Browsing by Author "U. Saxena"

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Effect of nonstoichiometry on the electrical conductivity of erbium oxide films
(1978) U. Saxena; O.N. Srivastava
Thin films of erbium oxide show a thickness dependent amorphous‐to‐crystalline transformation. Both, the amorphous and crystalline phases are found to be nonstoichiometric with respect to oxygen content. electrical measurements of I‐U characteristics of these as‐grown films in a sandwich configuration, Al‐erbium oxide‐Al/Au, exhibit a voltage controlled negative resistance region (VCNR). The nature of the characteristic is independent of electrode material, electrode polarity, and vacuum conditions. It is postulated that the inherent nonstoichiometry of the film gives rise to a ‘built in’ localized energy band in the forbidden gap of the oxide films which is responsible for the occurrence of VCNR. Copyright © 1978 WILEY‐VCH Verlag GmbH & Co. KGaA
Electron microscope studies of phase transformations in thin films of Tin Diselenide
(1984) A.M. Fernandez‐Samuel; U. Saxena; O.N. Srivastava
The present investigations explore the structural characteristics of the thin films of tin diselenide (SnSe2). Unlike bulk, only sparse studies are available on the structural characteristics in the thin form. SnSe2 is known to exhibit almost exclusively the 2H CdI2 type structure in bulk (single crystal) form. Particular emphasis on phase formation and structural transformations obtained in the thin film form has been given. The results reveal that all as‐deposited films are amorphous in nature and undergo an amorphous → crystalline transformation upon pulse annealing (∼200 °C), the crystalline phase exhibiting the 2H bulk structure. Further pulse annealing at higher temperatures (∼300 to 500 °C) results in thermal decomposition of this 2H phase, giving rise to several other phases, namely SnSe (orthorhombic), SnSe (cubic) and Sn2Se3 (tetragonal) phases, the SnSe phases exhibiting variations in lattice parameters. In addition, a new incommensurate phase of SnSe has been obtained. A model is proposed for the formation of this phase from the orthorhombie SnSe phase. The final product of decomposition is found to be tetragonal tin. Copyright © 1984 WILEY‐VCH Verlag GmbH & Co. KGaA
On the thickness dependence of the electrical resistivity of vacuum-evaporated erbium thin films
(1976) U. Saxena; O.N. Srivastava
[No abstract available]
Particle size dependence on the structural, transport and optical properties of charge-ordered Pr0.6Ca0.4MnO3
(Elsevier Ltd, 2015) Satyam Kumar; G.D. Dwivedi; J. Lourembam; Shiv Kumar; U. Saxena; A.K. Ghosh; H. Chou; Sandip Chatterjee
Abstract Structural, transport and optical properties of nano-crystalline Pr0.6Ca0.4MnO3 have been investigated to emphasize on the semiconducting properties of charge-ordered manganite. Rietveld refinement of X-ray diffraction pattern of Pr0.6Ca0.4MnO3 nanoparticles show that due to increase in sintering temperature, MnO6 octahedra elongated along z-direction and compressed in x-y plane. Both Mn-O-Mn angles are found to decrease with increasing sintering temperature. Fourier transform infrared (FTIR) spectroscopy measurements reveal that the stretching and bending vibration of Mn-O-Mn is responsible for the change in Mn-O-Mn bond length and bond angle respectively. With increasing sintering temperature, these vibrations tend to increase, which resulted in the further distortion of MnO6 octahedra. Magnetic measurements suggest that charge ordering is established and system becomes antiferromagnetic with increasing particle size. Resistivity behavior of Pr0.6Ca0.4MnO3 nanoparticles clearly exhibit semiconducting nature of these systems, which is due to the formation of charge-ordered state of Mn3+ and Mn4+. Estimated optical band-gap of ∼3.7 eV for Pr0.6Ca0.4MnO3 nanocrystals, makes it a potential candidate for wide band-gap magnetic semiconductors. © 2015 Published by Elsevier B.V.
Phase transformations in erbium oxide films
(1977) U. Saxena; O.N. Srivastava
[No abstract available]
Reply to comment on "on the thickness dependence of the electrical resistivity of vacuum-evaporated erbium thin films"
(1977) U. Saxena; O.N. Srivastava
[No abstract available]
Structural, transport and optical properties of (La0.6Pr0.4)0.65Ca0.35MnO3 nanocrystals: A wide band-gap magnetic semiconductor
(Royal Society of Chemistry, 2015) Satyam Kumar; G.D. Dwivedi; Shiv Kumar; R.B. Mathur; U. Saxena; A.K. Ghosh; Amish G. Joshi; H.D. Yang; Sandip Chatterjee
(La0.6Pr0.4)0.65Ca0.35MnO3 system has been synthesized via a sol-gel route at different sintering temperatures. Structural, transport and optical measurements have been carried out to investigate (La0.6Pr0.4)0.65Ca0.35MnO3 nanoparticles. Raman spectra show that Jahn-Teller distortion has been decreased due to the presence of Ca and Pr in A-site. Magnetic measurements provide a Curie temperature around 200 K and saturation magnetization (MS) of about 3.43μB/Mn at 5 K. X-ray photoemission spectroscopy study suggests that Mn exists in a dual oxidation state (Mn3+ and Mn4+). Resistivity measurements suggest that charge-ordered states of Mn3+ and Mn4+, which might be influenced by the presence of Pr, have enhanced insulating behavior in (La0.6Pr0.4)0.65Ca0.35MnO3. Band gap estimated from UV-Vis spectroscopy measurements comes in the range of wide band gap semiconductors (∼3.5 eV); this makes (La0.6Pr0.4)0.65Ca0.35MnO3 a potential candidate for device application. This journal is © The Royal Society of Chemistry 2015.
Unusual thickness dependence of the dielectric constant of erbium oxide films
(1976) U. Saxena; O.N. Srivastava
The thickness dependence of the dielectric constant of Er2O3 films was studied. It was found that for films less than 700 Å thick and greater than 1300 Å the dielectric constant ε shows the usual behaviour of increasing with increasing thickness and then assuming the limiting bulk value. However, for films of intermediate thicknesses (700-1300 Å) the dielectric constant first decreases, attains a minimum value and then increases to obtain the bulk value. It is suggested, based on electron microscope observations, that this curious thickness dependence of ε arises because of the transformations from (i) amorphous to crystalline and (ii) f.c.c. type to b.c.c. type crystalline phases in the films. © 1976.