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Multi-level phase-change can not only increase the storage density of phase-change memory, but also has important applications in neuromorphic computing. However, its realization is still very challenging. Here, we study the phase-change behaviors of four bilayer films, Sb7Se3(x nm)/Ge50Te50(90 nm), with x = 30, 50, 70, and 100. The expected three-level phase change is not observed at all. Raman and XRD characterizations reveal the occurrence of interlayer coupling between Sb7Se3 and Ge50Te50 layers. De-coupling structure films, Sb7Se3(x nm)/Si(50 nm)/Ge50Te50(90 nm), are proposed and prepared, where the amorphous Si layer serves as de-coupling the interlayer coupling. Three-level phase change is observed in all de-coupling Sb7Se3(x nm)/Si(50 nm)/Ge50Te50(90 nm) films. Our works demonstrate the effect of interlayer coupling on multi-level phase-change, and the de-coupling idea provides a design way for multi-level phase-change materials. [ABSTRACT FROM AUTHOR]

The crystalline–amorphous–crystalline transition process of an oxygen-tuned Sb phase-change material has been obtained by employing ab initio molecular dynamic calculations. By analyzing the local atomic arrangement and the electron structure of the SbO system, the intrinsic mechanism is explored to comprehend the material function: (1) ultrafast crystallization and difficulty in creating a glassy state of a pure Sb material might be caused by the resonance bonding of linear arrangement Sb atoms in the rhombohedral phase; (2) the impurity oxygen atoms break the medium and long-range linear arrangement of the Sb network by steric effects and change the electronic structure of these Sb atoms bonded to oxygen atoms, i.e., the obvious increase in electron localization and the great decrease in state distribution around the Fermi surface due to the high electronegativity of oxygen. These factors set an effective barrier for crystallization and improve the amorphous stability and, thus, data retention. The present research and scheme provide important insights into the engineering and manipulation of a phase-change material through first-principles calculations toward non-volatile phase change memory. [ABSTRACT FROM AUTHOR]

This work presents the optimization of the crystallization behavior and reliability of Sn15Sb85 thin films by doping Sm element. The phase transition behaviors induced by thermal were investigated by in situ resistance measurement. With the addition of Sm element, Sn15Sb85 film 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 (∼103 Ω), 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 Sn15Sb85 film 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)0.905 films can successfully achieve the complete SET and RESET reversible operation process with high operating speed (200 ns) and low power consumption (1.6 × 10−10 J). The results suggest that doping the proper concentration of Sm element will be an effectual solution to adapt and optimize the crystallization properties of Sn15Sb85 phase change material. [ABSTRACT FROM AUTHOR]

This work systematically investigates the improvement in phase change properties and crystallization mechanism of Sb films after Y doping. Compared with the pure Sb thin films, Sb51Y49 has a higher crystallizing temperature (203 ℃) and longer failure time (766 s), indicating that Y doping can enhance the thermal stability of the material in the amorphous state effectively. Y exists in an amorphous state and has an inhibitory effect on grain growth, resulting in smaller grain sizes in Y-doped Sb films. The crystallization mechanism was investigated using the Johnson–Mehl–Avrami model, and the crystallization mechanism of the Sb films before and after Y-doping is unchanged and always dominated by growth, a property that is favorable for achieving fast phase transitions. The phase change cells based on Sb51Y49 films exhibit a lower energy consumption (3.5 × 10–11 J) than the conventional Ge2Sb2Te5 material. The findings demonstrate the potential of Y-doped Sb as a phase change material with excellent crystallization rate and energy consumption performance. [ABSTRACT FROM AUTHOR]

The effects of yttrium dopants on the phase change behavior and microstructure of Sn15Sb85 films have been systematically investigated. The yttrium-doped Sn15Sb85 film has the higher phase transition temperature, ten year data retention ability and crystallization activation energy, which represent a great improvement in thermal stability and data retention. X-ray diffraction, transmission electron microscopy and x-ray photoelectron spectroscopy reveal that the amorphous Sn and Y components restrict the grain growth and decrease the grain size. Raman mode typically associated with Sb is altered when the substance crystallized. Atomic force microscopy results show that the surface morphology of the doped films becomes smoother. T-shaped phase change storage cells based on yttrium-doped Sn15Sb85 films exhibit the lower power consumption. The results demonstrate that the crystallization characteristics of Sn15Sb85 film can be tuned and optimized through the yttrium dopant for the excellent performances of phase change memory. [ABSTRACT FROM AUTHOR]

Sb100−xErx thin films were prepared by magnetron sputtering, and the electrical and thermal properties were investigated by in situ resistance measurement. Both the crystallization temperature and data retention increased significantly with the increase in Er content, revealing an improvement in thermal stability. The resistance drift index decreased with the increase in Er concentration, indicating better reliability for device application. X-ray diffraction and transmission electronic microscopy showed that Sb100−xErx materials precipitate a single Sb crystalline phase and the crystallization process is inhibited by Er doping content. A T-shaped phase change memory cell based on Sb96.9Er3.1film exhibited faster operation speed and lower energy consumption than traditional Ge2Sb2Te5, showing promise for application in electronic storage. [ABSTRACT FROM AUTHOR]

