Optical properties of CdTe nanoparticle thin films studied by spectroscopic ellipsometry

Thin films of CdTe nanoparticles were prepared by carrier gas evaporation technique in Ar partial pressures in the range of 10(-6)-10(-2) mbar. The size dependent complex dielectric spectra (epsilon (E) = epsilon(1)(E) + iepsilon(2)(E)) of these cluster-deposited nanoparticle thin films were investigated in the energy range 0.6-5 eV with a spectroscopic ellipsometer. The influence of Ar partial pressure on the particle size was estimated from the shift in the fundamental absorption edge and corroborated through x-ray diffraction data. The size of the particles ranged from 18 to 13 nm and showed a systematic decrease with increase in the Ar carrier gas partial pressure (10(-6)-10(-2) mbar). The average microstructural changes such as percentage of voids and extent of disorder occurring as a function of average particle size were analysed using a multilayer model with effective medium approximation. The correlation between the microstructure and the optical response was studied by evaluating standard sum rules. The average number of electrons per atom participating in the optical transition from valence band to conduction band decreased with reduction in particle size. Also the average energy gap, i.e. the splitting between the bonding and anti-bonding orbitals decreased as the partial pressure of Ar increased indicating the increase in surface states due to decrease in particle size. Increase in average bond-length and a decrease in the average co-ordination number were also observed in the specimens as a function of Ar partial pressure.