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![a) XRD patterns of the highly (0 0 l) oriented [Sb 2 Te 1 –GeTe]n superlattices, n is the number of the periodic structure. The XRD data of the layered Sb 2 Te 1 crystal are included for comparison. The lattice constants, a, listed above the curves were calculated by the Rietveld refinement procedure. b) The in-plane biaxial strain of GeTe layer versus the Sb x Te 1–x layer thickness . The strained levels in all strained superlattices meet the demand for atomic switching.](profile/Robert-Simpson-27/publication/293634435/figure/fig2/AS:327814520819712@1455168480087/a-XRD-patterns-of-the-highly-0-0-l-oriented-Sb-2-Te-1-GeTen-superlattices-n-is-the.png)
a) XRD patterns of the highly (0 0 l) oriented [Sb 2 Te 1 –GeTe]n superlattices, n is the number of the periodic structure. The XRD data of the layered Sb 2 Te 1 crystal are included for comparison. The lattice constants, a, listed above the curves were calculated by the Rietveld refinement procedure. b) The in-plane biaxial strain of GeTe layer versus the Sb x Te 1–x layer thickness . The strained levels in all strained superlattices meet the demand for atomic switching.
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Van der Waals heterostructure superlattices of Sb2 Te1 and GeTe are strain engineered to promote switchable atomic disordering, which is confined to the GeTe layer. Careful control of the strain in the structures presents a new degree of freedom to design the properties of functional superlattice structures for data storage and photonics applicatio...
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Citations
... Therefore, the surface energy of these crystal planes is arranged as follows: 1010 > 0116 > 1120 > 1013 > (0001). It has been suggested that the minimum surface energy supports (000l) orientation formation in Sb 2 Te thin film [20]. This implies that the surface energy is not the most important reason for prismatic preference. ...
... This may be an acceptable explanation. Zhou et al. [20] suggested that Sb 2 Te lattice with compressed c-axis is more stable because of the restricted atom free space. Therefore, the lattice strain is an important factor for the prismatic preference, and the prismatic texture could be maintained in Sb 2 Te thin film. ...
We investigated the texture formation of Sb2Te thin films for phase change memory applications. The Sb2Te thin films with different thicknesses were deposited on Si (100) wafers by the magnetron sputtering method. As-deposited Sb2Te thin films were annealed at various temperatures and times. The texture characterization was performed by using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Experimental results show that the annealed Sb2Te thin films exhibit the 112¯0 and 101¯0 prismatic texture. The formation of prismatic texture is induced by the lattice strain, surface energy, and coarse grains, in which the lattice strain is the essential origin of prismatic preference. Electronic transport properties of Sb2Te thin films were monitored by a physical property measurement system (PPMS). It was found that the formation of prismatic texture promotes the increase of carrier mobility. The stability of the film–substrate interface was also assessed by calculating mismatch. The prismatic-preferred Sb2Te thin films exhibit higher mismatch with a silicon wafer, reducing the interface stability.
... Mater. 2023, 2208065 [6,[18][19][20][32][33][34][35][36][37][38][39][40][41][42] GST-based, GeTe-based, and superlattice-like PCMs are usually able to store information with several to tens of nanoseconds. SbTe-based PCMs can be operated with shorter pulses (several nanoseconds), and 700-ps is achieved by the Sc-Sb-Te PCM. ...
... The data marked with star were obtained from Figures S2, S3, S9, and S10, Supporting Information, while other data were obtained from previous studies (refs. [6,[18][19][20], and [32][33][34][35][36][37][38][39][40][41][42]). ...
