  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 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 (Tm) after which, if it is cooled rapidly enough to below the glass
transition temperature (Tg), 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. When
heated above Tg with an intermediate power laser pulse for a significant amount
of time, then this favors the shift back to the energetically favorable
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.
