Application of specific heat measurements to the study of structural changes and concentration fluctuations in the molten semiconductors

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Using the grant, we have renewed some of the electronic modules of the scanning adiabatic calorimeter installed in the laboratory about two decades ago. We use this calorimeter to measure the specific heat of massive specimen (~ 0.1 mole) above the liquidus temperature. To complement the above measurements, the specific heat in the super-cooled region below the melting temperature is measured with the modulation method using a small specimen (~ 0.005 mole) sealed in capillary molten quartz cell. By combining both data, we can obtain the specific heat of molten metals and alloys in the wide range of temperature up to 800℃.We measured the specific heat of Ge-Te alloys around the Te rich eutectic composition (15 at.% Te) that shows the most sharp transition among molten alloys undergoing continues structural change. In the meantime, we obtained the density by the high energy γ-ray attenuation method and measured the sound velocity. We could determine all the thermodynamic response functio … More ns characterizing the structural changes in the Ge-Te system, therefore. Main results are:1. the crossover temperature T* indicated by a peak in the specific heat as a function of temperature reached a maximum at the eutectic composition, Ge_<15>Te_<85>.2. the width of the specific heat becomes narrowest at the same composition.From these results we showed that the structural change in the eutectic Ge-Te alloy does not induce the concentration fluctuations and therefore it is a kind of isomorphic type. Further we proposed a new idea that the width of the specific heat can be interpreted as fluctuations in the crossover temperature T*. (J. Non-Cryst. Solids 312-314 (2002) 212)To investigate the structural evolution in the atomic scale, I analyzed the neutron diffraction data together with the above-mentioned thermodynamic data in collaboration with Drs. C. Bergman (L2MP CNRS, Marseille) and C. Bichara (CRM2C CNRS, Marseille). It is shown that the structural change undergone by the liquid mainly consists in a modification of the first-neighbor shell of the Ge atoms and the Peierls distortion mechanism plays an important role in the structural change. (Phys. Rev. B. 67 (2003) 104202-104202-5)) Less