Yttrium-Doped Sb2Te3: A Promising Material for Phase-Change Memory

The Effect of Sputtering Sequence Engineering in Superlattice-like Sb-Rich-Based Phase Change Materials

This paper presents a comprehensive investigation into the thermal stability of superlattice-like (SLL) thin films fabricated by varying the sputtering sequences of the SLL [Ge8Sb92(25nm)/GeTe(25nm)]1 and SLL [GeTe(25nm)/Ge8Sb92(25nm)]1 configurations. Our results reveal significantly enhanced ten-year data retention (Tten) for both thin films measured at 124.3 °C and 151.9 °C, respectively. These values surpass the Tten of Ge2Sb2Te5 (85 °C), clearly demonstrating the superior thermal stability of the studied SLL configurations. Interestingly, we also observe a distinct difference in the thermal stability between the two SLL configurations. The superior thermal stability of SLL [GeTe(25nm)/Ge8Sb92(25nm)]1 is attributed to the diffusion of the Sb precipitated phase from Ge8Sb92 to GeTe. This diffusion process effectively reduces the impact of the Sb phase on the thermal stability of the thin film. In contrast, in the case of SLL [Ge8Sb92(25nm)/GeTe(25nm)]1, the presence of the Sb precipitated phase in Ge8Sb92 facilitates the crystallization of GeTe, leading to reduced thermal stability. These findings underscore the significant influence of the sputtering sequence on the atomic behavior and thermal properties of superlattice-like phase change materials. Such insights provide a robust foundation for the design and exploration of novel phase change materials with improved thermal performance.

Influence of samarium modification on the phase-change performance and phase structure of tin antimonide

This work presents the optimization of the crystallization behavior and reliability of Sn15Sb85thin films by doping Sm element. The phase transition behaviors induced by thermal were investigated byin situresistance measurement. With the addition of Sm element, Sn15Sb85film exhibits the superior crystallization temperature (232 °C) and data conservation (172.32 °C for 10 years), larger activation energy of crystallization (4.91 eV) and crystalline resistance (∼103Ω), which contributes to the increased thermal stability of the amorphous state and decrease in the programming energy. The Sm-doping can broaden the energy band gap from 0.55 to 1.07 eV. The amorphous Sm and Sn compositions could retard grain growth and refine grain size from 21.13 to 11.13 nm, combining with x-ray diffraction and x-ray photoelectron spectroscopy. The surface morphology of Sn15Sb85film becomes smoother after Sm doping as determined by atomic force microscopy images, resulting in the improved interfacial reliability. Phase change memory devices based on Sm0.095(Sn15Sb85)0.905films can successfully achieve the complete SET and RESET reversible operation process with high operating speed (200 ns) and low power consumption (1.6 × 10-10J). The results suggest that doping the proper concentration of Sm element will be an effectual solution to adapt and optimize the crystallization properties of Sn15Sb85phase change material.

Surface effects on the crystallization kinetics of amorphous antimony

Elemental antimony (Sb) is regarded as a promising candidate to improve the programming consistency and cycling endurance of phase-change memory and neuro-inspired computing devices. Although bulk amorphous Sb crystallizes spontaneously, the stability of the amorphous form can be greatly increased by reducing the thickness of thin films down to several nanometers, either with or without capping layers. Computational and experimental studies have explained the depressed crystallization kinetics caused by capping and interfacial confinement; however, it is unclear why amorphous Sb thin films remain stable even in the absence of capping layers. In this work, we carry out thorough ab initio molecular dynamics (AIMD) simulations to investigate the effects of free surfaces on the crystallization kinetics of amorphous Sb. We reveal a stark contrast in the crystallization behavior between bulk and surface models at 450 K, which stems from deviations from the bulk structural features in the regions approaching the surfaces. The presence of free surfaces intrinsically tends to create a sub-nanometer region where crystallization is suppressed, which impedes the incubation process and thus constrains the nucleation in two dimensions, stabilizing the amorphous phase in thin-film Sb-based memory devices.

Top-down patterning of topological surface and edge states using a focused ion beam

The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ2 switches from 1 to 0. These states offer prospects for quantum electronics; however, a method is needed to spatially-control ℤ2 to pattern conducting channels. It is shown that modifying Sb2Te3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ2 = 1 → ℤ2 = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics.

