Improvement of Bipolar Switching Properties of Gd:SiOx RRAM Devices on Indium Tin Oxide Electrode by Low-Temperature Supercritical CO2 Treatment

N A N O E X P R E S S

Open Access

Improvement of Bipolar Switching

Properties of Gd:SiO

_x

RRAM Devices on

Indium Tin Oxide Electrode by

Low-Temperature Supercritical CO

₂

Treatment

Kai-Huang Chen

, Kuan-Chang Chang

, Ting-Chang Chang

, Tsung-Ming Tsai

, Shu-Ping Liang

, Tai-Fa Young

,

Yong-En Syu

and Simon M. Sze

Abstract

Bipolar switching resistance behaviors of the Gd:SiO2resistive random access memory (RRAM) devices on indium

tin oxide electrode by the low-temperature supercritical CO2-treated technology were investigated. For physical and

electrical measurement results obtained, the improvement on oxygen qualities, properties of indium tin oxide electrode, and operation current of the Gd:SiO2RRAM devices were also observed. In addition, the initial metallic

filament-forming model analyses and conduction transferred mechanism in switching resistance properties of the RRAM devices were verified and explained. Finally, the electrical reliability and retention properties of the Gd:SiO2

RRAM devices for low-resistance state (LRS)/high-resistance state (HRS) in different switching cycles were also measured for applications in nonvolatile random memory devices.

Keywords: Nonvolatile memory, Gadolinium, Supercritical CO2, Resistive switching, Silicon oxide

Background

Many nonvolatile memory devices for ferroelectric ran-dom access memory (FeRAM), magnetic ranran-dom access memory (MRAM), and phrase change memory (PCM) are widely discussed for applications in the smart memory cards, electronic devises, and portable electrical devices [1–8]. Among these memory devices, various metals doped into silicon-based oxide thin films are widely and considerably discussed for the resistive random access memory (RRAM) devices because of its great compatibil-ity in integrated circuit (IC) processes, high operation speed, long retention time, and low operation voltage [9– 13]. Recently, the transparent ITO electrode of the various memory devices are widely discussed and investigated be-cause of its compatibility and integrated in system on panel concept applications [14–17]. The high thermal budget and fabrication cost of rapid temperature

annealing (RTA) and conventional furnace annealing (CFA) post-treatment methods were widely used for appli-cations in dielectric thin films reformed and passivated the defects [15–18]. However, the excellent liquid-like properties of the supercritical CO2 fluid (SCF) process

have attracted considerable research in efficiently trans-porting H2O molecules diffusion into the microstructures

of thin films at a low-temperature treatment [19–21]. To discuss the SCF-treated ITO electrode on bipolar switching properties of RRAM devices, the ITO/ Gd:SiO2/TiN structure was treated by low-temperature

SCF treatment. In addition, the electrical transferred conduction mechanism of the initial metallic filament-forming model was explained to bipolar switching prop-erties of RRAM devices on ITO electrode in this study.

Methods

The metal-insulator-metal (MIM) structure of Gd:SiO2

thin film RRAM devices was fabricated and prepared by SiO2 and gadolinium co-sputtering technology on the

TiN/Ti/SiO2/Si substrate. The sputtering power was

fixed with an rf power of 200 W and a DC power of

* Correspondence:[email protected];[email protected]

1

Department of Electrical Engineering and Computer Science, Tung Fang Design Institute, Kaohsiung, Taiwan, Republic of China

3_{Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan,}

Republic of China

Full list of author information is available at the end of the article

© 2016 Chen et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

10 W. The 200-nm-thick ITO electrode was deposited on Gd:SiO2film to form ITO/Gd:SiO2/TiN structure. In

addition, the ITO/Gd:SiO2/TiN structure sample was

placed in the supercritical fluid system, which was mixed with 5 vol.% pure H2O and 5 vol.% propyl alcohol,

injected at 3000 psi and 150 °C for 2 h. The bipolar switching operation current versus applied voltage (I–V) characteristics of Gd:SiO2 RRAM devices are measured

by Agilent B1500 semiconductor parameter analyzer. The X-ray photoelectron spectroscopy (XPS) is used to analyze the chemical composition and bonding of thin films, respectively.

Results and Discussion

To investigate the SCF-treated ITO electrode effect, the bipolar resistance switching behavior of the Gd:SiO2

RRAM devices was discussed and observed in Fig. 1. After the initial forming process of−10 V in Fig. 1b, the Gd:SiO2RRAM devices exhibited a low-resistance state

(LRS). Then, a high-resistance state (HRS) was forming by high negative bias. To define the set process state, the RRAM devices exhibited the LRS for applying a large negative bias than the set voltage. For reset process state, a gradual current decrease was presented in LRS to HRS for the bias to positive over the reset voltage. For inverted set/reset state properties of the Gd:SiO2RRAM

devices, we suggested the transferred electron early cap-tured by the lots of oxygen vacancy in top ITO electrode and formed the oppositely metallic filament [22]. The operation current of the Gd:SiO2 RRAM devices for

using SCF-treated ITO electrode was lower than that for

the nontreated electrode of others. In order to further discuss the initial metallic filament path diagram model, the electrical transferred mechanisms of RRAM devices for the SCF-treated ITO electrode were discussed and investigated.

