The sub-threshold electron transport properties of amorphous (a-) germanium telluride (GeTe) phase change material (PCM) ultra-thin films are investigated by using ab initio molecular dynamics, density function theory, and Green’s function simulations. The simulation results reproduce the trends in measured electron transport properties, e.g. current-voltage curve, intra-bandgap donor-like and acceptor-like defect states, and p-type conductivity. The underlying physical mechanism of electron transport in ultra-scaled a-PCM is unraveled. We find that, though the current-voltage curve of the ultra-scaled a-PCM resembles that of the bulk a-PCM, their physical origins are different. Unlike the electron transport in bulk a-PCM, which is governed by the Poole-Frenkel effect, the electron transport in ultra-scaled a-PCM is largely dominated by tunneling transport via intra-bandgap donor-like and acceptor-like defect states.

The electron transport properties of ultra-scaled amorphous phase change material (PCM) GeTe are studied using non-equilibrium Green’s function (NEGF). The inelastic electron-phonon scattering is included using Born approximation. It is shown that, in ultra-scaled PCM device with 6 nm channel length, less than 4% of the energy carried by the incident electrons from the source is transferred to the atomic lattice before reaching the drain, indicating that the electron transport is largely elastic. Our simulation results show that the inelastic electron-phonon scattering, which plays an important role to excite trapped electrons in bulk PCM devices, exerts very limited influence on the current density value and the shape of current-voltage curve of ultra-scaled PCM devices. The analysis reveals that the Poole-Frenkel law and the Ohm’s law, which are the governing physical mechanisms of the bulk PCM devices, cease to be valid in the ultra-scaled PCM devices.

A charge-injection GeTe/Sb2Te3 superlattice phase change memory (PCM or PRAM) has been developed as a candidate for a non-volatile memory that replaces NAND flash memory. It differs from PRAM with the conventional material of GeSbTe, and is therefore named “TRAM (topological switching random access memory)”. First principle calculations showed a resistance change in the GeTe/Sb2Te3 superlattice was enhanced by charge injection. The fabrication and analyses of a one-resistor TEG revealed that the superlattice structure was maintained after 1M endurance, which proved the occurrence of non-melting resistance change in TRAM. The reset current of TRAM was found to be less than 1/5 of that of conventional PRAM. Furthermore, TRAM enables a set -speed of 10 ns and reset -operation by DC-sweep to be achieved, which experimentally proved the atomic movement in TRAM can be enhanced by charge injection.

The crystallization properties of the phase change material (PCM) germanium telluride (GeTe) are investigated. It is shown that the critical nucleus radius of a crystalline cluster is smaller than 1.4nm when the annealing temperature is lower than 600K, indicating an extremely promising scaling scenario. It is revealed that the elastic energy, which is largely ignored in existing PCM crystallization studies, plays an important role in determining various crystallization properties and the ultimate scaling limit of the PCM. By omitting the influence of elastic energy, the critical formation energy (critical nuclei radius) will be underestimated by 41.7% (22.4%), and the nucleation rate will be overestimated by 74.2% when the annealing temperature is 600 K. The methodology proposed here is capable of quantitatively calculating the nucleation rate and growth speed of amorphous PCM from first principle calculations, which is relevant to computational design and optimization of PCM.