By dc and rf sputtering, we deposited thin films of GeTe, SbTe, GeSb which have four main compositions for each binary system. These materials were chosen because present phase change optical recording technology uses GeTe-based alloys, especially GeTeSb, as well as these materials have also many interesting aspects in the view of physics.
In sputtering, base pressure was $2 × 10^{-6}$ torr, and process pressure was $2 × 10^{-3}$ torr. Thin films of two thicknesses were prepared ; 50-nm films for observing laser-induced phase change and 200-nm films for measuring the optical and structural properties. The composition was controlled by deposition rate.
Reflectance, absortptance, and transmittance of the samples were measured by a spectrophotometer in the range of 2700~8300Å wavelength. We determined refractive indices n, k and dielectric constants using an ellipsometer. In addition, we observed absorption coefficient curve and achieved Tauc's plot to get the optical band gap by extrapolation. We have measured x-ray diffraction patterns with increasing temperature. And, we observed the x-ray patterns were changed from amorphous broad ones to those of crystalline which shows sharp peaks. The samples went through crystallization at some crystallization temperature after which x-ray patterns showed sharp crystalline peak. The crystallization temperature and corresponding latent heat was determined in important stoichiometric compositions or eutectic compositions of these binary systems using a DSC(Differential Scanning Calorimeter).
For real applications as phase change optical recording media, We tested and observed the laser-induced fast crystallization behavior of some samples from nanosecond time scale to milisecond time scale, and the results of these were compared with computer simulation.
From these experimental results, we had two conclusions. One is that the optical band gap depends dominantly on composition of the sample. This was explained by a simple model based on the fact that number of bondings for each type differs with varying composition. The other conclusion is that the computer thermal simulation based on the thermal conduction equation and the JMA(Johnson-Mehl-Avrami) crystallization equation worked well with the experiments in case that proper simulation parameters were used. So these two equation can be used in describing and predicting overall features of phase change, and this simulation can be applied to other systems in the future.
