Structural optical and electrical properties of pb doped Cds thin films deposited by spray pyrolysis technique

Abstract

Lead doped Cadmium Sulfide (Cd1-xPbxS) thin films for different values of x (x = 0.00, 0.05, 0.10, 0.15, 0.20) have been prepared by spray pyrolysis technique on glass substrates at 300 0C substrate temperature. The surface morphology, structural, optical and electrical properties of the as-deposited and annealed films are studied in details. The scanning electron microscopy (SEM) micrographs of Cd1-xPbxS films have been taken for different doping concentrations on glass substrate at 300 ºC and annealed at 450 ºC for one hour. The SEM micrographs of as-deposited film show uniform surface and deposition covers the substrate well. After annealing the surface roughness is increased. It means that sprayed particles (atoms) are absorbed onto the substrate to form clusters as the primary stage of nucleation. The Energy Dispersive X-ray (EDX) data of the Cd1-xPbxS thin films were taken. From EDX data, atomic weight % of Cd and S for x= 0.00 films is found to be 54.79 % and 45.21% respectively. It is observed that the Cd1-xPbx S films are stoichiometric. The crystalline structures of the as-deposited and annealed of Cd1-xPbxS films were studied by X-ray diffraction (XRD). All the samples have Hexagonal crystal structure. The unit cell volume decreases from 259.7305 Å3 to 223.8349 Å3 and the grain size decreases from 8.048 nm to 3.018 nm with the increase in doping concentration of Pb. After annealing at 450ºC for one hour, unit cell volume decreases from 223.660 Å3 to 222.690Å3 The spectral absorption coefficient (α) of the (Cd and the grain size increases from 4.527nm to 9.215 nm. 1-xPbx S) film was determined from spectral data of transmittance. It is observed that the absorption edge shifts toward the higher energy on annealing and this shifting in absorption edge has been attributed to Moss-Burnstein shift. The direct band gap energies for films of different doping concentrations were determined and found to be 2.40 eV to 2.23 eV. After annealing the direct band gap energies were reduced from 2.36 eV to 2.00 eV. Resistivity of Cd1-xPbxS has been measured for a number of films of different doping concentrations within the temperature range of 305 to 475 K. The resistivity of Cd1-xPbxS films are found to increase from 109 to 1014 • -cm with the doping concentration of Pb. The resistivity of the films becomes stable and reversible on repeated heating and cooling. This behavior remains unaltered on annealing at a higher temperature. Conductivity has been calculated from resistivity measurements. The conductivity of the films of different doping concentration initially decreases with the increase in temperature and reaches to a minimum at 430 K. The conductivity of Cd1-xPbxS films found to decrease from 10-11 to 10-14 mho-cm-1 with the doping concentration of Pb.The position of minimum conductivity shifts towards the higher temperature region with the increase in doping concentration. Form the nature of the conductivity curve of Cd1-xPbxS film it may be suggested that more than one types of conduction mechanisms are involved. The conductivity is metallic up to temperature 430 K, after that it (Cd1-xPbxS) behaves like a semiconductor. Activation energy in both the regions is quite low. In the metallic region the activation energy is ΔE1= 0.64 to 0.70 eV and in the semiconductor region, ΔE2 = 0.76 to 0.90 eV. The low values of activation energy may be associated with the localized levels hopping due to excitation of carriers from donor band to the conduction band.