Improved thermal stability of C-doped Sb2Te films by ...

Thin Solid Films 615 (2016) 345–350

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Improved thermal stability of C-doped Sb2Te films by increasing degree of disorder for memory application Guoxiang Wang a,b,⁎, Xiang Shen a,b, Qiuhua Nie a,b, Hui Wang a,b, Yegang Lu a,b, Daotian Shi a,b a b

Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province,Ningbo 315211, China

a r t i c l e

i n f o

Article history: Received 29 December 2015 Received in revised form 16 June 2016 Accepted 25 July 2016 Available online 26 July 2016 Keywords: Data storage materials Thin films Phase transitions Thermal stability Optical properties

a b s t r a c t The structural stability of carbon (C) incorporated Sb2Te films was investigated during crystallization process. Variations in the transition temperature for the as-deposited films during crystallization show that these films exhibit their enhanced amorphous stability due to C incorporation, while more C content will lead to a difference in the degree of disorder in the crystalline state. XPS data reveals that C atoms do not bond with Sb and Te atoms and only present in the form ofCCbonds. According to XRD and TEM results, C atoms presents amorphous and this can increase the degree of disorder in the crystalline films. The Sb2Te nanocrystals were surrounded by an amorphous C phase. A subsequent Raman analysis further provides the direct evidence of improvement in the degree of disorder in the crystalline state. The laser-induced crystallization process of C37.4(Sb2Te)62.6 reveals that the degree of disorder in the crystalline state is relatively high and the reliability during the repetitive laser meltquenching cycles is confirmed with fast crystallization as well as a low melting point of only 353 °C. Increasing degree of disorder in the Sb2Te films by C addition can improve the phase-change behavior and make this film suitable for data storage applications. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In the optical storage, rewritable optical disks, including compact disc (CD), digital versatile disks (DVD), blue-ray disks (BD), have been developed in the early stage of commercial memory devices [1,2]. In the electrical storage, the electrical phase change memory (PCM), which was thought to be one of the most promising candidates for the next generation nonvolatile memory because of its excellent properties, such as nonvolatility, high density and compatibility with complementary metal-oxide semiconductor (CMOS) process [3–5]. Among various materials available for optical disks or PCM, Te-based chalcogenide thin films are promising for data-storage applications because of their reversible phase transition between amorphous and crystalline states. For example, previous studies have shown that the Ag-In-Sb-Te alloys [6] are the most commonly utilized phase-change media in optical DVD-RAM, and they could also be applied for electrical PCM. Another family, the ternary alloy Ge2Sb2Te5 (GST) has also been widely studied due to its outstanding optical or electrical performance [7]. However, the issues, such as high RESET current, poor thermal stability and long operating time, remain to be solved in these materials. Therefore further optimization of the materials properties is still a main subject of the research. It is well known that, low crystallization temperature (~144 °C) and high melting temperature (~545 °C) in Sb2Te material usually leads to stability problems for the applications at high temperatures and high power consumption, respectively [8]. However, Sb2Te exhibits higher

http://dx.doi.org/10.1016/j.tsf.2016.07.059 0040-6090/© 2016 Elsevier B.V. All rights reserved.

crystallization speed than GST due to its growth-dominated crystallization mechanism [8]. Here, we propose low-conductive carbon (C) doped Sb2Te chalcogenide alloys as possible candidate to optimize the crystallization characteristic and properties of Sb2Te based on three reasons. Firstly, there are some examples of RESET current reduction when the impurities are introduced in a phase-change host, like Tidoped Sb2Te [9], Si-doped Sb2Te [10,11]. This effect could be due to the fact that low-conductive inclusions replace part of the programming volume and minimize the heat loss in the phase-change layer [12], and/or since the doping impurities increase the dynamic electrical resistance of the chalcogenide material [13]. Secondly, doping impurities could also improve thermal stability, as shown in Cu-doped Sb2Te [14] and in Zn-doped Sb2Te [15]. The benefit of doping for thermal stability could be justified by the fact that the dopants, arranged in a disordered configuration inside the phase-change material, could pile up at the grain boundaries, suppressing the growth of the crystalline grains [16]. Finally, considering that CVD could be most widely used tool to fabricate PCM devices, carbon dopants can be easily incorporated from precursors and coreactants during the deposition. Thus, it is essential to understand how carbon doping can modify the properties of the material. Previously, C-doping effect has been shown in couples of works such as: C-In3Sb1Te2 PCM [17] and C-GeTe PCM [18], but focusing on the effect of C-doped on pure Sb2Te phase change material has not been done before. Here we report a systematic investigation on the effect of C-doping on the structure, electrical, thermal and optical

