Atomic Migration Induced Crystal Structure Transformation and Core

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Letter pubs.acs.org/NanoLett

Atomic Migration Induced Crystal Structure Transformation and Core-Centered Phase Transition in Single Crystal Ge2Sb2Te5 Nanowires Jun-Young Lee,†,‡ Jeong-Hyeon Kim,†,‡ Deok-Jin Jeon,†,‡ Jaehyun Han,†,‡ and Jong-Souk Yeo*,†,‡ †

School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University, Incheon 406-840, Republic of Korea

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S Supporting Information *

ABSTRACT: A phase change nanowire holds a promise for nonvolatile memory applications, but its transition mechanism has remained unclear due to the analytical difficulties at atomic resolution. Here we obtain a deeper understanding on the phase transition of a single crystalline Ge2Sb2Te5 nanowire (GST NW) using atomic scale imaging, diffraction, and chemical analysis. Our cross-sectional analysis has shown that the as-grown hexagonal close-packed structure of the single crystal GST NW transforms to a metastable face-centered cubic structure due to the atomic migration to the pre-existing vacancy layers in the hcp structure going through iterative electrical switching. We call this crystal structure transformation “metastabilization”, which is also confirmed by the increase of set-resistance during the switching operation. For the set to reset transition between crystalline and amorphous phases, high-resolution imaging indicates that the longitudinal center of the nanowire mainly undergoes phase transition. According to the atomic scale analysis of the GST NW after repeated electrical switching, partial crystallites are distributed around the core-centered amorphous region of the nanowire where atomic migration is mainly induced, thus potentially leading to low power electrical switching. These results provide a novel understanding of phase change nanowires, and can be applied to enhance the design of nanowire phase change memory devices for improved electrical performance. KEYWORDS: Phase change nanowire, crystal structure transformation, metastabilization, atomic migration, phase transition, phase change memory

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existing grain boundaries, various defects, and mixed crystal structures and crystallographic orientations for the analysis at atomic resolution. Thus, a bottom-up grown single crystalline phase change nanowire can be used as a defect-free phase change material.7−12 In addition, the phase change NW has benefits such as low drift of resistance in the amorphous phase,13 high scalability with reduced diameter,14 efficient switching performance,15 and strong heterogeneous surface nucleation with a large surface-to-volume ratio.16 Bottom-up grown Ge2Sb2Te5 nanowires13 basically have hexagonal close-packed (hcp) structure. The hcp structure is energetically more stable and has higher conductivity, compared to the metastable fcc structure.17 However, the hcp structure is not suitable for practical PCM devices due to a large volume change during the phase transition from amorphous to crystalline.18,19 On the other hand, the metastable fcc structure in thin film based PCM device has been studied well for

e−Sb−Te alloys are one of the most promising materials for phase change random access memory (PRAM) applications.1,2 Phase change materials can efficiently convert its phase from crystalline to amorphous in terms of phase transition speed, repeatability, and reversibility.3 Furthermore, large differences in electrical and optical properties between the two phases are the most attractive properties for PRAM applications. Thanks to these intrinsic properties, the Ge−Sb− Te alloys or chalcogenide ternary compounds have already been used in various optical data storage devices such as compact disk (CD), digital versatile disk (DVD), and Blue-ray disk. Particularly, the large difference in the electrical resistance of Ge2Sb2Te5 (GST) between its crystalline phase (set state) and amorphous phase (reset state) enables its application to nonvolatile memories. Practical thin-film based GST phase change memory (PCM) devices use phase transitions between crystalline phase of metastable face-centered cubic (fcc) and amorphous phase.4−6 Analysis on the structural change of GST during the phase change process is important for improving the performance of PRAM device. However, polycrystalline characteristics of annealed GST thin-film have various obstacles such as pre© 2016 American Chemical Society

Received: May 31, 2016 Revised: September 2, 2016 Published: September 22, 2016 6078

