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Changes in electrical and structural properties of phase-change Ge-Sb-Te films by Zr addition
Journal of Non-Crystalline Solids 452 (2016) 9–13
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Changes in electrical and structural properties of phase-change Ge-Sb-Te films by Zr addition Zengguang Li a,b, Yegang Lu a,b,⁎, Yadong Ma a,b, Sannian Song c, Xiang Shen b,d, Guoxiang Wang b,d, Shixun Dai b,d, Zhitang Song c a
Faculty of Electrical Engineering and Computer Science, Ningbo University Zhejiang, 315211, China Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province,Ningbo 315211, China State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China d Laboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Zhejiang 315211, China b c
a r t i c l e
i n f o
Article history: Received 1 June 2016 Received in revised form 30 July 2016 Accepted 3 August 2016 Available online xxxx Keywords: Phase transformation Crystallization Thermal stability Band gap
a b s t r a c t In this paper, the effect of Zr on the phase change properties of Ge2Sb2Te5 (GST) is systemically studied for phasechange random access memory. The sheet resistance ratio between amorphous and crystalline states achieves four to five orders of magnitude. The crystalline resistance, crystallization temperature (Tc) and the 10 years data-retention of Zr-GST films increase with the Zr concentration. Zr-GST films are crystallized into a single phase without phase separation due to the Zr bonding with Sb and Te. With the increasing annealing temperature, the transformation from face-centered cubic (fcc) to hexagonal is suppressed when the Zr atomic content is higher than 6%, which is ascribed to the lack formation of the Te-Te pairs. The wide band gap of the amorphous Zr-GST films is favorable to reduce the threshold current. The incorporating Zr atoms are embedded in the inner atomic-scale structure of the GST, which contributes to performance improvement of the GST material for phasechange random access memory. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phase-change random access memory (PCRAM) is considered to be one of the most feasible candidates for the next-generation nonvolatile memory due to its advantages such as high density, high speed, low power consumption, and good thermal stability [1–3]. PCRAM is based on the reversible phase change between amorphous (RESET) and crystalline (SET) of chalcogenide materials induced by Joule heating to store information [4]. Chalcogenide materials have attracted much attention as a new advanced and replaceable technology material since their phase change behavior plays a key role in the practical operation of PCRAM. Among various chalcogenide phase change materials, Ge2Sb2Te5 (GST) has been regarded as the most promising alternative for PCRAM owing to its good trade-off between crystallization speed and thermal stability [5]. However, there are several issues that need to be solved. Firstly, its relatively low crystallization temperature (~160 °C) leads to an unsatisfied thermal stability with 10-years dataretention temperature of ~85 °C [6,7], and thus poses the problem associated with thermal crosstalk of the materials [8]. The low 10-years data-retention temperature can't meet the demand for the automobile ⁎ Corresponding author at: Faculty of Electrical Engineering and Computer Science, Ningbo University, Zhejiang 315211, China. E-mail address: [email protected] (Y. Lu).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.08.006 0022-3093/© 2016 Elsevier B.V. All rights reserved.
electronics (at least 120 °C). Secondly, high melting temperature (~ 620 °C) and low crystalline resistivity cause a high RESET current [9], which leads to a high power consumption. And the high RESET current has become the urgent issue to be resolved for high-density onchip integration, since drive capability of the transistor or diode is also scaled down. In recent years, many efforts have been made to address these issues. One of the effective ways is to dope a small amounts of the elements into GST such as Sn [10], N [11], Zn [12], O [13] and GaSb [14]. The addition of these elements into GST can improve the thermal stability and crystalline resistivity, but doping may bring phase separation which could lead to performance degradation in PCRAM devices [15]. It is reported that Zr-Sb-Te would form a single phase and Zr atoms bond with the surrounding Sb and Te atoms well in the crystal structure [16]. And the bonded Zr atoms may serve as substitutional impurities which replace some Sb and Te atoms in the crystal lattice, which would certainly improve the thermal properties of Zr-Sb2Te film [17]. These results motivate us to clarify whether Zr would be an alternative dopant for improving the properties of GST alloy. And what the role of Zr in structure transition and phase-change properties of GST should be addressed to assess the Zr-GST for PCRAM. In this work, we experimentally investigated the effects of Zr dopants on the thermal stability, crystallization behavior, structural, optical and electrical properties of Zr-GST phase change material.
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2. Experimental Zr-GST films were deposited on SiO2/Si(100) and quartz substrates by the magnetron co-sputtering method using separate Zr and GST alloy targets of 50 mm in diameter. The base and working pressures were prior to 2.0 × 10−4 Pa and 0.3 Pa, respectively. The RF power on the GST target was fixed to be 60 W, while the dc power on the Zr target was tuned in a range from 3 to 9 W in order to obtain the different Zr doping content. The composition of the as-deposited film was measured by energy dispersive spectroscopy (EDS). The samples in this study can be denoted by Zr4(GST)96, Zr6(GST)94, Zr9(GST)91 and Zr12(GST)88. The structures of the as-deposited and annealed thin films were examined by the X-ray diffraction (XRD) (Bruker D2 PHASER diffractometer). The sheet resistance of the as-deposited film as a function of the elevated temperature (non-isothermal) was in situ measured using a fourpoint probe in a homemade vacuum chamber. The optical transmittance (Top) in the spectral range 300–2500 nm was obtained by a Perkin-Elmer Lambda 950 UV-VIS-NIR spectrophotometer. The absorption coefficient (α) was calculated using the general relation α= −(1/ d) ln (Top), where d is the thickness of the film measured by the surface profiler. The optical band gap (Eopt) was evaluated using the absorption properties, which were expressed as (αhv)1/2 = B1/2(hv − Eopt), where hν is the energy of the incident photon and B is a parameter that depends on the electronic transition probability. Raman scattering spectroscopy was recorded at room temperature using a backscattering configuration. An Ar ion laser with a wavelength of 785 nm was used as an excitation source. The number of scans is 10 and the acquisition time is 1 s. The power density on the sample was kept at low levels (∼0.2 mW μm−2) in order to avoid any structural deformation induced by laser radiation. The chemical bonding states of samples were confirmed by x-ray photoelectron spectroscopy (XPS). In order to exclude the influence of the film surface, XPS was performed after Ar+ ion etching (4 kV) for 3 min. 3. Results and discussion Fig. 1(a) shows the temperature dependence of the sheet resistance Rs at a heating rate of 50 K min−1. With the increasing temperature, a continuous decrease in resistance is observed for all the films, which could be explained by the thermally activated hopping transport mechanism [18]. When the temperature reaches their respective crystallization temperature (Tc), phase transition from amorphous to crystalline phase occurs, which leads to an abrupt drop in resistance, as shown in Fig. 1(a). The obtained Tc increases with increasing Zr concentration, being 175 ± 2, 185 ± 2, 200 ± 2 and 210 ± 2 °C for Zr4(GST)96, Zr6(GST)94, Zr9(GST)91, and Zr12(GST)88 films, respectively, much higher than that of GST(~ 160 °C). In the inset of Fig. 1(a), we found that Tc increases almost linearly with Zr concentration in a range from 0 to 12 at.%. The addition of Zr brings a larger number of extra bonds into the GST due to Zr bonding with Sb and Te [17], and it causes a
