Search

A model, used for the calculation of glass transition temperature as a function of material composition, is devised to study the effects of Al and Cu doping on the crystallization properties of several phase change materials, like SbTe and GeSb. The results of the analysis show that Al doping leads to a tremendous increase in the glass transition temperature of the substance, though it decreases with the doping of Cu metal.

GeTe is a well-know prototypical phase change material, while pure Sb is described as a material with strong resonant bonding properties which are indicative of a fast switching phase change material. We have studied the crystallization times and crystallization temperatures of phasechange materials along the GeTe-Sb pseudobinary line with compositions Ge1SbxTe1. It was observed that alloys with x ≈1-2 show the highest crystallization temperature and resistance in the crystalline phase (indicative of low reset current) but also the slowest crystallization speed. As many phase change materials, Ge1SbxTe1 materials also suffer from conflicting requirements between speed, thermal stability of the amorphous phase, and low reset current. The Ge-Sb-Te phase-change material compositions must be carefully designed to optimize device performance for a specific application. Ge1Sb6Te1 material is a promising candidate for storage-class memory with fast speed and a crystallization temperature that is substantially higher than Ge2Sb2Te5 thus promises good data retention. [ABSTRACT FROM AUTHOR]

A LaCoO3/SrTiO3 heterostructure grown on Si (001) is shown to provide electrically switchable ferromagnetism, a large, electrically tunable magnetoresistance, and a vehicle for achieving and probing electrical control over ferromagnetic behavior at submicron dimensions. Fabrication of devices in a field-effect transistor geometry enables application of a gate bias voltage that modulates strain in the heterostructure via the converse piezoelectric effect in SrTiO3, leading to an artificial inverse magnetoelectric effect arising from the dependence of ferromagnetism in the LaCoO3 layer on strain. Below the Curie temperature of the LaCoO3 layer, this effect leads to modulation of resistance in LaCoO3 as large as 100%, and magnetoresistance as high as 80%, both of which arise from carrier scattering at ferromagnetic-nonmagnetic interfaces in LaCoO3. Finite-element numerical modeling of electric field distributions is used to explain the dependence of carrier transport behavior on gate contact geometry, and a Valet-Fert transport model enables determination of spin polarization in the LaCoO3 layer. Piezoresponse force microscopy is used to confirm the existence of piezoelectric response in SrTiO3 grown on Si (001). It is also shown that this structure offers the possibility of achieving exclusive-NOR logic functionality within a single device. [ABSTRACT FROM AUTHOR]

Thin films of the phase change material Ge–Sb with Ge concentrations between 7.3 and 81.1 at. % were deposited by cosputtering from elemental targets. Their crystallization behavior was studied using time-resolved x-ray diffraction, Auger electron spectroscopy, differential scanning calorimetry, x-ray reflectivity, profilometry, optical reflectivity, and resistivity versus temperature measurements. It was found that the crystallization temperature increases with Ge content. Calculations of the glass transition temperature (which is a lower limit for the crystallization temperature Tx) also show an increase with Ge concentration closely tracking the measured values of Tx. For low Ge content samples, Sb x-ray diffraction peaks occurred during a heating ramp at lower temperature than Ge diffraction peaks. The appearance of Ge peaks is related to Ge precipitation and agglomeration. For Ge concentrations of 59.3 at. % and higher, Sb and Ge peaks occurred at the same temperature. Upon crystallization, film mass density and optical reflectivity increase as well as electrical contrast (ratio of resistivity in amorphous phase to crystalline phase) all showed a maximum for the eutectic alloy (14.5 at. % Ge). For the alloy with 59.3 at. % Ge there was very little change in any of these parameters, while the alloy with 81.1 at. % Ge behaved opposite to a typical phase change alloy and showed reduced mass density and reflectivity and increased resistivity. [ABSTRACT FROM AUTHOR]

