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 139

Fig. 4.7 Schematic

cross-section of the memory

element [20]

Top electrode Pt

Conductive

metal oxide

Bottom electrode Pt

Tunnel oxide

2-3nm

between 380 and 450

C and result in a fully crystallized conductive metal oxide

and tunnel barrier. Quasi-static I–V curves of these stacks between ˙2 V are essen-

tially symmetric and the observed current scales with the tunnel oxide thickness.

Asymmetric behavior is observed at bias voltages higher than C2 V to the top elec-

trode and a regime of negative differential current indicates the programming of the

device into an OFF state. The operation is bipolar, i.e., a reversed voltage is needed

to erase this state. Applying voltage pulses of ˙3 V for pulse times between 1sto

10 s result in a change of the resistance of about 10. Cycling endurance is given

as 10

cycles.

Oxygen ion transport between the conductive metal oxide and the tunnel barrier

was proposed as the key step in the operation of the memory device structure. Based

on the ion migration model on an atomic scale for high electric ﬁelds an exponential

relation between the drift velocity of an ion in a dielectric material and the electric

ﬁeld is found [20]. Due to the very high electric ﬁelds in the range of 10 MV/cm

(3 V/3 nm tunnel oxide thickness) the oxygen ions are mobile enough even at room

temperature to travel for a few nanometers. Under positive bias at the top electrode

oxygen ions migrate from the conductive metal oxide to the tunnel oxide and get

trapped. The excess negative charge accumulates in the tunnel oxide leading to a

decrease in the tunneling current through the tunnel oxide. The conductive metal

oxide ABO

compensates this charge loss by a metal cation valence change in the

cationic sublattice, which is a reduction process:

2

! A



1x



2

30:5x

C 0:5xO

2

Figure 4.8 illustrates the write (RESET) operation.

Upon bias reversal the oxygen ions are transferred back to the conductive

metal oxide and the tunnel barrier recovers and the memory element switches

back to the ON state (SET or erase process). In [20] materials have not been dis-

closed. However, suitable candidate materials for the conductive metal oxide include

1x

MnO

(PCMO) in which the Mn

=Mn

valence change can compen-

sate for change in oxygen stoichiometry. With respect to the tunnel oxide not many

materials can withstand electric ﬁelds in the order of 10 MV/cm for prolonged pe-

riods of time [21]. Based on the published results on Time-Dependent Dielectric

Breakdown (TDDB) investigations for gate oxides Al

is a possible candidate

for the tunnel oxide material as theoretical dielectric breakdown ﬁelds of 11.2–

13.8 MV/cm have been reported [21].

140 R. Bruchhaus and R. Waser

Fig. 4.8 Schematic

representation of the write

(RESET) process. Oxygen

ions migrate under a high

electric ﬁeld from the

conductive metal oxide into

the tunnel oxide. A valence

change in the cationic

sublattice compensates the

charge loss by the migrated

oxygen ions [20]

Top electrode Pt

Conductive

metal oxide

Bottom electrode Pt

Tunnel

oxide

−

Fig. 4.9 I–V curves of the switching effects in the Mo/Sm/La

0:7

0:3

MnO

/Pt and

Mo/La

0:7

0:3

MnO

/Pt heterostructure [22]

4.3.2 Distributed Bipolar Switching in Complex Perovskites

A true interface-related switching effect is also claimed for the system Mo=Sm=

0:7

0:3

MnO

=Pt [22]. Figure 4.9 gives the layer sequence and the I–V

characteristics.

4 Bipolar Resistive Switching in Oxides for Memory Applications 141

Insertion of a thin Sm metal ﬁlm between the complex oxide and the Mo top

electrode yielded a considerable decrease of the current levels and a signiﬁcant im-

provement of the R

off

ratio. Initially the layer stack is in the ON state. The

RESET process to the OFF state is observed during positive bias to the top Mo elec-

trode. At negative bias the sample switches back to the ON state. A R

off

ratio

of 78 M=40 k D 1;950 was observed. An endurance of 1,000 switching cycles

was experimentally veriﬁed. The switching mechanism proposed in the paper is also

related to oxygen ion migration. Under positive bias to the top Mo electrode oxygen

ions migrate from the La

0:7

0:3

MnO

(LCMO) to the Sm metal ﬁlm and form a

thin oxide ﬁlm at the interface. The thin oxide ﬁlm forms an isolating layer thus

preventing the ﬂow of the electrical current. On negative bias the process is reversed

and the thin oxide layer removed and the ON state recovered. However, no proof is

given in the paper for the formation of the thin Sm oxide, which is claimed to be

responsible for the bistable switching.

