An Investigation into the Kinetics and Mechanism of Phase Transitions in Optical Phase Change Ternary Alloy $Ge_2Sb_2Te_5$

Abstract

Ternary alloys of Ge-Sb-Te (GST) have been extensively studied due to their unique ability display a reversible change in their phase upon stimulation by optical pulses i.e amorphous (a-GST) to crystalline (c-GST) and vice-versa. The two phases exhibit remarkably different electrical and optical properties like conductivity, reflectivity, refractive index and optical loss, this coupled with their high phase switching speeds, low power phase switching, large switching cycles, large measurable optical and electrical contrast, and phase stability makes GST alloys stand out from other phase change materials (PCM). GST alloys have already found extensive use in optical disks and electronic memories due to their non-volatility and zero static power consumption, but the precise mechanism of the phase change is not clearly understood. The phase change mechanism has usually been attributed to the optical pulse, usually a high power short pulse laser, heating up the c-GST alloy to above its melting temperature (T m ) after which, if it is cooled rapidly enough to below the glass transition temperature (T g ), the atoms are fixed in place due to the drastic reduction in their mobility, resulting in a phase which exhibits structure similar to a frozen liquid and lacks long range order i.e amorphous. If alternatively the a-GST is heated above T g with a intermediate power laser pulse for a significant amount of time to induce nucleation, then this favours the shift back to the energetically favourable crystalline phase. With the growing interest in next generation data storage technology for photonic and neuromorphic computing, the research into understanding and improving the properties of GST alloys has also been rekindled. In this term paper we hope to investigate and gain a deeper understanding and appreciation of the kinetics and underlying atomistic mechanism of this phase transition from c-GST to a-GST and vice-versa which is only now becoming clearer.