Thermal Barrier Phase Change Memory - ACS Applied Materials

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Functional Nanostructured Materials (including low-D carbon)

Thermal Barrier Phase Change Memory Jiabin Shen, Shilong Lv, Xin Chen, Tao Li, Sifan Zhang, Zhitang Song, and Min Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18473 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Materials & Interfaces

Thermal Barrier Phase Change Memory Jiabin Shen1,2, Shilong Lv1, Xin Chen1,3, Tao Li1,2, Sifan Zhang1,2, Zhitang Song1, Min Zhu1* 1State

Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-System and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China 2University of the Chinese Academy of Sciences, Beijing 100080, People’ s Republic of China 3School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China *e-mail: [email protected](M.Z.)

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Abstract Phase change memory is widely considered as the most promising candidate as storage class memory (SCM), bridging the performance gaps between DRAM and Flash. However, high required operation current remains the major limitation for the SCM application, even after using defect engineering materials, e. g. Ti-doped Sb2Te3. Here, we demonstrate ~87% current can be reduced by spatially separating Sb2Te3 and TiTe2 layer thanks to the semimetallic TiTe2, severing as a thermal barrier in the Reset process. Moreover, the stable crystalline TiTe2 layer provides ordered interface to speed up the crystallization process of amorphous Sb2Te3 layer, enabling ~10 ns ultrafast crystallization speed. Outstanding device lifetime, up to ~ 2×107 cycles, has been obtained, twice as long as alloy-based cell. Correlative electron microscopy and atom probe tomography evidence that TiTe2/Sb2Te3 multilayer can keep layer-stacked structure, avoiding phase segregation found in alloy and strong element intermixing in GeTe/Sb2Te3 superlattice, which enables excellent cyclability. This study suggests adding semimetallic layer in the phase change layer, like TiTe2 and TiSe2, can yield a phase change memory with superior properties. KEYWORDS: thermal barrier phase change material, atom probe tomography, Ti-Sb-Te, TiTe2, TiTe2/Sb2Te3, high endurance, low energy consumption


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1. INTRODUCTION Current computer generally uses hierarchical storage architecture, namely, SRAM/DRAM to temporarily keep data, and SSD (Solid State Drive)/Disk to permanently store information.1,2 There is a huge operating speed gap between DRAM and SSD, more than 105 times, severely slowing the computing speed.2 In 2008, Storage Class Memory (SCM) has been proposed by IBM to fill the performance gap. Phase change memory (PCM) is a foremost emerging memory candidate for SCM owing to its relatively fast operation speed, good scalability and multilevel storage ability.3 Differ from DRAM and SSD, storing data as charge, PCM uses physical structural difference in the chalcogenides to store digital information.4 These chalcogenides, typically GeTe-Sb2Te3 (GST) pseudo-binary alloy,5 undergo a rapid and reversible phase transition between amorphous and crystalline structures induced by varying electrical pulses. However, GST-based PCM requires mA-scale current at 90 nm node to melt the crystalline phase, called Reset operation, which imposes a stringent requirement on the selector device and also leads to too high-power consumption.6 Thus, the large programming current is still a key issue that limits the wide application of PCM. Many efforts have been made to decrease the Reset current of GST based PCM. One popular way is to reduce the effective contact area between the bottom electrode and chalcogenide film by replacing mushroom structure with edge-contact-type,7 ring-shape contract,8,9 confined10 or μTrench11 ones. Indeed, the Reset current has been dramatically reduced to μA level, however, a constant ~40 MA/cm2 current density is required to operate the PCM device, far beyond the maximum tolerable density (~8 MA/cm2) for metal wires in circuit.6 Another way is through improving the resistance of the crystalline phase by doping engineering. C, N, SiO2 as well as SiC have been generally incorporated into GST, which results in 30-60% current reduction.12,13 Unfortunately, this method often leads to serious phase segregation after repeated operation, which distinctly shortens the cell life-time.14,15,16,17 In 2011, a



