Crystallisation behaviour of phase change materials: comparison between nucleation- and growth-dominated crystallisation

We have studied the crystallisation behaviour of stoichiometric GeSbTe versus (doped) eutectic SbTe materials. For pseudo-binary compositions on the GeTe-Sb₂Te₃ tie-line, the nucleation time and the complete erasure time (CET) of an amorphous mark are of the same order of magnitude, typically in the range 20-100 ns. For compositions close to the eutectic Sb₆₉Te₃₁ the nucleation time exceeds 100 μs, 4 orders of magnitude longer than the CET. The nucleation probability of doped SbTe materials was determined experimentally and a low temperature maximum was observed, in line with classical nucleation theory. The large difference in the nucleation and growth rates of both material classes has important consequences for the erase mechanism of amorphous marks in practical phase change media: for stoichiometric GeSbTe materials mark erasure is nucleation-driven, whilst it is growth-driven for eutectic SbTe compositions. For growth-driven mark erasure, we observed a strong dependence of the CET on amorphous mark size. This dependence must be quantified, to be able to compare the crystallisation speed of different growth-dominated materials. Therefore, we developed a method in which the size of a written amorphous mark was deduced from the measured modulation level.