disturbance in the crystallization behavior of the whole system. Thus, the Tc of Zr-GST films increase with the concentration of Zr. The increase in crystallization temperature could contribute to improve the thermal stability of the material. It is known that the crystallization speed could be estimated from the steepness of decrease in sheet resistance. The decrease in sheet resistance around the crystallization temperature becomes flatter with the increasing concentration of Zr. It indicates that the addition of Zr could slow down the crystallization process of GST material. It is noted that a second drop of the resistance is found in the GST and Zr4(GST)96 films, owing to the transformation from face-centered-cubic (fcc) to hexagonal phase. And the transition is suppressed when the Zr concentration is over 6%. It provides a clear evidence for increased transition temperatures and thermal stability. For all compositions, a resistance contrast of four to five orders of magnitude between the amorphous and crystalline state is helpful in achieving a high ON/OFF ratio [19]. In addition, the crystalline resistance increases with the concentration of Zr, which is favorable to the low power consumption for Zr-GST thin film. The thermal stability of the film can be estimated by data retention which is judged by the time-dependent isothermal change in resistance. Fig. 1(b) shows the data retention characteristics for Zr-GST films. The failure time (t) is defined as the time when the resistance decreases to half of its initial value at a specific temperature. The maximum temperature for 10-years data retention can be extrapolated by fitting the data according to an Arrhenius equation t = τ exp (Ea/KBT), where τ, Ea, and KB are a proportional time constant, activation energy for crystallization, and the Boltzmann constant, respectively. The data retention temperature for 10 years of the amorphous Zr4(GST)96, Zr6(GST)94, Zr9(GST)91, and Zr12(GST)88 films are estimated to be about 92.8 ± 0.1, 100.4 ± 0.1, 113.0 ± 0.1 and 120.3 ± 0.1 °C, respectively, which are much higher than that of GST(82.1 ± 0.1 °C), suggesting the significant improvement in thermal stability of the GST by the Zr addition. Therefore, PCRAM based on Zr-GST could store the information far longer time than pure GST. Fig. 2(a) shows the XRD patterns of the Zr9(GST)91 films at different annealing temperatures. No diffraction peak was observed for the asdeposited Zr9(GST)91 film even annealed at 150 °C, indicating the amorphous nature in the as-deposited state. The characteristic diffraction peaks appear at 200 °C, suggesting the crystallization of the film. The Tc of the Zr9(GST)91 film is between 150 and 200 °C, which is consistent with the results of R-T. The crystal structure can be identified as the fcc phase even the film annealed at 350 °C. Fig. 2(b)–(d) shows the XRD patterns of the Zr-GST films annealed at 200, 300, and 350 °C, respectively. The peaks of (200) and (220) from fcc phase are observed for all the studied samples except the peak of (220) for the Zr12(GST)88 film at 200 °C, as shown in Fig. 2(b). The peak intensity of these alloys decreases with the increase of Zr indicating the increasing disorder degree in the alloys. According to Scherrer's equation [20], the mean crystallite size is inversely proportional to the full width at half maximum (FWHM) of the diffraction peak. It is shown that the FWHM of the peaks (200 and 220) increases with the concentration of Zr, suggesting
Fig. 1. (a) Sheet resistance as a function of temperature. (b) The Arrhenius extrapolation of 10-years data retention for GST and Zr-GST films.
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Fig. 2. (a) XRD pattern of Zr9(GST)91 film at different annealing temperature. XRD patterns of Zr-GST films annealed at (b) 200, (c) 300, and (d) 350 °C.
that the Zr addition suppresses the grain growth. With the annealing temperature increasing to 300 °C, the Zr12(GST)88 films crystallized into fcc phase, as shown in Fig. 2(c). It indicates that the onset transition temperature from amorphous to fcc phases increases gradually with increasing Zr concentration. It is interesting that the phase transition from fcc to hexagonal phases occurs for Zr4(GST)96 film with the annealing temperature increasing to 350 °C, while the crystal structure of the other samples are still kept in fcc phase, as shown in Fig. 2(d). It is suggested that Zr atoms can serve as a center for suppression of the fcc to hexagonal phase transition for the GST film with higher Zr concentration, leading to a one-step crystallization process. These results indicate that GST film with higher Zr-doping concentration has a higher crystallization temperature, as well as higher thermal stability in fcc phase. It is well known that phase separation will cause device failure, and this phenomenon has been observed in the N-GST and In-doped GeSbTe [11,21,22]. Crystalline Zr-GST film shows a single phase without phase separation, which will improve cycle life of the PCRAM. Fig. 3 shows the plots of (α(v)hv)1/2 versus hv for the amorphous and crystalline Zr9(GST)91 films annealed at 200 and 300 °C. The inset figure is the corresponding Vis\\IR transmission spectra. The transmittance decreases with the increase of annealing temperature. The optical band gap of amorphous Zr9(GST)91 film (0.76 eV) is higher than that of GST (0.63 eV) [23]. The widening band gap of the Z-GST film in the amorphous phase is importance for the application in PCRAM [24]. A suitable voltage is expected to trigger phase transition from amorphous to crystalline states of the material. The carriers fill the trap states in the amorphous phase at the appropriate voltage, which results in the formation of conductive path, and consequently leads to the desired phase transition [25]. The presence of such trap states requires a large band gap in the amorphous phase [23]. When the Zr9(GST)91 film is crystallized from amorphous to fcc phase (200 °C), the band gap decreases from 0.76 to 0.51 eV. With the annealing temperature further increasing to 300 °C, it reduces to 0.45 eV. The decrease in optical band gap corresponds directly to a decrease in activation energy for electrical conduction and better conductivity [26]. B1/2 refers slope in the extended region and it is the measure of disorder [27]. A higher value of B1/2 indicates less disorder. B1/2 increases with annealing temperature
indicating a decrease in disorder, which due to the change of long range order to short range order of the crystalline phase. The vibration modes of bonds were analyzed using Raman spectroscopy. Fig. 4(a)–(d) show the Raman spectra of the as-deposited, 150, 200, and 350 °C annealed Zr-GST films, respectively. The Raman spectra are dominated by a broad band covering the region 100–190 cm−1 for all the as-deposited films, as shown in Fig. 4(a). Two broad peaks overlap with each other in this region with one peak at 125 cm−1 (peak A) and the other at 150 cm−1 (peak B). Peak A is associated with the A1 mode of GeTe4-nGen (n = 1,2) tetrahedral modes [28], and peak B is ascribed to Sb\\Te bonds' vibrations in the SbTe3 units [29]. In addition, a weak broad Raman band located at 220 cm−1 (peak C) is assigned to the F2 mode of GeTe4 tetrahedra [30]. In the low-frequency region of the spectra, a peak at 56 cm−1 (peak D) can be observed, which is related to the E mode of GeTe4 tetrahedra [30]. All Zr-GST films exhibit similar Raman spectra in terms of peak position. However, the intensity of the peak at 125 cm− 1 decreases with increasing Zr content, suggesting that the addition of Zr leads to a higher degree of disorder in the amorphous phase of the material. In fact, the higher the disorder level of the
Fig. 3. Plots of (αhν1/2) vs. hν for the Zr9(GST)91 films. Inset figure is the corresponding Vis\ \IR transmission spectra.
