The Ge2Sb2Te5 thin films deposited by a sputtering method on SiO2∕Si substrates were annealed through a rapid thermal annealing process and performed a high-resolution transmission electron microscopy study in order to investigate the atomic arrangement of the metastable Ge2Sb2Te5. The metastable rocksalt structure having face-centered-cubic lattice was confirmed by high-resolution transmission electron microscopy images and simulated images on the directions of ⟨100⟩, ⟨110⟩, and ⟨211⟩ zone axes. According to the position of Ge and Sb in the metastable rocksalt structure, the atomic distribution alters when observed in different direction and this causes change in the charge-density distribution, resulting in different images in a high-resolution transmission electron microscopy. It is expected that as the crystallization proceeds, the Ge and Sb atoms tend to position themselves on a specific plane. From this aspect, the ordered structure model of the metastable Ge2Sb2Te5 was proposed by varying the position of the Ge and Sb atoms.

Original Paper 1638 M. S. Youm et al.: Effects of excess Sb on crystallization of δ-phase SbTe binary thin films

We investigated the properties of a Ge-Bi-Te ternary chalcogenide thin film which was deposited on a SiO 2 /Si substrate by varying RF-sputtering power on the GeTe and Bi target. The aim was to search for an appropriate candidate for a new phase change memory. Various analyses are conducted in order to investigate the composition, phase separation, and crystallization behavior of the Ge-Bi-Te alloy. The XRD results of each annealed sample showed that the Ge-Bi-Te alloy crystallized into Ge 2 Bi 2 Te 5 , GeBi 2 Te 4 , GeBi 4 Te 7 phase at around 300 • C according to Ge content and expelled amorphous Ge crystallized as a single phase over 400 • C. Combining these with the differential scanning calorimetry (DSC) results, we demonstrated that T c and T m of the Ge-Bi-Te alloy are respectively higher and lower than those of conventional Ge-Sb-Te (GST) films. All the phases, including not only various Ge-Bi-Te ternary phases but also the Ge phase crystal structure, were also confirmed with highresolution transmission electron microscopy (HR-TEM) images and diffraction patterns. It is noted that some of the Ge 2 Bi 2 Te 5 grains show specific facetted planes such as {0113}, {0112}, and {0001}. Through successive analyses, we revealed the structural evolution of the Ge-Bi-Te alloy according to Ge contents and confirmed the potential of the Ge-Bi-Te alloy for phase-change random access memory (PRAM) applications.

A new theoretical description of coherent and incoherent multiple scattering of electrons at a single-crystal surface partially covered with a disordered overlayer is reported. We use a formalism in which occupation operators specify the occupancy of a chemisorption site. This theory is based upon two approximations: (i) a mean-field-like approximation allows one to separate forward-and backward-scattering amplitudes of the overlayer into coherent and incoherent parts; (ii) the incoherent scattering amplitude of the adsorbate plus substrate system is approximated by its first-order expansion in the incoherent scattering amplitude of the overlayer alone. Hence, a simple expression for the incoherent relative intensity is abtained. It appears as a quadratic form in thep,,, factors Kr'(p, k' c k), which are the effective amplitudes for the pmax possible occupancies of a chemisorption site. The matrix r ( p , p ' , k' +k) from which this quadratic form is derived is nothing other than the Fourier transform on the lattice of the site-occupancy pair correlation function for a pair of sites with occupancies p and p'. Obviously, all information concerning the statistical distributionof atomsor moleculesin the overlayer isembodied in thismatrix. Asanexample, the particular case of one kind of atom adsorbed on a square lattice surface is examined in the low-coverage limit, when the site-occupancy pair correlation function has a range of about the interatomic distance. I Permanent and present address:

The microstructures of once melt-quenched Ge2Sb2Te5 (GST) phase change contact dimensions are directly investigated with high resolution transmission electron microscopy (HR-TEM) by applying reset pulse of 7–13V. The ovonic threshold switching voltage is decreased from 4.1to2.8V when the as-deposited GST cells are once melt quenched by 10V. HR-TEM reveals that there are hexagonal nuclei in the once melt-quenched GST and the GST can be partially left in not the amorphous but the crystalline state when the molten GST is not swiftly quenched, which is an origin of the switching failure.