Phase Change Memory Applications Research Articles

Simultaneous ultra-long data retention and low power based on Ge10Sb90/SiO2 multilayer thin films

In this article, Ge10Sb90/SiO2 multilayer thin films were prepared to improve thermal stability and data retention for phase change memory. Compared with Ge10Sb90 monolayer thin film, Ge10Sb90 (1 nm)/SiO2 (9 nm) multilayer thin film had higher crystallization temperature and resistance contrast between amorphous and crystalline states. Annealed Ge10Sb90 (1 nm)/SiO2 (9 nm) had uniform grain with the size of 15.71 nm. After annealing, the root-mean-square surface roughness for Ge10Sb90 (1 nm)/SiO2 (9 nm) thin film increased slightly from 0.45 to 0.53 nm. The amorphization time for Ge10Sb90 (1 nm)/SiO2 (9 nm) thin film (2.29 ns) is shorter than Ge2Sb2Te5 (3.56 ns). The threshold voltage of a cell based on Ge10Sb90 (1 nm)/SiO2 (9 nm) (3.57 V) was smaller than GST (4.18 V). The results indicated that Ge10Sb90/SiO2 was a promising phase change thin film with high thermal ability and low power consumption for phase change memory application.

Thermal stability and crystallization kinetics of Bi doped Si15Te85-xBix (0 ≤ x ≤ 2) chalcogenide glassy alloys

Bulk Si15Te85-xBix (0 ≤ x ≤ 2) chalcogenide glassy alloys were prepared by well-established melt quenching technique. Thermal stability and crystallization kinetics of these alloys were investigated by employing differential scanning calorimetry (DSC) technique at different heating rates, namely, 10, 15, 20 and 25 K/min under non-isothermal condition. Thermal parameters such as glass transition (Tg), onset crystallization (Tc) and peak crystallization (Tp) temperatures were observed. Double crystallization peaks observed in the DSC thermogram refer to the instability and phase separated network in the glasses. Various kinetic parameters such as thermal stability (ΔT), enthalpy (ΔHc), entropy (ΔS), specific heat (ΔCp) and fragility index are deduced. The calculated kinetic parameters suggest that the stability of glassy samples decreases with the increase in Bi addition. The activation energies of glass transition (Eg), and crystallization (Ec) are calculated using relevant kinetic formulae. We further discuss on the kinetics of the synthesized materials relevant for their applications in phase change memory (PCM) material

Reduction of thermal conductivity in YxSb2–xTe3 for phase change memory

Thermal conductivity (κ) is one of the fundamental properties of materials for phase change memory (PCM) application, as the set/reset processes strongly depend upon heat dissipation and transport. The κ of phase change materials in both amorphous and crystalline phases should be quite small, because it determines how energy-efficient the PCM device is during programming. At a high temperature, the electronic thermal conductivity (κe) is always notable for semiconductors, which is still lacking for antimony telluride under doping in the literature as far as we know. In this paper, using density functional theory and Boltzmann transport equations, we report calculations of lattice thermal conductivity κL and electronic thermal conductivity κe of the yttrium doped antimony telluride. We show that the average value of thermal conductivity decreases from ∼2.5 W m−1 K−1 for Sb2Te3 to ∼1.5 W m−1 K−1 for Y0.167Sb1.833Te3. This can be attributed to the reduced κL and κe, especially the κe at high temperature (near melting point). We further point out that the increased effective mass of carriers and the flat valance band edge are responsible for the decrease of κe. The reduced thermal conductivity is highly desirable for the decrease of heat dissipation and transport in PCM operations, which can increase the density of memory and reduce energy consumption.

Electrical and Thermal Conductivity and Conduction Mechanism of Ge2Sb2Te5 Alloy

Ge2Sb2Te5 alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge2Sb2Te5 alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge2Sb2Te5 alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge2Sb2Te5 alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge2Sb2Te5 alloy accounts for the thermal conduction mechanism.

Phase change behavior of pseudo-binary ZnTe-ZnSb material

The phase change behavior of pure ZnTe and pseudo-binary ZnTe-ZnSb material has been investigated systemically in terms of the electrical, thermal and optical properties. The results show that ZnTe is a composition with fast continuous crystallization, wide optical band gap and large crystalline resistance but thermally unstable and poor phase-switching ability. The pseudo-binary ZnTe-ZnSb material inherits the characteristics of fast crystallization and wide optical band gap from ZnTe and good thermal stability from ZnSb, respectively, and even shows better phase change properties with higher crystallization temperature and larger crystalline resistance. From these results, we conclude that ZnTe-ZnSb can be one of the most promising materials for phase change memory application.