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Fig. 4. Raman spectra of the GST and Zr-GST films: (a) as-deposited, (b) 150, (c) 200, and (d) 350 °C.
Zr-GST amorphous state, the higher the energy required to arrange it in an ordered crystalline form [31]. The Raman spectra of as-deposited and 150 °C-annealed Zr-GST films show similar shape, suggesting the amorphous nature. For the samples annealed at 200 °C, the Raman spectra still maintain broadening nature, while the peaks at 125 cm−1 (peak A) and 220 cm−1 (peak C) disappear, as shown in Fig. 4(c), due to the annealing-induced crystallization. In contrast, a new peak at 105 cm−1 (peak E) appears, which is related to the A1 mode of GeTe4 corner-sharing tetrahedral [32]. The results reveal that the GeTe component of GST alloys is mainly responsible for the phase transition [32]. At 350 °C, peak E at 105 cm−1 remains almost unchanged. Peak B at 150 cm−1 disappears and a new sharp peak F at ∼ 172 cm−1 appears for both the GST and Zr4(GST)96 films, as shown in Fig. 4(d), which is ascribed to the phase transformation from fcc to hexagonal phases. Peak F may be assigned to Te-Te pairs, similar to glasses in the Ge-Se-Te system [33]. The new peak F cannot be observed in the Zr-GST films with Zr concentration larger than 6 at.% due to the suppression of the fcc-tohexagonal phase transformation. Since adequate Zr atoms replace some Sb and Te atoms in the crystal lattice, the Zr\\Sb and Zr\\Te bonds exist in abundance for the Zr-GST films with Zr concentration higher than 6 at.%. As a result, the formation of Te-Te pairs suffers from obstacles to some extent, and thus the fcc-to-hexagonal phase transformation is suppressed. XPS spectra were further applied to investigate the binding state of Zr9(GST)91, as shown in Fig. 5. The binding energy scale was calibrated
using the value of 284.8 eV of the C1s core level. The banding energy of Zr 3d5/2 and 3d3/2 for Zr9(GST)91 are about 182.7 and 185.2 eV respectively, higher than those for pure Zr (178.7, 179.9 eV) [34], suggesting that Zr atoms bond with other elements. The peak positions of Sb 3d in Fig. 5(b) shift to lower binding energy compared with GST. Similar phenomenon can be observed in Te 3d spectra of the Zr9(GST)91 and GST films, as shown in Fig. 5(c). Since the electronegativity of Zr (1.33) is smaller than that of Sb (2.05) and Te (2.10), the decrease of binding energy implies that Zr atoms bond with both Sb and Te atoms. Similar results were reported in Zr-Sb-Te alloy [16]. Therefore, the incorporating Zr atoms are embedded in the inner atomic-scale structure of GST which plays an import role in structure transition and phase-change properties of Zr-GST. 4. Conclusions In summary, the crystallization behavior, structural, optical and electrical properties of the Zr-GST films were investigated systematically. The crystalline resistance, crystallization temperature and 10-years data-retention temperature of the Zr-GST films, which are higher than those of the GST, increase with increasing Zr concentration. The ZrGST films are crystallized into single fcc phase without phase separation, which help to enhance endurance of the PCRAM. The hexagonal structure can be suppressed for Zr-GST with high Zr content due to the formation inhabitation of the Te-Te pairs. Zr9(GST)91 film shows higher
Fig. 5. XPS spectra for 350 °C-annealed GST and Zr9(GST)91 films: (a) Zr 3d, (b) Sb 3d, and (c) Te 3d.
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band gap in comparison with GST, which is favorable to enhance the switching properties. All these advantages enable Zr-GST films to be a promising candidate for PCRAM. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 61306147, 61377061), the Science and Technology Public Project of Zhejiang Province (Grant No. 2016C31077), and Ningbo Municipal Natural Science Foundation of China (Grant No. 2014A610121), and sponsored by K. C. Wong Magna Fund in Ningbo University. References [1] A.V. Kolobov, P. Fons, A.I. Frenkel, A.L. Ankudinov, J. Tominaga, T. Uruga, Understanding the phase-change mechanism of rewritable optical media, Nat. Mater. (2004) 703–708. [2] K. Tanaka, S. Iizima, M. Sugi, Y. Okada, M. Kikuchi, Thermal effect on switching phenomenon in chalcogenide amorphous semiconductors, Solid State Commun. 8 (1970) 387–389. [3] M. Wuttig, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824–832. [4] H.P. Wong, S. Raoux, S. Kim, J. Liang, J.P. Reifenberg, B. Rajendran, M. Asheghi, K.E. Goodson, Phase change memory, Proc. IEEE 98 (2010) 2201–2227. [5] S. Raoux, W. Wełnic, D. Lelmini, Phase change materials and their application to nonvolatile memories, Chem. Rev. 110 (2010) 240–267. [6] M. Zhu, L. Wu, F. Rao, Z. Song, C. Peng, X. Li, D. Yao, W. Xi, S. Feng, Phase change characteristics of SiO2 doped Sb2Te3 materials for phase change memory application, Electrochem. Solid-State Lett. 14 (2011) H404–H407. [7] W. Czubatyj, S.J. Hudgens, C. Dennison, C. Schell, T. Lowrey, Nanocomposite phasechange memory alloys for very high temperature data retention, IEEE Electron Device Lett. 31 (2010) 869–871. [8] R.E. Simpson, M. Krbal, P. Fons, A.V. Kolobov, J. Tominaga, T. Uruga, H. Tanida, Toward the ultimate limit of phase change in Ge2Sb2Te5, Nano Lett. 10 (2010) 414–419. [9] S.W. Ryu, H.K. Lyeo, J.H. Lee, Y.B. Ahn, G.H. Kim, C.H. Kim, S.G. Kim, S.H. Lee, K.Y. Kim, J.H. Kim, W. Kim, C.S. Hwang, H.J. Kim, SiO2 doped Ge2Sb2Te5 thin films with high thermal efficiency for applications in phase change random access memory, Nanotechnology 22 (2011). [10] G. Singh, A. Kaura, M. Mukul, S.K. Tripathi, Electrical, optical, and thermal properties of Sn-doped phase change material Ge2Sb2Te5, J. Mater. Sci. 48 (2013) 299–303. [11] K.B. Borisenko, Y. Chen, S.A. Song, D.J.H. Cockayne, Nanoscale phase separation and building blocks of Ge2Sb2Te5N and Ge2Sb2Te5N2 thin films, Chem. Mater. 21 (2009) 5244–5251. [12] G. Wang, Q. Nie, X. Shen, R.P. Wang, L. Wu, J. Fu, T. Xu, S. Dai, Phase change behaviors of Zn-doped Ge2Sb2Te5 films, Appl. Phys. Lett. 101 (2012). [13] S. Privitera, E. Rimini, R. Zonca, Amorphous-to-crystal transition of nitrogen- and oxygen-doped Ge2Sb2Te5 films studied by in situ resistance measurements, Appl. Phys. Lett. 85 (2004) 3044–3046. [14] Y. Lu, Z. Zhang, S. Song, X. Shen, G. Wang, L. Cheng, S. Dai, Z. Song, Performance improvement of Ge-Sb-Te material by GaSb doping for phase change memory, Appl. Phys. Lett. 102 (2013).