DOI: 10.1021/acs.nanolett.6b02188 Nano Lett. 2016, 16, 6078−6085

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Figure 1. Metastabilization and core-centered phase transition of Ge2Sb2Te5 nanowire-based PCM device. (a) A scanning electron microscopy image and (b) a panoramic set of cross-sectional TEM images of GST NW-based PCM device after reset-operation. HRTEM images of a GST NW show (c) a crystalline region in the left side, (d) an amorphous region at the center, and (e) a crystalline region in the right side. (f) A schematic image of GST NW with Si3N4 encapsulation and WN electrodes on SiO2/Si substrate. (For the GST NW, the area with light green represents a crystalline region and the brighter area at the center represents an amorphous region.)

decades and proven to be suitable.17,18,20,21 Thus, if the GST NW has a metastable fcc crystal structure, it can be applied for a practical phase change device. Although the stabilization process from fcc to hcp using thermal annealing is wellknown,17 the opposite process, crystal structure transformation from hcp to fcc, has not been clearly investigated before. Here we suggest experimental observation that supports the crystal structure transformation from the as-grown hcp to the metastable fcc. We have investigated the crystal structure transformation of a GST NW with both direct imaging at atomic resolution and electrical characterization. Cross-sectional transmission electron microscopy (TEM) analysis has shown that the crystalline region of the GST NW consists of fcc structure. The observed fcc structure in the repetitively switched GST NW implies a structural transformation from the original hcp structure to a metastable fcc structure through the iterated switching operation. We define this phenomenon as a “metastabilization” because the structure changes from the stable hcp to the metastable fcc. Our result from a spherical aberration (CS) corrected scanning transmission electron microscopy (STEM) (JEOL-ARM200F) shows that the Ge

and Sb atoms migrate into the pre-existing vacancy layers in hcp structures through rearrangements in its crystal structure. Consistent increase of electrical resistance at its crystalline state also supports the transition from hcp to fcc. We also analyze the partial crystallites in amorphous region and discuss coreconcentrated phase change behavior of the GST NW with changes in atomic concentration. We fabricated a PCM device consisting of GST NWs and tungsten nitride (WN) electrodes (Figure 1a). The GST NWs were grown by the vapor−liquid−solid (VLS) method7 using a chemical vapor deposition (CVD) tool (Supporting Information (SI) section 1, Figure S1). The as-grown GST NWs formed a stable hcp structure with the lattice parameter, a = 4.22 Å and c = 16.56 Å (Figure S2). We formed a metal contact on a selected GST NW and measured the electrical property of the GST NW-based PCM unit cell. The phase and corresponding resistance of the sample were repeatedly changed by an electrical switching pulse from a high resistive condition (the reset state for an amorphous phase) to a low resistive condition (the set state for a crystalline phase) and vice versa. After the switching process was repeated 20 times, the 6079

DOI: 10.1021/acs.nanolett.6b02188 Nano Lett. 2016, 16, 6078−6085

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Nano Letters NW PCM device was stopped after the reset-operation. The cross-sectional TEM sample of the GST NW along its longitudinal direction was prepared and analyzed at atomic resolution. The phase change of the GST NW occurred at the center region of the NW. Figure 1b shows the cross-sectional view of the NW imaged by high-resolution TEM (HRTEM), having a smudge in the center region of the NW where the light gray color at the center indicates an amorphous phase while the remaining part of the NW shows a crystalline phase in dark gray color. This result contrasts with an as-grown single crystalline GST NW, showing a uniform color across its longitudinal cross sections (Figure S3). Figure 1c,e, and d clearly show the difference in crystallinity at the regions pointed by the blue and red arrows, respectively; they show the crystalline and amorphous regions of the NW, respectively, with their diffraction patterns in their respective insets. The spots in the diffraction patterns referring crystallinity are much clearer in the dark gray region (the inset of Figure 1c,e) than in the light gray region (the inset of Figure 1d). Unusual features we can observe from the cross-sectional images are as follows: In the crystalline region, linear bands of contrasts parallel to the basal plane (0001) were observed (Figure 1e). In the amorphous region, only a narrow portion (the middle part of Figure 1d) at the longitudinal center of NW was observed to be crystalline. To figure out various features in the cross-sectional HRTEM images of the GST NW, we have investigated the change of crystal structure (metastabilization), the core-centered phase transition behavior, the change in atomic concentration, and the role of partial crystallites at the amorphous region as schematically shown in Figure 1f. First of all, we have examined the linear bands of contrasts in crystalline region (Figure 1e). These linear contrasts represent the distributions of discrete vacancy layers. Further investigation on the linear bands using atomic resolution imaging and structural analysis has led to a finding of metastabilization in the GST NWs (Figure 2). Theoretically, the Petrov model,22 Kooi and De Hosson (KH) model,23 and Matsunaga model24 predict highly ordered vacancy layers in an hcp structure of GST NWs. An hcp structure has a pair of adjacent Te layers and a vacancy layer between them in the middle of the unit cell.24 The periodic vacancy layers are confirmed by a STEM image of as-grown nanowire.25 Our analysis using STEM high-angle annular darkfield (HAADF) imaging with zone axis of [011̅0] (Figure 2a,d) and [0001] (Figure S3) also indicates that an as-grown GST NW has an intrinsic hcp structure. However, the vacancy layers are neither seen periodic (Figure 2b,e) nor seen to be present (Figure 2c,f) in the crystalline region of the GST NW after switching operation. Some of the missing vacancy layers can be explained by atomic migration from the original structure of hcp (Figure 2g,h). The Ge, Sb, and Te atoms at the stable phase of an hcp structure are known to occupy octahedral sites (Figure 2g).1 Electrical pulses for switching operation can induce the Ge and Sb atoms to migrate from the intrinsic octahedral sites of Ge/ Sb-mixed layers to the tetrahedral sites of vacancy layers (Figure 2h). The rearrangements of Ge and Sb atoms transform the stable hcp structure into a metastable fcc structure (Figure 2i). We termed this phenomenon as metastabilization to reflect the characteristic of the crystal structure transformation.

