DSpace at VNU: Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films

Current Applied Physics 14 (2014) 1707e1712

Contents lists available at ScienceDirect

Current Applied Physics
journal homepage: www.elsevier.com/locate/cap

Correlation between crystallinity and resistive switching behavior of
sputtered WO3 thin films
Thi Bang Tam Dao a, Kim Ngoc Pham a, Yi-Lung Cheng b, Sang Sub Kim c,
Bach Thang Phan a, d, *
a

Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam
Department of Electrical Engineering, National Chi-Nan University, Nan-Tou, Taiwan, ROC
Department of Materials Science and Engineering, Inha University, Republic of Korea
d
Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam
b
c

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 9 June 2014
Received in revised form
13 August 2014
Accepted 10 October 2014
Available online 18 October 2014

The as-deposited WO3 thin films were post-annealed at different temperatures (300  C and 600  C) in air
to investigate a correlation between crystallinity and switching behavior of WO3 thin films. Associating
the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change
contributes to the variation of the switching behaviors of the WO3 thin films. The as-deposited WO3 films
with low crystalline structure are preferred for random Ag conducting path, resulting in large switching
ratio but fluctuating IeV hysteresis, whereas the annealed WO3 films with crystallized compact structure
limits Ag conducting path, favoring the stable IeV hysteresis but small switching ratio. It is therefore
concluded that electrochemical redox reaction-controlled resistance switching depends not only on
electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide.
© 2014 Elsevier B.V. All rights reserved.

Keywords:
Resistive random access memory (ReRAM)
WO3 thin films
Electrochemical redox
Crystallinity
Annealing

1. Introduction
Recent research has demonstrated that resistive random access
memory (ReRAM) is promising candidate for future non-volatile
memories. Oxide-based ReRAM structures exploit the functionality of capacitor structures where the oxide materials, such as
ternary oxides (Cr-doped SrTiO3, Cr-doped SrZrO3, Pr0.7Ca0.3MnO3,
etc.) [1e6], binary oxides (NiO, TiO2, CuOx, HfO2, ZrOx, ZnO, Nb2O5,
Al2O3, WOx) [7e12] are sandwiched between two metal electrodes.
Even though these materials show promising properties, the
involved switching mechanisms are still content of current
research activities. The study of the film structure-order is important in obtaining a clear understanding of its revealed switching
properties. For example, TiO2 has various crystalline phases and

also various resistive switching characteristics have been observed
in the amorphous, anatase, and rutile structures [13e18]. Lee et al.,
also observed that the epitaxial binary oxide NiO shows bipolar
switching while the polycrystalline NiO shows unipolar switching
[19]. Improved crystallinity with increasing ZnO layer thickness

* Corresponding author. Faculty of Materials Science, University of Science,
Vietnam National University, Ho Chi Minh City, Vietnam.
E-mail address: (B.T. Phan).
/>1567-1739/© 2014 Elsevier B.V. All rights reserved.

reduced the number of extended defects, which then reduced the
number of available sites for conduction path formation despite the
increased density of oxygen-related defects contributing to the
path formation, resulting in an increased set voltage in the high
resistive state [20]. Shang et al., reported that bipolar resistive
switching of WO3 thin films can be improved by in situ oxygen
annealing, which is attributed to the decrease in the surface density
states [21]. Syu et al., shows that the resistance switching behavior
of WOx - RRAM devices is unstable because the diverse oxidation
state provided the stochastic conduction paths. By introducing a
silicon element, Si interfusion in WOx resistance switching layer
can effectively localize the filament conduction paths to improve
the resistance switching property [22]. Jang et al., tuned the
switching characteristics by changing the additional oxygen content (d) of the WO3þd oxide. As the value of d varies, the switching
becomes to be unstable [23]. It is therefore suggested that in preparing oxide-based ReRAM devices, especially in device scaling,
careful control of crystallinity would be important. It is known that
WO3 has many polymorphs, depending temperature and preparation conditions (monoclinic, triclinic, orthorhombic, hexagonal and
tetragonal) [24]. The high diversity of physical parameters (e.g.,
crystal structure and density) and chemical parameters (e.g.,

