Gentaro Ohbayashi

Crystal structure of GeTe and Ge2Sb2Te5 meta-stable phase

Abstract Direct X-ray diffraction measurement of the erased state of the Ge–Sb–Te recording layer in a four-layered phase change optical disk, which was produced by an optical disk drive, was performed. It was identified as an fcc crystal structure. In order to carry out the detailed crystal structure analysis by the powder X-ray diffraction method with Rietveld refinements, somewhat larger amount of the fcc crystal powder was prepared from deposited 10 μm thick films. It revealed that Ge2Sb2Te5 belongs to the NaCl type structure (Fm3m) with the 4a site including 20% vacancies. The conclusion was supported by the results of the density measurements with Grazing Incidence of X-ray Reflectivity.

Local structure of amorphous GeTe and PdGeSbTe alloy for phase change optical recording

The local structures of amorphous Ge0.52Te0.48 and Pd0.01Ge0.17Sb0.26Te0.56 were examined by extended x-ray absorption fine structure. In amorphous GeTe film, only Ge atoms were observed in the nearest-neighbor of Te atoms. The average coordination number around Te atoms in amorphous GeTe is 1.5 which is close to twofold coordination. These results support a chemically ordered structure model. In amorphous PdGeSbTe film, Ge and Sb (and/or Te) atoms were observed in the nearest-neighbor of Te atoms at distances of 2.61 and 2.84 A, respectively. Debye–Waller factors of the Te–Ge bonds in TeGe and PdGeSbTe are 0.076 and 0.081 A, respectively, and are larger than that of Te-(Sb and/or Te); 0.063 A. From these results, we argue that the softened Te–Ge bond plays an important role in the elementary process of crystallization of GeTe and PdGeSbTe alloys. The total average coordination number of Pd atoms in amorphous PdGeSbTe alloy is 4.0. This result suggests Pd atoms play the role of cross-linking element.

PdGeSbTe Alloy for Phase Change Optical Recording

Phase change optical recording disks using a Pd–Ge–Sb–Te quaternary alloy demonstrated high crystallization speed and long-term thermal stability of the amorphous recording marks. This alloy film can be crystallized by a short duration laser pulse of less than 100 ns. It is applicable to a single beam overwrite optical recording system. The crystallized portion of this recording layer on the disk is assigned to single phase and polycrystalline face-centered-cubic (fcc) crystals by transmission electron diffraction. A small amount of Pd atoms (typically 0.2 to 3 at.%) in this alloy improve the thermal stability of amorphous marks.

Effect of Photo-active Compound Structure on Photosensitivity of Positive Photosensitive Polyimide

The photo-active compound (PAC) in positive tone photosensitive polyimide (posi PSPI) was investigated in order to obtain good photo lithographic performance (small development loss and good photosensitivity). When the PAC structure was changed posi PSPI showed different levels of photosensitivity and development loss. The development loss and photosensitivity showed a good correlation. The lower development loss posi PSPI showed lower photosensitivity. From the molecular orbital calculation, the PAC in the lower development loss posi PSPI had a lower dipole moment than that in the larger development loss posi PSPI. In addition, dissolution rate measurement (DRM) results suggest that posi PSPI may have two layers from the point of dissolution. The dissolution rate of the surface layer in posi PSPI was shown to be lower than that of a bottom layer. This suggests that PAC using the lower development loss posi PSPI may exist mainly on its surface. These results are quite important for the design of good posi PSPI.

Reliability of the Phase Change Optical Disk

Thermal stability of the Al-alloy reflective layer is essential for the archival life of a rewritable phase change optical disk. The new Al–Hf–Pd alloy reflective layer provides excellent thermal stability. A disk using this Al-alloy and a Pd–Ge–Sb–Te alloy recording layer is extremely stable. The bit-error-rate (BER) of the recorded signal did not change substantially after an acceleration test of 6,800 h under the conditions of 90° C, 80%RH. X-ray diffraction analysis showed that the new additive components (Hf, Pd) of the Al-alloy prevented the growth of the Al crystals.