significant step forward was the proposal to spatially separate GeTe and Sb2Te3, also called interfacial PCM (IPCM).18 IPCM device demonstrated ~50% reduction in Reset current. Recently, these GeTe/Sb2Te3 superlattices were found to completely reconfigures into an ordered GST structure after high temperature annealing.19 Since 2012, we have applied novel Ti-Sb-Te phase change materials in PCM to replace conventional GST alloy.20,21 PCM cells using Ti-Sb-Te film have demonstrated 82% reduction of Reset current compared to GST-based device thanks to the robust Ti-centered octahedra in both amorphous and crystalline states.22 Further investigation showed that Ti-centered octahedra not only exists inside the hexagonal Sb2Te3 lattice after the substitution of partial Sb by Ti, but also forms TiTe2 nano-lamellae inside the grain boundary.23 Since crystalline TiTe2 has quite lower thermal conductivity (0.12 W/mK) and higher electric conductivity (2.2E4 S/m) than those of crystalline Sb2Te3 (0.78 W/mK, 3.7E3 S/m),23,24 TiTe2 was believed to act as low-resistance thermal barrier to induce energy-inexpensive amorphization process.23 This suggests that it is beneficial to spatially separate Sb2Te3 and TiTe2 to lower the programming current. Interestingly, both TiTe2 and Sb2Te3 have hexagonal (HEX) crystalline structures with van der Waals gaps (vdWs), whereas their atomic stacking units between vdWs are different. HEX Sb2Te3 is stacked by quintuple layer along the c axis, -v-Te-Sb-Te-Sb-Te-v- (v represents vdWs),22 while TiTe2 is characterized by triple layer stacking rule, -v-Te-Ti-Te-v-.23 This implies that TiTe2 and Sb2Te3 can be used to construct superlattice-like materials along the c axis. In this work, we investigate device performances of TiTe2/Sb2Te3 multilayer-based PCM cells in detail. Element distribution in TiTe2/Sb2Te3 multilayers was studied by correlative electron microscopy and atom probe tomography. Molecular dynamic calculation and thermal simulation were employed to explain the enhanced device performances.

2. EXPERIMENTAL SECTION Film Deposition The thickness of TiTe2 triple layer is ~0.65 nm, and ~1 nm for Sb2Te3 quintuple


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layers. 2.6 nm-thick TiTe2/8 nm-thick Sb2Te3 multilayers were alternately deposited by physical vapor deposition. TiTe2 layer was fabricated by co-sputtering of Ti and Te targets with 50 DC power and 17 W RF power, respectively. The Sb2Te3 layer was deposited by a single Sb2Te3 alloy target with 7 W RF power. The growth velocities of TiTe2 and Sb2Te3 layer were 5.6 nm/min and 0.8 nm/min, respectively. In this work, TiTe2/Sb2Te3 film with total ~140 nm thickness was used in the PCM device, the cross-sectional transmission electron microscopy (TEM) image of which was presented Figure 1(a). Energy dispersive X-ray spectroscopy (EDX) mappings in Figure 1(b)-(f) confirmed the obtained film was stacked by alternative TiTe2 and Sb2Te3 layers. The device was annealed at 250 oC for 30 minutes to get the crystalline multilayer, the device performance of which was characterized by a parameter analyzer (Keithley 2400C) and a pulse generator (Tektronic AWG5200B). Cross-sectional specimens of film and operated cells were prepared using a dual-beam focused ion beam (FIB, FEI Helios Nanolab 600). To study the element diffusion in the crystalline film, 196 nm-thick multilayer deposited on silicon substrate was annealed at 250 oC for 30 minutes under vacuum, and then 600 nm-thick Sb2Te3 were deposited as a protective layer. GeTe/Sb2Te3 superlattice samples were fabricated by molecular beam epitaxy technology. The thickness of each GeTe and Sb2Te3 layer was 2.5 nm and 2 nm, respectively.

Figure 1. Structure of TiTe2/Sb2Te3 multilayer. (a)-(f) Scanning TEM-High angle annular dark field (HAADF) image and energy dispersive X-ray spectroscopy (EDX) mappings of crystalline TiTe2/Sb2Te3 multilayers. (g) X-ray diffraction patterns of TiTe2/Sb2Te3 multilayers annealed at different temperatures.



Thermal Simulations Thermal simulation was performed by Comsol 5.3 software. The thermal transfer obeyed the equation:25   T  Q  cT / t

where κ the thermal conductivity, T the temperature of the system, Q the Joule heat, ρ the density of each material, and c the specific heat. Here, we used κ of crystalline TiTe2, 0.12 W/mK,26 and c-Sb2Te3, ~0.78 W/mK.27 Meanwhile, ρ of c-TiTe2 (~6.3 g/cm3)28 and c-Sb2Te3 (~6.5 g/cm3)29, and c of c-TiTe2 (~1.80 J/cm3K)30 and c-Sb2Te3 (~1.02 J/cm3K)31, were used. The thickness of TiTe2/Sb2Te3 film was 190.8 nm, and each TiTe2 and Sb2Te3 layer was 7.8 nm and 24 nm, respectively. Moreover, 33 ns (@ 1.5 mA) current pulse was applied in analogy. Molecular dynamic calculation Theoretical simulations employed the density functional theory (DFT) with Vienna Ab initio Simulation Package (VASP).32 The projector augmented wave (PAW)33 potentials were used with the generalized gradient approximations (GGA) of Perdew−Burke−Ernzerhof (PBE) mexchange−correlation functional.34 Superlattice model with 3 nm-thick Sb2Te3 layer and 1.5 nm-thick TiTe2 layer was heated from 600 K to 1500K at 10 K/ps. The energy cutoff was 180 eV and the time step was 3 fs. Correlative TEM and Atom Probe Tomography Analysis Needled-sharped tips were prepared from crystalline TiTe2/Sb2Te3 multiple layers by standard lift-out procedure, using a dual-beam focused ion beam. These tips were mounted on a half-cut molybdenum TEM grid, further sharpened to a diameter of the apex less than 70 nm. Tips of GeTe/Sb2Te3 superlattices were prepared in the same way. Cs-TEM (JEOL JEM-ARM300F microscopy) was used for analyzing the morphology and element distribution of these tips with HAADF mode at 80 and 300 kV. Subsequently, tips were put into a 2.7×10-11 mbar high vacuum in a laser-assisted local electrode atom probe (LEAP 5000 XR, Cameca Instruments). A DC voltage of 2-8 kV with 45 pJ-50 pJ laser pulse was applied on the needle-shaped tips. The


