Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2 Te3 Superlattices Used in Interfacial Phase-Change Memory (iPCM) Devices

GeTe/MoTe2 Van der Waals Heterostructures: Enabling Ultralow Voltage Memristors for Nonvolatile Memory and Neuromorphic Computing Applications

Advanced electronic semiconducting Van der Waals heterostructures (HSs) are promising candidates for exploring next-generation nanoelectronics owing to their exceptional electronic properties, which present the possibility of extending their functionalities to diverse potential applications. In this study, GeTe/MoTe2 HS are explored for nonvolatile memory and neuromorphic-computing applications. Sputter-deposited Ag/GeTe/MoTe2/Pt HS cross-point devices are fabricated, and they demonstrate memristor behavior at ultralow switching voltages (VSET: 0.15 V and VRESET: -0.14 V) with very low energy consumption (≈30 nJ), high memory window, long retention time (104 s), and excellent endurance (105 cycles). Resistive switching is achieved by adjusting the interface between the Ag top electrode and the heterojunction switching layer. Cross-sectional transmission electron microscope images and conductive atomic force microscopy analysis confirm the presence of a conducting filament in the heterojunction switching layer. Further, emulating various synaptic functions of a biological synapse reveals that GeTe/MoTe2 HS can be utilized for energy-efficient neuromorphic-computing applications. A multilayer perceptron is implemented using the synaptic weights of the Ag/GeTe/MoTe2/Pt HS device, revealing high pattern accuracy (81.3%). These results indicate that HS devices can be considered a potential solution for high-density memory and artificial intelligence applications.

Space-Confined Growth of Ultrathin P-Type GeTe Nanosheets for Broadband Photodetectors

As p-type phase-change degenerate semiconductors, crystalline and amorphous germanium telluride (GeTe) exhibit metallic and semiconducting properties, respectively. However, the massive structural defects and strong interface scattering in amorphous GeTe films significantly reduce their performance. In this work, two-dimensional (2D) p-type GeTe nanosheets are synthesized via a specially designed space-confined chemical vapor deposition (CVD) method, with the thickness of the GeTe nanosheets reduced to 1.9 nm. The space-confined CVD method improves the crystallinity of ultrathin GeTe by lowering the partial pressure of the reactant gas, resulting in GeTe nanosheets with excellent p-type semiconductor properties, such as a satisfactory on/off ratio of 105 . Temperature-dependent electrical measurements demonstrate that variable-range hopping and optical-phonon-assisted hopping mechanisms dominate transport behavior at low and high temperatures, respectively. GeTe devices exhibit significantly high responsivity (6589 and 2.2 A W-1 at 633 and 980 nm, respectively) and detectivity (1.67 × 1011 and 1.3 × 108 Jones at 633 and 980 nm, respectively), making them feasible for broadband photodetectors in the visible to near-infrared range. Furthermore, the fabricated GeTe/WS2 diode exhibits a rectification ratio of 103 at zero gate voltage. These satisfactory p-type semiconductor properties demonstrate that ultrathin GeTe exhibits enormous potential for applications in optoelectronic interconnection circuits.

Nanoscale fabrication of heterostructures in thermoelectric SnTe

Enhancing the performance of thermoelectric materials is demanded to develop strategies for introducing multidimensional microstructures into materials to induce full-scale phonon scattering while ensuring electrical transport performance. Herein, a previously unreported rhombohedral h-SnTe (R3̄m) has been achieved in the nanoscale dimension by the electron beam irradiation of β-SnTe (Fm3̄m) materials. The h-SnTe structure contains interlayer van der Waals gaps and exhibits metallic behavior evaluated by density-functional theory calculations, which coherently appears in the narrow-band semiconductor β-SnTe matrix. Our results provide a strategy for modifying the properties of SnTe-based thermoelectric materials and designing nanostructured chalcogenide heterostructures via electron beam irradiation.

Designing a Multilayered Oxygen Barrier Structure to Tackle Oxidation Challenges in Phase-Change Memory for Improved Reliability

Phase-change memory (PCM) is considered one of the most promising candidates for universal memory. However, during the manufacturing process of PCM, phase-change materials (PCMs) encounter severe oxidation, which can cause degraded performance and reduced stability of PCM, hindering its industrialization process. In this work, a multilayered oxygen barrier (MOB) structure is proposed to tackle this challenge. Material characterization shows that the MOB structure can significantly reduce the extent of oxidation of PCMs from around 70% to as low as around 10%, achieving a remarkably low level of oxidation. Moreover, the material in the MOB structure exhibits notable enhancements in crystallization temperature and cycling capability. The improved stability is attributed to the oxygen barrier effect and the suppression of elemental segregation within the material, which are both conferred by the MOB structure. In summary, this work provides an effective solution to address the oxidation of PCMs, offering valuable guidance for realizing a high-reliability PCM in practical production.

