Showing papers by "V. Damodara Das published in 1987"

Size and temperature effects on the Seebeck coefficient of thin bismuth films

Abstract: The electrical resistivity and the Seebeck coefficient of thermally evaporated thin bismuth films of thicknesses from 300 to 1900 A\r{} have been measured in the temperature range 300--470 K. The latter is negative and its magnitude is found to increase initially with increasing temperature, reach a maximum, and then decrease with a further rise in temperature. The temperature at which the Seebeck coefficient is maximum is found to be thickness dependent, decreasing with increasing thickness. The observed dependence is explained by considering that the Fermi energy is temperature dependent. Bismuth films show a negative temperature coefficient of resistivity. The thickness dependence of the electrical resistivity and the Seebeck coefficient of simultaneously prepared films are analyzed using the newer effective mean-free-path model. From the analysis, important material constants like the mean free path, the electron concentration, and the effective mass of electrons have been evaluated.

Electrical conductivity and thermoelectric power of amorphous Sb2Te3 thin films and amorphous-crystalline transition

Abstract: The results of electrical conductivity and thermoelectric studies on antimony telluride, a promising thermoelectric material, in the thin film state are reported. Films were vacuum-deposited on to clean glass substrates with thickness between 50 and 200 nm and studied in the temperature interval 300 to 470 K. On heating the as-grown films, there is a sharp fall both in the Seebeck coefficient and the electrical resistivity at around 340 to 370 K for all the films. This is attributed to an amorphous to crystalline transition, which is confirmed by X-ray diffractogram and electron diffraction patterns.[/p]

In situ electrical conductivity and amorphous‐crystalline transition in vacuum‐deposited amorphous thin films of a Se50Te50 alloy

Abstract: In situ electrical conductivity measurements have been made on vacuum‐deposited amorphous thin films of various thicknesses of a Se50Te50 alloy in the temperature range 300–430 K. From the electrical conductivity, x‐ray studies, and electron diffraction studies it is found that the as‐grown films are amorphous and undergo an irreversible amorphous‐crystalline transition on heating in situ. The amorphous‐crystalline transition takes place in the temperature range 320–360 K for the different films. However, no systematic variation of the transition temperature with the thickness of the films is observed. All the films except the thinnest ones have a sharp transition temperature. X‐ray and electron diffraction analyses show that the Se50Te50 films above 360 K are polycrystalline. Above 360 K the electrical conductivity of the polycrystalline Se50Te50 films varies as an exponential function of reciprocal temperature.

Effect of applied field and temperature on the aging of copper discontinuous films studied by the repeated deposition technique

Abstract: Presented are the results of investigations on the effect of an applied DC electric field on the post-deposition resistance changes of island copper films on glass. Based on the functional dependence of the film resistance on time, an agglomeration rate is defined with the theory of mobility coalescence being invoked to explain the resistance increase of the films after deposition. Repeated deposition was employed until the film resistance became steady, after which measurements of resistance versus temperature were made. The agglomeration rate for film of a particular resistance decreased as the number of deposition cycles was increased, due to formation of large, less mobile islands. For one of the higher-resistance films, the agglomeration rate showed a drastic fall for a particular value of deposition cycle number indicating that large-scale coalescence (LSC) had occurred. It was found that the presence of a field does not alter the post-deposition coalescence process in island copper films, possibly due to the relative immobility of the copper islands. The initial resistances of the film with and without the field were, however, vastly different. The agglomeration rates of films of different initial resistances showed an oscillatory behaviour corroborating earlier findings on copper films. The resistance-temperature curves showed a minimum below which the film showed a negative temperature coefficient of resistance (TCR) and a positive TCR above the temperature of transition. The transition temperature is found to shift to higher values as the films approach the discontinuous-semi-continuous structure transition. The resistance-temperature behaviour is explained if one considers that the negative TCR region is due to an increase in the number of thermally activated charged islands while the positive TCR region is due to enhanced mobility of the copper islands at elevated temperatures leading to coalescence and thus to an increase in the average inter-island spacing and the resistance of the film.

Ageing studies of discontinuous copper and silver thin films

Abstract: The results of experiments carried out on the post-deposition resistance changes in discontinuous films of copper and silver with and without overlayers of SiO and Al2O3 are presented. The changes in the sheet resistance of the films with time and pressure were studied for the above combinations. Mobility coalescence is assumed to be responsible for the resistance increase of an uncovered copper film of initial resistance 1.9 MΩ/□. On exposure to the atmosphere, it was found that an Ag/SiO combination of initial resistance of 0.1 MΩ/□ achieved stability in the sheet resistance much quicker than a Cu/Al2O3 combination of initial resistance 20 MΩ/□. The fall in resistance of the Cu/Al2O3 composite is attributed to the formation of Al2(OH)6 due to the interaction of Al2O3 with the water vapour in atmosphere. Copper films with and without overlayers of Al2O3 show an abrupt increase in the sheet resistance as a function of pressure at a pressure of about 5 × 10−2 torr with the maximum rate of change of resistance occurring at higher pressure for the higher resistance film. This indicates that the overlayer of Al2O3 is very porous in nature. Field effect studies were carried out on an uncovered copper film of initial resistance 10 MΩ/□ and the behaviour was found to be ohmic up to a field of 800 V cm−1.

In situ electrical conductivity and amorphous-crystalline transition in vacuum-deposited thin films of Se20Te80 alloy

Abstract: The electrical conductivity of thin films of Se80Te20 polycrystalline alloy vacuum-deposited at room temperature on glass substrates has been studied duringin situ heating and cooling cycles. From the electron diffraction of as-grown films it is seen that the studied films are amorphous at room temperature. The electrical conductivity and electron diffraction studies showed that the as-grown amorphous thin films undergo an amorphous-crystalline transition in the temperature range 340 to 360 K. Upon cooling, the films appear to undergo a crystalline crystalline transition around the same temperatures. There does not appear to be any dependence of the amorphous-crystalline transition temperature on the thickness of the films. However, high-resistance films (thinner films) have a well-defined transition temperature while the low-resistance films (thicker films) have a broader transition. The electrical conductivity of polycrystalline Se80Te20 films above 360 K appears to be an exponential function of reciprocal temperature.

Ageing and field effect studies on discontinuous silver films at near liquid nitrogen temperatures

Abstract: The post deposition resistance changes in discontinuous silver films deposited in a vacuum of 2 × 10−6 torr on glass substrates maintained at near liquid nitrogen temperatures have been studied. Reduced agglomeration rates in comparison with films studied at room temperature were obtained, supporting the thermally assisted mobility coalescence model explaining the post deposition resistance increase. The non-linearI-V characteristics of one of the films followed by observations of resistance changes before and after field effect measurements on the other films have been explained as arising due to field-induced structural changes. The investigations of the variation of film resistance with temperature revealed a transition temperature. A fall in resistance with increasing temperature below the transition temperature has been explained by an increase in the number of thermally charged islands. The increase in resistance with temperature above the transition temperature is due to an increase in the thermally assisted mobility coalescence.