Annotated Bibliographies

Developmental mechanism for the resistance change effect in perovskite oxide-based resistive random access memory consisting of Bi2Sr2CaCu2O81d bulk single crystal

A. Hanada,1 K. Kinoshita,1,2,a) K. Matsubara,1 T. Fukuhara,1 and S. Kishida1,2 1Graduate School of Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8552, Japan 2Tottori University Electronic Display Research Center, 2-522-2 Koyama-Kita, Tottori 680-0941, Japan

(Received 2 April 2011; accepted 7 September 2011; published online 24 October 2011)

Resistive random access memory (ReRAM) structures of M/Bi2Sr2CaCu2O8þd (Bi-2212) bulk

single crystal/Pt (M¼Al, Pt) were prepared and their memory characteristics and superconducting properties were evaluated. The resistance change effect developed only in the Al/Bi-2212/Pt

structure and was enhanced with decreasing critical temperature by annealing in Ar atmosphere.

Due to the large resistance anisotropy of bulk Bi-2212 single crystals, the resistance change effect

was confirmed to occur at the interface between the Al electrode and the Bi-2212 single crystal.

These results indicate that introduction of an oxygen-depleted layer to the Bi-2212 single crystal is

required to develop the resistance change effect, which could be achieved by the deposition of

electrodes with low Gibbs free energy and raising the temperature to exceed the activation energy

for oxygen ions to move from Bi-2212 to the electrode. A model is proposed to explain the resistive

switching of perovskite oxide-based ReRAM by generation/recovery of the oxygen-depleted layer.

The resistance change effect developed also in the Pt/Bi-2212/Au structure annealed in hydrogen

gas, in which an oxygen-depleted layer is formed with the assistance of catalytic effect of Pt on the

surface of the Bi-2212 at the Pt/Bi-2212 interface, proving the validity of the model. VC 2011 American Institute of Physics. [doi:10.1063/1.3651465]

I. INTRODUCTION

Resistive random access memory (ReRAM) has a struc-

ture where a transition metal oxide (TMO) is sandwiched

between top and bottom electrodes. This simple structure

enables high integration and has attracted attention as a non-

volatile memory to replace flash memory. The resistance

change effect of perovskite oxide-based ReRAM, in which

perovskite oxides such as Pr1�xCaxMnO3 (PCMO) 1–5

and

SrTiO3 (Refs. 6 and 7) are used as the TMO layer, is thought

to be related to the migration of oxygen ions. However, the

switching mechanism is yet to be clarified.

One factor that hinders elucidation of the mechanism

could be attributed to the use of thin films. The presence of

grain boundaries may affect electrical conduction and the

migration of oxygen ions when polycrystalline thin films are

used. 3,4,6,8

Mismatching with a substrate introduces stress

into the film and complicates elucidation of the mechanism,

even if epitaxially grown TMO films are used. 1,5,7,9,10

Therefore, introduction of a bulk single crystal as the

TMO layer would be effective for elucidation of the switch-

ing mechanism. In addition, the superconducting critical

temperature (Tc) for a single crystal of the high temperature Bi2Sr2CaCu2O8þd (Bi-2212) superconductor is strongly

dependent on the oxygen content. 11,12

Therefore, the depend-

ence of the resistance change effect on the oxygen content

can be obtained by evaluating the relation between the

resistance change effect and Tc. The large crystalline anisot- ropy of Bi-2212 single crystal enables cleavage of the crystal

into thin plates with clean and flat surfaces. 13,14

Above all,

for elucidation of the resistance change effect, it is most

important to specify where the resistance change occurs.

In this paper, the resistance change effect of ReRAM

was investigated using M/Bi-2212 single crystal/Pt (M¼Al, Pt) structures. It was clarified that the resistance change

effect was caused at the interface of Al and Bi-2212 in the

Al/Bi-2212/Pt structure and was enhanced by annealing the

structure. This suggested that the resistance change effect

occurs in the oxygen-depleted layer of the Bi-2212 single

crystal formed at the interface between an electrode with low

Gibbs free energy and a Bi-2212 single crystal.

II. EXPERIMENTAL

Bi-2212 single crystals were grown using the vertical

Bridgman method 15,16

and were cleaved under the ambient

atmosphere to produce thin plates with typical dimensions of

2.0�1.5�0.02 mm3. All the cleaved crystals were annealed in flowing O2 at 500

�C for 20 min to induce uniform oxygen content. Al and Pt electrodes were deposited, respectively,

on both of the surfaces (a-b plane) of the Bi-2212 single crystals by sputtering to produce M-TE/Bi-2212/Pt-BE

(M¼Al, Pt) structures, where TE and BE represent top and bottom electrodes. The size and thickness of both the TE and

BE were 1.0�1.0 mm2 and 100 nm, respectively. Current- voltage (I-V) characteristics were measured using a semicon- ductor parameter analyzer (Agilent 4155 C).