Enhanced stretchability towards a flexible and wearable reflective display coating using chalcogenide phase change materials

Reflective color switching coatings based on chalcogenide phase change material (PCM) is becoming one of the most promising display technologies in the future. However, it is still a challenge to kindly control the stress and enhance the stretchability for flexible display coatings. Here, we report crack-reduced reflective color coatings on a flexible substrate by using buckling structure to regulate the distribution of vacancies in PCM. It significantly suppresses the formation of cracks and improves the robustness of optical and electrical properties during stretching of the display device, which opens the doors of opportunity for phase change display applications in a wide range of flexible and wearable display fields.

Atomic Layer Deposition of Sb2 Te3 /GeTe Superlattice Film and Its Melt-Quenching-Free Phase-Transition Mechanism for Phase-Change Memory

Atomic layer deposition (ALD) of Sb₂ Te₃ /GeTe superlattice (SL) film on planar and vertical sidewall areas containing TiN metal and SiO₂ insulator is demonstrated. The peculiar chemical affinity of the ALD precursor to the substrate surface and the 2D nature of the Sb₂ Te₃ enable the growth of an in situ crystallized SL film with a preferred orientation. The SL film shows a reduced reset current of ≈1/7 of the randomly oriented Ge₂ Sb₂ Te₅ alloy. The reset switching is 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, enhances the electromigration of Ge along the [111] direction of FCC structure, which enables such a significant improvement. Set operation switches the amorphous to the (111)-oriented FCC structure.

Uncovering Thermal and Electrical Properties of Sb2Te3/GeTe Superlattice Films

Superlattice-like phase change memory (SL-PCM) promises lower switching current than conventional PCM based on Ge₂Sb₂Te₅ (GST); however, a fundamental understanding of SL-PCM requires detailed characterization of the interfaces within such an SL. Here we explore the electrical and thermal transport of SLs with deposited Sb₂Te₃ and GeTe alternating layers of various thicknesses. We find up to an approximately four-fold reduction of the effective cross-plane thermal conductivity of the SL stack (as-deposited polycrystalline) compared with polycrystalline GST (as-deposited amorphous and later annealed) due to the thermal interface resistances within the SL. Thermal measurements with varying periods of our SLs show a signature of phonon coherence with a transition from wave-like to particle-like phonon transport, further described by our modeling. Electrical resistivity measurements of such SLs reveal strong anisotropy (∼2000×) between the in-plane and cross-plane directions due to the weakly interacting van der Waals-like gaps. This work uncovers electrothermal transport in SLs based on Sb₂Te₃ and GeTe for the improved design of low-power PCM.

A first-principles study of the switching mechanism in GeTe/InSbTe superlattices

Interfacial Phase Change Memories (iPCMs) based on (GeTe)₂/Sb₂Te₃ superlattices have been proposed as an alternative candidate to conventional PCMs for the realization of memory devices with superior switching properties. The switching mechanism was proposed to involve a crystalline-to-crystalline structural transition associated with a rearrangement of the stacking sequence of the GeTe bilayers. Density functional theory (DFT) calculations showed that such rearrangement could be achieved by means of a two-step process with an activation barrier for the flipping of Ge and Te atoms which is sensitive to the biaxial strain acting on GeTe bilayers. Within this picture, strain-engineering of GeTe bilayers in the GeTe-chalcogenide superlattice can be exploited to further improve the iPCM switching performance. In this work, we study GeTe-InSbTe superlattices with different compositions by means of DFT, aiming at exploiting the large mismatch (3.8%) in the in-plane lattice parameter between GeTe and In₃SbTe₂ to reduce the activation barrier for the switching with respect to the (GeTe)₂-Sb₂Te₃ superlattice.

Local structure and phase change behavior in interfacial intermixing GeTe-Sb2Te3 superlattices

Ge/Sb atomic intermixing in interfacial cationic layers is a common phenomenon for GeTe-Sb₂Te₃ superlattice (GST-SL) used in memory devices. In this paper, we explored the effect of Ge/Sb intermixing on the phase change behavior of GST-SL upon the heating-quenching procedure. Four interfacial intermixing models of Kooi, Ferro, Petrov and inverted Petrov with different Ge/Sb intermixing ratios (25/75, 50/50 and 75/25) were developed based on the ab initio molecular dynamics. The structural evolution indicated that the Ge/Sb interfacial intermixing could facilitate the structure changes especially for 50/50 Ge/Sb intermixed models. When quenching from 1500 K, more 4-fold Ge-centered octahedrons were produced than tetrahedrons, and the electron localization function further proved that the distorted of Ge(Sb)-centered 6-fold octahedrons were caused by the asymmetrical interactions of Ge-Ge/Sb and Ge-Te. A relatively large Te p  orbital contribution in coexisted Ge/Te layer led to a narrower bandgap. In addition, different Ge/Sb atom intermixed ratio which affected the electronic local structure, led to the discrepancy in the initial atom movement of Sb or Ge movement near the gap. The present studies enrich the understanding of Ge/Sb interfacial atomic intermixing effects in GST-SL structural changes.