More examples of interface type resistive switching in transition metal oxides

have been described in a recent review paper by Sawa [16]. To our knowledge,

Baikalov et al. [23] were the ﬁrst who proposed that the resistance switching is

related to the metal/oxide interface in their Pr

0:7

0:3

MnO

samples with Ag elec-

trode. Based on the slow dynamics and the fact that the process can be accelerated

by higher voltages it was speculated that electrochemical processes like oxygen va-

cancy creation and/or migration may be involved [23]. However, no experimental

evidence was given in the paper.

In general, the memory elements described for RRAM have a capacitor-like MIM

structure in which an isolating wide bandgap semiconductor is sandwiched between

metal electrodes. Schottky barriers are expected to form at the interface between the

metal and the semiconductor due to the difference in workfunction. Sawa et al. [24]

investigated the p-type semiconductor Pr

0:7

0:3

MnO

(PCMO) in contact with dif-

ferent metal electrodes like SrRuO

(SRO), Pt, Au, Ag, and Ti. Among these, only

the sample Ti=PCMO=SRO=.100/SrTiO

-substrate with Ti electrode showed recti-

fying I–V characteristics and resistance switching. Nb-doped SrTiO

(Nb:STO) was

chosen as an example for a n-type semiconductor [25]. The results are summarized

in Fig. 4.10.

For the p-type PCMO the rectifying I–V characteristics is observed for the metal

(Ti) with the lowest work function of about 4.3 eV. On the other hand, the n-type

Nb:STO is rectifying for the SRO (and Au) electrode and resistance switching is

demonstrated for the SRO electrode. An additional series of experiments were per-

formed to further narrow down the interface width. Ti=Sm

0:7

0:3

MnO

(n unit

cells: u.c.)=La

0:7

0:3

MnO

[Ti/SCMO(n)/LSMO] ﬁlms were deposited on (100)

SrTiO

single crystals with a 80-nm thick SRO bottom electrode. The SCMO is

insulating due to a narrow effective one-electron bandwidth, whereas the LSMO is

metallic due to a wide one-electron bandwidth [26, 27]. The 20-nm Ti top electrode

was deposited ex situ and protected by additional 380 nm of Au [28]. Fig. 4.11 gives

the I–V characteristics of the sample series with increasing number of unit cells n

from the top to the bottom. In the left panel the current is drawn in a linear scale,

the right panel gives the same curve with logarithmic scale.

142 R. Bruchhaus and R. Waser

Fig. 4.10 (a) I–V characteristics of M/p-type PCMO/SRO samples with M D Ti, Au, and SRO.

Only the sample with Ti electrode has a rectifying I–V curve and exhibits bipolar resistance switch-

ing. (b) I–V characteristics of M/n-type Nb:STO/Ag samples with M D Ti,Au,andSRO.The

samples with SRO (and Au) have a rectifying I–V characteristics. Bipolar resistance switching is

shown for the sample with SRO electrode. The insets in the upper part give the layer sequence and

in the lower part a sketch of the Schottky barrier at zero bias is given [16]

The pure Ti/LSMO .n D 0/ shows a slightly nonlinear but symmetric behavior.

Inserting a thin SCMO layer .n D 1/ results in a rectifying behavior similar to that

described in the earlier paper [24]. Together with the rectifying behavior a resistance

switching effect is observed, which becomes more obvious in the semilogarithmic

representation on the right side of Fig. 4.11. With increasing SCMO layer thickness

(n D 3, 5, and 250) both the rectifying as well as the switching behavior improve.

