A Unified Physical Model of Switching Behavior in Oxide-Based RRAM

N. Xu, B. Gao, L.F. Liu*, Bing Sun, X.Y. Liu, R.Q. Han, J.F. Kang

**, and B. Yu

†

Institute of Microelectronics, Peking University, Beijing 100871, P.R. China; *E-mail: lfliu@ime.pku.edu.cn;

**E-mail: kangjf@pku.edu.cn

†NASA Ames Research Center, Moffett Field, CA 94035, USA

Abstract Excellent bipolar resistive switching (RS) behavior was

achieved in TiN/ZnO/Pt resistive random access memory (RRAM) devices. A unified physical model based on electrons hopping transport among oxygen vacancies along the conductive filaments (CFs) is proposed to elucidate the RS behavior in the RRAM devices. In the unified physical model, a new reset mechanism due to the depletion of electrons in oxygen vacancies and the recovery of electron-depleted oxygen vacancies (VO

+)

with non-lattice oxygen ions (O2

) is proposed and identified. Introduction

Resistive switching in transition-metal oxides such as NiOx[1], TiO2[2], WOx[3] has attracted extensive attention due to its potential application in future universal memory technologies. Recently, significantly improved resistive switching characteristics have been achieved by introducing new materials and cell structures [1-3]. However, understanding transport and switching mechanisms is still a great challenge. The formation and rupture of conductive filaments (CFs) in oxide layer has been adopted to elucidate resistive switching behavior by many researches but the switching mechanism is still unclear [4,5]. In this paper, TiN/ZnO/Pt RRAM devices were fabricated and excellent bipolar RS characteristics is demonstrated. A unified physical model is proposed to elucidate the RS behavior in the RRAM devices, where new understanding on the set, reset and switching failure behaviors are discussed.

Experiments ZnO based RRAM devices were fabricated. About 30nm ZnO

films were deposited on Pt/Ti/SiO2/Si substrates by reactive sputtering followed by a 450

oC furnace annealing in O2/N2

mixture ambient for 20min. After top electrodes (TiN, Ti, W) were deposited at room temperature, devices were patterned by the traditional lithography technique to form isolated square-shape memory cells, with the size varied from 10×10m

2

to 200×200m2. Electrical measurements were performed using

Agilent4156C and Agilent4284 at different substrate temperatures to evaluate the switching characteristics of the RRAM devices.

Resistive Switching Characteristics Devices with different top electrodes were fabricated. Only

the devices with TiN top electrode (TiN devices) show high device yield and reproducible RS behavior. The bipolar RS characteristics of the TiN devices are shown in Figs.1-3. The measurement of the Electrical Pulse Induced Resistance Switching (EPIRS) behavior indicated that the 50×50m

2

devices can be set and reset using a 4V/<20ns pulse and a -4V/60ns pulse, respectively. These characteristics illustrate that TiN/ZnO/Pt stack is a promising candidate for emerging high-performance nonvolatile data storage applications.

Fig. 4 shows the cell-area dependence of the resistance values in HRS (RHRS) and in LRS (RLRS). Fig. 5 shows the relation of RLRS to the current-compliance for the last set process (ICOMP). The weak cell-area dependence of RLRS compared to RHRS and the ICOMP-dependent RLRS values match the CF mechanism [5]. A reversible multi-level resistive switching behavior from HRS to LRS (shown in Fig.6 and the insert) was observed in the TiN devices when a 1V durable voltage stress was applied on the TiN-TE. This multi-level resistive switching is very close to a dielectric soft-breakdown phenomenon, suggesting the set process is equivalent to a soft-breakdown under a low durable voltage stress. Fig. 7 shows the relaxation current as a function of stress time under a stress voltage lower than set voltage, which can be fitted well by the Pillai model [6], indicating the polarization effect caused by ions migration in dielectrics occurs.

The conduction mechanism in LRS was investigated based on the temperature dependence of the DC conductance (Fig.8) and the frequency dependence of the AC conductance (Fig.9). The

decreased DC resistance in LRS with increased temperature and the well-fitted frequency response of AC conductance by the Mott’s formula suggest that the conduction transport in LRS is electron hopping through localized oxygen vacancies [7]. The XPS O1s core levels spectra of ZnO film is shown in Fig.10, indicating that non-lattice oxygen ions (O

2) exist in the ZnO

film surface [8]. These O2

can be movable when the electrical field at interface is sufficient high (10

7 V/cm) [9].

Physical Model and Prediction Based on the above observations, a unified physical model is

proposed to explain the conduction in LRS/HRS and the switching between LRS/HRS as shown in Fig. 11. In the unified physical model, 1) the conduction of LRS and HRS is due to electron hopping transport among localized oxygen vacancies (VO

+) in the CFs; 2) the switching between LRS and HRS is due

to the formation and rupture of the CFs; 3) SET process is similar as dielectric soft breakdown which generates and move oxygen vacancies to form CFs, like a percolation effect; 4) RESET is due to the depletion of electrons in some VO

+ along

CFs @VRESET and the recovery of the electron-depleted VO+

with O

2; 5) Switching Failure between LRS/HRS is due to

insufficient non-lattice oxygen ions (O2

) to recover the electron-depleted VO

+ after multi-cycles reset process. The

transport and switching characteristics can be simulated by using the flow as shown in Fig. 12.

