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A new write strategy (WS) optimization method with a two-step search process for Blu-ray Disc (BD) recording is developed to shorten the optimization time. This method is realized by the WS optimization system, which is constructed with an optical pickup, a disc tester, and the WS optimization algorithm. The optimization is executed according to the two-step search process along the mathematical axis, which is experimentally derived from the sample WS parameters. As the experimental result, the optimization time is reduced by nearly two-thirds from that achieved using the conventional method performed by the experts. All the jitter values of the playback signal derived from the recorded marks are smaller than the 7% target value and the effectiveness of this new method is experimentally confirmed.

The structural and optical properties of GaN epilayers grown on intentionally strained sapphire (0001) substrates by metalorganic chemical vapor deposition (MOCVD) were investigated. Intentionally tensile strained sapphire substrates were prepared by the implantation of 2.4 MeV energy Cl þ and As þ -ions with a 10 15 cm À2 dose where the projection ranges (R p ) were 1.16 mm and 0.95 mm, respectively. It was found from Raman spectroscopy that the compressive stress normally existed in the GaN epilayer/sapphire was decreased by the use of a Cl þ -ion-implanted sapphire substrate. The intentionally tensile strained sapphire surface can result in a superior crystalline GaN epilayer on top of it. However, excessively roughened and modified surface by As þ -ions degraded the GaN epilayer. Not only the ionic radius but also the chemical species of implanted ions in sapphire affected the crystal quality of GaN epilayers. Therefore, we concluded that the properly tensile-strained sapphire (0001) substrate could improve the properties of GaN epilayers grown by MOCVD.

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