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detection rate was 0.5%. The measuring temperature was 50 K. The detection Applied high voltage and laser pulse, the surface atoms would be ionized, the field evaporated and finally projected onto a position-sensitive detector (PSD). The x- and y-coordinates of the ions or ionic clusters were collected by the PSD, as well as the z-coordinate, which then were used to reconstruct the 3D mapping. Moreover, these ions and ionic clusters can be chemically identified based on time-of-flight mass spectrometry.35,36 3D maps of these tips were reconstructed, with the actual sizes of tips from Cs-TEM analysis, and analyzed by software IVAS 3.8.2. In this paper, detector efficiency and k factor are 0.50 and 3.30, respectively. Initial Tip Radii of TiTe2/Sb2Te3 multilayer and GeTe/Sb2Te3 superlattice are 73.85 nm and 49.19 nm based on TEM and SEM images, and corresponding Shank Angle are 12.54 degree and 7.86 degree, respectively. Both Sphere-Cone Radius Ratios have tangential continuity. The image compression factors of TiTe2/Sb2Te3 is 3.0 to keep the same layer thickness obtained from HADDF image, and that of GeTe/Sb2Te3 is 1.9.

3. RESULTS AND DISCUSSIONS

Figure 2 Comparison of Reset Current. Reset current of 190 nm-electrode PCM cells using Ge2Sb2Te5 alloy, Ti0.4Sb2Te3 alloy and TiTe2/Sb2Te3 multilayer. The Reset current of GeTe/Sb2Te3 IPCM with 75 nm-electrode in ref.18 is also shown for



comparison. X-ray diffraction patterns of as-deposited TiTe2/Sb2Te3 multilayers in Figure 1(g) show weak diffraction peaks at 26.32o and 56.32o, corresponding to (0 0 9) crystal faces of hexagonal (HEX) Sb2Te3 and (0 0 4) one of HEX TiTe2, respectively. Annealed at 100 oC for 3 minutes, another diffraction peak belonging to (1 1 0) TiTe2 appears at 48.13o, which suggests TiTe2 film is in the polycrystalline states. The TiTe2 grains hardly grow with increasing temperature, proved by similar peak intensity, whereas Sb2Te3 grains dramatically crystallizes into polycrystalline states. This is because TiTe2 film (2.6 nm/layer) is much thinner than Sb2Te3 film (8 nm/layer). TiTe2/Sb2Te3 multilayers were used in PCM cell, which then annealed at 250 oC for 30 min to achieve the crystalline state. Figure 2 displays the Reset current required for programming TiTe2/Sb2Te3 multilayer based PCM cell. For comparison, device performances of PCM devices using GST, Sb2Te3 (ST), Ti0.4Sb2Te3 (TST) alloy devices, with the same device structure, are also presented here. As shown in Figure 2, initially, all the cells are in the low resistance state. Applied 1000 ns-width pulse current, the resistance of these cells remains until the current reaches to a certain value, followed by a cliff-like increase. Consequently, these cells transform to a high resistance state. The current value, at which the resistance reaches to half of the highest resistance values, is called Reset current. For GST-based PCM cell, as presented in Figure 2, a large current, 5.8 mA, is required for the Reset operation, far beyond the current provided by selector device (Ge2Sb2Te5>GeTe). After inserting TiTe2 layer in ST, forming TBPCM, the programming speed profoundly decreases to 10 ns, four times faster than ST-based one. Moreover, lower voltage is required to complete the crystallization process. Obviously, TBPCM possesses a Set operation speed comparable to TST. The nanosecond switching feature of the latter is enabled by the robust Ti-centered octahedra in both amorphous and crystalline states, which reduces the nucleation time and ultimately accelerates the operation speed. But why the TBPCMs also have such rapid operation speed?

Figure 5 Structural evolution of TiTe2/Sb2Te3 multilayer at 600 K, 900 K, 1200 K and 1500 K. Sb2Te3 layer becomes disorder at 900 K but TiTe2 layer remains intact even at 1500 K. From the structural evolution of TiTe2/Sb2Te3 multilayer at different ambient temperature in Figure 5 obtained from molecular dynamic (MD) simulations39, we can find easily that ST layer almost completely disorders at 900 K. This is because the



low melting point of ST layer, ~893 K. Noticeably, the TiTe2 layer remains intact against thermal fluctuations even at ~1500 K owing to its ~1490 K melting point, 600 K higher than ST layer. This means that the crystalline TiTe2 layer would survive after the Reset operation. These TiTe2 layers in the TBPCM provide ordered interfaces to disorder ST layers, speeding up the crystallization process of amorphous ST, which enables the 10 ns switching speed.