Fig. 13 shows the calculated electrons occupation rates in VO+

along CFs under various applied voltage based on the electron hopping transport among VO

+. Electrons depletion in VO

+

(defined as the occupation rate reaches a low level) near the cathode can be observed when the applied voltage reaches a critical value (VRESET). Since the electron-depleted VO

+ (positive

charged) can significantly increase its capture section to O2

(negative charged), the recover probability of VO

+ with O

2

significantly increases. This supports the new proposed reset mechanism.

Fig.14 shows the simulated reset behaviors of the RRAM devices with a single CF and multiple (10) CFs. The sharp reset process is observed in the single CF device. The simulated reset behavior was confirmed by the measured I-V curves as shown in Fig.15. Since the scaled cell size can cause the reduction of CF numbers, the scaled RRAM devices can achieve better reset characteristics.

Fig.16 shows the measured I-V curves of the device when the resistive switching failure occurs. The simulated I-V curves are shown in the insert, assuming the switching failure is due to insufficient non-lattice oxygen ions to recover the electron-depleted oxygen vacancies under a reset voltage. The agreement between measured and simulated I-V curves supports the proposed switching failure mechanism.

Based on the switching failure mechanism, we can deduce that increasing the non-lattice oxygen ions density or the storing ability of non-lattice oxygen ions will improve the switching cycle endurance. This deduction was partly identified by excellent switching characteristics of the TiN devices compared to other devices since TiN is regarded as oxygen reservoir [2]

Conclusion A unified physical model including new understandings on the

reset and resistive switching failure mechanisms is proposed based on the hopping transport among oxygen vacancies along the conductive filaments (CFs). Based on the model it can be deduced that 1) increasing the O

2 density and storing O

2 ability

will benefit to improve the switching cycle endurance; 2) the scaled cell size of RRAM device will help to increase the resistance window and set/reset speed. Acknowledgment: This work is partly supported by 973 Program (2006CB302700) and NSFC (90407023)

978-1-4244-1805-3/08/$25.00 © 2008 IEEE 2008 Symposium on VLSI Technology Digest of Technical Papers 100

Reference [1]K. Tsunoda et al. Tech. Dig. Int. Electron Device Meet. 2007, p.767-770. [2]Masayuki Fujimoto et al. APL 89,223509(2006) [3]ChiaHua Ho et al. IEEE Symp. on VLSI Technol.2007 p228-229 [4]Rainer Waser et al. Nature materials,6. p834-840(2007) [5] D. Lee et al. Tech. Dig. Int. Electron Device

Meet. 2006, p.796 [6]P. Pillai et al. European Polymer Journal,17,p.611 (1981) [7]N.F.Mott Electronic Processes in Non-crystalline Materials, Clarendon Press, Oxford 1979 [8]P.-T. Hsieh et al. Appl.Phys.A 90,317-321(2007) [9]R. Dong et al. APL, 90,182118(2007)

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.2

10-5

10-4

10-3

10-2

HRS

Pt as anode

TiN as anode

Initial

10th

100th

500th

Cu

rren

t (A

)

Voltage (V)

LRS

START

SETRESET

101

102

103

104

10-5

10-4

10-3

10-2

Sa

mp

lin

g c

urr

en

t (A

)

Retention Time (s)

LRS

HRS

T = 300K

Stress 500mV

W=L=50um

-200 -100 0 100 200 3000

10

20

30 RESET

Read Read

Erasing -4V 60ns

Time (ns)

0.0

1.5

3.0

4.5

0

10

20

Read

SET

Response c

urr

ent (m

A)

0.0

1.5

3.0

4.5

Read

Applie

d V

olta

ge (V

)

4V 20nsProgrammingTemp. = 300K

Read @ 500mV

HRS

LRS

101

102

103

104

105

106

107

40000100002500400225

Device Area (um2)

Re

sis

tan

ce

Va

lue

(O

hm

)

100

Fig.1 I-V curves of TiN/ZnO/Pt device

for initial and 10th, 100th and 500th

DC cycles using double voltage

sweeping mode with ICOMP=5mA.

Fig. 2 Memory data retention in HRS

and LRS, the current values were

tested under a high durable stress

(500mV) by using sampling mode

Fig. 3 EPIRS’ respondent current

under applied switching voltage on TE

(TiN). a) 4V/20ns Set pulse, and b)

-4V/60ns Reset pulse.

Fig. 4 Area dependences of the resistance

values in HRS and LRS. The weak area

dependence in LRS supports the

conductive filament (CF) mechanism.