The BE was grounded and a bias voltage was applied to

the TE. The compliance current was set to 60 mA, where the

compliance current is a limiting value to which a currenta)Electronic mail: [email protected].

0021-8979/2011/110(8)/084506/5/$30.00 VC 2011 American Institute of Physics110, 084506-1

JOURNAL OF APPLIED PHYSICS 110, 084506 (2011)

flow through the M-TE/Bi-2212/Pt-BE structure is limited

during the set process [resistive switching from a high

resistance state (HRS) to a low resistance state (LRS)]. The

samples were annealed in 100% Ar atmosphere at 300 and

400 �C for 60 min to control Tc, and the correlation between the resistance change effect and Tc was investigated. In addi- tion, a Ag electrode, used for monitoring interface resistance

between the M-TE and the Bi-2212 crystal and between the

Pt-BE and the Bi-2212 crystal, was formed on the same side

of the Bi-2212 single crystal as the TE. The Pt-TE/Bi-2212/

Au-BE structure was annealed in ArþH2 (Ar:H2¼19:1) atmosphere at 400 �C for 10 min to introduce an oxygen- depleted layer into the surface of the Bi-2212 single crystal

in the vicinity of the Pt-TE by reduction effect of H2 gas

with the assistance of the catalytic effect of Pt. 17–19

III. RESULTS AND DISCUSSION

Figure 1(a) shows the I–V characteristics of the as- prepared Pt-TE/Bi-2212/Pt-BE structure and those annealed

in Ar atmosphere at 300 �C for 60 min and at 400 �C for 60 min. The voltage was ramped up from 0 V to +1.0 V, and

then down to 0 V in steps of 10 mV. The voltage was then

ramped down from 0 V to �1.0 V, and then back to 0 V in steps of 10 mV. No resistance change effects and no signifi-

cant differences were observed in the I–V characteristics, independent of annealing temperature.

Figure 1(b) shows the I–V characteristics of the as- prepared Al-TE/Bi-2212/Pt-BE structure and those annealed

in Ar atmosphere at 300 �C for 60 min and at 400 �C for 60 min. Set and reset switching occurred by application of

positive and negative voltages, respectively, where reset rep-

resents a switching from LRS to HRS. The ratio of RHRS to RLRS (RHRS/RLRS) increased with increasing annealing temperature, where RHRS and RLRS represent the resistances in the HRS and LRS, respectively. RHRS/RLRS of the as- prepared sample was 2, whereas those annealed at 300 and

400 �C were increased to 10 and 20, respectively. In addi- tion, the initial resistances of the as-prepared Pt-TE/Bi-2212/

Pt-BE and Al-TE/Bi-2212/Pt-BE structures were 5.1 X and 1.3 kX, respectively, and those of the Al-TE/Bi-2212/Pt-BE structures annealed at 300 and 400 �C were 2.5 and 12.7 kX, respectively. The resistance of the as-prepared Al-TE/

Bi-2212/Pt-BE structure is higher than that of the as-

prepared Pt-TE/Bi-2212/Pt-BE structure, and the resistance

of the Al-TE/Bi-2212/Pt-BE structure increased with the

annealing temperature. The Gibbs free energies of Pt and Al

at 300 (600) K are 92.852 (9.356) kJ/mol and �1690.973 (�1717.192) kJ/mol,20 respectively; therefore, the increase in the initial resistance was caused by reduction of Bi-2212

due to oxidation of the Al electrode. This is consistent with

the large oxygen diffusion coefficient of Bi-2212

(1.6�10�17 cm2/s), even at 300 K.21 These results suggest that reduction of Bi-2212 is required for the development of

the resistance change effect and that the effect was enhanced

by the extent of reduction.

