Ramanathan Sh. (Ed.) Thin Film Metal-Oxides: Fundamentals and Applications in Electronics and Energy

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4 Bipolar Resistive Switching in Oxides for Memory Applications 159

Fig. 4.23 I–V plot of a 12-nm thick SiO

ﬁlm diffused with Cu. The SET process is observed

under positive bias at about C0:9 V and the RESET at 0:15 V. The voltage sweep started at

0:75 VtoC1 V and then back to 0:75 V. A current compliance limits the current to 5A.

4.5.2 Metallic Filament Formation with Cu- and Ag-Electrode

The use of Cu or Ag as an electrode material on oxide ﬁlms can trigger that a

completely different switching mode comes into play. Figure 4.23 gives the I–V

characteristics of several bipolar switching loops of a 12 nm thick Cu-doped SiO

memory element [69].

The SiO

ﬁlm was deposited by electron beam evaporation on a W bottom elec-

trode. The Cu top electrode was 35–45 nm thick. Thermal diffusion at 610

Cwas

applied to drive some of the Cu into the SiO

. The I–V plot clearly indicates a bipo-

lar switching behavior in conjunction with a metal electrode and an oxide; however,

the results need to be understood within the framework of a completely different

switching mode. The switching can be explained by an electrochemical metalliza-

tion effect. Now the cations are the electrochemically active and migrating species

and the oxide plays the role as an solid electrolyte. Cu ions are electrochemically re-

duced at the cathode and a thin metallic ﬁlament grows between the top and bottom

electrode. Thus, in this case the switching is due to the migration and electrochem-

ical redox reaction of cations in a solid electrolyte [3]. The same effect is observed

for Cu-doped WO

[70].

4.5.3 Bipolar and Unipolar Switching in One Sample

An even more interesting effect was observed in the switching behavior of amor-

phous TiO

ﬁlms deposited by atomic layer deposition (ALD) [71]. With Cu

electrodes uni-polar as well as bipolar switching was observed, Fig. 4.24.

160 R. Bruchhaus and R. Waser

Fig. 4.24 Fifty

nanometer-thick amorphous

TiO

ﬁlm with Cu top

electrode. (a) unipolar

switching, current

compliance 20 mA. (b)

Bipolar switching, current

compliance 20 A[71]

Simply the adjustment of the current compliance during the quasi static I–V

sweep the switching effect could be directed either to a unipolar or bipolar mode.

A high current compliance of 20 mA favored the unipolar switching which is char-

acterized by Joule heating effects [5]. During the voltage induced partial breakdown

a conductive ﬁlament is formed, which may consist of suboxides [72]. Other pro-

posals for the composition of the ﬁlament include the transport of electrode metal

into the ﬁlm or carbon from residual organics [11]. During the reset operation the

ﬁlament is disrupted thermally by the huge amount of heat generated locally at high

current density. We name this type of unipolar switching Thermo-Chemical Mem-

ory (TCM) effect. The ﬁlamentary nature of the switching in, for example, TiO

has

been conﬁrmed by Choi [55].

Using the low current compliance a bipolar I–V characteristic is observed and it

is proposed that, again, a completely different switching mode comes into play. First

of all, the onset for the SET process is shifted to about 0.5 V and the RESET process

occurs at 150 mV. The asymmetry and the low voltage for the RESET process in

conjunction with the low overall currents (3 orders of magnitude smaller than in the

unipolar case) are indicators for formation and rupture of metallic ﬁlaments based

4 Bipolar Resistive Switching in Oxides for Memory Applications 161

on cation migration and electrochemical redox reactions as described for Cu=SiO

(like in Fig. 4.23), Cu=WO

and the CBRAM (ECM) based on the GeS(e) solid

electrolytes [3,70].

4.6 Device Performance

The bistable resistance characteristics with resistive switching induced by voltage

pulses is a promising approach for future nonvolatile memory technologies. Due

to the localization of the switching effect either in the vicinity of interfaces with

a distance of several nanometers or the formation of ﬁlaments with a diameter of

less than 20 nm [4] an excellent scaling behavior of the memory element could

be demonstrated. In addition, future memory technologies are expected to be low

power and offer fast random read/write access. The memory cell consists of the

memory element and a select device like a transistor or a diode. In a high-density

memory chip the memory cells are arranged in large arrays of crossing wordlines

and bitlines, which are connected to the driver and sense ampliﬁer circuits [73]. For

the storage cells based on resistance switching also simple cross-point structures in

which the memory element is arranged at the cross points of the wordlines and bit-

lines have been demonstrated [74,75]. The advantage of the cross-point structure is

the small footprint of 4F2/cell (F is the feature size of the technology node) and that

the layers can be stacked and thus even increase the storage density. A select device

can efﬁciently decouple the memory element and reduce cross talk and disturb sig-

nals from neighboring memory cells, Fig. 4.25. Memory cells with select transistor

have a larger footprint of 6–8F2 [73].

