Phase Change Memory Applications Research Articles

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.

Simultaneously high thermal stability and low power based on Cu-doped GeTe phase change material

The Cu-doped GeTe material was investigated systematically for its potential application in phase change memory. Compared with GeTe, Ge40Cu20Te40 material had higher crystallization temperature (258 °C) and activation energy for crystallization (3.78 eV). After Cu adding, the band gap energy increased and the crystallization was restrained. A smoother surface for Ge40Cu20Te40 material was achieved due to smaller grain size. The phase change memory device based on Ge40Cu20Te40 material could have electrical switching with the RESET voltage of 2.65 V and pulse width 5 ns. The good thermal stability and low power demonstrated the promising application for Ge-Cu-Te material in phase change memory.

Interplay between Structural and Thermoelectric Properties in Epitaxial Sb2+xTe3 Alloys

AbstractIn recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X‐ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb2 and Sb4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned.

Structural and thermoelectric properties of Se doped In2Te3 thin films

The Se-Te based chalcogenides exhibit novel property of Phase Change Memory (PCM) which has potential applications in electrical non-volatile memories. These materials are also suitable in thermal to electrical energy conversions and, hence, of potential interest in energy sustainability as thermoelectric devices. In this study, the Se doped In2Te3 thin films were prepared by thermal evaporation and were annealed at 250 °C and 300 °C in Argon gas. The X-ray diffraction spectra show that thermal annealing leads to the phase transitions in Se doped In2Te3 into binary phases of In2Se3 and In2Te3. The surface morphology of the films exhibits the grains of spherical nature. Annealing also decreases the energy band gap due to the presence of two phases. From the four probe and photoconductivity measurements, a large contrast in electrical resistance between the amorphous and crystalline states is found with a variation of a few orders of magnitude. The electrical transport properties such as the electrical resistivity, Seebeck coefficient and the power factor were measured in the temperature range from 300 K to 430 K. All the deposited and annealed thin films exhibit n-type conductivity with the Seebeck coefficient ranging from -338 μVK-1 to -510 μVK-1. An increase in thermoelectric power of 25% is observed in the 300 °C annealed films in comparison to the as-deposited films. Moreover, the lower Se doped In2(Te0.96Se0.04)3 compound exhibits a better thermoelectric performance compared to the In2(Te0.90Se0.1)3 composition. This study shows the multifunctional nature of Se doped In2Te3 both for PCM and thermoelectric applications.

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.

Effect of cerium doping on the crystallization behavior of ZnSb for phase-change memory application

In the paper, the Ce doping is proposed to improve the thermal stability and increase the resistance in ZnSb thin film for phase-change memory (PCM) application. By Ce doping, it is proved that the resistance of amorphous and crystalline state, crystallization temperature (Tc), 10-year lifetime temperature (Tten) and activation energy (Ea) were significantly increased in ZnSb films. The improvement of thermal stability and the reduction of reset current were achieved. Through the experimental investigation, the new formation of Ce–Sb bonding by Ce doping was found in the Sb hexagonal structure, which might be the key reasons for the enhanced performance.

Unique interface-driven crystallization mechanism and element-resolved structure imaging of ZnO-Ge2Sb2Te5 nanocomposites

Amorphous Ge2Sb2Te5 thin films doped with ZnO were prepared and their crystallization behaviors were investigated. Our results showed that thermal annealing of amorphous ZnO-Ge2Sb2Te5 nanocomposites produced face centered-cubic Ge2Sb2Te5-nanocrystals embedded between interfaces. An increase in crystallization temperature and electrical resistance ratio were attributed to the increase of specific interfacial energies and inhomogeneous strain at the oxide/Ge2Sb2Te5 interfaces. The formation of the interfaces not only accelerated crystallization, but also limited the grain growth due to the one-dimensional growth mode. We proposed a crystallization model to elucidate the derived kinetic mechanism of the nanocrystal growth in the Ge2Sb2Te5 matrix. These experimental observations suggest that ZnO-Ge2Sb2Te5 nanocomposites are a good candidate for the applications in phase change memory.

