Browsing by Author "K. Asokan"

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A study on swift (∼100 MeV) heavy (Si8+) ion irradiated crystalline Si-solar cell
(2006) P.C. Srivastava; S.P. Pandey; K. Asokan
A study on irradiation (of ∼100 MeV, Si-ion) induced degradation effects of crystalline silicon solar cells has been carried out. The cell characteristics show that the cell efficiency decreases to ∼30% (of its pre-irradiated value) after the irradiation (of swift heavy ions of, 10 12 or 1013 or 1014 ions cm-2 fluences). The reduced efficiency of the irradiated solar cells is recovered back (nearly half of the pre-irradiated efficiency) after annealing at 300 °C. The IR spectroscopic studies on the irradiated and annealed solar cells show the presence of di-vacancy and their complexes with impurities like hydrogen, carbon and oxygen. The swift heavy ion irradiation damage in solar cells causing a heavy loss of the cell efficiency has been attributed to these electrically active complexes which shall cause the carrier removal/compensation phenomena for the observed decrease of the cell efficiency. These complexes dissociate on annealing to change their role as donors/acceptors to cause the observed recovery of the cell efficiency on annealing. © 2005 Elsevier B.V. All rights reserved.
Enhanced Tc in Sn-added Bi-2-1-2-2 superconductor
(1992) B.R. Sekhar; N.L. Saini; P. Srivastava; K. Asokan; K.B. Garg; K. Ramakrishna; O.N. Srivastava; G.L. Dwivedi
Addition of Sn to superconducting (BiPb)2Sr2Ca1Cu2Oy, has been studied extensively using EDAX, SEM and electron diffraction. It is found that addition of Sn increases Tc to an extent of 7 K unlike Sn substitution. The present study indicates that Sn occupies the Cu site. © 1992.
High energy heavy ion irradiation in semiconductors
(Elsevier Science Publishers B.V., 1999) P.C. Srivastava; S.P. Pandey; O.P. Sinha; D.K. Avasthi; K. Asokan
Pd/n-Si and Pd/n-GaAs devices have been irradiated from high energy (approximately 100 MeV) heavy ions of Au7+ (gold) and Si7+ (silicon) to study the irradiation effects in these junction devices on semiconductor substrates. The devices have been characterized from I-V and C-V studies for electronic flow characterization. It has been found that the devices become high resistive on the irradiation and the substrates change the conductivity type from n- to p- on the irradiation of fluence of approximately 1012-1013 ions/cm2. The change in conductivity type has been understood as a result of creation of deep acceptors on the irradiation.
Pd/Si device characteristics on 100 MeV gold ions irradiation
(Elsevier Ltd, 1997) P.C. Srivastava; S.P. Pandey; D.K. Avasthi; K. Asokan
Pd/p-Si and Pd/n-Si devices were irradiated from 100 MeV gold (7+) ions for varying doses (∼ 1011-1013 ions cm-2). The devices were characterized from I-V and C-V studies. It has been found that there is a change of conductivity type i.e. from n to p at a probed depth of ∼ 8 μm which is approximately the stopping range of the gold ions in silicon. A deep acceptor state (∼ 0.61 eV above V.B. edge) with a peak density of ∼ 109 cm-2 is observed for p-type irradiated devices at ∼ 3 μm which is attributed to the displacement damage caused by the high energy heavy ion irradiation. © 1998 Elsevier Science Ltd. All rights reserved.
Role of ion beam excitations on quasi one-dimensional magnetic system of Mn-doped LiCuVO4
(Elsevier Ltd, 2015) Abhishek Kumar; G.D. Dwivedi; Shiv Kumar; P. Shahi; K.K. Shukla; A.K. Ghosh; K. Asokan; D. Kanjilal; R.K. Singh; A.K. Nigam; Sandip Chatterjee
Abstract Induction of short range ferromagnetic ordering has been observed in the quasi one dimensional antiferromagnetic LiCuVO4 system with doping of Mn in the octahedral (Cu) site. Though, magnetic ordering is not stable enough as further increase of Mn-concentration, magnetic ordering gets deteriorated. This might be the case that Mn2+ ions, with strong magnetic moment as compared to Cu2+ ions, enhance the ferromagnetic coupling between the nearest neighbor atoms of quasi-one-dimensional magnetic LiCuVO4 system. Ferromagnetic ordering in LiCu0.95Mn0.05VO4 system also degraded after high energy ion beam excitation which creates defects and may disturb the short range ferromagnetic ordering in its near locality but it does not have much effect on the long range antiferromagnetic ordering. Irradiation causes no change in Raman modes of LiCu0.95Mn0.05VO4 system, while it produces some new vibrational modes in intermediate and high frequency region of LiCu0.9Mn0.1VO4 system. Above results have been understood based on competitions between ferromagnetic nearest neighbor (NN) coupling and antiferromagnetic next nearest neighbor (NNN) coupling in CuO2 chain. © 2015 Elsevier B.V.
Structural and optical properties of ZnO and ZnO:Fe nanoparticles under dense electronic excitations
(2013) Shiv Kumar; K. Asokan; Ranjan Kr. Singh; S. Chatterjee; D. Kanjilal; Anup K. Ghosh
We report on the changes in structural, morphological, and optical properties of sol-gel derived ZnO and ZnO:Fe nanoparticles due to dense electronic excitations produced by heavy ion irradiations using 200 MeV Ag +15 ion beams. X-ray diffraction studies with Rietveld refinement show that the samples are single phase and tensile strain has been developed in the ion-irradiated samples. The Raman spectroscopy measurements show that ion-irradiation results in microscopic structural disorders and breaking of translational symmetry giving rise to local distortions in the lattice. Atomic force microscopy studies show that roughness of the pellets increases strongly for pure ZnO as compared with Fe-doped ZnO due to ion-irradiation. Fourier transform infrared analysis confirms tetrahedral coordination of O ions surrounding the Zn-ions and surface modification of the nanoparticles. The UV-Vis spectroscopy measurements show that the band gap increases on Fe doping which may be due to 4s-3d and 2p-3d interactions and the Burstein-Moss band filling effect. The band gap decreases after irradiation which can be interpreted on the basis of creation of some new localized energy states above the valence band. Photoluminescence (PL) intensity is enhanced and two new emission bands viz. a blue band at ∼480 nm (related to surface defects) and a green band at ∼525 nm (related to O vacancies) are observed in ion-irradiated nanoparticles. The enhancement of PL-intensity in irradiated samples is attributed to the increase of different defect states and Zn-O bonds on the surfaces of the irradiated nanoparticles arising from surface modification. © 2013 AIP Publishing LLC.