Impact of Pressure on the Resonant Bonding in Chalcogenides

Resonant bonding has been appreciated as an important feature in some chalcogenides. The establishment of resonant bonding can significantly delocalize the electrons and shrink the band gap, leading to low electrical resistivity and soft optical phonons. Many materials that exhibit this bonding mechanism have applications in phase-change memory and thermoelectric devices. Resonant bonding can be tuned by various means, including thermal excitations and changes in composition. In this work, we manipulate it by applying large hydrostatic-like pressure. Synchrotron X-ray diffraction and density functional theory reveal that the orthorhombic lattice of GeSe appears to become more symmetric and the Born effective charge has significantly increased at high pressure, indicating that resonant bonding has been established in this material. In contrast, the resonant bonding is partially weakened in PbSe at high pressure due to the discontinuity of chemical bonds along a certain lattice direction. By controlling res...

Atomic Layer Deposition of GeTe and Ge–Sb–Te Films Using HGeCl3, Sb(OC2H5)3, and {(CH3)3Si}2Te and Their Reaction Mechanisms

In this paper, a new atomic layer deposition (ALD) process for depositing binary GeTe and ternary Ge–Sb–Te thin films is reported, where HGeCl3 and ((CH3)3Si)2Te were used as Ge and Te precursors, respectively. The precursors reacted together to form the films at a low substrate temperature of 50–100 °C, without involving any additional reactive process gas. HCl elimination from the Ge precursor to form the divalent Ge intermediate, GeCl2, is proposed to explain the formation of 1:1 composition stoichiometric GeTe films. The GeTe films are promising for use in phase change memory applications. Ternary Ge–Sb–Te films were deposited by combining the GeTe ALD process with a previously developed ALD process for Sb2Te3 films, where Sb(OC2H5)3 and ((CH3)3Si)2Te were employed respectively as the Sb and Te precursors. However, the composition of the ternary GeSbTe films deviated slightly from the desired GeTe–Sb2Te3 pseudobinary composition suggesting that a certain unwanted reaction was involved between the prev...

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.

One-step phase transition and thermal stability improvement of Ge2Sb2Te5 films by erbium-doping

The crystallization behavior of Er-doped Ge2Sb2Te5 phase-change materials is investigated systemically for phase change memory application. It is observed that Er dopants can serve as a center for suppression of face-centered-cubic to hexagonal phase transition of Ge2Sb2Te5 films, leading to a one-step crystallization process. The crystallization temperature, 10-year data retention ability and crystalline resistance of Ge2Sb2Te5 films can be significantly increased. Raman spectra suggest that GeTe component is mainly responsible for the phase transition in Er-doped Ge2Sb2Te5 films.

Improved thermal stability of Sb materials by SiO2 doping for ultra-fast phase change memory application

SiO2-doped Sb material is proved to be a promising candidate for phase change memory (PCM) use because of its high crystallization temperature (∼239 °C), large crystallization activation energy (4.05 eV), and good data retention ability (162 °C for 10 years). The band gap is broadened and the grain size is refined by SiO2 doping. The reversible resistance transition can be achieved by an electric pulse as short as 5 ns for the PCM cell based on SiO2-doped Sb material. A lower operation power consumption (the energy for RESET operation 1.1 × 10−11 J) is obtained. In addition, SiO2-doped Sb material shows a good endurance of 2.5 × 105 cycles.

Improving the thermal stability and phase change speed in Sb70Se30 films through Er doping

For the phase change memory (PCM) application, it is very desirable to possess better comprehensive performance in phase change materials, such as high stability, ultra-faster phase change and low power consumption. In this work, Er doped Sb70Se30 phase change materials were synthesized by magnetron sputtering technique. Compared with pure Sb70Se30 films, it was found that the crystallization temperature (T c) increased from 207 to 221 °C by Er doping, revealing the improvement of thermal stability. In addition, the increasing resistance of amorphous and crystalline state indicated much lower power consumption for PCM. More importantly, the Er doping significantly inhibits the formation of Sb–Se phase, which is to result in much faster phase change speed. Therefore, this work elucidated that the SbSe materials with Er doping were promising candidates possessing better data retention, lower power consumption and faster phase change speed for PCM application.

