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 149

Fig. 4.15 (a) Conductivity map of the SrTiO

surface. Spots with enhanced conductivity are

present on the surface. The inset shows a spot with a diameter of 1–2 nm in which the highest cur-

rents are found in a region typically of the size of an edge-type dislocation. (b) Line scan A ! >B

of a selected spot before and after application of a negative bias tip voltage which switches the

sample between the HRS .1:4 10

/ and the LRS .3:2  10

/ [39]

SrRuO

bottom electrodes on single-crystals of SrTiO

as a substrate material [43].

No thermal pretreatment is necessary for the thin ﬁlm to induce the electroformation

using the C-AFM tip by the same way as for the single crystals. In the thin ﬁlm sam-

ple the bottom SrRuO

electrode may serve as an oxygen source or sink depending

on the polarity of the applied voltage. The experiments have to be performed under

high vacuum conditions to observe the switching. Figure 4.16 gives a direct com-

parison of the received I–V curves for the single crystals and the thin ﬁlms.

Within the single crystal a three dimensional network of dislocations is as-

sumed which can serve as path for the migration of oxygen vacancies is depicted

(Fig. 4.16a). On the surface these conducting spots can be accessed by the tip of

the C-AFM. In the thin ﬁlm the conducting spots appear only at the top surface

150 R. Bruchhaus and R. Waser

Fig. 4.16 Resistance switching in SrTiO

single crystals and thin ﬁlms. (a) Single crystal sample

with a thickness of 50 m and a three-dimensional network of dislocations acting as conductive

spots, (b) 10-nm thick SrTiO

thin ﬁlm on a lattice matched SrRuO

bottom electrode with con-

ducting spots on the surface of the sample, (c) I–V curves for different dislocations with bistable

switching from nonmetallic to the metallic state within a range of different threshold voltages (V

var.), (d) similar kind of bistable switching observed for conducting spots on the surface of a 10-nm

thick SrTiO

thin ﬁlm [39]

(Fig. 4.16b). The I–V curves of the single crystal sample and the thin ﬁlm sample

are looking very similar (Fig. 4.16c, d), which may indicate that the switching in

the bulk single crystalline sample may occur very close to the surface only sev-

eral nanometers apart. Switching may correspond to an electrochemical reaction

of “opening” and “closing” the part of a dislocation in contact with the top elec-

trode or, in the C-AFM work, with the tip [39, 43]. The “opening” corresponds to

a depletion of oxygen vacancies in a region at the electrode interface and a corre-

sponding oxidation of the Ti ions, while the “opening” is related to an attraction of

oxygen vacancies and a reduction of the Ti ions, which lead to an increase in the

local conduction and a decrease of the potential barrier. This hypothesis has been

conﬁrmed for TiO

thin ﬁlms [44]. The importance of the interface region for the

switching effects has been discussed already in this contribution with respect to the

interface effects in conjunction with the manganites. For the uni-polar switching in

binary transition metal oxides a mechanism related to the opening and closing of a

“faucet” close to the metal electrode interface has been proposed [45].

Filamentary type of switching was also demonstrated for 1% Nb-doped thin

ﬁlms epitaxially deposited on (100) SrTiO

and (110) NdGaO

substrates [46].

A substrate temperature of 700

C and a low oxygen partial pressure of

4 Bipolar Resistive Switching in Oxides for Memory Applications 151

Fig. 4.17 Reversible bistable resistance switching in epitaxial Nb-doped SrTiO

. A regular array

of conducting protrusions can be repeatably switched with the tip of a C-AFM under high vacuum

conditions. a ! bandd! c: scan with 3V, b ! dandc! a: scan with C2V. The current

compliance is set to 1A to avoid tip damage [46]

approximately 10

6

mbar are the key process parameters to receive well conducting

Nb-doped SrTiO

ﬁlms [47, 48]. C-AFM under high vacuum conditions revealed

that the sample surface is covered with a regular array of little protrusions between

30–50nm in diameter and a height between 1–4 nm. These protrusions are attributed

to the three-dimensional growth under the conditions mentioned above. The center

of these protrusions turned out to be conductive while the boundaries are almost

nonconductive. Most interestingly these protrusions form a well-ordered array of

conducting spots. The conducting spots can be switched by proper bias voltages

between two stable states with a R

off

ratio of approximately 50, Fig. 4.17.

