Effect of thickness and crystallization on the optical energy gap of some chalcogenide phase-change thin films

The chalcogenide GeSbTe semiconductor compounds are widely used in rewritable optical disk systems such as CD-RW, PD and DVD-RAM. The readout and write/erase properties are strongly dependent on the optical constants (refractive index n and extinction coefficient k, absorption coefficient a) of the phase-change layer. Therefore, the accurate determination of the optical constants of these materials is important, not only in order to know the basic mechanisms underlying these phenomena, but also to exploit and develop their interesting technological applications. One of the most important factors determining the optical constants is the optical absorption edge and consequently the band gap E_g^opt. In this work, E_g^opt have been measured on some typical chalcogenide phase change thin films. The effect of film thickness and thermal annealing on E_g^opt is discussed. The type of the optical transition process and its corresponding E_g^opt value may provide a fundamental access to the wide optical application of these chalcogenide thin films.