The Effect of Carbon Doping on the Crystal Structure and Electrical Properties of Sb2Te3

As a new generation of non-volatile memory, phase change random access memory (PCRAM) has the potential to fill the hierarchical gap between DRAM and NAND FLASH in computer storage. Sb₂Te₃, one of the candidate materials for high-speed PCRAM, has high crystallization speed and poor thermal stability. In this work, we investigated the effect of carbon doping on Sb₂Te₃. It was found that the FCC phase of C-doped Sb₂Te₃ appeared at 200 °C and began to transform into the HEX phase at 25 °C, which is different from the previous reports where no FCC phase was observed in C-Sb₂Te₃. Based on the experimental observation and first-principles density functional theory calculation, it is found that the formation energy of FCC-Sb₂Te₃ structure decreases gradually with the increase in C doping concentration. Moreover, doped C atoms tend to form C molecular clusters in sp² hybridization at the grain boundary of Sb₂Te₃, which is similar to the layered structure of graphite. And after doping C atoms, the thermal stability of Sb₂Te₃ is improved. We have fabricated the PCRAM device cell array of a C-Sb₂Te₃ alloy, which has an operating speed of 5 ns, a high thermal stability (10-year data retention temperature 138.1 °C), a low device power consumption (0.57 pJ), a continuously adjustable resistance value, and a very low resistance drift coefficient.

Ultra-Stable, Endurable, and Flexible Sb2TexSe3-x Phase Change Devices for Memory Application and Wearable Electronics

Flexible memory and wearable electronics represent an emerging technology, thanks to their reliability, compatibility, and superior performance. Here, an Sb₂Te_xSe_3-x (STSe) phase change material was grown on flexible mica, which not only exhibited superior nature in thermal stability for phase change memory application but also revealed novel function performance in wearable electronics, thanks to its excellent mechanical reliability and endurance. The thermal stability of Sb₂Te₃ was improved obviously with the crystallization temperature elevated 60 K after Se doping, for the enhanced charge localization and stronger bonding energy, which was validated by the Vienna ab initio simulation package calculations. Based on the ultra-stability of STSe, the STSe-based phase change memory shows 65 000 reversible phase change ability. Moreover, the assembled flexible device can show real-time monitoring and recoverability response in sensing human activities in different parts of the body, which proves its effective reusability and potential as wearable electronics. Most importantly, the STSe device presents remarkable working reliability, reflected by excellent endurance over 100 s and long retention over 100 h. These results paved a novel way to utilize STSe phase change materials for flexible memory and wearable electronics with extreme thermal and mechanical stability and brilliant performance.

Unraveling Crystallization Mechanisms and Electronic Structure of Phase-Change Materials by Large-Scale Ab Initio Simulations

Ge-Sb-Te ("GST") alloys are leading phase-change materials for digital memories and neuro-inspired computing. Upon fast crystallization, these materials form rocksalt-like phases with large structural and vacancy disorder, leading to an insulating phase at low temperature. Here, a comprehensive description of crystallization, structural disorder, and electronic properties of GeSb₂ Te₄ based on realistic, quantum-mechanically based ("ab initio") computer simulations with system sizes of more than 1000 atoms is provided. It is shown how an analysis of the crystallization mechanism based on the smooth overlap of atomic positions kernel reveals the evolution of both geometrical and chemical order. The connection between structural and electronic properties of the disordered, as-crystallized models, which are relevant to the transport properties of GST, is then studied. Furthermore, it is shown how antisite defects and extended Sb-rich motifs can lead to Anderson localization in the conduction band. Beyond memory applications, these findings are therefore more generally relevant to disordered rocksalt-like chalcogenides that exhibit self-doping, since they can explain the origin of Anderson insulating behavior in both p- and n-doped chalcogenide materials.

Designing Conductive-Bridge Phase-Change Memory to Enable Ultralow Programming Power

Phase-change material (PCM) devices are one of the most mature nonvolatile memories. However, their high power consumption remains a bottleneck problem limiting the data storage density. One may drastically reduce the programming power by patterning the PCM volume down to nanometer scale, but that route incurs a stiff penalty from the tremendous cost associated with the complex nanofabrication protocols required. Instead, here a materials solution to resolve this dilemma is offered. The authors work with memory cells of conventional dimensions, but design/exploit a PCM alloy that decomposes into a heterogeneous network of nanoscale crystalline domains intermixed with amorphous ones. The idea is to confine the subsequent phase-change switching in the interface region of the crystalline nanodomain with its amorphous surrounding, forming/breaking "nano-bridges" that link up the crystalline domains into a conductive path. This conductive-bridge switching mechanism thus only involves nanometer-scale volume in programming, despite of the large areas in contact with the electrodes. The pore-like devices based on spontaneously phase-separated Ge13 Sb71 O16 alloy enable a record-low programming energy, down to a few tens of femtojoule. The new PCM/fabrication is fully compatible with the current 3D integration technology, adding no expenses or difficulty in processing.