A detailed HRTEM analysis was performed on the Nb-doped SrTiO

ﬁlm grown

on the SrTiO

substrate to gain insight onto the ﬁne structure of the ﬁlm. The ﬁlm

is grown coherently without any misﬁt dislocations at the interface. However, de-

fect rich clusters with a density of about 10

3

and lateral dimensions between

2–20 nm have been found. It is proposed that the clusters correspond to defect rich

areas in which Ti

=Ti

redox reactions are facilitated which lead to the different

conducting states. Based on the C-AFM and HRTEM results the defects found in

the Nb-doped SrTiO

ﬁlms are different from those in the undoped SrTiO

[39,43]

described in the previous paragraph. However, the exact mechanism remains unclear

and other effects related to surface and possible space charge layers may be in place

as well [49].

152 R. Bruchhaus and R. Waser

No bistable resistance switching was observed for epitaxial SrTiO

thin ﬁlms on

-Si substrates grown by MBE [50]. In the bias voltage range 2–4 V only very

small currents in the range of pA were measured in small areas. The size of these

areas is in the range of 10–50 nm. These areas do not correlate with any topographic

features. As the bias voltage is increased additional conducting areas appear; how-

ever, in contrast to the results described above these areas are not associated with

localized defects like dislocations. Thermionic emission appears to be the dominant

mechanism for the observed currents. It can be speculated that the growth conditions

prevented the formation of ﬁlms with the ability for resistance switching. Another

possible explanation is the missing oxygen reservoir in contact with the SrTiO

ﬁlm

in this work as the Si substrate is not able supply any oxygen ions needed to control

the conducting path along the dislocations [39].

4.4.4 Bipolar Switching Effects in Binary Transition Metal Oxides

The formation of conducting paths due to oxygen vacancy migration was also ob-

served for rutile .TiO

/ single crystals [51]. The ﬁeld induced motion results in

a dark blue color indicating a reduced state of the Ti-ion as described earlier. It

was observed that the oxygen vacancies drift much faster in the (001) direction

than in the (110) and (100) direction. However, no information is given if these

conducting areas are related to extended defects as described for undoped SrTiO

single-crystals [39].

Bipolar resistance switching effects were also described for TiO

thin ﬁlms [44,

52–59]. Figure 4.18 gives an example for 50 switching loops of a 50-nm thick TiO

ﬁlm with a Pt top and bottom electrode [44].

The TiO

ﬁlm actually has a bilayer structure. An oxygen-deﬁcient TiO

2x

layer is deposited ﬁrst and a near stoichiometric TiO

layer is the second layer

(Fig. 4.19a). This bilayer structure intentionally introduces a gradient of the oxy-

gen vacancy concentration in the sample. Resistance switching is observed at ˙2 V

and a R

off

ratio of about 1,000 can be read from the log I versus voltage repre-

sentation in the inset of Fig. 4.18. The blue curve gives the current for a maximum

voltage of 1:5 V. As indicated by the lower current level multiple resistive states

can be written to the memory element by different voltage levels. The switching

mechanism of this device proposed in [44] is based on the migration of oxygen

vacancies in the electric ﬁeld. The mentioned bilayer structure intentionally intro-

duces a gradient of the oxygen vacancy concentration in the sample. The oxygen

vacancy concentration in the oxygen-deﬁcient part TiO

2x

is higher compared to

the near stoichiometric TiO

part. Due to the higher doping level of oxygen vacan-

cies at the bottom electrode interface an Ohmic contact is formed. The top electrode

interface between the near stoichiometric TiO

and the Pt is nonohmic rectifying

due to the Schottky-like barrier at the interface. Application of a negative bias volt-

age to the top electrode attracts the oxygen vacancies into the near stoichiometric