Observation of van der Waals reconfiguration in superlattice phase change materials

Phase change memory (PCM) is a leading candidate for nonvolatile memory applications in the big data era. However, the high power consumption, caused by melting GeTe-Sb2Te3-like phase change materials, hinders their applications. A significant step is the proposal to spatially separate GeTe and Sb2Te3 in the form of a superlattice, enabling a higher operating speed and better cyclability at reduced switching energy. However, the physical origin is under intensive debate. Recently, the swapping of the SbTe terminating layers nearest to the van der Waals (vdWs) gap has been claimed to be the mechanism for the superlattice. Here, we reported a direct atomic-scale chemical identification of two kinds of vdWs reconfigurations together with atomic simulations. The vdWs reconfigurations, which occurred at the GeTe and Sb2Te3 boundary, were demonstrated to change the electrical properties and turn this semiconductor into a conductor, leading to the resistance contrast. Besides, strong intermixing of Ge and Sb atoms was directly observed; in the most severe cases, ∼50% of Ge in the GeTe layer diffused into the adjacent Sb2Te3 layer. Our work paves the way for deeper understanding of the phase transition of the GeTe/Sb2Te3 superlattice and the future design of non-volatile memories towards dynamic random access-like memories.

In situ observations of the reversible vacancy ordering process in van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb2Te3 superlattices

Chalcogenide-based thin films are employed in data storage and memory technology whereas van der Waals-bonded layered chalcogenide heterostructures are considered to be a main contender for memory devices with low power consumption. The reduction of switching energy is due to the lowering of entropic losses governed by the restricted motion of atoms in one dimension within the crystalline states. The investigations of switching mechanisms in such superlattices have recently attracted much attention and the proposed models are still under debate. This is partially due to the lack of direct observation of atomic scale processes, which might occur in these chalcogenide systems. This work reports direct, nanoscale observations of the order-disorder processes in van der Waals bonded Ge-Sb-Te thin films and GeTe-Sb2Te3-based superlattices using in situ experiments inside an aberration-corrected transmission electron microscope. The findings reveal a reversible self-assembled reconfiguration of the structural order in these materials. This process is associated with the ordering of randomly distributed vacancies within the studied materials into ordered vacancy layers and with readjustment of the lattice plane distances within the newly formed layered structures, indicating the high flexibility of these layered chalcogenide-based systems. Thus, the ordering process results in the formation of vacancy-bonded building blocks intercalated within van der Waals-bonded units. Moreover, vacancy-bonded building blocks can be reconfigured to the initial structure under the influence of an electron beam, while in situ exposure of the recovered layers to a targeted electron beam leads to the reverse process. Overall, the outcomes provide new insights into local structure and switching mechanism in chalcogenide superlattices.

In situ observations of the reversible vacancy ordering process in van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb2Te3 superlattices

Chalcogenide-based thin films are employed in data storage and memory technology whereas van der Waals-bonded layered chalcogenide heterostructures are considered to be a main contender for memory devices with low power consumption. The reduction of switching energy is due to the lowering of entropic losses governed by the restricted motion of atoms in one dimension within the crystalline states. The investigations of switching mechanisms in such superlattices have recently attracted much attention and the proposed models are still under debate. This is partially due to the lack of direct observation of atomic scale processes, which might occur in these chalcogenide systems. This work reports direct, nanoscale observations of the order-disorder processes in van der Waals bonded Ge-Sb-Te thin films and GeTe-Sb2Te3-based superlattices using in situ experiments inside an aberration-corrected transmission electron microscope. The findings reveal a reversible self-assembled reconfiguration of the structural order in these materials. This process is associated with the ordering of randomly distributed vacancies within the studied materials into ordered vacancy layers and with readjustment of the lattice plane distances within the newly formed layered structures, indicating the high flexibility of these layered chalcogenide-based systems. Thus, the ordering process results in the formation of vacancy-bonded building blocks intercalated within van der Waals-bonded units. Moreover, vacancy-bonded building blocks can be reconfigured to the initial structure under the influence of an electron beam, while in situ exposure of the recovered layers to a targeted electron beam leads to the reverse process. Overall, the outcomes provide new insights into local structure and switching mechanism in chalcogenide superlattices.