Phase change memory (PCM) is one of the most promising candidates for next‐generation data‐storage technology, the programming speed of which has enhanced within a timescale from milliseconds to subnanosecond (∼500 picoseconds) through decades of effort. As the potential applications of PCM strongly depend on the switching speed, namely, the time required for the recrystallization of amorphous chalcogenide media, the finding of the ultimate crystallization speed is of great importance both theoretically and practically. Here, through systematic analysis of discovered phase change materials and ab initio molecular dynamics simulations, we first predict elemental Sb‐based PCM may have a superfast crystallization speed. Indeed, such cells experimentally present extremely fast crystallization speeds within 360 ps. Remarkably, the recrystallization process is further speeded up as the device shrinks, and a record fast crystallization speed of only 242 ps is achieved in 60 nm‐size devices. These findings open opportunities for DRAM‐like and even cache‐like PCM using appropriate storage materials. This article is protected by copyright. All rights reserved
... Subsequent studies, however, revealed a crystalline-to-amorphous phase-transition behavior in a similar SL structure, indicating the possible controversy in the switching mechanism. Different mechanisms such as enhanced thermal efficiency, [10][11][12][13][14] stacking fault [15] or strain-assisted transition, [16,17] and partial amorphization [18] have also been suggested in more recent works. Still, studies so far lack a detailed analysis of the crystalline state in the set state. ...
... The reset and set processes occur in the (111)-oriented FCC GST and amorphous states, respectively. Therefore, the low-current amorphization due to the high thermal efficiency of SL suggested by other groups [4,13,17] cannot occur in this case. ...
This work demonstrates the atomic layer deposition (ALD) of Sb2 Te3 /GeTe superlattice (SL) film on planar and vertical sidewall areas containing TiN metal and SiO2 insulator. The peculiar chemical affinity of the ALD precursor to the substrate surface and the two-dimensional nature of the Sb2 Te3 enabled the growth of an in-situ crystallized SL film with a preferred orientation. The SL film showed a reduced reset current of ∼ 1/7 of the randomly oriented Ge2 Sb2 Te5 alloy. The reset switching was induced by the transition from the SL to the (111)-oriented face-centered-cubic (FCC) Ge2 Sb2 Te5 alloy and subsequent melt-quenching-free amorphization. The in-plane compressive stress, induced by the SL-to-FCC structural transition, enhanced the electromigration of Ge along the [111] direction of FCC structure, which enabled such a significant improvement. Set operation switched the amorphous to the (111)-oriented FCC structure. This article is protected by copyright. All rights reserved.
... Indeed, the GeTe layers embedded within the Sb 2 Te 3 or Sb 2 Te 1 scaffold exhibit premelting, which is similar to amorphization but limited to the GeTe layers. The Sb 2 Te 3 films remain crystalline and the layered structure is uncompromised [22,30]. It is, therefore, important to confirm whether the Ti 3.6 -(Sb 2 Te 3 ) 96.4 -GeTe superlattice also exhibits premelting. ...
Phase change memory devices are typically reset by melt-quenching a material to radically lower its electrical conductance. The high power and concomitantly high current density required to reset phase change materials is the major issue that limits the access times of 3D phase change memory architectures. Phase change superlattices were developed to lower the reset energy by confining the phase transition to the interface between two different phase change materials. However, the high thermal conductivity of the superlattices means that heat is poorly confined within the phase change material, and most of the thermal energy is wasted to the surrounding materials. Here, we identified Ti as a useful dopant for substantially lowering the thermal conductivity of Sb2Te3-GeTe superlattices whilst also stabilising the layered structure from unwanted disordering. We demonstrate via laser heating that lowering the thermal conductivity by doping the Sb2Te3 layers with Ti halves the switching energy compared to superlattices that only use interfacial phase change transitions and strain engineering. The thermally optimized superlattice has (0 0 l) crystallographic orientation yet a thermal conductivity of just 0.25 W/m.K in the "on" (set) state. Prototype phase change memory devices that incorporate this Ti-doped superlattice switch faster and and at a substantially lower voltage than the undoped superlattice. During switching the Ti-doped Sb2Te3 layers remain stable within the superlattice and only the Ge atoms are active and undergo interfacial phase transitions. In conclusion, we show the potential of thermally optimised Sb2Te3-GeTe superlattices for a new generation of energy-efficient electrical and optical phase change memory.
