Formation-Polarity-Dependent Improved Resistive Switching Memory Performance

Using IrOx/GdOx/WOx/W Structure

Debanjan Jana, Siddheswar Maikap�, Ta Chang Tien1, Heng Yuan Lee2,

Wei-Su Chen2, Frederick T. Chen2, Ming-Jer Kao2, and Ming-Jinn Tsai2

Thin Film Nano Tech. Lab., Department of Electronic Engineering, Chang Gung University, Kweishan, Taoyuan 333, Taiwan1Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan2Electronic and Opto-Electronic Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan

Received September 26, 2011; revised November 28, 2011; accepted November 29, 2011; published online April 20, 2012

The formation-polarity-dependent improved resistive switching memory performance using a new IrOx/GdOx/WOx/W structure has been

investigated. The memory device has been observed by both high-resolution transmission electron microscopy and energy dispersive X-ray

spectroscopy. The thicknesses of the GdOx and WOx layers are observed to be approximately 15 and 5.5 nm, respectively. All layers are also

analyzed by X-ray photoelectron spectroscopy. The resistive switching mechanism is filament formation/rupture in the high-� GdOx layer, which is

controlled by the oxygen ion migration in bilayer GdOx/WOx films under negative and positive formation polarities. Excellent uniformity of SET/

RESET voltages, low/high resistance states, and switching cycles have been observed under positive formation polarity owing to the charge

trapping/detrapping in the high-� GdOx switching layer. The memory device shows a long endurance of >104 times, and extrapolated 10-year

data retention at 85 �C. This device shows great potential for future nonvolatile memory (NVM) applications.

# 2012 The Japan Society of Applied Physics

1. Introduction

Resistive switching random access memory (ReRAM) is apromising candidate for the next generation of nanoscalenonvolatile memory applications owing to its simplestructure with high-� material, high speed with lowoperation voltage, and process compatibility with that ofthe present complementary metal–oxide–semiconductors(CMOSs).1–5) To achieve the requirements of the IndustrialTechnology Research for Semiconductor (ITRS) roadmap,many materials such as SrZrO3,

6) Pr0:7Ca0:3MnO3,7) HfO2,

8)

Al2O3,9) ZrO2,

10,11) MnO2,12) CuO,13) NiO,14,15) TiO2,

16) andWOx

17) have been reported by many groups. According tothe reported results in the literature, two major resistanceswitching mechanisms have been demonstrated. One is themetallic filament formation/dissolution in solid-electrolytebased materials using a metal such as silver (Ag) or copper(Cu) as mobile ions and the other is oxygen vacancy filamentformation/rupture in oxide-based materials.18,19) Recently,many rare-earth metal oxides, such as Gd2O3,

20) andSm2O3

21) which can be used as a high-� gate insulator foradvanced CMOS technology in the future, have shownresistive switching phenomena suitable for future nanoscalenonvolatile memory applications. One of the rare-earthmetal oxides of high-� Gd2O3 is promising because of itshigh dielectric constant of 16, large energy band gap of5.3 eV and good chemical and thermal stabilities22) withSi. Furthermore, this high-� Gd2O3 material has also beeneconsider for flash memory device applications.23) Theresistive switching characteristics using high-� Gd2O3

materials have rarely been reported.22) To improve theresistive switching performance, gadolinium (Gd) has alsobeen implanted in the high-� HfO2 film24) and Cu–MoOx/GdOx bilayer has also been reported.25) According to ourview, the resistive switching memory characteristics usinga GdOx layer in an IrOx/GdOx/WOx/W structure have notyet been reported. Furthermore, formation polarity on thepristine device is one of the important steps towardsachieving high-uniformity ReRAM application. In this

study, formation-polarity-dependent improved resistiveswitching parameters, such as SET/RESET voltages, lowresistance state (LRS), high resistance state (HRS), uni-formity, switching cycles, and extrapolated 10-year dataretention at 85 �C have been investigated. The switchingmechanism is also explained. The high-� GdOx material hasbeen characterized by high-resolution transmission electronmicroscopy (HRTEM), energy-dispersive X-ray (EDX)spectroscopy, and X-ray photoelectron spectroscopy (XPS)analyses.