Figure 6 Microstructure of TiTe2/Sb2Te3-based PCM after repeated operations. (a) TEM image of the device in the Reset operation. (b) and (c) High-resolution TEM images of the areas marked by blue and red rectangle, respectively. The insets are the corresponding fast Fourier transformation patterns. To check the state of TiTe2 layers after the Reset operation, the microstructure of TiTe2/Sb2Te3 cell after several Set-Reset cycles is investigated, as shown in Figure 6. The cell is in the Reset state. Figure 6(a) is the bright field (BF) image of the cell. As shown in Figure 6(c), the TiTe2 and ST layers above the tungsten electrode are still in


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the crystalline states, which is due to the lower temperature compared the concentered films as illustrated in Figure 3. Further selected area diffraction pattern indicates that the TiTe2 grain, highlighted by a green rectangle, is in (0 0 2) crystal face, and ST is in (0 0 15) crystal face. This means both layers growth along the c axis, forming superlattice-like structure in some areas. Noticeably, there are some ST layers found in the amorphous state, as shown in Figure 6(b), leading to the high resistance of the cell. The adjacent TiTe2 are also in the amorphous phase, which means the actual temperature, induced by electric current, is higher than the melting point. Even so, the crystalline TiTe2 layer found in Figure 6(c) still can provide an ordered interface to speed up the crystallization process. Similar melt-amorphized region has been observed in GeTe/ST superlattice after using large current (1.25 mA).18 Since 106 endurance, as shown in Figure S1. As we reported before, ST based PCM cell only can be repeatedly operated for ~200 times.24 This means the inserted TiTe2 layers significantly prolong the device life-time. The cyclability performance of TBPCM is also better than TST alloy-based PCM cell (~1×107 cycles).21 From these endurance results (Figure 7 and Figure S1), clearly, the failure of TBPCM is owing to the losing Set operation ability. This failure mechanism is called Reset-stuck, which is known to be ascribed to the formation of void after repeated operation.40 As shown in Figure 6, there is a big void in the W electrode of our cell owing to the preparation problem. As a result, the phase change layer would easily form void after repeated operation, thereby leading to the Reset-stuck failure. The failure mechanism is essentially different from that for TST based cells (Set-stuck),21 which is owing the phase segregation of TiTe2 and ST.23

Figure 8 Morphology and elements distribution of needle-shaped TBPCM by Cs-TEM analysis. (a) HAADF image of TBPCM tip. 600 nm-thick Sb2Te3 is deposited as protecting layer (P-L) in the preparation process. (b)-(g) HAADF image and corresponding EDX mappings of TBPCM and adjacent Silicon substrate.


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To analyze element distribution in TiTe2/Sb2Te3 TBPCM, we performed correlative Cs-TEM and atom probe tomography (APT) investigation. GeTe/Sb2Te3 superlattice is also investigated. The state-of-the-art APT equipment, assisting with pulse laser, can provide three dimensional (3D) mapping of semiconductor, with high elemental sensitivity(>1 ‰), which has been widely used to analyze atomic diffusion at the interfaces.35,36,41 Needle-shaped specimens of crystalline sample, with a diameter of the apex no more than 70 nm, were prepared by focused ion beam and then investigated by Cs-TEM, as shown in Figure 8 and Figure S2. 600 nm-thick ST was deposited as protecting layer in the preparation process. The high angle annular dark field (HAADF) image of crystalline TBPCM film in Figure 8(b) displays alternative dark and bright layers. This is because quite different atomic numbers (Z) of Ti and Sb atoms (ZTi=22, ZSb=51), which determines the intensity in HAADF mode (roughly proportional to Z1.7). Further EDX mappings in Figure 8 (c)-(d) confirm the ordered stacking TiTe2/Sb2Te3 TBPCM.

Figure 9 Reconstructed three-dimensional mappings and element distributions of GeTe/Sb2Te3 superlattice and TiTe2/Sb2Te3 TBPCM. (a)-(b) Reconstructed three-dimensions mappings of GeTe/Sb2Te3 superlattice and TiTe2/Sb2Te3 TBPCM, respectively. To reduce the influence of ion beams in the tip preparation process, we