To specify where the resistance change effect occurs,

two extreme cases shown in Figs. 2(a) and 2(b) are discussed

for TE/TMO/BE structures which have a monitoring elec-

trode (ME) on the same side of the TMO layer as TE. Figure

2(a) shows the TE/TMO/BE structure using the TMO layer

with small resistance anisotropy. When bias voltage is

applied between the TE and ME, current dominantly flows

through the BE. This is due to the fact that a film thickness

of a TMO layer is, in general, much smaller than a TE-ME

distance. Therefore, we can regard Fig. 2(a) as the circuit

given by connecting the ME/TMO/BE and BE/TMO/TE

structures in series. In this case, resistances of the TE/TMO

and TMO/BE interfaces cannot be measured independently

because current flows through the both interfaces. On the

other hand, Fig. 2(b) shows the TE/TMO/BE structure using

the TMO layer with large resistance anisotropy such as a

Bi-2212 bulk single crystal as the TMO layer. When

bias voltage is applied between the TE and ME, a current

FIG. 1. (Color online) I-V characteristics of as-prepared and annealed (a) Pt-TE/Bi-2212/Pt-BE and (b) Al-TE/Bi-2212/Pt-BE structures. The device

structures are shown in the insets.

FIG. 2. (Color online) Schematics to explain current paths in TE/TMO/BE

structures which have ME for the TMO layers with (a) small and (b) large

resistance anisotropies.

084506-2 Hanada et al. J. Appl. Phys. 110, 084506 (2011)

dominantly flows along the surface of the Bi-2212 (a-b plane) between TE and ME due to its large resistance anisot-

ropy (qc/qab > 10 3 ).

13 Therefore, the resistances of the TE/

TMO and TMO/BE interfaces can be obtained directly by

the measurement of resistances between TE-ME and ME-

BE, respectively.

To specify where the resistance change of the Al-TE/Bi-

2212/Pt-BE structure takes place, a sample with a Ag-ME

(D) on the same side of the Bi-2212 single crystal as the TE

was prepared, as shown in Fig. 3(a). Set voltages (þV) and reset voltages (�V) were alternately applied between terminals A and C (A-C). At the same time, resistances

between terminals A and D (A-D) and between terminals C

and D (C-D) were measured. Terminal B was located at a

different position than terminal A on the Al electrode, and

the resistance between terminals A and B (A-B) was also

measured. The results are shown in Fig. 3(b). The resistance

between A-C was alternately switched between low and high

resistance by application of +V and �V, respectively. The resistance between A-D changed in accordance with the

change of resistance between A-C. In contrast, the resistan-

ces between A-B and C-D were invariably independent of

the resistance between A-C. The results indicate that the

resistance change of the Al-TE/Bi-2212/Pt-BE structure

occurs at the Al-TE/Bi-2212 interface.

Figure 4(a) presents the resistivity-temperature (q-T) characteristics of an as-prepared Bi-2212 single crystal and

those annealed in Ar atmosphere at 300 �C for 60 min and 400 �C for 60 min. The q-T measurement was performed using four-terminal method with four Pt electrodes. The inset

shows four Pt electrodes formed side by side on the surface

of the Bi-2212 single crystal, where the pairs of outer and

inner electrodes were used as the current and voltage termi-

nals, respectively. An enlarged view of the q-T characteris- tics around Tc is shown in the inset. No significant change in the q-T characteristics was observed, independent of the annealing temperature. The same measurements were per-

formed for a sample on which an Al electrode was deposited

between the voltage terminals and the results are shown in

Fig. 4(b). The Tc of the as-prepared Bi-2212 single crystal with the Al electrode was 84 K, which was lower than the Tc of 88 K for the sample without the Al electrode. In addition,

the Tc of the Bi-2212 single crystal with the Al electrode was

decreased to 83 and 75 K when annealed in Ar atmosphere

at 300 and 400 �C for 60 min, respectively. It is well known that both the resistivity in the normal conducting state and Tc are strongly dependent on the oxygen content of the Bi-2212

crystal; 11,12

the resistivity increases with decreasing oxygen

content, whereas Tc decreases. An increase in the resistivity and decrease in Tc were observed only in the sample with the Al electrode, which suggests that the Al electrode removes

oxygen from the Bi-2212 single crystal and an oxygen-

depleted layer is formed in the Bi-2212 single crystal in the

vicinity of the Al electrode.

Free energies for the states of AlþBi2Sr2CaCu2O8þd1 (a) and AlOxþBi2Sr2CaCu2O8þd2 (b) are represented in Fig. 5. A shift of the energy state from a to b corresponds to a reduction of the Bi-2212 single crystal due to oxidation of

the Al electrode by oxygen diffusion from the Bi-2212 single

crystal to the Al electrode. The reaction rate, v ! exp(-Ea/ kBT), is dependent on the annealing temperature T, and it is necessary to exceed the activation energy Ea, for the reaction from a to b to proceed. Accordingly, the resistance of the Al-TE/Bi-2212/Pt-BE structure is increased with increasing

T. In contrast, oxygen ions will not move at the interface between the Pt electrode and the Bi-2212 crystal, because Pt

is not easily oxidized. The reason for the higher resistance

measured in the as-prepared Al-TE/Bi-2212/Pt-BE structure

FIG. 3. (Color online) (a) Al-TE/Bi-2212/Pt-BE structure with a Ag-ME

(terminal D) and (b) resistances measured between A-B, A-D, and D-C

when the resistance between A-C was changed.

