Phase-change materials show a large resistance contrast between the crystalline and the amorphous phase. This property is used by phase-change memory to store information in a non-volatile manner. Switching between the amorphous and the crystalline state is achieved via Joule heating by applying an electrical pulse. To reach the amorphous phase (RESET procedure, pulse duration several ns), the material is heated above the melting temperature and then quenched into the amorphous phase leading to the high-resistance state. To set the low-resistance, crystalline state (SET procedure, pulse duration ~50-500 ns) the material is heated up to the crystallization temperature followed by a comparatively slow cooling.
In a conventional single-level cell (SLC), one bit (1 and 0) is stored by a fully amorphous, high-resistance or fully crystalline, low-resistance portion of the cell. To reach higher storage densities, multiple levels of resistance have to be stored in one single cell (MLC = Multilevel cell) by varying the ratio between amorphous and crystalline material in the cell. MLC is hindered by the so-called resistance drift, which appears as a slow but steady resistance increase in the amorphous phase.
The goal of DIASPORA is to gain a better understanding of the underlying physics of this resistance drift.
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A comprehensive thermoelectric model was developed to capture the characteristics of phase change memory devices. A remarkable achievement was the satisfying matching of simulation and experimental data for doped-Ge2Sb2Te5 memory cells, validating the accuracy of the proposed model. Another key insight was the significant role played by the various thermo-electric components in addition to Joule heating. The reason being the large temperature gradients created in the nanoscale devices. The study was presented as a talk at the SISPAD conference in Washington DC, USA in September 2015.
With regards to electrical transport, we thoroughly investigated the low field regime of the current-voltage characteristics in amorphous phase change materials. We developed a unified model based on multiple-trapping transport together with 3D Poole–Frenkel emission from a two-center Coulomb potential to describe the so-called sub-threshold transport regime. This result was published in the New Journal of Physics. We also discovered a new conduction regime at higher fields and this study was reported in the Journal of Applied Physics. The temporal evolution of electrical transport on a micro-second time scale was also studied employing nanoscale line-cells fabricated at RWTH. The results were presented at E\PCOS 2014 in Marseille.
To gain insights into the nature of the DoS for the various phase change materials, dark- and photoconductivity measurements were conducted over a wide range of temperatures and light flux intensities. These measurements together with simulations provided strong indications for the DoS of AIST being substantially different from the other phase change materials. These results were presented at the MRS Spring Meeting 2015 in San Francisco. To study the impact of structural relaxation on the DoS and in particular the bandgap, infrared spectra were measured on amorphous thin films of the three different phase change materials (GeTe, GST, AIST). We found a widening of the bandgap upon annealing accompanied by a decrease of the optical dielectric constant epsilon infinity for all three materials. Quantitative comparison with existing experimental data from the RWTH work group revealed that the temporal evolution of bandgap and activation energy for electrical conductivity can be decoupled. This phenomenon regarding the link between the DoS and electrical transport demonstrates a possibility to identify new phase change materials with reduced resistance drift. The study was published in Nature Scientific Reports and was presented as oral contribution to the E\PCOS 2015 conference in September in Amsterdam.
High quality modulated photo-conductivity (MPC) measurements on microscale GeSbTe and AgInSbTe devices were performed, including an advanced measurement methodology with varying light fluxes. The in-depth analysis of the measurement results for the Gaussian defect in GeSbTe compare well to results found in literature. However, while previous studies consider the capture coefficients of the valence bandtail states in amorphous PCM as constant, our results clearly indicate that these states in GeSbTe and AgInSbTe exhibit non-constant capture characteristics. This result is a striking input for future efforts to understand the link between the density of states and electrical conduction, since the latter one is naturally governed by the trap and release processes of charge carriers.
Ab Initio Molecular Dynamics (AIMD) simulations were performed to study the effect of rapidly quenching molten phase-change materials to lower temperatures. The observation of the following crystallization process under varying quenching conditions resulted in the central outcome the research: the dependence of the stability of an amorphous state on holding temperature, quenching rate and density or stress respectively. Those simulations were accompanied by a thorough series of melt-quenching experiments on according nano-scale devices, which show the potential of a class of materials that has so far not been seriously considered for applications.
Over the four years, we have made significant steps towards a deep understanding of electrical transport and resistance drift in amorphous phase change materials. Our work is expected to have a significant impact on MLC PCM technology and potentially even other emerging applications of phase change memory devices such as non-von Neumann computing.