Zhitang Song

One order of magnitude faster phase change at reduced power in Ti-Sb-Te

To date, slow Set operation speed and high Reset operation power remain to be important limitations for substituting dynamic random access memory by phase change memory. Here, we demonstrate phase change memory cell based on Ti0.4Sb2Te3 alloy, showing one order of magnitude faster Set operation speed and as low as one-fifth Reset operation power, compared with Ge2Sb2Te5-based phase change memory cell at the same size. The enhancements may be rooted in the common presence of titanium-centred octahedral motifs in both amorphous and crystalline Ti0.4Sb2Te3 phases. The essentially unchanged local structures around the titanium atoms may be responsible for the significantly improved performance, as these structures could act as nucleation centres to facilitate a swift, low-energy order-disorder transition for the rest of the Sb-centred octahedrons. Our study may provide an alternative to the development of high-speed, low-power dynamic random access memory-like phase change memory technology.

Si-Sb-Te materials for phase change memory applications

Si-Sb-Te materials including Te-rich Si₂Sb₂Te₆ and Si(x)Sb₂Te₃ with different Si contents have been systemically studied with the aim of finding the most suitable Si-Sb-Te composition for phase change random access memory (PCRAM) use. Si(x)Sb₂Te₃ shows better thermal stability than Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ in that Si(x)Sb₂Te₃ does not have serious Te separation under high annealing temperature. As Si content increases, the data retention ability of Si(x)Sb₂Te₃ improves. The 10 years retention temperature for Si₃Sb₂Te₃ film is ~393 K, which meets the long-term data storage requirements of automotive electronics. In addition, Si richer Si(x)Sb₂Te₃ films also show improvement on thickness change upon annealing and adhesion on SiO₂ substrate compared to those of Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ films. However, the electrical performance of PCRAM cells based on Si(x)Sb₂Te₃ films with x > 3.5 becomes worse in terms of stable and long-term operations. Si(x)Sb₂Te₃ materials with 3 < x < 3.5 are proved to be suitable for PCRAM use to ensure good overall performance.

Carbon-doped Ge2Sb2Te5 phase change material: A candidate for high-density phase change memory application

Carbon-doped Ge2Sb2Te5 material is proposed for high-density phase-change memories. The carbon doping effects on electrical and structural properties of Ge2Sb2Te5 are studied by in situ resistance and x-ray diffraction measurements as well as optical spectroscopy. C atoms are found to significantly enhance the thermal stability of amorphous Ge2Sb2Te5 by increasing the degree of disorder of the amorphous phase. The reversible electrical switching capability of the phase-change memory cells is improved in terms of power consumption with carbon addition. The endurance of ∼2.1 × 104 cycles suggests that C-doped Ge2Sb2Te5 film will be a potential phase-change material for high-density storage application.

Nitrogen-implanted Ge2Sb2Te5 film used as multilevel storage media for phase change random access memory

Ge2Sb2Te5 films were deposited by RF magnetron sputtering on Si(100)/SiO2 substrates. N+ ion was implanted into Ge2Sb2Te5 films. Two obvious steps were observed in the resistance–temperature curve of the Ge2Sb2Te5-N film with a minor nitrogen implant dose. The two steps may change into one step because the phase transition from FCC to hexagonal structure was suppressed by nitrogen implantation if the nitrogen implant dose is higher than 4.51 × 1016 cm−2. The favourite nitrogen implant dose is about 6.44 × 1015 to 1.92 × 1016 cm−2 in our study. This phenomenon is very important for multilevel storage. Three-level storage with Ge2Sb2Te5-N media for chalcogenide random access memory (C-RAM) can be performed easily, and hence, the capacity of C-RAM will be dramatically increased.

Ga14Sb86 film for ultralong data retention phase-change memory

Ga14Sb86 film has been studied to explore its suitability as a novel active material for phase change memory application. With a crystallization temperature about 220 °C, Ga14Sb86 film has the activation energy of crystallization larger than 4.6 eV obtained both by nonisothermal and isothermal methods. This leads to an ultralong data retention, which is characterized by the temperature for ten years data retention at 162 °C. The reversible phase change can be realized by a pulse as short as 20 ns. Ga14Sb86-based cell shows a good endurance up to 3.2x105 SET-RESET cycles during endurance test.

Sn12Sb88 material for phase change memory

Phase change memory cell based on Sn12Sb88 film shows reversible phase change abilities between high and low resistance states. We calculate the resonance character of crystalline SnSb material, which proves that SnSb is a potential phase change candidate. Sn12Sb88 is the suitable composition that has faster crystallization speed, higher crystallization temperature, and larger crystallization activation energy but lower melting point than those of Ge2Sb2Te5 material. Hence, phase change memory cell using this composition is able to show quicker set operation speed, better data retention ability, and lower reset power consumption than those of the Ge2Sb2Te5 based cell.

Phase-Change Memory Materials by Design: A Strain Engineering Approach

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 applications.

Multilevel Data Storage Characteristics of Phase Change Memory Cell with Doublelayer Chalcogenide Films (Ge2Sb2Te5 and Sb2Te3)

Phase change memory (PCM) cells with monolayer chalcogenide film (Ge2Sb2Te5 or Sb2Te3) and doublelayer chalcogenide films (Ge2Sb2Te5 and Sb2Te3) were successfully fabricated. The PCM cell with doublelayer structure like W/Ge2Sb2Te5(30 nm)/Sb2Te3(60 nm)/TiN/Al shows superior performances to monolayer ones, which is mostly referred to the reduction of set voltage value and reset voltage value, and the ability of multilevel data storage. Theoretical simulations of the temperature distribution of the PCM cell with the doublelayer structure were also investigated.

Pressure-induced reversible amorphization and an amorphous-amorphous transition in Ge2Sb2Te5 phase-change memory material

Ge2Sb2Te5 (GST) is a technologically very important phase-change material that is used in digital versatile disks-random access memory and is currently studied for the use in phase-change random access memory devices. This type of data storage is achieved by the fast reversible phase transition between amorphous and crystalline GST upon heat pulse. Here we report pressure-induced reversible crystalline-amorphous and polymorphic amorphous transitions in NaCl structured GST by ab initio molecular dynamics calculations. We have showed that the onset amorphization of GST starts at approximately 18 GPa and the system become completely random at approximately 22 GPa. This amorphous state has a cubic framework (c-amorphous) of sixfold coordinations. With further increasing pressure, the c-amorphous transforms to a high-density amorphous structure with trigonal framework (t-amorphous) and an average coordination number of eight. The pressure-induced amorphization is investigated to be due to large displacements of Te atoms for which weak Te–Te bonds exist or vacancies are nearby. Upon decompressing to ambient conditions, the original cubic crystalline structure is restored for c-amorphous, whereas t-amorphous transforms to another amorphous phase that is similar to the melt-quenched amorphous GST.

Programming voltage reduction in phase change memory cells with tungsten trioxide bottom heating layer/electrode

A phase change memory cell with tungsten trioxide bottom heating layer/electrode is investigated. The crystalline tungsten trioxide heating layer promotes the temperature rise in the Ge(2)Sb(2)Te(5) layer which causes the reduction in the reset voltage compared to a conventional phase change memory cell. Theoretical thermal simulation and calculation for the reset process are applied to understand the thermal effect of the tungsten trioxide heating layer/electrode. The improvement in thermal efficiency of the PCM cell mainly originates from the low thermal conductivity of the crystalline tungsten trioxide material.