Figure 2. Metastabilization. Schematic images of (a) an as-grown GST NW (the blue rod, a single crystal GST NW which has stable hcp crystal structure); (b) a pulse induced GST NW (the green region, a rearranged hcp crystal structure, that is, a metastable fcc structure); (c) a repeatedly switched GST NW (the green region, corresponds to a metastable fcc crystal structure). Atomic resolution HAADF image of (d) the as-grown GST NW at zone axis of [011̅0], (e) the pulse induced NW at zone axis of [011̅0], and (f) fully metastabilized region at zone axis of [112̅]. Schematics of hcp structure of GST crystal with (g) a conventional hcp model, (h) a transitional state of metastabilization, and (i) an fcc structure.

While it is well-known that the GST thin films can have a metastable state depending on the atomic composition,17,26 the transformation of GST compounds from a stable hcp structure to a metastable fcc structure has not been studied previously. To the best of our knowledge, this is the first report on the experimental verification of metastabilization driven with electric field on GST NWs. The metastabilization phenomenon is supported by various evidence such as the disappearance of vacancy layer, the change of distance between the Te layers, the change of peak intensities in Ge and Sb layers adjacent to the vacancy layer, and the increase of set-resistance in GST NWs. The migrations of Ge and Sb atoms are related to weak bonding energy of Te−Te bond.27,28 A Te−Te bonding is based on van der Waals type interaction,29 thus weaker than covalent Te−Ge and Te−Sb bonding (Te−Te bonding is expressed by dotted lines in Figure 2g). Consequently, the Ge and Sb atoms adjacent to Te−Te bonding in an hcp structure can easily locate themselves into vacant tetrahedral sites located between Te layers once their migrations are electrically induced. Furthermore, because Ge and Sb atoms have relatively higher electromigration rate30 and atomic mobility31 than Te atoms, the disordering is mainly caused by Ge and Sb atoms 6080