Crystallization behavior of scalable phase change materials can be studied on nanoscale structures. In this paper, high density ordered phase change nanodot arrays were fabricated using the lift-off technique on a self-assembled diblock copolymer template, polystyrene-poly(methyl-methacrylate). The size of the nanodots was less than 15 nm in diameter with 40 nm spacing. This method is quite flexible regarding the patterned materials and can be used on different substrates. The crystallization behavior of small scale phase change nanodot arrays was studied using time-resolved x-ray diffraction, which showed the phase transition for different materials such as Ge15Sb85, Ge2Sb2Te5, and Ag and In doped Sb2Te. The transition temperatures of these nanodot samples were also compared with their corresponding blanket thin films, and it was found that the nanodots had higher crystallization temperatures and crystallized over a broader temperature range. [ABSTRACT FROM AUTHOR]

The crystallization behavior of ultrathin phase change films was studied using time-resolved x-ray diffraction (XRD). Thin films of variable thickness between 1 and 50 nm of the phase change materials Ge2Sb2Te5 (GST), N-doped GST, Ge15Sb85, Sb2Te, and Ag- and In-doped Sb2Te were heated in a He atmosphere, and the intensity of the diffracted x-ray peaks was recorded. It was found for all materials that the crystallization temperature increases as the film thickness is reduced below 10 nm. The increase depends on the material and can be as high as 200 °C for the thinnest films. The thinnest films that show XRD peaks are 2 nm for GST and N-GST, 1.5 nm for Sb2Te and AgIn-Sb2Te, and 1.3 nm for GeSb. This scaling behavior is very promising for the application of phase change materials to solid-state memory technology. [ABSTRACT FROM AUTHOR]

We have used time-resolved x-ray diffraction to study the amorphous-crystalline phase transition in 20–80 nm particles of the phase change materials Ge2Sb2Te5, nitrogen-doped Ge2Sb2Te5, Ge15Sb85, Sb2Te, and Sb2Te doped with Ag and In. We find that all samples undergo the phase transition with crystallization temperatures close to those of similarly prepared blanket films of the same materials with the exception of Sb2Te that shows the transition at a temperature that is about 40 °C higher than that of blanket films. Some of the nanoparticles show a difference in crystallographic texture compared to thick films. Large area arrays of these nanoparticles were fabricated using electron-beam lithography, keeping the sample temperatures well below the crystallization temperatures so as to produce particles that were entirely in the amorphous phase. The observation that particles with diameters as small as 20 nm can still undergo this phase transition indicates that phase change solid-state memory technology should scale to these dimensions. [ABSTRACT FROM AUTHOR]

The texture of CoSi2 films on Si(001) substrates has been examined using synchrotron based high-resolution pole figure analysis. We find that axiotaxy, a recently recognized type of texture which was observed in NiSi thin films on Si(001), is also occurring in CoSi2 layers. Typical of axiotaxy texturing, the CoSi2 pole figures show symmetrical patterns of lines. Calculations show that these features on the pole figures are created by three off-normal fiberlike texture components. These off-normal fiber orientations are a consequence of the alignment of CoSi2<110> planes with Si<110> planes because of very similar d spacings, which causes a periodic arrangement along Si<100> directions in the plane of the interface. This one-dimensional periodicity may provide a lower interfacial energy; however, the periodic arrangement along the Si<100> directions may also be indicative of growth anisotropy. The influence of Ti alloying has been investigated by using in situ x-ray diffraction measurements, as well as pole figure analysis. These results show that the nucleation of both the CoSi and CoSi2 phases is delayed to higher temperatures for the 5 at. % Ti alloyed films. The texture of the CoSi2 phase is also influenced by the addition of Ti. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High density ordered phase change nanodot arrays are prepared by using the lift-offs technique on a self-assembled diblock copolymer template, polystyrene-poly(methyl-methacrylate). The transition temperatures of these nanodot samples are compared with their corresponding blanket thin films and it is shown that the nanodots have higher crystallization temperatures and crystallized over a broad temperature range.

X-ray diffraction techniques using synchrotron radiation play a vital role in the understanding of structural behavior for a wide range of materials important in microelectronics. The extremely high flux of X-rays produced by synchrotron storage rings makes it possible to probe layers and interfaces in complicated stacked structures, characterize low-atomic-weight materials such as polymers, and study in situ phase transformations, to name only a few applications. In this paper, following an introduction to synchrotron radiation, we describe the capabilities of the IBM/MIT X-ray beamlines at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). A range of techniques are introduced, and examples of their applicability to the study of microelectronics-related materials phenomena are described. [ABSTRACT FROM AUTHOR]