2. Experimental Methods

Tungsten (W) metal as a bottom electrode (BE) wasdeposited by sputtering on SiO2 (200 nm)/Si 8-in. wafers.The thickness of the W layer was approximately 100 nm. Toform the ReRAM device, the SiO2 layer with a thickness of�150 nm was deposited. Then, a small via-hole with anactive area of 2� 2 �m2 was formed by standard lithogra-phy. Photoresist (PR) was used to design the pattern, andwas open at the active and top electrode (TE) regions. Then,high-� Gd2O3 film with a thickness of 15 nm was depositedby an E-beam evaporator using pure Gd2O3 granules withsizes from 2 to 3mm. The deposition rate of Gd2O3 was0.2 �A/s and the power was 400W. Then, IrOx with athickness of �300 nm was deposited as a TE by rfsputtering. An Ir target was used. The ratio of Ar to Ogases was 1 : 1 (i.e., 25 : 25 sccm). The deposition powerand chamber pressure were 50W and 20mTorr, respec-tively. During deposition of the Gd2O3 and IrOx films,oxygen diffuses into the W layer, which results a WOx

formation on surface of the W layer. This process has thealso the advantage of creating the GdOx/WOx bilayer. Thelift-off process was used to fabricate the ReRAM device. Aschematic view of our IrOx/GdOx/WOx/W ReRAM deviceis shown in Fig. 1. The sweeping voltage was applied on theTE and the BE was grounded during measurement. Themicrostructure and thicknesses of all layers in the IrOx/GdOx/WOx/W device were observed by HRTEM with aoperation voltage of 200 kV. The memory devices for TEMobservation were prepared using an FEI Helios-400s systemwith an operating voltage of 5 kV and a Gaþ ion source. All�E-mail address: sidhu@mail.cgu.edu.tw

Japanese Journal of Applied Physics 51 (2012) 04DD17

04DD17-1 # 2012 The Japan Society of Applied Physics

REGULAR PAPERDOI: 10.1143/JJAP.51.04DD17

elements in the memory device were confirmed by EDXanalysis. Material compositions were studied by XPS withan Al K� monochrome X-ray and an energy of 1486.6 eV.The analyzed area was 650 �m in diameter. The vacuum inthe analytic chamber was 1� 10�9 Torr. For W layeranalysis, the Gd2O3 layer with a thickness of �10 nm wasetched out with an energy of 3 keV and Ar ion sputtering for100 s. All spectra were calibrated by the C 1s peak at anenergy of 284.6 eV. The memory characteristics wereexamined using the HP 4156C semiconductor parameteranalyzer. To investigate the charge-trapping characteristics,capacitance–voltage (C–V ) hysteresis was measured withthe HP 4284A LCR meter.

3. Results and Discussion

Figure 2(a) shows the HRTEM image of the ReRAMdevice. All layers are observed clearly. The thickness ofthe GdOx and WOx layers are approximately 15 and 5.5 nm,respectively. The GdOx film shows small crystalline grainswhile the WOx film is polycrystalline. The d-spacing of thecrystalline GdOx nanograin is 1.92 �A. It is very important tonote that the filament can be guided by the Gd-richnanograin or nanocrystal grain boundary. The EDX spectraconfirm the presence of expected elements of Ir, Gd, W, andO in respective layers, as shown in Fig. 2(b). The oxygencounts at an energy of �0:52 eV for the IrOx (81), GdOx

(92), and WOx (63) layers are higher than that of the Wlayer (29). Chemical compositions of GdOx and WOx layersare investigated by XPS, as shown in Fig. 3. Figure 3(a)represents the chemical bonding states of Gd2O3 film. TheGd 3d5=2 and Gd2O3 3d5=2 peaks are located at 1186.73 and1189 eV, respectively, which confirms the Gd:Gd2O3

mixture. Ahren et al.26) reported the same chemical bondingstates at 1186 and 1188 eV in the high-� Gd2O3 film. Theheight and area ratios of Gd:Gd2O3 are found to be 1 : 0:93and 1 : 0:89, respectively, which suggests a Gd-rich GdOx

film. Figure 3(b) shows the WO3 element with an energypeak at 35.48 eV. Katoh and Takeda27) reported the samebinding energy at 35.5 eV for the WO3 or WOx layer on thesurface of W BE. Hence the GdOx/WOx bilayer structure isobserved.