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Nano Letters occupying the vacancies. The atomic movement changes the stacking sequence of Te−Te layers in hcp structure from Te−v (vacancy layer)−Te to Te−Ge/Sb/v (mixed layer)−Te.32,33 As a result, the periodic vacancy layers in the hcp structure are continuously filled with neighboring Ge and Sb atoms. The atomic resolution images for the GST NW with pulse induced structural change (Figure 2e) and after the repeated switching (Figure 2f) show that the originally tetrahedral sites with vacancy layers disappear and change to octahedral sites of mixed vacancy/Ge/Sb while the crystal structure is rearranged to accommodate Ge and Sb atoms between Te and Te layers (Figure 2i). This rearrangement process is expected to spread vacancies from the intrinsic vacancy layers to nearby Ge/Sb layers, triggering the metastabilization of Ge2Sb2Te5 crystal to the well-known fcc structure as shown by the atomic resolution images along [112̅] zone axis (Figure 2f) and the diffraction patterns (the inset of Figure 2f). The similarity and difference between hcp and fcc structures of GST have been studied in theoretical researches.24,34 An fcc structure with the viewing direction of [112̅] is similar to a typical hcp structure with the viewing direction of [011̅0], except for the periodic presence of vacancy layers between Te− Te bonding. The fcc structure of metastable GST is known to have vacancy of 20% in Ge/Sb site.24,35 The distribution of vacancies can vary from partially ordered28,34 to randomly distributed36 in the metastable fcc structure. In contrast, the stable hcp structure has been reported to have highly ordered vacancy layers.22−24,34 Therefore, the disappearance of vacancy layers is a clear evidence of the metastabilization to fcc structure for the repeatedly phase changed GST NW. The transformation from the initial hcp structure22−24,34 to fcc structure is more clearly identified by examining a location where the vacancy layer disappears (Figure 3a). Specifically, the distance between Te layers is an important indicator of the transition. According to the conventional hcp structure, the planar distance between Te layers along [0001] direction is 0.253−0.286 nm with a vacancy layer formed between Te−Te atoms (interatomic distance of 0.354−0.375 nm).24,34,37 On the other hand, the planar distance between Te layers with a Ge/Sb layer is longer, that is, 0.339−0.365 nm (the interatomic distance of 0.290−0.320 nm for Te−Ge and Te−Sb atoms).23,24,34,37 Figure 3a shows a periodic stacking of Ge, Sb atoms (small and dim points) and Te atoms (larger and brighter points) with a partial vacancy layer (a dark linear space between Te layers along the middle on the left side). The measured distance of 0.274 nm between Te layers containing the vacancy layer agrees with the predicted value (Figure 3b). On the other hand, the measured distance of 0.342 nm between Te layers where the vacancy layer disappears is longer (Figure 3c). The increase of the distance from 0.274 nm to 0.342 matches with the theoretically predicted distance of Te−v−Te where Ge and Sb migrated to the vacancy layer.32 Therefore, the results verify the formation of Ge/Sb mixed layer where a vacancy layer was originally located. Peak intensities extracted from Figure 3a also provide another critical observation. The migration of Ge/Sb atoms can also be verified by examining the peak intensities of presumably Ge/Sb mixed layer. In Figure 3b,c, the high peak intensities refer to Te atoms and the low peak intensities indicate Ge/Sb atoms. The peak intensities from Ge/Sb mixed layer adjacent to the vacancy layer (indicated as a and a′ in Figure 3b) are relatively lower in Ge/Sb mixed layer adjacent to the formed Ge/Sb layer (indicated as b and b′ in Figure 3c).

Figure 3. Migration of Ge/Sb atoms and increase of the set-resistance. (a) Atomic resolution STEM-high-angle annular dark-field (HAADF), inverse Fourier transformed, image of the crystalline region where the vacancy layer is gradually filled with the migrated Ge/Sb atoms. Intensity graphs (b) with the vacancy layer (along the red line in (a)) and (c) without a vacancy layer (along the green line in (a)). (d) The change of the set-resistance of the GST NW based PCM device while set-pulses induced repeatedly.

This implies that the Ge/Sb atoms have migrated to fill the vacancy layer in the middle, resulting in the decreased concentration of Ge/Sb atoms in the directly adjacent Ge/Sb mixed layers. This migration of Ge/Sb atoms leads to the formation of a new stacking sequence, thus producing the rearranged crystal structure. This trend in Ge/Sb peak intensities is consistently observed throughout the crystalline region of the GST NW, thus supporting the mechanism of the metastabilization. Another way to characterize metastabilization is to examine its electrical properties. Specifically, resistance can be an indicator of the crystallinity of a structure. In case of the lattice structures of GST, its hcp structure has lower resistivity compared to its fcc structure. We have conducted the switching operation of the GST NW PCM device and measured its resistance in a pulse mode probe station, which enables measuring the resistance between each pulse. We alternatively applied the set and reset pulses with increasingly larger voltages for investigating the switching performance. Notably, upon alternatively applying the set and reset pulses, the resistance of the GST NW at the set state was gradually increased from the initial resistance of 2.44 to the resistance of 20 kΩ (Figure 3d). The set pulse amplitude was limited up to 1.55 V to keep it lower than the threshold voltage of 1.75 V for the phase transition from crystalline to amorphous phase. In general, the set-resistance usually does not change by the 6081