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mainly analyze the inner areas of tips. (c)-(d) One dimensional concentration profiles of the region of interest in GeTe/Sb2Te3 superlattice (black dashed zone in Figure 9a) and TiTe2/Sb2Te3 TBPCM (red dashed zone in Figure 9b), respectively. The above tip was subsequently measured by APT, the results of which is shown in Figure 9. To reduce the influence of ion beams in the tip preparation process, we mainly analyze the inner parts of the needle-shaped tips. Figure 9(a) and (b) show reconstructed

three-dimensions

mappings

of

GeTe/Sb2Te3

superlattice

and

TiTe2/Sb2Te3 TBPCM, respectively. Obviously, the 3D mappings are characterized by multilayer stacking sequence, exhibiting different colors. Yet, each GeTe, TiTe2 and Sb2Te3 layer in GeTe/Sb2Te3 superlattice and TiTe2/Sb2Te3 TBPCM is thicker than the set value. This implies mutual element diffusion between adjacent layers. To get quantitative information, one dimensional concentration profiles of GeTe/Sb2Te3 superlattice and TiTe2/Sb2Te3 TBPCM is displayed in Figure 9(c) and (d), respectively. The highest concentration of Ge is found in the center of GeTe layer, ~30 at. %, which is ~20 at. % lower than the ideal one. It decreases dramatically toward ST layer. Similarly, the higshest concentration of Sb in ST layer is only ~22 at. %, half of the ideal value. The concentration of diffused Ge in the ST layer is ~20 at. %, almost equal to that of Sb. The stoichometry of diffused ST layer is close to Ge2Sb2Te5 (Ge: Sb: Te=22.5 at. %: 22.5 at. %: 55 at. %). This clearly shows the strong mutual intermixing of Ge/Sb atoms in the GeTe/Sb2Te3 superlattice, which agrees well with reported TEM results. This is due to quite close formation energy between

ordered-layer

structure

(-Sb-Te…-Te-Ge-)

and

mixed-layer

one

(Te-Ge/Sb-Te), just 0.018 eV/atom energy difference.42 In the case of TiTe2/Sb2Te3 layer (Figure 9(d)), the most serious situation is found in the TiTe2 layer, which contains ~15 at. % Sb diffused from adjacent ST layer. The diffusion of Ti atoms into the ST is also found in the TiTe2/Sb2Te3 interface, which disappears in the center of ST layer. This diffusion behavior can be directly observed from ion maps in Figure 10. From this figure, we can find that, ST PCM prefers to be


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evaporated as molecular, such as Sb2Te, Sb2Te3 and SbTe2, rather than single ion, like Sb and Te, significantly different from metals (Al, Fe) and other non-PCM semiconductor (Si, GeSe).36 Interestingly, TiTe2/Sb2Te3 film presents orientated diffusion behavior, more specifically, Sb tends to diffuse along the growth direction (Figure 10(a)-(c)), whereas Ti mainly to move along the opposite direction (Figure 10 (d)). Normalized concentration profile in Figure S3 further shows that the diffusion length of Ti against growth direction is 3.92 nm, more than twice that in another direction, only 1.8 nm, whereas Sb (Ge) diffusion in GeTe/Sb2Te3 is irregular in both directions (Figure 9c). All the TEM-APT investigations have demonstrated that TiTe2/Sb2Te3 can avoid the strong intermixing of cation found in GeTe/Sb2Te3 superlattice. This is owing to that the maximum Ti concentration for replacing Sb atom is ~10 at. %,43 while excess Ti atoms prefer to form TiTe2.23 The TiTe2/Sb2Te3 multilayer without strong intermixing is possibly responsible for its long device lifetime.

Figure 10 Ion maps of the TiTe2/Sb2Te3 TBPCM. Ion maps of (a) SbTe2, (b) Sb2Te3, (c) and (d) Ti ions. First three ion maps show Sb tends to diffuse along the growth direction, while Ti likes to move to the opposite direction from the last one.

4. CONCLUSIONS After observing the phase segregation of TiTe2 in Ti-Sb-Te PCM, which plays an important role in lowering the power consumption, we spatially separate Sb2Te3 and



TiTe2, namely, TiTe2/Sb2Te3 multilayers TBPCM. Our TiTe2/Sb2Te3-based TBPCM devices (with ~190 nm tungsten-heating electrode) have demonstrated ~10 ns ultrafast operation speed owing to the ordered interface provided by stable crystalline TiTe2 during Set operation. The required Reset current of the cell is just 0.5 mA after applying TiTe2/Sb2Te3 multilayer, which only 1/3 for TST based PCM and 1/10 of GST-based one. Moreover, the current is even 0.35 mA lower than GeTe/Sb2Te3 IPCM with 50 nm electrode. The ultralow programming current is attributed to the TiTe2 layer, severing as a thermal barrier in the Reset process. Outstanding cyclability, ~2×107 times, has been achieved. This is because the TiTe2/Sb2Te3 multilayer remain layer-stacked structure, without strong element intermixing. The TiTe2/Sb2Te3 film used in this work is multilayer rather than superlattice due to the limitation of film preparation equipment. With ~11% lattice mismatch, TiTe2 have been proved to form superlattice with Sb2Te3.44 We expect the device performance of TiTe2/Sb2Te3 TBPCM to be further improved using superlattice film, potentially realizing DRAM-like (fast, low power consummation and long-life) PCM applications.