TiO

and toward the top electrode interface. The vacancies are expected to diffuse

4 Bipolar Resistive Switching in Oxides for Memory Applications 153

Fig. 4.18 Experimental (solid) and modeled (dotted) switching loops of the Pt/TiO

-TiO

2x

/Pt

switching element. The inset gives the same result in a log-scale. The blue curve is a lower current

I–V curve to demonstrate that multiresistive states are possible [44]

through preferred channels like grain boundaries and thus leading to conductive

paths penetrating the electronic barrier and leading to a conducting channel. The

device is switched to the ON state (Fig. 4.19b). The symmetric exponential ON

current is controlled by tunneling through a residual barrier. Under positive bias to

the top electrode the oxygen vacancies are repelled from the top interface which

results in outdiffusion from the conducting path recovering the electronic barrier of

the OFF state (Fig. 4.19c). This reset process is clearly different from the reset pro-

cess described for unipolar switching in which the rupture of the ﬁlament is induced

by Joule heating [7]. It is important to note that the switching process occurs only

at the rectifying interface between the stoichiometric TiO

part of the bilayer and

the electrode. Under bias voltage the oxygen vacancy concentration and distribution

will also change at the bottom electrode interface; however, this change is not sig-

niﬁcant enough to lead to a change of the ohmic behavior. According to this model

the rectiﬁcation and switching polarities are determined by the initial distribution of

the oxygen vacancies. Upon reversing the fabrication sequence and depositing the

near stoichiometrc TiO

layer ﬁrst and the TiO

2x

layer as the second layer the po-

larity for the SET and RESET process are reversed as sketched in Fig. 4.19d–f. Now

the bottom interface layer is the rectifying active layer for the switching process and

the positive voltage drives the oxygen vacancies into the TiO

thus modifying the

154 R. Bruchhaus and R. Waser

TiO

2-x

TiO

2-x

TiO

2-x

TiO

2-x

TiO

2-x

TiO

2-x

Fig. 4.19 Schematic representation of resistance switching in titanium oxide thin ﬁlms with oxy-

gen vacancy gradient. (a) As deposited double layer structure with an oxygen vacancy rich TiO

2x

layer as the ﬁrst layer and a near stoichiometric TiO

as the second layer. (b) A negative bias volt-

age is applied to the top electrode and oxygen vacancies migrate into the TiO

layer and, ﬁnally

forming a conductive path along preferred channels (c) Upon bias reversal the oxygen vacancies

are pulled out again and the OFF state recovers. (d)–(f) give the schematic switching process when

the layer sequence and the voltage polarity are reversed

electronic barrier layer and switching the device to the ON state (Fig. 4.19e). Upon

bias reversal the oxygen vacancies are pulled out again and the HRS state is re-

covered (Fig. 4.19f). This experiment clearly demonstrated that the interplay of the

TiO

and TiO

2x

layers is the key for the resistive switching of the multilayer stack.

Figure 4.20 gives a schematic representation of the I–V curves received when the

bias voltage is applied to the top Pt electrode.

Oxygen ion migration is discussed as the possible mechanism for bistable switch-

ing in 2.5-nm thick anatase ﬁlms received by surface oxidation of a TiN ﬁlm with

4 Bipolar Resistive Switching in Oxides for Memory Applications 155

Current lin. scale

voltage

TiO

2-x

Current lin. scale

voltage

TiO

2-x

TiO

2-x

TiO

2-x

TiO

2-x

TiO

2-x

TiO

2-x

Fig. 4.20 Schematic I–V curve for a titanium oxide layer with oxygen vacancy gradient (a)the

oxygen vacancy rich TiO

2x

layer is deposited ﬁrst. Under negative bias oxygen vacancies move

into the near stoichiometric TiO

layer and form a conductive path. (b) The oxygen vacancy rich

TiO

2x

layer is deposited last and under positive bias to the top electrode the conductive path is

formed in the near stoichiometric TiO

layer [44]

ozone [57, 58]. Initially the device was in the OFF state and could be switched by

very short 20 ns wide pulses with 2 V to the ON state. For the reverse process a

30 ns long pulse with C2:2 V was applied. However, details about the possible role

of grain boundaries in TiN as an oxygen reservoir and the impact of the extremely

high nominal electric ﬁelds of about 10MV/cm on the anatase ﬁlm remain unclear.