According to the relationship of the Schottky emission equation, J ¼ A  T2_{exp−q ϕ} Β− ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiqEi=4πεi p   =KT   , where T is the absolute temperature, ΦB is the Schottky barrier

height, εi is the insulator permittivity, K is Boltzmann’s

constant, and A* is Richardson constant. The I–V switching curve of the Gd:SiO2 RRAM devices was

transferred to ln(I/T2)− V1/2and ln(I) − ln(V) curve to fit the Schottky emission and the ohmic conduction mechanism. In Fig. 2, the Gd:SiO2 RRAM devices for

LRS/HRS in the set state exhibited the ohmic con-duction mechanism for low applied voltage. In Fig. 2a for 0.3~0.5 V, the LRS/HRS of Gd:SiO2 RRAM

de-vices all exhibited the Schottky emission conduction by ln(I/T2)− V1/2 curve fitting for the temperature of 300–350 K [23, 24]. If the J–E curves obey the Schottky emission model, the fitting curves should be straight in this figure. In Fig. 3a, the LRS/HRS of Gd:SiO2 RRAM devices in the reset state also

exhib-ited the ohmic conduction mechanism by ln(I) − ln(V) curve and the Schottky emission conduction mechan-ism by ln(I/T2)− V1/2 curve fitting.

To analyze the oxygen element of the chemical com-position characteristics in ITO electrode, the mole frac-tion of stannum (Sn), indium (In), and oxygen (O), in the ITO thin film was 5.08, 47.76, and 47.15 %, respect-ively, calculated from the peak areas of XPS spectra. For

Fig. 1 The typicalI–V switching characteristics of the Gd:SiO2thin film RRAM devices for (a) the initial forming process and (b) In3+3d5/2of ITO

electrode in XPS spectra

the SCF-treated ITO electrode, we found that the mole fraction of Sn, In, and O elements was 4.7, 18.32, and 76.98 %, respectively. The mole fraction of the oxygen element increased from 47.15 to 76.98 %. The increase of oxygen ion qualities and decrease of the electric conductivity of SCF-treated ITO electrode were also proved and verified in the XPS spectra. In Fig. 1b, the In1+3d5/2 peaks of ITO electrode that shifted two

valences to In3+3d5/2 effect was caused and improved

by oxidation ability and binding energy of SCF treat-ment. The oxidation ability and repaired damaged effect of ITO electrode of Gd:SiO2RRAM devices improved by

SCF treatment process were found [15–17].

As discussed above, the electrical transferred mecha-nisms ofI–V curves results, the metal filament path dia-gram model of the Gd:SiO2 RRAM devices was

described. To the initial metallic filament path-forming process for the negative applied voltage, the uniform Fig. 2 TheI–V switching curves of the Gd:SiO2RRAM devices using SCF-treated ITO electrode for LRS/HRS state in set state. (a) ln(I/T2)-V1/2curve

fitting and (_{b) the reliability properties for different switching cycle}

Fig. 3 TheI–V switching curves of the Gd:SiO2RRAM devices using SCF-treated ITO electrode for LRS/HRS state in reset state. (a) ln(I/T2)-V1/2curve

oxygen ions existed in Gd:SiO2 thin film of the RRAM

devices for the set state are shown in Fig. 4a. To con-tinuously apply negative voltage, lots of oxygen ions were accompanied into the ITO electrode. The metallic filament path increased and exhibited Schottky emission conduction mechanism. In Fig. 4b, the oxygen ions in ITO electrode return back to Gd:SiO2thin film for the

initial reset state exhibited the ohmic conduction mech-anism for the low voltage applied. Then, the metallic filament path was decreased by oxygen ion oxidation

and exhibited Schottky emission conduction mechanism for continuously applying positive voltage.

For the electrical reliability properties, the on/off ratio in I–V curves of the Gd:SiO2RRAM devices was

mea-sured and obtained for the different switching cycle. In Fig. 2b, no significant changes in the current values for 104 s were observed. In addition, the switching cycling measured another type of the retention characteristics shown in Fig. 3b. The slight fluctuation of the resistance in the LRS/HRS and the stable switching property of

Fig. 4 The electrical transferred mechanisms and metallic filament path diagram of the Gd:SiO2RRAM devices using SCF-treated ITO electrode for

a set state under the negative voltage and b reset state under the positive voltage

105cycles exhibited the reliability properties of the non-volatile Gd:SiO2RRAM devices applications.

Conclusions

In conclusion, the bipolar resistance switching charac-teristics and low power consumption of Gd:SiO2

RRAM devices for ITO top electrode were achieved by using a low-temperature supercritical CO2

treat-ment. The switching resistance mechanisms in the SCF-treated ITO electrode of RRAM devices for HRS/LRS were proved and investigated by electrical transferred mechanisms and a metallic filament path diagram model. Finally, no significant changes of the operation current of the electrical reliability properties in Gd:SiO2RRAM devices for on/off state were

main-tained to 104 s. For the retention characteristics, the slight fluctuation of resistance in the LRS/HRS states and the stable switching property of 105 cycles were also found.

Competing interests

The authors declare that they have no competing interests. Authors’ contributions

KHC and KCC designed and performed the experimental work, explained the obtained results, and wrote the paper. TCC, TMT, conceived the study and participated in its design and coordination. KHC, SPL, and TFY, helped in writing of the paper and participated in the experimental work. All authors read and approved the final manuscript.

Acknowledgements

This work was performed at the National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in the Kaohsiung-Pingtung area and was supported by the National Science Council of the Republic of China under Contract Nos. MOST 104-2633-E-272 -001 -MY2, and MOST 103-2633-E-272 -001.

Author details

1_{Department of Electrical Engineering and Computer Science, Tung Fang}

Design Institute, Kaohsiung, Taiwan, Republic of China.2Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China.3_{Department of Physics, National Sun}

Yat-Sen University, Kaohsiung, Taiwan, Republic of China.4_Advanced

Optoelectronics Technology Center, National Cheng Kung University, Tainan, Taiwan, Republic of China.5_{Department of Mechanical and}

Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China.6_{Department of Electronics Engineering and}

Institute of Electronics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China.

Received: 11 August 2015 Accepted: 21 January 2016

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