Origin of Hopping Conduction in Graphene-Oxide-Doped Silicon Oxide Resistance Random Access Memory Devices

IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 5, MAY 2013 677

Origin of Hopping Conduction in

Graphene-Oxide-Doped Silicon

Oxide Resistance Random

Access Memory Devices

Kuan-Chang Chang, Rui Zhang, Ting-Chang Chang, Senior Member, IEEE, Tsung-Ming Tsai, J. C. Lou,

Jung-Hui Chen, Tai-Fa Young, Min-Chen Chen, Ya-Liang Yang, Yin-Chih Pan, Geng-Wei Chang,

Tian-Jian Chu, Chih-Cheng Shih, Jian-Yu Chen, Chih-Hung Pan, Yu-Ting Su, Yong-En Syu,

Ya-Hsiang Tai, and Simon M. Sze, Life Fellow, IEEE

Abstract— In this letter, a double-active-layer (Zr:SiO

/

C:SiO

)

resistive

switching

memory

device

with

a

high

ON

/

resistance ratio and small working current (0.02 mA),

is presented. Through the analysis of Raman and Fourier

transform infrared spectroscopy spectra, we find that graphene

oxide exists in the C:SiO

layer. It can be observed that

Zr:SiO

/C:SiO

structure has superior switching performance

and higher stability compared with the single-active-layer

(Zr:SiO

) structure, which is attributed to the existence of

graphene oxide flakes formed during the sputter process. I–V

characteristics under a series of increasing temperature were

analyzed to testify the carrier hopping distance variation, which

is further verified by our graphene oxide redox reaction model.

Index Terms— conduction, graphene oxide, hopping, redox

reaction, resistance random access memory (RRAM).

I. I

T

O SURMOUNT the technical and physical limitation

issues of conventional charge-storage-based memories

Manuscript received December 19, 2012; revised February 20, 2013; accepted February 26, 2013. Date of publication March 27, 2013; date of current version April 22, 2013. This work was supported by the National Science Council of the Republic of China under Contract NSC-101-2120-M-110-002, and Contract NSC 101-2221-E-110-044-MY3. The review of this letter was arranged by Editor T. San.

K. C. Chang, T. M. Tsai, Y.-C. Pan, T.-J. Chu, and C.-H. Pan are with the Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan.

R. Zhang and J. C. Lou are with the School of Software and Microelec-tronics, Peking University, Beijing 100871, China.

T.-C. Chang, M.-C. Chen, J.-Y. Chen, Y.-T. Su, and Y.-E. Syu are with the Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, and also with the Advanced Optoelectron-ics Technology Center, National Cheng Kung University, Taiwan (e-mail: [email protected]).

J.-H. Chen and C.-C. Shih are with the Department of Chemistry, National Kaohsiung Normal University, Kaohsiung, Taiwan.

T.-F. Young and Y.-L. Yang are with the Department of Mechanical & Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan.

G. W. Chang and Y. H. Tai are with the Department of Photonics and the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan.

S. M. Sze is with the Department of Physics, National Sun Yat-Sen Univer-sity, Kaohsiung 80424, Taiwan, and also with the Department of Electronics Engineering, National Chiao Tung University, Hsinchu 300, Taiwan.

Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2013.2250899

[1]–[6], the resistance random access memory (RRAM) is

con-structed of an insulating layer sandwiched by two electrodes.

This structure is a great potential candidate for next-generation

nonvolatile memory due to its superior characteristics such

as lesser cost, simple structure, high-speed operation, and

nondestructive readout [7]–[16].

Recent years have witnessed many breakthroughs in the

research on graphene as well as a significant advance in

the mass production of this material. This one-atom-thick

fabric has many supreme properties, like extreme mechanical

strength and high electronic and thermal conductivities.

In our research, double resistive switching layers with a

sandwiched structure of Pt/Zr:SiO

/C:SiO

/TiN were

fab-ricated. Meanwhile, single-resistive-switching-layer RRAM

(Pt/Zr:SiO

/TiN) was also fabricated so as to make a

com-parison. From the analysis of Raman and Fourier transform

infrared spectroscopy (FTIR) spectra, graphene oxide was

found, and that was the reason why Zr:SiO

/C:SiO

structure

showed better performance of resistive switching

characteris-tics and had lower forming voltage.

As hopping conduction mechanism was found in current

fitting of Zr:SiO

/C:SiO

RRAM, I

−V measurement under

a series of increasing temperature was performed to further

obtain carrier hopping distance. Meanwhile, the hopping

dis-tance variation was demonstrated by our graphene oxide redox

reaction model.

II. E

S

The experimental specimens were prepared as follows. For

the single-active-layer specimen, the Zr:SiO

thin film (around

20 nm) was deposited on the TiN/Ti/SiO

/Si substrate by

cosputtering with the pure SiO

and Zr targets. However, for

the double-resistive-switching-layer specimen, first a C:SiO

film (around 6 nm) was deposited by cosputtering with the

SiO

and C targets. Then the layer of Zr:SiO

(around

14 nm) was deposited with the same RF power, argon ambient

and working pressure as the antecedent single-Zr:SiO

-layer

specimen

Ultimately, the Pt top electrode of 200 nm thickness was

deposited on both specimens by DC magnetron sputtering.

678 IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 5, MAY 2013

Fig. 1. Blue and red curves are the resistive switching characteristics of Zr:SiOxfilm and Zr:SiOx/C:SiOxfilm, respectively. The current of Zr:SiOx/

C:SiOxfilm is reduced on both high resistance state (HRS) and low resistance

state (LRS). The figure on bottom left is a forming curve, which shows that Zr:SiOx/C:SiOx film has smaller forming voltage.

