Effects of the Sb 2Te 3 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 Sb 2Te 3 alloy was used as an additional layer below or above the recording Ge 2Sb 2Te 5 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 Sb 2Te 3 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 Sb 2Te 3 layer is essentially to induce crystallization of Ge 2Sb 2Te 5 recording layer from (110) plane of Sb 2Te 3 crystals. This is due to the fact that the crystallization temperature of Sb 2Te 3 crystal is 85 °C below that of Ge 2Sb 2Te 5 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 Sb 2Te 3 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 Sb 2Te 3 layer on disk properties, the results show that under the 10 nm Ge 2Sb 2Te 5 layer thickness, the Sb 2Te 3-assisted disks with lower Sb 2Te 3 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 Sb 2Te 3-assisted disks require no initialization process, because the as-deposited disks can be directly written and erased.