Browsing by Author "P.C. Srivastava"

Now showing 1 - 20 of 94

A study on CoFe/p-Si interfacial structures before and after swift heavy ion irradiation
(Bellwether Publishing, Ltd., 2015) Arvind Kumar; P.C. Srivastava
Interfacial structures of CoFe/p-Si have been studied before and after the swift heavy ion (∼100 MeV, Ni7+) irradiation to investigate its electronic and magnetic behavior. X-ray diffraction (XRD), atomic force microscopy, magnetic force microscopy and magnetization characteristics (M-H) from vibrating sample magnetometer (VSM) techniques have been used for the above. XRD data have confirmed the formation of the CoFe alloy phase along with the silicide phases of Fe and Co. It is observed that there is an irradiation-induced growth in crystallite but surface remains smooth with a surface roughness of ∼34 nm. A very significant increase has been observed in the magnetization and that too with irradiation dose as compared with unirradiated ones, maintaining their superparamagnetic behavior. The results could be understood due to the role played by various magnetic phases in the structure. The magnetic field sensitivity on electronic transport across the structures has also increased in a significant manner after the irradiation as compared with unirradiated ones. The magnetic field sensitivity has resulted in an MR of 20%. The results could be understood due to the irradiation induced interfacial intermixing to result in increased magnetic phases of silicide for the observed significant magnetic behavior on the irradiation. © 2015 Taylor & Francis.
A study on exchange coupled structures of Fe/NiO and NiO/Fe interfaced with n- and p-silicon substrates
(2012) Neelabh Srivastava; P.C. Srivastava
Interfacial structures of ferromagnetic (FM)/antiferromagnetic (AF) (Fe/NiO) and AF/FM (NiO/Fe) on n- and p-Si substrates have been realized by sequential deposition of FM and AF layers on the silicon substrates by electron beam evaporation technique. The structures have been characterized from x-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM), and M-H characteristics. It has been found that there is a strong interfacial intermixing to form the various oxide and silicide phases of Fe 2O 3, β-Fe 2O 3, β″-Fe 2O 3, NiSi, Ni 3Si, and Fe 5Si 3. AFM micrographs show the granular morphology of the top layer of the structure, with a large grain size of ∼400 nm, however, the XRD data show the crystallite size of ∼20 to 70 nm. It seems that the crystallites are clustered to form larger grains. MFM features show a large domain size corresponding to AFM grain size for Fe/NiO/Si structure and very small domain of nanometer size for NiO/Fe/Si structure (having NiO as a top layer). M-H characteristics show that the magnetic behavior is only significant for Fe/NiO/nSi structure with a significant coercivity and exchange bias than for all other interfacial structures of Fe/NiO/pSi, NiO/Fe/pSi, and NiO/Fe/nSi. Thus, it has been found that Fe/NiO/nSi structure can be used in magneto-electronic device applications. It seems that the observed result of significant exchange bias and coercivity is due to the microstructural and chemical structure changes in the antiferromagnetic layer along with the roughness (data as obtained from AFM). © 2012 American Institute of Physics.
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.
A study on swift (∼100 MeV) heavy ion irradiated Ni films on Si substrates
(2007) J.K. Tripathi; P.S. Pandey; P.C. Srivastava
Ni thin films (∼50 nm) on silicon substrates have been irradiated from 100 MeV swift heavy ions of Fe7+ with a fluence of 1012 ions cm-2. SEM studies show a nice feature of interwoven grains which looks like a knitted network which has been resolved as a spherical grainy structure from AFM studies. Chemical phase identification of the grains has been done from XRD studies and it is found that there is a formation of the Ni2Si silicide phase having average grain size of ∼70 nm. The devices have also been characterized from I-V characteristics before and after the irradiation at varying temperature from LN2 to room temperature. The current across the irradiated interface has increased by two orders of magnitude as compared to the unirradiated ones and show a nearly temperature independent behaviour. MR (magnetoresistance) has been studied from the current flow data in magnetic fields up to 10 kG. Unirradiated devices do not show any effect on current transport in external magnetic field. M-H characteristics of the irradiated devices show the typical magnetic behaviour of nano particles like superparamagnetic behaviour. The MR features has been related to the M-H variations. The observed results show the formation of magnetic nano grains due to interfacial intermixing in these devices of Ni/n-Si. The role of swift heavy ions for nano grain fabrication has been discussed and the observed properties have been understood by considering the formation of a nano magnetic granular phase. © 2007 Elsevier B.V. All rights reserved.
