We present thermodynamic crystallization and melting models and calculate phase change velocities in Ge2Sb2Te5 based on kinetic and thermodynamic parameters. The calculated phase change velocities are strong functions of grain size, with smaller grains beginning to melt at lower temperatures. Phase change velocities are continuous functions of temperature which determine crystallization and melting rates. Hence, set and reset times as well as power and peak current requirements for switching are strong functions of grain size. Grain boundary amorphization can lead to a sufficient increase in cell resistance for small-grain phase change materials even if the whole active region does not completely amorphize. Isolated grains left in the amorphous regions, the quenched-in nuclei, facilitate templated crystal growth and significantly reduce set times for phase change memory cells. We demonstrate the significance of heterogeneous melting through 2-D electrothermal simulations coupled with a dynamic materials phase change model. Our results show reset and set times on the order of ~1 ns for 30 nm wide confined nanocrystalline (7.5 nm -25 nm radius crystals) phase change memory cells.Solids tend to melt heterogeneously: the liquid phase initially forms at high energy sites such as grain boundaries and material interfaces. While many materials heat ~20% above their melting temperature (Tmelt) before the liquid phase forms within the bulk solid, heterogeneous melting may occur below Tmelt 1 . In this manuscript, we consider the impacts of heterogeneous melting on phase change memory (PCM). PCM is a non-volatile memory technology which stores information as the low resistivity crystalline or high resistivity amorphous phase of a material (Fig. 1). PCM retention, endurance, and speed depend on the physics underlying crystallization and melting. We model temperature and grain size dependent phase change velocities in Ge2Sb2Te5 (GST), a common phase change material, based on kinetic and thermodynamic parameters. We incorporate heterogeneous melting into a finite element phase change model coupled with electrothermal physics 2-7 and show that it can account for the experimentally demonstrated PCM performance improvement with decreasing grain size 8,9 .Tmelt is the temperature at which the Gibbs free energy difference between bulk liquid and crystalline phases (Δglc) is zero. However, melting becomes thermodynamically favorable below Tmelt at crystal interfaces. The Gibbs free energy of a spherical crystal surrounded by liquid (ΔGcrys) is calculated by classical nucleation theory aswhere r is the crystal radius and γlc is the energy penalty at a liquid-crystal interface (Fig. 2a). (1) has extrema at r = 0 and r = rc, the critical radius:(2) Crystals with r < rc are subcritical and can reduce ΔGcrys by shrinking, i.e. melting. rc increases with T: Δglc decreases with increasing T, crossing 0 at Tmelt. γlc is difficult to measure in GST and often used as a fitting parameter; however, γlc increases with T in metals as well as in t...

No abstract

No abstract

Herein, logic function implementations are computationally demonstrated using lateral and vertical multicontact phase‐change devices integrated with complementary metal–oxide–semiconductor (CMOS) circuitry, which use thermal cross‐talk as a coupling mechanism to implement logic functions at smaller CMOS footprints. Thermal cross‐talk during the write operations is utilized to recrystallize the previously amorphized regions to achieve toggle operations. Amorphized regions formed between different pairs of write contacts are utilized to isolate read contacts. Typical expected reduction in CMOS footprint is ≈50% using the described approach for toggle‐multiplexing, JK‐multiplexing, and 2 × 2 routing. The switching speeds of the phase‐change devices are in the order of nanoseconds and are inherently nonvolatile. An electrothermal modeling framework with dynamic materials models is used to capture the device dynamics, and current and voltage requirements.

No abstract