... Another exotic characteristic of iPCM structure is that it can exhibit a topologically nontrivial band structure, and its properties can be engineered through strain controlling approach. 244,245 Then in 2019, Okabe et al. investigated the switching mechanism of GeTe−Sb 2 Te 3 iPCM and found that the thermal properties of iPCM account only for ∼13% reduction of RESET current change when compared with traditional GST alloys. 246 Another significant cause of the reduced RESET energy is the void migration process, in which the random voids distributed in iPCM move and concentrate around BEC, leading to smaller BEC area and RESET current. ...
Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.
... Previous reports showed that when the GeTe strain is controlled in the superlattice structure, the bonding strength of the Ge-Te changes, and the crystallization energy changes accordingly. [47][48][49] However, in the trigonal GST structure, no structure exists in which the GeTe layer is strained between Sb 2 Te 3 layers. The SLs in the structure consisting solely of t-GST after heat treatment no longer had fast and low energy switching characteristics because of the t-GST stability. ...
Interfacial phase-change memory (iPCM), comprising alternating layers of two chalcogenide-based phase-change materials—Sb2Te3 (ST) and GeTe (GT)—has demonstrated outstanding performance in resistive memories. However, its comprehensive understanding is controversial. Herein, the phase-change characteristic of iPCM is identified using atomic scale imaging, X-ray diffraction, and chemical analysis with first-principles density functional theory (DFT) calculations. By inducing laser pulsing, the ST/GT superlattice structure in the low-resistance state tends to reversibly convert into the modified metastable face-centered cubic (fcc) GeSbTe structure in the high-resistance state. This transition is driven by Ge atom rearrangement to pre-existing vacancy layers and ordered vacancy-layer formation. DFT atomistic modeling shows that the resistance difference of 10² orders between low- and high-resistance states is a direct consequence of the intercalation of Ge atoms into the vacancy layer. These results provide insights into iPCM phase-change mechanisms and phase-change random access memory design with low energy and high speed.
... As the number of grain boundaries increases, the crystal diffusion and slippage can be reduced. Hence, the residual stress in the bulk of films can be degraded [24,25]. Moreover, the increased grain boundaries provide a phonon and electron scattering center, and the decreased thermal and electrical conductivity will improve the energy efficiency of the Joule heating [26]. ...
Phase change memory (PCM), due to the advantages in capacity and endurance, has the opportunity to become the next generation of general-purpose memory. However, operation speed and data retention are still bottlenecks for PCM development. The most direct way to solve this problem is to find a material with high speed and good thermal stability. In this paper, platinum doping is proposed to improve performance. The 10-year data retention temperature of the doped material is up to 104 °C; the device achieves an operation speed of 6 ns and more than 3 × 105 operation cycles. An excellent performance was derived from the reduced grain size (10 nm) and the smaller density change rate (4.76%), which are less than those of Ge2Sb2Te5 (GST) and Sb2Te3. Hence, platinum doping is an effective approach to improve the performance of PCM and provide both good thermal stability and high operation speed.
... Apart from thermal annealing 17 , a variety of approaches have emerged for tuning disorder in GST compounds. These include laser excitation 18,19 , focused electron beam irradiation 20 , tuning of chemical composition 21,22 , interface templates [22][23][24] , or the application of pressure [25][26][27][28][29][30] , strain 31,32 , and voltage pulses [33][34][35] . Ion irradiation facilitates precisely tailoring disorder in solids via nuclear collision cascades of the impinging ions; thus, multiple studies have investigated the effect of defect engineering of GST compounds [36][37][38][39][40][41][42][43][44] . ...
Phase-change materials serve a broad field of applications ranging from non-volatile electronic memory to optical data storage by providing reversible, repeatable, and rapid switching between amorphous and crystalline states accompanied by large changes in the electrical and optical properties. Here, we demonstrate how ion irradiation can be used to tailor disorder in initially crystalline Ge2Sb2Te5 (GST) thin films via the intentional creation of lattice defects. We found that continuous Ar ion irradiation at room temperature of GST films causes complete amorphization of GST when exceeding 0.6 (for rock-salt GST) and 3 (for hexagonal GST) displacements per atom (n_dpa). While the transition from rock-salt to amorphous GST is caused by progressive amorphization via the accumulation of lattice defects, several transitions occur in hexagonal GST upon ion irradiation. In hexagonal GST, the creation of point defects and small defect clusters leads to disordering of intrinsic vacancy layers (van der Waals gaps) that drives the electronic metal-insulator transition. Increasing disorder then induces a structural transition from hexagonal to rock-salt and then leads to amorphization. Furthermore, we observed different annealing behavior of defects for rock-salt and hexagonal GST. The higher amorphization threshold in hexagonal GST compared to rock-salt GST is caused by an increased defect-annealing rate, i.e., a higher resistance against ion-beam-induced disorder. Moreover, we observed that the recovery of defects in GST is on the time scale of seconds or less at room temperature.
