James Howard Coombs

Laser‐induced crystallization phenomena in GeTe‐based alloys. I. Characterization of nucleation and growth

The laser‐induced crystallization behavior of GeTe‐based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal‐growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as‐deposited and melt‐quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as‐deposited layer can differ by over an order of magnitude from that of the melt‐quenched amorphous layer. The as‐deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of...

Laser‐induced crystallization phenomena in GeTe‐based alloys. II. Composition dependence of nucleation and growth

The laser‐induced crystallization behavior of GeTe‐based amorphous alloys has been measured with a novel multipulse laser technique. This enables the composition dependence of the nucleation rate and crystal growth speed to be independently followed. Two types of crystallization are investigated. The first involves single‐phase crystallization of quaternary alloys based on Ge39Sb9Te52, in which the composition dependence of nucleation and growth is followed as Se, S, Sn, and Si are included. Both the nucleation rate and crystal‐growth speed vary exponentially with the composition, and a correlation is found between crystallization behavior and bond strengths. The second involves multiphase crystallization in the GeSbTe ternary system. It is shown that the observed variations in crystallization behavior primarily arise from the composition dependence of nucleation rather than crystal growth. The implications of this finding for the importance of long range diffusion during crystallization in the GeSbTe sys...

Laser‐induced crystallization phenomena in GeTe‐based alloys. III. GeTeSe alloys for a CD compatible erasable disk

A novel multipulse laser technique has been used to study the crystallization behavior of the GeTeSe ternary system, and the dependence of the nucleation and growth rates on composition has been related to the optical and structural properties. We show that the Se content is dominant in determining both the nucleation rate and the amorphous optical constants, while the crystal‐growth speed depends primarily on the single or multiphase nature of the resultant crystalline state. In this respect compositions on the GeTe‐GeSe line are unique in that they are single‐phase for a Se content up to 25%. This characterization of both the optical and crystallization behavior has allowed us to identify materials suitable for use in a CD‐compatible erasable optical disk based on the phase change principle, in which local amorphous regions in a crystalline film represent data bits. Readout compatibility with a CD imposes extremely stringent conditions on the reflection from the amorphous and crystalline states as well ...

Scanning optical microscopy: a powerful tool in optical recording

A Scanning Optical Microscope (SOM) provides valuable information on the physical and optical properties of an optical recording system. In particular, alternative detection modes such as Differential Phase Contrast detection provide a means of separating the phase and amplitude parts of the local reflectivity, and the SOM is thereby able to distinguish some of the processes involved in optical recording. In this paper we describe how a SOM can be used in optical recording research and development, and give examples of its application.

CD-compatible erasable disk

We have experimentally demonstrated the feasibility of a fully backward compatible erasable Compact Disc. Such discs require new phase change materials; we have identified such materials by means of novel selection criteria, finding that a range of GeTeSe alloys possess both the required recording and optical properties.