Subodh K. Gautam

Correlations of charge neutrality level with electronic structure and p-d hybridization

The formation of charge neutrality level (CNL) in highly conducting Cadmium oxide (CdO) thin films is demonstarted by the observed variation in the band gap upon annealing and doping. It may be explained by the observation that Tin (Sn) doping breaks the perfect periodicity of CdO cubic crystal structure and creates virtual gap states (ViGS). The level of local CNL resides at the branch point of ViGS, making the energy at which native defect’s character changes from predominantly donor-like below CNL to predominantly acceptor-like above the CNL and a schematic band diagram is developed to substantiate the same. Further investigations using soft x-ray absorption spectroscopy (SXAS) at Oxygen and Cadmium edges show the reduction of Sn4+ to Sn2+. The analysis of the spectral features has revealed an evidence of p-d interaction between O 2p and Cd 4d orbitals that pushes the valence band minima at higher energies which is symmetry forbidden at г point and causing a positive valance band dispersion away from the zone centre in the г ~ L, K direction. Thus, origin of the CNL is attributed to the high density of the Oxygen vacancies as confirmed by the change in the local electronic structure and p-d hybridization of orbitals.

Carrier transport mechanism of highly-sensitive niobium doped titanium dioxide/p-Si heterojunction photodiode under illuminations by solar simulated light

Nano-crystalline Nb doped anatase TiO2 (NTO) thin film was deposited on p-type Si substrate for fabrication of n-NTO/p-Si heterojunction photodiode using RF magnetron sputtering technique. Rutherford backscattering spectrometry and Raman spectroscopy results suggest the substitutional incorporation of Nb5+ ions in anatase TiO2 lattice, which is evidenced from large stiffening of Eg(1) and softening of B1g Raman modes. The current density-voltage (J-V) characteristics are measured and important diode parameters of n-NTO/p-Si heterojunction diode are determined, such as ideality factor, barrier height, and series resistance. Diode exhibits excellent behavior under dark condition as rectification ratio is found to be ∼7 × 102 with high forward current density ∼1.54 A/cm2 at 5 V. The n-NTO/p-Si heterojunction works as an efficient photodiode in reverse bias under simulated solar light illumination with high contrast ratio ∼225 at −2 V and very high photo responsivity ∼2.7 A/W at −5 V. The high photo responsiv...

Anomalous behavior of B1g mode in highly transparent anatase nano-crystalline Nb-doped Titanium Dioxide (NTO) thin films

The effect of Niobiumdoping and size of crystallites on highly transparent nano-crystalline NiobiumdopedTitanium Dioxide (NTO) thin films with stable anatase phase are reported. The Nbdoping concentration is varied within the solubility limit in TiO2 lattice. Films were annealed in controlled environment for improving the crystallinity and size of crystallites. Elemental and thickness analysis were carried out using Rutherford backscattering spectrometry and cross sectional field emission scanning electron microscopy. Structural characteristics reveal a substitutional incorporation of Nb+5 in the TiO2 lattice which inhibits the anatase crystallites growth with increasing the doping percentage. The micro-Raman (MR) spectra of films with small size crystallites shows stiffening of about 4 cm−1 for the Eg(1) mode and is ascribed to phonon confinement and non-stoichiometry. In contrast, B1g mode exhibits a large anomalous softening of 20 cm−1 with asymmetrical broadening; which was not reported for the case of pure TiO2 crystallites. This anomalous behaviour is explained by contraction of the apical Ti-O bonds at the surface upon substitutional Nb5+doping induced reduction of Ti4+ ions also known as hetero-coordination effect. The proposed hypotheses is manifested through studying the electronic structure and phonon dynamics by performing the near edge x-ray absorption fine structure(NEXAFS) and temperature dependent MR down to liquid nitrogen temperature on pure and 2.5 at.% doped NTO films, respectively.

High efficiency hybrid solid state blended dyes sensitized solar cell based on zinc oxide nanostructures

This paper reports on the development of high efficiency hybrid solid state blended dye sensitized solar cell (HssBDSSC) using ultrasonification process. Cell synthesized using conducting organic polymer as organic hole-transporting material together with BDs, while Poly (3-Octylthiopehene) (P3OT) and zinc oxide nanostructures (ns-ZnO) as electron-transporting material. BD is made in a composition, so that it can absorbed light in the entire visible region and provided the high electron injection. The HssBDSSC device structure such as indium tin oxide (ITO)/PEDOT:PSS/ns-ZnO+P3OT/BDs/Electrode is developed and an ultrathin layer of PEDOT:PSS is deposited on ITO for making Schottky contact. Device shows a very high conversion efficiency of 4.67%, Fill Factor (FF) 52.9%, open-circuit photo voltages 0.58 V and short-circuit photocurrents 15.1 mA/cm2 under 100 mW/cm2 visible band illumination. We propose an energy level alignment diagram to understand the photophysics of hybrid nanocomposites, which include ch...

Multifunctional hybrid diode: Study of photoresponse, high responsivity, and charge injection mechanisms

A multifunctional hybrid heterojunction diode is developed on porous silicon and its current density-voltage characteristics reveal a good rectification ratio along with other superior parameters such as ideality factor, barrier height and series resistance. The diode also functions as an efficient photodiode to manifest high photosensitivity with high responsivity under illumination with broadband solar light, UV light, and green light. The diode is also carefully scrutinized for its sensitivity and repeatability over many cycles under UV and green light and is found to have a quick response and extremely fast recovery times. The notable responsivity is attributed to the generation of high density of excitons in the depletion region by the absorption of incident photons and their separation by an internal electric field besides an additional photocurrent due to the charging of polymer chains. The mechanisms of generation, injection and transport of charge carriers are explained by developing a schematic ...

The role of ion irradiation in activating silent Raman modes via tuning in plasmonic behaviour and surface disorder of Au/ZnO/Pt NFG system

The perceptible progression of Raman modes of zinc oxide (ZnO) is studied in nanostructures film gap (Au (10 nm)/ZnO (70 nm)/Pt (50 nm)) system with 1.2 MeV Xe ion irradiation. Unattainable silent Raman modes of ZnO turn out to be strongly visible after ion irradiation. The creation of ion-beam–induced lattice disorder, defects, and impurities in a ZnO layer leads to breakdown the translational crystal symmetry that results in the origin of silent modes. The formation of hot-spots in the ZnO layer of the NFG system also supports the enhancement of the intensity of Raman modes. Overall results are attributed to combined effects of lattice disorder, defects, and impurities along with plasmonic effect and explained in the framework of elastic-thermal-spike formation.