Crystallization kinetics of Ga-Sb-Te films for phase change memory

Ga-Sb-Te ternary thin films were prepared by co-sputtering using targets GaSb and Sb₈Te₂. Crystallization processes of these films were studied by measuring the temperature-dependent electrical resistance during non-isothermal heating. The activation energy (E_a), rate factor (K_o), and kinetics exponent (n) were deduced from Kissinger and Ozawa's plots, respectively. The crystallization temperatures (T_x = 108∼319°C) increase with increasing GaSb contents. The activation energy (1.82∼7.52 eV) increases with increasing Ga content until 31.6 at.% then it decreases. The crystallization mechanisms of compositions A∼D (Ga_17∼32Sb_71∼62Te_12∼6), as evidenced from n values, are nuclei formation and subsequent crystal growth; the nucleation rate decreases with grain growth. While composition E (Ga_38.2Sb_57.7Te_4.1) reveals an n value lower than 1.5, it implies the one-dimensional crystal growth from the nuclei. The crystallization time of each composition was evaluated using JMA equation using all derived kinetics parameters. Composition C (Ga_26.4Sb_65.2Te_8.4), showing the shortest crystallization time, is suggested for phase change RAM applications.     

Crystal structure of GeTe and Ge2Sb2Te5 meta-stable phase

Direct X-ray diffraction measurement of the erased state of the Ge–Sb–Te recording layer in a four-layered phase change optical disk, which was produced by an optical disk drive, was performed. It was identified as an fcc crystal structure. In order to carry out the detailed crystal structure analysis by the powder X-ray diffraction method with Rietveld refinements, somewhat larger amount of the fcc crystal powder was prepared from deposited 10 μm thick films. It revealed that Ge₂Sb₂Te₅ belongs to the NaCl type structure (Fm m) with the 4a site including 20% vacancies. The conclusion was supported by the results of the density measurements with Grazing Incidence of X-ray Reflectivity.     

Materials science and engineering of phase change random access memory

Having monopolised the optical data storage industry since the very beginning, phase change materials are now being intensively explored for next-generation electronic data storage, referred to as phase change random access memory (PCRAM). Because phase change materials are electrically programmable; capable of reversibly switching between two stable structural phases of contrasting electrical properties, besides data storage they also enable data computation. For these reasons, PCRAM envisages to overcome both miniaturisation and data flow bottlenecks, challenges which current silicon charge-based technology is failing to cope with. This review, while reasoning the need for a switch to a newer data storage technology, and comparing PCRAM with other data storage and computation platforms, comprehensively takes stock of the benefits and challenges associated with PCRAM. This review also critically investigates and associates the materials science and physics, such as the atomic structure and bonding, thermodynamics and kinetics of the phase transformation, with the PCRAM device characteristics and performance. Various device design-concepts and requirements are reviewed. Recent advances, and evolution of newer platforms, including those relating to neuromorphic computing and photonic memory are also described.

This is the winning review of the 2017 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged     

磁控溅射沉积制备SnSe薄膜及其热电性能研究

因为晶体结构以及热电性能各向异性,硒化锡(SnSe)沿b轴方向表现出优异的热电性能,受到业内的广泛关注.但关于SnSe薄膜研究的报道较少.本研究利用磁控溅射技术,将SnSe沉积到Si/SiO2基底得到SnSe薄膜,分析了沉积温度对SnSe薄膜结构和热电性能的影响.结果显示:沉积温度升高,晶粒尺寸相应增加,薄膜的结晶质量...     

Crystallization and electrical characteristics of Ge1Cu2Te3 films for phase change random access memory

Phase change random access memory (PCRAM) requires an advanced phase change material to lower its power consumption and to enhance its data retention and endurance abilities. The present work investigated the crystallization behaviors and electrical properties of Ge₁Cu₂Te₃ compound films with a low melting point of about 500 °C for PCRAM application. Sputter-deposited Ge₁Cu₂Te₃ amorphous films showed a high crystallization temperature of about 250 °C. The Ge₁Cu₂Te₃ amorphous film showed an electrical resistance decrease of over 10²-fold and exhibited a small increase in thickness of 2.0% upon crystallization. The Ge₁Cu₂Te₃ memory devices showed reversible switching behaviors and exhibited a 10% lower power consumption for the reset operation than the conventional Ge₂Sb₂Te₅ memory devices. Therefore, the Ge₁Cu₂Te₃ compound is a promising phase change material for PCRAM application.     

