From a Fundamental Understanding of Phase Change Materials to Optimization Rules for Nonvolatile Optical and Electronic Storage

Abstract

Phase change materials are commercially used in rewritable optical storage and investigated as non-volatile electronic storage. A short laser or current pulse of high intensity melts a sub-micron sized spot of crystalline material before quenching it to the amorphous state. A second pulse of lower intensity but longer duration may recrystallise and erase that bit. Since reflectivity and conductivity of the amorphous state are lower, a third even weaker laser or current pulse can be used to read out the state of the bit without changing it. As recrystallisation is the slowest process involved, materials with a small structural difference between the crystalline and amorphous phase promise higher data transfer rates. Such structural similarity however limits the optical and electronic contrast between the phases and the stability against spontaneous recrystallisation. This contradiction makes the development of phase change media a challenge that despite commercial applications still heavily relies on empirical approaches. This summary of recent experiments and ab-initio calculations reflects first steps toward an atomistic understanding of phase change materials.