Phase Change Memory Applications Research Articles

Ultrafast crystallization in nanoscale phase change film of monobasic antimony

The property of Sb phase change thin film is investigated systematically. Sb thin film has an appropriate crystallization temperature (~165 °C) and data retention (110 °C for 10 year). Irradiated by laser pulse, the Sb thin film can achieve the crystallization operation within 6.05 ns. The monoclinic structure is confirmed by X-Ray Diffraction and the bond vibration mode of A7 phase is demonstrated by Raman scattering spectrum. With the crystallization, the band gap becomes narrow and the polycrystalline structure gradually forms. The Sb-based phase change memory cells are prepared. An ultrafast programing operation (~5 ns) and good endurance of 1.4 × 107 cycles are achieved. The results reveal that Sb thin film has promising potential in phase change memory application.

Insulator–metal transition and ultrafast crystallization of Ga40Sb60/Sn15Sb85 multiple interfacial nanocomposite films

The phase change behavior of Ga40Sb60/Sn15Sb85 multiple interfacial film was studied systematically for its application in phase change memory. The thermal stability of amorphous state was evaluated using the Arrhenius formulas. The evolution of phase structure indicates that the Sb phase exists in the Sb-rich Ga40Sb60/Sn15Sb85, Ga40Sb60 and Sn15Sb85 films. The change in density and thickness during crystallization was obtained by X-ray reflectance. The nanoscale change in roughness of the Ga40Sb60/Sn15Sb85 film was confirmed by atomic force microscopy. The rate for crystallization and amorphization transformation was measured by a picosecond laser pump-probe system. The phase change memory device based on [Ga40Sb60 (7 nm)/Sn15Sb85 (3 nm)]5 thin film was fabricated. An ultra-fast switching and low power were achieved for Ga40Sb60/Sn15Sb85 multiple interfacial film.

Improvement of phase change speed and thermal stability in Ge5Sb95/ZnSb multilayer thin films for phase change memory application

Superlattice-like Ge5Sb95/ZnSb (SLL GS/ZS) thin films were fabricated by radio-frequency magnetron sputtering technology. [GS(4 nm)/ZS(10 nm)]4 thin films display excellent thermal stability, and their crystallization temperature, activation energy, and 10-year data retention temperature are 253 °C, 3.78 eV and 157 °C, respectively, which are higher than traditional Ge2Sb2Te5 (GST) thin films. Slight fluctuation (3.5%) in thin film volume during phase transition process indicates good reliability. Ultrafast crystallization speed (9.2 ns) in the [GS(4 nm)/ZS(10 nm)]4 thin film was validated, which contributes to accomplish rapid storage of data in phase change memory cell. Moreover, phase change memory cells based on [GS(4 nm)/ZS(10 nm)]4 thin films can achieve reversible SET and RESET operations with 20 ns width pulse.

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.

Regulating phase change behavior and surface characteristics of Sn15Sb85 thin film by oxygen doping

In this paper, oxygen doped Sn15Sb85 thin films were proposed to reduce the power consumption for phase change memory (PCM) application. Compared with Sn15Sb85, oxygen doped Sn15Sb85 thin film had higher crystallization temperature (168 °C–255 °C) and broader energy band gap (1.23–1.55 eV). X-ray diffraction patterns and transmission electron microscope showed that the crystallization of thin film was suppressed and grains became smaller when oxygen was added. After oxygen doping, the surface roughness decreased from 13.6 to 2.5 nm. Antimony oxide formed to enhance the thermal stability. In comparison to Ge2Sb2Te5, oxygen doped Sn15Sb85 had an ultra-fast phase transition speed (3.9 ns) confirmed by laser picosecond technology. The result of differential scanning calorimetry revealed that oxygen doped Sn15Sb85 had a lower melting temperature (494 °C). PCM cells based on the oxygen doped Sn15Sb85 thin film were fabricated to evaluate the electrical characteristics as well. The results indicated that the oxygen doped Sn15Sb85 thin film had great potentiality in PCM application.

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.