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G. Wang et al. / Thin Solid Films 615 (2016) 345–350

Sheet resistance (Ω/

)

7 (a) 10

5

10

Sb2Te C16.3(Sb2Te)83.7 C27.6(Sb2Te)72.4

3

10

C37.4(Sb2Te)62.6 C44.1(Sb2Te)55.9

0

100 200 Temperature (OC)

300

diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectra and Transmission electron microscopy (TEM). The optical band gap (Eopt) was also evaluated using the absorption properties, which were expressed as α(v) ⋅ hv = B(hv − Eopt)2 [19], where α(v) is absorption coefficient, hv is the energy of incident photon and B is a parameter that depends on the electronic transition probability. A static laser tester (PST-1, NANOSstorage Co. Ltd., Korea) with a wavelength of 658 nm was used to characterize crystallization behavior. The change of the laser power ranged from 5 to 70 mW, and that of the laser pulse width was from 5 to 250 ns. The relationship between optical reflectivity and temperature changes under laser irradiation was monitored in real time by Optical Power Analyzer (OPA-1200) and the melting temperature was carefully determined. 3. Results and discussion

(b) Time to failure (Sec.)

10

10 years data retention C16.3(Sb2Te)83.7

8

C27.6(Sb2Te)72.4 10

C37.4(Sb2Te)62.6

6

4

10

2

5.5 9

10

eV 5.4 6 4. eV 0 3. 4eV 79 eV

C44.1(Sb2Te)55.9

120.6OC 101.3OC

127.1OC 10

91.9OC

0

24

26

28

30

32

-1

1/kBT(eV )

400

-1

300

Sb2Te C16.3(Sb2Te)83.7

(a)

Fig. 1. (a) Sheet resistance as a function of temperature for undoped and C-doped Sb2Te films; (b) The Arrhenius extrapolation at 10-yr of data retention for undoped and Cdoped Sb2Te films; (c) Plots of (αhv)1/2 vs. hv for the as-deposited undoped and Cdoped Sb2Te films.

C16.3(Sb2Te)83.7 C27.6(Sb2Te)72.4 C37.4(Sb2Te)62.6 C44.1(Sb2Te)55.9

10 performances of Sb2Te films. This study may be useful for understanding crystallization behavior of the phase change materials.

(b)

Pure Sb2Te and C-doped Sb2Te films with a thickness of 200 nm were deposited on quartz and SiO2/Si (100) substrates by magnetron cosputtering method using separated C and Sb2Te alloy targets. In each run of the experiment, the chamber was evacuated to 1.2 × 10−4 Pa and then Ar gas was introduced to 0.3 Pa for the film deposition. The concentration of C dopant in the C-doped Sb2Te films was measured using energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The sheet resistances of as-deposited films as a function of elevated temperature (non-isothermal) and time at specific temperatures (isothermal) were in situ measured using a four-point probe in a homemade vacuum chamber. The structure of as-deposited and annealed C-doped Sb2Te thin films was examined by X-ray

Sb2Te

Intensity (a.u.)

2. Experimental

20

30 40 2θ (degree)

50

60

(114)

1.4

(023)

1.2

(016) (110)

1.0 hv (eV)

(103)

V 0e

4e 0.4

0 .5

0.8

Sb2Te

C37.4(Sb2Te)62.6

(004) (005)

0.6

V

C27.6(Sb2Te)72.4 0.2 8e V 0. 30 eV

200

Intensity (a.u.)

(αhv)1/2 (cm eV)1/2

(c)

