Dynamics of amorphization induced in crystalline Ge₁Sb ₂Te₄ films by single femtosecond pulses

The dynamics and the conditions of amorphous transitions induced in a Ge₁Sb₂Te₄ system upon a single femtosecond (fs) pulse melting were studied by real-time reflectivity measurements. The system has a multilayer structure of 100 nm ZnS-SiO₂/(15-100 nm) Ge ₁Sb₂Te₄/120nm ZnS-SiO₂/0.6mm polycarbonate substrate. It is shown that under optimum conditions amorphization is completed within 900 ps. The thickness of the phase change layer plays an important role in controlling the heat flow conditions in the system upon a fs pulse irradiation. The use of the fs laser pulse leads to a situation in which the pulse energy is deposited within a very short time in a thin surface layer, leading to heating or melting. The so-generated steep temperature gradient is subsequently smoothed by heat diffusion toward the substrate. The relative thermal process and effects are estimated. The calculated results are consisted with those from real-time reflectivity measurements. The mechanism of crystalline to amorphous transition triggered by single fs laser pulses is discussed.