Atom Probe Tomography of Phase Change Materials

  High resolution correlative APT-TEM investigations showing the partial laser-induced crystallization of amorphous Ge2Sb2Te5 tip I. Physikalisches Institut A Figure 1. High resolution correlative APT-TEM investigations showing the partial laser-induced crystallization of amorphous Ge2Sb2Te5 tip

Atom probe tomography is one of the few techniques available which can address major issues encountered while working with phase change materials (PCMs), such as the mechanism of switching repeatability and its failure. However, until now, only very few studies have been performed. Here, laser-assisted atom probe tomography (APT) is used to systematically study various crystalline phase change materials and their amorphous counterparts. The technique utilizes picosecond UV laser pulses to field evaporate the atoms from the apex of the tip. This is potentially detrimental when studying phase change materials, which are characterized by ultrafast crystallization. Indeed, for one of the PCMs characterized by a very low crystallization temperature (~120°C) we observe partial crystallization of amorphous Ge2Sb2Te5 tips during studies in the atom probe tomography when applying laser energies higher than 30 pJ and base-temperatures higher than 50K. However, other amorphous materials investigated in our group are found to be stable under the laser pulses in the APT. More importantly, atom probe tomography investigations show strikingly different evaporation behaviors for amorphous and crystalline phase change materials.

  High resolution correlative APT-TEM investigations showing the partial laser-induced crystallization of amorphous Ge2Sb2Te5 tip I. Physikalisches Institut A Figure 2. High resolution correlative APT-TEM investigations showing the partial laser-induced crystallization of amorphous Ge2Sb2Te5 tip

Superlattice structures display an even enhanced switching performance compared with the standard PCMs. Many chalcogenides are topologically non-trivial, opening up an ambiguity in transport channels for the common polycrystalline materials. For example, the topological properties have recently been proposed to dominate the conductivity contrast upon switching in GeTe-Sb2Te3 superlattices.

In the “Atomprobe” group, the relationship between the composition and the structure of these supperlattices is understood by applying correlative microscopy approach, i.e. the combination of the atom probe tomography (APT) with the transmission electron microscopy (TEM) as shown in Figure 2. This gives direct information about inter-diffusion between supperlattices’ constituents at the nanoscale, quality of the epitaxial films and defects formation in each deposited film.