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 data retention capacity (141.2 °C for 10 years), revealing that Y doping improves amorphous thermal stability. X-ray diffraction and X-ray photoelectron spectroscopy analysis show that the addition of yttrium could inhibit grain growth and restrict the grain size due to the formation of amorphous Y and Ge components. X-ray reflectivity results show that yttrium doping results in less volume change, which predicts the enhanced performance stability of the device. A T-shaped phase change memory cell based on the Y0.26(Ge1Sb9)0.74 films exhibits a faster operation speed (100 ns) and lower power consumption (2.4 × 10−10 J) than traditional Ge2Sb2Te5 materials. The results reveal that Y-doped Ge1Sb9 is a phase change memory material with good structural properties and device performance. [ABSTRACT FROM AUTHOR]

Sb-rich films, such as Ge10Sb90, having ultra-fast phase change speed are promising chalcogenide materials for phase change memory (PCM) applications. However, it is difficult to accurately observe the phase change properties of ultrathin Sb-rich films due to their volatilization at higher temperatures. In this work, we establish a strategy to characterize ultrathin phase change behavior based specifically on Al/Ge10Sb90 superlattice-like (SLL) structures. It is confirmed that the Al layers, which can form a retardant layer without phase change behavior, can efficiently inhibit the volatilization of Ge10Sb90 films. In addition, the crystallization temperature can be modulated by varying the thickness ratio in SLL structures. In particular, the film of [Al(10nm)/Ge10Sb90(2nm)]5 is one of the promising candidates owing to their high crystallization temperature, good operating temperature for 10 years and excellent surface roughness. [ABSTRACT FROM AUTHOR]

Effects of the titanium dopant on the physical properties and structure of SbSe thin films were systematically investigated by experiments and first-principles calculations. The amorphous-to-polycrystalline transformation induced by heat was examined by in situ electrical resistance measurements. With the incorporation of titanium atoms, both the crystallization temperature and electrical resistance increase, revealing the improvement of the amorphous thermal stability and programing energy consumption. X-ray diffraction, transmission electron microscopy, and density functional theory calculations illustrate that a small amount of titanium dopant can inhibit the grain growth and refine the crystal size. The shift of Raman modes associated Sb upon the crystallization was observed. X-ray reflectivity and atomic force microscopy results prove the smaller volume fluctuation and the smoother surface morphology, meaning the better interfacial property and reliability of titanium-doped SbSe materials. Phase change memory cells based on titanium-doped antimony-selenium were fabricated to evaluate the electrical performance as well. All these results indicate that the suitable incorporation of the titanium element will be an effective method to optimize the physical properties and tune the structure of the SbSe phase change material. [ABSTRACT FROM AUTHOR]

Titanium-doped SnSb4 phase-change thin film has been experimentally investigated for phase-change random access memory (PCRAM) use. The crystallization temperature, amorphous/crystalline resistance, and data retention of the SnSb4 thin film can be significantly enhanced by doping with titanium, improving the thermal stability of the amorphous state and reducing power consumption. The effect of titanium doping on electrical transport in the thin film was explored using the Hall system. X-ray diffraction and transmission electron microscopy confirmed that the grain size of the thin film was reduced as the titanium content was increased. Grain refinement improved the density changes of the thin films before and after the phase transition. The consequent change in chemical bonding states after adding titanium indicated that Ti was bonded to Sb in the SnSb4 lattice structure, wrapping around the Sb grains in an amorphous state. PCRAM cells based on Ti0.10SnSb4 thin film can be realized with a whole operation window by a 50 ns width pulse, and the operating power consumption was lower than that of a Ge2Sb2Te5 (GST) PCRAM cell of similar dimensions. [ABSTRACT FROM AUTHOR]

The superlattice-like Sn 15 Sb 85 /Ti phase change films were fabricated by magnetron sputtering, and the effects of Ti layer on the phase change properties and crystal structure of the Sn 15 Sb 85 films were systematically researched. With the addition of Ti layers, the crystallization temperature of the films increased, indicating their thermal stability and data retention were improved. X-ray diffraction results reflect that the presence of Ti layers restrains grain growth and refines grain size. X-ray photoelectron spectroscopy analysis reveals that the incorporation of Ti layers enhances the interatomic binding energy and improves the stability. X-ray reflectivity analysis and atomic force microscopy indicate that the Sn 15 Sb 85 /Ti films have smaller volume change and smoother surface than pure Sn 15 Sb 85 films, implying the better interfacial property and reliability. The T-type phase change memory cell based on Sn 15 Sb 85 /Ti thin film has faster operation speed (100 ns) and lower power consumption (2.6 ×10−12 J) than the conventional Ge 2 Sb 2 Te 5 material. The results show that the superlattice-like Sn 15 Sb 85 /Ti film will be a potential candidate with outstanding properties in thermostability and power consumption. • Proposing the Sn 15 Sb 85 /Ti thin film with superlattice-like structure. • Performance optimization and structural analysis of film. • PCM cells based on Sn 15 Sb 85 /Ti film was prepared and tested. [ABSTRACT FROM AUTHOR]

