Srinivasa Rao Gunti

Te-rich Si15Te85-xGex (1 ≤ x ≤ 11) glasses are found to exhibit an anomalous phase separations with germanium composition. The structural transformation of o-GeTe crystalline phase from o-GeTe with a = 11.76 Å, b = 16.59 Å, c = 17.44 Å, to high pressure o-GeTe with a new reduced lattice parameters a = 10.95 Å, b = 4.03 Å, c = 4.45 Å, is observed at Tc3 in the composition range 6 ≤ x ≤ 11. Raman studies support the possible existence of high pressure o-GeTe phase which is observed in X-ray diffraction experiments.

We have investigated thermal properties of bulk Si15Te85-xAgx (4 ≤ x ≤ 20) glasses in detail, through alternating differential scanning calorimetry experiments. The composition dependence of thermal parameters reveal the signatures of rigidity percolation and chemical threshold at compositions x = 12 and x = 19, respectively. The stability and glass forming ability of these glasses have also been determined using the data obtained from different thermodynamic quantities and it is found that the Si15Te85-xAgx glasses in the region 12 ≤ x ≤ 17 are more stable when compared to other glasses of the same series. Further, the blueshift observed in Raman spectroscopy investigations, in the composition range 12 ≤ x ≤ 13, support the occurrence of stiffness threshold in this composition range. All Si15Te85-xAgx (4 ≤ x ≤ 20) glasses are found to exhibit memory type switching (for sample thickness 0.25 mm) in the input current range 3–9 mA. The effect of rigidity percolation and chemical thresholds on switching voltages are observed at x = 12 and 19, respectively.

An interesting topic for quite some time is an intermediate phase observed in chalcogenide glasses, which is related to network connectivity and rigidity. This phenomenon is exhibited by Si-Te-In glasses also. It has been addressed here by carrying out detailed thermal investigations by using alternating differential scanning calorimetry technique. An effort has also been made to determine the stability of these glasses using the data obtained from different thermodynamic quantities and crystallization kinetics of these glasses. Electrical switching behavior by recording I-V characteristics and variation of switching voltages with indium composition have been studied in these glasses for phase change memory applications.

A new phase-change memory material, in bulk, has been prepared by melt-quenching technique, which has a better glass forming ability. This sample is set and resettable relatively easily for several cycles at 2mA SET and RESET input currents, and is likely to be a suitable material for phase-change memory applications. Raman scattering studies have been undertaken during the SET and RESET operations to elucidate the local structural transformations that occur during these operations.

Alternating Differential Scanning Calorimetric (ADSC) and electrical switching studies have been undertaken on Ge20Se80−xBix glasses (1 ≤ x ≤ 13), to understand the effect of topological thresholds on thermal properties and electrical switching behavior.

It is found that the compositional dependence of glass transition temperature (Tg), crystallization temperature (Tc1) and thermal stability (∆T) of Ge20Se80−xBix glasses show anomalies at a composition x = 5, the rigidity percolation/stiffness threshold of the system. Further, unusual variations are also observed in different thermal properties, such as Tg, Tc1, ∆T, ∆Cp and ∆HNR, at the composition x = 10, which indicates the occurrence of chemical threshold in these glasses at this composition.

Electrical switching studies indicate that Ge20Se80−xBix glasses with 5 ≤ x ≤ 11 exhibit threshold switching behavior and those with x = 12 and 13 show memory switching. A sharp decrease has been noticed in the switching voltages with bismuth concentration, which is due to the more metallic nature of bismuth and the presence of Bi+ ions. Further, a saturation is seen in the decrease in VT around x = 6, which is related to bismuth phase percolation at higher concentrations of Bi.