13
[15] S.W. Nam, C. Kim, M.H. Kwon, H.S. Lee, J.S. Wi, D. Lee, T.Y. Lee, Y. Khang, K.B. Kim, Phase separation behavior of Ge(2)Sb(2)Te(5) line structure during electrical stress biasing, Appl. Phys. Lett. 92 (2008). [16] N. Soheilnia, K.M. Kleinke, H. Kleinke, Crystal structure, electronic structure, and physical properties of two new antimonide-tellurides: ZrSbTe and HfSbTe, Chem. Mater. 19 (2007) 1482–1488. [17] Y. Zheng, Y. Cheng, M. Zhu, X. Ji, Q. Wang, S. Song, Z. Song, W. Liu, S. Feng, A candidate Zr-doped Sb2Te alloy for phase change memory application, Appl. Phys. Lett. 108 (2016) 210. [18] D. Ielmini, Y.G. Zhang, Evidence for trap-limited transport in the subthreshold conduction regime of chalcogenide glasses, Appl. Phys. Lett. 90 (2007). [19] C. Wang, S. Li, J. Zhai, B. Shen, M. Sun, T. Lai, Rapid crystallization of SiO2/Sb80Te20 nanocomposite multilayer films for phase-change memory applications, Scr. Mater. 64 (2011) 645–648. [20] Y. Yin, H. Sone, S. Hosaka, Dependences of electrical properties of thin GeSbTe and AgInSbTe films on annealing, Jpn. J. Appl. Phys. 44 (2005) 6208. [21] C. Kim, D. Kang, T.-Y. Lee, K.H.P. Kim, Y.-S. Kang, J. Lee, S.-W. Nam, K.-B. Kim, Y. Khang, Direct evidence of phase separation in Ge2Sb2Te5 in phase change memory devices, Appl. Phys. Lett. 94 (2009). [22] S.J. Park, M.H. Jang, S.-J. Park, M.-H. Cho, D.-H. Ko, Characteristics of phase transition and separation in a In–Ge–Sb–Te system, Appl. Surf. Sci. 258 (2012) 9786–9791. [23] E.M. Vinod, K. Ramesh, K.S. Sangunni, Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys, Sci. Rep. 5 (2015). [24] W. Welnic, A. Pamungkas, R. Detemple, C. Steimer, S. Blugel, M. Wuttig, Unravelling the interplay of local structure and physical properties in phase-change materials, Nat. Mater. 5 (2006) 56–62. [25] 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. [26] C. Das, M.G. Mahesha, G. Mohan Rao, S. Asokan, Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films, Appl. Phys. A Mater. Sci. Process. 106 (2012) 989–994. [27] E.M. Vinod, R. Naik, A.P.A. Faiyas, R. Ganesan, K.S. Sangunni, Temperature dependent optical constants of amorphous Ge2Sb2Te5 thin films, J. Non-Cryst. Solids 356 (2010) 2172–2174. [28] P. Nemec, A. Moreac, V. Nazabal, M. Pavlista, J. Prikryl, M. Frumar, Ge–Sb–Te thin films deposited by pulsed laser: an ellipsometry and Raman scattering spectroscopy study, J. Appl. Phys. 106 (2009). [29] I. Watanabe, S. Noguchi, T. Shimizu, Study on local structure in amorphous SbS films by Raman scattering, J. Non-Cryst. Solids 58 (1983) 35–40. [30] K.S. Andrikopoulos, S.N. Yannopoulos, G.A. Voyiatzis, A.V. Kolobov, M. Ribes, J. Tominaga, Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition, J. Phys. Condens. Matter 18 (2006) 965–979. [31] G.B. Beneventi, L. Perniola, V. Sousa, E. Gourvest, S. Maitrejean, J.C. Bastien, A. Bastard, B. Hyot, A. Fargeix, C. Jahan, J.F. Nodin, A. Persico, A. Fantini, D. Blachier, A. Toffoli, S. Loubriat, A. Roule, S. Lhostis, H. Feldis, G. Reimbold, T. Billon, B. De Salvo, L. Larcher, P. Pavan, D. Bensahel, P. Mazoyer, R. Annunziata, P. Zuliani, F. Boulanger, Carbon-doped GeTe: a promising material for phase-change memories, Solid State Electron. 65-66 (2011) 197–204. [32] A.N. Kolobov, P. Fons, J. Tominaga, Phase-change optical recording: past, present, future, Thin Solid Films 515 (2007) 7534–7537. [33] A.H. Moharram, M.A. Hefni, A.M. Abdel-Baset, Short and intermediate range order of Ge20Se80 − xTex glasses, J. Appl. Phys. 108 (2010). [34] J.F. Moulder, J. Chastain, R.C. King, Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data, Chem. Phys. Lett. 99 (1963) 7–10.
