Justification

The recent Development on PCM
technology is due to the fast switching between amorphous and crystalline phases
of Ge2 Sb2Te5 (GST) chalcogenide materials. The
high operating speed and low power consumption makes them promising candidate
for next generation PCM memories. But the trouble in these materials is to
enhance the thermal stability so that they can retain the stored date in high
temperature also. Temperature dependence of chalcogenide material lacks the
stored data at high temperature. Glass transition temperature is most widely
used parameter to estimate the thermal stability of chalcogenide alloys. Number
of relation has been reported to relate the glass transition temperature Tg
and some other easily measurable quantities. P. K. Thiravikraman1 reported
the dependence of Tg on molar volume Vm of chalcogenide
alloys, using the fact that there is huge decrease in the viscosity of glasses when
it undergoes a transition from supercooled liquid to glassy state . This
decrease in viscosity is due to structural changes happening at Tg .
GST consists of 2D covalent structure and these structural units are held
together by van-der waal forces. Since van der waal bonds are weaker than covalent
bond. So it can be expected that van der waal bonds are breaking at transition
temperature. The van der waal bond energy should be comparable with heat energy
kT at transition temperature. Based on this assumption and solving the integral
for van der waal bond energy it can be concluded that transition temperature Tg
varies 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 4th
element should be less than the average atomic mass of GST.

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The estimation of glass
transition temperature has been reported by many people in a variety of ways.
But all the reported correlation accounts the dependence of Tg on
connectedness, compactness and quality of connection in the network.  L. Tichy and H. Ticha2 reported the
dependence of mean bond energy E on glass transition temperature. In order to
know the glass transition temperature of materials it is also important to
measure the connectedness and quality of connection in the network. In past few
years a lot of work has been done on conventional GST in doping a small amount
of elements such as Ag, Ti, N, Al, O, Sn, Ni and zinc in Ge2Sb2Te5
alloy in order to enhance the thermal stability for PCM applications. It has
been reported that relatively weak bond strength can enhance the crystallization
speed e.g. bond energy of Ge-Te bond is 397kJ/mol which is large as compared to
Cd-Te bond (100kJ/mol). Addition of Cadmium to GST increases the band gap and
decreases the molar volume. Due to lower electronegativity and approximately
similar size to average size of Ge, Sb and tellurium elements find it possible
alternative.

 The thermal stability of GST can also be
improved by introducing Pr to GST. Lower electronegativity of Preseodymium
decreases the mean bond energy of cress linked structure and increases the
crystallization speed. Due to higher coordination of Pr than mean coordination
of GST alloy increases the degree of crosslinking. Yu-Jen Hung et al 3
reporter the experimental evidence of drastic increase in transition
temperature upto 236?C with 8% Cerium doped GST alloy. 

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