SbSe/ZnSb stacked thin films with multi-level phase transition for high density phase change memory applications

As a highly integrated non-volatile memory device, phase change memory (PCM) has attracted considerable attention. In this paper, SbSe/ZnSb (SS/ZS) stacked thin films were developed for high density phase change memory applications. SS/ZS stacked thin films show two pronounced resistance steps with the increase in temperature and possess excellent thermal stability. The X-ray diffraction reveals that Sb, Sb2Se3 phases crystallize first, hexagonal ZnSb phase forms then at higher temperatures. The stacked thin films exhibit subtle variations in thickness and roughness after crystallization. Ultrafast crystallization speed (9.68 ns) in the SS(25 nm)/ZS(25 nm) thin film was validated by picosecond laser pump-probe system. Phase-change memory cells based on SS(25 nm)/ZS(25 nm) stacked thin film can realize multi-level data storage and the whole SET and RESET operations can be implemented with a 20 ns electrical pulse. Thus, SS/ZS stacked thin films have advantages for multi-level data storage capability, better thermal stability, and fast phase change speed, and are therefore good candidates for high density PCM device.

Effects of SiO2 interlayers on the phase change behavior in the multilayer Zn15Sb85/SiO2 materials

In this study, the multilayer Zn15Sb85/SiO2 materials were investigated for phase change memory application with the intention of improving thermal stability and switching speed. The Zn15Sb85/SiO2 materials exhibited a high crystallization temperature (Tc∼230 °C), good data retention (T10 ∼ 160 °C), small density change (∼3.3%), flat surface and rapid amorphization speed (∼1.34 ns). T-shaped phase change devices based on Zn15Sb85/SiO2 materials achieved low RESET voltage (VRESET∼1.3 V) in programming operation. The results showed that the multilayer Zn15Sb85/SiO2 material was a promising phase change material with high thermal ability, low power consumption and rapid switching speed.

Identifying optimal dopants for Sb2Te3 phase-change material by high-throughput ab initio calculations with experiments

Doping is used to improve the overall performance of rhombohedral Sb2Te3 for the application in phase-change memory (PCM). However, it is difficult and laborious to obtain the optimal dopants using the time-consuming trial and error way. By high-throughput ab initio calculations with experiments, the present work identified Scandium, Yttrium and Mercury as the optimal dopants for Sb2Te3 to reduce the power consumption of Sb2Te3-based PCM. Using Sc as an example the doping effect on the structure and property of Sb2Te3 has been extensively investigated. The band gap of Sb2Te3 linearly increases with increasing the Sc concentration by ab initio calculations, and our experimental results confirm the theoretical findings. Furthermore, based on the semi-classical Boltzmann transport theory, Sc can significantly decrease the electrical conductivity and thermal conductivity of Sb2Te3, and therefore reduced energy consumption of Sb2Te3-based PCM is expected. Finally, the ab initio molecular dynamics simulations demonstrate that Scandium can significantly improve the thermal stability of amorphous Sc-doped Sb2Te3, and hence the enhanced data reliability of Sb2Te3-based PCM. The present work provides fundamentals for fast screening optimal dopants to enhance the overall performance of Sb2Te3 for PCM, and the present method can be extended to other semiconductor materials.

Rare Earth Doping Brings Thermal Stability Improvement in Zn0.15Sb0.85 Alloy for Phase Change Memory Application

In this paper, the rare earth (Er or Sm) doped Zn0.15Sb0.85 films were synthesized by magnetron sputtering. The crystallization temperature (Tc), activation energy (Ea) and the temperature for ten year data retention (Tten) of the thin films were increased significantly by increasing rare earth dopants, revealing thermal stability improvement. Importantly, it was found that the Sm doping was a more effective way to increase Tc and Tthan compared with Er doping due to its lower electronegativity. Therefore, the Zn0.15Sb0.85 alloys by rare earth doping, which exhibited excellent thermal stability, were the promising candidates for phase change memory application.