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Changes in electrical and structural properties of phase-change Ge-Sb-Te films by Zr addition Zengguang Li a,b, Yegang Lu a,b,⁎, Yadong Ma a,b, Sannian Song c, Xiang Shen b,d, Guoxiang Wang b,d, Shixun Dai b,d, Zhitang Song c a
Faculty of Electrical Engineering and Computer Science, Ningbo University Zhejiang, 315211, China Key Laboratory of Photoelectric Materials and Devices of Zhejiang Province,Ningbo 315211, China State Key Laboratory of Functional Materials for Informatics, Laboratory of Nanotechnology, Shanghai Institute of Micro-system and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China d Laboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Zhejiang 315211, China b c
a r t i c l e
i n f o
Article history: Received 1 June 2016 Received in revised form 30 July 2016 Accepted 3 August 2016 Available online xxxx Keywords: Phase transformation Crystallization Thermal stability Band gap
a b s t r a c t In this paper, the effect of Zr on the phase change properties of Ge2Sb2Te5 (GST) is systemically studied for phasechange random access memory. The sheet resistance ratio between amorphous and crystalline states achieves four to five orders of magnitude. The crystalline resistance, crystallization temperature (Tc) and the 10 years data-retention of Zr-GST films increase with the Zr concentration. Zr-GST films are crystallized into a single phase without phase separation due to the Zr bonding with Sb and Te. With the increasing annealing temperature, the transformation from face-centered cubic (fcc) to hexagonal is suppressed when the Zr atomic content is higher than 6%, which is ascribed to the lack formation of the Te-Te pairs. The wide band gap of the amorphous Zr-GST films is favorable to reduce the threshold current. The incorporating Zr atoms are embedded in the inner atomic-scale structure of the GST, which contributes to performance improvement of the GST material for phasechange random access memory. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phase-change random access memory (PCRAM) is considered to be one of the most feasible candidates for the next-generation nonvolatile memory due to its advantages such as high density, high speed, low power consumption, and good thermal stability [1–3]. PCRAM is based on the reversible phase change between amorphous (RESET) and crystalline (SET) of chalcogenide materials induced by Joule heating to store information [4]. Chalcogenide materials have attracted much attention as a new advanced and replaceable technology material since their phase change behavior plays a key role in the practical operation of PCRAM. Among various chalcogenide phase change materials, Ge2Sb2Te5 (GST) has been regarded as the most promising alternative for PCRAM owing to its good trade-off between crystallization speed and thermal stability [5]. However, there are several issues that need to be solved. Firstly, its relatively low crystallization temperature (~160 °C) leads to an unsatisfied thermal stability with 10-years dataretention temperature of ~85 °C [6,7], and thus poses the problem associated with thermal crosstalk of the materials [8]. The low 10-years data-retention temperature can't meet the demand for the automobile ⁎ Corresponding author at: Faculty of Electrical Engineering and Computer Science, Ningbo University, Zhejiang 315211, China. E-mail address: [email protected] (Y. Lu).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.08.006 0022-3093/© 2016 Elsevier B.V. All rights reserved.
electronics (at least 120 °C). Secondly, high melting temperature (~ 620 °C) and low crystalline resistivity cause a high RESET current [9], which leads to a high power consumption. And the high RESET current has become the urgent issue to be resolved for high-density onchip integration, since drive capability of the transistor or diode is also scaled down. In recent years, many efforts have been made to address these issues. One of the effective ways is to dope a small amounts of the elements into GST such as Sn [10], N [11], Zn [12], O [13] and GaSb [14]. The addition of these elements into GST can improve the thermal stability and crystalline resistivity, but doping may bring phase separation which could lead to performance degradation in PCRAM devices [15]. It is reported that Zr-Sb-Te would form a single phase and Zr atoms bond with the surrounding Sb and Te atoms well in the crystal structure [16]. And the bonded Zr atoms may serve as substitutional impurities which replace some Sb and Te atoms in the crystal lattice, which would certainly improve the thermal properties of Zr-Sb2Te film [17]. These results motivate us to clarify whether Zr would be an alternative dopant for improving the properties of GST alloy. And what the role of Zr in structure transition and phase-change properties of GST should be addressed to assess the Zr-GST for PCRAM. In this work, we experimentally investigated the effects of Zr dopants on the thermal stability, crystallization behavior, structural, optical and electrical properties of Zr-GST phase change material.
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2. Experimental Zr-GST films were deposited on SiO2/Si(100) and quartz substrates by the magnetron co-sputtering method using separate Zr and GST alloy targets of 50 mm in diameter. The base and working pressures were prior to 2.0 × 10−4 Pa and 0.3 Pa, respectively. The RF power on the GST target was fixed to be 60 W, while the dc power on the Zr target was tuned in a range from 3 to 9 W in order to obtain the different Zr doping content. The composition of the as-deposited film was measured by energy dispersive spectroscopy (EDS). The samples in this study can be denoted by Zr4(GST)96, Zr6(GST)94, Zr9(GST)91 and Zr12(GST)88. The structures of the as-deposited and annealed thin films were examined by the X-ray diffraction (XRD) (Bruker D2 PHASER diffractometer). The sheet resistance of the as-deposited film as a function of the elevated temperature (non-isothermal) was in situ measured using a fourpoint probe in a homemade vacuum chamber. The optical transmittance (Top) in the spectral range 300–2500 nm was obtained by a Perkin-Elmer Lambda 950 UV-VIS-NIR spectrophotometer. The absorption coefficient (α) was calculated using the general relation α= −(1/ d) ln (Top), where d is the thickness of the film measured by the surface profiler. The optical band gap (Eopt) was evaluated using the absorption properties, which were expressed as (αhv)1/2 = B1/2(hv − Eopt), where hν is the energy of the incident photon and B is a parameter that depends on the electronic transition probability. Raman scattering spectroscopy was recorded at room temperature using a backscattering configuration. An Ar ion laser with a wavelength of 785 nm was used as an excitation source. The number of scans is 10 and the acquisition time is 1 s. The power density on the sample was kept at low levels (∼0.2 mW μm−2) in order to avoid any structural deformation induced by laser radiation. The chemical bonding states of samples were confirmed by x-ray photoelectron spectroscopy (XPS). In order to exclude the influence of the film surface, XPS was performed after Ar+ ion etching (4 kV) for 3 min. 3. Results and discussion Fig. 1(a) shows the temperature dependence of the sheet resistance Rs at a heating rate of 50 K min−1. With the increasing temperature, a continuous decrease in resistance is observed for all the films, which could be explained by the thermally activated hopping transport mechanism [18]. When the temperature reaches their respective crystallization temperature (Tc), phase transition from amorphous to crystalline phase occurs, which leads to an abrupt drop in resistance, as shown in Fig. 1(a). The obtained Tc increases with increasing Zr concentration, being 175 ± 2, 185 ± 2, 200 ± 2 and 210 ± 2 °C for Zr4(GST)96, Zr6(GST)94, Zr9(GST)91, and Zr12(GST)88 films, respectively, much higher than that of GST(~ 160 °C). In the inset of Fig. 1(a), we found that Tc increases almost linearly with Zr concentration in a range from 0 to 12 at.%. The addition of Zr brings a larger number of extra bonds into the GST due to Zr bonding with Sb and Te [17], and it causes a