Electrical and structural properties of TaSiN electrode for phase change random access memory

Characteristics of tantalum silicon nitride (TaSiN) thin films have been investigated as an electrode material for Ge₂Sb₂Te₅ chalcogenide phase change material. The films were deposited by co-sputtering system in which the ratio of tantalum nitride to silicon was controlled by the plasma power on each target. The TaSiN films showed tunable resistivity from 260 to 560 μΩ cm with increasing Si content. From the evaluation of PRAM cell structures consisting of the TaSiN and the Ge₂Sb₂Te₅, we found that the SET voltages are nicely correlated with the resistivity of the TaSiN. Moreover, the sensing margin (resistance ratio: R_SET/R_RESET) turned out to be good for practical application.     

Nano composite Si2Sb2Te film for phase change memory

Growth-dominant Sb₂Te material with large crystal grain is converted to the nano composite material after Si doping. The increase of Si content in Si_xSb₂Te material helps to further diminish the grain size, form more uniform grain distribution, and enhance the thermal stability of the amorphous phase. Si₂Sb₂Te crystallizes into a nano composite structure [amorphous Si + crystalline Sb₂Te (< 20 nm grain size)] without any Te or Sb phase segregation, which ensures better operation stability for the application in T-shaped phase change memory device. Comparing to Ge₂Sb₂Te₅ film, Si₂Sb₂Te film shows better data retention ability (10 years at 397 K). Meanwhile, electrical measurements prove that phase change memory cell based on Si₂Sb₂Te film also has low power consumption than that of the Ge₂Sb₂Te₅ film based cell.     

Determination of optical parameters of GeTe semiconductor films after thermal treatment

The optical parameters of GeTe semiconductor films after various thermal treatments have been measured using a novel method. A comparative study using a spectrum ellipsometer is presented. The optical parameters of the films were extracted precisely by data analysis and corrections have been made to previous calculations. Calculations based on the spectral ellipsometry measurements are presented finally, and the complex refractive index curves of the samples in the spectral range from 250 to 830 nm have been obtained.     

Picosecond laser pulse-driven crystallization behavior of SiSb phase change memory thin films

Transient phase change crystallization process of SiSb phase change thin films under the irradiation of picosecond (ps) laser pulse was studied using time-resolved reflectivity measurements. The ps laser pulse-crystallized domains were characterized by atomic force microscope, Raman spectra and ellipsometrical spectra measurements. A reflectivity contrast of about 15% can be achieved by ps laser pulse-induced crystallization. A minimum crystallization time of 11 ns was achieved by a low-fluence single ps laser pulse after pre-irradiation. SiSb was shown to be very promising for fast phase change memory applications.     

Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2Te3 Superlattices Used in Interfacial Phase‐Change Memory (iPCM) Devices

Van der Waals layered GeTe/Sb₂Te₃ superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so‐called interfacial phase‐change memory (iPCM) devices. GeTe/Sb₂Te₃ SLs are made by periodically stacking ultrathin GeTe and Sb₂Te₃ crystalline layers. The mechanism of the resistance change in iPCM devices is still highly debated. Recent experimental studies on SLs grown by molecular beam epitaxy or pulsed laser deposition indicate that the local structure does not correspond to any of the previously proposed structural models. Here, a new insight is given into the complex structure of prototypical GeTe/Sb₂Te₃ SLs deposited by magnetron sputtering, which is the used industrial technique for SL growth in iPCM devices. X‐ray diffraction analysis shows that the structural quality of the SL depends critically on its stoichiometry. Moreover, high‐angle annular dark‐field‐scanning transmission electron microscopy analysis of the local atomic order in a perfectly stoichiometric SL reveals the absence of GeTe layers, and that Ge atoms intermix with Sb atoms in, for instance, Ge₂Sb₂Te₅ blocks. This result shows that an alternative structural model is required to explain the origin of the electrical contrast and the nature of the resistive switching mechanism observed in iPCM devices.     

