A transmission electron microscopy study on the atomic arrangement and grain growth of hexagonal structured Ge2Sb2Te5

The atomic arrangement and grain growth of the hexagonal structured Ge₂Sb₂Te₅ were investigated by a transmission electron microscopy study. Unlike the isotropic crystallization of face-centered-cubic (fcc) structured Ge₂Sb₂Te₅, the hexagonal structured Ge₂Sb₂Te₅ grain was preferably grown to a large degree with a specific direction. As a result, we have revealed that the grain growth occurred parallel to the (0 0 0 1) plane, and identified the atomic arrangement of the hexagonal structured Ge₂Sb₂Te₅ having nine cyclic layers by analyzing the high-resolution transmission electron microscopy images and simulated images obtained in the direction of zone axis.     

Crystallization of Ge2Sb2Te5 phase—change optical disk media

In this paper, the crystallization behaviour of amorphous Ge₂Sb₂Te₅ thin films is investigated using differential scanning calorimetry), x-ray diffraction and optical transmissivity measurements. It is indicated that only the amorphous phase to face-centred-cubic phase transformation occurs during laser annealing of the normal phase-change structure, which is a benefit for raising the phase-change optical disk's carrier-to-noise ratio (CNR). For amorphous Ge₂Sb₂Te₅ thin films, the crystallization temperature is about 200℃ and the melting temperature is 546.87℃. The activation energy for the crystallization, E_a, is 2.25eV. The crystallization dynamics for Ge₂Sb₂Te₅ thin films obeys the law of nucleation and growth reaction. The sputtered Ge₂Sb₂Te₅ films were initialized by an initializer unit. The initialization conditions have a great effect on the reflectivity contrast of the Ge₂Sb₂Te₅ phase-change optical disk.     

Effective method for preparation of oxide-free Ge2Sb2Te5 surface: An X-ray photoelectron spectroscopy study

Cleaning the surfaces of the as-deposited Ge₂Sb₂Te₅ was studied by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray diffraction (XRD). The mixed native oxides on the as-deposited Ge₂Sb₂Te₅ surface can be easily removed by dipping Ge₂Sb₂Te₅ in de-ionized water for 1 min, while the surface morphology remains unchanged after cleaning. Native oxides only re-grow after exposure to air for more than 4 min. Although dipping in water leads to a surface layer deficient in Ge and Sb, the surface composition of Ge₂Sb₂Te₅ can recover to its stoichiometric value after annealing at 200 °C in vacuum. The phase remains amorphous at room temperature after dipping in water, and changes to fcc and hcp after annealing at 100 and 220 °C, respectively.     

Characteristics of Si-doped Sb2Te3 thin films for phase-change random access memory

The characteristics of Si-doped Sb₂Te₃ thin films were investigated using differential scanning calorimetry (DSC), four-point probe technique, X-ray diffraction (XRD) analysis and high resolution transmission electron microscopy (HRTEM). It is found that the as-deposited Sb₂Te₃ film in our study is partly crystallized. Silicon doping increases the crystallization temperature and resistivity of Sb₂Te₃ film significantly. XRD and HRTEM analyses indicated that some of the doped Si atoms substitute for Sb or Te in the lattice, while others exists at the grain boundaries in the form of amorphous phase, which may be responsible for grain size reduction and high crystalline resistivity of Si-doped specimens. Compared with the conventional Ge₂Sb₂Te₅ film, Si-doped Sb₂Te₃ films exhibit lower melting temperature and higher crystalline resistivity, which is beneficial to RESET current reduction of phase-change random access memory (PRAM). These results show the feasibility of Si-doped Sb₂Te₃ films in PRAM application.     

Effects of Si doping on the structural and electrical properties of Ge2Sb2Te5 films for phase change random access memory

The effects of Si doping on the structural and electrical properties of Ge₂Sb₂Te₅ film are studied in detail. Electrical properties and thermal stability can be improved by doping small amount of Si in the Ge₂Sb₂Te₅ film. The addition of Si in the Ge₂Sb₂Te₅ film results in the increase of both crystallization temperature and phase-transition temperature from face-centered cubic (fcc) phase to hexagonal (hex) phase, however, decreases the melting point slightly. The crystallization activation energy reaches a maximum at 4.1 at.% and then decreases with increasing dopant concentration. The electrical conduction activation energy increases with the dopant concentration, which may be attributed to the increase of strong covalent bonds in the film. The resistivity of Ge₂Sb₂Te₅ film shows a significant increase with Si doping. When doping 11.8 at.% of Si in the film, the resistivity after 460 °C annealing increases from 1 to 11 mΩ cm compared to the undoped Ge₂Sb₂Te₅ film. Current-voltage (I-V) characteristics show Si doping may increase the dynamic resistance, which is helpful to writing current reduction of phase-change random access memory.     