During positive bias (forward direction) the device switches from the OFF state to

the ON state. In the reverse direction (negative bias) the switching from the ON state

to the OFF state is observed. For the sample with n D 5 aR

off

ratio of 6  10

is observed. The resistance switching can also be induced by voltage pulses. Fast

voltage pulses of 100 ns lead to small R

off

ratios, ratios of more than 10 are

measured with a pulse length of 100 s for the sample stack with n D 5 [28].

Rectiﬁcation and resistance switching was also observed in a series of samples

with SrTiO

single crystals n-doped with different levels of Nb of 0.002, 0.1, 0.2,

1 and 2%. [29]. SRO was used as the high workfunction metal electrode. The rec-

tifying characteristics is in agreement with that of a conventional n-type Schottky

diode. A clear resistance switching behavior was found for the 0.1, 0.2, and the

4 Bipolar Resistive Switching in Oxides for Memory Applications 143

Fig. 4.11 I–V characteristics of Ti/SCMO(n)/LSMO samples with n D 0,1,3,5,250drawnwith

a linear I scale (left) and a logarithmic I scale (right)[28]

1% Nb doping level in the forward bias direction. The sample with the smallest Nb

concentration shows some minor switching effect and no switching is observed for

the sample with 2% Nb. The resistance is switched from the ON state to the OFF

state under reverse (positive) bias stress. The SET process from the OFF state to the

ON state is observed under forward (negative) bias stress.

However, the proof of a Schottky barrier at the interface between properly

selected metals and semiconducting oxides alone does not explain the resistance

switching effect. For this some asymmetric modulation of the Schottky barrier as

a response to the applied voltage is needed. As a possible model for the switch-

ing effect, a charging effect of the Schottky-like interface is proposed [24, 25, 29].

Charging and discharging of trapping sites may modify the electronic properties of

the junction. This model is further supported by the ﬁnding that proper interface

engineering by doping can suppress the resistance switching effect. Insertion of a

2-nm thick undoped SrTiO

layer suppresses the switching by blocking the tun-

neling path but leaves the rectifying I–V characteristic unchanged. Heavy electron

doping by the insertion of 2 nm La

0:25

0:75

TiO

leads to a quasi Ohmic behavior

without switching and rectiﬁcation [29]. The substitution of La

for Sr

provides

electrons into the conduction band. Now the SRO/La:STO/Nb:STO can be regarded

as a metal/n

/n-junction reducing the Schottky barrier width. Electrons possibly

can pass through the barrier by direct tunneling. These ﬁndings are summarized in

Fig. 4.12.

144 R. Bruchhaus and R. Waser

Fig. 4.12 Interface engineering of SrRuO

/X (2 nm)/SrTi

1x

.x D 0:01/ samples. Inser-

tion of the highly doped La

0:25

0:75

TiO

layer suppresses rectiﬁcation and switching. Insertion

of 2 nm undoped isolator SrTiO

supresses the switching but leaves the rectifying behavior un-

changed. The induced changes are schematically depicted in the band diagrams [29]

The possibility of electrochemical redox reactions and migration of oxygen ions

is ruled out due to experimental ﬁnding that the Ti/LSMO sample did not show

the resistance switching [28]. However, an oxygen vacancy diffusion mechanism

was proposed by Nian et al. [30]forPr

0:7

0:3

MnO

(PCMO) ﬁlms grown on Pt

bottom electrodes on TiN=SiO

=Si substrates. The PCMO ﬁlms were grown by rf

sputtering at 500

C under either oxygen rich .Ar W O

D 2 W 1/ or oxygen deﬁcient

(Ar only) conditions. The PCMO ﬁlms were 400 nm thick and Ag was deposited

by sputtering as the top electrode material. Based on the ﬁnding that the resistiv-

ity of ReAMnO

3•

such as LaBaMnO

increases by several orders of magnitude

when the oxygen content is reduced by 5%[31] it is proposed that under positive

voltage pulses oxygen ions are moved into the vacancies located at the metal elec-

trode interface (switching from OFF to ON). With negative bias the oxygen ions are

moved toward the metal interface and piling up there leaving a region with increased

oxygen vacancies which deﬁnes the OFF state. The local overdrive at the electrode

interface leads to a local nonequilibrium distribution which explains the resistance

decay observed in the I–V characteristic. The observation that an oxygen-annealing

step improved the resistance switching in metal-La

0:7

0:3

MnO

-metal heterostruc-

tures [32] supports the model that oxygen vacancies may play a key role in the

4 Bipolar Resistive Switching in Oxides for Memory Applications 145

resistance switching mechanism in oxides. For the bipolar switching effects based

on formation and rupture of ﬁlaments, discussed later in this chapter, the distinct

role of oxygen vacancies has been demonstrated.