disturbance in the crystallization behavior of the whole system. Thus, the Tc of Zr-GST films increase with the concentration of Zr. The increase in crystallization temperature could contribute to improve the thermal stability of the material. It is known that the crystallization speed could be estimated from the steepness of decrease in sheet resistance. The decrease in sheet resistance around the crystallization temperature becomes flatter with the increasing concentration of Zr. It indicates that the addition of Zr could slow down the crystallization process of GST material. It is noted that a second drop of the resistance is found in the GST and Zr4(GST)96 films, owing to the transformation from face-centered-cubic (fcc) to hexagonal phase. And the transition is suppressed when the Zr concentration is over 6%. It provides a clear evidence for increased transition temperatures and thermal stability. For all compositions, a resistance contrast of four to five orders of magnitude between the amorphous and crystalline state is helpful in achieving a high ON/OFF ratio [19]. In addition, the crystalline resistance increases with the concentration of Zr, which is favorable to the low power consumption for Zr-GST thin film. The thermal stability of the film can be estimated by data retention which is judged by the time-dependent isothermal change in resistance. Fig. 1(b) shows the data retention characteristics for Zr-GST films. The failure time (t) is defined as the time when the resistance decreases to half of its initial value at a specific temperature. The maximum temperature for 10-years data retention can be extrapolated by fitting the data according to an Arrhenius equation t = τ exp (Ea/KBT), where τ, Ea, and KB are a proportional time constant, activation energy for crystallization, and the Boltzmann constant, respectively. The data retention temperature for 10 years of the amorphous Zr4(GST)96, Zr6(GST)94, Zr9(GST)91, and Zr12(GST)88 films are estimated to be about 92.8 ± 0.1, 100.4 ± 0.1, 113.0 ± 0.1 and 120.3 ± 0.1 °C, respectively, which are much higher than that of GST(82.1 ± 0.1 °C), suggesting the significant improvement in thermal stability of the GST by the Zr addition. Therefore, PCRAM based on Zr-GST could store the information far longer time than pure GST. Fig. 2(a) shows the XRD patterns of the Zr9(GST)91 films at different annealing temperatures. No diffraction peak was observed for the asdeposited Zr9(GST)91 film even annealed at 150 °C, indicating the amorphous nature in the as-deposited state. The characteristic diffraction peaks appear at 200 °C, suggesting the crystallization of the film. The Tc of the Zr9(GST)91 film is between 150 and 200 °C, which is consistent with the results of R-T. The crystal structure can be identified as the fcc phase even the film annealed at 350 °C. Fig. 2(b)–(d) shows the XRD patterns of the Zr-GST films annealed at 200, 300, and 350 °C, respectively. The peaks of (200) and (220) from fcc phase are observed for all the studied samples except the peak of (220) for the Zr12(GST)88 film at 200 °C, as shown in Fig. 2(b). The peak intensity of these alloys decreases with the increase of Zr indicating the increasing disorder degree in the alloys. According to Scherrer's equation [20], the mean crystallite size is inversely proportional to the full width at half maximum (FWHM) of the diffraction peak. It is shown that the FWHM of the peaks (200 and 220) increases with the concentration of Zr, suggesting
Fig. 1. (a) Sheet resistance as a function of temperature. (b) The Arrhenius extrapolation of 10-years data retention for GST and Zr-GST films.
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Fig. 2. (a) XRD pattern of Zr9(GST)91 film at different annealing temperature. XRD patterns of Zr-GST films annealed at (b) 200, (c) 300, and (d) 350 °C.
that the Zr addition suppresses the grain growth. With the annealing temperature increasing to 300 °C, the Zr12(GST)88 films crystallized into fcc phase, as shown in Fig. 2(c). It indicates that the onset transition temperature from amorphous to fcc phases increases gradually with increasing Zr concentration. It is interesting that the phase transition from fcc to hexagonal phases occurs for Zr4(GST)96 film with the annealing temperature increasing to 350 °C, while the crystal structure of the other samples are still kept in fcc phase, as shown in Fig. 2(d). It is suggested that Zr atoms can serve as a center for suppression of the fcc to hexagonal phase transition for the GST film with higher Zr concentration, leading to a one-step crystallization process. These results indicate that GST film with higher Zr-doping concentration has a higher crystallization temperature, as well as higher thermal stability in fcc phase. It is well known that phase separation will cause device failure, and this phenomenon has been observed in the N-GST and In-doped GeSbTe [11,21,22]. Crystalline Zr-GST film shows a single phase without phase separation, which will improve cycle life of the PCRAM. Fig. 3 shows the plots of (α(v)hv)1/2 versus hv for the amorphous and crystalline Zr9(GST)91 films annealed at 200 and 300 °C. The inset figure is the corresponding Vis\\IR transmission spectra. The transmittance decreases with the increase of annealing temperature. The optical band gap of amorphous Zr9(GST)91 film (0.76 eV) is higher than that of GST (0.63 eV) [23]. The widening band gap of the Z-GST film in the amorphous phase is importance for the application in PCRAM [24]. A suitable voltage is expected to trigger phase transition from amorphous to crystalline states of the material. The carriers fill the trap states in the amorphous phase at the appropriate voltage, which results in the formation of conductive path, and consequently leads to the desired phase transition [25]. The presence of such trap states requires a large band gap in the amorphous phase [23]. When the Zr9(GST)91 film is crystallized from amorphous to fcc phase (200 °C), the band gap decreases from 0.76 to 0.51 eV. With the annealing temperature further increasing to 300 °C, it reduces to 0.45 eV. The decrease in optical band gap corresponds directly to a decrease in activation energy for electrical conduction and better conductivity [26]. B1/2 refers slope in the extended region and it is the measure of disorder [27]. A higher value of B1/2 indicates less disorder. B1/2 increases with annealing temperature
indicating a decrease in disorder, which due to the change of long range order to short range order of the crystalline phase. The vibration modes of bonds were analyzed using Raman spectroscopy. Fig. 4(a)–(d) show the Raman spectra of the as-deposited, 150, 200, and 350 °C annealed Zr-GST films, respectively. The Raman spectra are dominated by a broad band covering the region 100–190 cm−1 for all the as-deposited films, as shown in Fig. 4(a). Two broad peaks overlap with each other in this region with one peak at 125 cm−1 (peak A) and the other at 150 cm−1 (peak B). Peak A is associated with the A1 mode of GeTe4-nGen (n = 1,2) tetrahedral modes [28], and peak B is ascribed to Sb\\Te bonds' vibrations in the SbTe3 units [29]. In addition, a weak broad Raman band located at 220 cm−1 (peak C) is assigned to the F2 mode of GeTe4 tetrahedra [30]. In the low-frequency region of the spectra, a peak at 56 cm−1 (peak D) can be observed, which is related to the E mode of GeTe4 tetrahedra [30]. All Zr-GST films exhibit similar Raman spectra in terms of peak position. However, the intensity of the peak at 125 cm− 1 decreases with increasing Zr content, suggesting that the addition of Zr leads to a higher degree of disorder in the amorphous phase of the material. In fact, the higher the disorder level of the
Fig. 3. Plots of (αhν1/2) vs. hν for the Zr9(GST)91 films. Inset figure is the corresponding Vis\ \IR transmission spectra.