Systematic materials design for phase-change memory with small density changes for high-endurance non-volatile memory applications

The optical properties of transition-metal-based ternary tellurides have been systematically examined by density functional theory simulations. Based on an empirical law, which correlates density change with the optical contrast between the amorphous and crystalline phases of chalcogenides, the density changes occurring upon crystallization for hypothetical materials have been predicted. Cr2Ge2Te6 was found theoretically to be one of the best materials for phase-change memory applications owing to its small density change. This novel criterion will be useful in the screening of materials yet to be experimentally realized, and will help target promising materials for high-endurance phase-change memory applications.

Superlattice‐like Zn 15 Sb 85 /Ga 30 Sb 70 thin films for low power and ultrafast phase change memory application

In this study, the amorphous-to-crystalline transitions of superlattice-like (SLL) Zn 15 Sb 85 /Ga 30 Sb 70 thin films were investigated by in situ film resistance measurement. The thermal stability, resistance and band gap of the SLL Zn 15 Sb 85 /Ga 30 Sb 70 thin film perform well. The data retention temperature for 10 years increases from 177°C of [Zn 15 Sb 85 7/Ga 30 Sb 70 3] to 220°C of [Zn 15 Sb 85 2/Ga 30 Sb 70 8]. The X-ray diffraction patterns show the presence of a large amount of Sb phase in the Zn 15 Sb 85 /Ga 30 Sb 70 thin films, which causes a rapid phase transition. The surface morphology is observed by atomic force microscopy, which indicates that the thickness of Ga 30 Sb 70 in the SLL thin films can affect the grain size and inhibit the crystallisation process. For the Zn 15 Sb 85 /Ga 30 Sb 70 thin films, an ultra-short switching time of around 2 ns is achieved. These results indicate that the SLL Zn 15 Sb 85 /Ga 30 Sb 70 thin films have a promised application in phase change memory.

Yttrium-doped Sb2Te as high speed phase-change materials with good thermal stability

We propose Y0.03(Sb2Te)0.97 (YST), prepared by magnetron sputtering, as a phase-change material for the application in phase change memory (PCM). The thermal stability of Sb2Te is increased by Y doping, with the crystallization temperature and 10-year data retention increase from 148.7 °C to 187.7 °C and from 55.0 °C to 114.5 °C, respectively. The grain refinement of the crystalline films is realized thanks to the inclusion of Y. On the device level, the phase transition of YST can be triggered by a 20 ns electric pulse, making it a promising candidate for PCM devices with both high operation speed and excellent thermal stability.

Local structure origin of ultrafast crystallization driven by high-fidelity octahedral clusters in amorphous Sc0.2Sb2Te3

Phase-change material Sc0.2Sb2Te3 (SST) can remarkably boost the writing speed of memory devices due to the extremely fast crystallization. It was demonstrated that the fast crystallization is because Sc stabilizes the 4-fold rings which act as precursors of nuclei in the amorphous phase. In this work, by using first-principles molecular dynamics simulations, we studied the local structures in the liquid and amorphous SST at various temperatures. The results reveal that Sc-centered configurations are almost in the form of stable octahedral clusters, which enhances the local order of the amorphous phase. Different from Sb- and Te-centered clusters which have lower coordination numbers, Sc-centered clusters are mainly in high-coordinated octahedral structures. These Sc-centered octahedral clusters present a high stability in supercooled liquid and amorphous states, remarkably reducing the incubation time of nucleation and speeding up the crystallization. Our study reveals the role of Sc atoms in the liquid and amorphous structure, paving the way for the application of Sc-based phase-change memory.

High thermal stability and fast speed phase change memory by optimizing GeSbTe with Scandium doping

Phase change materials along the GeTe-Sb isoelectronic tie line are proved to be promising host material for optimization, based on which the well-known golden composition is developed. Here, Sc doped Ge2Sb1Te2 has been proposed for phase change memory (PCM) application, showing higher thermal stability and faster operation speed than those of the golden composition. The fast speed of 40 ns, high 10-year data retention of 160 °C, and good endurance of 6×105 cycles have made Sc0.2Ge2Sb1Te2 a promising candidate for PCM application. The impact of Sc on the microstructure is believed to be essential for those improvements in PCM.