valence state of W ions and composition) makes the research more


1708

T.B.T. Dao et al. / Current Applied Physics 14 (2014) 1707e1712

complex and more interesting for the variation of switching
properties. From this point of view, in this study, we reported a
correlation between crystallinity and switching behavior of sputtered WO3 thin films.
2. Experimental
The WO3 thin films were fabricated using the DC sputtering
technique, from metallic W targets on Pt/Ti/SiO2/Si substrates. The
deposition process of 300-nm-thick WO3 thin films was executed
under the total pressure PArþO2 of 7 Â 10À3 Torr at 300  C, and the
mixture ratio of oxygen to argon gas, PO2/PArþO2, was fixed at 90%.
Before the top electrode deposition, the as-deposited WO3 films
were annealed at various temperatures (300  C and 600  C) in air
for 5 h. During the deposition of the 100-nm-thick top electrode
(Ag) in an argon environment at 7 Â 10À3 Torr, a mask was used for
top electrode patterning. The crystalline phases of the thin films
were characterized in qÀ2q mode by D8 Advance (Bruker) X-ray
diffractometer (XRD) with Cu Ka radiation (l ¼ 0.154 nm) and
Fourier transform infrared spectroscopy (FTIR). The surface morphologies of the films were obtained using scanning electron microscopy (FESEM). X-ray photoelectron spectroscopy (XPS) was
used to investigate the chemical state of the films. Deconvolution of
the XPS spectra included Shirley baseline subtraction was carried
out using the least squares curve fitting program. The profile of the
peaks was taken as a Gaussian function. Currentevoltage (IeV)
measurements were carried out using a semiconductor characterization system (Keithley 4200 SCS) and probe station. The IeV
curves was obtained under voltage sweep mode with the 0 V / e

Vmax / 0 V / þ Vmax / 0 V voltage profile, sweep speed is normal
mode and step voltage is 0.02 V. The read voltage for endurance test
is 0.5 V. The Pt bottom electrode was biased and the Ag top electrode was grounded.
3. Results and discussion
The XRD patterns of both the as-deposited WO3 and the
annealed WO3 thin films are shown in Fig. 1. Representative
diffraction peaks at 2q ¼ 22.87, 23.74 , 24.4 , 26.8 , 29.4 , 34.12 ,
and 49.99 can be clearly identified. The as-deposited WO3 thin
films have two visible peaks, the board peak at 2q ¼ 22.87 and the
other peak at 2q ¼ 29.4 . The 300  C e annealed WO3 thin films
have four peaks at 2q ¼ 23.74 , 24.4 , 29.4 and 34.12 . Among

Fig. 1. XRD of (a) as-deposited WO3 film, (b) 300  C e annealed WO3 film, and (c)
600  C e annealed WO3 film.

those 4 peaks, the intense peak locates at 2q ¼ 24.4 , while the
intensity of peak at 2q ¼ 29.4 decreases. There are six diffraction
peaks, 2q ¼ 23.74 , 24.4 , 26.8 , 29.4 , 34.12 , 49.99 , observed from
the 600  C e annealed WO3 thin films with two intense peaks at
2q ¼ 24.4 , 34.12 . In order to classify the crystal type of those WO3
thin films from the above diffraction peaks, we investigated the
card number LCPDS of Triclinic phase, Monoclinic, Orthohombic
phase, Hexagonal phase, Tetragonal phase. Based on the card
number LCPDS, those mentioned diffraction peaks are characteristic peaks of (002), (020), (200), (120), (112), (220) and (400)
planes of Monoclinic phases. The board and low intensity of (002)
peak around 2q ¼ 22.87, indicating that the as-deposited WO3 thin
film is low crystallinity. In contrast, the XRD patterns of the
annealed WO3 thin films reveal the sharp and intense (200) peak at
24.4 , implying an improvement of the crystalline by the annealing
treatment. It is therefore noted that annealing the as-deposited

