Electrical properties of the amorphous interfacial layer between Al electrodes andepitaxial NiO filmsJae Hyuck Jang, Ji-Hwan Kwon, Seung Ran Lee, Kookrin Char, and Miyoung Kim Citation: Applied Physics Letters 100, 172101 (2012); doi: 10.1063/1.4704917 View online: http://dx.doi.org/10.1063/1.4704917 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural and electrical properties of pure and Cu doped NiO films deposited at various oxygen partial pressures AIP Conf. Proc. 1512, 640 (2013); 10.1063/1.4791200 Switching of nanosized filaments in NiO by conductive atomic force microscopy J. Appl. Phys. 112, 064310 (2012); 10.1063/1.4752032 Origin of resistivity change in NiO thin films studied by hard x-ray photoelectron spectroscopy J. Appl. Phys. 109, 124507 (2011); 10.1063/1.3596809 I-V measurement of NiO nanoregion during observation by transmission electron microscopy J. Appl. Phys. 109, 053702 (2011); 10.1063/1.3553868 Structure and spectroscopic properties of C–Ni and C N x – Ni nanocomposite films J. Appl. Phys. 98, 034313 (2005); 10.1063/1.2001746

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Electrical properties of the amorphous interfacial layer betweenAl electrodes and epitaxial NiO films

Jae Hyuck Jang,1 Ji-Hwan Kwon,1 Seung Ran Lee,2 Kookrin Char,2 and Miyoung Kim1,a)

1Research Institute of Advanced Materials, Department of Materials Science and Engineering,Seoul National University, Seoul 151-744, Korea2Department of Physics and Astronomy and Center for Strongly Correlated Materials Research,Seoul National University, Seoul 151-747, Korea

(Received 21 January 2012; accepted 31 March 2012; published online 23 April 2012)

The amorphous interfacial layer (a-IL) between Al electrode and epitaxial NiO films were studied

using electron energy-loss spectroscopy (EELS) and energy-dispersive x-ray spectroscopy. Two

distinct properties were found in the a-IL, i.e., a lower metallic and an upper insulating layer.

EELS results revealed that the metallic Ni atoms were responsible for the conducting nature of the

lower oxide amorphous layer. The resistance behavior of Al/a-IL/epi-NiO was changed from a

high to a low resistance state after forming process. The resistance change could be explained by

the formation of a nanocrystalline metal alloy in the insulating amorphous layer. VC 2012 AmericanInstitute of Physics. [http://dx.doi.org/10.1063/1.4704917]

There has been increasing interest in the interface

between a metal electrode and a transition metal oxide

(TMO) in electrical devices, in particular, in resistance

switching random access memory (ReRAM) devices. Fre-

quently, an amorphous interfacial layer (a-IL) is created

between the metal and the oxide after deposition of metals

onto oxides or during a bias voltage sweep.1 Resistive

switching, induced in a metal/oxide/metal stack by an elec-

tric field, is often attributed to an interfacial reaction,2–5

especially in a bipolar system where bias polarity determines

the switching direction. Although many models suggest ionic

transport to or from the interface, e.g., oxygen migration6

and oxygen-deficient cluster at the interface,7 few micro-

structural investigations having high spatial resolution have

been reported.

NiO is one of the most intensively studied materials for

ReRAM because it is easily manufactured and exhibits supe-

rior electrical properties such as retention and endurance.8,9

We studied NiO thin films grown epitaxially (epi-NiO) on a

SrRuO3 bottom electrode as a model system to characterize

the electrical properties of an amorphous interfacial layer. Al

electrodes easily form an a-IL on the oxide due to the low

oxidation enthalpy.10 The switching behavior of ReRAM

devices of this epi-NiO depends on the metal electrodes; re-

sistance switching occurs with a Pt top electrode while no

switching occurs with an Al electrode.11

We used electron energy-loss spectroscopy (EELS) and

energy-dispersive x-ray spectroscopy (EDS) to investigate

the origin of the electrical properties of Al/epi-NiO. The a-

IL between the epi-NiO films and the Al electrode was com-

posed of Al, Ni, and O. The electronic structures of those

elements in the a-IL were compared for epi-NiO films grown

at two different temperatures, which exhibited different elec-

trical properties. Additionally, we examined structural

changes in the a-IL after applying a voltage and related these

changes to the electrical properties. Interestingly, the chemi-

cal states of diffused Ni atoms at the a-IL determined the

electrical properties of the devices.