Ge/Sb2Te3 nanocomposite multilayer films for high data retention phase-change random access memory application

The amorphous-to-crystalline transition of Ge/Sb₂Te₃ nanocomposite multilayer films with various thickness ratios of Ge to Sb₂Te₃ were investigated by utilizing in situ temperature-dependent film resistance measurements. The crystallization temperature and activation energy for the crystallization of the multilayer films increased with the increase in thickness ratio of Ge to Sb₂Te₃. The difference in sheet resistance between amorphous and crystalline states could reach as high as 10⁴ Ω/□. The crystallization temperature and activation energy for the crystallization of Ge/Sb₂Te₃ nanocomposite multilayer films was proved to be larger than that of conventional Ge₂Sb₂Te₅ film, which ensures a better data retention for phase-change random access memory (PCRAM) use. A data retention temperature for 10 years of the amorphous state [Ge (2 nm)/Sb₂Te₃ (3 nm)]₄₀ film was estimated to be 165 °C. Transmission electron microscopy (TEM) images revealed that Ge/Sb₂Te₃ nanocomposite multilayer films had layered structures with clear interfaces.     

In situ transmission electron microscopy study of the nucleation and grain growth of Ge2Sb2Te5 thin films

The nucleation and grain growth of the Ge₂Sb₂Te₅ (GST) thin films were studied using high voltage electron microscope operated at 1250 kV. As a result, we have found that 2 nm-sized nucleus forms as a cluster which atoms are arranged regularly at the stage of nucleation prior to the formation of grains having crystal structure. The high-resolution transmission electron microscopy study and fast-Fourier transformations revealed that coexistence of face-centered-cubic (FCC) and hexagonal structure occurs, and formation of twin defect is found in the hexagonal structure during the grain growth as the annealing temperature is increased. GST grain having the hexagonal structure grow from the surface, and the growth proceeded perpendicular to the [0 0 0 1], namely the path parallel to the (0 0 0 1) plane. Consequently, grain growth to a large-scale result in a lengthened shape.     

相变存储材料Ge1Sb2Te4和Ge2Sb2Te5薄膜的结构和电学特性研究

采用射频磁控溅射方法制备了两种用于相变存储器的Ge₁Sb₂Te₄和Ge₂Sb₂Te₅相变薄膜材料,对其结构、电学输运性质和恒温下电阻随时间的变化关系进行了比较和分析.X射线衍射(XRD)和原子力显微镜(AFM)的结果表明:随着退火温度的升高,Ge₁Sb₂Te₄薄膜逐步晶化,由非晶态转变为多晶态,表面出现均匀的、 关键词： 硫系相变材料 1Sb₂Te₄')" href="#">Ge₁Sb₂Te₄ 2Sb₂Te₅')" href="#">Ge₂Sb₂Te₅     

High thermal stability and low thermal conductivity in Ga30Sb70/Sb80Te20 nanocomposite multilayer films

The reliability characteristics and thermal conductivity of Ga₃₀Sb₇₀/Sb₈₀Te₂₀ nanocomposite multilayer films were investigated by isothermal resistance and transient thermoreflectance (TTR) measurements, respectively. The crystallization temperature and activation energy for the crystallization can be modulated by varying the layer thickness of Ga₃₀Sb₇₀. A data retention time of ten years of the amorphous state [Ga₃₀Sb₇₀ (3 nm)/Sb₈₀Te₂₀ (5 nm)]₁₃, [Ga₃₀Sb₇₀ (5 nm)/Sb₈₀Te₂₀ (5 nm)]₁₀, and [Ga₃₀Sb₇₀ (10 nm)/Sb₈₀Te₂₀ (5 nm)]₇ was estimated when ambient temperature is 137, 163, and 178 °C, respectively. Ga₃₀Sb₇₀/Sb₈₀Te₂₀ nanocomposite multilayer films were found to have lower thermal conductivity in both the amorphous and crystalline state compared to Ge₂Sb₂Te₅ film, which will promise lower programming power in the phase-change random access memory.     

