined as the reset voltage. Interestingly, the resistance of the as-deposited device is slightly lower than the HRS resistance after reset, and the initial operating voltage is also compa-rable with the set voltage, which means that the as-deposited device exhibits classical forming-free behavior. It should be noted that the RS performance of our device is significantly distinct from the forming-required behavior of ZrO 2 films deposited by radio-frequency magnetron sputter with similar device structure [23, 24]. This may be attributed to different

Figure 1. (a) Schematic diagram of the Ti/ZrO 2/Pt device. (b) Typical I –V curves of the initial voltage sweep and switch sweep for the

as-deposited devices. The I –V

curves for both (c) positive and (d) negative voltage sweep regions in double-logarithmic plot.

fabrication processes and the oxygen-getting ability of thicker Ti electrode, which creates abundant oxygen-vacancy-related defects within the ZrO2 films.

To further clarify the conduction and switching mecha-nism of the memory device, the representative I–V curve was replotted in double logarithmic plots, as shown in figures 1(c) and (d). Similar conduction behaviors are observed for both voltage polarity branches. The I–V relationship in HRS shows that the charge transport behavior is in good agreement with classical trap-controlled space charge limited conduction (SCLC), which consists of three portions: the Ohmic region (I∝V), the Child’s law region (I∝V2), and the steep current increase region [5, 25]. Analogously, the conduction behavior in LRS follows a linear Ohmic behavior at low voltage with the addition of a quadratic term at higher voltage, i.e. I∝aV + bV2. The Ohmic conduction in the very low voltage range would be due to the fact that thermally generated free car-riers inside the film are predominant over the injected charge carriers, which is followed by the trap-filled SCLC behavior. Then, due to the formation of conducting paths and trapping of the injected charges, an abrupt current increase occurs. M oreover, conductive paths comprising trapped electrons could form at local region of high-density defects in LRS. Afterward, part of the conductive paths will be ruptured through a de-trapping process by applying a negative voltage and then the device is switched to HRS again. Therefore, the fundamental switching mechanism of the device is related to the carrier trapping and de-trapping processes in oxygen-vacancy-related defects [26, 27].

Figure 2(a) depicts the I–V curves of the ZrO2 films annealed in N2 ambient. Surprisingly, the initial resistance (R ini) of the structure is about 30 Ω (ON state), which is even lower than the LRS resistance during a cycling opera-tion. This may be attributed to the high density of defects in ZrO2 film induced during thermal annealing in an oxygen-deficiency environment [28]. A first negative voltage reset process was performed to initialize the device by rupturing the pre-existed CFs, and the device was switched to HRS.

After the first reset process, stable and reproducible bipolar RS characteristics were obtained. It should be mentioned that this thermal-forming behavior is essentially the same as the electrical forming process but does not need to subject to high voltage, which is very promising for NVM applications [12].

A similar SCLC conduction mechanism is observed, which corroborates well with that of the as-deposited counterparts (data not shown here). Hence, it can be deduced that thermal treatment only affects the initial resistance by inducing more oxygen vacancies. While the interface between Ti and ZrO2 film and the charge traps dominant the switching mechanism. We note that the electrical transport behavior in the initial ON state shows distinct features from the stable switching cycles, which exhibit a completely linear Ohmic conduction character through the pre-existed CFs. The positive resistance tempera-ture coefficient of 2.65 × 10?3 K?1 (inset of figure 2(a)) with metal-like conducting behavior was obtained in this study by variable temperature measurement. The relatively larger αmay be derived from the generation of metallic Zr0 and abun-dant oxygen vacancies during N2 annealing [21]. Furthermore, these CFs may distributed randomly and dispersedly in the film, different from the steady LRS with locally concentrated conductive paths. That means the dimension of CFs in the initial ON state might stronger than that formed in the sub-sequent LRS and the preexisted CFs are multiply connected, as inferred from the larger reset power needed for the first reset process (figure 2(a)). Besides, the first reset operation of the initial ON state features a strong abrupt reset process followed by a gradual one, which is primarily attributed to the easily annihilation of the conductive paths due to the abundance of the oxygen vacancies [27]. In contrast, a high voltage (~11 V) forming process is needed for the control sample, which is annealed in O2 ambient, as shown in the inset of figure 2(a). This observation indicates that lesser oxygen vacancies remained because of oxygen compensating effects after annealing in O2-rich atmosphere, and further verified that the defects levels can be effectively engineered by annealing conditions. Figure 2(b) compares the typical stable I–V char-acteristics of the as-deposited and N2 annealed ZrO2 films with Ti/ZrO2/Pt structure. It can be observed that the resistance Figure 2.(a) Typical I–V curves of the first reset sweep and

switch sweep for the N2 annealed devices. The inset (i) shows

the resistance temperature coefficient α of the initial ON state

is 2.65 × 10?3 K?1. The inset (ii) depicts the forming process

of the control sample annealed in O2 ambient. (b) Stable I–V characteristics of the ZrO2 films before and after annealing in N2

ambient.

ratio is higher for the devices after annealing. The distinct forming-free features must be related to the microscopic alter-nation produced by the thermal annealing treatment.

In order to verify the crystallinity of the N 2 annealed devices, TEM was utilized to examine the microstructure directly. Figure 3(a) shows the corresponding cross-sectional TEM image for the N 2 annealed ZrO 2 films deposited on Pt electrode. As expected, the thickness of the ZrO 2 film is approximate 50 nm and apparent locally crystallized regions with different orientations can be observed, suggesting a polycrystalline nature induced by the annealing process. Figures 3(b) and (c) further illustrate the corresponding sur-face morphology and roughness in AFM observations. The average surface roughness of the ZrO 2 films before and after annealing is about 1.08 nm and 1.59 nm, respectively. The var-ying surface roughness may impact on the RS behavior of the memory devices.

