Effect of firing atmosphere and bottom electrode on resistive switching mode in TiO2 thin films

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Effect of

ﬁring atmosphere and bottom electrode on resistive switching mode in TiO

2

thin

ﬁlms

Chun-Hung Lai

a,

⁎

, Chih-Yi Liu

b

, Cheng-Hsing Hsu

c

, Yi-Mu Lee

a

, Jenn-Sen Lin

d

, Hsiwen Yang

e a

Department of Electronic Engineering, National United University, Miaoli 360, Taiwan

b_{Department and Institute of Electronic Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan} c

Department of Electrical Engineering, National United University, Miaoli 360, Taiwan d

Department of Mechanical Engineering, National United University, Miaoli 360, Taiwan e

Department of Materials Science and Engineering, National United University, Miaoli 360, Taiwan

a b s t r a c t

a r t i c l e i n f o

Available online 17 September 2012 Keywords:

TiO2

Resistive switching Sol–gel

Space-charge-limited

Sol–gel deposited Ag/TiO2/Pt tri-layers exhibit unipolar resistive switching (URS) with a negative turn-on voltage whenfired in air. Variation in both the turn-on voltage and the on-state current at turn-off threshold is observed in successive current–voltage (I–V) measurements. Based on the same composition and processing flow, bipolar resistive switching (BRS) showing positive turn-on voltage by contrast is obtained by applying two alternative procedures: one procedure under oxygen ambience and another one in which indium–tin oxide is used as the bottom electrode. Conduction mechanisms induced by reduction/oxidation reactions explain howfiring ambient and bottom electrode affect the switching mode, as well as the distinct turn-on voltage polarity. URS and BRS are related to the amount of ambient oxygen and the arrangement of internal oxygen vacancy, thus determining whether the direction-independent thermochemical heating or polarity-dependent electrochemical oxidation near the anode interface is responsible for thefilament rupture. Additionally, the space-charge-limited transport is analyzed to examine how URS and BRS-activated samples significantly differ from each other. Importantly, the temperature-dependent I–V data helps to elucidate the dominant carrier behavior in the regime of low and high electricfield.

1. Introduction

Owing to its simple structure and fast switching speed, resistive random access memory (RRAM) has been investigated widely for its potential nonvolatile storage application in mobile devices [1,2]. Resistance switching (RS) phenomena have been observed in various perovskite and transition metal oxides[3_–5]. Two conduction states can be obtained by applying an appropriate programming voltage sweep or pulse. The set operations switch a memory cell in its high-resistance state (off) to low-resistance state (on), while the reset operations restore the cell to off state. Notably VSETand VRESETdenote

switching voltages for the turn-on and turn-off processes, respectively, and ISET/IRESETfor the corresponding currents. Additionally, unipolar

resistance switching (URS) represents RS achieved by VSETand VRESET

of the same polarity, whereas bipolar resistance switching (BRS) displays polarity dependence[6,7]. URS and BRS differ in resistance ratio, magnitude and stability in the switching voltage and, especially, reset current[8]. This large reset current inhibits the integration of RRAM devices, which are frequently observed in URS. BRS has a more

stable current–voltage hysteresis and does not require the current compliance settings during turn-on transition[8].

Theﬁlament model stipulates that the formation and rupturing of conductive paths in an insulating matrix are responsible for the RS effect. The driving mechanism of URS differs from that of BRS in the set/reset process by thermal reduction/oxidation (redox) or that by the electrochemical migration of oxygen ions, respectively[9]. For BRS, bias polarity indicates whether the charged particles are attracted or repelled at electrode interface, while the reset process induced by Joule heating appears to be directionless for URS. The current compliance setting in URS is always higher than that in BRS

[8–11]. The on state is a low-resistance state accompanied by the reduction reaction at VSET. VSETpolarity denotes a higher potential

at the top or bottom side. Whether the defect has a positive valence into (or negative valence out of) the thinﬁlm, at the top or bottom side, depends on the VSET polarity. Oxygen-related defects, such as

oxygen ions or vacancies, generally dominate the carrier transport in metal oxides. VSETpolarity also helps to determine which one is the

plausible one. Carrier transport arising from cation or anion migration leads to conduction of different activation energies [12,13]. This feature illustrates how electrode materials affect the RS mode in addi-tion to the high or low Schottky barrier by work funcaddi-tion difference

[11]. Inﬂuenced by composition, fabrication, and electrode property,

Thin Solid Films 529 (2013) 430–434

⁎ Corresponding author.

E-mail address:[email protected](C.-H. Lai).

http://dx.doi.org/10.1016/j.tsf.2012.09.025

Contents lists available atSciVerse ScienceDirect

Thin Solid Films

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