In situ AFM and Raman spectroscopy study of the crystallization behavior of Ge2Sb2Te5 films at different temperature

Using in situ atomic force microscope (AFM) and Raman spectroscopy, the real-time crystallization properties of Ge₂Sb₂Te₅ films at different temperature were characterized. The given AFM topograph and phase images revealed that the structure of amorphous Ge₂Sb₂Te₅ films began to change at a temperature of as low as 100 °C. When the temperature reached 130 °C, some crystal fragments had formed at the film surface. Heating up to 160 °C, the size of the visible crystal fragments increased, but decreased at a higher temperature of 200 °C. When the Ge₂Sb₂Te₅ film was cooled down to room temperature (RT) from 200 °C, the crystal fragments divided into crystal grains due to the absence of heating energy. The Raman spectra at different temperature further verified the structure evolution of the Ge₂Sb₂Te₅ film with temperature. This work is of significance for the preparation of Ge₂Sb₂Te₅ films and the erasing of data.     

基于Si掺杂Sb2Te3薄膜的相变存储器研究

采用磁控三靶(Si，Sb及Te)共溅射法制备了Si掺杂Sb₂Te₃薄膜，作为对比，制备了Ge₂Sb₂Te₅和Sb₂Te₃薄膜，并且采用微加工工艺制备了单元尺寸为10μm×10μm的存储器件原型来研究器件性能.研究表明，Si掺杂提高了Sb₂Te₃薄膜的晶化温度以及薄膜的晶态和非晶态电阻率，使得其非晶态与晶态电阻率之比达到10⁶，提高了器件的电阻开/关比；同Ge₂Sb₂Te₅薄膜相比，16at% Si掺杂Sb₂Te₃薄膜具有较低的熔点和更高的晶态电阻率，这有利于降低器件的RESET电流.研究还表明，采用16at% Si掺杂Sb₂Te₃薄膜作为存储介质的存储器器件原型具有记忆开关特性，可以在脉高3V、脉宽500ns的电脉冲下实现SET操作，在脉高4V、脉宽20ns的电脉冲下实现RESET操作，并能实现反复写/擦，而采用Ge₂Sb₂Te₅薄膜的相同结构的器件不能实现RESET操作. 关键词： 相变存储器 硫系化合物 2Te₃薄膜')" href="#">Si掺杂Sb₂Te₃薄膜 SET/RESET转变     

Morphological characteristics of amorphous Ge2Sb2Te5 films after a single femtosecond laser pulse irradiation

The morphology of materials resulting from laser irradiation of the single-layer and the multilayer amorphous Ge₂Sb₂Te₅ films using 120 fs pulses at 800 nm was observed using scanning electron microscopy and atomic force microscopy. For the single-layer film, the center of the irradiated spot is depression and the border is protrusion, however, for the multilayer film, the center morphology changes from a depression to a protrusion as the increase of the energy. The crystallization threshold fluence of the single-layer and the multilayer film is 22 and 23 mJ/cm², respectively.     

Phase stability and electronic structure of Si2Sb2Te5 phase-change material

On the basis of an ab initio computational study, the present work provide a full understanding on the atomic arrangements, phase stability as well as electronic structure of Si₂Sb₂Te₅, a newly synthesized phase-change material. The results show that Si₂Sb₂Te₅ tends to decompose into Si₁Sb₂Te₄ or Si₁Sb₄Te₇ or Sb₂Te₃, therefore, a nano-composite containing Si₁Sb₂Te₄, Si₁Sb₄Te₇ and Sb₂Te₃ may be self-generated from Si₂Sb₂Te₅. Hence Si₂Sb₂Te₅ based nano-composite is the real structure when Si₂Sb₂Te₅ is used in electronic memory applications. The present results agree well with the recent experimental work.     

