High-speed Phase Change Memory Research Articles

Investigation of Ru-doped Sb2Te alloy for high-speed and good thermal stability phase change memory applications

Investigation of Ru-doped Sb2Te alloy for high-speed and good thermal stability phase change memory applications

Ultrafast crystallization mechanism of amorphous Ge15Sb85 unraveled by pressure-driven simulations

Crystallization rate determines the switching speed of phase-change memory for next-generation cache-type applications. Using ab initio molecular dynamics simulations, we studied the pressure-driven rapid crystallization behavior of amorphous Ge15Sb85 (a-GS), a technologically important Te-free phase-change material (PCM) that exhibits rapid growth-controlled phase transition. We concentrated on the fluctuations and competitions between the short-range order (SRO) and medium-range order (MRO) in a-GS under different pressures. The results reveal that the a-GS at zero pressure is composed of Ge-centered tetrahedrons and defective Sb-centered octahedrons, and these special SRO motifs are connected by 5-fold rings. The pressure-driven rapid phase transition of a-GS involve two stages: the incubation stage and the fast growth stage. In the incubation stage, Ge-centered tetrahedrons transform into octahedrons and meanwhile Sb-centered octahedrons increase as well. The disordered 5-fold rings evolve into a specific ordered spatial conformation, in which four atoms lie on one plane and one atom is pushed out with a dihedral angle of 90°. During the fast growth stage, the formation of 4-fold rings is linked with the existing defective octahedrons, requiring only minimum atomic movements with disappearance of 5-fold rings. Using pressure, we reveal this transient process which bridges the missing gap in the formation of new MRO clusters that are yet unknown in experiments. The unique crystallization mechanism of a-GS under pressure is of great significance for the design of high-speed phase-change memory.

Investigation of V2O5/Ge8Sb92 multilayer thin film for high-data-retention and high-speed phase change memory applications

In this paper, the V2O5/Ge8Sb92 multilayer thin films were prepared and investigated. Compared with Ge8Sb92, V2O5/Ge8Sb92 film had a higher crystallization temperature and a larger conductivity activation energy. The surface roughness for V2O5/Ge8Sb92 film was small before and after crystallization. The crystallization of V2O5/Ge8Sb92 was inhibited and the grain size was only 5.0 nm. A higher stability and smaller grain made V2O5/Ge8Sb92 multilayer film a potential application in high-density phase change memory.

Heterogeneous nucleus-induced crystallization for high-speed phase change memory applications

Phase change memory has high potential for next-generation nonvolatile memory technology. The effect of lower layer Sb2Te (ST) on the crystallization process of Ge2Sb2Te5 (GST) is investigated by observing the change of the microstructure. Compared to the GST-based device, this paper achieved a faster operation speed (20 ns) with stable SET and RESET resistances via the addition of ST under the GST. The crystallization of lower layer ST occurs prior to that in the upper layer GST due to lower crystallization temperature, and the crystal grains of ST can supply the heterogeneous nuclei to reduce the nucleation time and enhance the crystallization speed of GST, which accounts for the improved operation speed. In addition, the effect of atomic migration on the GST/ST interface and the device performance is also investigated.

Characterization of Ge ions implantation in Sb2Te3 thin films for high speed phase change memory application

Ge ion implantation is adopted for doping of Sb2Te3 thin films and modifying the amorphous to crystal transition kinetics. The crystallization temperatures, crystallization activation energies of crystallization, and thermal stability improve with the Ge dose increasing from 5 × 1015 to 3.0 × 1016 Ge-ions/cm2. In the case of 1.0 × 1016 Ge-ions/cm2 dose, GexSb2Te3 phase change material exhibits a higher crystallization temperature (∼180 °C), a larger crystallization activation energy (∼3.7 eV), and a better data retention ability (∼102 °C for 10 years) in comparison with Ge2Sb2Te5. A reversible switching between Set and Reset can be realized by an electric pulse width as short as 10 ns for GexSb2Te3 based phase change memory.

Sb-rich CuSbTe material: A candidate for high-speed and high-density phase change memory application

Sb-rich CuSbTe material is proposed to meet the requirements of low-power consumption and high-speed of phase change memory application. Sb-rich CuSbTe material is based on Sb-rich SbTe (Sb3Te) material which has a fast crystallization speed. The poor amorphous thermal stability of Sb3Te material can be solved by the addition of Cu. Cu0.25Sb3Te-based device is expected to achieve fast access speed (6 ns) and maintain a high data retention (10-year: 85 °C) in comparison with that of Ge2Sb2Te5 (10-year: 78 °C). With the doping of Cu, the power consumption and the grain size of Sb3Te-based device are greatly reduced. Under the pulse width of 6 ns, the RESET voltage is only 4.1 V. Moreover, the operation cycles is close to 1 × 105, and the resistance ratio is nearly two orders of magnitude. Finally, the XRD, TEM and XRR measurements were carried out to obtain the microscopic analysis for Cu0.25Sb3Te alloy.

