DSpace at VNU: Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films

Correlation between crystallinity and resistive switching behavior ofBach Thang Phana,d,* a Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh

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Correlation between crystallinity and resistive switching behavior of

Bach Thang Phana,d,*

a Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam

b Department of Electrical Engineering, National Chi-Nan University, Nan-Tou, Taiwan, ROC

c Department of Materials Science and Engineering, Inha University, Republic of Korea

d Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam

a r t i c l e i n f o

Article history:

Received 9 June 2014

Received in revised form

13 August 2014

Accepted 10 October 2014

Available online 18 October 2014

Keywords:

Resistive random access memory (ReRAM)

WO 3 thin ﬁlms

Electrochemical redox

Crystallinity

Annealing

a b s t r a c t

The as-deposited WO3thinﬁlms were post-annealed at different temperatures (300C and 600C) in air

to investigate a correlation between crystallinity and switching behavior of WO3thinfilms Associating the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change contributes to the variation of the switching behaviors of the WO3thinfilms The as-deposited WO3films with low crystalline structure are preferred for random Ag conducting path, resulting in large switching ratio butfluctuating IeV hysteresis, whereas the annealed WO3films with crystallized compact structure limits Ag conducting path, favoring the stable IeV hysteresis but small switching ratio It is therefore concluded that electrochemical redox reaction-controlled resistance switching depends not only on electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide

1 Introduction

Recent research has demonstrated that resistive random access

memory (ReRAM) is promising candidate for future non-volatile

memories Oxide-based ReRAM structures exploit the

function-ality of capacitor structures where the oxide materials, such as

ternary oxides (Cr-doped SrTiO3, Cr-doped SrZrO3, Pr0.7Ca0.3MnO3,

etc.)[1e6], binary oxides (NiO, TiO2, CuOx, HfO2, ZrOx, ZnO, Nb2O5,

Al2O3, WOx)[7e12]are sandwiched between two metal electrodes

Even though these materials show promising properties, the

involved switching mechanisms are still content of current

research activities The study of theﬁlm structure-order is

impor-tant in obtaining a clear understanding of its revealed switching

properties For example, TiO2 has various crystalline phases and

also various resistive switching characteristics have been observed

in the amorphous, anatase, and rutile structures[13e18] Lee et al.,

also observed that the epitaxial binary oxide NiO shows bipolar

switching while the polycrystalline NiO shows unipolar switching

[19] Improved crystallinity with increasing ZnO layer thickness

reduced the number of extended defects, which then reduced the number of available sites for conduction path formation despite the increased density of oxygen-related defects contributing to the path formation, resulting in an increased set voltage in the high resistive state [20] Shang et al., reported that bipolar resistive

annealing, which is attributed to the decrease in the surface density states[21] Syu et al., shows that the resistance switching behavior

of WOx- RRAM devices is unstable because the diverse oxidation state provided the stochastic conduction paths By introducing a silicon element, Si interfusion in WOxresistance switching layer can effectively localize theﬁlament conduction paths to improve the resistance switching property [22] Jang et al., tuned the switching characteristics by changing the additional oxygen con-tent (d) of the WO3 þdoxide As the value ofdvaries, the switching becomes to be unstable[23] It is therefore suggested that in pre-paring oxide-based ReRAM devices, especially in device scaling, careful control of crystallinity would be important It is known that

prepara-tion condiprepara-tions (monoclinic, triclinic, orthorhombic, hexagonal and tetragonal) [24] The high diversity of physical parameters (e.g., crystal structure and density) and chemical parameters (e.g., valence state of W ions and composition) makes the research more

* Corresponding author Faculty of Materials Science, University of Science,

Vietnam National University, Ho Chi Minh City, Vietnam.

E-mail address: pbthang@hcmus.edu.vn (B.T Phan).