Epi-NiO films were deposited on a SrRuO3/SrTiO3

(SRO/STO) substrate using the pulsed laser deposition

method. The epi-NiO films were prepared at substrate tem-

peratures of 500 and 700 �C. In the pristine states, the latter

was conducting while the former was insulating. The high re-

sistance of the epi-NiO(500) turned into a low resistance

state when a bias voltage was applied.11 Details of the depo-

sition process are available in the literature.12 After deposi-

tion of the epi-NiO, Al metal was deposited onto it by DC

magnetron sputtering. The current–voltage characteristics of

the Al/epi-NiO were measured using a Keithley 4200 semi-

conductor system. EDS was used (Tecnai F20, Tecnai 136-5

model, EDAX Corporation) to obtain the composition of the

a-IL between the Al metal and the epi-NiO. The energy-loss

near-edge structures (ELNES) of Ni L2,3, Al L2,3, and O Kedges were measured using the Gatan Imaging Filter system

(GIF Tridiem 863). The full width at half maximum of the

zero-loss peak was approximately 1.0 eV.

Figure 1(a) shows a transmission electron microscopy

(TEM) image of the Al/epi-NiO(500)/SRO/STO sample. The

thickness of the epi-NiO film in Fig. 1(a) is about 20 nm. The

NiO film had interfaces with two different metals, i.e., a top

Al electrode and a bottom SRO electrode. A 5-nm-thick

amorphous layer was observed at the interface with the Al

metal (Figure 1(b)), while a sharp interface was observed

between epi-NiO and SRO, in agreement with earlier

work.11 The electrical behavior of the Al/epi-NiO(500)/SRO

changed from insulating to metallic after the forming process

and did not switch to a high resistance state with additional

voltage sweeps.11,13 Lattice fringes were observed in some

of the a-IL after the forming process (Figure 1(c)), indicating

partial crystallization of the a-IL. In Fig. 1(c), the interfacial

layers can be divided into two regions: the upper part of the

a-IL is mainly amorphous but with some crystallinity, while

the lower part of the a-IL has a more nanocrystalline struc-

ture. The crystal structure was identified as the NixAl1�x

intermetallic phase. In the case of poly-NiO films, thea)E-mail: mkim@snu.ac.kr.

0003-6951/2012/100(17)/172101/3/$30.00 VC 2012 American Institute of Physics100, 172101-1

APPLIED PHYSICS LETTERS 100, 172101 (2012)

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formation of elemental Ni or a Ni-Pt phase with a Pt elec-

trode12 and the clustering of elemental Ni at the interface

between NiO and Ag metal14 were previously reported. The

formation of a Ni–Al intermetallic phase in the a-IL by the

applied voltage is attributed to the low free energy of forma-

tion for Al2O3 (�1.056 MJ/mol at 298 K).15–17 This low free

energy drives the separation of Ni from the NiO substrate to

provide oxygen for the formation of Al2O3. In the case of Al/

epi-NiO(700), which shows metallic property even in the

pristine state,13 we found that the crystalline Al electrode

partly encroached on the upper region of the amorphous

layer (Figure 1(d)). This Al intrusion was not observed in the

pristine epi-NiO(500). It is worth mentioning that two dis-

tinctive a-ILs were observed for all samples, before and after

the forming process of the epi-NiO(500), and the pristine

epi-NiO(700).