... Apart from thermal annealing [17], a variety of approaches have emerged for tuning disorder in GST compounds. These include laser excitation [18,19], focused electron beam irradiation [20], tuning of chemical composition [21,22], interface templates [22][23][24], or the application of pressure [25][26][27][28][29][30], strain [31,32], and voltage pulses [33][34][35]. Ion irradiation facilitates precisely tailoring disorder in solids via nuclear collision cascades of the impinging ions; thus, multiple studies have investigated the effect of defect engineering of GST compounds [36][37][38][39][40][41][42][43][44]. ...
Phase-change materials serve a broad field of applications ranging from non-volatile electronic memory to optical data storage by providing reversible, repeatable, and rapid switching between amorphous and crystalline states accompanied by large changes in the electrical and optical properties. Here, we demonstrate how ion irradiation can be used to tailor disorder in initially crystalline Ge 2 Sb 2 Te 5 (GST) thin films via the intentional creation of lattice defects. We found that continuous Ar ⁺ -ion irradiation at room temperature of GST films causes complete amorphization of GST when exceeding 0.6 (for rock-salt GST) and 3 (for hexagonal GST) displacements per atom ( n dpa ). While the transition from rock-salt to amorphous GST is caused by progressive amorphization via the accumulation of lattice defects, several transitions occur in hexagonal GST upon ion irradiation. In hexagonal GST, the creation of point defects and small defect clusters leads to the disordering of intrinsic vacancy layers (van der Waals gaps) that drives the electronic metal–insulator transition. Increasing disorder then induces a structural transition from hexagonal to rock-salt and then leads to amorphization. Furthermore, we observed different annealing behavior of defects for rock-salt and hexagonal GST. The higher amorphization threshold in hexagonal GST compared to rock-salt GST is caused by an increased defect-annealing rate, i.e., a higher resistance against ion-beam-induced disorder. Moreover, we observed that the recovery of defects in GST is on the time scale of seconds or less at room temperature.
... Since the pioneering work conducted by Ovshinsky [327], these materials have attracted extensive attention and are considered the leading candidates for non-volatile memory applications [328]. Most studies of PCMs focus on design of novel systems [329,330], switching speed [331,332], scalability of the nanosized devices [333,334], and material design [319,335]. Among them, the switching from the amorphous to crystalline state, namely the crystallization, is deemed as the key factor affecting the writing speed [336,337]. ...
The past two decades have witnessed the rapid development of nanocalorimetry, a novel materials characterization technique that employs micromachined calorimetric sensors. The key advances of this technique are the ultrahigh scanning rate, which can be as high as 10⁶ K/s, and the ultrahigh heat capacity sensitivity, with a resolution typically better than 1 nJ/K. Nanocalorimetry has attracted extensive attention in the field of materials science, where it is applied to perform quantitative analysis of rapid phase transitions. This paper reviews the development of nanocalorimetry over the last three decades and summarizes its applications to various materials ranging from polymers to metals. The glass transition and crystallization of non-crystalline materials, melting and solidification of metallic droplets, and solid-state phase transitions of thin films are introduced as typical examples. Furthermore, nanocalorimetry coupled with structural characterization techniques, such as transmission electron microscopy and synchrotron X-ray diffraction, is presented. Finally, current challenges and future outlooks for the technique are discussed. Given the unique attributes of the technique, we expect nanocalorimetry to attract increasing attention, especially with regard to characterization of fast phase transitions and evaluation of size effects.
