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.     

In situ TEM observations of irradiation-induced phase change in tungsten

Electron irradiation-induced phase change in tungsten was examined by in situ high resolution electron microscopy. In samples prepared by a chemical polishing technique using a NaOH aqueous solution, an α to β phase change was induced under 200 keV electron irradiation at 773 K. It is suggested that sodium atoms deposited on the tungsten sample from the solution, rather than oxygen atoms, may play an essential catalytic role in inducing the phase change.     

The impact of heater-recess and load matching in phase change memory mushroom cells

Two-dimensional finite element simulations with rotational symmetry are used to analyze the impact of the bottom electrode recess on the reset operation of phase change memory elements with mushroom cell geometry (Ge(2)Sb(2)Te(5) (GST) film over a patterned TiN pillar). Temperature dependent materials parameters are used for GST and TiN, and the latent heat of fusion in melting of GST is included to model melting. The results of this study indicate that a lower reset current and a more favorable thermal profile may be achieved by extending the active region of GST down into the pillar, due to the heat confinement. It is shown that the current through cells with an insufficient load condition for maximum power transfer can be maintained at a level lower than that which is sufficient for reset operation for extended periods of time due to the non-linear nature of temperature dependent electrical conductivity of GST. These results suggest that if the load condition is not matched, excessive voltage levels or pulse durations would be necessary to achieve successful reset operation across cell arrays.     

Investigation of inter-diffusion in bilayer GeTe/SnSe phase change memory films

A metal-chalcogenide layer, SnSe, is inserted between the memory layer GeTe and the top electrode to form a phase change memory cell. The GeTe layer exhibits ovonic threshold switching at a threshold field of ~ 110 V/μm. For subsequent implementation into applications and reliability, material inter-diffusion and sublimation are examined in bilayer phase change films of GeTe/SnSe. Transmission electron microscopy and parallel electron energy loss spectroscopy analyses reveal Sn migration to the GeTe layer, which is responsible for lowering the rhombohedral to cubic structural transformation temperature in GeTe. Incongruent sublimation of SnSe and GeTe is observed at temperatures higher than 500 °C. Severe volatilization of Se results in the separation of a metallic Sn phase. The use of Al₂O₃ as a capping layer has been found to mitigate these effects.     

Visualization of the process of metal heating and melting in the anode zone in an arc discharge with a tungsten electrode

Pictures of an electric arc burning in argon, obtained by means of a digital camera, within the different domains of wavelengths of the visible spectrum are presented. Maps of the thermal fields of the heating spot are plotted, and the plasma temperature in the anode arc zone is calculated. The application potential of the digital image technology in the visible wavelength domain for analysis of the processes directly in the anode arc zone and for estimation of the arc column parameters is shown.     

Metastable phase diagram on heating in quenched Ti-Nb high-temperature shape memory alloys

Journal of Materials Science - Metastable phase diagrams of β (BCC)-Ti high-temperature shape memory alloys (HTSMAs) have been investigated extensively, where however β→isothermal...     

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.     

Characterizations of tungsten carbide as a non-Pt counter electrode in dye-sensitized solar cells

Tungsten carbide particles as a counter electrode for dye-sensitized solar cells are prepared by heating layered tungsten oxides at 700, 800, and 900 °C. In particular, the sample heated at 800 °C reveals dominant WC structure and relatively high specific surface area. The improved photoelectrochemical properties of the sample prepared at 800 °C may be due to both catalytic activity of WC phase and high active sites for iodide reduction in comparison with other electrodes.     

Resistance switching characteristics of HfO2 film with electrode for resistance change random access memory

The electrode dependent resistance switching behaviors of amorphous HfO2 films grown by atomic layer deposition were systematically investigated. The low and high resistance states were successfully achieved for all the metal-insulator-metal resistor systems with Mo, Ru, and Pt symmetric electrodes. The characteristic reset and set voltages as well as the dynamic resistance ratio of the resistor device are strongly dependent on the electrode material with different work function. In addition, the different features for switching voltages with electrode are shown with annealing temperature.     

Changes in the physicomechanical state of an elastic layer with memory of the shape under bending and heating

On the basis of the model proposed earlier by the authors, we determine the phase and stress state of a layer characterized by the direct martensite transformation occurring under bending followed by the reverse transformation in the process of heating. It is shown that the nonuniform residual stress state is formed in the layer as a result of changes in its phase composition. These stresses gradually decrease in the course of heating, which is explained by the recovery of the original phase state of the material of the layer.Pidstryhach Institute for Applied Problems in Mechanics and Mathematics, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 3, pp. 90–95, May–June, 1995.     

基于Sn掺杂Ge2Sb2Te5的相变存储器器件单元存储性能

采用0.18μm标准工艺制备出基于Sn掺杂Ge2Sb2Te5相变材料的相变存储器器件单元,利用自行设计搭建的电学测试系统研究了其存储性能.结果表明:Sn的掺杂没有改变Ge2Sb2Te5的相变特性,其相变阚值电压和阈值电流分别为1.6V和25μA;实现了器件单元的非晶态(高阻)与晶态(低阻)之间的可逆相变过程;器件单元中相变材料结晶所需电流最低为1.78mA(电流宽度固定为100ns)、结晶时间大于80ns(电流高度固定为3mA);相变材料非晶化脉冲电流宽度为30ns时,所需电流大于3.3mA;与Ge2Sb2Te5相比,Sn的掺杂降低了SET操作的脉冲电流宽度,提高了结晶速度,有利于提高相变存储器的存储速度.     