Suppression for an intermediate phase in ZnSb films by NiO-doping

The structural evolution and phase-change kinetics of NiO-doped ZnSb films are investigated. NiO-doped ZnSb films exhibit a single-step crystallization process, which is different from that of undoped ZnSb. NiO-doped ZnSb can directly crystallize into a stable ZnSb phase at temperatures greater than 320 °C with suppression of a metastable ZnSb phase. These characteristics enlarge the amorphous/crystalline resistance ratio by approximately five orders of magnitude. Moreover, NiO doping of ZnSb films increases crystallization temperature from 260 to 275 °C, improves data retention temperature from 201.7 to 217.3 °C and increases crystalline activation energy from 5.64 to 6.34 eV. The improvement of the thermal parameters in the nanocomposite can be attributed to stable ZnSb grain growth refinement owing to the dispersion of NiO particles in the sample matrix. This provides additional nucleation sites and produces more ZnSb/NiO interfaces, which can initiate the nucleation and accelerate crystallization. The kinetic exponent n decreases from 1.12 to 0.44, which confirms the ultrafast one-dimensional growth and heterogeneous phase transition of the NiO-doped ZnSb films. The improved thermal stability, larger resistance ratio and direct transition to a stable phase with ultrafast one-dimensional crystal growth indicate the good potential of these materials in phase-change memory applications.

Extremely High Contrast Multi-Level Resistance States of In3 SbTe2 Device for High Density Non-Volatile Memory Applications

Current-controlled promotion of crystallization offers stable multi-level resistances after threshold switching process which are being exploited for multi-bit phase change memory applications. In this paper, ultra-high contrast multi-level resistance characteristics of In3SbTe2 device is demonstrated by the means of systematically varying the current-controlled crystallization process for a wide range of currents starting from few μA to 40 μA and the corresponding resistance levels ranging from 10 MΩ to 10 kΩ, respectively. Also, the conduction process in the amorphous phase and the stability of multiple resistance levels have been analyzed using trap-limited sub-threshold electrical transport models, and the effective crystalline thicknesses of various resistance states were estimated. Furthermore, a systematic increment of programming current unfolds more than eight discrete stable resistive states with a remarkably larger programming margin (∼570 times) between amorphous and crystalline (set) state revealing the extraordinary multi-bit programming capabilities of In3SbTe2 device for high-density data storage applications.

Surface Energy Driven Cubic-to-Hexagonal Grain Growth of Ge2Sb2Te5 Thin Film

Phase change memory (PCM) is a promising nonvolatile memory to reform current commercial computing system. Inhibiting face-centered cubic (f-) to hexagonal (h-) phase transition of Ge2Sb2Te5 (GST) thin film is essential for realizing high-density, high-speed, and low-power PCM. Although the atomic configurations of f- and h-lattices of GST alloy and the transition mechanisms have been extensively studied, the real transition process should be more complex than previous explanations, e.g. vacancy-ordering model for f-to-h transition. In this study, dynamic crystallization procedure of GST thin film was directly characterized by in situ heating transmission electron microscopy. We reveal that the equilibrium to h-phase is more like an abnormal grain growth process driven by surface energy anisotropy. More specifically, [0001]-oriented h-grains with the lowest surface energy grow much faster by consuming surrounding small grains, no matter what the crystallographic reconfigurations would be on the frontier grain-growth boundaries. We argue the widely accepted vacancy-ordering mechanism may not be indispensable for the large-scale f-to-h grain growth procedure. The real-time observations in this work contribute to a more comprehensive understanding of the crystallization behavior of GST thin film and can be essential for guiding its optimization to achieve high-performance PCM applications.

Study on the physical properties and structure of titanium antimony thin films for phase change memory application

The phase change behaviors of titanium antimony thin films were examined as a function of Ti concentration by in situ electrical measurement. With the increment in titanium content, the crystallization temperature enhanced, while the electrical conductivity reduced. The sharp decline in carrier density is responsible for the drop of the electrical conductivity before and after crystallization. The amorphous and polycrystalline state was confirmed by means of selected area electron diffraction. The shift of Raman modes associated with Sb upon the phase transition was observed. The surface morphology and density fluctuation were obtained from atomic force microscopy and X-ray reflectivity. The interfacial adhesion strength between the titanium antimony thin films and silicon dioxide was implemented by Nano Indenter®. Based on Arrhenius plot and Johnson–Mehl–Avrami model, the crystallization kinetics including the crystallization activation energy, the crystallization mechanism and the Avrami coefficient were investigated as well. The obtained values of Avrami indexes illustrate one-dimensional growth-dominated mechanism of titanium antimony thin films.