Figure 4 shows the typical current–voltage (I–V ) char-acteristics of the IrOx/GdOx/WOx/W memory device.Initially, the virgin state of the device is found to be

insulating. In order to activate the resistive switching, aninitial soft breakdown process (forming) is carried out byapplying negative and positive biases on the TE for negativeand positive formation polarities, respectively. A formingvoltage of approximately �6:4V is needed to activateresistive switching behavior for negative polarity, whereasthe forming voltage of approximately +3.6V is needed forpositive polarity (not shown here). Considering the workfunctions of IrOx (�m � 5:2 eV28)) and W (�m � 4:3{4:93eV29)) electrodes, the electron injection from the IrOx

electrode is lower than that of the W electrode owing tothe higher barrier height at the IrOx/GdOx interface. Thisreveals that the magnitude of formation voltage undernegative polarity is higher than that under positive polarity.This is one of the reasons for improving the resistiveswitching parameters by lowering the formation voltage orby selecting the formation polarity. Typical I–V hysteresischaracteristics with five consecutive cycles after negativeformation and first RESET operation are shown in Fig. 4(a).The SET and RESET voltages are found to be �2:4 and+1.9V, respectively. A huge cycle-to-cycle variation of theI–V switching characteristics is observed, which may be dueto the high formation voltage, i.e., many weak conductionpaths created in the switching material or the currentovershoot effect without series transistor or a combine of thetwo may cause the switching fluctuations. On the other hand,the stable and repeatable switching characteristics could be

Fig. 1. (Color online) Schematic view of our resistive switching memory

device in IrOx/GdOx/WOx/W structure. Device size is 2� 2 �m2. The

bottom electrode (BE) was grounded during measurement.

GdOx ~ 15 nm

WOx ~ 5.5 nmW (BE)

IrOx (TE)(a)

Nanograin

1 2 5 10

W

Gd

W

Cu

W

IrIrOx layer

GdOx layer

WOx layer

W layer

Ir

Ir

Ir

Ir

O Gd

W

WWW

WW

WIn

ten

sity

(ar

b. u

nit

s)

Energy (keV)

O

Ir

O

(b)

Fig. 2. (Color online) (a) Cross-sectional high-resolution TEM image

of the resistive switching memory device. (b) EDX analysis of different

layers.

D. Jana et al.Jpn. J. Appl. Phys. 51 (2012) 04DD17

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maintained even after an I–V sweep of more than 700 cyclesunder positive formation, as shown in Fig. 4(b). The SETand RESET voltages are +1.8 and �1:8V, respectively. Toelucidate the current transport mechanism of the IrOx/GdOx/WOx/W memory device, the I–V curves are plottedin log–log scale (not shown here). Under negative formationpolarity on the TE, both the LRS and HRS show ohmicconduction behaviors with a slope of approximately 1.1. TheLRS shows ohmic behavior because of conducting filamentformation in the GdOx layer and the HRS also shows ohmicbehavior because the holes can pass through the GdOx grainboundary. The switching mechanism is as follows. Undernegative formation polarity on the TE, the Gd–O bonds willbe broken easily on the grain boundary releasing oxygenions (O2�). Because of negative polarity on the TE, theoxygen ions will move from the GdOx layer to the WOx

layer, and it will be hidden in the WOx layer. Inconsequence, the oxygen vacancies (Vo) will move fromthe WOx layer to the GdOx layer. Then the oxygen vacancyfilament will be formed in the GdOx layer or the oxygenvacancy filament can be guided by the Gd-rich GdOx

nanograin. Thus the device switches to the LRS. Theconducting filament will be ruptured by applying positivebias to the TE and the device switches to the HRS. In this

