Anil K. Bhatnagar

Structural and optical investigations on CdS thin films grown by chemical bath technique

Cadmium sulphide (CdS) thin films have been prepared by using a chemical bath deposition (CBD) method. Structural analysis (XRD and TEM) revealed that the films showed hexagonal structure with lattice constants a=0.4015 nm and c=0.6545 nm. On annealing, the films exhibited secondary phase of beta -CdS (cubic) along with hexagonal phase, but density of dislocations decreased. The stacking faults have been observed in the crystalline arrangement by high resolution transmission electron microscopy (HRTEM). The direct band gaps of as-grown and annealed films were found to be 2.42 and 2.62 eV with sub band gap of 2.35 eV, respectively, which were confirmed by optical analysis. On annealing, the resistivity of the films decreased from 3 x 10(6) to 1 x 10(3) Omega cm. Multiple Cd and S defects were observed by employing photoluminescence (PL) method. The intensities of emission peaks for annealed films differed with that of as-grown films. As was discussed with giving reasons.

Linear optical properties of niobium-based tellurite glasses

Abstract 10·Na 2 O– x ·Nb 2 O 5 –(90− x )·TeO 2 ( x =0, 5, 10 and 20) glasses have been prepared and some of their physical and optical properties are investigated. The refractive index variation with wavelength is measured using a novel technique, white light interferometry. Optical band gaps and Urbach energies are estimated from the optical absorption spectra. Optical parameters viz., dispersion energy, average oscillator energy, Abbes number and third order nonlinear susceptibility values are estimated from the dispersion of the refractive index. Respective refractive index and estimated third order nonlinear optical susceptibility values are larger than for the tellurite glasses reported earlier. Relatively large refractive index values obtained for the present glasses are attributed to the hyperpolarisability of the Nb–O bands. The present study of the optical band gap and dispersion energies of these glasses suggests the decrease of covalency of the glass with the increase of niobium (Nb) content.

Optical, structural and electrical properties of tin doped indium oxide thin films prepared by spray-pyrolysis technique

Tin doped indium oxide (In2O3:Sn) or indium tin oxide (ITO) thin films have been successfully deposited by the low cost spray-pyrolysis method. Low sheet resistance and high mobility films were obtained when the films were deposited at the substrate temperature of 793 K. The direct optical bandgaps for the films deposited at 793 (a) and 753 K (b) were found to be 3.46 and 3.40 eV, respectively. Similarly, the indirect bandgaps for a- and b-type films were found to be 3.0 and 2.75 eV, respectively. The Burstein-Moss shift was observed in the films. The refractive index (n) and extinction coefficient (k) were found to be in the range of 2.1 to 1.1 and 0.6 to 0.01, respectively. The various scattering mechanisms such as lattice, ionized impurity, neutral impurity, grain boundary and alloy scattering due to variation of theoretical mobilities with temperature are discussed, in order to compare experimental results. In the lattice scattering mechanism, the quantum size effect phenomena were employed to estimate the energy dilation (EI). The a-type films exhibited SnO2 as secondary phase whereas b-type films showed single phase In2O3:Sn with high sheet resistance. The lattice constants were found to be 10.16 and 10.09 A for a- and b-type films, respectively.

Amorphous to crystalline transformation of Fe81B13.5Si3.5C2

The crystallization of Fe81B13.5Si3.5C2 (Metglas 2605SC) has been studied by resistivity, differential thermal analysis (DTA), Mossbauer and x‐ray techniques, and compared with reported results based on other techniques. The DTA shows in addition to the usual two peaks a third peak at 850 K paralleled by a change in the resistivity measurements at the same temperature. The crystallization is found to take place gradually in the temperature region 750–810 K with ∝‐iron precipitating first. Mossbauer studies provide evidence for ∝‐Fe, Fe2B, and a species with a hyperfine magnetic field ∼314 kOe which could not be unequivocally established. X‐ray data shows evidence for the presence of Fe3C in addition to ∝‐Fe and Fe2B.The crystallization of Fe81B13.5Si3.5C2 (Metglas 2605SC) has been studied by resistivity, differential thermal analysis (DTA), Mossbauer and x‐ray techniques, and compared with reported results based on other techniques. The DTA shows in addition to the usual two peaks a third peak at 850 K paralleled by a change in the resistivity measurements at the same temperature. The crystallization is found to take place gradually in the temperature region 750–810 K with ∝‐iron precipitating first. Mossbauer studies provide evidence for ∝‐Fe, Fe2B, and a species with a hyperfine magnetic field ∼314 kOe which could not be unequivocally established. X‐ray data shows evidence for the presence of Fe3C in addition to ∝‐Fe and Fe2B.