Figures 2(a) and 2(b) show the EDS line profiles of Al/

a-IL/epi-NiO(500) before and after the forming process,

respectively. A scanning transmission electron microscopy

(STEM) image is inserted in Fig. 2(a). A significant amount

of Ni diffusion into the a-IL was observed (Figure 2(a)). The

intensity of Ni increased at the lower part of the interfacial

layer after the forming process in the Fig. 2(b), indicating a

drift of Ni atoms by the positive bias. Additionally, oxygen

accumulation in the upper a-IL by the bias was also

observed. This variation in the atomic concentration in the

a-IL influenced the crystalline and electronic structure of the

a-IL and affected the electrical properties.

EEL spectra revealed the electronic structure of the a-

IL. Figures 3(a) and 3(b) show Ni L2,3 edges of the Al/a-IL/

epi-NiO film before and after the forming process, respec-

tively. The Ni L2,3 edge of the NiO film (black line) in Fig. 3

shows two major peaks associated with transitions from the

2p1/2 orbitals near 873 eV (L2 edge) and from the 2p3/2 orbi-

tals near 855 eV (L3 edge). The symmetric L3 edge and the

energy-loss interval between L2 and L3 of about 17.5 eV indi-

cate stoichiometric NiO.18 In contrast, the Ni L2,3 edges from

the lower a-IL show a skewed shape for the Ni L3 edge, a

broadening of the sharp L3 edge, and also a lower relative in-

tensity (L3/L2 ratio) compared to that for epi-NiO. All of

these features represent the reduced oxidation states of

NiO1�x, and the shape of the spectra is similar to metallic Ni

edges.19–21 Similar behavior was observed for the nanocrys-

talline structure at the upper part of the interfacial layer.

Therefore, we believe that the metallic nanocrystalline struc-

ture in the a-IL was the origin of the low resistance state in

the a-IL after the forming process. The Ni L2,3 edges of the

lower a-IL, before and after the forming process, were very

similar (Figure 3(b)). This implies that the lower interfacial

layer, both in the pristine state and in the forming state, is

metallic. Note that the lower amorphous layer was composed

of Al, Ni, and O, and the Ni atoms diffused from the NiO

substrate. The EELS showed that the O atoms that were ini-

tially bonded to Ni atoms in the substrate bonded more

strongly to Al atoms in the electrode and left the Ni in the

metallic state. The percolation of these Ni atoms in the lower

amorphous oxide layer made conducting paths for electrons.

In the upper layer, however, the onset energy of the Ni L2,3

of a pristine sample (Figure 3(b)) was shifted to higher

energy, indicating a non-metallic state. These results suggest

that the upper interfacial layer was responsible for the insu-

lating property of the Al/a-IL/epi-NiO(500) in the pristine

sample. This also explains origin of the metallic nature of

Al/a-IL/epi-NiO(700) in the pristine state, i.e., crystalline Al

in the upper a-IL and metallic Ni in the lower a-IL.

The electronic structure was further studied by observ-

ing the Al edges. Figure 4(a) shows the Ni M and Al L spec-

tra of the upper a-IL before and after the forming process.

The Al L2,3 peak shape changed from broad to sharp peaks

after the forming process, as shown in Fig. 4(a), which

reflects the variation of the Al bonding nature in the a-IL.

Peak “a” in Fig. 4(a) denotes Al coordination with oxygen.

FIG. 1. (a) and (b) Conventional and high-resolution TEM images, respec-

tively, of Al/a-IL/epi-NiO(500)/STO substrate of the pristine sample, (c)

high-resolution TEM image of Al/a-IL/epi-NiO(500) of the forming sample,

and (d) high-resolution TEM image of Al/a-IL/epi-NiO(700) of the pristine

sample.

FIG. 2. (a) and (b) EDS line profiles of the pristine and forming samples,

respectively. Annular dark field image of Al/a-IL/epi-NiO/SRO is inserted

in (a), and shown the EDS line and drift control box.