FIG. 4. (Color online) q-T characteristics of Bi-2212 single crystal meas- ured using the four-terminal method with four Pt electrodes (a) before and

(b) after Al deposition between the voltage terminals. Enlarged views of the

q-T characteristics around Tc are shown in the insets.

084506-3 Hanada et al. J. Appl. Phys. 110, 084506 (2011)

than that in the as-prepared Pt-TE/Bi-2212/Pt-BE structure is

attributed to the sputtering energy during Al deposition. The

radiant heat of the Ar plasma and kinetic energy of sputter-

ing particles raised the temperature of the sample, which

enabled partial reaction from a to b. Shono et al.3 reported that a 10 nm thick TiOx layer was naturally formed at the as-

deposited interface between Ti and PCMO without annealing

of the sample. Therefore, the reaction from a to b is caused by deposition of an electrode with low Gibbs free energy

onto the Bi-2212 and heating it to a temperature correspond-

ing to the activation energy. No resistance change occurred

in Pt-TE/Bi-2212/Pt-BE structure, which suggests that the

introduction of an oxygen-depleted layer into Bi-2212 is

required for the development of the resistance change effect.

Figure 6 shows schematics that indicate how the resist-

ance change effect develops and how the resistive switching

between the LRS and HRS occurs. First, an electrode with

low Gibbs free energy (Al) receives oxygen from the

Bi-2212 single crystal, and an oxygen-depleted layer is

formed in Bi-2212 in the vicinity of the Al-TE, which results

in the HRS [Fig. 6(a)]. Second, by applying positive voltage

to the TE, oxygen ions in the bulk of the single crystal are

drawn to the TE side by coulombic forces with the assistance

of Joule heat. Therefore, the oxygen-depleted layer is partly

recovered and the LRS is attained [Fig. 6(b)]. Application of

negative voltage to the TE results in the movement of oxy-

gen ions into the partly recovered oxygen-depleted layer of

the single crystal bulk and the oxygen-depleted layer is

formed again to give the HRS [Fig. 6(c)]. If the resistance

change occurred due to redox reaction of the Al-TE at the

Al-TE/Bi-2212 interface, the relationship between the resist-

ance change and the bias polarity opposite to that observed

in this study should be observed. 3,4

Therefore, the resistance

change effect is caused by generation/recovery (reduction/

oxidation) of the oxygen-depleted Bi-2212 layer formed at

the interface between Al and Bi-2212. The most direct way

to prove the validity of the proposed resistance change model

is to show the development of the resistance change effect

simply by inserting the oxygen-depleted Bi-2212 layer at the

interface between a high Gibbs free energy electrode such as

Pt and Bi-2212 layer. Utilizing the catalytic effect of Pt that

drastically enhances reduction reaction of hydrogen, 17

we

can introduce the oxygen-depleted Bi-2212 layer into the Pt/

Bi-2212 interface by annealing a Pt-TE/Bi-2212/Au-BE

structure in H2 atmosphere. Since the Au is resistant to

hydrogen and has high Gibbs free energy of �42.447 kJ/mol at 300 K,

20 the Bi-2212 layer is not reduced at the Bi-2212/

Au interface during the H2 annealing. Figure 7 shows the I-V characteristics of the as-prepared Pt-TE/Bi-2212/Au-BE

structure and those annealed in Ar and ArþH2 atmospheres at 400 �C for 60 and 10 min, respectively. Resistance change did not occur in the as-prepared sample and that annealed in

Ar atmosphere. On the other hand, the resistance change

effect was observed in the sample annealed in ArþH2 atmos- phere. Here, set and reset switching occurred by application

of positive and negative voltages, respectively.