case, O2� ions will move from the WOx layer toward theGdOx switching layer and oxidize the conducting filament.Basically, conducting filament formation/rupture is due tooxygen ion migration. Under positive formation polarity onthe TE, the Gd–O bonds will also be broken, releasing theO2� into the GdOx layer. Owing to the positive polarity onthe TE, the O2� will move toward the GdOx/TE interface.This suggests that the GdOx layer is separated into twolayers, i.e., oxygen-vacancy and oxygen-rich GdOx layers.The oxygen-rich layer will be formed near the TE and theoxygen-vacancy layer will be formed near the WOx layer.The conduction channels can be formed in the oxygenvacancy layer of GdOx switching material. The oxygenvacancy, as a positive charge, gives rise to the LRS with aseries insulator near the TE. The HRS occurs as a result ofelectron trapping or detrapping holes in the oxygen-vacancylayer of GdOx switching material via O2� migration from theoxygen-rich layer at the GdOx/TE interface towards theoxygen-deficient layer. Therefore, charge trapping anddetrapping phenomena lead to the LRS and HRS, respec-tively. To investigate the conduction mechanism of ourmemory device under positive formation polarity, I–Vcurves are plotted on a log–log scale (not shown here). TheLRS and HRS both show space-charge-limited current

-4 -2 0 2 410-9

10-7

10-5

10-3

10-1 5 cycles

5th cycle

Cu

rren

t (A

)

Sweeping Voltage (V)

1

2 3

4

VSET : -2.4V

VRESET : +1.9V

(a)

-3 -2 -1 0 1 2 3

10-5

10-4

10-3

10-2

700 Cycles

700th Cycle

Cu

rren

t (A

)

Sweeping voltage (V)

1

2

3

4

VSET :1.8V

VRESET : -1.8V

(b)

Fig. 4. (Color online) I–V characteristics of our memory device (a) for

negative SET polarity, and (b) for positive SET polarity.

1180 1185 1190 1195

As-receivedDe-convolutedBackgroundResultant

Gd 3d5/2 ~1186.73 eVIn

ten

sity

(ar

b. u

nit

)

Binding energy (eV)

Gd2O3 3d5/2

~1189 eV

(a)

30 32 34 36 38 40

As-receivedDe-convolutedBackgroundResultant

Inte

nsi

ty (

arb

. un

it)

Binding energy (eV)

WO3 ~ 35.48 eV(b)

Fig. 3. (Color online) XPS characteristics of GdOx and WOx layers.

(a) XPS spectra of Gd 3d and Gd2O3 3d, and (b) WO3 4f core-level

electrons.

D. Jana et al.Jpn. J. Appl. Phys. 51 (2012) 04DD17

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conduction (SCLC), which consists of an ohmic region(I1V ) and a Child’s law region (I1V2). This suggests thatthe switching mechanism is based on the trapping anddetrapping of charges between the WOx and oxygen-richGdOx layers under positive formation polarity. The chargetrapping and detrapping phenomena in the high-� GdOx

layer is explained below.The charge-trapping properties have been investigated

through C–V measurements in an IrOx/GdOx/SiO2/n-Simetal–insulator–semiconductor (MIS) structure, as shown inFig. 5. After cleaning the p-Si surface, the SiO2 tunnelingoxide with a thickness of 3 nm was deposited. Then, a high-�GdOx charge-trapping layer with a thickness of 20 nm wasdeposited by an E-beam evaporator. The capacitors wereannealed by RTA at 900 �C for 1min in N2 ambient. TheIrOx metal gate was deposited using a shadow mask with anarea (A) of 13:84� 10�14 cm2. Clockwise C–V hysteresis isobserved under a sweeping gate voltage of �10V. Becauseof the electron/hole trapping in the high-� GdOx film,the hysteresis memory window of 2V is observed. Thecharge-trapping density (Nt) in the switching layer can becalculated as30)

Nt ¼ �V � Cox

q � A ; ð1Þ

where Cox (24.8 pF) is the capacitance in the accumulationregion, �V (¼ 2V) is the hysteresis memory window, and q(1:602� 10�19 C) is the electronic charge. Using eq. (1), ahigh charge trapping (or defects) density of 2:34� 1013

cm�2 is obtained, which will control the resistive switchingcharacteristics.

Uniformity of both SET/RESET voltages and HRS/LRSis very important for the realization of memory deviceapplication. Using 20 randomly selected devices, SET/RESET voltages and HRS/LRS have been measured forpositive formation polarity with different device sand 100cycles, as shown in Fig. 6. The device-to-device averagevalues of (standard deviations) SET and RESET voltages are+1.8 (0.079) and �1:81V (0.075), while their cycle-to-cyclevalues are +1.87 (0.042) and �1:84V (0.041), respectively.