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Figure 4. Core-centered phase transition. (a) The change of the reset-resistance of the GST NW based PCM device as a function of reset pulse amplitude. (b) Pulse induced switching performance of the NW device. (c) (top) A panoramic set of STEM-HAADF images of GST NW after resetoperation, (bottom) a STEM-HAADF image of the center region (the green square), a HRTEM image of a partially crystalline region (the blue square), and STEM-HAADF image of the core part of the center region (the red and the magenta square). Schematic images of the GST NW with different states: (d) as-fabricated, (e) after set-operation, and (f) after reset-operation.

iteration of switching cycles if sufficiently long falling time is used (950 ns in our case). However, our experimental results have shown the tendency of the increased set-resistance. This indicates a crystal structure transformation from hcp to fcc in the crystalline region of the GST NW because the electrical conductivity of GST is 2230 S/cm for hcp structure and 22− 382 S/cm for fcc structure (the wide range of variation in the electrical conductivity of fcc is related to the distribution of Ge and Sb atoms and vacancies in the lattice structure).17,20,21,38,39 The difference in conductivity between hcp and fcc corresponds to the difference in device resistance of approximately 103 and 104−105 Ω considering the physical scale of our GST NW device (the length of 2.5 μm, the diameter of 133 nm, and the contact resistance around 1.5 kΩ). Because the estimated resistance values for different structures agree with the measured set-resistance values, we can interpret the gradual increase in resistance as a progressive transformation from the single crystalline GST NW having hcp structure to the one having fcc structure.

The gradual increase in the set resistance also supports that lattice disordering is the governing phenomena during setoperation. Applied electric pulses induce two phenomena at the same time: annealing by Joule heating and lattice disordering by electrically driven atomic migration. In general, Joule heating can induce amorphous to crystalline fcc transformation and then to hcp structure if sufficient temperature can be reached. Further stabilization from fcc to hcp crystal should have been observed as the decrease of set-resistance with the increasing number of pulses. But this was apparently not the case because electrically driven atomic migration induces the lattice disordering and crystal structure transformation from hcp to fcc with the number of pulses. On the other hand, the Joule heating at the pulse amplitude smaller than 1.55 V was not strong enough to anneal the crystal structure back from fcc to hcp. To support the explanation on the phenomena, we have measured the set resistance using another GST NW at a fixed set-pulse amplitude of 1.3 V (Figure S4). The set-resistance continues to increase with the cumulative number of set-pulses, 6082

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Figure 5. Atomic migration at the core of the longitudinal center in GST nanowire. (a) A TEM image of the center part of the GST NW. STEMEDS line scanning data of the phase transition area along (b) the longitudinal axis (A-A′) and the vertical line, (c) B−B′ and (d) C−C′. (e) A collection of STEM-EDS point scanning data for the amorphous region including the partial crystallites (along the vertical line, D−D′). (f) A STEMHAADF image (inverse Fourier transformed) of remaining crystalline area, pseudomatched with crystal lattices of Te atoms (the yellow balls) and mixed Ge/Sb atoms (the gray balls).

which indicates that the electric field driven lattice disordering continuously transforms the crystal structure to a metastable fcc with higher resistance. To verify the metastabilization phenomenon more accurately, we analyzed another GST NW that is sampled after setoperation and before reset condition is applied. Figure S5 shows atomic resolution images of the GST NW in the setstate. The whole region of GST NW has metastable fcc structure in the atomic resolution STEM-HAADF image with a viewing direction of [011] (Figure S5b,c). This fully metastabilized GST NW indicates that GST NW has been transformed to the metastable structure without any mechanical failure or partial melting while the set pulses were applied. We also checked the same phenomena in different GST NWs (Figure S6). The experimental results have so far indicated that the GST NW undergoes metastabilization in crystalline region via atomic migration. This unconventional mechanism from the single crystal phase change nanowire can potentially lead to the benefits such as more efficient and stable nonvolatile memory devices. First, the metastabilized nanowire is expected to provide lower reset power for phase transition in a single crystal GST NW based PCM device. Because the fcc structure of GST is less stable than hcp structure,24,34,40−42 it requires less power to reset from metastable crystalline phase to amorphous phase compared to the hcp structure. For decades, various doping methods have been suggested for reducing the reset power consumption.5,6,43−46 The crystal structure transformation from hcp to fcc with the iterated switching of GST NW has a benefit of lowering reset power consumption without additional doping process. In addition, the metastabilization from hcp to