Bismuth doping effect on the phase-change characteristics of nitrogen-doped GeTe films

The microstructures and electrical properties of 8.4% nitrogen-doped GeTe and GeBi(6 at.%)Te films thermally annealed in N₂ atmosphere were investigated. With the addition of Bi to N-doped GeTe films, the initial crystallization temperature was reduced and crystallization speed slowed. The N-doped GeBiTe films showed a rapid increase in crystallite size compared to the N-doped GeTe films. The formation energy of the nucleus may be lower due to the Bi atoms and the growth speed may be slower.     

Investigation of phase transition behaviors of the nitrogen-doped Sb-rich Si-Sb-Te films for phase-change memory

The phase transformation properties of the nitrogen-doped Sb-rich Si-Sb-Te films were investigated in detail. It was found that the addition of N atoms into the Si-Sb-Te films increases the temperature for phase transition from the amorphous phase to a stable hexagonal structure and enhances the sheet resistance of the films following grain refinement. The surface topography of the crystalline films was improved by doping nitrogen atoms. The activation energy for crystallization of the films was increased from 1.84 to 2.89 eV with the increased nitrogen content from 0 to 21 at.%, which promises an improved thermal stability. A prolonged data lifetime up to 10 years at 149.4 °C was realized. From the device performance point of view, the N-doped Si-Sb-Te film with a moderate nitrogen content was preferable for the phase-change memory applications due to its advantage of higher reliability.     

Crystallization behavior and resistance change in eutectic Si15Te85 amorphous films

Crystallization process and the corresponding electrical resistance change were investigated in eutectic Si₁₅Te₈₅ amorphous thin films. The Si₁₅Te₈₅ amorphous film showed two-stage crystallization process upon heating. In the first stage, the Si₁₅Te₈₅ amorphous crystallized into Te crystals at 175 °C. In the second stage, the residual amorphous phase crystallized into Si₂Te₃ crystals at above 300 °C accompanying the resistance drop. Before the second crystallization, the electrical resistance once increased in the temperature range of about 250-295 °C. This phenomenon can be explained by considering the formation of amorphous phase with a high electrical resistivity.     

Analyzing residual stress in bilayer chalcogenide Ge2Se3/SnTe films

Chalcogenide thin film stacks of polycrystalline SnTe and amorphous Ge₂Se₃ are analyzed for residual stress using X-ray diffraction. The as-deposited film stacks are annealed at different temperatures and the thermal dependence of stress is investigated. Stress evaluations are performed by employing the sin²ψ technique using a 2D area detector system. As-deposited samples are found to be compressively stressed and exhibit increasingly tensile thermal stress with increasing annealing temperature. Onset of crystallization of the bottom Ge₂Se₃ layer is indicated by a sharp drop in the stress level in the 270 °C-360 °C temperature range, due to volume shrinkage associated with the crystallization. Diffraction patterns of samples annealed at different temperatures indicate compositional changes that are attributed to inter-diffusion of ions between the two layers. The XRD profiles of samples annealed at 360 °C and 450 °C indicate the formation of a Ge₂Se₃-SnTe solid solution. It is suggested that both, residual stress and temperature dependent compositional changes affect the measured d spacings.     

Effects of oxygen or nitrogen doping on the microstructures, bonding, electrical, thermal properties and phase change kinetics of GeSb9 films

This study investigated the phase change kinetics, thermal, structural, electrical properties, and chemical bonding state of 50-nm-thick oxygen or nitrogen doped GeSb₉ films prepared by sputtering. Up to 6.8 at.% O or 8.7 at.% N doping, as measured by secondary ion mass spectrometry, the crystallization temperatures, lattice constants, and the resistivity of both crystalline and amorphous films increased with oxygen or nitrogen concentration , while the grain sizes decreased. Nitrogen atoms bonded to Ge and formed GeN₄ nitrides, and both the GeO₂ and Sb₂O₃ co-existed when oxygen concentration reached 8.5 at.%. Crystallization could be triggered by a laser pulse of duration 12 ns for all films. Both GeSb₉-O and GeSb₉-N films had fast crystallization speed, good thermal stability, and improved resistivity and are therefore potential candidates as active layers for phase-change memory.     