To study the real-time crystallization behavior of C-doped Sb2Te films, we measured the change in sheet resistance (Rs) as a function of temperature for various compositions with a heating rate fixed of 40 °C/min and the results were shown in Fig. 1(a). The Rs of the films was observed to increase with C content in the entire annealing process. This was ascribed to the increase in the degree of disorder in the material. It has been well documented that, the increasing disorder in the film tends to turn the delocalized electronic states into localized states, leading to stronger electron scattering thus higher resistance [20,21]. On the other hand, the Rs of the films decreased with increasing of temperature, and exhibited a quick drop at the crystallization temperature (Tc). Based on the results in Fig. 1(a), the Tc values for pure Sb2Te, C16.3(Sb2Te)83.7, C27.6(Sb2Te)72.4, C37.4(Sb2Te)62.6 and C44.1(Sb2Te)55.9 films were determined to be ~144, ~155, ~164, ~173 and ~175 °C, respectively. It is obvious that Tc of the films increases with C-doping concentration increasing and all Tc values are higher than that of GST (~150 °C) reported in the literatures [1,4,5], indicating an outstanding amorphous stability of the films. Furthermore, the abrupt decrease of Rs for pure Sb2Te, C16.3(Sb2Te)83.7, C27.6(Sb2Te)72.4 and C37.4(Sb2Te)62.6 is in contrast with a rather gradual profile of Rs for C44.1(Sb2Te)55.9

Sb2Te

C16.3(Sb2Te)83.7

C27.6(Sb2Te)72.4 C37.4(Sb2Te)62.6 C44.1(Sb2Te)55.9

10

20

30 40 2θ (degree)

50

60

Fig. 2. XRD patterns of undoped and C-doped Sb2Te films: (a) as-deposited and (b) 200 °C.


films, implying that more C content will lead to a difference in the degree of disorder in the crystalline state. The crystallization process was investigated by measuring isothermal changes of the film resistance in situ at various temperatures. The maximum temperature for the 10-year data retention period can be extrapolated by fitting the data with the Arrhenius equation t = τexp(Ea/ kBT), where τ, Ea, and kB are the proportional time coefficient, crystalline activation energy, and Boltzmann's constant, respectively. The failure time (t) is defined as the time when the film resistance reaches half of its initial value at a specific isothermal temperature T. The value of data retention as shown in Fig. 1(b) increases monotonously from 91.9 °C to 127.1 °C with C-doping content increasing, which are higher that Sb2Te (52 °C) [22] and GST (89 °C) [22]. Among them, the data retention temperature for 10 years (T10year) was 120.6 °C for the C37.4(Sb2Te)62.6 film with a larger activation energy of 5.46 eV, which

347

is sufficient for potential applications in automotive electronics (at least 10 years at 120 °C).19 Fig. 1(c) shows the plots of (αhv)1/2 vs. hv for as-deposited Sb2Te and C-doped Sb2Te films. By extrapolating the linear portion of the curves to zero absorption, the indirect optical band gaps are determined to be from 0.28 to 0.5 eV. It is found that, the optical band gap of the film increases with increasing C content. The increased optical band gap may be explained by the increase of randomness in atomic configuration of the material [23]. The carbon addition into Sb2Te film presents in the form of amorphous content which has been confirmed in XRD and TEM. More C-doping will exhibit more wrong-bonds (or heteropolar bonds), which can form the localized states in amorphous solids and thus increase the disorder level, modifying the optical band gap of the amorphous films [24].

Fig. 3. (a) Bright-field TEM, (b) SAED and (c) HRTEM image of Sb2Te film annealed at 200 °C for 2 min; (d) Bright-field TEM and (e) HRTEM image of C37.4(Sb2Te)62.6 film annealed at 200 °C for 2 min. Inset is electron diffraction pattern of amorphous region.


C27.6(Sb2Te)72.4

Intensity (a.u.)

C37.4(Sb2Te)62.6

275 (b)

280 285 290 Binding energy (eV)

Sb 3p

295

C27.6(Sb2Te)72.4 C37.4(Sb2Te)62.6

Intensity (a.u.)

Sb2Te

760 780 Binding energy (eV) (c)

Te 3d

800

C27.6(Sb2Te)72.4 C37.4(Sb2Te)62.6

Intensity (a.u.)

Sb2Te

570

575 580 Binding energy (eV)

585

Fig. 4. XPS spectra for 200 °C-annealed Sb2Te and C-doped Sb2Te films: (a) C1 s, (b) Sb 3p, and (c) Te 3d.