Improved thermal stability and power consumption performances of Ge1Sb9 phase change thin film via doping yttrium

The effect of yttrium doping on the phase transition properties and crystal structure of Ge1Sb9 thin films was studied. Y-doped Ge1Sb9 thin films have higher crystallization temperature (218 °C) and...

Multi-level phase-change memory with ultralow power consumption and resistance drift

By controlling the amorphous-to-crystalline relative volume, chalcogenide phase-change memory materials can provide multi-level data storage (MLS), which offers great potential for high-density storage-class memory and neuro-inspired computing. However, this type of MLS system suffers from high power consumption and a severe time-dependent resistance increase ("drift") in the amorphous phase, which limits the number of attainable storage levels. Here, we report a new type of MLS system in yttrium-doped antimony telluride, utilizing reversible multi-level phase transitions between three states, i.e., amorphous, metastable cubic and stable hexagonal crystalline phases, with ultralow power consumption (0.6-4.3 pJ) and ultralow resistance drift for the lower two states (power-law exponent < 0.007). The metastable cubic phase is stabilized by yttrium, while the evident reversible cubic-to-hexagonal transition is attributed to the sequential and directional migration of Sb atoms. Finally, the decreased heat dissipation of the material and the increase in crystallinity contribute to the overall high performance. This study opens a new way to achieve advanced multi-level phase-change memory without the need for complicated manufacturing procedures or iterative programming operations.

First-principles study of the liquid and amorphous phases of Sb2Te phase change memory material

We have investigated the local structure of liquid and amorphous phases of Sb₂Te phase change memory material by the means of density functional theory-molecular dynamics simulations. The models of liquid and amorphous states were generated by quenching from the melt. The results show that the local environment of liquid Sb₂Te is a mixed bonding geometry, where the average coordination numbers (CNs) of Sb and Te atoms are 4.93 and 4.23, respectively. Compared with crystalline state, there are more Sb-Sb bonds (∼53%) and less Sb-Te bonds (∼42%) with the presence of Te-Te bonds (∼5%) in liquid Sb₂Te. Therefore, the formation of homopolar bonds and the breaking of heteropolar bonds are important structural transformations in melt process. For amorphous Sb₂Te, the average CNs of Sb and Te atoms are 4.54 and 3.57, respectively. They are mostly in an octahedral environment, similar to the case in crystalline phase. The fractions of Sb-Sb, Te-Te, and Sb-Te bonds are ∼52%, ∼2%, and ∼46%, respectively. Thus, the increase in the fraction of octahedron accompanied with the decrease in average CN is the major structural changes in quenching process. Furthermore, the octahedral geometry in both the crystalline and amorphous Sb₂Te increases the local structural similarity, facilitating the rapid low-energy crystallization.

High performance broadband photodetectors based on Sb2Te3/n-Si heterostructure

With the rapid development of optoelectronic devices, photodetectors have triggered unprecedented promise in the field of optical communication, environmental monitoring, biological imaging, chemical sensing. At the same time, there is a higher requirement for photodetectors. It is still a huge challenge for photodetectors that possess excellent performance, low cost and broad range photoresponse from ultraviolet to infrared. In this work, a facile, low cost growth of Sb₂Te₃ thin film using magnetic sputtering was performed. After rapid annealing treatment, the crystallinity of the thin film was transformed from amorphous to polycrystalline. Ultraviolet-visible-infrared absorption study of the thin film revealed broad absorption range, which is ideal for use in broadband photodetectors. Such photodetectors can find many important applications in communication, data security, environmental monitoring and defense technology etc. A prototype photodetector, consisting of Sb₂Te₃/n-Si heterostructure, was produced and characterized. The device demonstrated a significant photoelectric response at a broad spectral range of between 250 and 2400 nm. The maximum responsivity and detectivity of the device were 270 A W^-1 and 1.28 × 10¹³ Jones, respectively, under 2400 nm illumination. Therefore, the results showed the potential use of Sb₂Te₃ thin film in developing high performance broadband photodetectors.

