Investigation of electrical characteristics of the In3Sb1Te2 ternary alloy for application in phase‐change memory

A simple layered phase‐change random access memory (PRAM) cell was fabricated using the In₃Sb₁Te₂ alloy. The overall resistance value of the reset state was about 70 times larger than that of the set state. The resistance difference between the amorphous and crystalline state was fairly well maintained after 10² cycles. Interestingly, the measured current–voltage (I –V) curve showed three obvious steps in the crystalline state. By means of high temperature X‐ray diffractometry (HTXRD) and differential scanning calorimetry (DSC) experiments, we confirmed that the current steps originate from successive structural transformations of the In₃Sb₁Te₂ ternary alloy. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and magnetic properties of the phase transition in absorbed and non-absorbed copper(II) trans-1,4-cyclohexanedicarboxylate

A large amount of toluene can be absorbed into the 1-dimensional tunnels in copper(II) trans-1,4-cyclohexane dicarboxylate (Cu(trans-1,4-OOCC₆H₁₀C)) at room temperature, and it can be desorbed by evacuation at elevated temperatures. The reversible absorption/desorption behavior of toluene was studied by magnetic susceptibility measurements using SQUID magnetometer and by powder X-ray diffractometry with high-energy synchrotron radiation at SPring-8, Japan. The first order phase transition was observed at 160 K in the magnetic susceptibility curve for the empty (non-absorbed or desorbed) sample, and the structural difference between the low and high temperature phases was detected in the powder X-ray diffraction patterns. On the other hand, no phase transition was observed in fully toluene-absorbed sample, and the structure was similar to that of the low temperature phase of the empty sample. The absorption/desorption behavior observed in the present study coincides with the results of heat capacity measurements by adiabatic calorimetry reported previously.     

Formation of relaxed SiGe on the buffer consists of modified SiGe stacked layers by Si pre-intermixing

High-quality relaxed SiGe films on Si (0 0 1) have been demonstrated with a buffer layer containing modified SiGe (m-SiGe) islands in ultra-high vacuum chemical vapor deposition (UHV/CVD) system. The m-SiGe islands are smoothened by capping an appropriate amount of Si and the subsequent annealing for 10 min. This process leads to the formation of a smooth buffer layer with non-uniform Ge content. With the m-SiGe-dot multilayer as a buffer layer, the 500-nm-thick uniform Si_0.8Ge_0.2 layers were then grown. These m-SiGe islands can serve as effective nucleation centers for misfit dislocations to relax the SiGe overlayer. Surface roughness, strain relaxation, and crystalline quality of the relaxed SiGe overlayer were found to be a function of period's number of the m-SiGe-dot multilayer. By optimizing period number in the buffer, the relaxed Si_0.8Ge_0.2 film on the 10-period m-SiGe-dot multilayer was demonstrated to have a threading dislocation density of 2.0 × 10⁵ cm⁻² and a strain relaxation of 89%.     

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×1Oμm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500 ns的电脉冲下实现SET操作,在脉高4 V、脉宽20 ns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.     

Effect of alkali leaching on the surface structure of Ni3Al catalyst

The surface structure of the alkali-leached single-phase Ni₃Al powder was investigated by X-ray diffraction, BET (Brunauer-Emmett-Teller) surface area analysis, electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction. It was found that fine Ni particles of several nm in diameter were formed on the outer surface layer of the Ni₃Al powder after the alkali leaching process. The surface of the Ni particles was covered with a thin layer of Ni oxides and hydroxide, Ni₂O₃, NiO and Ni(OH)₂, and these Ni oxides and hydroxide can be easily reduced by hydrogen to the metallic nickel that is catalytically active. The inside of the Ni₃Al powder remained as the original Ni₃Al ordered structure after alkali leaching. Having heat resistant properties, the Ni₃Al phase can serve as a support of the fine Ni particles and provide the structural and thermal stabilities to the fine Ni particles.     

Structural phase transformation of GaN under high-pressure: an exact exchange study

The phase transformation of GaN under high pressure is theoretically studied in the KS-DFT framework using the EXX method. Both KLI and LDA pseudopotentials were tried. The LDA-PP result for the transition pressure is 41.7 GPa while that of the KLI-PP is 41.5 GPa. Both results are in a very good agreement with the latest experimental value of 42 GPa. The effect of the nonlinear core correction is found to be small but not negligible.     

Ab initio study of phase transition and thermodynamic properties of PtN

The transition phase of PtN from zincblende (ZB) structure to rocksalt (RS) structure is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The transition phase from the ZB structure to the RS structure occurs at the pressure of 18.2 GPa, which agrees well with other calculated values. Moreover, the dependences of the relative volume V/V₀ on the pressure P, the Debye temperature Θ and heat capacity C_V on the pressure P, together with the heat capacity C_V on the temperature T are also successfully obtained.     

