Alternative analytical descriptions of the temperature dependence of the energy gap in cadmium sulfide

Experimental data available from published literature for the temperature dependence of the energy gap in CdS (A‐gap) are fitted numerically on the basis of four different analytical models. In analogy to earlier applications to many other semiconductor materials, the conventional models due to Varshni (1967) and Viña et al. (1984) are found here also for CdS to provide some rough fits of experimental data. A high‐precision fit is shown to be given by a novel analytical expression of the form {article}{empty}{document}Eg (T) = Eg (0) - (a/2)(√(2)ᵖ + θ ᵖ - θ),{document} where α is the familiar (limiting) gap shrinkage coefficient and θ tbond; ħω/ k represents the effective phonon energy on temperature scale. The corresponding low‐temperature T p ‐power exponent p is found for CdS to be nearly equal to 2.5. Thus we are concerned with a fractional exponent whose magnitude is markedly larger than its conventional value of 2. The associated θ value of about 150 K suggests that the main contribution to the gap shrinkage effect in CdS should be due to acoustic phonons ranging energetically slightly below the middle of the total phonon energy spectrum.