5 10 15 20 25 30

80

120

160

200

Set Current Compliance (mA)

LR

S R

esis

tan

ce

(O

hm

)

TiN as Anode

Pt as Anode

W = L = 50um

T = 300K

Read @ 500mV

0 200 400 600 800 1000 1200

102

103

104

105

-1.5 -1.2 -0.9 -0.6 -0.3 0.0

10-5

10-4

10-3

10-2

2

Cu

rre

nt

(A)

Voltage (V)

Reset operation

after the breakdown

1

LRS

HRS

2nd@1072.2s

1st@481.07s

Sampling @1V stress

Time (s)

Re

sis

tan

ce

Va

lue

(O

hm

)

0 2000 4000 6000 8000 1000022.0

24.0

26.0

28.0

30.0

32.0

34.0

0 2000 4000 6000 8000 1000010

-5

10-4

10-3

10-2

HRS

Sam

plin

g c

urr

en

t (A

)

Retention Time (s)

LRS

0( ) n

aI A t t

Sa

mp

lin

g C

urr

en

t (u

A)

Time (s)

Measured

Simulated

Stress = 500mV

Temp. = 300K

W = L = 50um

101

102

103

104

440410390360330310

HRS

AVG

MIN

Re

sis

tan

ce

Va

lue

(O

hm

)

Temperature (K)

MAX

W=L=50um

Read@500mVLRS

300

Fig. 5 Dependence of LRS

resistance values on the previous

set current compliance, supporting

the CF mechanism [5].

Fig. 6 The multi-level resistive switching from

HRS to LRS @1V stress is similar to a dielectric

soft breakdown (SB). The insert shows the device

can be reset from LRS to HRS after SB

Fig. 7 Relaxation current as a

function of stress time under a

500mV stress voltage applied on

TiN top electrode.

Fig. 8 Temperature dependence of RHRS

and RLRS. The reduced RLRS with

increased temperature supports the

electrons hopping transport.

102

103

104

105

106

0.00

0.04

0.08

0.12

4 2

2 4

5( ) ( )[ ( )] [ln( )]

96

ph

F

eN E kT

AC

Co

nd

ucta

nce

(m

S)

Frenquency (Hz)

Measured data

Fitting curves

W = L = 50 um

Temp. = 300K

AC ampl. = 10mV

526 528 530 532 534 536 538

1016 1018 1020 1022 1024 1026 1028

530.37eV

O 1s Non-lattice

OxygenLattice

Oxygen 531.99eV

Zn 2p

In

ten

sity

(Arb

.)

Binding Energy (eV)

1021.9eV

Fig. 9 Frequency dependence of

the AC conductance in LRS,

which can be fitted by the Mott’s

electrons hopping theory.

Fig. 10 The XPS spectra of Zn 2p

and O 1s core levels in ZnO film.

Non-lattice oxygen ions were

observed in the ZnO film.

Fig. 11 Schematic views of the unified

physical model for the conduction

transport in and the switching processes

between LRS and HRS.

Fig. 12 Simulation flow for the current transport in

LRS/HRS and switching process based on the

unified model, where f, W, R are electron occupation

rate, hopping rate, and hopping distance.

0.0 0.5 1.0 1.5

10-2

10-1

100

101

0.0 0.5 1.0 1.5

10-3

10-2

10-1

100

(b)

9 CFs rupture

the final

CF rupture

10 CFs

HRS

Voltage (V)

Cu

rre

nt

(Arb

. )

Reset

LRS

Single CF

(a)

-1.6 -1.2 -0.8 -0.4 0.010

-8

10-7

10-6

10-5

10-4

10-3

10-2

Cu

rre

nt (A

)

Voltage (V)

1st

2nd

3rd

Sharp Reset

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

10-4

10-3

10-2

10-1

Failure

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

10-2

10-1

100

Curr

ent (A

rb. U

nits)

Voltage (V)

Reset

Failure State

LRS

Cu

rren

t (A

)

Voltage (V)

12 3

5 4Current Compliance

W = L = 50um

T = 300K

Reset

Fig. 13 Calculated electrons

occupation rate in the oxygen

vacancies of CF as a function of depth

and applied voltage. The electron

depletion under a critical voltage near cathode occurs in LRS

Fig. 14 Simulated I-V curves of a) single CF and

b) multiple CFs (10) in the RRAM device. The

slower I-V slope of reset process similar to the

measured curves was simulated compared to

single CF. This means that reduced CFs with the scaled cell size can cause fast reset process

Fig. 15 Measured I-V curves of

Multi-step Reset phenomena. The

last voltage sweeping cycle during

Reset caused an abrupt current

decrease, which is seldom observed in stable Reset process.

Fig. 16. Measured I-V curves when

the switching failure occurs after

continuous cycles. The insert shows

simulated I-V curves by assuming

insufficient O2

to recover the electron-depleted VO

+

Oxygen ions drift

Delete sites

Depleted?

Recover?

978-1-4244-1805-3/08/$25.00 © 2008 IEEE 2008 Symposium on VLSI Technology Digest of Technical Papers 101