Atomistic simulation of the point defects in B2-type MoTa alloy

The formation and migration mechanisms of three different point defects (mono-vacancy, anti-site defect and interstitial atom) in B₂-type MoTa alloy have been investigated by combining molecular dynamics (MD) simulation with modified analytic embedded-atom method (MAEAM). From minimization of the formation energy, we find that the anti-site defects Mo_Ta and Ta_Mo are easier to form than Mo and Ta mono-vacancies, while Mo and Ta interstitial atoms are difficult to form in the alloy. In six migration mechanisms of Mo and Ta mono-vacancies, one nearest-neighbor jump (1NNJ) is the most favorable due to its lowest activation and migration energies, but it will cause a disorder in the alloy. One next-nearest-neighbor jump (1NNNJ) and one third-nearest-neighbor jump (1TNNJ) can maintain the ordered property of the alloy but require higher activation and migration energies, so the 1NNNJ and 1TNNJ should be replaced by straight [1 0 0] six nearest-neighbor cyclic jumps (S[1 0 0]6NNCJ) or bent [1 0 0] six nearest-neighbor cyclic jumps (B[1 0 0]6NNCJ) and [1 1 0] six nearest-neighbor cyclic jumps ([1 1 0]6NNCJ), respectively. Although the migrations of Mo and Ta interstitial atoms need much lower energy than Mo and Ta mono-vacancies, they are not main migration mechanisms due to difficult to form in the alloy.     

应用于相变存储器的Cu-Ge3Sb2Te5薄膜的结构及相变特性研究

采用磁控溅射法制备了不同Cu含量的Cu-Ge₃Sb₂Te₅薄膜, 原位测试了薄膜电阻与温度的关系, 并利用X射线衍射仪、透射电镜、透过和拉曼光谱仪分别研究了 Cu-Ge₃Sb₂Te₅薄膜的晶体结构、微结构、禁带宽度及成键情况. 结果表明, Cu-Ge₃Sb₂Te₅薄膜的结晶温度和结晶活化能随着Cu含量的增加而增大, Cu的加入有效改善Ge₃Sb₂Te₅薄膜的热稳定性和10年数据保持力. 随着Cu含量的增加, 非晶态Cu-Ge₃Sb₂Te₅薄膜的禁带宽度逐渐减小. 同时, 拉曼峰从129 cm^-1向127 cm^-1处移动, 这是由于Cu–Te极性键振动增强的缘故. Cu-Ge₃Sb₂Te₅结晶为均匀、相互嵌套的六方Cu₂Te和Ge₂Sb₂Te₅相.     

Disorder-order phase transition induced by size effect in bulk La2/3Sr1/3MnO3

Bulk La_2/3Sr_1/3MnO₃ ceramic samples prepared by thermal decomposition are investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). An abnormal phenomenon, where three kinds of La_2/3Sr_1/3MnO₃ phases with different structures and the same composition coexist in the same grain, has been observed. Besides the stable rhombohedral majority phase, the two other phases are a simple cubic structure with a=0.389 nm and a new hexagonal structure with a=0.544 nm, c=0.668 nm. The simple cubic phase is a residual phase of high-temperature due to the size effect and bondage of twin boundary. Image simulations have suggested that the new hexagonal phase is the La-Sr ordered structure with space group , which is converted from the disordered simple cubic phase. The formation mechanism of the ordered phase is explained from volume energy and interface energy considerations.     

Structure and topography modifications of austenitic steel surfaces after friction in sliding contact

Friction experiments between two austenitic stainless steel (AISI 304L) surfaces in sliding contact were carried out under very low loads in two liquid environments, namely demineralized water and methanol, in order to study the correlation between surface damage (wear and surface topography) and structural modifications (phase formation and microstructure). The particularity of our approach was to perform the tests under Hertzian pressures, which were several orders of magnitude lower than the elastic limit of stainless steel. The structural modifications produced during friction were analysed by X-ray diffraction and transmission electron microscopy and the surface topography was studied by scanning electron microscopy and three-dimensional (3D) profiling. Whatever the experimental conditions investigated, the morphology of the damage observed on both surfaces consisted of very fine, smooth and parallel grooves typical of an abrasive wear process of a ductile material caused by the ploughing action of hard particles. From the beginning of the tests, the transformation of austenite into martensite was observed in the superficial layers and the dominant presence of martensite was identified in the wear debris. These results suggest that, under our experimental conditions, abrasion is the dominant mechanism of material removal. Received: 12 March 2002 / Accepted: 3 May 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/4949-6692, E-mail: jean.paul.riviere@univ-poitiers.fr     

Nontraditional temperature shift of the fundamental absorption edge for layered substances and the ferroelastic phase transition in Cs3Bi2I9

We have carried out investigations of the exciton reflection spectra of Cs₃Bi₂I₉ layered crystals as a function of temperature. For the first time for the layered substances we have found nontraditional temperature shift of the energy gap E_g(T) described by the Varshni formula. We have registered a transition region in the temperature broadening of the half-width, H(T), of the exciton band with increasing of temperature in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. We have also found a surge in H(T) at the ferroelastic phase transition point      

Structural Transition of Gd2O3：Eu Induced by High Pressure

The structural transition of bulk and uano-size Gd2O3：Eu are studied by high pressure energy disperse x-ray diffraction （XRD） and high pressure photoluminescence. Our results show that in spite of different size of Gd2O3 particles, the cubic structure turns into a possible hexagonal one above 13.4 GPa. When the pressure is released, the sample reverses to the monoclinic structure. No cubic structure presents in the released samples. That is to say, the compression and relaxation of the sample leads to the cubic Gd2O3：Eu then turns into the monoclinic one.     