FIG. 3. (a) Ni L2,3 edge from the NiO films and the lower/upper a-IL after

the forming process. Annular dark field image of Al/a-IL/epi-NiO is

inserted. (b) Ni L2,3 from the lower/upper a-IL of the pristine sample.

172101-2 Jang et al. Appl. Phys. Lett. 100, 172101 (2012)

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Octahedral coordination is shown near 78 eV, and tetrahedral

coordination appears near the pre-peak of “a.” A broad “a”

peak indicates that the Al atoms in the upper a-IL part have

both types of bonding, i.e., tetrahedral and octahedral coordi-

nation. Additionally, peak “b” is related to the Al–O bond

length.22 The broader peak “b” in the pristine sample indi-

cates different Al–O bond lengths from that in the forming

sample. The Al L2,3 edge after forming is similar to spinel-

type Al2O3 (Figure 4(a)).23 Moreover, the intensity of the Ni

M edge decreased after the forming process. This is consist-

ent with the EDS results of Fig. 2(a).

In the case of the lower a-IL, the Al L2,3 edge of the pris-

tine sample clearly showed Al–O bonding (Figure 4(b)). In-situ x-ray photoelectron spectroscopy also revealed that

Al–O bonding was present at the initial stage of Al sputtering

on the TiO2 films, at an Al thickness of 3 nm, after which

metallic Al began to be deposited.1 This is consistent with

Al atoms bonded with oxygen atoms that derived from the

epi-NiO during the deposition process. After the forming

process, the peaks of Al L2,3 became sharper. Therefore, Al

atoms, having relatively unstable bonding with O in the

lower a-IL part, form the more stable Al2O3 during the form-

ing process. This stable Al2O3 is similar to that found in the

upper part of the a-IL (Figure 4(a)). Therefore, the Al L2,3

edge indicates that stable Al2O3 forms in the a-IL during the

forming process, while the crystalline Ni–Al phase simulta-

neously forms in the a-IL. This crystalline structure is the or-

igin of the conducting path through the amorphous layer

between the Al and the epi-NiO after the forming process,

which maintains crystalline phase with additional voltage

sweeps. For this reason, the Al/a-IL/epi-NiO sample could

not switch to a high resistance state due to the metallic nano-

crystalline structure in the a-IL.24

In summary, we investigated the amorphous layer in Al/

a-IL/epi-NiO(500) (pristine and forming samples) and pris-

tine Al/a-IL/epi-NiO(700) using microscopic analysis to

determine the electrical properties of the samples. EDS

results indicated that the positive bias voltage on the Al elec-

trode caused drift of Ni and O atoms in the a-IL. A metallic

nanocrystalline structure in the a-IL was observed after the

forming process; EELS confirmed the Ni–Al intermetallic

phase and the metallic Ni character. The metallic nature of

the formed epi-NiO(500) sample could be explained by the

formation of a Ni–Al intermetallic phase, while that of the

pristine epi-NiO(700) sample by Al intrusion in the upper

interface layer. Interestingly, the electronic structures deter-

mined by the Ni L edge indicated that the lower interfacial

layer was metallic in all samples, even in the pristine amor-

phous sample. Al had both octahedral and tetrahedral coordi-

nation in the pristine a-IL, whereas the Al bonding state

became stable Al2O3 bonding during the forming process.

This helped to create permanent Ni–Al intermetallic con-

ducting paths at the interface between the Al and the epi-

NiO.

This work was supported by a grant from the National

Research Foundation of Korea, funded by the Ministry of

Education, Science and Technology (Grant No.

20110016477).

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FIG. 4. (a) Ni M and Al L2,3 edges from the upper a-IL of the pristine and

forming samples; (b) Ni M and Al L2,3 edges from the lower a-IL of the pris-

tine and forming samples.

172101-3 Jang et al. Appl. Phys. Lett. 100, 172101 (2012)

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