To specify where the resistance change of the Pt-TE/Bi-

2212/Au-BE structure takes place, a sample with a Ag-ME

(D) was prepared, as shown in Fig. 8(a). Set voltages (þV) and reset voltages (�V) were alternately applied between A-C. The results are shown in Fig. 8(b). The resistance

between A-C was alternately switched between low and high

resistance by application of of þV and �V respectively. The resistance between A-D changed in accordance with the

change of resistance between A-C. In contrast, the resistan-

ces between A-B and C-D were invariably independent of

the resistance between A-C. The results indicate that the

FIG. 5. (Color online) Free energies for the states of

AlþBi2Sr2CaCu2O8þd1 a and AlOxþBi2Sr2CaCu2O8þd2 b in the interface between Al-TE and Bi-2212 and the activation energy for oxygen ions to

move from Bi-2212 to the electrode.

FIG. 6. (Color online) Schematics to indicate the oxygen movement until

stabilization of the initial state (a) and during the resistive switching to LRS

(b) and to HRS (c).

FIG. 7. (Color online) I-V characteristics of the as-prepared Pt-TE/Bi-2212/ Au-BE structure and those annealed in Ar and ArþH2 atmospheres at 400 �C for 60 and 10 min, respectively. The device structure of Pt/Bi-2212/ Au is shown in the inset.

084506-4 Hanada et al. J. Appl. Phys. 110, 084506 (2011)

resistance change of the Pt-TE/Bi-2212/Au-BE structure

occurs at the Pt-TE/Bi-2212 interface. In addition, resistive

switching did not be observed in a Pt-TE/Bi-2212/Au-BE

structure in which Pt-TE was deposited after ArþH2 anneal- ing. Therefore, it was shown that an oxygen-depleted

Bi-2212 layer was formed in the Bi-2212 single crystal in

the vicinity of the Pt-TE due to the reduction effect of the

ArþH2 annealing18,19 and the catalytic effect of the Pt-TE.

17 These results as well as the bias polarity depend-

ence of the resistance change indicate that the resistance

change is caused by generation/recovery of the oxygen-

depleted Bi-2212 layer as shown in Fig. 6. A similar

resistance change effect was also reported in Ag/PCMO

structures. 2

The resistance change of Ag/PCMO structures

was reported to be caused by destruction/repair of the con-

ductive Mn-O chain caused by a change in the concentration

of oxygen ions near the Ag/PCMO interface, and which was

enhanced in a PCMO film grown under oxygen deficient

conditions compared with that grown under oxygen rich

condition. Accordingly, a reduction of oxide ions near the

structure interface plays a key role in the development of the

resistance change effect by providing space for oxygen ions

to migrate. A similar scenario may also be applicable to

other perovskite oxides, independent of whether the TMO is

a single crystal or poly crystalline. Considering the Gibbs

free energies of the oxides and electrodes, the resistance

change effect of perovskite oxide-based ReRAM can be con-

trolled by application of an appropriate annealing tempera-

ture that exceeds the activation energy for oxygen diffusion

from the TMO to the electrode.

IV. CONCLUSION

Perovskite oxide-based ReRAM was prepared using

Bi2Sr2CaCu2O8þd bulk single crystal for the TMO layer. The

resistance change effect was observed in the Al-TE/Bi-2212/

Pt-BE structure. An introduction of the Bi-2212 bulk single

crystal enabled clarification that the resistance change effect

occurs at the Al/Bi-2212 interface. An increase in the

resistivity and decrease in the Tc with increased annealing temperature were confirmed, and the resistance change effect

(RHRS/RLRS) was enhanced by increased annealing temperature.

These results indicate that the introduction of an oxygen-

depleted layer into the Bi-2212 single crystal is required for

the development of the resistance change effect, which can

be achieved by deposition of a low Gibbs free energy elec-

trode in order to form an oxygen-depleted layer at the TMO

surface. The resistance change effect developed even in the

Pt-TE/Bi-2212/Au-BE structure, which has high Gibbs free

energy electrodes, by inserting the oxygen-depleted layer

into the surface of the Bi-2212 single crystal at the vicinity

of the Pt-TE. This excludes the possibility that resistance

change occurs due to the oxidation/reduction of the Al-TE.

The resistance change of ReRAM is caused by the migration

of oxygen ions under application of an intense electric field,

and it is thought that the set/reset processes are caused by re-

covery/generation of the oxygen-depleted layer. This resist-

ance change model provides a guideline for the selection of

oxide and electrode materials for perovskite oxide-based

ReRAM.

ACKNOWLEDGMENTS

This study was supported by Grant-in-Aid for Young

Scientists B (No. 23760313).

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FIG. 8. (Color online) (a) Pt-TE/Bi-2212/Au-BE structure with a Ag-ME

(terminal D) and (b) resistances measured between A-B, A-D, and D-C

when the resistance between A-C was changed.

084506-5 Hanada et al. J. Appl. Phys. 110, 084506 (2011)

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