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.     

Effect of Mo doping on phase change performance of Sb2Te3

Mo, as a dopant, is doped into SbTe to improve its thermal stability. It is shown in this paper that the Mo-doped Sb₂Te₃ (Mo_0.26Sb₂Te₃, MST) material possesses phase change memory (PCM) applications. MST has better thermal stability than Sb₂Te₃(ST) and will crystallize only when the annealing temperature is higher than 250 ℃. With the good thermal stability, MST-based PCM cells have a fast crystallization time of 6 ns. Furthermore, endurance up to 4×10⁵ cycles with a resistance ratio of more than one order of magnitude makes MST a promising candidate for PCM applications.     

Combinatorial study of phase transformation characteristics of a Ti-Ni-Pd shape memory thin film composition spread in view of microactuator applications

Phase transformation characteristics of a Ti-Ni-Pd shape memory thin film composition spread have been investigated. The thin film composition spread was fabricated from elemental targets using an ultra-high vacuum combinatorial magnetron sputter-deposition system and subsequent annealing at 500 °C for 1 h in situ. Automated temperature-dependent resistance measurements (R(T)), energy dispersive X-ray analysis (EDX) and X-ray diffraction measurements (XRD) have been applied for the high-throughput characterization of the composition spread. Reversible phase transformations within the measurement range of −40 to 250 °C within the Ti-Ni-Pd system were observed for compositions with Ti content between 50 and ∼59 at.%. For Ti-richer films, Ti₂Ni and Ti₂Pd precipitates are inhibiting reversible phase transformations. The transformation temperatures and the thermal hysteresis were determined from R(T) measurements. Rising transformation temperatures with increasing Pd content and significantly lower thermal hysteresis for the B2-B19, compared to the B2-R-B19′ transformations were found in good agreement with published data. For low Pd contents (<7-12 at.%, depending on the Ti content) two-stage B2-R-B19′ transformations were observed. Compositions with higher Pd contents showed a single-stage B2-B19 transformation. Increasing Ti content within the B2-B19 transformation region results in a linear increase of the thermal hysteresis and decreasing transformation temperatures.     

Systematic theoretical investigations for contribution of lattice constraint to novel atomic arrangements in alloy semiconductor thin films

The atomic arrangements in zinc blende structured GaN_xAs_1−x thin films coherently grown on V-grooved substrates are theoretically investigated using empirical interatomic potentials and Monte Carlo simulation. The resultant atomic arrangements in GaN_xAs_1−x strongly depend on concentration x and substrate lattice parameter a_sub. Surface segregation of As or N is mainly found in GaN_xAs_1−x with large lattice mismatch to the substrate. On the other hand, the novel atomic arrangements such as layered segregation or ordered structure are found in GaN_xAs_1−x at the specific region such as (x, a_sub) = (0.5, 5.3), (0.3, 5.3), and (0.3, 5.1). This specific region corresponds to that with negative excess energy and with sufficient N and As atoms remaining in thin film layers even after their surface segregation. The formation of the novel atomic arrangements is discussed in terms of bond lengths in the surface layers. These results suggest that various novel atomic arrangements in alloy semiconductor thin films appear depending on x and a_sub which control degree of lattice constraint.     

Bundles and foils of WS2 nanotubes

In this paper, the synthesis of multigram portions of WS₂ nanotubes and large foils consisting of aligned nanotubes is described. For this purpose, a fluidized bed reactor, in which WO₃ powder is fed from the top and the reaction gases are blown from below maintaining the powder aloft, is described. The reaction mechanism is briefly discussed. Received: 20 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Investigation of structural phase transition in polycrystalline SrTiO3 thin films by Raman spectroscopy

Polycrystalline SrTiO₃ thin films were prepared by pulsed laser deposition technique. The phonon properties and structural phase transition were studied by Raman spectroscopy. The first-order Raman scattering, which is forbidden in SrTiO₃ single crystal, has been observed in the films, due to the structural distortion caused by strain effect and oxygen vacancies. The Fano-type line shape of TO₂ phonon reveals the existence of polar microregions in the STO thin films. The evolution of TO₂ and TO₃ phonons with temperature shows the occurrence of a structural phase transition at 120 K related to the formation of polar macroregions in the films.