CHARACTERZATION OF Ge-Sb-Te PHASE-CHANGE MEMORY MATERIALS

摘要

Phase-change memory materials are promising for the next generation of non-volatile flash memory that will serve in new mobile computing entertainment and other handheld electronics. Among them are chalcogenide glasses Ge-Sb-Te (GST) which can exist in two separate structural states - amorphous and crystalline. Switching of the material from one to another state can be done by heating applying an electrical pulse or by exposure to intense laser beam. In the present work we report the changes of optical parameters of amorphous Ge_1Sb_2Te_4, Ge_1Sb_4Te_7, and Ge_2Sb_2Te_5 thin films under heat treatment and light exposure. The illumination with white during 1 hour does not change the transmission spectra of the as-deposited amorphous film. The spot of phase change transformation of the amorphous material was observed when the film was illuminated with UV laser pulses. From the transmission spectra T=f(λ) the optical constants (Absorption coefficient α, optical band gap E_g, refractive index n, the average electronic energy gap E_0 and the dielectric oscillator strength E_d were calculated. For Ge_1Sb_2Te_4 the value of E_0 is smaller than optical band gap E_g=1.08 eV obtained from the Tauc plot. Large values of the refractive index n are obtained for smaller E_0=0.931 eV and for large E_d=7.448 eV. The anealing of the amorphous Ge_2Sb_2Te_5 thin film at T=100 ℃ during t=4 min shifts the transmission spectra in the low frequency region. The anealing at higher temperatures makes the thin film non-transparent, e.g. take place the process of crystalization.