Effect of Ionic Bond in Recording Materials on Characteristics of Phase-Change Optical Disk
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Effect of Ionic Bond in Recording Materials on Characteristics of Phase-Change Optical Disk Motoyasu Terao Storage Technology Research Center, R&D Group, Hitachi Ltd. 1-280, Higashi-koigakubo Kokubunji, Tokyo, Japan Phone: +81-42-323-1111 ex.3255, Fax: +81-42-327-7765, E-mail: [email protected] Abstract This paper discusses a crystallization model that takes difference of covalent bond and ionic bond into consideration.
Ionic bond makes crystallization easier
than covalent bond because ionic bond is more flexible than covalent bond. The diameter and spaces around large minus-ions should also be taken into consideration.
Elements having electronic negativity far from that of Te or Sb
has a tendency to form ionic bonds.
For example, Tb and In are considered to
make chemical bonds having more ionic characteristics than Ge.
In the case of
already reported Sn or Bi addition, large ion diameter seems to increase crystallization speed. 1. Introduction Phase-change optical disks are widely used because they have many merits. Those are possibility of read-out by reflectivity difference that is the same as ROM disk, possibility of one-beam overwrite, possibility of double layer disk, etc. However, these properties strongly depend on material characteristics, and to control amorphous material characteristics is difficult.
Small atomic % elements
in the chalcogenide material seem to have essential effects to crystallization characteristics.
However, there is no easily understandable model to explain 1)
Why GeTe and Ge2Sb2Te5 have quite different crystallization characteristics.
2)
Why addition of small amount of metallic element to the chalcogenide material changes the crystallization speed.
3) Why a few% of Ge addition to Sb-rich
chalcogenide material stabilizes amorphous state of recording marks. 4) Why Sb-rich recording material crystallizes without nucleation. Usually, phase-change is treated by JMA (Johnson Mehr Avrami) equation, but discrepancy between the theory and experiment often appears because JMA equation is a macroscopic equation.
In this paper, we try to explain dependence
of phase-change disk characteristics on recording film material introducing difference between covalent bond and ionic bond.
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2. Experiment Two major compositions of Phase change recording material are Te rich composition, for example, Ge2Sb2Te5, and Sb rich composition, for example, Ag4In8Sb65Te23.
Comparison of Te rich and Sb rich composition has been
carried out experimentally. In the case of Te rich composition, several compound compositions, for example Ge2Sb2Te5, exist.
Compound-composition recording material is
advantageous for fast crystallization because only one kind of crystal is formed and long-range atom diffusion to form more than two kinds of crystals from homogeneous amorphous state is not necessary.
Although bulk Te has
hexagonal crystal structure, we proposed a NaCl-type crystal structure model
1)
as
shown in Fig.1, because result of the structure analysis by electron beam dif
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