Alternating bismuth/selenium multilayered films prepared by thermal evaporation show an exceptionally high optical recording sensitivity energy threshold of 0.5 nJ for 50 ns pulses because the absorbed light heats the film until it begins to melt, at which time an exothermic reaction between the Bi and Se releases more energy. Electron microscopy reveals that the bits written on the multilayered Bi/Se films are very clean. The intrinsic signal-to-noise ratio, corrected for the substrate defects, of a digital signal recorded in these films was about 55 dB and was the same as or slightly better than that of a 300 A pure Te film on a glass substrate. An earlier study by AT&T Bell Laboratories showed exceptionally good environmental stability of the Bi/Se bilayers; some Bi/Se bilayer films deposited more than 10 years ago still show little sign of degradation. For write pulse energies below those required to form a hole, the write beam causes the reflectivity of the film to increase because of Bi2Se3 formation. The thermal annealing of the film at 220°C for 1 min results in a similar reflectivity increase. This reflectivity increase effect could be used to write format information, and hole formation could then be used to record user data.
S. Y. Suh,
H. G. Craighead,
"Optical Writing Characteristics Of Multilayered Bismuth/Selenium Thin Films," Optical Engineering 26(6), 266524 (1 June 1987). https://doi.org/10.1117/12.7974109