The effect of partial crystallization on phase-change optical storage

The degree of partial crystallization of phase change materials for rewritable optical data storage is shown to affect the subsequent formation of amorphous marks. Experiments with Ge₂Sb_2.5Te₅ (GST) and AgInSbTe (AIST) phase-change materials demonstrate the difference between nucleation-dominated and growth-dominated processes in the uniformity of crystalline states and in the subsequent formation of amorphous marks. For the GST material, experimental measurements along with thermal and optical modeling show that the reduced reflectivity and increased absorption of the partially crystalline state increases the final amorphous mark size. An 8% decrease in reflectivity and a 11% increase in mark diameter is measured between the amorphous marks formed in the fully crystallized, high reflectivity (R = 43%) state and partially crystallized, low reflectivity (R = 30%) state. In contrast, the AIST material, which is growth-dominated, does not crystallize uniformly at reflectivity levels below full crystallization. This is shown to result in a mixture of amorphous and crystalline regions rather than a partially crystalline state. Further, the final amorphous mark size does not have the inverse relation with initial reflectivity measured and modeled for the GST material.; Novel reflectivity measurements and images of amorphous state marks are completed by combining Photon Tunneling Microscopy (PTM) with technology for microscope testing of optical disks. Reflectivity measurements of amorphous marks between 200 nm and 500 nm are made with a 680-nm laser diode to a first-surface GST phase-change disk. A two-laser static tester provides measurements of the change in reflectivity during the write process. The effect of air gap on the coupling of propagating and evanescent waves is measured. A PTM with an effective numerical aperture of 1.4 and blue illumination at 435.8 nm is used to image amorphous marks with a resolution of 190 nm. The reflectivity measurements and images are used to demonstrate the inverse dependence between the initial reflectivity and the final amorphous mark size for a nucleation-dominated GST phase-change optical recording disk.