We measured XRD patterns of the films in order to understand the effect of C-doping on the crystalline structure, and the results were shown in Fig. 2. The broad peaks in the XRD patterns of the asdeposited C-doped Sb2Te films correspond to the amorphous phase as shown in Fig. 2(a). In Fig. 2(b), the Sb2Te film is completely crystallized into a single crystalline Sb2Te with a hexagonal structure at the annealing temperature to 200 °C. C16.3(Sb2Te)83.7 film annealed at 200 °C exhibits the similar the diffraction peaks as the Sb2Te film, and no other crystalline or C precipitation can be observed. As the content of C increases, the diffraction peaks (004) (005) (114) are suppressed gradually in the C27.6(Sb2Te)72.4, C37.4(Sb2Te)62.6 and C44.1(Sb2Te)55.9 films. The intensities of the diffraction peaks generally decrease with increasing carbon content in Fig. 2(b), suggesting that the size of the crystalline grains have been significantly suppressed by the carbon doping. Fig. 3 shows a bright-field TEM, selected-area electron diffraction (SAED) pattern and high-resolution TEM (HRTEM) images of Sb2Te and C37.4(Sb2Te)62.6 films after 2 min heating at 200 °C. We can see that the nanocrystals were observed for the annealed Sb2Te film in Fig. 3(a) with a grain size in the range of 20–30 nm. Through indexing the SAED pattern in Fig. 3(b), the crystallized Sb2Te film can be assigned to Sb2Te phase which is not changed in the C37.4(Sb2Te)62.6 film. As revealed in the XRD result, the nano-crystals in the annealed

C37.4(Sb2Te)62.6 film were still confirmed to be Sb2Te and grain size has been reduced to 10–20 nm in Fig. 3(d). In HRTEM images as shown in Fig. 3(c) and (e), clear atomic arrangement can be observed and only Sb2Te crystal phase can be found. The Sb2Te nanocrystals were surrounded by an amorphous phase which could be also confirmed in electron diffraction pattern as shown in the inset of Fig. 3(e). The analysis of EDS (attached to TEM) indicates that the amorphous regions are enriched in C. This implies that the C atoms exist at grain boundaries and suppress the grain growth. In order to further confirm the present form of C atoms in the crystalline state, we adopted the XPS spectra to investigate the binding state of C-doped Sb2Te films and the results were shown in Fig. 4(a)–(c). It is obvious that the peak positions of C1 s for C27.6(Sb2Te)72.4 and C37.4(Sb2Te)62.6 locate at 284.8 eV, which is the same as CC binding energy [25] and almost keep no change with increasing C concentration as shown in Fig. 4(a), indicating that C atoms may be not bond with Sb and Te atoms. This point can be also confirmed further in Fig. 4(b) and (c). The peak positions of Sb 3p and Te 3d exhibit no shift in binding energy after C doping. This indicates that the Sb and Te atoms in Sb Te bonds are not replaced by C atoms. It reveals that the C atoms only present in the form of CC bonds. Due to no C precipitation in XRD and TEM results, C atoms presents amorphous and this can increase the degree of disorder in the crystalline film. Fig. 5 shows Raman spectra of undoped and C-doped Sb2Te thin films. Spectral responses of amorphous samples (see Fig. 5(a)) present a characteristic peak around 141 cm−1, which can be ascribed to the vibration of amorphous Sb Te bonds [26]. The Raman spectra of the C-doped Sb2Te films compared with that of pure Sb2Te film suggests that adding carbon leads to a higher degree of disorder in the amorphous phase of the material, featuring a broader Raman peak at 141 cm−1. Raman spectra of C-doped Sb2Te films annealed at 200 °C as shown in Fig. 5(b) shows different structural characteristics assigned to crystallized materials. Amorphous Sb Te and crystalline Sb Te bonds are marked in the figure as “a” and “c”, respectively. For 200 °C-annealed Sb2Te films, three crystalline peaks ascribed to crystalline Sb Te bonds are found, indicating that Sb2Te has sufficiently crystallized at 200 °C. The

(a)

-1

Sb2Te

141cm

C16.3(Sb2Te)83.7

Normalized intensity

(a) C 1s

C27.6(Sb2Te)72.4 C37.4(Sb2Te)62.6

100 (b) Normalized intensity

348

200 300 Raman shift (cm-1)

c a+c c a+c a+c c c c c c c

100

400

Sb2Te C16.3(Sb2Te)83.7 C27.6(Sb2Te)72.4 C37.4(Sb2Te)62.6

200 300 Raman shift (cm-1)

400

Fig. 5. Raman spectra of (a) amorphous and (b) 200 °C-annealed undoped and C-doped Sb2Te films.


