Phase change materials (PCM) are a technologically important materials class. There are two mature main application areas for PCM. The first deals with their use in optical discs for data storage. The second application area is in electronics and is known as phase change random access memory (PRAM). These memory elements exploit the resistance contrast between the amorphous and crystalline phases of PCM.
Great advances have been made in understanding the fundamental properties of PCM by studying amorphous and polycrystalline films. Nevertheless the answer to some questions, such as the determination of the band structure requires the use of very advanced characterization techniques, such as angle resolved photoemission spectroscopy, that are limited to single crystals or epitaxial thin films.
In recognition of this need the epitaxial growth of PCM was pioneered using molecular beam epitaxy (MBE). Here I review the progress we made in the growth of crystalline phase change materials by MBE. First I discuss the crystal structures of Ge2Sb2Te5, the prototype phase change material and then I focus on the growth of epitiaxial GST films and GeTe/Sb2Te3 superlattices both on amorphous and single crystalline substrates. Finally I show the device performances of such ordered films.