Photothermal Radiometry applied in nanoliter melted tellurium alloys

Abstract

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.