A.P.B. Sinha

Photoconductive CdS Films by a Chemical Bath Deposition Process

A new technique for the preparation of CdS thin films by chemical bath deposition is described. The CdS films prepared by this method have many of the advantages of those made by the spray deposition method, such as the ease of coating large areas and simplicity of the process. The stoichiometry is easy to maintain in both these methods. The films are formed on rotating substrates from a bath containing cadmium salt in a complex form. Electrical, photoconducting, and optical properties such as I--V characteristics, decay of photoconductivity, spectral response of photoconductivity, and optical absorption were studied. Addition of cations like Ag, Cu. In, and Zn to the deposition bath makes the films amorphous. Such amorphous layers have a density of state of the order of 10/sup 19/--10/sup 20//cm/sup 3//eV near the Fermi level, as determined by photodecay and field effect measurements. The films produced are strong, adhering tightly to the substrates, show little or no degradation, and are photosensitive as deposited.

Photoelectron-spectroscopic study of nickel, manganese and cobalt selenides

Photoelectron-spectroscopic studies (XPS and UPS) have been carried out to investigate electronic structure and chemical bonding in the monoselenides of nickel, manganese and cobalt. Binding-energy values and chemical shifts are reported from the XPS measurements for Ni 2p and 3s, Mn 2p and 3s, Co 2p and 3s and Se 3p and 3d. The electronic structure and bonding in the monoselenides are interpreted from an analysis of the valence-band spectra. The combined XPS and UPS data suggest transfer of electrons from the metal orbitals (Ni, Mn and Co) to selenium, contrary to bonding schemes proposed previously in the literature. Magnetic-susceptibility measurements performed by us indicate NiSe to be diamagnetic and MnSe and CoSe to be highly paramagnetic at room temperature. The presence of shake-up satellites in the XPS spectra for MnSe and CoSe and their absence for NiSe are discussed in relation to these magnetic properties.

Deposition of improved optically selective conductive tin oxide films by spray pyrolysis

Antimony-doped SnO2 films with a resistivity as low as 9×10−4 Ωcm were prepared by spray pyrolysis. Structural, electrical and optical properties were studied by varying the antimony concentration, film thickness and deposition temperature. About 94% average transmission in the visible region and about 87% infrared reflectance were obtained for antimony-doped SnO2 films by a systematic optimization of the preparation parameters. As the best combination, an average transmission of 88% in the visible region and an infrared reflectance of 76% was possible for the doped SnO2 films.

Effect of temperature on the structure of manganites

Abstract The effect of temperature on the structure of some “normal” manganites has been studied by high temperature X-ray diffraction and by differential thermal analysis. At rcom temperature they are isomorphous with hausmannite, with c/a > 1. On heating to elevated temperatures, the c,a, and c/a parameters are found to remain constant until just before a critical transformation temperature whereupon c/a falls abruptly to unity, coinciding with the structure transforming from tetragonal to cubic. This abrupt transformation is reminiscent of a first order process. The structure of the tetragonal forms at various temperatures, and the cubic form just after transition, have been determined by X-ray diffraction. The theoretical model for the variation of distortion with temperature has been discussed, and a correlation between the transition temperature and the cell distortion has been shown. An anomalous behaviour has been observed for magnesium manganite because of its tendency to change its cation distribution and alter, on heating, from a “normal” to a “random” structure. The site preference energy for the cations in MgMn2O4 has been calculated from the change in cation distribution with temperature. The D.T.A. data support the transition temperatures of various manganites as obtained by X-ray and also lead to the determination of the entropy of transformation.

Studies on thin films of cadmium sulphide prepared by a chemical deposition method

A new method of chemical deposition consisting of the formation of thin films of a substance at the interface of a solution and a gas was used to form thin films of semiconducting and photoconducting CdS. The structural, electrical and optical properties of these films were studied. A special feature is the cubic zinc-blende structure of the films. A possible explanation for the optical and electrical properties of these films is given.

Crystal Distortion in Ferrite-Manganites

The origin of the distortion of spinels from cubic to tetragonal symmetry is examined in the case of copper ferrites and a series of manganite-ferrite systems. The degree of distortion is found to depend on the temperature and the net fraction of cations forming appropriately orientated dsp2 bonds in octahedral sites. Expressions derived agree satisfactorily with the experimental data.

Interaction of oxygen with Zr76Fe24 metglass: An X-ray photoelectron spectroscopy study☆

Abstract Core level X-ray photoelectron spectra of Zr76Fe24 metglass have been studied under various oxygen exposures and at different temperatures. The formation of both ZrO and ZrO2 was observed even at 200 °C. Angle-dependent studies revealed that the ratio of ZrO to ZrO2 increases as the grazing angle of electron takeoff is approached, up to 100 °C and at 200 °C this ratio decreases as the grazing angle is approached. The Fe 2p 3 2 level did not show any binding energy shift.

Study of the thermal decomposition of iron and barium citrates

Abstract Iron citrate decomposes to give α-Fe2O3 as an end product at 460°C via a four-step process. Barium citrate has been found to decompose to a stable barium carbonate at 412°C. It is, however, observed that a mixture of iron and barium citrates decomposes to BaO and either γ-Fe2O3 or Fe3O4 or a mixture of these two oxides. Thus, the mixture does not decompose to give α-Fe2O3 and BaCO3 as obtained in the individual decomposition of iron citrate and barium citrate, respectively. The iron and barium oxides obtained as a result of decomposition of the mixture then chemically combine to form barium ferrite at 600°C.

HgSr2CuO4+δ: A new 78 K superconductor by Mo substitution

Abstract A new series of strontium-based (Hg, Mo)-1201 superconducting compounds has been synthesized. The highest Tc of the series (78 K) is found for the nominal composition (Hg0.85Mo0.15)Sr2CuO4+δ, with lattice constants: a = 3.787 A and c = 8.844 A . X-ray diffraction and SEM with EDX analysis confirm that Hg and Mo are indeed incorporated in the structure. High-pressure oxygenation at 250 bar and 325°C, and also at 1 bar, reduces the Tc indicating that the samples are overdoped. The a-axis increases and c-axis decreases with increasing Mo concentration, suggesting that Mo substitutes in the Hg-O layer rather than in the CuO2 layer.

(Mg,Pb)Sr2(Y,Ca)Cu2O7, a new 60 K superconductor in the Pb-1:2:1:2 family

Abstract (Mg,Pb)Sr2(Y,Ca)Cu2O7, a new superconductor in the Pb-1:2:1:2 family, has been synthesized. The Y:Ca ratio and annealing conditions (oxygen pressure/temperature) have been optimized. Addition of silver oxide aids in increasing the superconducting volume fraction and raising Tc to 60 K. Since this Tc is comparable to that of other Pb-1:2:1:2 compounds, it is unlikely that Mg2+ replaces Cu in the CuO2 planes, in spite of its small radius ( r=0.72 A ); in that case a substantial depression of Tc would be expected. Instead, Mg2+ appears to go into the PbO layer in a rock-salt surrounding, in analogy with other divalent elements.