Transmission electron microscopy for advanced materials research
Bart J. Kooi and J.Th.M. De Hosson
Dept. of Applied Physics, Materials Science Centre and Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen.
An introduction is given on Transmission Electron Microscopy (TEM) as the most powerful tool nowadays to relate properties of materials to their structure and chemistry on the atomic and nanometer scale. A TEM study of phase change materials used in rewritable CD/DVDs and for future non-volatile memories will serve as main example.
In phase-change optical recording (i.e. on CDs and DVDs), Ge2Sb2Te5 is currently most widely used as the active medium for rewritable information storage. Sub-micron amorphous areas embedded in a crystalline surrounding act as bits of information. A relatively high laser power is used to write the amorphous spots and medium and low powers are used for erasing (re-crystallization) and reading, respectively. Due to the increasing demands on data storage capacity and transfer rates, improved phase-change materials are searched for. Using TEM with in-situ heating, we studied crystallization of amorphous thin films (in-between 10 and 70 nm thick) of Sb3.6Te with 0, 4 and 16 at.% Ge in comparison with Ge2Sb2Te5. Adding Ge to Sb3.6Te strongly increases the crystallization temperature (Tc); 95, 155 and 220 0C for 0, 4 and 16 at.% Ge, respectively. Adding Ge also increases the nucleation rate, leading to smaller crystallites and increases the activation energy for growth. The main aim of the talk is to show the importance of TEM in advanced materials research.