Resistive switching, which is attributed to the interface, was also observed for

SrTiO

ﬁlms deposited on SrTiO

substrates doped with 0.7% Nb [33]. In this

work, notable structural distortions were detected at the SrTiO

/0.7% Nb W SrTiO

interface by HRTEM. The switching process is interpreted as a trapping–detrapping

process of carriers in the interfacial region. This is concluded by comparison with

samples deposited on 0.1% Nb-doped SrTiO

substrates which did not switch and

had a well-matched interface structure with sharp contrast [33]. An interface-related

switching was also reported for La

1x

1Cx

MnO

single crystals with Ti electrode

[34]. Bistable switching was observed in Pr

1x

MnO

.x D 0:3/ (PCMO) thin

ﬁlms at room temperature [35]. These ﬁlms were grown in a polycrystalline state on

the highly (111) oriented Pt electrode. Evaporated Ag was used as the top electrode

material. The sample exhibited bipolar switching with a R

off

ratio between 10

and 100. The switching voltage was less than 2 V for the 150-nm thick ﬁlms. The

I–V characteristics was characterized in the frame of a trap-controlled space-charge

limited current (SCLC) mechanism [35].

4.4 Localized Bipolar Switching Effects in Transition

Metal Oxides

4.4.1 Filamentary Switching in Cr-Doped SrTiO

Single Crystals

Clear evidence for a localized conducting ﬁlament-based bipolar switching behavior

in transition metal oxides was obtained on single-crystal samples [36]. 0.2mol%

Cr-doped SrTiO

single crystals with Pt electrodes deposited on the surface with

a separation of 50 m were exposed to an electric ﬁeld of 10

V=cm for 30 min

(“conditioning process”). The initially isolating crystal becomes conducting and can

be switched between an ON state and an OFF state by application of bipolar bias

voltages, Fig. 4.13.

Negative bias voltages switch the sample from the OFF state to the ON state and

the opposite polarity triggers the RESET process. The ﬁnding that both the ON state

and the OFF state indicate a metallic behavior in the resistance versus temperature

plot of Fig. 4.13b are in contrast to models that describe the resistance switch-

ing as an alteration of a Schottky barrier [24] or nonpercolative metallic domains

[37]. For both models a nonmetallic temperature dependence is expected. Detailed

information about the microscopic nature of the conducting path were received

by micro-X-ray ﬂuorescence (XRF) and X-ray absorption near edge spectroscopy

(XANES) at the Cr and Ti absorption K-edges. Based on these measurements it was

concluded that the conditioning process forms a few micrometer wide path of oxy-

gen vacancies between the electrodes. The Cr ions in the lattice serve as a kind of

146 R. Bruchhaus and R. Waser

Fig. 4.13 (a) Bipolar resistance switching between the OFF state and the ON state after “con-

ditioning.” (b) Temperature dependence of the resistance for the ON state and the OFF state

indicating metallic behavior for both states. (c) IR thermal image of the memory cell with a cur-

rent of 5 mA showing an area of increased temperature in front of the anode and a ﬁlament like

conduction proﬁle between the anode and the cathode [36]

seed because the oxygen vacancies are preferentially associated with the Cr ions and

arranged along the conducting path like pearls in a pearl necklace. The Ti cations

compensate for the oxygen deﬁciency by reduction to Ti

.4n/C

species which is

equal to ﬁlling the Ti 3d band and leads to a metallic conduction [38]. The switch-

ing process itself is localized in the vicinity of the anode. Close to the anode is an

area of higher resistance as indicated by the higher temperature in the thermal mi-

croscopy. It is proposed that under negative bias pulses positively charged oxygen

vacancies migrate preferentially into this high-resistance interface region leading to

the ON state. Under positive bias the oxygen vacancies are repelled from the anode

region and the OFF state is recovered.