WO3 thin films enhance significantly crystallinity.
The FTIR spectra of both the as-deposited WO3 and the annealed
WO3 thin films are shown in Fig. 2. Tungsten oxide film comprises
OeWeO grains or crystals with terminal W]O bonds on their
boundaries. The as-deposited WO3 films consist of 5 bands at
621 cmÀ1, 669 cmÀ1, 730 cmÀ1, 954 cmÀ1 and 1109 cmÀ1. The
annealed WO3 films at 300  C have 4 bands at 621 cmÀ1, 730 cmÀ1,
954 cmÀ1 and 1109 cm-1. When the WO3 films were annealed up to
600  C, 6 bands are found at 621 cmÀ1, 730 cmÀ1, 804 cmÀ1,
866 cmÀ1, 954 cmÀ1 and 1109 cmÀ1. The number of bands shows
that the post-annealing treatment strongly affected the structure of
WO3 thin film.
The bands at 621 cmÀ1 and 730 cmÀ1 exist in all investigated
WO3 thin films. However, these two bands are competitive intensity, the band at 621 cmÀ1 is dominant in the as-deposited WO3
thin films, whereas the band at 730 cmÀ1 become well-defined and
comparative intensity in the 600  C -annealed WO3 thin films. All
the WO3 films show a band around 954 cmÀ1, which has been
assigned to the W6þ ¼ O stretching mode of terminal oxygen atoms
possibly on the surfaces of the cluster or micro-void structures in
the films [25]. The visible board band centered at 669 cmÀ1 is
ascribed to the low crystallinity material [26]. With increasing
annealing temperature, the peak at 669 cmÀ1 disappeares. The
peak at 730 cmÀ1 belongs to the stretching vibration of crystalline
WO3 [26]. The results indicate that the as-deposited WO3 thin films

Fig. 2. FTIR spectroscopy of (a) as-deposited WO3 film, (b) 300  C e annealed WO3
film, and (c) 600  C e annealed WO3 film.


T.B.T. Dao et al. / Current Applied Physics 14 (2014) 1707e1712

1709

are partially crystallized. This result is consistent to the XRD data
with the board (002) peak and the sharp (112) peak. With the
600  C e annealed WO3 thin films, both the sharp peaks at
730 cmÀ1 and 804 cmÀ1 are assigned as WeOeW stretching modes
in WO6 octahedral units and WO4 tetrahedral units, characterizing
the monoclinic phase [25,27e29]. The shorter WeOeW bonds are
responsible for the stretching mode at 804 cmÀ1, whereas the
longer bonds are the source of the 730 cmÀ1 peak. The peak at
866 cmÀ1 may be ascribed to WO3.nH2O [28]. In summary, as the
post-annealing temperature increases, the crystallinity of the film
tends to improve.
Fig. 3 shows superimposed O 1s photoelectrons spectra of both
the as-deposited WO3 and the annealed WO3 thin films. The core
level spectra of O 1s can be deconvoluted into two peaks corresponding to lattice oxygen/stoichiometric WO3 phase (LO, ~ 530 eV)
and non-lattice oxygen/non-stoichiometric WO3Àx phase (NLO,
~ 531 eV) [30e32].
Fig. 4 shows the XPS of the W 4f core level spectrum of both the
as-deposited WO3 and the annealed WO3 thin films. The W 4f7/2
and W 4f5/2 peaks of the W6þ ion were assigned to the peaks at
around 36 eV and 37.9 eV. These peaks coincide with literaturereported W 4f binding energies measured on similar WO3 thin
films [30e36]. In addition, the W 4f spectrum of all thin films
present the clear shoulder at around 34.5 eV, assigned to W5þ.
Among those thin films, only the 600  C e annealed WO3 thin films
have an additional shoulder at lower binding energy (~33.1 eV),
which is assigned to W4þ. The lower valence states of W ions (W5þ
and W4þ) indicate the presence of reduced WO3Àx phase. Since
oxygen vacancies exists in the films, the electronic near its adjacent

W atoms increases, creating a larger screening, which lowers the 4f
level binding energy. The two peaks located at higher binding energies (~39.7 eV and 41.3 eV) are assigned to W5þ and W6þ of W5p3/
2. Since the as-deposited WO3 films were annealed in air, the
annealing process just affected the crystallinity, not stoichiometry
of films. Therefore, oxygen vacancies exist in all the as-deposited
WO3 and the annealed WO3 films.

Fig. 4. XPS spectrum of the W4f core level of (a) as-deposited WO3 film, (b) 300  C e
annealed WO3 film, and (c) 600  C e annealed WO3 film.
Fig. 3. XPS spectrum of the O 1s core level of (a) as-deposited WO3 film, (b) 300  C e
annealed WO3 film, and (c) 600  C e annealed WO3 film.