C16.3(Sb2Te)83.7 film exhibits similar Raman peaks as pure Sb2Te film, which is in well agreement with the XRD observations, revealing that they have quite similar microstructures. With further increasing carbon content, the Raman peaks become broader and weaker in C27.6(Sb2Te)72.4 and C37.4(Sb2Te)62.6 films. The broadened Raman peaks are due to the increasing disorder induced by carbon doping, which would lead to strong localization of these vibrational modes. Moreover, the increasing disorder level would affect Sb and Te local environment, decrease the intensity of the crystalline peaks as shown in Fig. 5(b). It can be explained that C atoms may precipitate to the grain boundaries and stay in the amorphous phase, suppressing Sb2Te grain growth as evidenced by XRD, TEM and XPS. Thus, amorphous Sb Te bonds will remain in the district around the C atoms. Obviously, the 155 cm−1 peak dramatically disappears with increasing C content for C37.4(Sb2Te)62.6 film. This implied that the crystallization degree of the films is reduced with the addition of C.

(a) 70 0.078

III

50

0.068

ly c ryst alliz ed

0.058

40

plet e

30 20

I

II

10 50

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0.047 0.037 0.027 0.017 0.0067

Com

Laser power (mW)

60

-0.0035

250

Pulse width (ns) (b)

0.06

Optical contrast

0.04 Crystalline state

0.02 Optical switching

0.00 -0.02

Amorphous state -0.04 -0.06 0

10

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Number of cycles 1.0

Phase change

Crystalline

0.8 0.01 0.00

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dR/dT

Normalized reflectivity

(c)

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0.2

0

100

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Tm

353oC

300

400

Temperature (OC) 0.0

0

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Amorphous

400

Temperature ( C) O

Fig. 6. (a) Power-Time-Effect (PTE) diagram of C37.4(Sb2Te)62.6 film, (b) The optical switching behavior during 50 cycles of operation on the C37.4(Sb2Te)62.6 film, (c) The relationship between optical reflectivity power and temperature changes of C37.4(Sb2Te)62.6 film and the inset is the corresponding differential curve.

349

One of the main features in the phase change material is that a change in the optical reflectance can be induced by laser irradiation [27]. To examine whether C37.4(Sb2Te)62.6 film possess such properties, we measured a Power-Time-Effect (PTE) diagram as shown in Fig. 6(a), which showed the change in reflectance as functions of applied laser power and pulse width. The measurements were performed using a static tester with a wavelength of 650 nm, a numerical aperture of 0.6 and a focused beam size of around 1 μm. In zone I, the laser power and/or the pulse width was found to be insufficient to cause a phase change, so the change in the crystallization degree was negligible. However, in zone II, with higher power and/or longer pulse width, the crystallization was clearly observed. The film was completely crystallized as shown in red region and the crystallization degree reaches a maximum value of 0.068. Subsequently, as the laser power increases further, the film began to be into amorphization and the crystallization degree decreases in zone III. For the conventional GST film, the crystallization is essentially complete after 280 ns at the laser power of 70 mW with the maximum crystallization degree of 0.26 [28]. While for C37.4(Sb2Te)62.6 film, the film is completely crystallized at a lower power of 40 mW and shorter pulse width of 250 ns (250 ns @ 40 mW) with the maximum crystallization degree of 0.068, indicating that the degree of disorder in the crystalline state is relatively high. Our previous work [29] has revealed that the GST film undergoes a reversible phase transition between amorphous and crystalline states. To confirm that C37.4(Sb2Te)62.6 could be switched reversibly between the amorphous and crystalline states over many cycles. Fig. 6(b) shows that a reliable optical switching in C37.4(Sb2Te)62.6 film was maintained during 50 cycles of operation. It was found that a reversible repetitive optical switching behavior could be realized in the film with proper crystallization (250 ns @ 40 mW) and amorphization (100 ns @ 70 mW) condition. But, it is observed that optical constants in the Fig. 6(b) are not matched with Fig. 6(a) data. The reason is that the onset state is the melt-quenched amorphous materials by laser quenching during cyclic REEST & SET operation in Fig. 6(b). There is no nucleation required as the melt-quenched material typical surrounded by crystalline material. Furthermore, the melt-quenched material also contains tiny crystal nuclei in the amorphous region that are formed during the quenching process that will act as initiating clusters for the crystallization process. Thus, the melt-quenched amorphous C37.4(Sb2Te)62.6 film for optical switching test can have much shorter crystallization and amorphization times at the same laser power compared to the as-deposited amorphous film. So the optical contrast value is also relative small. As for the C37.4(Sb2Te)62.6 film, only when the crystallized film is heated above its melting temperature (Tm), the amorphization process will occur and drive it to the amorphous state. In order to obtain the melting temperature, the reflectivity curve of the C37.4(Sb2Te)62.6 varying with annealing temperature was shown in Fig. 6(c). It is found that the cliffy decreased in reflectivity corresponds to the crystallineto-amorphous phase transformation. According to the change in the reflectivity curve, the phase change temperature from crystalline to amorphous can be observed clearly. The differential curve versus temperature could be redrawn after determining the temperature region as shown in inset of Fig. 6(c) and we can obtain clearly the melting temperature about 353 °C, which is smaller than Sb2Te (~545 °C),8 and conventional GST (~620 °C) [30]. The lower melting temperature contributes a lot to reduction of the power. In the film, the amorphous Crich domain boundaries with higher resistance could serve as microheaters and produce much Joule heat under RESET pluses to melt the enclosed crystalline phase, meaning that less power consumption is required for the RESET operation. In fact, the similar mechanisms have been discussed in Si-Sb-Te materials, where the presence of amorphous domain boundaries is found to reduce the RESET current [31]. Noteworthy, the reflectivity is re-increased beyond 400 °C (amorphous melting region) as shown in the Fig. 6(c), since phase change materials experience ablation. The amorphization can be easy to realize due to its

