Large-scale two-dimensional
sheets of graphene-like germanium,
namely, germanene, have been epitaxially prepared on Ag(111) thin
films grown on Ge(111), using a segregation method, differing from
molecular beam epitaxy used in previous reports. From the scanning
tunneling microscopy (STM) images, the surface is completely covered
with an atom-thin layer showing a highly ordered long-range superstructure
in wide scale. Two types of protrusions, named hexagon and line, form
a (7√7 × 7√7)R19.1° supercell
with respect to Ag(111), with a very large periodicity of 5.35 nm.
Auger electron spectroscopy and high-resolution synchrotron radiation
photoemission spectroscopy demonstrate that Ge atoms are segregated
on the Ag(111) surface as an overlayer. Low-energy electron diffraction
clearly shows incommensurate “(1.35 × 1.35)”R30° spots, corresponding to a lattice constant of
0.39 nm, in perfect accord with close-up STM images, which clearly
reveal an internal honeycomb arrangement with corresponding parameter
and low buckling within 0.01 nm. As this 0.39 nm value is in good
agreement with the theoretical lattice constant of free-standing germanene,
conclusively, the segregated Ge atoms with trivalent bonding in honeycomb
configuration form a characteristic two-dimensional germanene-like
structure.
We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb2Te3] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200 °C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.
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