D. R. Rao

Structural and optical properties of CdSexTe1 − x thin films grown by electron beam evaporation

Abstract CdSe x Te x − 1 (0≤ x ≤1) ternary thin films have been deposited on glass substrates at room temperature by electron beam evaporation, using the source materials prepared in our laboratory by direct reaction of high purity elemental Cd(99.9999%), Se(99.999%) and Te(99.999%). These alloy films independent of composition, are polycrystalline, single phase (zinc blende structure) with strong preferential orientation of the crystallites (30–50 nm size) along (111) direction. Linear variation of lattice constant with composition ( x ) is observed. Absorption measurements show quadratic variation of bandgap with composition ( x ). Refractive indices, determined from transmission spectra, are in the range 2.43–2.52 depending upon the composition of the films.

Derivative spectra of polycrystalline Zn3P2 thin films

Abstract Polycrystalline Zn3P2 thin films prepared by e-beam evaporation technique on glass substrates kept at 300°C, have exhibited low resistivity value, nearly stoichiometry and good adherence which are very promising features for device applications. Optical studies namely, optical absorption and derivative spectra, have been made which helped to understand not only the nature of the optical band gap (1.55 eV) but also the details of the other transitions related to the band structure (tetragonal) of this compound semiconductor. The results indicated the direct optical transitions at energies greater than 1.70 eV and the energy values for spin-orbit and crystal field splitting are found to be 0.16 and 0.02 eV respectively.

Preparation and characterization of Zn3P2-Cd3P2 solid solutions

Solid solutions of Zn3P2-Cd3P2 systems of the type (ZnxCd1−x)3P2 have been prepared by direct reaction of the constituent elements (Zn, Cd, P) for values of x equal to 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. X-ray diffraction data indicate that all the systems crystallize in tetragonal (α) phase only, exhibiting preferred orientation along the (220) and (224) directions. The lattice parameters, a and c, and the interplanar spacing, d, vary linearly with x, obeying Vegards law. The systems show minimum conductivity at room temperature for composition corresponding to x values in the range 0.4–0.6. Electrical conductivity for all systems is measured in the temperature range 100–450 K. In view of the estimated (low) values of the activation energy the conduction process in the different temperature regions has been attributed to the presence of shallow trapping levels in the systems.

Preparation and characterization of Cd3P2 thin films

Cd3P2 thin films have been deposited on glass substrates by electron beam evaporation technique. The amorphous as‐deposited films, after annealing in vacuum in the temperature range 180–200 °C, developed the crystallinity. Such polycrystalline films exhibited preferential orientation along the (224 400) direction. The films indicated n‐type conductivity with resistivity value around 1.6 Ω cm at room temperature. The composition, ionicity, and Madelung constant, calculated from x‐ray photoelectron spectroscopy data, are Cd3P1.7, 0.08, and 1.68, respectively. From the optical (R and T) and modulation (derivative) spectra, the different parameters, namely, refractive index (n∼3.83), band gap (Eg=0.61 eV), band splitting parameters (crystal field ΔCF=0.03 eV, spin‐orbit Δ0=0.08 eV) have been obtained. Photoluminescence emission related to band‐to‐band transition is observed around 0.51 eV.

Effect of grain size on TL yield of polycrystalline BaS: Bi phosphors

Abstract Effect of grain size on thermoluminescence (TL) of BaS: Bi (0.36% by wt.) phosphors was studied by X-irradiation at RT (30° C). It is observed that the the intensity of phosphorescence and TL output decrease with the decrease of the grain size of the powders, which has been attributed to the loss of incident excitation energy due to scattering.

On the stability of TL traps of alumina

Thermoluminescence (TL) of heat-treated (in the temperature range 450 to 1400° C) α-Al2O3 powders have been studied by X-irradiation at room temperature (27° C). The TL patterns have indicated essentially two groups of traps; one in the range 50 to 250° C and the other in the range 200 to 450° C. Apart from other thermal stability characteristics, preferential bleaching by light in the wavelength region of 540 to 630 nm has been observed only in the case of the first group of TL traps. It is concluded that the TL phenomena in α-Al2O3 are in general controlled by trace impurities rather than intrinsic lattice defects and the TL traps of the system may be associated with a distribution of trapped holes (O−, O°) stabilized by trace impurities.

Derivative spectra of polycrystalline Zn1−xCdxSe thin films

Derivative spectra have been recorded at room temperature to determine the energies at the lowest gap (E0) and spin-orbit splitting parameter (Δ0) for the electron beam deposited polycrystalline Zn1−xCdxSe (0 < x < 1) thin films on glass substrates as a function of composition (x). The spectra clearly indicated the well resolved transitions due to E0 and E0 + Δ0 gaps. A quadratic relation of E0 and E0 + Δ0 with x is observed, giving respectively the bowing parameters as 0.46 and 0.52 eV. The variation of Δ0 with x at Γ point is very small and found to be in the range from 0.38 to 0.43 eV.

On the energy storage properties of cadmium phosphate (α-Cd3(PO4)2)

Abstract Thermally Stimulated Luminescence (TSL) techniques have been used to control the C/H2 reduction process of Cd3(PO4)2, the parent chemical, used in the preparation of Cd3P2, a II3 - V2 compound semiconductor. TSL glow curves (obtained after excitation by UV (200 nm) at room temperature, X-rays and recorded at 20°C min-1 heating rate) for the different phosphates (commercial as well as those prepared in the laboratory) indicated two glow peaks around 80 and 160°C. These are attributed to the presence of background impurities and the host lattice defects respectively. The calculated trapping parameters E and (s′n0) are in the range 0.5–0.35 eV and 105–108 s-1 respectively.