Thin films from cross section Bi2Te3-Sb2Te3 were studied in view of glass-crystal reversible phase change recording by using laser diode irradiation. By adjusting the spherical aberration of the focalization lens and using a Raman microprobe, reflectivity and vibrational profiles of the laser spot were performed in order to determine the most sensitive region of the illuminated area. The optical switch was then analyzed in optimized conditions over the whole cross section. When considering identical cycling parameters, the latter appears to be inverted at a critical composition, which depends critically on the layer thickness.