Contents lists available atScienceDirect Current Applied Physics

j o u r n a l h o me p a g e : w w w e l s e v i e r c o m/ l o ca t e / c a p

http://dx.doi.org/10.1016/j.cap.2014.10.009

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complex and more interesting for the variation of switching

properties From this point of view, in this study, we reported a

correlation between crystallinity and switching behavior of

sput-tered WO3thinﬁlms

2 Experimental

technique, from metallic W targets on Pt/Ti/SiO2/Si substrates The

deposition process of 300-nm-thick WO3thinﬁlms was executed

under the total pressure PArþO2of 7 103Torr at 300C, and the

mixture ratio of oxygen to argon gas, PO2/PAr þO2,wasﬁxed at 90%

Before the top electrode deposition, the as-deposited WO3 ﬁlms

were annealed at various temperatures (300C and 600C) in air

for 5 h During the deposition of the 100-nm-thick top electrode

(Ag) in an argon environment at 7 103Torr, a mask was used for

top electrode patterning The crystalline phases of the thinﬁlms

were characterized inq2qmode by D8 Advance (Bruker) X-ray

diffractometer (XRD) with Cu Ka radiation (l ¼ 0.154 nm) and

Fourier transform infrared spectroscopy (FTIR) The surface

mor-phologies of theﬁlms were obtained using scanning electron

mi-croscopy (FESEM) X-ray photoelectron spectroscopy (XPS) was

used to investigate the chemical state of theﬁlms Deconvolution of

the XPS spectra included Shirley baseline subtraction was carried

out using the least squares curveﬁtting program The proﬁle of the

peaks was taken as a Gaussian function Currentevoltage (IeV)

measurements were carried out using a semiconductor

character-ization system (Keithley 4200 SCS) and probe station The IeV

Vmax/ 0 V / þ Vmax/ 0 V voltage proﬁle, sweep speed is normal

mode and step voltage is 0.02 V The read voltage for endurance test

is 0.5 V The Pt bottom electrode was biased and the Ag top

elec-trode was grounded

3 Results and discussion

diffraction peaks at 2q¼ 22.87, 23.74, 24.4, 26.8, 29.4, 34.12,

and 49.99 can be clearly identiﬁed The as-deposited WO3 thin

ﬁlms have two visible peaks, the board peak at 2q¼ 22.87and the

other peak at 2q¼ 29.4 The 300Ce annealed WO3thinﬁlms

have four peaks at 2q¼ 23.74, 24.4, 29.4 and 34.12 Among

those 4 peaks, the intense peak locates at 2q¼ 24.4, while the

intensity of peak at 2q¼ 29.4decreases There are six diffraction

peaks, 2q¼ 23.74, 24.4, 26.8, 29.4, 34.12, 49.99, observed from

the 600Ce annealed WO3thinﬁlms with two intense peaks at

2q¼ 24.4, 34.12 In order to classify the crystal type of those WO3

thinﬁlms from the above diffraction peaks, we investigated the card number LCPDS of Triclinic phase, Monoclinic, Orthohombic phase, Hexagonal phase, Tetragonal phase Based on the card number LCPDS, those mentioned diffraction peaks are character-istic peaks of (002), (020), (200), (120), (112), (220) and (400) planes of Monoclinic phases The board and low intensity of (002) peak around 2q¼ 22.87, indicating that the as-deposited WO3thin

ﬁlm is low crystallinity In contrast, the XRD patterns of the annealed WO3thinﬁlms reveal the sharp and intense (200) peak at 24.4, implying an improvement of the crystalline by the annealing treatment It is therefore noted that annealing the as-deposited

WO3thinﬁlms enhance signiﬁcantly crystallinity

The FTIR spectra of both the as-deposited WO3and the annealed

WO3thinﬁlms are shown inFig 2 Tungsten oxideﬁlm comprises

621 cm1, 669 cm1, 730 cm1, 954 cm1 and 1109 cm1 The annealed WO3ﬁlms at 300C have 4 bands at 621 cm1, 730 cm1,

954 cm1and 1109 cm-1 When the WO3ﬁlms were annealed up to

600 C, 6 bands are found at 621 cm1, 730 cm1, 804 cm1,

866 cm1, 954 cm1and 1109 cm1 The number of bands shows that the post-annealing treatment strongly affected the structure of

WO3thinﬁlm

The bands at 621 cm1and 730 cm1exist in all investigated

in-tensity, the band at 621 cm1is dominant in the as-deposited WO3 thinfilms, whereas the band at 730 cm1become well-defined and comparative intensity in the 600C -annealed WO3thinfilms All

assigned to the W6þ¼ O stretching mode of terminal oxygen atoms possibly on the surfaces of the cluster or micro-void structures in theﬁlms [25] The visible board band centered at 669 cm1is ascribed to the low crystallinity material [26] With increasing

peak at 730 cm1belongs to the stretching vibration of crystalline

WO3[26] The results indicate that the as-deposited WO3thinﬁlms

Fig 1 XRD of (a) as-deposited WO 3 ﬁlm, (b) 300 C e annealed WO 3 ﬁlm, and (c)

e annealed WO ﬁlm.