Transistor T and the memory element R form a serial connection between PL

and BL. Select transistors can supply only a certain amount of drive current. For an

180 nm technology 100 A for a transistor with a width/length ratio of 180/180nm

is a typical value [76]. Thus, set as well as reset currents need to be smaller than the

maximum drive current of the array transistor to achieve reliable operation. Drive

currents can be increased by using array transistors with larger width; however,

Fig. 4.25 1Resistor-

1Transistor (1R1T)-memory

cell of the memory element R

and the select transistor T

connected to Bitline BL,

wordline WL, and the

plateline PL

162 R. Bruchhaus and R. Waser

the area penalty is severe for large arrays. In addition, as the array transistor T

and the memory element R form a voltage divider the resistance ratio is impor-

tant. The channel resistance for a transistor in an 180 nm technology is about 1 k

[76]. Again, for reliable operation the LRS of the memory element must be higher

than the transistor resistance. The supply voltage for future technology nodes is ex-

pected to approach 1 V meaning that low voltage switching is required to integrate

the resistive memory elements into a standard CMOS process [77].

A switching voltage as low as 0.5 V was reported for epitaxial Cr-doped SrZrO

ﬁlms [13]. For TiO

in a cross-point array prepared by nanoimprint lithography

switching at 1.5 V was reported for single devices [74]. ˙3 V is the switching volt-

age reported for a La

0:7

0:3

MnO

ﬁlm [22]. Fast switching times as low as 10 ns

have been reported for PCMO ﬁlms using 3.2 V pulses [78]. Very thin TiO

layers

could be switched to the LRS with a 20 ns pulses with 2 V and back to the HRS with

30 ns pulses with an amplitude of 2.2 V [58]. An endurance of more than 10

switch-

ing cycles and a remarkable retention stability of >1;000hat150

C were reported

for the memory elements based on TaO

ﬁlms. The fully integrated 1T1R memory

cells fabricated in an 0:18 m CMOS process could be switched with <170Afor

the set as well as for the reset process [63].

For the conductive metal oxide with tunnel barrier system described in the section

of the localized bipolar switching typical operation voltages are ˙3 V, the pulse

duration for switching is typically 1–10s and an endurance of 10

is reported [20].

O was integrated into the standard CMOS architecture by thermal oxidation

of a 0:18m via after the CMP process [64, 65]. Figure 4.26 gives the schematic of

the memory cell, a schematic cross-section of the cell array and a cross-sectional

TEM of the memory element. The Cu

O ﬁlm is only 12 nm thick and the top elec-

trode (TE) is formed by Ti/TiN. The Cu bottom electrode is connected to the select

transistor. Detailed reliability studies are needed to assess this approach. The ther-

mal stability of Cu

O which is reported to convert into CuO and Cu at 300

Cmay

limit the application of this approach [79].

Fig. 4.26 (a) 1T1R memory cell, (b) cross-sectional representation of the arrangement of the

memory cells in the array, (c) TEM of the memory element with the thin Cu

O layer on top of the

Cu via [64]

4 Bipolar Resistive Switching in Oxides for Memory Applications 163

These ﬁrst results look very promising. However, currently only very limited

reliability data is available in the literature. Work related to the reliability of the

memory elements has just started and need to be studied in much more detail. The

underlying microscopic mechanisms which may limit the reliable operation need to

be identiﬁed and understood. This work needs to be done on fully integrated test

chips prepared in a state-of-the-art technology node. Only with these test chips suf-

ﬁcient statistical data can be received and the interaction with the CMOS BEOL

process tested and understood. Many oxides under discussion for the use in RRAM

are sensitive against hydrogen or, more general, reducing conditions. On the other

hand, hydrogen is present in many processes during the BEOL. Very efﬁcient en-

capsulation technologies are needed to preserve the sophisticated defect gradients

used in some of the previously described material systems. In terms of data reten-

tion, typically 10 years at 85

C is speciﬁed for a nonvolatile memory. In terms of

endurance, the number of applications increases with the number of switching cy-

cles. Ultimately, a DRAM-like endurance performance with 10

switching cycles

is the target.