Change in Crystallization Mechanism of Sb Film by Doping VO2 for Ultraretention and High-Speed Phase-Change Memory

Pure Sb thin film has attracted great interest for phase-change memory owing to its appealing properties such as simple composition, fast crystallization, and sufficient electrical contrast between...

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.

Layer thickness dependence of the phase separation and phase change properties of Ge2Sb2Te5/TiN superlattice-like thin films

The dependence on the layer thickness of the structural and phase change properties of Ge2Sb2Te5/TiN superlattice-like thin films are investigated. The phase separation of binary Sb2Te3 and GeTe phases is investigated using X-ray diffraction and Raman spectral analysis. The Ge2Sb2Te5/TiN interface is believed lead to phase separation, as well as the interface effects that facilitate the phase separation process in the superlattice-like films. In this investigation of the phase change properties, the critical temperature, 10-year lifetime temperature, phase change speed, and optical band gap are measured. We found that the phase change properties of Ge2Sb2Te5 are greatly improved by increasing the layer thickness as a result of the phase separation suppression in the structure. This superlattice-like thin film may have potential in high-speed phase change memory applications.

Excellent thermal stability owing to Ge and C doping in Sb2Te-based high-speed phase-change memory

The contradictory nature between transition speed and thermal stability of phase-change materials has always been the key limitation to the achievement of wide applications under harsh conditions. Ge2.3Sb2.0Te phase-change alloy is proposed here to feature high thermal stability (10 year data retention above 220 °C) and fast switching speed (SET programming speed up to 5 ns) for electronic storage. In mushroom-shaped device cells, the nanocomposite materials implement an endurance life of nearly 1 × 105 cycles. Such operation speed among high-temperature alloys is the best ever reported. And the moderate incorporation of C offers intriguing benefits that include enhanced thermal stability and reduced RESET voltage in the above-mentioned Ge-rich Sb2Te-based memory cells. Through microscopic analysis, the local segregation of C dopants can further refine the crystalline grains and thus induce a lower volume change and roughness upon heating. These properties are crucial with regard to the application potential in high-performance and high-density embedded memories.

MoSbTe for high-speed and high-thermal-stability phase-change memory applications

Mo-doped Sb1.8Te materials and electrical devices were investigated for high-thermal-stability and high-speed phase-change memory applications. The crystallization temperature (tc = 185 °C) and 10-year data retention (t10-year = 112 °C) were greatly enhanced compared with those of Ge2Sb2Te5 (tc = 150 °C, t10-year = 85 °C) and pure Sb1.8Te (tc = 166 °C, t10-year = 74 °C). X-ray diffraction and transmission electron microscopy results show that the Mo dopant suppresses crystallization, reducing the crystalline grain size. Mo2.0(Sb1.8Te)98.0-based devices were fabricated to evaluate the reversible phase transition properties. SET/RESET with a large operation window can be realized using a 10 ns pulse, which is considerably better than that required for Ge2Sb2Te5 (∼50 ns). Furthermore, ∼1 × 106 switching cycles were achieved.

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

A weak electric field-assisted ultrafast electrical switching dynamics in In3SbTe2 phase-change memory devices

Prefixing a weak electric field (incubation) might enhance the crystallization speed via pre-structural ordering and thereby achieving faster programming of phase change memory (PCM) devices. We employed a weak electric field, equivalent to a constant small voltage (that is incubation voltage, Vi of 0.3 V) to the applied voltage pulse, VA (main pulse) for a systematic understanding of voltage-dependent rapid threshold switching characteristics and crystallization (set) process of In3SbTe2 (IST) PCM devices. Our experimental results on incubation-assisted switching elucidate strikingly one order faster threshold switching, with an extremely small delay time, td of 300 ps, as compared with no incubation voltage (Vi = 0 V) for the same VA. Also, the voltage dependent characteristics of incubation-assisted switching dynamics confirm that the initiation of threshold switching occurs at a lower voltage of 0.82 times of VA. Furthermore, we demonstrate an incubation assisted ultrafast set process of IST device for a low VA of 1.7 V (∼18 % lesser compared to without incubation) within a short pulse-width of 1.5 ns (full width half maximum, FWHM). These findings of ultrafast switching, yet low power set process would immensely be helpful towards designing high speed PCM devices with low power operation.