Hall mobility of amorphous Ge2Sb2Te5

The electrical conductivity, Seebeck coefficient, and Hall coefficient of three-micron-thick films of amorphous Ge₂Sb₂Te₅ have been measured as functions of temperature from room temperature down to as low as 200 K. The electrical conductivity manifests an Arrhenius behavior. The Seebeck coefficient is p-type with behavior indicative of multi-band transport. The Hall mobility is n-type and low (near 0.07 cm²/V s at room temperature).     

Performance improvement of Sb2Te3 phase change material by Al doping

Al doped Sb₂Te₃ material was proposed to improve the performance of phase-change memory. Crystallization temperature, activation energy, and electrical resistance of the Al doped Sb₂Te₃ films increase markedly with the increasing of Al concentration. The additional Al-Sb and Al-Te bonds enhance the amorphous thermal stability of the material. Al_0.69Sb₂Te₃ material has a better data retention (10 years at 110 °C) than that of Ge₂Sb₂Te₅ material (10 years at 87 °C). With a 100 ns width voltage pulse, SET and RESET voltages of 1.3 and 3.3 V are achieved for the Al_0.69Sb₂Te₃ based device.     

Enhanced growth of low-resistivity cobalt silicide by using a Co/Au/Co trilayer film on Si0.8Ge0.2 virtual substrate

Formation of Co germanosilicides on Si_0.8Ge_0.2 virtual substrates with a Co/Au/Co sandwich thin film after different heat treatments has been investigated. The sequence of phase formation is the same as the reaction of blanket Co thin film with (001)Si. The presence of thin interposing Au layers was found to significantly enhance the formation of low-resistivity CoSi₂ on (001)Si_0.8Ge_0.2 substrates. The formation temperature of CoSi₂ phase in the Co/Au/Co/(001)Si_0.8Ge_0.2 samples was lowered by about 200 °C compared to that of Co/(001)Si_0.8Ge_0.2 samples. From TEM and EDS analysis, some of Au atoms were found to diffuse from the original interface position to disperse within the CoSi₂ layers during silicidation reactions. The mechanisms for the enhanced formation of CoSi₂ in the Co/Au/Co/Si_0.8Ge_0.2 system were explained in the context of classical nucleation theory.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Effects of Ge doping on the properties of Sb2Te3 phase-change thin films

Ge-doped Sb₂Te₃ films were prepared by magnetron sputtering of Ge and Sb₂Te₃ targets on SiO₂/Si (1 0 0) substrates. The effect of Ge doping on the structure was studied in details by X-ray diffraction, differential scanning calorimetry, and X-ray photoelectron spectroscopy measurements. It is indicated that Ge atoms substitute for Sb/Te in lattice sites and form Ge-Te bonds, moreover, a metastable phase was observed in Ge-doped specimens. Both crystallization temperature and resistivity of amorphous Sb₂Te₃ increase after Ge doping, which are beneficial for improving room temperature stability of the amorphous state and reducing the SET current of chalcogenide random access memory.     

Effects of carbon doping on chemical states of amorphous Ge2Sb2Te5, measured with synchrotron radiation

We fabricated and analyzed the chemical states of carbon-doped (5.2–13.2 at.%) Ge₂Sb₂Te₅ thin films on Si substrates using high-resolution, X-ray photoelectron spectroscopy with synchrotron radiation. Thin films were completely amorphous and their phase-change temperature was 150 °C higher than for un-doped GST. As the carbon doping concentration increased, new chemical states of Ge 3d with 29.9 eV and C 1s with 283.7 eV core-levels were observed. The doped carbon was bonded only with Ge in GST and doping was saturated at 8.7 at.%.     

氮掺杂Ge2Sb2Te5相变存储器的多态存储功能

通过反应溅射的方法，制备了N掺杂的Ge₂Sb₂Te₅(N-GST)薄膜，用作相变存储器的存储介质.研究表明，掺杂的N以GeN的形式存在，不仅束缚了Ge₂Sb₂Te₅ (GST)晶粒的长大也提高了GST的晶化温度和相变温度.利用N-GST薄膜的非晶态、晶态面心立方相和晶态六方相的电阻率差异，能够在同一存储单元中存储三个状态，实现相变存储器的多态存储功能. 关键词： 相变存储器 多态存储 N掺杂 2Sb₂Te₅')" href="#">Ge₂Sb₂Te₅