Characterization of phase transition in silver photo-diffused Ge2Sb2Te5 thin films

Surface activity of thermally evaporated amorphous chalcogenide films of Ge₂Sb₂Te₅ has been investigated. Silver (Ag) is readily deposited on such films from appropriate aqueous ionic solution and Ag diffuses into the films upon irradiation with energetic photons. The composition of Ge₂Sb₂Te₅ thin films and the amount of Ag photo-diffused has been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. The composition of the films was found to be very close to the bulk used to deposit films and the amount of Ag photo-diffused was ∼ 0.38 at. %. X-ray diffraction and temperature dependent sheet resistance studies have been used for the structural analysis of the bulk alloy, as-deposited, Ag photo-diffused and annealed films at different temperatures. The films remain amorphous after Ag photo-diffusion into the amorphous Ge₂Sb₂Te₅ films. The reflectivity, reflectivity contrast and extinction coefficient of the crystalline and amorphous photo-diffused thin films are presented. The optical band gaps of the amorphous and crystalline photo-diffused (Ge₂Sb₂Te₅)_100−xAg_x=0.38 phase change thin films have also been calculated from absorption data using UV-VIS spectroscopy.     

Electrical characteristics and detailed interfacial structures of Ag/Ni metallization on polycrystalline thermoelectric SnSe

SnSe is a promising thermoelectric material with a high figure of merit in single crystal form, which has stimulated continuous research on polycrystalline SnSe. In this study, we investigated a metallization techniques for polycrystalline SnSe to achieve highly efficient and practical SnSe thermoelectric modules. The Ag/Ni metallization layers were formed on pristine polycrystalline SnSe using various deposition technique: sputter coating Ni, powder Ni and foil Ni by spark plasma sintering. Structural analysis demonstrated that the microstructure and contact resistance could be different according to the metallization process, despite using the same metals. The Ag/Ni metallization layer using foil Ni acted as an effective diffusion barrier and minimized electrical contact resistance(2.3 × 10~(-4) Ω cm~2). A power loss in the thermoelectric module of only 5% was demonstrated using finite element simulation.     

The effects of S-doping concentration on the photocatalytic performance of SnSe/S-GO nanocomposites

The effects of S-doped graphene oxide (S-GO) on the photocatalytic performance of SnSe nanostructures have been investigated. Different concentrations of S-doping as 2S-GO, 4S-GO, and 6S-GO (2, 4, and 6% in weight) have been synthesized. Characterization results indicated sulfur not only has successfully placed in the GO structure and a part of the GO sheet has been changed into reduced GO (rGO) by sulfur doping but also the surface morphology of the GO sheets has been changed from a smooth surface to fractured crack surfaces. The results showed that the increase of sulfur content caused the morphology of the SnSe nanostructures was changed from nanoparticles (NPs) into nanorods (NRs). The photocatalytic activity of the samples to degrade dyes under the visible-light irradiation conditions was carried out and it was observed an enhancement photocatalytic performance for the SnSe/2S-rGO nanocomposites in comparison to the other samples. More than 95% of dyes were degraded by the SnSe/2S-rGO nanocomposites for only 60 min. Brunauer–Emmett–Teller (BET) and electrical measurement results indicated the textural properties and conductivity of GO sheets were improved by sulfur doping. In addition, the photogenerated electron lifetime (τ_r) of the SnSe/rGO and SnSe/S-rGO nanocomposites has been measured by the Bode phase plot and it was observed a lifetime of τ_r = 71.1 and 31.7 μs for the SnSe/S-rGO and SnSe/rGO nanocomposites, respectively.     

Phase transformations in sputtered Ni-Mn-Ga magnetic shape memory alloy thin films

Ni-Mn-Ga magnetic shape memory alloy films have been prepared by the DC magnetron sputtering technique. As-deposited films show a quasi-amorphous structure that crystallizes at ~ 500 K. Crystallization study using Kissinger's analysis reveals a relatively low activation energy indicating partial crystallinity in the films. In situ X-ray diffraction studies show reversible martensite phase transformations, and phase segregation to non-transforming L1₂ precipitates at higher temperatures. It was observed that the phase segregation can be suppressed by low temperature heat treatment.