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low melting temperature, while evaporation may be occurred with the increase of temperature, thus reflectivity will be influenced by element segregation or Si substrate. 4. Conclusions The present work demonstrates that C-doped Sb2Te films exhibit higher crystallization temperature, and thus superior thermal stability than pure Sb2Te film. The optical band gap of amorphous films increases from 0.28 to 0.5 eV with the C addition. Raman analysis reveals that the addition of carbon can increase the disorder level of the amorphous material, which can widen optical band gap and suppress the growth of crystalline grains, resulting in an enhancement in electrical resistance and crystallization temperature of the phase change films. A laserinduced crystallization reveals that the C37.4(Sb2Te)62.6 film has a high degree of disorder in the crystalline state and exhibits a low crystalline degree of 0.068. Under proper crystallization (250 ns @ 40 mW) and amorphization (100 ns @ 70 mW) condition, the film can realize the reversible phase transformation and its melting point is only 353 °C. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 61306147), the Natural Science Foundation of Zhejiang Province, China (Grant No. LQ15F040002), the Public Project of Zhejiang Province (Grant No. 2014C31146), the Zhejiang Open Foundation of the Most Important Subjects (Grant No. xkx11532), and the Young Leaders of the academic climbing project of the Education Department of Zhejiang Province (Grant No. pd2013092), and was sponsored by the K. C. Wong Magna Fund at Ningbo University. References [1] T. Ohta, K. Nishiuchi, K. Narumi, Y. Kitaoka, H. Ishibashi, N. Yamada, T. Kozaki, Overview and the future of phase-change optical disk technology, Jpn. J. Appl. Phys. 39 (2000) 770–774. [2] M. Wuttig, Phase change materials: the importance of resonance bonding, Phys. Status Solidi 8 (2009) 1820–1825. [3] H.F. Hamann, M. O'Boyle, Y.C. Martin, M. Rooks, H.K. Wickramasinghe, Ultra-highdensity phase-change storage and memory, Nat. Mater. 5 (2006) 383–387. [4] M. Zhu, M.J. Xia, F. Rao, X.B. Li, L.C. Wu, X.L. Ji, S.L. Lv, Z.T. Song, S.L. Feng, H.B. Sun, S.B. Zhang, One order of magnitude faster phase change at reduced power in Ti-Sb-Te, Nat. Commun. 5 (2014) 4086–4090. [5] M. Wuttig, D. Lusebrink, D. Wamwangi, W. Welnic, M. Gilleen, R. Dronskowski, The role of vacancies and local distortions in the design of new phase-change materials, Nat. Mater. 6 (2007) 122–128. [6] M. Wuttig, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824–832. [7] S. Guo, Z.G. Hu, X.L. Ji, T. Huang, X.L. Zhang, L.C. Wu, Z.T. Song, J.H. Chu, Temperature and concentration dependent crystallization behavior of Ge2Sb2Te5 phase change films: tungsten doping effects, RSC Adv. 4 (2014) 57218–57222. [8] M.H.R. Lankhorst, B.W.S.M.M. Ketelaars, R.A.M. Wolters, Low-cost and nanoscale non-volatile memory concept for future silicon chips, Nat. Mater. 4 (2005) 347–352.

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