ASSOCIATED CONTENT Support Information Figure S1 shows endurances of TiTe2/Sb2Te3 based PCM cell. Figure S2 shows HAADF image and corresponding EDX mappings of TBPCM and adjacent Silicon substrate and Sb2Te3 protecting layer. Figure S3 shows nomalized Ti concentration distribution along Z axis.

ACKNOWLEDGMENTS Financial Supported by National Key Research and Development Program of C hina (2017YFB0206101, 2017YFB0405601), Strategic Priority Research Program of the Chinese Academy of Sciences (XDPB12), National Natural Science Foundat ion of China (61504157). M. Zhu acknowledges support by Hundred Talent Pr ogram (Chinese Academy of Sciences) and Shanghai Pujiang Talent Program (


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18PJ1411100).



REFERENCES (1) Atwood, G., Engineering. Phase-Change Materials for Electronic Memories. Science 2008, 321, 210-211. (2) Burr, G. W.; Breitwisch, M. J.; Franceschini, M.; Garetto, D.; Gopalakrishnan, K.; Jackson, B.; Kurdi, B.; Lam, C.; Lastras, L. A.; Padilla, A.; Rajendran, B.; Raoux, S.; Shenoy, R. S., Phase Change Memory Technology. J. Vac. Sci. Technol., B. 2010, 28, 223-262. (3) Andry, P. S.; Tsang, C. K.; Webb, B. C.; Sprogis, E. J.; Wright, S. L.; Dang, B.; Manzer, D. G., Fabrication and Characterization of Robust Through-Silicon Vias for Silicon-Carrier Applications. Ibm J. Res. Dev. 2008, 52, 571-581. (4) Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. (5) Yamada, N.; Ohno, E.; Nishiuchi, K.; Akahira, N.; Takao, M., Rapid‐phase Transitions of GeTe‐Sb2Te3 Pseudobinary Amorphous Thin Films for An Optical Disk Memory. J. Appl. Phys. 1991, 69, 2849-2856. (6) Wong, H. S. P.; Raoux, S.; Kim, S.; Liang, J.; Reifenberg, J. P.; Rajendran, B.; Asheghi, M.; Goodson, K. E., Phase Change Memory. Proc. IEEE 2010, 98, 2201-2227. (7) Ha, Y. H.; Yi, J. H.; Horii, H.; Park, J. H.; Joo, S. H.; Park, S. O.; Chung, U. I.; Moon, J. T., An Edge Contact Type Cell for Phase Change RAM Featuring Very Low Power Consumption. Symp. VLSI Technol, Dig. Technol Pap. 2003, pp 175-176. (8) Chen, Y. C.; Rettner, C. T.; Raoux, S.; Burr, G. W.; Chen, S. H.; Shelby, R. M.; Salinga, M.; Risk, W. P.; Happ, T. D.; McClelland, G. M.; Breitwisch, M.; Schrott, A.; Philipp, J. B.; Lee, M. H.; Cheek, R.; Nirschl, T.; Lamorey, M.; Chen, C. F.; Joseph, E.; Zaidi, S.; Yee, B.; Lung, H. L.; Bergmann, R.; Lam, C., Ultra-Thin Phase-Change Bridge Memory Device Using GeSb. IEEE Int. Electron Devices Meet. 2006, 531-534. (9) Oh, J. H.; Park, J. H.; Lim, Y. S.; Lim, H. S.; Oh, Y. T.; Kim, J. S.; Shin, J. M.; Park, J. H.; Song, Y. J.; Ryoo, K. C.; Lim, D. W.; Park, S. S.; Kim, J. I.; Kim, J. H.; Yu, J.; Yeung, F.; Jeong, C. W.; Kong, J. H.; Kang, D. H.; Koh, G. H.; Jeong, G. T.; Jeong, H. S.; Kim, K., Full Integration of Highly Manufacturable 512Mb PRAM Based on 90nm Technology. IEEE Int. Electron Devices Meet. 2006, 1-4. (10) Hwang, Y. N.; Lee, S. H.; Ahn, S. J.; Lee, S. Y.; Ryoo, K. C.; Hong, H. S.; Koo, H. C.; Yeung, F.; Oh, J. H.; Kim, H. J.; Jeong, W. C.; Park, J. H.; Horii, H.; Ha, Y. H.; Yi, J. H.; Koh, G. H.; Jeong, G. T.; Jeong, H. S.; Kim, K., Writing Current Reduction for High-Density Phase-Change RAM. IEEE Int. Electron Devices Meet. 2003, 893-896. (11) Pellizzer, F.; Benvenuti, A.; Gleixner, B.; Kim, Y.; Johnson, B.; Magistretti, M.; Marangon, T.; Pirovano, A.; Bez, R.; Atwood, G., A 90nm Phase Change Memory Technology for Stand-Alone Non-Volatile Memory Applications. Symp. VLSI Technol, Dig. Technol Pap. 2006, pp 122-123. (12)Czubatyj, W.; Lowrey, T.; Kostylev, S., Current Reduction in Ovonic Memory Devices. EPCOS 2006.