fcc can improve the mechanical robustness of PCM device because a stable hcp structure undergoes a large volume change during the phase transition, which can lead to a mechanical failure.18,19 To change from low resistive crystalline phase to highly resistive amorphous phase, reset pulses were applied with the duration of 150 ns. A reset pulse amplitude larger than 1.75 V resulted in amorphization of GST NW. Measured average reset and set resistances were ∼107 and ∼104 Ω, respectively, and their ratios were larger than 103 (Figure 4a and Figure S7). We also measured the repeatability of electrical switching from set state to reset state and vice versa (Figure 4b). Thus, the result from the switching operation indicates that the GST NW device is suitable for phase change memory applications. By the applied reset pulse, an amorphous region was formed around the longitudinal center of the GST NW (Figure 4c, top panel). At the reset state after the repeated switching cycles shown in Figure 4b, the amorphous region was found to have small and discontinuous portions of partial crystallites as shown with the TEM images and the diffraction patterns in Figure 4c and Figure S8. The partial crystallites in the phase transition region can be explained by nonmelting phase change mechanism. In our nanowire geometry, simulation results (Figure S9) indicate that we cannot effectively reach the melting temperature of GST nanowire in the center region though we indeed observe a phase transition. This result shows that the phase transition of GST NW does not need to include a fully melted liquid state during reset operation. Instead, what we have seen is the change in the atomic concentration near the core-centered region of the GST NW due to the electrically induced atomic 6083

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STEM-EDS analysis (Figure 5e) show the Te deficient region in partially crystalline state after the repeated switching cycles. In this region, thin layers of crystallites were aligned along the longitudinal direction of NW in a disconnected form as shown in Figures 4c and 5e. Because nanowires are influenced by surface effects, bulk atoms within the nanowire generally provide higher conductivity, resulting in more migration of charged ions around the core under the applied field. This can explain the oval shape of the amorphous region in the center of NW. More migration along the core of NW may lead to the disparity in the concentration of atoms in GST, thus changing the condition for phase transition locally in the NW. With this change, some part of the core region in GST NW is believed to remain crystalline for the same reset pulse condition. As shown in Figure 5f, structures formed of five atomic layers were consistently seen at the remaining partial crystallites in the amorphous region using an atomic resolution STEM-HAADF image. This type of structure is not predicted from the crystal structure of Ge2Sb2Te5. Because of the substantial atomic migration during the iterated reset and set switching processes, original stoichiometry in Ge2Sb2Te5 changes to relatively lower concentration of Te and higher concentration of Ge/Sb providing 5-layered atomic sequence, Te−Ge/Sb−Te−Ge/ Sb−Te-v with a pitch of 1 nm (Figure 5f and Figure S11). Conductive paths bridging these crystallites upon a set pulse should be easier than transforming the entire amorphous region to crystalline. Therefore, the presence of these crystallites in amorphous region is believed to lower the threshold power for phase transition. Furthermore, the formation of partial crystallites can potentially allow the control of the phase transition process, thus providing an approach to multilevel cell phase change memory using bottom-up grown nanowires. This work has shown for the first time a metastabilization of crystal structure transformation from hcp to fcc in a single crystal Ge2Sb2Te5 nanowire using a direct imaging at atomic resolution. Migration of Ge and Sb atoms from the octahedral site of the Ge/Sb layers to the tetrahedral site of the intrinsic vacancy layer in hcp was verified with atomic resolution STEM analysis. The atomic migration results in the transition to a metastable structure as confirmed from the direct imaging as well as the electrical measurements. The set-resistance values of the GST NW correlate well with the structural change from hcp to fcc during the metastabilization. In addition, this work has shown a novel finding on the core-centered phase transition in GST NW and the presence of partial crystallites in the core of the amorphous region using atomic resolution STEM imaging and chemical analysis. These results extend our understanding on the phase transition behavior of a single crystal phase change nanowire device and open up new possibilities for nanowirebased memory applications.