Fig 2 FTIR spectroscopy of (a) as-deposited WO 3 ﬁlm, (b) 300 C e annealed WO 3

ﬁlm, and (c) 600 e annealed WO ﬁlm.

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are partially crystallized This result is consistent to the XRD data

with the board (002) peak and the sharp (112) peak With the

730 cm1and 804 cm1are assigned as WeOeW stretching modes

in WO6octahedral units and WO4tetrahedral units, characterizing

the monoclinic phase[25,27e29] The shorter WeOeW bonds are

responsible for the stretching mode at 804 cm1, whereas the

longer bonds are the source of the 730 cm1peak The peak at

866 cm1may be ascribed to WO3.nH2O[28] In summary, as the

post-annealing temperature increases, the crystallinity of theﬁlm

tends to improve

Fig 3shows superimposed O 1s photoelectrons spectra of both

the as-deposited WO3and the annealed WO3thinﬁlms The core

level spectra of O 1s can be deconvoluted into two peaks

corre-sponding to lattice oxygen/stoichiometric WO3phase (LO, ~ 530 eV)

~ 531 eV)[30e32]

Fig 4shows the XPS of the W 4f core level spectrum of both the

as-deposited WO3and the annealed WO3thinﬁlms The W 4f7/2

and W 4f5/2peaks of the W6þion were assigned to the peaks at

around 36 eV and 37.9 eV These peaks coincide with

ﬁlms [30e36] In addition, the W 4f spectrum of all thin ﬁlms

present the clear shoulder at around 34.5 eV, assigned to W5þ

Among those thinﬁlms, only the 600Ce annealed WO3thinﬁlms

have an additional shoulder at lower binding energy (~33.1 eV),

which is assigned to W4þ The lower valence states of W ions (W5þ

and W4þ) indicate the presence of reduced WO3 xphase Since

oxygen vacancies exists in theﬁlms, the electronic near its adjacent

W atoms increases, creating a larger screening, which lowers the 4f

level binding energy The two peaks located at higher binding

en-ergies (~39.7 eV and 41.3 eV) are assigned to W5þand W6þof W5p3/

2 Since the as-deposited WO3 ﬁlms were annealed in air, the

annealing process just affected the crystallinity, not stoichiometry

ofﬁlms Therefore, oxygen vacancies exist in all the as-deposited

WO3and the annealed WO3ﬁlms

Fig 3 XPS spectrum of the O 1s core level of (a) as-deposited WO 3 ﬁlm, (b) 300 C e

ﬁlm, and (c) 600 e annealed WO ﬁlm.

Fig 4 XPS spectrum of the W4f core level of (a) as-deposited WO 3 film, (b) 300 C e annealed WO 3 film, and (c) 600 C e annealed WO 3 film.

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The surface morphology of the WO3thinﬁlms was examined by

FESEM, as shown inFig 5 The surface structure of the as-deposited

WO3thinﬁlm is porous with unclear irregularly grains, whereas

morphology with clear grains Meanwhile, the morphology and the

porosity of as-depositedﬁlms are greatly affected by the annealing

temperature: the higher the annealing temperature is, the more

visible the grain boundaries are and the compacter the structure is

According to the XRD, FTIR, XPS and FESEM analyses, the

crys-tallinity was improved as well as the grain become visibly as

annealing temperature increasing

Fig 6shows the IeV characteristics of the as-deposited Ag/WO3/

Pt device and the annealed Ag/WO3/Pt devices All devices showed

the bipolar resistance switching It is worthwhile to point out that

no forming process is necessary for activating the switching effect

Based on the IeV hysteresis, the initial high-resistance state (HRS)

was changed to a low-resistance state (LRS) as a negative bias (0/

e 1.5 V) applied to the Pt bottom electrode The device remained in the LRS for subsequently descending, and the LRS was progressively changed to the HRS only by a voltage sweep in the positive voltage region (0/ þ 2 V) Among the investigated WO3thinﬁlms, only

IeV curves of 600 Ce annealed WO3thinﬁlms superimpose to

Fig 5 FESEM images of (a) as-deposited WO 3 ﬁlm, (b) 300 C e annealed WO 3 ﬁlm,

Fig 6 IeV characteristics of (a) as-deposited WO 3 ﬁlm, (b) 300 C e annealed WO 3

ﬁlm, and (c) 600 C e annealed WO 3 ﬁlm.