Characteristics of Sb6Te4/VO2 Multilayer Thin Films for Good Stability and Ultrafast Speed Applied in Phase Change Memory**Supported by the National Natural Science Foundation of China under Grant No 11774438, the Natural Science Foundation of Jiangsu Province under Grant No BK20151172, the Qing Lan Project, the Opening Project of State Key Laboratory of Silicon Materials under Grant No SKL2017-04, the Opening

The Sb6 Te4/VO2 multilayer thin films are prepared by magnetron sputtering and the potential application in phase change memory is investigated in detail. Compared with Sb6 Te4, Sb6 Te4/VO2 multilayer composite thin films have higher phase change temperature and crystallization resistance, indicating better thermal stability and less power consumption. Also, Sb6 Te4/VO2 has a broader energy band of 1.58 eV and better data retention (125°C for 10 y). The crystallization is suppressed by the multilayer interfaces in Sb6 Te4/VO2 thin film with a smaller rms surface roughness for Sb6 Te4/VO2 than monolayer Sb4 Te6. The picosecond laser technology is applied to study the phase change speed. A short crystallization time of 5.21 ns is realized for the Sb6 Te4(2 nm)/VO2 (8 nm) thin film. The Sb6 Te4/VO2 multilayer thin film is a potential and competitive phase change material for its good thermal stability and fast phase change speed.

Colloidal Phase-Change Materials: Synthesis of Monodisperse GeTe Nanoparticles and Quantification of Their Size-Dependent Crystallization.

Phase-change memory materials refer to a class of materials that can exist in amorphous and crystalline phases with distinctly different electrical or optical properties, as well as exhibit outstanding crystallization kinetics and optimal phase transition temperatures. This paper focuses on the potential of colloids as phase-change memory materials. We report a novel synthesis for amorphous GeTe nanoparticles based on an amide-promoted approach that enables accurate size control of GeTe nanoparticles between 4 and 9 nm, narrow size distributions down to 9–10%, and synthesis upscaling to reach multigram chemical yields per batch. We then quantify the crystallization phase transition for GeTe nanoparticles, employing high-temperature X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. We show that GeTe nanoparticles crystallize at higher temperatures than the bulk GeTe material and that crystallization temperature increases with decreasing size. We can explain this size-dependence using the entropy of crystallization model and classical nucleation theory. The size-dependences quantified here highlight possible benefits of nanoparticles for phase-change memory applications.

Improvement of thermal stability of antimony film by cerium addition for phase change memory application

Compared with pure Antimony (Sb) materials, Cerium (Ce) doped Sb alloys is proved to be a promising candidate with better thermal stability for phase change memory application. In this paper, the Ce doped Sb films were synthesized by magnetron sputtering. The crystallization temperature (Tc), activation energy (Ea) and the temperature for 10 years data retention (Tten) all increased significantly by increasing Ce dopants, meaning a much better thermal stability. These results may ascribe to disturbing crystallization process from the existence of Ce–Sb bond.

(Invited) Sputter Growth of Chalcogenide Superlattice Films for Future Phase Change Memory Application

Ge-Sb-Te ternary alloys are key materials for phase-change random access memory (PCRAM). In PCRAM, data recording relies on reversible switching between amorphous and crystalline phases by electrical pulse induced Joule heating. We have proposed and developed GeTe/Sb2Te3 superlattice phase change memory, which is known as interfacial phase change memory (iPCM), and have demonstrated a significant reduction in switching energy and much longer endurance compared to conventional alloy-type phase change memory. Since this superlattice is composed of atomically aligned GeTe and Sb2Te3 multilayers, fabricating a highly-oriented film is crucial. Furthermore, in order to utilize such material for novel devices in an industrial setting, reliable processes that allow large area deposition of uniform films is critical. Sputtering is one of the most useful, technologically friendly, and reliable methods for thin film fabrication. In this work, the growth mechanism of layered chalcogenide films is discussed. The superlattice films were grown on various substrate materials by radio-frequency magnetron sputtering. Ge-Te and Sb-Te alloy targets were used. Before the growth of the film, Ar sputter cleaning was carried out to remove a native oxide from the substrate surface. An Sb2Te3 seed layer was grown initially at room temperature followed by alternate deposition of GeTe and Sb2Te3 layers at high temperature. The crystal structure of the films as well as the degree of orientation were evaluated by x-ray diffraction (XRD) while the microstructure was observed by transmission electron microscopy (TEM). We found there is a strong dependency on substrate materials in terms of out-of-plane orientation of the superlattice film. On the basis of the obtained results, we will discuss the growth mechanisms of layered chalcogenide films and propose optimal growth conditions for future phase change memory applications. This work was supported by JST-CREST (JPMJCR14F1).

