Tsuneyuki Research Group

Research

"First-principles calculations for amorphization of phase change materials by irradiating ultrashort pulse laser "

Reversible optical devices, such as Blu-ray Disc, have been used all over the world. These devices exploit reversible phase transition of phase change materials, which are used as recording films, between amorphous and crystalline phases to record and delete data. Recent experimental results proposed that Ge₂Sb₂Te₅, which is one of the phase change materials, is fast amorphized without melting by irradiation of ultrashort pulse laser, whose duration time is about 10^-13 s. These experimental results showed the potential of faster encoding devices. However, we need more understanding of this amorphization to realize them.

Then, we theoretically simulated the amorphization of phase change materials to resolve the atomic dynamics. We carried out first-principles calculations based on density functional theory for electrons at finite-temperature. As results, we found that the structure changes to a highly coordinated structure shortly after irradiation of the laser, subsequently via thermal relaxation, changes to the amorphous phase which is stable at room temperature. Our calculation results are inconsistent with a conventional model which suggests the amorphization via a low coordinated structure. However, the amorphization resulted from our calculations is similar to pressure-induced amorphization with respect to the highly coordinated intermediate structure and non-melting transition. Based on our calculations results, we are going to propose new amorphization model which has similarity with pressure-induced amorphization.