Page 20 of 24

Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13)Hubert, Q.; Jahan, C.; Toffoli, A.; Navarro, G.; Chandrashekar, S.; Noe, P.; Blachier, D.; Sousa, V.; Perniola, L.; Nodin, J. F.; Persico, A.; Kies, R.; Maitrejean, S.; Roule, A.; Henaff, E.; Tessaire, M.; Zuliani, P.; Annunziata, R.; Pananakakis, G.; Reimbold, G.; De Salvo, B., Lowering the Reset Current and Power Consumption of Phase-Change Memories with Carbon-Doped Ge2Sb2Te5. IEEE Int. Mem. Worksh. 2012, 1-4. (14)Borisenko, K. B.; Chen, Y.; Cockayne, D. J. H.; Song, S. A.; Jeong, H. S., Understanding Atomic Structures of Amorphous C-Doped Ge2Sb2Te5 Phase-Change Memory Materials. Acta Mater. 2011, 59, 4335-4342. (15) Song, S. A.; Zhang, W.; Sik Jeong, H.; Kim, J. G.; Kim, Y. J., In Situ Dynamic HR-TEM and EELS Study on Phase Transitions of Ge2Sb2Te5 Chalcogenides. Ultramicroscopy 2008, 108, 1408-1419. (16)Kim, Y.; Jeong, K.; Cho, M. H.; Hwang, U.; Jeong, H. S.; Kim, K., Changes in the Electronic Structures and Optical Band Gap of Ge2Sb2Te5 and N-Doped Ge2Sb2Te5 during Phase Transition. Appl. Phys. Lett. 2007, 90, 171920. (17)Ryu, S. W.; Lyeo, H. K.; Lee, J. H.; Ahn, Y. B.; Kim, G. H.; Kim, C. H.; Kim, S. G.; Lee, S. H.; Kim, K. Y.; Kim, J. H.; Kim, W.; Hwang, C. S.; Kim, H. J., SiO2 Doped Ge2Sb2Te5 Thin Films with High Thermal Efficiency for Applications in Phase Change Random Access Memory. Nanotechnology 2011, 22, 254005. (18)Simpson, R. E.; Fons, P.; Kolobov, A. V.; Fukaya, T.; Krbal, M.; Yagi, T.; Tominaga, J., Interfacial Phase-Change Memory. Nat. Nanotechnol. 2011, 6, 501-505. (19)Momand, J.; Wang, R.; Boschker, J. E.; Verheijen, M. A.; Calarco, R.; Kooi, B. J., Interface Formation of Two- and Three-Dimensionally Bonded Materials in the Case of GeTe-Sb2Te3 Superlattices. Nanoscale 2015, 7, 19136-19143. (20)Zhu, M.; Wu, L. C.; Song, Z. T.; Rao, F.; Cai, D. L.; Peng, C.; Zhou, X. L.; Ren, K.; Song, S. N.; Liu, B.; Feng, S. L., Ti10Sb60Te30 for Phase Change Memory with High-Temperature Data Retention and Rapid Crystallization Speed. Appl. Phys. Lett. 2012, 100, 824. (21)Zhu, M., Ti-Sb-Te Phase Change Materials: Component Optimisation, Mechanism and Applications. Springer Theses: 2017. (22)Zhu, M.; Xia, M. J.; Rao, F.; Li, X. B.; Wu, L. C.; Ji, X. L.; Lv, S. L.; Song, Z. T.; Feng, S. L.; Sun, H. B.; Zhang, S. B., One Order of Magnitude Faster Phase Change at Reduced Power in Ti-Sb-Te. Nat. Commun. 2014, 5, 4086. (23)Rao, F.; Song, Z.; Cheng, Y.; Liu, X.; Xia, M.; Li, W.; Ding, K.; Feng, X.; Zhu, M.; Feng, S., Direct Observation of Titanium-Centered Octahedra in Titanium-Antimony-Tellurium Phase-Change Material. Nat. Commun. 2015, 6, 10040. (24)Xia, M.; Zhu, M.; Wang, Y.; Song, Z.; Rao, F.; Wu, L.; Cheng, Y.; Song, S., Ti-Sb-Te Alloy: a Candidate for Fast and Long-Life Phase-Change Memory. ACS Appl. Mater. Interfaces 2015, 7, 7627-7634. (25)Kang, D.-H.; Ahn, D.-H.; Kim, K.-B.; Webb, J. F.; Yi, K.-W., One-Dimensional Heat Conduction Model for An Electrical Phase Change Random Access Memory Device with An 8F2 Memory cell (F=0.15 μm). J. Appl. Phys. 2003, 94, 3536-3542. (26)Chiritescu, C.; Mortensen, C.; Cahill, D. G.; Johnson, D.; Zschack, P., Lower



