Vincent Schick

Thermal characterization of the SiO2-Ge2Sb2Te5 interface from room temperature up to 400°C

The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, was measured using a photothermal radiometry experiment. The two phase changes in the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the face centered cubic (fcc) crystalline state at 130°C and then to the hexagonal crystalline phase (hcp) at 310°C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.

Thermal resistance at Al-Ge2Sb2Te5 interface

Ge2Sb2Te5 is a phase change material candidate to constitute the active element of future nonvolatile memory devices. The evolution of the thermal resistance at the interface between an aluminum thin layer and Ge2Sb2Te5 is studied using the time resolved pump probe technique from room temperature to 400 °C. The thermal resistance is influenced by the amorphous to crystalline phase change occurring in Ge2Sb2Te5. The decrease in the thermal resistance from the amorphous to the crystalline phase is well explained by the diffuse mismatch model asymptotic form for high temperature. The large increase of the interface thermal resistance between fcc and hcp crystalline states is explained by the fast and significant grain growth and species inter-diffusion during this second phase change. This leads to the formation of an interfacial layer whose chemical and mechanical intrinsic properties have been measured in order to model the thermal resistance in the hcp state.

Photothermal Radiometry applied in nanoliter melted tellurium alloys

We report on thermal measurements of molten materials at the nanoliter scale. An experimental setup of Photothermal Radiometry (PTR), formerly developed for solid state measurements, has been adapted for this purpose. The material is a chalcogenide glass-type tellurium alloy, Ge2Sb2Te5 (GST), amorphous at room temperature, and that becomes crystalline at 130°C. The same material, brought to its melting temperature Tm, about 600°C, becomes amorphous after rapid cooling. Since the liquid is the precursor phase of the amorphous state, its characterization is of paramount importance. Thin film PTR characterization was first performed in solid state by measuring the GST thermal conductivity evolution during the structural phase changing, from the amorphous phase to its crystalline phase. In order to characterize the melt at high temperature, a lightly Ge-doped Te alloy sample was secondly fabricated. This latter tellurium alloy melts at a lower temperature, (Tm~450°C, as for pure tellurium) than GST. A random lattice of hemispherical tellurium structures, 500 nm in radius, was grown by MOCVD technique on a thermally oxidized silicon substrate. The hemispheres were then embedded in a 500 nm SiO2 protecting layer in order to prevent evaporation during the melting. A 30 nm cap layer of Pt was then evaporated on the SiO2 as thermal transducer for the laser beam. Measurements have been performed from room temperature up to 650°C. SEM and XRD measurements performed after annealing, have shown that these samples withstood the thermal stress up to 300°C. At temperatures above 380°C some Te is still present in the hemispherical structures, but a part of it has reacted with Pt to form PtTe by migration through the SiO2 matrix. Experiments carried out at temperatures below 300°C have shown an anomalous behaviour of the thermal contact resistance between the tellurium alloy and the oxide interface.

Temperature-dependent thermal characterization of Ge2Sb2Te5 and related interfaces by the photothermal radiometry technique

The thermal conductivity of Ge2Sb2Te5 (GST) layers, as well as the thermal boundary resistance at the interface between the GST and amorphous SiO2, were measured using a PhotoThermal Radiometry experiment. The two phase-changes of the Ge2Sb2Te5 were retrieved, starting from the amorphous and sweeping to the fcc crystalline state at 130 °C and then to the hcp crystalline state at 310 °C. The thermal conductivity resulted to be constant in the amorphous phase, whereas it evolved between the two crystalline states. The thermal boundary resistance at the GST-SiO2 interface was estimated to be higher for the hcp phase than for the amorphous and fcc ones.

Identification of the temperature-dependent thermal boundary resistance at a metal-phase change material

The time-resolved pump-probe technique has been implemented to study the temperature-dependent thermal boundary resistance (TBR) at Al–Ge2Sb2Te5 (GST) interface, which is between metal and phase change semiconducting material. This study is made possible due to the accurate knowledge of the thermal properties (thermal conductivity and specific heat in the 20–400°C temperature range) of the GST layer. The measure of the acoustic oscillation damping permits characterization of the adhesion at the Al–GST interface, a quantity that can be related to the temperature-dependent TBR in the 25–300°C range.