4.7 Summary, Conclusions, and Outlook

Numerous emerging memory technologies have been pursued in the last decade

and memory chips of the most advanced technologies like FeRAM and MRAM

are available on the market in millions of parts. Now, these parts have proven re-

liable operation over many years and are well-established in niche markets. The

next emerging memory technology to master the breakthrough from development

laboratories to mass production and market is expected to be phase change RAM

(PCRAM) in the near future. Compared to these emerging memory technologies

RRAM based on valency change effects in transition metal oxides described in this

chapter is immature.

A key question for the further evolution is the scaling potential compared to the

standard mass memory devices like DRAM and ﬂash and the more “established”

emerging memory technologies and the inherent scaling limits. The ultimate scaling

limit is given by the tunneling distance between neighboring memory cells includ-

ing leakage currents between the wordlines and bitlines [80]. Obviously, the current

approaches are far away from this limit. Another and probably earlier scaling limit is

given by the lateral extension of the switching effect in the memory element. In this

respect, the approaches based on the distributed switching effects appear advanta-

geous compared to the localized switching effects as long as lateral dimensions are

so large that edge effects do not play the dominant role. The variety of different

classes of oxides in combination with suitable electrodes offering switching effects

based on different mechanisms offers a huge potential for optimization. At the same

time these structures need to be fabricated in a very cost-efﬁcient way and excellent

control of material properties is needed to guarantee reliable operation as a non-

volatile memory device.

164 R. Bruchhaus and R. Waser

Among the presented oxides which exhibit bipolar switching no clear frontrunner

can be identiﬁed based on the published data. Much more research effort within

projects covering material properties and integration challenges is needed to explore

the potential of the bipolar resistive switching effect in oxides. A deeper understand-

ing of the microscopic switching mechanisms and the effects limiting the reliability

of the memory devices are needed.

Acknowledgment We gratefully acknowledge valuable discussions with G. Bednorz and I. Meijer

(IBM Research, Zurich), M. Kund and R. Symanczyk (Qimonda AG, Munich), U-In Chung and

B. Bae (Samsung Electronics), V. Zhirnov (SRC), J. Yi (Hynix Semiconductor), A. Sawa (CERC,

Tsukuba), H. Hwang (Gwangju Institute of Science and Technology), R. Dittmann, K. Szot (Re-

search Center Juelich), and D. Strukov and R.S. Williams (HP Labs, Palo Alto).

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Chapter 5

Complex Oxide Schottky Junctions

Yasuyuki Hikita and Harold Y. Hwang

Abstract A Schottky junction is not only an important heterostructure for

electronic and optical devices, but is also an ideal system for studying fundamental

interface physical phenomena such as band-offsets, band bending, and interface

states. In this chapter, we focus our attention on the complex oxides with the

perovskite structure. This family of materials exhibit unique physical properties in-

cluding ferroelectricity, magnetism and high temperature superconductivity. We will

describe how to incorporate the strong interaction between charge, lattice and spins,

absent in conventional semiconductors, in describing oxide Schottky junctions, and

present new oxide speciﬁc functionalities. After a brief overview of the history

of complex oxide Schottky junctions (Sect. 5.1), the basics of Schottky junctions

are reviewed (Sect. 5.2). The speciﬁc features of oxide Schottky junctions will be

introduced focusing on band bending and band alignment mechanisms (Sect. 5.3)

followed by examples of novel functionalities achievable in oxide Schottky junc-

tions (Sect. 5.4). This Chapter will close with a summary and a future outlook

(Sect. 5.5).

5.1 Introduction

In this chapter, recent progress made in the ﬁeld of complex oxide Schottky

junctions will be introduced. The oxides of interest here are mainly those with the

perovskite structure which have generated substantial progress in fundamental as

well as applications oriented research. A number of detailed reviews on the physical

properties of perovskites are available [1–3] and here only a brief introduction will

be presented.

The perovskite structure is composed of three different elements with a chemical

formula ABO

, in which the B-site ion is usually a transition metal ion coordinated

octahedrally by the oxygen ions and the A-site is occupied by an alkaline-earth

Y. Hikita (



) and H.Y. Hwang

University of Tokyo, Department of Advanced Materials Science, Graduate School of Frontier

Sciences, Japan

e-mail: hikita@k.u-tokyo.ac.jp

S. Ramanathan (ed.), Thin Film Metal-Oxides: Fundamentals and Applications

in Electronics and Energy, DOI 10.1007/978-1-4419-0664-9

 Springer Science+Business Media, LLC 2010

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