4 Bipolar Resistive Switching in Oxides for Memory Applications 147

4.4.2 Filament Fine Structure Analysis with Conductive-Tip AFM

Undoped SrTiO

single crystals were used for conductive-tip atomic force

microscope (C-AFM) investigations of the ﬁlament ﬁne structure [39]. SrTiO

is a well-suited material for this purpose as it has a simple cubic structure, it is

a model band insulator and widely investigated for many years. In addition, high

quality single crystals are easily available. C-AFM turned out to be an invaluable

tool in a twofold sense. First it allows one to examine very small features with a

sufﬁcient spatial resolution and, secondly, the tip can be used as nano-sized elec-

trode to locally apply bias voltages and study the local conductivity at the same

time. Dislocations and the preferred oxygen transport along these defects play the

key role to explain the bistable switching observed in this material. A three step

process was developed to activate the highly isolating single crystals and transform

them into a stable resistance switching material [39]. The treatment starts with an

annealing under reducing conditions then a holding time under ambient conditions

and, ﬁnally, application of an electric ﬁeld under ultrahigh vacuum [40–42]. After

this sequence of treatments the sample material is in a metallic state and exhibits

bistable switching under application of a bias current, Fig. 4.14.

Both, the OFF state and the ON state exhibit a metallic resistance versus tem-

perature behavior which is indicated by a positive temperature coefﬁcient. The

ﬁlamentary type of conduction can already be observed in the optical microscope.

Inspection using an optical microscope reveals streaks formed between the cath-

ode and the anode in various segments. The streaks are arranged orthogonal along

the f100g direction of the crystal. At spots where a streak happened to cross the

surface an increased conductivity was observed by C-AFM. By observation under

higher magniﬁcation close to the anode gas bubbles lift the anode metallization

which result by electrochemical oxidation of the oxygen ions to form gaseous

.[39]

The C-AFM is an excellent tool to map spatial variation of electrical conductance

at the sample surface. Using step one and two of the preparation method described

above conducting spots could be observed by C-AFM, Fig. 4.15.

The spots are typically 2–4nm in diameter and the density is around 10

2

With the C-AFM the electroformation process could be initiated at the surface.

A negative voltage applied to the tip resulted in a considerable increase in con-

ductivity over several orders of magnitude. These spots can be repeatably switched

between the OFF state and the ON state by application of appropriate bipolar bias

voltages. A range of at least 3 orders of magnitude was observed after reading the

states with a read voltage of 0.1 V well below the switching voltage. Direct evidence

that these conducting spots are related to the dislocations were received on samples

which have been subjected to a chemical etch treatment before the forming heat

treatment described above. The chemical etching makes the dislocations visible as

etch pits and the conducting spots are found within the center of these etch pits. The

studies performed on undoped single crystalline SrTiO

revealed the key role of de-

fects in the form of dislocations and their role as preferential path for the transport of

oxygen to serve as a model to explain the ﬁlamentary bistable resistance switching.

148 R. Bruchhaus and R. Waser

Fig. 4.14 V–I curves taken under current control mode to avoid damage to the samples and the

C-AFM tip. Bistable switching between the OFF state and the ON state is observed within a crit-

ical current window. The inset gives an example for a multistep transition which is occasionally

observed [39]

4.4.3 Filament Fine Structure Analysis on Thin Film Samples

However, for the use of resistive memory elements in a high-density RRAM with

feature size in the sub 100 nm range the active oxide layer in the MIM structure

must be fabricated as a thin ﬁlm. Results received on single crystals are very useful

for the basic understanding of the switching mechanisms and due to high quality of

the crystal the impact of side-effects like grain-boundaries,internal stresses from the

deposition process, integration damage and pronounced impact from the interface

between oxide and electrode are suppressed. Fortunately, SrTiO

can be deposited

in high-quality on lattice matched substrates and electrodes. Results received on

these high-quality epitaxially grown thin ﬁlms can be compared with the results on

the single crystals and the impact of sample thickness studied and thereby the actual

depth in which the ﬁlamentary switching process occurs narrowed down. Pulsed

laser deposition was used to deposit 10-nm thick SrTiO

thin ﬁlms on 100-nm thick