Showing papers by "V. Damodara Das published in 1990"

Thickness dependence of the phase transition temperature in Ag2Se thin films

Abstract: Thin films of silver selenide (Ag2Se) between thicknesses of about 700 and 2200 A have been prepared on glass substrates at room temperature in a vacuum of 5×10−5 Torr. After vacuum annealing the films (at about 373 K for 3 h) electrical resistivity measurements on these films have been carried out in vacuum. From the increase in the rate of decrease of resistance with temperature, the phase transition temperatures (orthorhombic to body‐centered cubic) of the different films have been located. It is found that the phase transition temperature of the thin films is a function of thickness increasing with a decrease in the thickness. This observation has been explained by a recently developed theory [V. Damodara Das and D. Karunakaran, J. Phys. Chem Solids 46, 551 (1985)] of phase transitions in thin films modified further. Also, an order‐of‐magnitude value of the difference in the function of specific surface and interfacial energies of the two phases has also been determined using the theory.

Thermoelectric power of annealed β-Ag2Se alloy thin films: temperature and size effects ― possibility of a new (β2) phase at low temperatures

Abstract: Thermoelectric power of annealed β‐Ag2Se thin films of different thicknesses has been measured both while heating and cooling by the integral method. It is found that it remains practically constant (in β‐Ag2Se phase) during heating while it is a function of temperature while cooling. The thermoelectric power in both heating and cooling cycles is a function of inverse thickness of the films. The difference in behavior between Ag2Se films during heating and cooling is attributed to the possible transformation to monoclinic phase during cooling from the original orthorhombic phase during heating. The inverse thickness dependence has been explained by the size effect theories. Important material parameters like carrier concentration, Fermi energy, effective mass of carriers, and energy dependence of the mean free path have been evaluated for the β‐Ag2Se (orthorhombic) phase.

A two-layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconductor thin films and application of the model to PbTe thin films

Abstract: A two‐layer model to explain the thickness dependence of conductivity and thermoelectric power of semiconducting thin films has been developed assuming that the film is a parallel combination of resistances of the three layers: The first is the interior ‘‘grain‐boundary’’ layer, and the other two, outer layers on opposite sides, whose conductivities are altered by the band bending (and is also affected by surface‐gas interactions). The equations obtained in this model lead to an inverse thickness dependence of both the conductivity and thermoelectric power of thin films. The model is applied to analyze the conductivity and thermoelectric variation with thickness of PbTe thin films and the parameters Ug, the energy dependence of the ‘‘grain‐boundary’’ mean free path lg, and σg the surface conductivity, have been evaluated.