Y-Doped Sb2Te3 Phase-Change Materials: Toward a Universal Memory

The disadvantages of high power consumption and slow operating speed hinder the application of phase-change materials (PCMs) for a universal memory. In this work, based on a rigorous experimental scheme, we synthesized a series of Y_xSb_2-xTe₃ (0 ≤ x ≤ 0.333) PCMs and demonstrated that Y_0.25Sb_1.75Te₃ (YST) is an excellent candidate material for the universal phase-change memory. This YST PCM, even being integrated into a conventional T-shaped device, exhibits an ultralow reset power consumption of 1.3 pJ and a competitive fast set speed of 6 ns. The ultralow power consumption is attributed to the Y-reduced thermal and electrical conductivity, while the maintained crystal structure of Sb₂Te₃ and the grain refinement provide the competitive fast crystallization speed. This work highlights a novel way to obtain new PCMs with lower power consumption and competitive fast speed toward a universal memory.

Sc-Centered Octahedron Enables High-Speed Phase Change Memory with Improved Data Retention and Reduced Power Consumption

Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. In the automotive application, intelligent system not only needs high efficiency to handle massive data processing but also good robustness to retain the existing data against high working temperature. In this work, Sc-doped GeTe is developed for PCM, which has achieved 120 °C data retention for 10 years, 6 ns operation speed, and 7 nJ low power consumption. The high data retention is attributed to the high coordination number of Sc and its strong bonds with Te atoms in the amorphous phase, which enhances the robustness of the atomic matrices. Sc-centered octahedrons in amorphous state provide a nucleation center, leading to fast crystallization. In the crystalline phase, Sc atoms occupy Ge vacancies to form a homogenous GeTe-like rhombohedral phase. The strong covalent-like Sc-Te bonds weaken the neighboring Ge-Te bonds, lowering energy for melting. Together with the increased energy efficiency originated from confined grain size, the reduced power consumption has been achieved. The improvements in data retention, speed, and power efficiency have made Sc-doped GeTe a promising candidate for high-performance automobile electronics application.

Origin of high thermoelectric performance with a wide range of compositions for BixSb2−xTe3 single quintuple layers

The high power factor of a p-type BST single QL is ensured by the robust multi-valley character of valence bands.

Stabilizing amorphous Sb by adding alien seeds for durable memory materials

We developed a doping strategy to improve the thermal stability of phase-change memory by adding alien tetrahedral seeds.

Atomic scale insight into the effects of Aluminum doped Sb2Te for phase change memory application

To date, the unpleasant trade-off between crystallization speed and thermal stability for most phase change materials is detrimental to achieve phase change memory (PCM) with both features of high-speed and good-retention. However, it is proved that Al doping in Sb2Te, served as storage media in PCM, favors both a high writing speed (6 ns) and a good retention (103 °C), as well as a low power consumption. Judging by experimental and theoretical investigations, doped Al atoms prefer to replace Sb in Sb2Te lattice, strongly bonded with 6 Te atoms, to form a homogeneous phase. While in amorphous Al doped Sb2Te (AST), Al atoms are in tetrahedral environment, firmly bonded with four Sb/Te atoms. The strong bonding in Al centered tetrahedron in amorphous AST can obstruct the collective motion of Sb atoms near the matrix boundary, leading to the improvement in thermal stability and the confinement in grain size.

Scandium doping brings speed improvement in Sb2Te alloy for phase change random access memory application

Phase change random access memory (PCRAM) has gained much attention as a candidate for nonvolatile memory application. To develop PCRAM materials with better properties, especially to draw closer to dynamic random access memory (DRAM), the key challenge is to research new high-speed phase change materials. Here, Scandium (Sc) has been found it is helpful to get high-speed and good stability after doping in Sb₂Te alloy. Sc_0.1Sb₂Te based PCRAM cell can achieve reversible switching by applying even 6 ns voltage pulse experimentally. And, Sc doping not only promotes amorphous stability but also improves the endurance ability comparing with pure Sb₂Te alloy. Moreover, according to DFT calculations, strong Sc-Te bonds lead to the rigidity of Sc centered octahedrons, which may act as crystallization precursors in recrystallization process to boost the set speed.

Metastable Stacking-Polymorphism in Ge2Sb2Te5

Metastable rocksalt structured Ge₂Sb₂Te₅ is the most widely used phase-change material for data storage, yet the atomic arrangements of which are still under debate. In this work, we have proposed metastable stacking-polymorphism in cubic Ge₂Sb₂Te₅ based on first-principles calculations. Our results show that cubic Ge₂Sb₂Te₅ is actually polymorphic, varying from randomly distributed vacancies to highly ordered vacancy layers; consequently, the electrical property varies between metallic and semiconducting. These different atomic stackings of cubic Ge₂Sb₂Te₅ can be obtained at different experimental synthetic conditions. The concept of stacking-polymorphic Ge₂Sb₂Te₅ provides important fundamentals for metastable Ge₂Sb₂Te₅ and is useful for tuning the performance of the phase-change materials.