Ge 2 Sb 2 Te 5 /ZnSb (GST/ZS) stacked thin films were proposed for high density phase change memories (PCM). Electrical and structural properties were studied by in - situ resistance measurements and X-ray diffraction (XRD), respectively. The films exhibited good thermal stability and two resistance steps during heating process. A picosecond laser pump-probe system was used to measure phase change speed. Phase change memory cells based on [GST(35 nm)/ZS(15 nm)] 1 thin film were fabricated to test and verify multi-level switch between set and reset states. [ABSTRACT FROM AUTHOR]

Phase change behavior in Al(SnSe) (x = 0.003, 0.010, 0.023) films were investigated by utilizing in situ resistance measurements. It is found that the crystallization temperatures and resistances increase with increasing of Al content. The analysis of X-ray diffraction indicates that the grain size decreases and the crystallization is suppressed by more Al doping. Al(SnSe) has an excellent thermal stability with the crystallization activation energy of 3.79 eV and the failure time is longer than that of GeSbTe film. The crystallization speed of Al(SnSe)film is faster than that of GST. The phase transition kinetics of Al(SnSe)films were investigated. The obtained values of Avrami indexes indicate that a one dimensional growth-dominated mechanism is responsible for the amorphous-crystalline transformation of Al(SnSe)film. We conclude that Al(SnSe) film is a good candidate for phase-change random-access memory applications with good thermal stability and high switching speed. [ABSTRACT FROM AUTHOR]

Comparing to un-doped ZnSb material, N-doped ZnSb material had higher crystallization temperature, lower conductivity and better data retention. The optical band gap was derived from the transmittance spectra and a significant increase was observed with increasing nitrogen doping concentration. The measurement of atomic force microscopy indicated that the crystallization was inhibited and the surface of thin films became smoother after N doping. Phase change memory devices based on N-doped ZnSb thin film were fabricated to test and verify their electrical properties. [ABSTRACT FROM AUTHOR]

Multi-level phase-change (MLPC) memory can not only further raise the storage density of high-density phase-change memory, but also have significant applications in neuromorphic computing. However, acquisitions of MLPC materials are very challenging. MLPC is only observed in a small number of phase-change materials and can't be designed determinately because the mechanism of MLPC emergence has been understood incompletely. Here, we investigate phase-change behaviors of six Ge 2 Sb 2 Te 5 (x nm)/Sb 7 Se 3 (60-x nm) bilayer films with x = 5, 10, 15, 20, 35, and 50. Three-level phase-change memory is observed for five bilayer films with x < 50, but not for one with x = 50, revealing the dependence of MLPC memory on the thickness ratio of two constituent layers composing bilayer films. A parallel resistance model is proposed to describe the measured sheet resistance of bilayer films and can be used to explain the thickness-ratio dependence of three-level phase-change memory very well. Subsequently, a design rule for MLPC memory films is given out unambiguously. Based on the parallel resistance model, MLPC memories are simulated for bilayer and trilayer films. Three-level memory indeed appears in bilayer films only when the design rule is met, confirming the validity of our design rule. Four-level memory is predicted in trilayer films as the design rule is obeyed. [Display omitted] • Multi-level phase-change behaviors of six bilayer phase-change films, Ge 2 Sb 2 Te 5 (x nm)/Sb 7 Se 3 (60-x nm) with x = 5, 10, 15, 20, 35 and 50, were studied by the measurement of sheet resistance as a function of heating temperature. • Appearance of three-level phase-change behaviors depended on the thickness ratio of Ge 2 Sb 2 Te 5 and Sb 7 Se 3 layers. Similar phenomena were reported previously in the literatures, but not understood and explained. • A parallel resistance model was proposed to describe the sheet resistance of bilayer films. Based on the model, the thickness-ratio dependence of three-level phase-change behaviors could be explained very well. • A design rule of multi-level phase-change multi-layer films was given out for the first time. Multi-level phase changes were simulated for bilayer films based on the sheet resistance model. It was found that three-level phase changes indeed emerged only when our design rule was met totally, conforming the validity of our design rule. Furthermore, a scheme of trilayer films was proposed. Its multi-level phase changes were simulated. Four-level phase changes were predicted only if the design rule was obeyed totally. • Multi-level phase-change films could be designed determinately under the guidance of the design rule, no longer be encountered and observed accidentally. Our results pave the way for determinate design of desired multi-level phase-change films. [ABSTRACT FROM AUTHOR]