1710

T.B.T. Dao et al. / Current Applied Physics 14 (2014) 1707e1712

The surface morphology of the WO3 thin films was examined by
FESEM, as shown in Fig. 5. The surface structure of the as-deposited
WO3 thin film is porous with unclear irregularly grains, whereas
both the annealed WO3 thin films are non-porous surface
morphology with clear grains. Meanwhile, the morphology and the
porosity of as-deposited films are greatly affected by the annealing
temperature: the higher the annealing temperature is, the more
visible the grain boundaries are and the compacter the structure is.
According to the XRD, FTIR, XPS and FESEM analyses, the crystallinity was improved as well as the grain become visibly as
annealing temperature increasing.
Fig. 6 shows the IeV characteristics of the as-deposited Ag/WO3/
Pt device and the annealed Ag/WO3/Pt devices. All devices showed
the bipolar resistance switching. It is worthwhile to point out that

no forming process is necessary for activating the switching effect.
Based on the IeV hysteresis, the initial high-resistance state (HRS)
was changed to a low-resistance state (LRS) as a negative bias (0 /

Fig. 6. IeV characteristics of (a) as-deposited WO3 film, (b) 300  C e annealed WO3
film, and (c) 600  C e annealed WO3 film.

Fig. 5. FESEM images of (a) as-deposited WO3 film, (b) 300  C e annealed WO3 film,
and (c) 600  C e annealed WO3 film.

e 1.5 V) applied to the Pt bottom electrode. The device remained in
the LRS for subsequently descending, and the LRS was progressively
changed to the HRS only by a voltage sweep in the positive voltage
region (0 / þ 2 V). Among the investigated WO3 thin films, only
IeV curves of 600  C e annealed WO3 thin films superimpose to


T.B.T. Dao et al. / Current Applied Physics 14 (2014) 1707e1712

each other, which seems to be influenced by its high crystalline
structure.
An endurance test has been carried out at reading voltage of
0.5 V, as shown in Fig. 7. All devices present a clear reversible
switching for over 100 cycles. The value of HRS of WO3 films decreases as increasing the annealing temperature. The lower resistance at higher annealing temperature is the result of the
improvement of crystallinity. It is noted that the annealing treatment strongly affected a switching ratio. As shown in Fig. 8, the
switching ratio of the as-deposited thin film is about 70, but the
switching ratio down to 30 and 6 for the thin films annealed at
300  C and 600  C, respectively. Because the switching ratio is
smaller than 10, the post-annealed thin films at 600  C cannot be
applied for ReRAM, although this structure show the stable

switching over hundreds of cycle. The film annealed at 600  C
shows higher crystallinity but its switching ratio is the lowest,
which may be ascribed to the crystallized compact structure of the
thin film.
Associating the results of XRD, FTIR, XPS and FESEM measurements, obviously, the annealing-caused microstructure change
contributes to the variation of the switching behaviors of the WO3
thin films.
Since the IeV curve of the LRS in log e log scale shows a linear
relationship between current and voltage (not shown here), in
addition to the nature of electrode, a reactive Ag electrode and an
inert Pt electrode along with the switching direction, it is suggested
that switching mechanism in both the as-deposited and annealed
WO3 thin films is controlled by the electrochemical redox reactions
[37,38], which is explained as follows.
By applying a negative voltage at the Pt bottom electrode (a
positive voltage at the Ag top electrode), an electrochemical reaction occurs in the anode (Ag), which oxidizes the Ag metal atoms to
Ag ions, the metal ions Agþ start from the top interface and easily
drift through the as-deposited low crystalline WO3 films to connect
the bottom electrode. At the Pt cathode, an electrochemical
reduction and an electro-crystallization of Ag occur. This electrocrystallization process results in the formation of an Ag filament,
which grow towards the Ag electrode. As a result, the Ag filaments
grow and connect the Ag top electrode, leading to HRS to LRS
switching. To reset the cell, a positive voltage is applied at the Pt
bottom electrode (a negative switching voltage at the Ag top
electrode), which leads to a dissolution of the Ag filament and LRS
to HRS occurs. As mentioned above, the as-deposited WO3 films

1711

Fig. 8. Switching ratio of as-deposited WO3 films and annealed WO3 films (300  C and