The entire electrical measurements of devices with the Pt

electrode of 250-

μm diameter were performed using Agilent

B1500 semiconductor parameter analyzer. Besides, FTIR and

Raman spectroscopy were used to analyze the chemical

com-position and bonding of these insulator materials, respectively.

III. R

D

The “forming process” is required to activate all of the

RRAM devices, using DC voltage sweeping with a

com-pliance current of 10

μA. Compared with Zr:SiO

RRAM,

Zr:SiO

/C:SiO

RRAM devices have smaller forming voltage

around 5–7 V (inset of Fig. 1), and the working current of the

Zr:SiO

/C:SiO

RRAM devices in both low-resistance state

(LRS) and high-resistance state (HRS) is lower than that of

the Zr:SiO

devices (Fig. 1).

To investigate the interesting phenomena, we utilized the

material spectra analyses to find out the reason of working

current reduction and better stability. The C:SiO

film was

analyzed by Raman spectroscopy, from which we found

typ-ical graphene oxide Raman spectra which comprised of a

higher G band peak and a lower D band peak (Fig. 2) [17].

In order to further testify the existence of graphene oxide and

find its chemical bonding type, FTIR was used to analyze

the C:SiO

film. Graphene-oxide-coupled OH peak can be

observed at the wave number of 3665 cm

, as shown in the

top right FTIR spectra of Fig. 2.

Through current fitting, we found that

single-resistive-switching-layer devices exhibit Poole–Frenkel conduction in

HRS and Ohmic conduction in LRS, while both LRS and HRS

are hopping conduction mechanisms for double-active-layer

devices. Then, we carried out I –V measurement under a series

of increasing temperature, which is shown in Fig. 3(a) for

state and in Fig. 3(c) for

state. According to the

relation-ship of hopping conduction, J

= q Naυ

e

e

,

where N , a,

φ

,

υ

, and d are density of space charge, mean of

hopping distance, barrier height of hopping, intrinsic vibration

frequency, and film thickness, respectively, we can draw out

Fig. 2. Raman spectra of C sp2 and C sp3 in C:SiOx film. It confirms

the existence of grapheme oxide. The blue curve on upper right is the corresponding FTIR spectra from which grapheme oxide coupling OH peak can be observed at the wavenumber of 3665 cm−1.

Fig. 3. (a) and (c) ON and OFF state current-voltage characteristics of Zr:SiOx/C:SiOxRRAM under increasing temperature. The calculated hopping

distance is 0.32 and 0.46 nm, respectively. (b) and (d) CorrespondingONand OFFstate activation energy under different voltage.

the curve

φ

-aV /2d with a vertical axis of In(I

) and a lateral

axis of 1000/T [Fig. 3(b) for

state and Fig. 3(d) for

state]. As d stands for hopping distance, we calculated from

the curves in Fig. 3(b) and (d) the corresponding hopping

distance for

state and

state as 0.324 and 0.468 nm,

respectively. The result showed an increase of 0.144 nm in the

hopping distance in HRS as compared with LRS.

On the basis of the electrical and material analyses, we

proposed a reaction model to explain the transfer of carrier

conduction mechanism of the Zr:SiO

/C:SiO

film as shown

in Fig. 4. The conductive filament will be formed in the

Zr:SiO

film after the forming process and it will be connected

with graphene oxide flake in the switching region of C:SiO

film. Meanwhile, oxygen-contained groups are removed from

graphene oxide and are driven to the direction of the bottom

electrode. The carrier will hop through carbon atoms within

CHANG et al.: ORIGIN OF HOPPING CONDUCTION 679

Fig. 4. Schematic diagram of graphene oxide redox reaction in Zr:SiOx/

C:SiOx RRAM. The hopping distance variation is due to the stretching of

carbon-carbon bond within the carbocycle.

the carbocycle in the C:SiO

film. If the bottom TiN electrode

is applied with a negative bias to perform a reset process,

oxygen atoms are repelled to the reverse direction of TiN

elec-trode and are absorbed by graphene oxide. With the absorption

of oxygen atoms, carbon-carbon bonds are stretched and

car-bocycle is enlarged, which results in longer hopping distance

of carriers In single-layer RRAM devices, in LRS, electrons

conduct through metal filaments from top electrode to bottom

electrode, and in HRS, electrons conduct through shallow

defects between the tip of the ruptured filament and the

bottom TiN electrode. But in double-layer devices, the

oxygen-contained groups’ absorption and desorption of graphene oxide

result in carbocycle deformation and thus the hopping distance

variation between

and

states in graphene-oxide-doped

silicon RRAM.

IV. C

By cosputtering C and Zr with SiO

we fabricated a

double-resistive-switching-layer RRAM that has obvious

lower working current and forming voltage. Both FTIR

and Raman spectra confirmed the existence of graphene

oxide. With the absorption and desorption of oxygen atoms,

carbon-carbon bonds within carbocycle are stretched, which

results in hopping distance variation and resistive switch. The

Zr:SiO

/C:SiO

structure has superior switching performance

and higher stability compared with the single-active-layer

(Zr:SiO

) structure, which is attributed to the redox reaction

of graphene oxide.

A

This work was performed at the National Science Council

Core Facilities Laboratory for Science and

Nano-Technology in the Kaohsiung-Pingtung area.

R

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