Epitaxial Phase Change Superlattices Designed for Investigation of Non-Thermal Switching

Funded by Leibniz Gemeinschaft within the Leibniz Competition (formerly SAW). Funding period: 01.07.2015–30.06.2019


The development of data-storage has been driven by an exponentially grown demand at the global level. As the ideal solution for all memory requirements a unified memory is envisioned: a fast, small, and non-volatile cell with a long lifetime and reduced power consumption. One promising memory concept which aims at that goal is based on phase-change materials (PCMs).


PCMs are similar to a chameleon that changes in a fast and reversible manner between two different states, namely the crystalline and the amorphous one. In virtue of the strong difference in physical properties exhibited by the two phases PCMs are already at the heart of optical storage technologies, e.g. digital versatile disc (DVD).


One impressive achievement has been accomplished when it was realized that PCM memory structures made of alternating GeTe and Sb2Te3 layers (PCM SLs) showed dramatically improved performance in terms of reduced switching energies, improved write-erase cycle lifetimes, and faster switching speeds. The outstanding memory performances were attributed to a transition between two different crystalline states. The identification of such states however is not fully agreed upon by different groups and the switching mechanism remains still a matter of controversy.


The main goal of the present proposal is to fabricate epitaxial PCM SLs with a designed sequence such that the interpretation of time-resolved experimental results will be directly associated to the grown structure, through an atomistic picture.


We will record data from the same samples as they change over time. The phase transformation we will investigate is based either on the formation and breaking of chemical bonds or on the rearrangement of atoms. Both processes take place on a very short time scale. Hence, almost all our experiments fall into the so-called "ultra-fast" time regime. 


We propose four different types of complementary time-resolved experiments on our tailored materials. We will study the response of the crystal lattice to an external trigger (optical or electrical) using optical and structural methods in order to address the behavior of the phase change. In such a way we can compare the optical and electrical switching mechanism.


Dr. Lutz Geelhaar

Head of Department

+49 30 20377-359