Reviews on Hetero-Phase Structural Fluctuation in Semiconductor Melts

During phase transitions in various systems, when changes in observable quantities of the system occur, they are often accompanied by pre-transitional or pre-cursor phenomena and, sometimes, display behavioral anomalies on the thermophysical properties. The origin of the phenomena is the formation of nuclei, or molecular complexes, of different competing thermodynamic phases inside the host phase. The formations of these nuclei are called the hetero-phase fluctuations. As opposed to the equilibrium homo-phase fluctuations, which can be described by equilibrium thermodynamics or statistical mechanics, systems with hetero-phase fluctuations are in quasi-equilibrium, or local equilibrium. In those phase transitions that one or more first derivatives of the relevant thermodynamic potentials, as a function of their variables, exhibit discontinuities may be called first-order, or discontinuous transitions. Those transitions in which the first derivatives of the thermodynamic potentials at the transition points remain continuous while only higher-order derivatives become divergent or change discontinuously, may be termed continuous transitions. In a normal scientific sense, a transition should exist over a region, not at one point and the hetero-phase fluctuation around a phase-transition point can be considered as a phenomenon that smears a transition into a region. A great number of experiments describing the hetero-phase states during the nucleation process has been reported in various phase transitions, which included solid–liquid, liquid–vapor, glass-liquid, martensitic, ferroelectric, superconducting and magnetic transitions. As one type of structural phase transitions, there were also experimental studies of the semiconductor–metal transition on the Se–Te, Ga2Te3, In2Te3 and Al2Te3 liquid solutions as well as on the HgTe-based pseudo-binary melts by the measurements of density, viscosity, electrical conductivity, thermal conductivity, Hall coefficient and mobility, thermoelectric power, coefficient of volume expansion, isothermal compressibility, Knight shift, magnetic susceptibility, relaxation phenomenon and structure factor. The results have confirmed the proposed description of inhomogeneous hetero-phase that the liquids consist of two well-defined domains, which have the same composition but different structures and transport properties. Investigations of the thermophysical properties on various semiconducting compound melts, such as the electrical conductivity, magnetic susceptibility, viscosity, density, etc., have been conducted to understand and to develop the fundamentals of the growth processes. Some interesting experimental results have been observed for those systems with solid phase of narrow homogeneity ranges such as the Zn–Te, Cd–Te, Mg–Bi, Tl–Te, In–Te and Ga-Te systems. In other cases of self-seeded melt growth of II–VI compounds, namely, CdTe, CdZnTe and ZnSe, there were a large number of experimental facts showing the correlation between melt state and crystal quality. It is believed that the crystalline quality can be improved if the melt was either markedly superheated or long-time soaked before the growth is commenced. It was suggested that the thermal history of how the solid sample was solidified governs the structure of the molten state near its melting point. These results suggest that the liquid state and structure of these narrow homogeneity range II–VI semiconductors need an in-depth examination to enhance the fundamental knowledge of hetero-phase fluctuations phenomena in the melts so as to improve the melt growth processes of these semiconductors.