600  C).

have more pores in the bulk than those WO3 films annealed at
300  C and 600  C. Therefore, as numerous randomly Ag metallic
path forms, resulting in fluctuating IeV hysteresis. In the postannealed WO3 thin films, the denser structure could not offer
extensive internal volume to conduct ions, the number of Ag conducting path is limited, resulting in stable IeV hysteresis and lower
switching ratio. In comparison, Syu et al., shows that the resistance
switching behavior of WOx e RRAM devices is unstable because the
diverse oxidation state of W ions (W6þ, W5þ, and W4þ) provided
the stochastic W conduction paths [22]. In their study, the WO3 thin
films were sputtered at room temperature and the authors do not
reported the crystalline structure of the WOx thin films. In general,
WO3 thin films prepared at room temperature are amorphous,
resulting in many voids for providing the stochastic conduction
paths. Our as-deposited WO3 thin films have low crystallinity with
only two valence states of W ions (W6þ, W5þ) show the fluctuating
switching, which is consistent to Syu's report [22]. However, our
600  C e annealed WO3 thin films have high crystallinity with
many valence states of W ions (W6þ, W5þ, and W4þ) show the
stable switching behavior (Figs. 7 and 8) or the stochastic Ag conduction paths are limited. It is suggested that the stochastic Ag
conduction paths are also controlled by the crystalline structure.
4. Conclusions
The as-deposited WO3 thin films were post-annealed at
different temperatures (300 and 600  C) in air to investigate the
effects of crystallinity on switching behaviors of the films. The asdeposited WO3 films are monoclinic phase with low crystallinity.
Annealing the films up to 600  C improve the crystallization. The
resistance switching mechanism is the Ag filament paths mediated
by electrochemical redox reactions, in which the Ag conducting
formation is influenced by the crystalline structure. The electrochemical redox reaction depends on crystalline structure of WO3
thin films. The as-deposited WO3 with low crystalline structure are

preferred for large switching ratio but fluctuating IeV hysteresis,
whereas the annealed WO3 with crystallized compact structure
favor the stable IeV hysteresis but small switching ratio.
Acknowledgment

Fig. 7. Endurance of (a) as-deposited WO3 film, (b) 300  C e annealed WO3 film, and
(c) 600  C e annealed WO3 film.

This work is financially supported by Vietnam National University in HoChiMinh City under Grant B2013-18-02.


1712

T.B.T. Dao et al. / Current Applied Physics 14 (2014) 1707e1712

References
[1] A. Beck, J.U. Bednorz, C. Gerber, Ch Rossel, D. Windmer, Appl. Phys. Lett. 77
(2000) 139.
[2] R. Waser, M. Aono, Nat. Mater. 6 (2007) 833.
[3] B.T. Phan, J. Lee, Appl. Phys. Lett. 93 (2008) 222906.
[4] B.T. Phan, J. Lee, Appl. Phys. Lett. 94 (2009) 232102.
[5] B.T. Phan, N.C. Kim, J. Lee, J. Kor. Phys. Soc. 54 (2009) 873.
[6] B.T. Phan, Taekjib Choi, A. Romanenko, Jaichan Lee, Solid-State Electron. 75
(2012) 43.
[7] B.J. Choi, D.S. Jeong, S.K. Kim, S. Choi, J.H. Oh, C. Rohde, H.J. Kim, C.S. Hwang,
K. Szot, R. Waser, B. Reichenberg, S. Tiedke, J. Appl. Phys. 98 (2005) 033715.
[8] K. Jung, H. Seo, Y. Kim, H. Im, J.P. Hong, J.W. Park, J.K. Lee, Appl. Phys. Lett. 90
(2007) 052104.
[9] A. Chen, S. Haddad, Y.C. Wu, Z. Lan, T.N. Fang, S. Kaza, Appl. Phys. Lett. 91
(2007) 123517.

