Andrzej Kusiak

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.

Numerical Inversion of Laplace Transform for Time Resolved Thermal Characterization Experiment

The aim of this technical brief is to test numerical inverse Laplace transform methods with application in the framework of the thermal characterization experiment. The objective is to find the most reliable technique in the case of a time resolved experiment based on a thermal disturbance in the form of a periodic function or a distribution. The reliability of methods based on the Fourier series methods is demonstrated.

Thermal conductivity measurement of a Sb2Te3 phase change nanowire

The c-axis thermal conductivity of a Sb2Te3 nanowire is measured using the scanning thermal microscopy technique within the 3ω mode. The contact parameters, in terms of boundary contact resistance and contact area radius, are measured in specific configurations, and the values found are assumed not to vary within the nanowire case. The method does not require handling or suspending the nanowire. The measured thermal conductivity at room temperature is found to be in a good agreement with that of the bulk, since the nanowire characteristic dimension in the diffusion direction is larger than the phonon mean free path.

Effect of a thin Ti interfacial layer on the thermal resistance of Ge2Sb2Te5-TiN stack

We study the dependence of the thermal resistance of TiN/Ge2Sb2Te5 stacks on Si in the presence or not of a thin Ti interfacial layer. While for TiN/Ge2Sb2Te5 almost ideal thermal properties of the interfaces are found, a different behaviour is measured for TiN/Ti/Ge2Sb2Te5. After exposure to temperatures up to 440 °C, the thermal resistance results to be lower than expected despite both the formation of the TiTe2 phase, the depletion of Te inside Ge2Sb2Te5, and the non complete development of the hexagonal structure. Those observations have been also validated on the SiO2/Ge2Sb2Te5 stack with and without Ti interfacial layer.

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.

Carbon epoxy composites thermal conductivity at 77 K and 300 K

The in-plane and in-depth thermal conductivities of epoxy-carbon fiber composites have been measured at 77 K and 300 K. The experimental technique rests on the hot disk method. The two thermal conductivities as well as the thermal contact resistance between the probe and the composite materials are estimated from measurement data and an analytical heat transfer model within the experimental configuration. The results obtained at 77 K explained well the ignition test results performed on the composites at 77 K with regards to liquid oxygen storage.

Thermal Diffusivity Estimation in a Picosecond Photoreflectance Experiment

The aim of this work is to provide an analytical expression for the thermal diffusivity of a material in the configuration of the picosecond photoreflectance experiment. It is shown that the thermal diffusivity can be estimated from the absorption depth of the pump beam together with the probe beam as well as the time when the two asymptotic behaviors of the impulse response cross. Thereby, it is not required to measure absolute values of incident heat flux and average temperature on the aiming area.

Thermal conductivity of carbon doped GeTe thin films in amorphous and crystalline state measured by modulated photo thermal radiometry

The thermal conductivity and thermal boundary resistance of GeTe and carbon doped GeTe thin films, designed for phase change memory (PCM) applications, were investigated by modulated photo thermal radiometry. It was found that C doping has no significant effect on the thermal conductivity of these chalcogenides in amorphous state. The thermal boundary resistance between the amorphous films and SiO2 substrate is also not affected by C doping. The films were then crystallized by an annealing at 450°C as confirmed by optical reflectivity analysis. The thermal conductivity of non-doped GeTe significantly increases after crystallization annealing. But, surprisingly the thermal conductivity of the crystallized C doped GeTe was found to be similar from that of the amorphous state and independent of C concentration. As for the amorphous phase, C doping does not affect the thermal boundary resistance between the crystalline GeTe films and SiO2 substrate. This behaviour is discussed thanks to XRD and FTIR analysis. In particular, XRD shows a decrease of crystalline grain size in crystalline films as C concentration is increased. FTIR analysis of the film before and after crystallization evidenced that this evolution could be attributed to the disappearing of Ge-C bonds and migration of C atoms out of the GeTe phase upon crystallization, limiting then the growth of GeTe crystallites in C-doped films.

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.

Thermal investigation of a phase change memory device at the nanoscale

New technologies in non-volatile memories have been developed for several years based on phase-change alloys out of which, the most known is the Ge2Sb2Te5. The thermal investigation of the microelectronics device at cell scale is relevant since the heat transfer is the main limiting aspect for the optimal functioning of the device. More particularly, the thermal resistance at interfaces between the constitutive materials is of primary importance. We implemented a scanning thermal microscopy experiment in the 3D mode that allowed to fully characterizing the thermal properties of the cell at the nanoscale. The results lead to understand the 3D heat diffusion in the cell and more particularly the role of the vertical and horizontal interfaces.