Electrovortex motion of a melt in dc furnaces with a bottom electrode

The effect of the Lorenz force on the origination of the electrovortex motion of a metal melt in dc electric steel furnaces with a bottom electrode is evaluated. The motion of a metal melt is described by equations of magnetic hydrodynamics for isothermal liquid, which are solved by the finite element method. The numerical solution is compared with theoretical results and experimental data of other authors. From analysis of the results obtained, conclusions are drawn that the Lorenz force has a marked effect on the origination of the vortex motion of a melt.     

Mathematical simulation of conjugate heat exchange in heating furnaces with a moving bottom

A mathematical model of conjugate heat exchange in heating furnaces with a moving bottom (ring furnaces and walking-beam furnaces) has been developed. The model allows one to determine the heating of steel blanks in these furnaces with account for all mechanisms of heat transfer in the high-temperature working region of a furnace, in its lining, and in the steel blanks. On the basis of this model, a more economical regime of operation, as compared to the existing ones, has been calculated for a walking-beam furnace and comparative analysis of the operating characteristics of a ring furnace and a walking-beam furnace has been performed. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 3, pp. 3–11, May–June, 2006.     

Reduced distribution of threshold voltage shift in double layer NiSi2 nanocrystals for nano-floating gate memory applications

We report on the fabrication and capacitance-voltage characteristics of double layer nickel-silicide nanocrystals with Si3N4 interlayer tunnel barrier for nano-floating gate memory applications. Compared with devices using SiO2 interlayer, the use of Si3N4 interlayer separation reduced the average size (4 nm) and distribution (+/- 2.5 nm) of NiSi2 nanocrystal (NC) charge traps by more than 50% and giving a two fold increase in NC density to 2.3 x 10(12) cm(-2). The increased density and reduced NC size distribution resulted in a significantly decrease in the distribution of the device C-V characteristics. For each program voltage, the distribution of the shift in the threshold voltage was reduced by more than 50% on average to less than 0.7 V demonstrating possible multi-level-cell operation.     

Electronic and atomic structure of Ge2Sb2Te5 phase change memory material

Electronic structure calculations are presented for various model structures of the crystalline and amorphous phases of Ge₂Sb₂Te₅. The structures are all found to possess a band gap of order 0.5 eV, indicating closed shell behaviour. It is pointed out that structural vacancies in A7-like Ge₂Sb₂Te₅ are not electronically active. In addition, A7-like structures do not support valence alternation pair defects, which are one model of the conduction processes in the amorphous phase in non-volatile memories.     

基于太阳能供热的石墨烯/石蜡复合相变材料蓄放热特性实验研究

由于太阳能间歇性和不稳定性的缺点,给其在供热方面的应用带来了局限,采用相变储能法可改善太阳能供热应用的缺点。对此以石蜡为基液,向其中添加石墨烯通过"两步法"制备出石墨烯质量分数分别为0.1%、0.3%和0.5%的复合相变材料,搭建复合相变材料蓄放热实验台并记录数据。结果表明:在相同的实验条件下,石墨烯/石蜡复合相变材料随着石墨烯质量分数的增加,熔化速率也随之增加,在添加量为0.5%时增加了14.15%;凝固速率也随石墨烯质量分数的增加而增加,但效果不如熔化速率明显;复合相变材料在圆管外的蓄放热过程中,位于圆管底部的材料熔化与凝固效果均不理想。     

Kinetics of germanium tetrachloride reduction with hydrogen in the presence of pyrolytic tungsten

Pyrolytic tungsten coatings have been produced on the surface of ash microspheres under steady-state conditions using tungsten hexacarbonyl as a precursor. The nanostructured composites thus obtained were characterized by X-ray diffraction and scanning electron microscopy. We have studied the kinetics of the catalytic reduction of germanium tetrachloride with hydrogen in the temperature range 423–973 K in the presence of the composites as catalysts and determined the reaction order and activation energy for the catalytic reduction of germanium tetrachloride with hydrogen.     

碳纳米管/石蜡相变复合材料研究进展

石蜡系相变材料具有较高的潜热值和单位质量储能密度,近年来引起了国内外学者的广泛关注与研究.作为中低温相变材料中重要的一类,石蜡系是制备室温及低温环境下相变复合材料的首选,但是导热率较低等缺陷阻碍了其进一步的工业化进程.碳纳米管具有独特的微观结构和优良的导热性能,故被认为是有望显著改善石蜡系相变材料热性能不足的重要候选材料之一,因此碳纳米管/石蜡相变复合材料的制备及性能研究成为热点问题.本文综述了近年来石蜡系相变材料与碳纳米管复合材料的研究现状,针对其制备设计、微胶囊化及实际应用等方面的进展进行系统归纳和评论,并对碳纳米管/石蜡相变复合材料所面临的挑战(制备工艺复杂、稳定性差、实际评估少等)及未来可能的研究重点(掺杂比、浸润性、经济性等)进行展望.