Similarly, device-to-device average values of (standarddeviations) HRS and LRS are 197 (12.95) and 8.24 k�(1.64), while their cycle-to-cycle values are 206.19 (16.21)and 8.64 k� (1.56), respectively. Excellent uniformity ofSET/RESET and HRS/LRS have been observed underpositive formation polarity, which is very useful fornanoscale nonvolatile memory applications.

Figure 7(a) shows the program/erase endurance perfor-mance of the memory device. Programming and erasingwere accomplished at +2.8 and �2:6V, respectively, withpulse time of 100 �s. The evolution of the resistances of thetwo well-resolved states up to 104 cycles is also shown. Theresistance values are taken recorded at +0.2V. Although theHRS and LRS show slight fluctuations, the resistance ratio ishigher than 10. Figure 7(b) shows that a good read disturbimmunity of 105 cycles with resistance ratio >10 is alsoachieved. Figures 8(a) and 8(b) shows the retentioncharacteristics of the memory device at room temperature(RT) and 85 �C for negative and positive formationpolarities, respectively. The resistances of LRS and HRSare found to be approximately 2.86 k� and 12.5M� fornegative formation polarity, and 7.2 and 265 k� for positiveformation polarity, respectively. The LRS under positiveformation polarity is higher than that under negative

-2 0 2 4 6

16

18

20

22

24

26C

apac

itan

ce (

pF

)

Sweeping gate voltage (V)

ΔV= 2 V

IrOx/GdO

x/SiO

2/n-Si structure

Fig. 5. (Color online) C–V characteristics of p-Si/SiO2/GdOx/IrOx

memory capacitors.

-2 -1 0 1 2

0

20

40

60

80

100

Cycle-to-CycleDevice-to-Device

Cu

mu

lati

ve p

rob

abili

ty (

%)

Voltage (V)

VRESET VSET

(a)

0 100 200

0

20

40

60

80

100

Cycle-to-Cycle Device-to-Device

Cu

mu

lati

ve p

rob

abili

ty (

%)

Resistance (kΩ)

LRS HRS

HRS/LRS ~ 30

(b)

Fig. 6. (Color online) (a) Cumulative probability of cycle-to-cycle and

device-to-device SET and RESET voltages. (b) Cumulative probability of

cycle-to-cycle and device-to-device HRS and LRS.

D. Jana et al.Jpn. J. Appl. Phys. 51 (2012) 04DD17

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formation polarity owing to the series resistance effect oroxygen-rich GdOx layer at the GdOx/IrOx interface. Boththe LRS and HRS resistances can be retained for 104 s withhigh resistance ratio >103 at RT and 85 �C for negativeformation, and with resistance ratio >10 at RT and 85 �C forpositive formation polarity. These findings indicate that thememory device is nonvolatile and stable at both RT and85 �C.

4. Conclusions

The formation-polarity-dependent improved resistiveswitching memory performance of the IrOx/GdOx/WOx/W structure was investigated. The thicknesses of the GdOx

and WOx layers were found to be approximately 15 and5.5 nm, respectively, from the HRTEM image. The ex-istences of both GdOx and WOx films were confirmed byEDX and XPS analyses. The device shows excellent SET/RESET and LRS/HRS uniformity, and good endurancecharacteristics of 104 cycles under positive formationpolarity. Also data retention is good at RT as well as85 �C with a resistance ratio of >103 for negative formationpolarity and >10 for positive formation polarity. Thismemory device shows great potential for future nanoscalenonvolatile memory device applications.

Acknowledgements

The authors are grateful to National Science Council (NSC)for financial support under contract number: NSC-98-2221-E-182-052-MY3. The authors are also grateful to Electronicand Opto-Electronic Research Laboratories (EOL), Indus-trial Technology Research Institute (ITRI), Hsinchu, Taiwanfor their support of part of the experiments. The authors aregrateful to MSSCORPS Co., Ltd., Hsinchu, Taiwan, for theirTEM support and discussion on our ReRAM device.

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