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each other, which seems to be inﬂuenced by its high crystalline

structure

An endurance test has been carried out at reading voltage of

0.5 V, as shown in Fig 7 All devices present a clear reversible

switching for over 100 cycles The value of HRS of WO3ﬁlms

de-creases as increasing the annealing temperature The lower

resis-tance at higher annealing temperature is the result of the

improvement of crystallinity It is noted that the annealing

treat-ment strongly affected a switching ratio As shown inFig 8, the

switching ratio of the as-deposited thinﬁlm is about 70, but the

switching ratio down to 30 and 6 for the thinﬁlms annealed at

300C and 600 C, respectively Because the switching ratio is

smaller than 10, the post-annealed thinﬁlms at 600C cannot be

applied for ReRAM, although this structure show the stable

shows higher crystallinity but its switching ratio is the lowest,

which may be ascribed to the crystallized compact structure of the

thinﬁlm

Associating the results of XRD, FTIR, XPS and FESEM

measure-ments, obviously, the annealing-caused microstructure change

contributes to the variation of the switching behaviors of the WO3

thinﬁlms

Since the IeV curve of the LRS in log e log scale shows a linear

relationship between current and voltage (not shown here), in

addition to the nature of electrode, a reactive Ag electrode and an

inert Pt electrode along with the switching direction, it is suggested

that switching mechanism in both the as-deposited and annealed

WO3thinﬁlms is controlled by the electrochemical redox reactions

[37,38], which is explained as follows

By applying a negative voltage at the Pt bottom electrode (a

positive voltage at the Ag top electrode), an electrochemical

reac-tion occurs in the anode (Ag), which oxidizes the Ag metal atoms to

Ag ions, the metal ions Agþstart from the top interface and easily

drift through the as-deposited low crystalline WO3ﬁlms to connect

the bottom electrode At the Pt cathode, an electrochemical

reduction and an crystallization of Ag occur This

electro-crystallization process results in the formation of an Agﬁlament,

which grow towards the Ag electrode As a result, the Agﬁlaments

grow and connect the Ag top electrode, leading to HRS to LRS

switching To reset the cell, a positive voltage is applied at the Pt

bottom electrode (a negative switching voltage at the Ag top

electrode), which leads to a dissolution of the Agﬁlament and LRS

300C and 600C Therefore, as numerous randomly Ag metallic path forms, resulting in ﬂuctuating IeV hysteresis In the

extensive internal volume to conduct ions, the number of Ag con-ducting path is limited, resulting in stable IeV hysteresis and lower switching ratio In comparison, Syu et al., shows that the resistance switching behavior of WOxe RRAM devices is unstable because the diverse oxidation state of W ions (W6þ, W5þ, and W4þ) provided the stochastic W conduction paths[22] In their study, the WO3thin ﬁlms were sputtered at room temperature and the authors do not reported the crystalline structure of the WOxthinﬁlms In general,

resulting in many voids for providing the stochastic conduction paths Our as-deposited WO3thinﬁlms have low crystallinity with only two valence states of W ions (W6þ, W5þ) show theﬂuctuating switching, which is consistent to Syu's report[22] However, our

600 C e annealed WO3 thin ﬁlms have high crystallinity with many valence states of W ions (W6þ, W5þ, and W4þ) show the stable switching behavior (Figs 7 and 8) or the stochastic Ag con-duction paths are limited It is suggested that the stochastic Ag conduction paths are also controlled by the crystalline structure

4 Conclusions

different temperatures (300 and 600C) in air to investigate the effects of crystallinity on switching behaviors of thefilms The as-deposited WO3films are monoclinic phase with low crystallinity Annealing thefilms up to 600C improve the crystallization The

resistance switching mechanism is the Agﬁlament paths mediated

by electrochemical redox reactions, in which the Ag conducting formation is inﬂuenced by the crystalline structure The electro-chemical redox reaction depends on crystalline structure of WO3

thinﬁlms The as-deposited WO3with low crystalline structure are preferred for large switching ratio butﬂuctuating IeV hysteresis,

favor the stable IeV hysteresis but small switching ratio

Acknowledgment

Uni-versity in HoChiMinh City under Grant B2013-18-02

Fig 7 Endurance of (a) as-deposited WO 3 ﬁlm, (b) 300 C e annealed WO 3 ﬁlm, and

Fig 8 Switching ratio of as-deposited WO 3 ﬁlms and annealed WO 3 ﬁlms (300 C and

600C).

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