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Fig. 4. Raman spectra of the GST and Zr-GST films: (a) as-deposited, (b) 150, (c) 200, and (d) 350 °C.
Zr-GST amorphous state, the higher the energy required to arrange it in an ordered crystalline form [31]. The Raman spectra of as-deposited and 150 °C-annealed Zr-GST films show similar shape, suggesting the amorphous nature. For the samples annealed at 200 °C, the Raman spectra still maintain broadening nature, while the peaks at 125 cm−1 (peak A) and 220 cm−1 (peak C) disappear, as shown in Fig. 4(c), due to the annealing-induced crystallization. In contrast, a new peak at 105 cm−1 (peak E) appears, which is related to the A1 mode of GeTe4 corner-sharing tetrahedral [32]. The results reveal that the GeTe component of GST alloys is mainly responsible for the phase transition [32]. At 350 °C, peak E at 105 cm−1 remains almost unchanged. Peak B at 150 cm−1 disappears and a new sharp peak F at ∼ 172 cm−1 appears for both the GST and Zr4(GST)96 films, as shown in Fig. 4(d), which is ascribed to the phase transformation from fcc to hexagonal phases. Peak F may be assigned to Te-Te pairs, similar to glasses in the Ge-Se-Te system [33]. The new peak F cannot be observed in the Zr-GST films with Zr concentration larger than 6 at.% due to the suppression of the fcc-tohexagonal phase transformation. Since adequate Zr atoms replace some Sb and Te atoms in the crystal lattice, the Zr\\Sb and Zr\\Te bonds exist in abundance for the Zr-GST films with Zr concentration higher than 6 at.%. As a result, the formation of Te-Te pairs suffers from obstacles to some extent, and thus the fcc-to-hexagonal phase transformation is suppressed. XPS spectra were further applied to investigate the binding state of Zr9(GST)91, as shown in Fig. 5. The binding energy scale was calibrated
using the value of 284.8 eV of the C1s core level. The banding energy of Zr 3d5/2 and 3d3/2 for Zr9(GST)91 are about 182.7 and 185.2 eV respectively, higher than those for pure Zr (178.7, 179.9 eV) [34], suggesting that Zr atoms bond with other elements. The peak positions of Sb 3d in Fig. 5(b) shift to lower binding energy compared with GST. Similar phenomenon can be observed in Te 3d spectra of the Zr9(GST)91 and GST films, as shown in Fig. 5(c). Since the electronegativity of Zr (1.33) is smaller than that of Sb (2.05) and Te (2.10), the decrease of binding energy implies that Zr atoms bond with both Sb and Te atoms. Similar results were reported in Zr-Sb-Te alloy [16]. Therefore, the incorporating Zr atoms are embedded in the inner atomic-scale structure of GST which plays an import role in structure transition and phase-change properties of Zr-GST. 4. Conclusions In summary, the crystallization behavior, structural, optical and electrical properties of the Zr-GST films were investigated systematically. The crystalline resistance, crystallization temperature and 10-years data-retention temperature of the Zr-GST films, which are higher than those of the GST, increase with increasing Zr concentration. The ZrGST films are crystallized into single fcc phase without phase separation, which help to enhance endurance of the PCRAM. The hexagonal structure can be suppressed for Zr-GST with high Zr content due to the formation inhabitation of the Te-Te pairs. Zr9(GST)91 film shows higher
Fig. 5. XPS spectra for 350 °C-annealed GST and Zr9(GST)91 films: (a) Zr 3d, (b) Sb 3d, and (c) Te 3d.
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band gap in comparison with GST, which is favorable to enhance the switching properties. All these advantages enable Zr-GST films to be a promising candidate for PCRAM. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 61306147, 61377061), the Science and Technology Public Project of Zhejiang Province (Grant No. 2016C31077), and Ningbo Municipal Natural Science Foundation of China (Grant No. 2014A610121), and sponsored by K. C. Wong Magna Fund in Ningbo University. References [1] A.V. Kolobov, P. Fons, A.I. Frenkel, A.L. Ankudinov, J. Tominaga, T. Uruga, Understanding the phase-change mechanism of rewritable optical media, Nat. Mater. (2004) 703–708. [2] K. Tanaka, S. Iizima, M. Sugi, Y. Okada, M. Kikuchi, Thermal effect on switching phenomenon in chalcogenide amorphous semiconductors, Solid State Commun. 8 (1970) 387–389. [3] M. Wuttig, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824–832. [4] H.P. Wong, S. Raoux, S. Kim, J. Liang, J.P. Reifenberg, B. Rajendran, M. Asheghi, K.E. Goodson, Phase change memory, Proc. IEEE 98 (2010) 2201–2227. [5] S. Raoux, W. Wełnic, D. Lelmini, Phase change materials and their application to nonvolatile memories, Chem. Rev. 110 (2010) 240–267. [6] M. Zhu, L. Wu, F. Rao, Z. Song, C. Peng, X. Li, D. Yao, W. Xi, S. Feng, Phase change characteristics of SiO2 doped Sb2Te3 materials for phase change memory application, Electrochem. Solid-State Lett. 14 (2011) H404–H407. [7] W. Czubatyj, S.J. Hudgens, C. Dennison, C. Schell, T. Lowrey, Nanocomposite phasechange memory alloys for very high temperature data retention, IEEE Electron Device Lett. 31 (2010) 869–871. [8] R.E. Simpson, M. Krbal, P. Fons, A.V. Kolobov, J. Tominaga, T. Uruga, H. Tanida, Toward the ultimate limit of phase change in Ge2Sb2Te5, Nano Lett. 10 (2010) 414–419. [9] S.W. Ryu, H.K. Lyeo, J.H. Lee, Y.B. Ahn, G.H. Kim, C.H. Kim, S.G. Kim, S.H. Lee, K.Y. Kim, J.H. Kim, W. Kim, C.S. Hwang, H.J. Kim, SiO2 doped Ge2Sb2Te5 thin films with high thermal efficiency for applications in phase change random access memory, Nanotechnology 22 (2011). [10] G. Singh, A. Kaura, M. Mukul, S.K. Tripathi, Electrical, optical, and thermal properties of Sn-doped phase change material Ge2Sb2Te5, J. Mater. Sci. 48 (2013) 299–303. [11] K.B. Borisenko, Y. Chen, S.A. Song, D.J.H. Cockayne, Nanoscale phase separation and building blocks of Ge2Sb2Te5N and Ge2Sb2Te5N2 thin films, Chem. Mater. 21 (2009) 5244–5251. [12] G. Wang, Q. Nie, X. Shen, R.P. Wang, L. Wu, J. Fu, T. Xu, S. Dai, Phase change behaviors of Zn-doped Ge2Sb2Te5 films, Appl. Phys. Lett. 101 (2012). [13] S. Privitera, E. Rimini, R. Zonca, Amorphous-to-crystal transition of nitrogen- and oxygen-doped Ge2Sb2Te5 films studied by in situ resistance measurements, Appl. Phys. Lett. 85 (2004) 3044–3046. [14] Y. Lu, Z. Zhang, S. Song, X. Shen, G. Wang, L. Cheng, S. Dai, Z. Song, Performance improvement of Ge-Sb-Te material by GaSb doping for phase change memory, Appl. Phys. Lett. 102 (2013).
