Periodic cycle number modulating effect on crystallization temperature in superlattice-like [Ge/Ge8Sb92]n phase-change films and exploration of mechanism

Superlattice-like (SLL) phase-change films provide more controllable parameters for the optimization of the performance of phase-change films, including the thickness of each constituent layer, the thickness ratio of two constituent layers and cycle number of periodicity. The effects of the first two parameters on the performance of SLL films have been studied widely. However, the influence of last parameter, cycle number of periodicity, was studied sparsely. In this study, we have studied the period number effect on crystallization temperature of SLL [Ge/Ge8Sb92]n films, and designed and fabricated a series of superlattice-like (SLL) [Ge/Ge8Sb92]n phase-change films. Their crystallization behaviors are studied by the measurement of temperature-dependent sheet resistance. We find that crystallization temperature decreases with increasing cycle number of periodicity, revealing period-cycle-number modulation effect. However, such the effect cannot be explained by current interface effect model. We test the existence of periodic structures of the crystallized SLL films by coherent acoustic phonon (CAP) spectroscopy. Apparent folded CAP modes related to SLL nanostructures are observed, implying the existence of excellent periodic structures or no alloying within one period in crystallized SLL films. Therefore, such period number manipulation effect cannot be explained by the cooperative effects of interface and alloying effects either, implying new mechanisms to be unveiled. We tentatively propose two kinds of possible long-range effects, built-in electric field and strain effects. Based on strain effect, our results can be explained phenomenologically.