Justification The recent Development on PCMtechnology is due to the fast switching between amorphous and crystalline phasesof Ge2 Sb2Te5 (GST) chalcogenide materials. Thehigh operating speed and low power consumption makes them promising candidatefor next generation PCM memories. But the trouble in these materials is toenhance the thermal stability so that they can retain the stored date in hightemperature also. Temperature dependence of chalcogenide material lacks thestored data at high temperature. Glass transition temperature is most widelyused parameter to estimate the thermal stability of chalcogenide alloys. Numberof relation has been reported to relate the glass transition temperature Tgand some other easily measurable quantities.
P. K. Thiravikraman1 reportedthe dependence of Tg on molar volume Vm of chalcogenidealloys, using the fact that there is huge decrease in the viscosity of glasses whenit undergoes a transition from supercooled liquid to glassy state . Thisdecrease in viscosity is due to structural changes happening at Tg .GST consists of 2D covalent structure and these structural units are heldtogether by van-der waal forces.
Since van der waal bonds are weaker than covalentbond. So it can be expected that van der waal bonds are breaking at transitiontemperature. The van der waal bond energy should be comparable with heat energykT at transition temperature.
Based on this assumption and solving the integralfor van der waal bond energy it can be concluded that transition temperature Tgvaries inversely with the fifth power of cube root of molar volume Vm. The selection of dopent for GST is so that the atomic mass of 4thelement should be less than the average atomic mass of GST. The estimation of glasstransition temperature has been reported by many people in a variety of ways.But all the reported correlation accounts the dependence of Tg onconnectedness, compactness and quality of connection in the network. L. Tichy and H. Ticha2 reported thedependence of mean bond energy E on glass transition temperature. In order toknow the glass transition temperature of materials it is also important tomeasure the connectedness and quality of connection in the network.
In past fewyears a lot of work has been done on conventional GST in doping a small amountof elements such as Ag, Ti, N, Al, O, Sn, Ni and zinc in Ge2Sb2Te5alloy in order to enhance the thermal stability for PCM applications. It hasbeen reported that relatively weak bond strength can enhance the crystallizationspeed e.g.
bond energy of Ge-Te bond is 397kJ/mol which is large as compared toCd-Te bond (100kJ/mol). Addition of Cadmium to GST increases the band gap anddecreases the molar volume. Due to lower electronegativity and approximatelysimilar size to average size of Ge, Sb and tellurium elements find it possiblealternative.
The thermal stability of GST can also beimproved by introducing Pr to GST. Lower electronegativity of Preseodymiumdecreases the mean bond energy of cress linked structure and increases thecrystallization speed. Due to higher coordination of Pr than mean coordinationof GST alloy increases the degree of crosslinking. Yu-Jen Hung et al 3reporter the experimental evidence of drastic increase in transitiontemperature upto 236?C with 8% Cerium doped GST alloy.