We have developed a semitransparent layer structure that is ideally suited to the laser-incident-side (front-side) recording layer of a rewritable dual-layer Blu-ray Disc (BD-RE). A TiO2 top film and a memory film of GeTe–Sb2Te3 characterize this layer structure. The TiO2 film has a large refractive index, which works to increase the transmittance of the entire recording layer to more than 50%. It simultaneously increases the reflectivity contrast, defined by (R cry-R amo)/(R cry+R amo), where R cry and R amo (R cry>R amo) are the reflectivities of the optical disc in the crystalline and amorphous parts, respectively. The GeTe–Sb2Te3 memory film has optical constants that are appropriate for balancing the transmittance between the crystalline part (T cry) and the amorphous part (T amo) in the laser wavelength region of approximately 405 nm. Our experimental dual-layer disc shows high transmittances of T cry=51.6% and T amo=52.0% and a high reflectivity contrast of 0.71 in the laser-incident-side layer. Dynamic examination showed that both recording layers have a good jitter value, satisfying the BD-RE specifications; more specifically, the laser-incident-side layer jitter value is 7.4% (spec.: 8.5%) and the back-side layer jitter value is 6.4% (spec.: 6.5%) through the front-side layer.

Nishihara, T., Kojima, R., Miyagawa, N., & Yamada, N. (2005). High-Transmittance Phase-Change Media with TiO2 Film for Rewritable Dual-Layer Blu-ray Discs. Japanese Journal of Applied Physics, 44(5R), 3037.