GeSbTe - Material Properties

Material Properties

GeSbTe is a ternary compound of germanium, antimony, and tellurium, with composition GeTe-Sb₂Te₃. In the GeSbTe system, there is a pseudo-line as shown where most of the alloys lie on. Moving down this pseudo-line, it can be seen that as we go from Sb₂Te₃ to GeTe, the melting point and glass transition temperature of the materials increase, crystallization speed decreases and data retention increases. Hence, in order to get high data transfer rate, we need to use material with fast crystallization speed such as Sb₂Te₃. This material is not stable because of its low activation energy. On the other hand, materials with good amorphous stability like GeTe has slow crystallization speed because of its high activation energy. In its stable state, crystalline GeSbTe has two possible configurations: hexagonal and a metastable face centered cubic (FCC) lattice. When it is rapidly crystallized however, it was found to have a distorted rocksalt structure. GeSbTe has a glass transition temperature of around 100 °C. GeSbTe also has many vacancy defects in the lattice, of 20 to 25% depending on the specific GeSbTe compound. Hence, Te has an extra lone pair of electrons, which are important for many of the characteristics of GeSbTe. Crystal defects are also common in GeSbTe and due to these defects, an Urbach tail in the band structure is formed in these compounds. GeSbTe is generally p type and there are many electronic states in the band gap accounting for acceptor and donor like traps. GeSbTe has two stable states, crystalline and amorphous. The phase change mechanism from high resistance amorphous phase to low resistance crystalline phase in nano-timescale and threshold switching are two of the most important characteristic of GeSbTe.