Date of Award

1996

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Science, Technology and Engineering

First Advisor

Dr Steven Hinckley

Abstract

Polycrystalline thin films of undoped CdS and In doped CdS (i.e. CdS:In) have been deposited by chemical bath on glass slides and high purity silicon wafers. The effect of different processing conditions (e.g. substrate temperature, doping and air annealing) on film growth, optical transmittance and electrical properties were correlated with the film microstrucutre. The mechanism of chemical bath deposition of CdS thin films from the ammonia-thiourea system is studied. The influence of reaction parameters (i.e. concentration of reactants and pH) on film growth were determined and modeled. Thin film growth is thermally activated with an energy ≈ 5 x 1023 eV·mol1 where it initiates a sequential reaction mechanism. A kinetic study is presented resulting in the formulation of a growth rate formula. As deposited CdS films exhibited high resistivities typically ≈ 105 - 016 Ω·cm that decreased with increasing [Cd] and [TU] concentrations. Optical band gap energies of the mixture hexagonal and cubic structures were ≈ 2.46eV. The band energies were found to be proportional to [Cd] concentrations and inversely proportional to [TU] concentrations. CdS:ln films were formed by in-situ chemical doping with In ions. The resulting films revealed a more organized crystal structure than the undoped films. A comparison of the optical transmittance spectra for undoped and doped films indicated improved film crystallinity and a marginal impurity absorption band at In dopant concentrations <1019 atoms·cm -3. Then-type conductivities were typically ~0.1 Ω-1·cm-1 (as deposited) and ~10Ω-1·cm-1 (air annealed at 200'C) band gaps differ in the range of2.46 to 2.37 eV. Films annealed in air at < 200°C exhibit a fast photoresponse, and a photoconductivity to dark current ratio of ≈104. Annealed films show an optical transmittance increase of about 8 - 12% at the band edge and caused the band edge and a shift in the band edge to higher wavelengths. Optical band gaps of the films were found to decrease from 2.46 cV for (as deposited) to 2.38 eV for films annealed at 395°C for 1 hr. Film resistivity decreased by a factor of ~105 as a result of impurity phases and improved crystal orientation. The higher conductivity of the air annealed films were attributed to the presence of the conducting CdO phase. XRD studies of the annealed films confirmed the presence of a mixture of CdO and CdSO, related phases.

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