[10] C.Y. Lin, C.Y. Wu, C. Hu, T.Y. Tseng, J. Electrochem. Soc. 154 (2007) G189.
[11] T. Le, H.C.S. Tran, V.H. Le, T. Tran, C.V. Tran, T.T. Vo, M.C. Dang, S.S. Kim, J. Lee,
B.T. Phan, J. Korean Phys. Soc. 60 (2012) 1087.
[12] J.B. Park, K.P. Biju, S.J. Jung, W.T. Lee, J.M. Lee, S.H. Kim, S.S. Park, J.H. Shin,
H.S. Hwang, IEEE Electron Device Lett. 32 (2011) 476.
[13] R. Dong, D.S. Lee, M.B. Pyun, M. Hasan, H.J. Choi, M.S. Jo, D.J. Seong, M. Chang,
S.H. Heo, J.M. Lee, Appl. Phys. A 93 (2008) 409.
[14] W.G. Kim, S.W. Rhee, Microelectron. Eng 86 (2009) 2153.
[15] S. Won, S. Go, K. Lee, J. Lee, Electron. Mater. Lett. 4 (2008) 29.
[16] H.Y. Jeong, J.Y. Lee, S.-Y. Choi, Appl. Phys. Lett. 97 (2010) 042109.
[17] M.H. Lee, K.M. Kim, G.H. Kim, J.Y. Seok, S.J. Song, J.H. Yoon, C.S. Hwang, Appl.
Phys. Lett. 96 (2010) 152909.
[18] K.P. Biju, X. Liu, E.M. Bourim, I. Kim, S. Jung, J. Park, H.S. Hwang, Electrochem.
Solid-State Lett. 13 (2010) H443.
[19] S.R. Lee, K. Char, D.C. Kim, R. Jung, S. Seo, X.S. Li, G.-S. Park, I.K. Yoo, Appl. Phys.
Lett. 91 (2007) 202115.

[20] Y.H. Kang, J.-H. Choi, T.I. Lee, W. Lee, J.-M. Myoung, Solid State Comm. 151
(2011) 1739.
[21] D.S. Shang, L. Shi, J.R. Sun, B.G. Shen, F. Zhuge, R.W. Li, Y.G. Zhao, Appl. Phys.
Lett. 96 (2010) 072103.
[22] Y.E. Syu, T.C. Chang, T.M. Tsa, G.W. Chang, K.C. Chang, Y.H. Tai, M.J. Tsai,
Y.L. Wang, S.M. Sze, Appl. Phys. Lett. 100 (2012) 022904.
[23] B.U. Jang, A.I. Inamdar, J.M. Kim, W. Jung, H.S. Im, H.S. Kim, J.P. Hong, Thin
Solid Films 520 (2012) 5451.
[24] T. Vogt, P.M. Woodward, B.A. Hunter, J. Solid State Chem. 144 (1999) 209.
[25] M.F. Daniel, B. Desbat, J.C. Lassegues, J. Solid State Chem. 67 (1987) 235.
[26] A. Cremonesi, D. Bersani, P.P. Lottici, Y. Djaoued, P.V. Ashrit, J. Non-Crystalline
Solids 345&346 (2004) 500.
[27] S.-H. Lee, M.J. Seong, H.M. Cheong, E. Ozkan, E.C. Tracy, S.K. Deb, Solid State

Ionics 156 (2003) 447.
[28] M. Gotic, M. Ivanda, S. Popovic, S. Music, Mat. Sci. Eng B77 (2000) 193.
[29] J. Gabrusenoks, A. Veispals, A. vonCzarnowski, K.-H. Meiwes Broer, Electrochim. Acta 46 (2001) 2229.
[30] K. Senthil, K. Yong, Nanotechnology 18 (2007) 395604.
[31] H.Y. Wong, C.W. Ong, R.W.M. Kwok, K.W. Wong, S.P. Wong, W.Y. Cheung, Thin
Solid Films 376 (2000) 131.
[32] G. Leftheriotis, S. Papaefthimiou, P. Yianoulis, A. Siokou, D. Kefalas, Appl. Surf.
Sci. 218 (2003) 275.
[33] C. Bigey, L. Hilaire, G. Maire, J. Catal. 184 (1999) 406.
[34] H. Wang, P. Xu, T. Wang, Mater. Des. 23 (2002) 331.
[35] A. Romanyuk, P. Oelhafen, Sol. Energy Mater. 90 (2006) 1945.
[36] T.D.L. Arcos, S. Cwik, A.P. Milanov, V. Gwildies, H. Parala, T. Wagner, A. Birkner,
D. Rogalla, H.eW. Becker, J. Winter, A. Ludwig, R.A. Fischer, A. Devi, Thin Solid
Films 522 (2012) 11.
[37] R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 21 (2009) 2632.
[38] D.S. Jeong, R. Thomas, R. Katiyar, J.F. Scott, H. Kohlstedt, A. Petraru, C.S. Hwang,
Rep. Prog. Phys. 75 (2012) 076502.