Optimum structure for HD-DVD optical disks containing Al-Ti/Si bi-layer recording system was identified by reflectivity simulation and dynamic test of disk samples. For the disk sample with optimized structure, the maximum partial response signal-to-noise ratio (PRSNR) of 19.1 dB, minimum simulated bit error rate (sbER) of 1.7 X 10(-7) and modulation >0.6 were achieved at the writing power (P-W) = 11.2 mW. Transmission electron microscopy (TEM) revealed that the polycrystalline granular clusters constitute the recording marks. Subsequent analyses evidenced that element mixing/alloy reactions occur in between Si and Al-Ti layers and the formation of Al3.21Si0.47 crystalline phase is responsible for the signal recording in the disk samples. (C) 2008 Elsevier B.V. All rights reserved.