Synthesis of SnSe2 thin films by thermally induced phase transition in SnSe

The synthesis of SnSe2 thin films, for phase change memory applications, generally require selenization and higher temperature. However, there are many critical problems associated with selenization, such as use of highly toxic H2Se/Se gas at high temperature, slow reaction rate and non-uniform composition of thin films. To overcome these problems, a novel and relatively low temperature synthesis of SnSe2 thin films has been reported here. Selenium rich Sn30Se70 (prepared by melt quenching technique) has been thermally evaporated in presence of Ar followed by vacuum annealing at 200 °C. An attempt has been made to elucidate the structural, optical and electrical properties of these thin films. X-ray diffraction and UV–Vis studies confirmed a phase change from orthorhombic SnSe to hexagonal SnSe2 followed by improvement in crystallinity (from ≈5 nm to 15 nm) and a red shift in optical band gap on annealing. Electrical studies revealed that SnSe phase have single thermal and photo activation energies in temperature range from −20 °C to 100 °C. A large contrast (four orders) in dark-conductivity [(3.19 ± 0.02) × 10−5Ω−1cm−1 to (5.16 ± 0.02) × 10−1Ω−1cm−1] between two phases of Sn30Se70, suggests its use in phase change memory devices. The SnSe2 phase is observed to be highly stable in the presence of visible light as compared to SnSe.

Controllable phase separation and improved grain growth mode of Mg-doped Sb7Te3 films

The effects of Mg on the structure and phase-change kinetics of Sb7Te3 were investigated for phase change memory application. The results revealed that the addition of Mg increased the crystallization temperature, and crystalline activation energy, while hindering grain growth and suppressing phase separation from Sb+Sb2Te, Sb2Te, to Sb phases. Laser-induced phase-change behavior showed four regions in the Mg-Sb7Te3 films: crystallization, transition toward stability, amorphization and ablation. A reversible switching between crystallization and amorphization occurred more rapidly in Mg-doped Sb7Te3 films than in conventional Ge2Sb2Te5. Evaluation of Johnson-Mehl-Avrami plots indicated that the crystallization mode changed from three, two to one-dimensional grain growth as the Mg-doping level increased. This phenomenon contributed to the rapid phase change of Mg-doped Sb7Te3 films.

Se-doped Ge10Sb90 for highly reliable phase-change memory with low operation power

Abstract

Carbon layer application in phase change memory to reduce power consumption and atomic migration

Phase change memory (PCM) cells with carbon buffer layer have been fabricated. Carbon layer reduced the heat dissipation through upper electrode, increasing the energy efficiency of PCM, reducing the power consumption. The element distribution in PCM cell after 3×105 cycles proved that the carbon layer has successfully prevented atoms diffusion. After carbon layer being applied, the local high temperature during RESET in phase change material has been lowered. Hence, the higher viscosity at lower temperature has reduced atomic migration, beneficial to a good endurance.

Synthesis and Screening of Phase Change Chalcogenide Thin Film Materials for Data Storage.

A combinatorial synthetic methodology based on evaporation sources under an ultrahigh vacuum has been used to directly synthesize compositional gradient thin film libraries of the amorphous phases of GeSbTe alloys at room temperature over a wide compositional range. An optical screen is described that allows rapid parallel mapping of the amorphous-to-crystalline phase transition temperature and optical contrast associated with the phase change on such libraries. The results are shown to be consistent with the literature for compositions where published data are available along the Sb2Te3-GeTe tie line. The results reveal a minimum in the crystallization temperature along the Sb2Te3-Ge2Te3 tie line, and the method is able to resolve subsequent cubic-to-hexagonal phase transitions in the GST crystalline phase. HT-XRD has been used to map the phases at sequentially higher temperatures, and the results are reconciled with the literature and trends in crystallization temperatures. The results clearly delineate compositions that crystallize to pure GST phases and those that cocrystallize Te. High-throughput measurement of the resistivity of the amorphous and crystalline phases has allowed the compositional and structural correlation of the resistivity contrast associated with the amorphous-to-crystalline transition, which range from 5-to-8 orders of magnitude for the compositions investigated. The results are discussed in terms of the compromises in the selection of these materials for phase change memory applications and the potential for further exploration through more detailed secondary screening of doped GST or similar classes of phase change materials designed for the demands of future memory devices.