Based on conduction mechanism analysis and the struc-ture measurement discussed above, a possible scenario of a forming-free switching model is proposed in figure 4. As shown in figure 4(a), it is inevitable that some defects such as oxygen vacancies and interstitials atoms exist naturally during the low-temperature and high-vacuum electron beam evaporation process. Moreover, owing to the strong oxygen-getting ability of the Ti electrode, abundant oxygen ions will be extracted from the ZrO 2 layer and reserved by the Ti top electrode, leaving a high density of oxygen vacancies to assist the formation of conductive paths during the set process [23]. This interface layer induced by the Ti electrode is suggested

to cause the formation of a space charge region and dominate the SCLC conduction behavior in the Ti/ZrO 2/Pt structure as confirmed in figure 1. All these aspects are responsible for the forming-free characteristic in the as-deposited nonstoi-chiometric cells. However, after N 2 annealing, the resistance is further reduced to the ON state. This phenomenon implies that many conductive paths preexisted in the matrix, as illus-trated in figure 4(b). It is expected that more oxygen vacancies will be induced during the thermal treatment in oxygen- deficiency ambient [21]. Therefore, the amount of charged defects may exceed the percolation threshold, and the film becomes conductive with extremely lower resistance [14]. In addition, more available oxygen vacancies will be stored in the interfacial oxygen reservoir and then the interfacial layer may become thicker. On the other hand, the crystallinity and surface roughness of ZrO 2 were increased by annealing. So the oxygen vacancies and metal defects prefer to form and accumulate around the extended defects, such as grain bound-aries and dislocations, which also contribute to the initial ON state [29]. Furthermore, a positive voltage sweep was tried but failed to reset the initial ON device. This demonstrates that electrochemical redox rather than Joule heating dominates the bias-polarity-dependent reset process. In the subsequent cycling measurements, the Ti electrode induced interfacial layer provides sufficient oxygen ions for inducing the redox reaction of the conductive paths near the Ti/ZrO 2 interface, leading to the reversible RS characteristics (figure 4(c)).

In addition, it was noted that the resistance ratio of the N 2 annealed film after the first reset operation is larger compared

Figure 3. (a) Cross-sectional TEM image for the N 2 annealed ZrO 2 films deposited on Pt electrode. AFM images for the samples (b) before

and (c) after N 2

annealing.

Figure 4. Possible scenario of forming-free switching mechanisms for the Ti/ZrO 2/Pt devices. (a) The as-deposited device with resistance comparable to the HRS. (b) The N 2

annealed device with initial ON state. (c) Stable RS processes during cycling measurement.

to that of the as-deposited device. This unique behavior may be attributed to the sufficient oxygen ions stored in the interfacial oxygen reservoir layer, which are available to n eutralize the oxygen vacancies during the first reset process. Meanwhile, high power is required during the first reset process. The large current flow through the filaments leads to a severe Joule heating effect, which enhances the migration of oxygen ions, leading to the sharp reset process as depicted in figure 2(a) [2]. Consequently, the pre-existed oxygen vacancy clusters were annihilated completely, and a high resistance ratio was obtained with larger HRS resistance.

To further evaluate the memory performance of the devices, the cycling endurance characteristics were measured for the as-deposited and N 2 annealed cells, respectively, as shown in figure 5. The resistance values were read out at 0.1 V in each sweep. Notably, it was found that the HRS of the device after N 2 annealing manifests visible scattering distribution during the endurance measurement, especially for the first 20 cycles. This behavior might be related to the large amount of oxygen ions stored in the oxygen reservoir layer for the N 2 annealed devices. As discussed above, it is probable that the available oxygen ions to neutralize the oxygen vacancies could not be well controlled during the reset operation due to their high density in nature. As a consequence, a stochastic incomplete rupture of filaments occurred, giving rise to a larger variation in HRS. However, after several endurance cycles, the amount of the oxygen ions stored in the reservoir layer declined to some extent, thus a more stable HRS is expected as dem-onstrated in figure 5. In addition, the improved crystallinity for the N 2 annealed devices would also responsible for the degraded HRS stability as reported earlier [30]. Although some fluctuations occurred, especially in HRS, reversible and reproducible resistive switching property is demonstrated for more than 100 cycles. A larger resistance ratio (~200) was guaranteed for the annealed devices in N 2 ambient, while the resistance ratio was about 30 for the as-deposited cells. These memory windows are high enough for the periphery circuits to probe the stored information.

4. Conclusion

In summary, distinct forming-free RS characteristics modu-lated by thermal treatment have been demonstrated in ZrO 2 RRAM device with a Ti top electrode. It is revealed that the voltage for the first HRS-to-LRS switching is close to the set voltage in the as-deposited films. However, after the oxygen-deficient ambient annealing, the initial resistance was further reduced to the ON state. A first low-voltage reset operation is required to initialize the device and a higher resistance ratio is observed in the subsequent cycle switching. This behavior is attributed to the nonstoichiometric component variation and partial crystallization during the thermal annealing. We infer that further optimizing the fabrication process and annealing parameters could accurately adjust the initial state and the resultant resistance ratio, which is highly desirable for prac-tical nonvolatile memory applications.Acknowledgments

The authors would like to thank Nanofabrication Facility and Platform for Characterization and Test of SINANO, Chinese Academy of Sciences. This work was supported by the China Postdoctoral Science Foundation (Grant No. 2014M551675), the Postdoctoral Science Foundation of Jiangsu Province (Grant No. 1302160C) and National Natural Science Founda-tion of China (Grant No. 11404373).References

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