We present experimental results of relaxation oscillations based on the TaO x threshold switching devices as a function of voltage, load resistance, and the parallel capacitance. Of particular interest are the dynamics of transitions between ON and OFF states of the device which impose an upper limit of the oscillation frequency. The dynamics have been captured by the finite element electrothermal model using only electrical conductivity vs temperature and thermally activated conductivities as input data. The model reproduced current and voltage waveforms and allowed for following the changes in current density and temperature distributions within the device during oscillations. Both undergo significant breathing-mode-type changes in each cycle as the current spontaneously constricts during the capacitance discharge/heating stage. The model points out the possible approaches toward achieving higher oscillation frequencies.

Phase change memory devices are a leading candidate for non-volatile memory arrays, as artificial synapses in neuromorphic circuits, and for in-memory computing. These applications are hampered by a large cycle-to-cycle variation of resistance that is likely associated with the crystallization mechanism of phase change materials. This work presents a direct observation by transmission electron microscopy of the crystallization process during the SET operation of the GeTe 100 nm mushroom cell. The process proceeded purely by growth from the periphery of the amorphous dome without nucleation. It is invariably associated with partial melting of the GeTe layer. The growth-dominated SET operation can completely crystallize the GeTe functional layer only by a gradual sweep down of the voltage across the device. The experimental results were correlated with a finite element model of heat and charge flow and the crystallization dynamics.

The phase change memory devices are used for the application in neuromorphic computing as artificial synapses. However, the cycle-to-cycle variability, likely associated with the inherently stochastic nature of the nucleation...

The density, crystallinity, and microstructure of reactively sputtered amorphous Ge2Sb2Te5 thin films have been assessed as a function of deposition temperature. The continuous density increase was observed with increasing substrate temperature between room temperature and 200 °C. The films deposited at room temperature are amorphous and exhibit a columnar structure with a lateral size of cells in the 10–15 nm range. Cells consist of high-density interior with boundaries with the density lower by ∼9% due to incorporation of pores. The pores and the columnar microstructure can be eliminated by deposition at 80 °C while still preserving the amorphous phase. The density of pore- and stress-free amorphous Ge2Sb2Te5 is 6.16 g/cm3 and is only 1.5% lower than the crystalline Ge2Sb2Te5 with NaCl structure.