Bipolar resistance switching was described for Mo-doped SrZrO

ﬁlms [60]. Mo

doping levels of 0.1, 0.2, and 0.3% were investigated and the ﬁlms with 0.2% ex-

hibited the most stable switching properties. It is speculated that the switching is

related to the formation and rupture of conducting paths composed of various kind

of defects.

A redox reaction process between Fe

and ”-Fe

was proposed to explain

the bipolar switching behavior in an iron-oxide-based resistance change memory

element [61, 62]. The iron-oxide system was chosen, because Fe

and ”-Fe

have a very similar crystal structure but the resistivity of these oxides is quite dif-

ferent. A 100-nm thick iron-oxide ﬁlm was deposited from a Fe

target and Pt

was used as top and bottom electrode. The as-deposited ﬁlm was transferred to a

mixed Fe

=”-Fe

ﬁlm by post deposition annealing at temperatures between

250–350

C. Initially the memory element is in the ON state. An initial forming pro-

cess with an electrical pulse with negative polarity to the top electrode is applied to

switch the sample into the OFF state. After electroforming an additional peak which

is attributed to an interfacial layer of ”-Fe

at the anode side was observed in the

Raman spectrum. It is proposed that a conducting ﬁlamentary path of Fe

pene-

trates the isolating interface layer [61]. The redox reaction can be summarized as:

156 R. Bruchhaus and R. Waser

Fig. 4.21 HX-PES spectra of a large thin Pt/TaOx/Pt test structure indicating different

5•

=TaO

2“

ratio in the ON state and OFF state [63]

3-Fe

C 2e



Set

!



Reset

2Fe

C O

2

Bistable, reversible resistance switching was demonstrated applying C2 V= 2:2 V

with 100 ns pulse width. Switching endurance of 3  10

cycles was demonstrated

as well as a fast switching time down to 10 ns. Data retention could be improved by

replacing the ”-Fe

with ZnFe

, which is speculated to suppress the oxygen

ion mobility and thereby improve the data retention.

Another system in which a gradient of the oxidation state of a transition metal

oxide is claimed by the authors to be the root cause for bistable resistance memory

operation is Ta-oxide [63]. A Pt=TaO

=Pt stack forms the memory element. Hard

X-ray Photoemission Spectroscopy (HX-PES) data were interpreted that during the

set process HRS-> LRS some of the Ta

5•

is reduced to TaO

2“

,Fig.4.21.

It is proposed by the authors that for the reset operation oxygen ions migrate from

the TaO

2“

toward the anode under the positive voltage bias and some Ta

5•

formed. This compound directly modiﬁes the Schottky barrier between the oxide

and the noble metal thus leading to the OFF state. Upon bias reversal Ta

5•

reduced back to TaO

2“

and the ON state is recovered:

C 2e



Set

!



Reset

2TaO

C O

2

It is argued that the TaO

2“

=Ta

5•

redox pair is a preferred one as the Gibbs en-

ergy G between these components is very small and thus facilitating the reaction.

Bipolar switching was also reported for Cu

O[32, 64–66]. Figure 4.22 gives a

typical I–V curve of an integrated Ti=Cu

O=Cu memory element. The Cu

Oﬁlm

has been formed by thermal oxidation of a Cu via hole [64,65].

4 Bipolar Resistive Switching in Oxides for Memory Applications 157

Fig. 4.22 Typical I–V curve of a Ti/Cu

O=Cu memory element connected to a select transistor.