Investigation of Sb65Se35/Sb multilayer thin films for high speed and high thermal stability application in phase change memory

Sb65Se35/Sb multilayer composite thin films were prepared by depositing the Sb65Se35 and Sb layers alternately. In situ resistance vs. temperature was measured and the crystallization temperature increased with thickening the Sb65Se35 layer in Sb65Se35/Sb thin films. The data retention temperature of 10 years increased greatly from 14 °C of pure Sb to 103 °C of [Sb65Se35(3 nm)/Sb(7 nm)]3. Also, the band gap was broadened and the surface became smoother. X-ray diffraction patterns for the studied materials revealed that Sb and Sb2Se3 phases coexisted in Sb65Se35/Sb thin films. Absorbing the advantages of the fast phase change for Sb, the [Sb65Se35(1 nm)/Sb(9 nm)]5 multilayer thin film had an ultrafast amorphization speed of 1.6 ns. The results indicated that Sb65Se35/Sb multilayer thin film was a potential phase change material for fast speed and good stability.

(Invited) Sputter Growth of Chalcogenide Superlattice Films for Future Phase Change Memory Applications

Ge-Sb-Te ternary alloys are key materials for phase-change random access memory (PCRAM). In PCRAM, data recording relies on reversible switching between the amorphous and crystalline phases by means of electrical pulse induced Joule heating. We proposed and developed GeTe/Sb2Te3 superlattice phase change memory, also known as interfacial phase change memory (iPCM), and have demonstrated a significant reduction in switching energy and much longer endurance compared to devices fabricated from conventional alloy-type phase change memory. In this work, we discuss the growth mechanisms of layered chalcogenide films and propose optimal growth conditions for future phase change memory applications.

Investigation of laser-irradiated structure evolution and surface modification by in situ Micro-Raman spectroscopy

The structure evolution and surface modification of the ZnSb, ZnSb-Bi and ZnSb-NiO phase-change films are detected by in situ Raman laser irradiation with the different laser power and irradiation time. The results show that undoped ZnSb films exhibit a phase transformation from amorphous to metastable phases at 250 mW. The Bi-doped ZnSb films exhibit two different crystallization behaviors as the laser power increases. The films containing low Bi-doping concentration exhibit an amorphous-to-metastable phase transition, while the films containing high Bi-doping concentration directly crystallize into a mixture of new Bi-related crystalline phases and metastable ZnSb phases. As for ZnO-doped ZnSb films, the structure evolution encounters an amorphous phase, metastable ZnSb phase or mixture of both. Identifying the structure evolution in the metastable phase by in-situ Raman measurement method is of great significance for phase change memory application.

Physical and electrical characteristics of GexSb100−x films for use as phase-change materials

In this study, we investigate a GeSb binary phase to be used as a high-speed phase-change material (PCM). In particular, by varying the Ge concentration from 10 at.% to 30 at.%, we study the effects of the PCM composition on the physical and electrical properties of sputtered GexSb100−x films. The Ge10Sb90 film assumes a crystal trigonal structure (Sb phase); however, phase separation occurs in the films at higher Ge concentrations (i.e., the Sb phase coexists with the crystalline Ge phase). As the Ge concentration increases, the grain size decreases, and the sheet resistances of both the amorphous and crystalline states increase. An increase in the Ge concentration in these GeSb films results in an increase in the crystallization temperature, activation energy, data retention, and band gap. The current–voltage measurements show that the threshold and RESET currents decrease with increasing Ge concentrations. In addition, we discuss the R–V curve, crystallization speed, and programming window. The results indicate that GeSb films possess favorable physical and electrical characteristics and can be used as PCMs in phase-change memory applications.