Limit to the Lattice Thermal Conductivity of Nanostructured Bi2Te3-Based Materials. J. Appl. Phys. 2009, 106, 634-126 (27)Chen, J.; Sun, T.; Sim, D.; Peng, H.; Wang, H.; Fan, S.; Hng, H. H.; Ma, J.; Boey, F. Y. C.; Li, S.; Samani, M. K.; Chen, G. C. K.; Chen, X.; Wu, T.; Yan, Q., Sb2Te3 Nanoparticles with Enhanced Seebeck Coefficient and Low Thermal Conductivity. Chem. Mater. 2010, 22, 3086-3092. (28)McTaggart, F. K.; Moore, A., The sulphides, Selenides, and Tellurides of Titanium, Zirconium, Hafnium, and Thorium. IV. Lubrication Properties of the Graphitic Chalcogenides. Aust. J. Chem. 1958, 11, 445-457. (29)Anderson, T. L.; Krause, H. B., Refinement of the Sb2Te3 and Sb2Te2Se Structures and Their Relationship to Nonstoichiometric Sb2Te3−ySey Compounds. Acta Crystallogr. B 1974, 30, 1307-1310. (30)Craven, R. A.; Di Salvo, F. J.; Hsu, F. S. L., Mechanisms for the 200 K Transition in TiSe2: A Measurement of the Specific Heat. Solid State Commun. 1978, 25, 39-42. (31)Yáñez-Limón, J. M.; González-Hernández, J.; Alvarado-Gil, J. J.; Delgadillo, I.; Vargas, H., Thermal and Electrical Properties of the Ge:Sb:Te System by Photoacoustic and Hall Measurements. Phys. Rev. B 1995, 52, 16321-16324. (32) Kresse, G.; Hafner, J., Ab Initiomolecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558-561. (33)Blöchl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50 (24), 17953-17979. (34)Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (35)Gault, B.; Moody, M. P.; Cairney, J. M.; Ringer, S. P., Atom Probe microscopy. Springer: 2012. (36)Zhu, M.; Cojocaru-Miredin, O.; Mio, A. M.; Keutgen, J.; Kupers, M.; Yu, Y.; Cho, J. Y.; Dronskowski, R.; Wuttig, M., Unique Bond Breaking in Crystalline Phase Change Materials and the Quest for Metavalent Bonding. Adv. Mater. 2018, 30, 1706735. (37)Verdy, A.; Navarro, G.; Sousa, V.; Noe, P.; Bernard, M.; Fillot, F.; Bourgeois, G.; Garrione, J.; Perniola, L., Improved Electrical Performance Thanks to Sb and N Doping in Se-rich GeSe-Based OTS Selector Devices. IEEE Int. Mem. Worksh. 2017, 84-87. (38)Perniola, L.; Sousa, V.; Fantini, A.; Arbaoui, E.; Bastard, A.; Armand, M.; Fargeix, A.; Jahan, C.; Nodin, J.-F.; Persico, A.; Blachier, D.; Toffoli, A.; Loubriat, S.; Gourvest, E.; Betti Beneventi, G.; Feldis, H.; Maitrejean, S.; Lhostis, S.; Roule, A.; Cueto, O.; Reimbold, G.; Poupinet, L.; Billon, T.; De Salvo, B.; Bensahel, D.; Mazoyer, P.; Annunziata, R.; Zuliani, P.; Boulanger, F., Electrical Behavior of Phase-Change Memory Cells Based on GeTe. IEEE Int. Electron Devices Meet. 2010, 31, 488-490. (39)Zhou, X.; Kalikka, J.; Ji, X.; Wu, L.; Song, Z.; Simpson, R. E., Phase-Change Memory Materials by Design: A Strain Engineering Approach. Adv. Mater. 2016, 28, 3007-3016.


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Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40)Li, W. Y.; Zhang, C.; Guo, X. P.; Li, C. J.; Liao, H. L.; Coddet, C., Study on Impact Fusion at Particle Interfaces and Its Effect on Coating Microstructure in Cold Spraying. Appl. Surf. Sci. 2007, 254, 517-526. (41)Müller, M.; Gault, B.; Field, M.; Sullivan, G. J.; Smith, G. D. W.; Grovenor, C. R. M., Interfacial Chemistry in An InAs/GaSb Superlattice Studied by Pulsed Laser Atom Probe Tomography. Appl. Phys. Lett. 2012, 100, 2545. (42)Sun, Z.; Zhou, J.; Ahuja, R., Structure of Phase Change Materials for Data Storage. Phys. Rev. Lett. 2006, 96, 055507. (43)Zhu, M.; Wu, L.; Rao, F.; Song, Z.; Xia, M.; Ji, X.; Lv, S.; Feng, S., The Micro-Structure and Composition Evolution of Ti-Sb-Te Alloy During Reversible Phase Transition in Phase Change Memory. Appl. Phys. Lett. 2014, 104, 063105. (44)Schmid, B. A.; Rostek, R.; Mortensen, C.; Johnson, D. C., Synthesis of (Sb2Te3)x(TiTe2)y Superlattices. ICT Thermo. 2005, 255-257.



TABLE OF CONTENTS GRAPHIC


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Thermal Barrier Phase Change Memory - ACS Applied Materials

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