For the SET process the positive bias is applied to the top electrode, while the bottom electrode

is grounded. For the RESET process the voltage is reversed. The transistor limits the current to

45 A. The inset gives the 1T1R memory cell [65]

In bulk form cuprous oxide .Cu

O/ is known as a nonstoichiometric p-type

semiconductor [67]. Compared to the bulk material electrodeposited Cu

Oﬁlms

show a very high resistivity in the range 10

–10

cm compared to 10

–10

cm

for the bulk material. This difference is attributed to space-charge-limited-current

conduction for the thin ﬁlms. For the 3:2-m thick ﬁlm it is shown that the con-

ductivity is controlled by an acceptor-type deep level which is compensated with

a donor-type level [67]. It is claimed that this model can be applied for the very

thin Cu

Oﬁlmsin[64,65]. Under applied electric ﬁeld at the trap-ﬁlled-limit volt-

age most of the traps become ﬁlled, which leads to a drastic increase in the current

which is observed in the I–V curve as the SET process. The OFF state is interpreted

as the state with empty traps. However, the trap levels and the trap-ﬁlled-limit volt-

ages .V

TFL

/ calculated from the measurements differ considerably for the different

references. Thus other effects may come into play and more detailed studies, which

also take mechanisms and interface effects described for other oxides into consider-

ation, are needed.

4.5 Impact of Electrode Materials on Bipolar Switching

The bipolar switching observed in transition metal oxides were grouped into dis-

tributed switching effects related to interfaces between the transition metal oxide

and the electrode and localized effects where the switching can be traced back to

158 R. Bruchhaus and R. Waser

the formation of localized ﬁlaments. For the distributed systems an impact from the

electrodes appears more obvious; however, it will be shown in this section that for a

certain electrode material the system switches to a completely different mode.

4.5.1 Workfunction of the Metal

Transition metal oxides are isolating relatively wide bandgap semiconductors and

for the application in RRAM devices they are 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.

For the p-type semiconductor Pr

0:7

0:3

MnO

(PCMO) a rectifying contact

and resistance switching was received for Ti (workfunction 4:3 eV) metal. With

other metals like Ag .4:3 eV/,Au.5:1 eV/,Pt.5:65 eV/,andSrRuO

(SRO,

5:3 eV) a linear I–V characteristic and no switching was observed [24]. This be-

havior can be understood according to the p-type Schottky contact model for the

metals Au, Pt, and the conducting oxide SRO as these metals have a signiﬁcantly

higher workfunction than Ti. However, the simple Schottky barrier approach can-

not explain the difference between Ag and Ti as these metals have about the same

work function. Here, additional effects need to be taken into consideration. Com-

pared to Ag, Ti has smaller electronegativity and an extreme afﬁnity to oxygen. It

is argued that by extraction of oxygen from the surface of the PCMO a high density

of interface states may cause a large degree of band bending at the Ti/PCMO inter-

face compared to the Ag/PCMO interface [24]. Ag or Cu electrodes always needs

special consideration as the use of these two metals may result in electrochemical

metallization-based effects. In this case, the metal ion is the migrating species and

undergoes electrochemical oxidation and reduction during the switching process.

An instructive example is given in 4.5.2. The received I–V curves need to be care-

fully reviewed to avoid misinterpretation.

For the n-type semiconductor Nb-doped SrTiO

the high work function metals

Au and SRO form contacts with rectifying I–V characteristics. For the lower work

function metal Ti a linear I–V curve is received [16]. Again, resistance switching

is observed only for the rectifying contact metals. An improvement of the R

off

ratio for differently prepared RuO

electrodes on Nb-doped SrTiO

single crystals

compared to Ru and Pt metal was reported by Hasan et al. [68].

These results suggest that the formation of a Schottky barrier plays an impor-

tant role for the resistance switching. However, obviously this is a necessary but

not sufﬁcient condition and during the switching process additional polarity depen-

dent effects come into play. It can be speculated that oxygen vacancies and their

migration in the applied electric ﬁeld may play a decisive role.