V. Damodara Das

Optical and electrical investigations of indium oxide thin films prepared by thermal oxidation of indium thin films

Indium oxide thin films have been prepared by thermal oxidation of vacuum‐deposited indium thin films in air in an open furnace at about 600 K. These indium oxide thin films prepared by thermal oxidation have been examined for optical transparency by measuring their optical absorbance as a function of wavelength. From the optical absorption data, optical band gap and the nature of the forbidden energy gap in the indium oxide thin films have been determined. Electrical conductivity measurements have also been carried out on the above oxide films as a function of temperature during heating and cooling cycles in vacuum. It is found that after the first heating, electrical conductivity increases to a significant extent due to removal of point defect clusters due to annealing which contribute to both carrier generation and scattering. From the thermoelectric power measurements carried out, it has been concluded that electrons are the majority carriers in these indium oxide thin films.

EFFECT OF SURFACE TREATMENT ON AN N-CDSE0.6TE0.4 THIN-FILM PHOTOANODE/POLYSULPHIDE ELECTROLYTE SOLAR CELL

Polycrystalline thin films of n-CdSe0.6Te0.4 were deposited in a vacuum of 5×10−5 Torr by thermal flash evaporation with a deposition rate of 20±1 A/s on indium oxide coated glass plates [σ=1.25×104 (Ω cm)−1] held at 200 °C. Application of surface treatment techniques such as annealing and photoelectrochemical etching on the films revealed that the films exhibit photoelectrochemical behavior with increased conversion efficiency and stability after treatment. Gartner’s model [Phys. Rev. 116, 84 (1954)] was used in the calculation of the solid state parameters of the films like the carrier concentration ND and minority carrier diffusion length Lp for different surface treatments. Chemical etching improves the efficiency and fill factor from 1.53% and 40% to 2.72% and 50% respectively, whereas photoelectrochemical etching improves further the efficiency to 3.83% and fill factor to 59% and the stability of the photoelectrode in the polysulphide electrolyte.

A study of band-bending and barrier height variation in thin film n-CdSe0.5Te0.5 photoanode/polysulphide junctions

Polycrystalline thin films of n-CdSe0.5Te0.5 were deposited in a vacuum of 5 × 10−5 torr by thermal flash evaporation with a deposition rate of 20 ± 1 As−1 on indium oxide coated glass plates [σ = 1.25 × 104 (Ωcm)−1] held at a temperature of 473 K. The change in the pH value of the polysulphide electrolyte from 12.5 to 8.0 leads to a fall in the extent of band bending from 680 meV to 420 meV and decrease in the barrier height from 690 meV to 430 meV. The barrier heights as calculated from Mott-Schottky plots and current-voltage plots are compared. From the reverse saturation current (I0) variation with temperature, it is found that the barrier height decreases from 690 meV to 430 meV with decrease in pH of the electrolyte from 12.5 to 8.0. The variation of diode ideality factor n, with temperature was also studied. It was found that the diode ideality factor n, decreases with increase of temperature, viz. from 3.00 to 2.00 with rise in temperature from 300 K to 333 K. This is due to the additional injection of electrons from the bulk into the junction region because of increase in temperature.

Thermoelectric power and electrical resistivity of crystalline antimony telluride (Sb2Te3) thin films: temperature and size effects

Crystalline Sb2Te3 thin films of different thicknesses have been prepared by subsequent annealing (at 500 K) of vacuum deposited, as‐grown, amorphous thin films of Sb2Te3 prepared on glass substrates at room temperature. Thermoelectric power and electrical resistivity of these annealed (crystalline) films have been determined as a function of temperature. The size dependence of thermoelectric power and electrical resistivity have been analyzed by the effective mean free path model of size effect. It is found that both the thermoelectric power and the electrical resistivity are linear functions of the reciprocal of thickness of the films. The data from the analyses of thermoelectric power and electrical resistivity have been combined to evaluate important material parameters such as carrier concentration, their mean free path, Fermi energy, and effective mass. The values of some of these are compared with the previous available values from literature.

Thickness dependence of the phase transition temperature in Ag2Se thin films

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.

Size and temperature effects on thermoelectric power of β‐tin thin films

Tin thin films of thicknesses in the range 500–7000 A have been prepared by vacuum deposition at room temperature at a pressure of 5×10−5 Torr on glass substrates. Thermal electromotive forces (emfs) of these films have been measured after aging as a function of temperature difference. It is found that the thermoelectric power of the films is independent of temperature in the range studied (300–425 °K). It is also found that the thermoelectric power of the films obeys the inverse thickness dependence predicted by size effect theories. The electronic mean free path is evaluated to be 530 A.

Colour centre studies in MoO3 films

Abstract The growth of defects in vacuum-evaporated MoO3 films has been studied by optical absorption in the present work. The as-grown films were subjected to different kinds of treatment such as thermal annealing, UV and X-ray irradiation, and their optical absorption spectra were recorded between 300 and 1500 nm at room temperature as well as at liquid N2 temperature The films were found to be amorphous from X-ray and electron-microscopic studies. From these studies it is observed that in addition to point defects, defect aggregates similar to the colloidal centres in silver and alkali halides, are present in these films.

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

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]

Annealing and thickness effects on the electrical resistance of vacuum-deposited tin antimonide alloy films

Tin antimonide alloy films, vacuum-deposited at room temperature onto glass substrates, were heated to a maximum temperature of about 300 °C and the changes in their electrical resistance with temperature were recorded. The initial lattice distortion energy spectra of the films have been determined from the resistance-temperature data. It is found that the F0(E)max and Emax values vary from 200×10-4 to 700×10-4ohms;cm/eV and from 1.55 to 1.74 eV respectively and that they depend upon the thickness of the film. The resistivities of the films due to thermal vibrations alone have been calculated and it is found that the calculated resistivity values for films of different thicknesses are in good agreement with the size effect theory.

Electrical conduction studies on thin films

thin films of different thicknesses were deposited by the flash evaporation method onto cleaned glass plates held at room temperature. Structural characterization was carried out using x-ray diffraction and transmission electron microscopy which revealed that the films are polycrystalline and the grain size increases with increasing thickness. Electrical resistivity was measured in the temperature range 300 - 450 K during two cycles of heating and cooling. During the first heating, irreversible behaviour of conductivity has been observed. Semiconductor-like behaviour has been observed in the annealed films and also during the first cooling and subsequent heating - cooling cycles. The activation energy for conduction (in annealed films) is found to be thickness dependent and this can be explained with the help of the grain-boundary trapping model. The thickness dependence of electrical resistivity (in annealed films) has been analysed using the effective mean free path model. From the analysis, important material constants like the mean free path and the electron concentration have been evaluated.