migration (which is discussed in detail later). Subsequently, reset pulses lead to the formation and propagation of prismatic dislocation loops8 with lattice disordering (SI section 7, Figure S10), thus resulting in a phase transition. Small portion of crystallites can still exist in this region of phase transition following the reset-operation. As such crystallites are observed to be discontinuous in the amorphous region, the NW with such state is measured to be highly resistive. Furthermore, the thin partial crystallites distributed at the core of the amorphous region in GST NW were observed in the atomic resolution STEM-HAADF image (the magenta square in Figure 4c). The overall change of the GST NW during the iterated set and reset process is summarized in Figure 4d−f. In the asfabricated condition, the whole region of GST NW has the stable crystalline phase with hcp crystal structure (Figure 4d). The crystalline phase of GST NW is then transformed from a stable structure to a metastable structure by applied pulses under the threshold value (Figure 4e). After a reset-operation, the metastable phase in the longitudinal center of the GST NW is changed to amorphous phase with the formation of discontinuous partial crystallites at the core part of the amorphous region (Figure 4f). These two phenomena, the metastabilization and the core-centered phase transition, occurred due to the same reason: the atomic migration and the lattice disordering by applied electric pulses. Figure 5 shows the change of atomic concentration in the amorphous region at the longitudinal center of the GST NW after reset-operation. We measured the atomic concentration in the amorphous region along the longitudinal direction of the nanowire using an energy-dispersive X-ray spectroscopy (EDS) as seen in Figure 5b (A-A′ line of Figure 5a). The Te concentration was decreased down to 12% and the Sb and Ge concentrations were increased up to 6% and 7%, respectively, in the measured area. The change in atomic concentration implies that Te atoms were migrated away while Ge and Sb atoms were relatively in place. Because the change occurred along the current path, we can hypothesize that the atoms were migrated by electric potential. As Te has higher electronegativity of 5.49 eV than Ge and Sb (4.6 and 4.85 eV, respectively47,48), Te atoms can be anions due to the redistribution of electrons48 in the amorphous region (Figure 5a). Then the anionic Te atoms are migrated along the reverse direction of the current path while electric pulses are applied repeatedly in amorphous region.26,49 We also analyzed the atomic migration in the amorphous region along the direction perpendicular to the NW axis (B−B′ in Figure 5a). According to the STEM-EDS intensity graph in Figure 5c, atomic concentrations were not uniform along the direction. Compared to the uniform intensity for Ge and Sb atoms along the NW diameter, the intensity for Te was reduced by 24% at the core of the amorphized center in the GST NW. This indicates the Te concentration is dominantly decreased by atomic migration at the core part of the GST NW. In contrast, the Te intensity at the outside of amorphous region is relatively uniform along the NW diameter (Figure 5d). The local area having the minimum concentration of Te was further analyzed using a point scan with STEM-EDS (Figure 5e). The STEM-EDS results indicate an inflow of Ge and Sb atoms and a substantial outflow of Te atoms. Figure 5e shows the image of the amorphous area having the partially crystalline region at the core where the Sb and Ge concentrations were increased by 7.9% and 3.6%, respectively, while Te concentration was decreased by 11.4%. The STEM-HAADF image and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02188. Methods for GST NW device fabrication, electrical property measurement, and cross-sectional TEM analysis; HRTEM images, STEM-HAADF images, STEMEDS spectra (PDF) 6084

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the MSIP (Ministry of Science, ICT, and Future Planning), Korea, under the “ICT Consilience Creative Program” (IITP-R0346-16-1008) supervised by the IITP (Institute for Information and communications Technology Promotion) and the “Mid-career Researcher Program” (NRF-2016R1A2B2014612) supervised by the NRF (National Research Foundation). This research was also supported by the National Research Project for Next Generation MLC PRAM Development by the Ministry of Knowledge Economy (MKE) of Korea.

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DOI: 10.1021/acs.nanolett.6b02188 Nano Lett. 2016, 16, 6078−6085