13
[15] S.W. Nam, C. Kim, M.H. Kwon, H.S. Lee, J.S. Wi, D. Lee, T.Y. Lee, Y. Khang, K.B. Kim, Phase separation behavior of Ge(2)Sb(2)Te(5) line structure during electrical stress biasing, Appl. Phys. Lett. 92 (2008). [16] N. Soheilnia, K.M. Kleinke, H. Kleinke, Crystal structure, electronic structure, and physical properties of two new antimonide-tellurides: ZrSbTe and HfSbTe, Chem. Mater. 19 (2007) 1482–1488. [17] Y. Zheng, Y. Cheng, M. Zhu, X. Ji, Q. Wang, S. Song, Z. Song, W. Liu, S. Feng, A candidate Zr-doped Sb2Te alloy for phase change memory application, Appl. Phys. Lett. 108 (2016) 210. [18] D. Ielmini, Y.G. Zhang, Evidence for trap-limited transport in the subthreshold conduction regime of chalcogenide glasses, Appl. Phys. Lett. 90 (2007). [19] C. Wang, S. Li, J. Zhai, B. Shen, M. Sun, T. Lai, Rapid crystallization of SiO2/Sb80Te20 nanocomposite multilayer films for phase-change memory applications, Scr. Mater. 64 (2011) 645–648. [20] Y. Yin, H. Sone, S. Hosaka, Dependences of electrical properties of thin GeSbTe and AgInSbTe films on annealing, Jpn. J. Appl. Phys. 44 (2005) 6208. [21] C. Kim, D. Kang, T.-Y. Lee, K.H.P. Kim, Y.-S. Kang, J. Lee, S.-W. Nam, K.-B. Kim, Y. Khang, Direct evidence of phase separation in Ge2Sb2Te5 in phase change memory devices, Appl. Phys. Lett. 94 (2009). [22] S.J. Park, M.H. Jang, S.-J. Park, M.-H. Cho, D.-H. Ko, Characteristics of phase transition and separation in a In–Ge–Sb–Te system, Appl. Surf. Sci. 258 (2012) 9786–9791. [23] E.M. Vinod, K. Ramesh, K.S. Sangunni, Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys, Sci. Rep. 5 (2015). [24] W. Welnic, A. Pamungkas, R. Detemple, C. Steimer, S. Blugel, M. Wuttig, Unravelling the interplay of local structure and physical properties in phase-change materials, Nat. Mater. 5 (2006) 56–62. [25] 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. [26] C. Das, M.G. Mahesha, G. Mohan Rao, S. Asokan, Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films, Appl. Phys. A Mater. Sci. Process. 106 (2012) 989–994. [27] E.M. Vinod, R. Naik, A.P.A. Faiyas, R. Ganesan, K.S. Sangunni, Temperature dependent optical constants of amorphous Ge2Sb2Te5 thin films, J. Non-Cryst. Solids 356 (2010) 2172–2174. [28] P. Nemec, A. Moreac, V. Nazabal, M. Pavlista, J. Prikryl, M. Frumar, Ge–Sb–Te thin films deposited by pulsed laser: an ellipsometry and Raman scattering spectroscopy study, J. Appl. Phys. 106 (2009). [29] I. Watanabe, S. Noguchi, T. Shimizu, Study on local structure in amorphous SbS films by Raman scattering, J. Non-Cryst. Solids 58 (1983) 35–40. [30] K.S. Andrikopoulos, S.N. Yannopoulos, G.A. Voyiatzis, A.V. Kolobov, M. Ribes, J. Tominaga, Raman scattering study of the a-GeTe structure and possible mechanism for the amorphous to crystal transition, J. Phys. Condens. Matter 18 (2006) 965–979. [31] G.B. Beneventi, L. Perniola, V. Sousa, E. Gourvest, S. Maitrejean, J.C. Bastien, A. Bastard, B. Hyot, A. Fargeix, C. Jahan, J.F. Nodin, A. Persico, A. Fantini, D. Blachier, A. Toffoli, S. Loubriat, A. Roule, S. Lhostis, H. Feldis, G. Reimbold, T. Billon, B. De Salvo, L. Larcher, P. Pavan, D. Bensahel, P. Mazoyer, R. Annunziata, P. Zuliani, F. Boulanger, Carbon-doped GeTe: a promising material for phase-change memories, Solid State Electron. 65-66 (2011) 197–204. [32] A.N. Kolobov, P. Fons, J. Tominaga, Phase-change optical recording: past, present, future, Thin Solid Films 515 (2007) 7534–7537. [33] A.H. Moharram, M.A. Hefni, A.M. Abdel-Baset, Short and intermediate range order of Ge20Se80 − xTex glasses, J. Appl. Phys. 108 (2010). [34] J.F. Moulder, J. Chastain, R.C. King, Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data, Chem. Phys. Lett. 99 (1963) 7–10.