Atomic layer deposition and tellurization of Ge-Sb film for phase-change memory applications.

We studied the atomic layer deposition (ALD) and the tellurization of Ge–Sb films to prepare conformal crystalline Ge–Sb–Te (GST) films and to achieve void-free gap filling for emerging phase-change memory applications. ALD Ge–Sb film was prepared by alternating exposures to GeCl2-dioxane and Sb(SiEt3)3 precursors at 100 °C. The growth rate was 0.021 nm per cycle, and the composition ratio of Ge to Sb was approximately 2.2. We annealed the ALD Ge–Sb films with a pulsed feeding of di(tert-butyl)tellurium. The ALD Ge–Sb films turned into GST films by the tellurization annealing. When the tellurization temperature was raised to 190 °C or higher temperatures, the Raman peaks corresponding to Ge–Sb bond and amorphous Ge–Ge bond disappeared. The Raman peaks corresponding to Ge–Te and Sb–Te bonds were evolved at 200 °C or higher temperatures, resulting in the phase transition temperature of 123 °C. At 230 °C or higher temperatures, the entire film was fully tellurized to form a GST film having a relatively uniform composition of Ge3Sb2Te6, and the carbon impurities in the as-deposited ALD Ge–Sb film were eliminated. As the tellurization temperature increases, the volume of the ALD film is expanded owing to the incorporation of tellurium, resulting in complete filling of a trench pattern by GST film after the tellurization at 230 °C.

Preparation of Ge-Sb-Te Thin Films by Tellurization of Ge-Sb Thin Film for Phase-Change Random-Access Memory Application

We report the preparation of crystalline Ge-Sb-Te (GST) compounds by pulsed tellurization annealing of amorphous sputtered Ge-Sb films in a di(tert-butyl)tellurium [(tBu)2Te] atmosphere. The cycle of feeding (tBu)2Te into the annealing chamber for 1 s and then pumping down for 60 s was repeated, which was to avoid tellurium deposition. We investigated the effect of annealing temperature and annealing cycle number on the composition and crystal structure of GST thin films. The thin film was not wholly tellurized at 160 or 190°C, while the entire film was uniformly tellurized and crystallized at 220°C to form a GST film having a composition ratio close to that of Ge1Sb2Te4. The phase change temperature gradually decreased as tellurization proceeded. In the case of 30 cycles at 220°C, in which the entire Ge-Sb film was tellurized, the resistance reduction was observed at 150°C. The diffusion of germanium from GST into the substrate was also discussed.

Crystallization Properties of Mg35Sb65/Sb Nanocomposite Multilayer Films for Phase Change Memory Application

The phase transition properties of Mg35Sb65/Sb multilayer thin films were studied, including the crystallization mechanism, surface morphology, atomic bonding mode and adhesion strength. With the increase of the thickness of Mg35Sb65 interlayers, Mg35Sb65/Sb thin film had better thermal stability and lower electrical conductivity. The growth-dominated crystallization mechanism made Mg35Sb65/Sb have ultra-fast phase change speed. The subtle change on surface morphology was ascribed to the grain growth and interface stress during crystallization. The scratch test revealed the adhesion strength contrast before and after crystallization. This work showed that Mg35Sb65/Sb multilayer film was a potential material with fast speed and good thermal stability for phase change memory application.

Fast switching and low drift of TiSbTe thin films for phase change memory applications

As the non-volatile storage medium of the next generation, phase-change random-access memory (PCRAM) has characteristics such as high density, low cost and a wide range of market applications. However, the write speed of GST-based PCRAM falls short of the current DRAM memories and its applications domain expansion is limited. In this article, TiSbTe (TST) shows good performance both in high speed and low drift due to its fast crystallization rate and titanium-centered stable octahedral structure. The promising properties of this new material are illustrated in a 4 Mb chip for embedded applications and tested at wafer level, and the results show that the crystallization speed of TST is improved by 80% and a smaller resistance drift is achieved, with the drift exponent coefficient of highest resistance level being as low as is 0.02, compared with GeSbTe (GST).