Investigating the improvement of resistive switching trends after post-forming negative bias stress treatment

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Investigating the improvement of resistive switching trends after post-forming negative

bias stress treatment

Hsueh-Chih Tseng, Ting-Chang Chang, Jheng-Jie Huang, Po-Chun Yang, Yu-Ting Chen, Fu-Yen Jian, S. M. Sze, and Ming-Jinn Tsai

Citation: Applied Physics Letters 99, 132104 (2011); doi: 10.1063/1.3645004

View online: http://dx.doi.org/10.1063/1.3645004

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Investigating the improvement of resistive switching trends after

post-forming negative bias stress treatment

Hsueh-Chih Tseng,1Ting-Chang Chang,1,2,a)Jheng-Jie Huang,1Po-Chun Yang,3 Yu-Ting Chen,3Fu-Yen Jian,4S. M. Sze,1,4,5and Ming-Jinn Tsai6

1

Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

2

Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

3

Department of Electro-Optical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

4

Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan

5

Department of Electrical Engineering, Stanford University, Stanford, California 94305-4085, USA

6

Electronics and Optoelectronics Research Laboratory, Industrial Technology Research Institute, Chutung, Hsinchu 310, Taiwan

(Received 22 May 2011; accepted 3 September 2011; published online 29 September 2011) This paper investigates the improvement of resistive switching trends after post-forming negative bias stress treatment of a Pt/Yb2O3/TiN device that has undergone positive bias forming process

for activation. After the treatment, characteristics of the conductive filament, such as the temperature dependence of resistivity and transition mechanism, undergo changes. Furthermore, this treatment causes the conductive filament to transform from being primarily composed of vacancies to being metallic Yb dominant, which not only reduces operation voltages such as Vset

and Vreset but also improves the on/off ratio. In reliability tests, the device has stable retention.

VC _{2011 American Institute of Physics. [doi:}_{10.1063/1.3645004}_]

Resistive random access memory (RRAM) has attracted considerable interest for the next generation of nonvolatile memory devices due to its simple structure, low operation voltage, and process compatibility with the current comple-mentary metal-oxide-semiconductor (CMOS) industry.1 Up to now, two dominant resistance switching mechanisms have been proposed. One is oxygen vacancy nucleation at a metal/ oxide interface.2,3The other mechanism is the conductive fil-ament (CF) model, which describes the formation/rupture of a metallic filament using a metal such as Cu or Ag to act as mobile ions in the oxide.4 Recently, rare earth (RE) metal oxides, which are used as a high-k gate insulator for advanced CMOS technology,5,6 have exhibited resistance switching phenomena.7,8One of the RE metal oxides, ytter-bium oxide (Yb2O3), is attractive as a gate dielectric in

CMOS devices because of its dielectric constant of 15, larger energy band gap (>5 eV), and predicted chemical and ther-mal stability with Si. Therefore, it also has been explored for semiconductor applications including memory devices, logi-cal devices, and optoelectronic devices.9The application of Yb2O3in the resistive switching field, however, has not been

sufficiently researched.

This work investigates the bipolar resistance switching characteristics of a Pt//Yb2O3/TiN structure by using an

additional negative bias stress (NBS) treatment after the pos-itive electroforming (PF) process which is necessary to acti-vate the RRAM device and which causes the resistance state (RS) transformation to be switchable. The power law rela-tionship of the low resistance state (LRS) indicates that the NBS treatment can change the transition mechanism; more-over, further confirmation of this can be found in the temper-ature dependence of LRS trends. In addition, a mechanism is

proposed to explain the influence of the presence or absence of NBS treatment on resistive switching characteristics of Yb2O3based RRAM of the device.

Yb2O3thin film of 20 nm thickness was deposited on a

TiN/Si substrate by reactive magnetron RF sputtering an Yb2O3target in Ar (30 SCCM) ambient at room temperature.

Then the Pt top electrode (TE) was deposited and patterned by the liftoff process. The without-NBS device was only PF treated, whereas the with-NBS device was used as a refer-ence with NBS treatment after the PF activation and will be referred to as without-NBS and with-NBS devices hereafter. All electrical characteristics were measured over an 8 lm 8 lm cell size by an Agilent B1500 semiconductor parameter analyzer. During these measurements, bias was applied to the TiN bottom electrode (BE) while the Pt TE was ground.

The transition between LRS and high resistance state (HRS) of the without-NBS and the with-NBS devices was observed by using the same dc voltage sweeping mode, as shown in Fig.1. During the PF process, as shown in the inset (i) of Fig.1(a), a positive dc bias of about 12 V was applied to BE with a current compliance of 1 lA. Figure1(a)shows that the resistance of the without-NBS device can be switched to HRS by applying negative bias of about2 V. In a subsequent sweep, the RS can be switched again to LRS by applying a predetermined positive bias of about 2 V with a 10 mA current compliance. During the NBS treatment, a higher and reverse polarity dc bias with a 10 mA current compliance was applied to the without-NBS device, as shown in Fig.1(a). First, when the bias increased to about 3 V, there was a first obvious transformation in RS, which suddenly transformed from LRS to NBS-HRSinitial(after the NBS treatment of first transforma-tion without bipolar switching operatransforma-tion) and caused the local CF narrowing due to the joule heating effect.10Subsequently, as the bias continued to increase at about 5 V, the RS

a)_{Author to whom correspondence should be addressed. Electronic mail:}

tcchang@mail.phys.nsysu.edu.tw.

