Thermal properties of selected II-VI semiconductors determined by photopyroelectric calorimetry technique

The review presents the results of the investigations of the thermal properties of selected II-VI binary, ternary, and quaternary semiconductors. All tested crystals were grown from the melt using the high-pressure vertical Bridgman-Stockbarger method with different composition The main objective of the work was systematic examination of the thermal properties of the crystals using contact (PPE photopyroelectric) and non-contact (active thermography) measurement techniques. As a result of the studies, the values of the thermal diffusivity and the effusivity of all tested samples were obtained. For the selected series of crystals, their heat capacity was also determined. The thermal conductivity was calculated using simple relationships that combine all the thermal parameters. In this way, a complete thermal characterization of the crystals was carried out. For several ternary mixed crystals diagrams of the thermal conductivity versus composition were analyzed applying model for mixed semiconducting crystals given by Sadao Adachi. Thanks to that a contribution of the thermal resistivity arising from the lattice disorder to the total resistivity of the crystal has been determined. Although the PPE method itself is fairly simple, each material class requires a different approach. For samples with strongly different conductivity or thickness, different frequency ranges and a suitable detector should be used. This is not a simple task for the samples with unknown thermal properties. It is to mention that the PPE technique is a contact method and its main disadvantage in case of solid samples is to provide good thermal contact between the sample and the detector. In practice, it is realized by gluing a sample to the detector with a small amount of different types of liquids. It has been shown that the influence of the coupling layer can be minimized by appropriate modification of the experimental system and proper selection of the coupling fluid. In this way, the photopyroelectric method has been satisfactorily adapted for the thermal characterization of II-VI crystals.