Crystallization Process of Superlattice-Like Sb/SiO2 Thin Films for Phase Change Memory Application**Supported by the National Natural Science Foundation of China under Grant No 11774438, the Natural Science Foundation of Jiangsu Province under Grant No BK20151172, the Changzhou Science and Technology Bureau under Grant No CJ20160028, the Qing Lan Project, the Opening Project of State Key Laboratory of Silicon Materials under

After compositing with SiO2 layers, it is shown that superlattice-like Sb/SiO2 thin films have higher crystallization temperature (∼240°C), larger crystallization activation energy (6.22 eV), and better data retention ability (189°C for 10 y). The crystallization of Sb in superlattice-like Sb/SiO2thin films is restrained by the multilayer interfaces. The reversible resistance transition can be achieved by an electric pulse as short as 8 ns for the Sb(3 nm)/SiO2(7 nm)-based phase change memory cell. A lower operation power consumption of 0.09 mW and a good endurance of 3.0 × 106 cycles are achieved. In addition, the superlattice-like Sb(3 nm)/SiO2(7 nm) thin film shows a low thermal conductivity of 0.13 W/(m·K).

Study on the phase change behavior of nitrogen doped Bi2Te3 films

Bi2Te3 has been widely used as thermoelectric material due to its overall good properties, such as low thermal conductivity, high electrical conductivity, and flexible atoms arrangement for structural optimization et al. However, its potential in phase change memory (PCM) application is not well evaluated due to its unstable amorphous state. In this work, stability of amorphous Bi2Te3 has been improved by N doping. Crystallization temperature of 175 °C and data retention of 43.6 °C have been achieved. Smaller and defective grains have been directly observed. Abnormal volume expansion of 7.2% has been observed after crystallization. Low-power consumption and good reversible phase change ability of N doped Bi2Te3 has been verified in PCM cell.

Scandium doped Ge2Sb2Te5 for high-speed and low-power-consumption phase change memory

To bridge the gap of access time between memories and storage systems, the concept of storage class memory has been put forward based on emerging nonvolatile memory technologies. For all the nonvolatile memory candidates, the unpleasant tradeoff between operation speed and retention seems to be inevitable. To promote both the write speed and the retention of phase change memory (PCM), Sc doped Ge2Sb2Te5 (SGST) has been proposed as the storage medium. Octahedral Sc-Te motifs, acting as crystallization precursors to shorten the nucleation incubation period, are the possible reason for the high write speed of 6 ns in PCM cells, five-times faster than that of Ge2Sb2Te5 (GST) cells. Meanwhile, an enhanced 10-year data retention of 119 °C has been achieved. Benefiting from both the increased crystalline resistance and the inhibited formation of the hexagonal phase, the SGST cell has a 77% reduction in power consumption compared to the GST cell. Adhesion of the SGST/SiO2 interface has been strengthened, attributed to the reduced stress by forming smaller grains during crystallization, guaranteeing the reliability of the device. These improvements have made the SGST material a promising candidate for PCM application.

Atomic diffusion in laser irradiated Ge rich GeSbTe thin films for phase change memory applications

The atomic diffusion and compositional variations upon melting have been studied by transmission electron microscopy and electron energy loss spectroscopy in Ge rich GeSbTe films, with a composition optimized for memory applications. Melting and quenching has been achieved by laser pulses, in order to study pure thermal diffusion without electric field induced electromigration. The effect of different laser energy densities has been investigated. The diffusion of Ge atoms in the molten phase is found to be a prominent mechanism and, by employing finite elements computational analysis, a diffusion coefficient of Ge on the order of 5 × 10−5 cm2 s−1 has been estimated.

Effect of selenium doping on the crystallization behaviors of GeSb for phase-change memory applications

We investigated the effect of Se doping on the physical and electrical properties of Ge10Sb90 films prepared by sputtering. We examined the crystal structure, chemical bonding, electrical and thermal properties, and volume shrinkage during the crystallization of Ge10Sb90 films doped concentrations of Se between 0 and 20 at.%. X-ray diffraction patterns showed that incorporating Se leads to phase separation at high Se concentrations. As the Se concentration increased, both the crystallization temperature (Tc) and the sheet resistance increased in both the amorphous and crystalline states. In contrast, the melting temperature (Tm) and grain size both decreased with the increasing Se. The volume shrinkage of the Se-doped Ge10Sb90 films was analyzed by X-ray reflectivity measurements. Our results demonstrate that Se-doped Ge10Sb90 films show a favorably low operation current and good thermal and mechanical stability, which strongly indicate their feasibility for phase-change memory applications.