Glass Transitions, Semiconductor-Metal Transitions, and Fragilities in Ge−V−Te ( V=As , Sb) Liquid Alloys: The Difference One Element Can Make

Abstract

Glass transition temperatures (T g ) and liquid fragilities are measured along a line of constant Ge content in the system Ge-As-Te, and contrasted with the lack of glass-forming ability in the twin system Ge-Sb-Te at the same Ge content. The one composition established as free of crystal contamination in the latter system shows a behavior opposite to that of more covalent system.Comparison of T g vs bond density in the three systems Ge-As-chalcogen differing in chalcogen i.e. S, Se, or Te, shows that as the chalcogen becomes more metallic, i.e. in the order S<Se<Te the bond density effect on T g becomes systematically weaker, with a crossover at <r> = 2.3.When the more metallic Sb replaces As at <r> greater than 2.3, incipient metallicity rather than directional bond covalency apparently gains control of the physics. This leads us to an examination of the electronic conductivity and, then, semiconductor-to-metal (SC-M) transitions, with their associated thermodynamic manifestations, in relevant liquid alloys. The thermodynamic components, as seen previously, control liquid fragility and cause fragile-tostrong transitions during cooling. We tentatively conclude that liquid state behavior in phase change materials (PCMs) is controlled by liquid-liquid (SC-M) transitions that have become submerged below the liquidus surface. In the case of the Ge-Te binary, a crude extrapolation to GeTe stoichiometry indicates that the SC-M transition lies about 20% below the melting point, suggesting a parallel with the intensely researched "hidden liquid-liquid (LL) transition", in supercooled water. In the water case, superfast crystallization initiates in the high fragility domain some 4% above the T LL which is located at ~15% below the (ambient pressure) melting point.

Liquid‐liquid phase transitions in glass‐forming systems and their implications for memory technology

“…Figure A shows the transition from semiconductor to metallic conductivity in Ge 15 Te 85 and Se 30 Te 70 melts. This behavior is representative of a large number of chalogenide melts as demonstrated long ago by Alekseev et al and recently discussed in relation to the M‐SC transition in PCMs by the present authors . The activation energy for conductivity E a is plotted in Figure B and exhibits a clear maximum for both melts.…”

“…On the other end, the high kinetic factor of fragile systems is disadvantageous in the glassy state as it may lead to crystallization of the amorphous bit at ambient temperature and loss of stored data. It was recently reported that this conundrum can be solved in PCMs such as AIST, Ge 15 Sb 85 or Ge 2 Sb 2 Te 5 where a sharp transition from fragile to strong occurs upon cooling, thereby providing the benefit of fast kinetics at high temperature and stabilization of the amorphous phase at low temperature . The structural origin of these L‐L transitions was characterized by employing femtosecond x‐ray diffraction to observe the change in local atomic environment before crystallization occurred on the nanosecond time scale.…”

“…In some cases the F‐S crossover can be extremely sharp, indeed discontinuous, as in the case of Si and models of water . Such anomalies have been linked to the presence of a liquid‐liquid phase transition which may be hidden below the melting point and can only be observed in the supercooled regime . The observation of such L‐L transitions can therefore be challenging as the liquid has a limited range of stability between crystallization at high temperature and vitrification at lower temperature .…”

“…While this type of L‐L transition has initially raised interest for its fundamental importance in understanding fluid thermodynamics, it has recently been suggested that it may play a key role in enabling advanced information storage technologies . Indeed, the phase memory alloys that are the central elements of ultrafast nonvolatile computer memories have been shown to exhibit L‐L transitions and the associated F‐S transitions .…”

“…These two opposite properties can only be achieved because the system undergoes a sharp L‐L (and F‐S) transition during cooling. In addition, this transition is associated with a metal‐to‐semiconductor (M‐SC) transition which is also essential to provide the contrast in electrical properties necessary to encode binary information in computer memories …”