0003-6951/2011/99(13)/132104/3/$30.00 99, 132104-1 VC2011 American Institute of Physics

APPLIED PHYSICS LETTERS 99, 132104 (2011)

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transformed again from the NBS-HRSinitialto NBS-LRSinitial

(after the NBS treatment of second transformation without bipolar switching operation), which is similar to the forming process and reconstructs the CF. Figure1(b)shows the differ-ent trend of bipolar resistive switching characteristic of the with-NBS device, different from the without-NBS device. In addition, a few cycles of unipolar switching phenomena appear momentarily, as shown in the inset of Fig.1(b).

The inset (i) of Fig.2shows that the LRSinitial(after PF treatment without bipolar switching operation) of the without-NBS device decreases when the ambient tempera-ture increases, as is typical of semiconductor behavior

prop-erties.11 This implies that the CF is the dominated by

vacancies.12In contrast, under the second RS transformation of the NBS treatment, the oxygen vacancies and oxygen ions can be re-generated. Because the number of the re-generated oxygen vacancies and diffusion ions are enough to cause the metallic CF formation, the NBS-LRSinitial of the with-NBS

device increases as ambient temperature rises, which indi-cates typical metallic behavior;11 furthermore, calculating the temperature coefficient of the resistance a indicates that a is about 9.74 104 K1 (the ideal a of metallic Yb is about 1.3 103 K1), as shown in Fig.2. This result indi-cates that the NBS treatment can generate the Yb metallic-type conductive path(s). However, these mobile oxygen ions drift to Pt and can be chemisorbed at the grain boundary or penetrate through the Pt layer during the NBS treatment.13,14 After bipolar stressing (switching), these mobile oxygen ions cannot fully drift back to the insulator and towards TiN;15

moreover, the CF characteristic changes from metallic behavior to semi-like behavior, as shown in the inset (ii) of Fig. 2, because the decrease in mobile oxygen ions acts to reconstruct the thinner redox reaction region during the bipolar-set process.

Figure3shows the power law relationship and current fitting between the without-NBS and with-NBS devices. According to the power law relationship (I¼ KVn) for the I-V plot of LRS, the switching mechanism before and after NBS treatment is different. Fig.3(a)shows that the value of n is about 1.78 for the without-NBS sample and about 1.22 for the with-NBS while both underwent the bipolar switching operation is due to the switching layer (SL) formation. How-ever, after the NBS treatment without bipolar switching operation, the value of n was about 0.97. If no SL exists dur-ing the switchdur-ing process, as in unipolar switchdur-ing,16 the n value would be about 1 due to the metallic filament forma-tion. In addition, the current fitting results also show that the LRS for the without-NBS device obeys the Schottky conduc-tion behavior due to the original-SL formaconduc-tion, as shown in Fig.3(b). The NBS treatment can cause the original-SL dis-integrating, which makes the LRS transition mechanism to follow the Ohmic conduction behavior (the slope is 1.0), as shown in Fig. 3(c); furthermore, because the reconstructed

FIG. 1. (Color online) (a) The I-V curve characteristics of the without-NBS sample and with-NBS treatment. The inset shows the PF treatment. (b) The I-V curve characteristics for the control sample. The inset shows unipolar characteristics. Both (a) and (b) show typical I-V curve characteristics with 10 mA compliance.

FIG. 2. (Color online) The temperature-dependent initial low resistance state in with-NBS samples without switching operation, the inset (i) without-NBS device and the inset (ii) with-NBS with the bipolar switching operation.

FIG. 3. (Color online) The LRS mechanism fitting result for (a) power law relationship, (b) Schottky mechanism of the without-NBS device, (c) Ohmic mechanism of the with-NBS without bipolar switching, and (d) Ohmic mechanism of the with-NBS with bipolar switching. And the inset of (b)–(d) shows the schematic diagram of the without-NBS and with-NBS mechanism.

132104-2 Tseng et al. Appl. Phys. Lett. 99, 132104 (2011)

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SL is thinner than the without-NBS device, the LRS transi-tion mechanism also follows the Ohmic conductransi-tion behavior (the slope is 1.12), as shown in Fig.3(d).

The comparisons in Fig. 4(a) show that the without-NBS device also has lower Vsetand Vreset. The decrease in mobile oxygen ions of the with-NBS device can be easily induced the redox reaction for a smaller Vset, and the metal-lic CF of the with-NBS device has a better electric conduc-tivity making the smaller Vreset induce the reset current. To test the device for practical memory application, reliability of retention was monitored. Figure4(b)shows the data reten-tion at 85C. The results reveal that the proposed treatment can retain stable HRS and LRS without degradation longer than 104s.

In conclusion, the effect of NBS on the resistive switch-ing characteristics of Pt/Yb2O3/TiN RRAM device has been

investigated. Based on experimental results, the NBS treat-ment can transform the CF from vacancy-dominated to me-tallic Yb dominant. Hence, the characteristics of resistance switching mechanism can be changed into redox reaction accompanying joule heating effect by applying the NBS treatment in order to obtain better bipolar switching charac-teristics such as lower set and reset voltages and a larger on/off ratio.

This work was performed at National Science Council Core Facilities Laboratory for Science and Nano-Technology in Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China

under Contract Nos.100-2120-M-110-003 and NSC-97-2112-M-110-009-MY3.

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FIG. 4. (Color online) (a) Set and reset voltage statistics for with-NBS samples for 100 cycles in dc sweeping mode. The inset shows the statistics of the without-NBS sample. (b) Retention characteris-tics for both with and without NBS sam-ples for both resistance states at 85C.

132104-3 Tseng et al. Appl. Phys. Lett. 99, 132104 (2011)