S.B. Shrivastava

Effect of Co doping on structural, morphological, electrical and optical properties of nanocrystalline zinc oxide films

We report the effect of cobalt doping on the structural, morphological, electrical and optical properties of nanocrystalline ZnO films febricated by chemical spray pyrolysis technique (CSPT). The structural studies reveals that films have strongly c-axis oriented wurtzite structure. However with the increase (at 5%) in Co doping percentage, an inclination from (0 0 2) to (1 0 1) orientation has been observed. This may be due to the fact that, in addition to substitutional sites, Co may start to occupy interstitial sites due to the deformation of lattice structure caused by substitution of nearby Zn atoms by other Co atoms. This will also result in an increase of lattice constant. The films appear to be homogeneous and single-phase material, where cobalt enters the ZnO structure as Co 2+ rather than forming metallic clusters. Self-assembly of particles are clearly displayed in Atomic Force Microscope (AFM) micrograph. The particle size and roughness of the films are found to decrease with the increase in Co doping percentage. Optical transmittance spectra showed red shift at higher doping. A sharp increase in transmittance occurs at 375 nm for all films, which correspond to band edge absorption. In addition to the band edge, absorption bands are also observed for all Co doped samples at 573, 606 and 666 nm, which are attributed to d-d transitions of tetrahedrally coordinate Co 2+ . The absolute strength of these absorption bands increases almost linearly with the increase in Co concentrations. Band gap values are found in the interval between 3.26 to 3.16 eV. Resistance are found to decrease at 15% of Cobalt doping.

INVESTIGATION OF Fe-DOPED AND -UNDOPED NiO NANOCRYSTALLINE FILMS

The NiO and Fe-doped NiO thin films have been prepared by using spray pyrolysis technique. The effects of change in the volumetric concentration and substrate temperature on the film characteristics have been investigated. With the increase in volumetric concentration, the grain size and the roughness of the film increase; while the number of voids has been reduced. In case of NiO thin films the activation energy was found to increase from 0.33 eV to 0.40 eV with the increase in the film thickness. The analysis of the data suggests that the density of states at the Fermi level increase with the thickness of the film. In Fe-doped NiO film the density of states was found to increase with the increase in the substrate temperature. The optical measurements show that the maximum transmittance (≈77.13%) occurs for the film corresponding to the lowest volumetric concentration. On increasing the substrate temperature the transmittance also increases, thus displaying the decrease in the film thickness.

Positron Annihilation in Ion-Implanted ZnO

The Diffusion Trapping Model has been used to obtain the positron annihilation Doppler broadening lineshape parameter in ZnO and O+, B+, N+, Al+ implanted ZnO films. The concentration of vacancy clusters is found to be related to the atomic number and the fluence of the implanted ion. The S-parameter is found to be largest in the case of implantation of Al+ ions and is minimum for the implantation of B+ ions. Thus, the vacancy clusters are found to be largest in the case of Al+ implantation. The calculated results have been compared with the experimental value.

Effect of Molarity of Precursor Solution on Nanocrystalline Zinc Oxide Thin Films

During the last two decades, the use of transparent conducting films of non-stoichiometric and doped metallic oxides for the conversion of solar energy into electrical energy has assumed great significance. A variety of materials, using various deposition techniques, has been tried for this purpose [1-3]. Among these various materials, zinc oxide (ZnO) is one of the prominent oxide semiconductors suitable for photovoltaic applications because of its high electrical conductivity and optical transmittance in the visible region of the solar spectrum [4]. Furthermore, thin films of ZnO have shown good chemical stability against hydrogen plasma, which is of prime importance in a-Si:H-based solar-cell fabrication. Thus, zinc oxide can serve as a good candidate for replacing SnO2 and indium tin oxide (ITO) films in Si:H-based solar cells. One of the outstanding features of ZnO is its large excitonic binding energy, i.e. 60meV, leading to the existence of excitons at room temperature and even at higher temperatures [5-8]. These unique characteristics have generated a wide range of applications of ZnO. For example, gas sensors [9], surface acoustic devices [10], transparent electrodes and solar cells. Many techniques are used for preparing the transparent conducting ZnO films, such as RF sputtering [11], evaporation [12], chemical vapour deposition [13], ion beam sputtering [14] and spray pyrolysis [15–18]. Among these, the spray pyrolysis technique has attracted considerable attention due to its simplicity and large-scale production combined with low-cost fabrication. By using this technique, one can produce large-area coatings without any need for ultra-high vacuum. Thus, the capital cost and the production cost of high-quality zinc oxide semiconductor thin films are lowest among all other techniques. In the present work, we have synthesized ZnO films by using the spray pyrolysis technique. A number of films have been prepared by changing the molarity of the precursor solution. The prepared films have been characterized with regard to their structural, morphological and electrical properties.

Slow Positron Annihilation in Ion-Implanted Silicon

The mechanism of slow positron annihilation in ion-implanted Si has been discussed in terms of the Diffusion-Trapping model (DTM). The trapping of positron has been considered in native vacancies (monovacancies) and ion induced vacancies i.e. vacancy clusters. The model has been used to calculate the Doppler broadening line shape parameter (S-parameter) as a function of incident positron energy for different ion-implanted Si. It has been found that at lower energies the monovacancies and vacancy clusters both contribute to the S-parameter while, with the increase in positron energy the vacancy clusters are reduced. The S-parameter is found to be dependent on the fluency of the implanted ions.