D. Raja Reddy

ESR and photoluminescence properties of Cu doped ZnS nanoparticles

Nanoparticles of Zn1-xCuxS with Cu concentrations of x=0.0, 0.1, 0.2, 0.3 and 0.4 were prepared by a co-precipitation reaction method from homogeneous solutions of zinc and copper salts. Both the ZnS and ZnS:Cu nanoparticles excited at about 370 nm exhibits a broad green emission band peaking around 491 nm, which confirms the characteristic feature of Zn2+ as well as Cu2+ ions as luminescent centers in the lattice. The TEM micrographs showed spherical morphology for ZnS nanocrystals and the average size of the particles was estimated to be around 8.5 nm. At liquid nitrogen temperature, ESR signal characteristic of Cu2+ ions was observed in samples of all concentrations. ESR spectra analysis also indicated that Cu2+ ions enter the host lattice by replacing Zn2+ ions with distorted tetrahedral site symmetry.

Laser-like mechanoluminescence in ZnMnTe-diluted magnetic semiconductor

Laser-like mechanoluminescence is reported in the spintronic material ZnMnTe. The luminescence was also observed when the material was exposed to UV and x-rays. Photoluminescence spectra consisted of peaks at 386.5 and 648 nm, which may be attributed to band gap, and Mn2+ levels, respectively. Lattice contraction, sharpness of the x-ray diffraction peak, and the blueshift in the energy gap confirm the nanostructure of the material. A possible mechanism for lasing is also suggested.

Vapour phase growth and surface topography of (ZnSe)x(CdTe)1−x single crystals

Abstract Single crystals—a few centimetres in length and 5 mm in diameter—of the ( ZnSe ) x ( CdTe ) 1− x system have been grown in the entire range of composition (0.00⩽ x ⩽ 1.00) using a self-sealing vapour phase growth technique. The optimum growth conditions for these crystals have been established. One hollow crystal with composition x =0.40 has been obtained. Growth spirals were observed on all the mixed crystals, suggesting a screw dislocation mechanism of crystal growth. The room temperature conductivity of these crystals decreased as the composition x is increased. The energy gap of these crystals varied nonlinearly with composition. All the crystals with x ⩽0.60 were p-type and the remaining were n-type.

Preparation and characterization of Mn doped sprayed p-CuInS2 films

Abstract CuInS2, a promising material for tandem solar cell realisation, is doped with Mn, a probing dopant for crystal fields and a creator of fluorescent levels in several crystal. 0.7, 1.2 and 1.9 at % of Mn doped p-CuInS2 films were prepared by spray pyrolysis. XRD studies confirmed the chalcopyrite structure with c = 11.02 and a = 5.51 A . The direct bandgap obtained from transmission spectra was found to increase with Mn content in the range 1.38–1.56 eV. Hall effect and conductivity studies in the temperature range 190–273 K showed a decrease of hole concentration with doping indicating the creation of donor levels by Mn. The observed rise in conductivity with temperature may be attributed to increase of hole mobility due to ionised impurity scattering.

Crystal growth and reflectivity studies of Zn1-xMnxTe crystals

Single crystals of Zn1-xMnxTe were prepared by vertical Bridgman crystal growth method for different concentrations of Mn. Chemical analysis and reflectivity studies were carried out for compositional and band structure properties. Microscopic variation in composition between starting and end compounds was observed from EDAX analysis. Linear dependence of fundamental absorption edgeE0as a function of Mn concentration (x) was expressed in terms of a straight line fit and a shift in E0 towards higher energy was observed in reflectivity spectra of Zn1-xMnxTe.

EPR and susceptibility studies of Zn1-xMnxTe crystals at room temperature.

Single crystals of Zn1-xMnxTe for x = 0.1, 0.25, 0.45, 0.5 and 0.6 were prepared using vertical Bridgman technique. EPR (electron paramagnetic resonance) spectra were recorded at room temperature (303 K) between 0 and 6 kG magnetic field and range of frequency 8.8-9.6 GHz. As the concentration of Mn increases the line width (DeltaH) and the number of spins (Ns) were increased. Susceptibility studies were carried out at room temperature in the range of dc magnetic field 0-10 kG using vibrating sample magnetometer (VSM). Non-linear variation in susceptibility as a function of concentration (x) was observed and was explained on the basis of sp-d and d-d exchange interactions between Mn2+ ions and ZnTe lattice ions. Both EPR and susceptibility studies confirm the paramagnetic state of Zn1-xMnxTe system at RT.

High-temperature TSC in sprayed CdS films

High-temperature thermally stimulated current (TSC) studies on sprayed CdS thin films are reported for the first time. Different illumination conditions were used. Three independent peaks were obtained in TSC curves and these are not due to three distinct defect states. The TSC curves in both vacuum and in atmospheric conditions seem to be due to photochemical reactions reported earlier in single crystals of CdSxSe1-x. Chemisorption of oxygen also seems to be responsible for the observed different TSC peaks under different illumination conditions.

Photochemical reactions in CdSxSe1-x single crystals

Thermally stimulated current (TSC) measurements in the temperature range 90-400K were carried out on vapour-phase grown CdSxSe1-x single crystals over the full range of x (1.00, 0.90, 0.70, 0.50, 0.30, 0.10 and 0.00) by continuously exciting the crystals from different temperatures while cooling (TEWC) in the range 400-90K. Large changes in the peak currents were observed for different illumination conditions. The existing photochemical models are not able to explain the TSC dependence on TEWC observed in the crystals. A tentative photochemical model which takes into account the variation of photochemical reaction with composition has been proposed.

Tailor made TSC peaks in sprayed CdS films

Abstract Trap levels in Cds or Cds like single crystals were studied by several workers using different techniques like Thermally Stimulated Conductivity (TSC), thermoluminescence etc. The trap parameters reported varied from author to author and various models were proposed to understand the trap structure. TSC studies on aged sprayed Cds films at high temperatures were reported by the present authors for the first time and the data was satisfactorily explained by a model proposed earlier. In this paper high temperature TSC of fresh as well as aged CdS films is reported. A continoous shifting of the TSC peak temperature with TEWC is observed in the case of fresh films. This is attributed to a continuous separation of electron hole pair traps with TEWC. However, in aged films such continuous separation was not observed in conformation with our earlier result. This was attributed to the locking of constituent electrons and holes in aged films. Thus the degradation in efficiency with time observed in CdS/Cu 2 S solar cells could be due to the presence of these locked traps.