Investigation of Cu–Sn–Se material for high-speed phase-change memory applications

Cu–Sn–Se material was prepared by magnetron sputtering to investigate its potential application in phase change memory. The amorphous-to-crystalline transition was studied by in situ film resistance measurements. Cu–Sn–Se material had lower activation energy for crystallization (1.60 eV) and higher crystallization resistance than SnSe. The amorphous Cu–Sn–Se had more narrow band gap compared to SnSe. After the adding of Cu, the crystallization of Cu–Sn–Se material was inhibited and the grain structure became more compact. The picosecond laser pulse measurement indicated that Cu–Sn–Se material had a fast phase change speed (3.36 ns). The results demonstrated that Cu–Sn–Se material was a promising phase change material which had low power and high speed application in phase change memory.

O-doped Sb materials for improved thermal stability and high-speed phase change memory application

The O-doped Sb materials were proved to have higher crystallization temperature (∼205 °C), larger crystallization activation energy (3.95 eV) and better data retention ability (143 °C for 10 years). The band gap was broadened by O-doping and the grain size was refined. The formation of Sb oxide increased the binding energy. The fast phase change speed was obtained for O-doped Sb materials by picosecond laser technology. After O-doping, the phase change film had a smaller surface roughness (1.05 nm).

Improved thermal stability of N-doped Sb materials for high-speed phase change memory application

Compared with pure Sb, N-doped Sb material was proved to be a promising candidate for the phase change memory (PCM) use because of its higher crystallization temperature (∼250 °C), larger crystallization activation energy (3.53 eV), and better data retention ability (166 °C for 10 years). N-doping also broadened the band gap and refined grain size. The reversible resistance transition could be achieved by an electric pulse as short as 8 ns for the PCM cell based on N-doped Sb material. A lower operation power consumption (the energy for RESET operation 2.2 × 10−12 J) was obtained. In addition, N-doped Sb material showed a good endurance of 1.8 × 105 cycles.

Uniform silicon carbide doped Sb2Te nanomaterial for high temperature and high speed PCM applications

Sb–Te alloy is widely considered as one of the important materials for phase change memory (PCM) with fast operation speed. However, the poor amorphous phase stability limits its wide application. In this work, silicon carbide (SiC) doped Sb2Te (Sb2Te-SiC) nanomaterial was proposed to improve the thermal stability of Sb2Te. It was found that the crystallization temperature of Sb2Te was remarkably improved from 149 °C to 251 °C. Accordingly, the temperature for 10-year data retention increases from 56.5 °C to 156.4 °C. X-ray diffraction and transmission electron microscope results indicate that the grain size of Sb2Te-SiC film was largely reduced with SiC doping. Besides, experimental results show the Sb2Te-SiC nanomaterial is very uniform and no phase separation was observed even after 300 °C annealing. Furthermore, the Sb2Te-SiC nanomaterial based PCM cells show fast operation speed of 7 ns and good endurance ability of 2.1 × 104 cycles.

Ni-doped GST materials for high speed phase change memory applications

In this paper, Ni-doped Ge2Sb2Te5 (GST) was investigated for high speed phase change memory applications. Compared with GST, Ni0.3Ge2.8Sb2.2Te4.7 film exhibits a higher crystallization temperature (∼217°C) and a better data retention ability (∼135°C for 10 years). A reversible switching between set and reset can be realized by an electric pulse as short as 6-ns for Ni0.3Ge2.8Sb2.2Te4.7 based phase change memory. Furthermore, Ni0.3Ge2.8Sb2.2Te4.7 based cell shows good endurance up to 1.5×104 SET–RESET cycles during endurance test.

Uniform Ti-doped Sb2Te3 materials for high-speed phase change memory applications

Compared with pure Sb2Te3, Ti0.32Sb2Te3 (TST) phase change material has larger resistance ratio, higher crystallization temperature and better thermal stability. The sharp decrease in mobility is responsible for the increasing amorphous and crystalline sheet resistance. The uniform crystalline structure of TST film is very benefit for the endurance characteristic. The Set and Reset operation voltages for TST-based phase change memory device are much lower than those of conventional Ge2Sb2Te5-based one. Remarkably, the device presents extremely rapid Set operation speed (∼6 ns). Furthermore, up to 1 × 106 programming cycles are obtained with stable Set and Reset resistances.

Phase change material W0.04(Sb4Te)0.96 for application in high-speed phase change memory

W-doped Sb4Te is proposed for high-speed phase change memory (PCM). The crystallization speed of W0.04(Sb4Te)0.96 is characterized in both the film and device, confirmed to be ∼30ns and 6ns. The crystallization temperature and data retention have been increased to 192 and 112°C. The melting point is 550°C, ∼75°C lower than that of GST. The grain size is controlled to be 30–50nm, lowering the stress and improving the cycling ability. The small grains, good thermal stability, high crystallization speed and low melting temperature have made the W0.04(Sb4Te)0.96 a potential candidate for high-speed PCM application.