Mössbauer study of a ferromagnetic metallic glass: Fe74Co10B16

Abstract Amorphous Fe74Co10B16 (METGLAS 2605CO) has been studied in the temperature range of 77 K – 700 K by Mossbauer spectroscopy. Its crystallization temperature is found to be 665 ± 5 K and Curie temperature is estimated to be 760±10 K. The observed rapid decrease in reduced hyperfine fields can be explained well by Handrichs model for amorphous ferromagnets if one assumes a temperature dependent δ, a measure of fluctuations in the exchange interactions in such solids.

The effect of sulfur concentration on the properties of chemical bath deposited CdS thin films

We study the structural, surface morphology and optical properties of chemical bath deposited (CBD) cadmium sulfide (CdS) thin films under the effect of variation of S/Cd ratio. CdS thin films have been successfully deposited by CBD technique with solutions containing S/Cd ionic concentration ratio of 5.0, 2.5, 1.0, 0.5 and 0.25. Single phase CdS, with a hexagonal structure, is observed for the concentration of S/Cd = 5.0, 2.5, 1.0 and 0.5 films while for the ratio of 0.25, the films exhibited a partially amorphous nature. These have been confirmed by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM) analyses. The band gaps of the films obtained by transmission and photoacoustic spectra are found to be in the range of 2.40 to 3.26 eV. The large variation of band gaps of the films with composition is discussed by employing quantum size effect phenomena. The transition levels of CdS are also studied using photoacoustic spectroscopy.

The influence of antimony impurity on optical and electrical properties of amorphous selenium

Amorphous Se1-xSbx alloys were prepared using a conventional rapid quenching technique. It is found that antimony can only be substituted up to approximately 10 at.% in selenium to produce Se-Sb glassy alloys. The photoacoustic technique is employed for the first time to determine the optical energy gaps of Se1-xSbx glasses. The optical energy gap of amorphous selenium is found to be 1.99+or-0.02 eV. It reduces drastically on substitution of 2 at.% Sb after which its variation with x is small. The activation energies of Se1-xSbx glasses have been determined from the temperature dependence of their electrical resistivities. The activation energy as a function of x also shows a sudden decrease in the range 0.01<or=x<or=0.02. Plausible explanations have been suggested based on the Street-Mott model for charged defect states in amorphous chalcogenides.

Microstructural study of yttria stabilized zirconia buffered sapphire for YBa2Cu3O7−δ thin films

Yttria stabilized zirconia (YSZ) buffer layers were grown by rf‐magnetron sputtering on the r (1102) plane of sapphire for YBa2Cu3O7−δ (YBCO) thin film deposition. Microstructural changes of YSZ buffer layers grown using different sputtering conditions (5, 10, and 20 mTorr; Ar/O2 gas mix ratio of 9:1 and 1:1) were monitored by atomic force microscopy (AFM). Films grown using a lower oxygen partial pressure (5 mTorr) and a higher Ar/O2 ratio (9:1) showed smooth surface morphology and the average surface roughness increased with an increase in oxygen partial pressure. YBCO films in situ grown by pulsed laser deposition on sapphire with a YSZ buffer layer deposited using optimized sputtering parameters (5 mTorr gas pressure, 9:1 Ar/O2 ratio) yielded the highest critical density, Jc≊4.5×106 A cm−2 at 77 K. An excellent correlation between microstructure and Jc has been found and AFM has proved to be important for the study of the microstructure of films.

A Mössbauer study of amorphous Fe40Ni40B20

Amorphous Fe40Ni40B20 (VITROVAC 0040) alloy has been investigated using 57Fe Mossbauer Spectroscopy. The Curie temperature Tc is found to be well defined and is 695 ± 1 K. The quadrupole splitting just above Tc is 0.64 mm sec−1. The crystallization temperature is 698 ± 2 K, close to but definitely above Tc. The average hyperfine field Heff(T) of the glassy state shows a temperature dependence of Heff(0)[1 − B32(T/Tc)32 − C52(T/Tc)52 − …] indicative of the existence of spin wave excitations. The values of B32 and C52 are found to be 0.40 and 0.06, respectively, for T/Tc ⩽ 0.72. At temperatures close to Tc, Heff(T) varies as (1 − T/Tc)β where β is one of the critical exponents and its value is found to be 0.29 ± 0.02.

Photoacoustic study of crystalline Se1−xTex semiconducting alloys

Abstract The photoacoustic technique has been applied for the first time to study the energy gap variation of mixed semiconducting Se1−xTex alloys as a function of x. The energy gaps of crystalline and amorphous selenium are found to be 1.87 eV and 2.00 eV, respectively. The energy gap of SeTe alloys is observed to vary linearly with x in the region 0.1 ≤ x ≤ 1. A steep change in the slope is observed at x ⋍ 0.1 and not at x ⋟ 0.6 as reported earlier by Beyer et al. A possible explanation for this behavior is suggested based on the band structure of Se and Te.