Effects of the Sb2Te3 crystallization-induced layer on crystallization behaviors and properties of phase change optical disk

Abstract

The conventional phase-change optical disk is generally fabricated by the sputtering process, which has a drawback of requiring an initialization process to change the as-deposited recording layer in the disk from amorphous to crystalline phases before the disk can be used for reading or writing. In order to develop an initialization-free process, the Sb2Te3, alloy was used as an additional layer below or above the recording Ge2Sb2Te5 layer to study its effect on crystallization behaviors of the recording layer. The layer structures were deposited on substrates of Si wafer, Cu-mesh to examine crystal structure (XRD), amorphous-to-crystal transformation (DSC) and microstructure (TEM). The complete disk specimens were prepared on PC board to measure their dynamic properties, such as reflectivity, jitter and modulation (dynamic tester); and to examine the effects of laser pulse duration time, position and thickness of Sb2Te3, layer on static reflectivity of the disk (static tester), where Avrami coefficient 'q' in J-M-A rate equation can be derived. The results show that effect of Sb2Te3 layer is essentially to induce crystallization of Ge2Sb2Te5, recording layer from (110) plane of Sb2Te3 crystals. This is due to the fact that the crystallization temperature of Sb2Te3 crystal is 85 degreesC below that of Ge2Sb2Te5 crystal, in addition to a lower lattice mismatch between two crystals. The is in agreement with the J-M-A kinetic analyses that the rate controlling step for amorphous-crystal transformation in disk specimens with Sb2Te3 layer over 15-nm thickness is mainly governed by nucleation with q = 2.53-2.79 > 2.5 in J-M-A equation. Regarding the effects of Sb2Te3 layer on disk properties, the results show that under the 10 nm Ge2Sb2Te5, layer thickness, the Sb2Te3 assisted disks with lower Sb2Te3 layer thickness between 13 and 20 nm show the best combination of reflectivity and modulation. The most important advantage of this process is that the Sb2Te3-assisted disks require no initialization process, because the as-deposited disks can be directly written and erased. (C) 2003 Elsevier B.V. All rights reserved.