Acceptor states in Pd/n-GaAs devices and effect of hydrogenation
(1991) P.C. Srivastava; S. Chandra; U.P. Singh
Pd/n-GaAs devices have been fabricated and electrically characterized, before and after hydrogenation. The technique of hydrogenation is novel in the sense that the hydrogen absorbed in Pd provides a source of hydrogen at room temperature for its injection into GaAs by diffusion. The effective donor density decreases on hydrogenation. The forward bias C-V characteristics show the presence of two acceptor states at approximately 0.52 and approximately 0.72 eV for the unhydrogenated devices and at approximately 0.83 and approximately 0.92 eV for the hydrogenated devices. The likely origin of these states is discussed.
AFM and electronic transport studies of swift heavy ion irradiated Mn/p-Si bilayer structure
(Elsevier, 2008) P.C. Srivastava; M.K. Srivastava; P.S. Pandey
Mn/p-Si structures have been realised by electron beam evaporation of manganese on etched and cleaned p-Si wafers. Bilayer structures have been irradiated by swift heavy ions (of 100 MeV Fe7+ having a fluence of 1 × 1013 ions/cm2). The electronic transport features across the bilayer of the structure (i.e. I-V characteristics across the Mn/p-Si interface) show a significant increase of current (by two orders of magnitude) for the irradiated ones as compared to un-irradiated ones. I-V characteristics across the interface has also been recorded in presence of in-plane (i.e., along the plane of the interface) magnetic field which show a significant magnetic field sensitivity for the irradiated ones. The surface morphological studies from AFM show a granular structure with open face having micro-particles in it, prior to the irradiation and round shaped embedded granular structure after the irradiation. XRD data show the formation of manganese silicide (Mn5Si2). The results are understood in the realm of interfacial intermixing which is tailored by the swift heavy ion irradiation. © 2008 Elsevier B.V. All rights reserved.
AFM studies of swift heavy ion-irradiated surface modification in Si and GaAs
(2003) P.C. Srivastava; V. Ganesan; O.P. Sinha
Atomic force microscopy (AFM) studies of Swift Heavy Ions (SHI ∼100 MeVSi7+ and Au7+) irradiated Si and GaAs surfaces have been performed for a variable fluence in the range of 1010-10 13 ions cm-2. The craters with piled up material, which is called hill, are clearly seen in the micrographs. A significant direct observation of amorphization (or melting due to SHI irradiation damage), plastic flow and subsequent recrystallization in the form of platelets has been made. The quantitative estimation of the features revealed that the volume of the craters for silicon ion irradiation is smaller than the gold ion irradiation. However, surface roughness has been found to be enhanced after the irradiation. Moreover, the GaAs surfaces were found to be less rough than the Si surface. The features are related to the difference in electronic energy loss of incident ions, thermal diffusivity, thermal conductivity and density of the target materials. © 2003 Elsevier Ltd. All rights reserved.
Al/n-Si diodes with low energy (∼100 eV) hydrogen ion implantation prior to metallization
(1994) P.C. Srivastava; C. Colluza; S. Chandra; U.P. Singh
[No abstract available]
Antiferromagnetic coupling in Co-doped ZnS
(Kluwer Academic Publishers, 2015) Prayas Chandra Patel; Surajit Ghosh; P.C. Srivastava
In this paper, we report room-temperature ferromagnetism in chemically synthesized Zn1−xCoxS (0 ≤ x ≤ 0.10) diluted magnetic semiconductor nanoparticles of ~3–5 nm. The incorporation of Co2+ ion for Zn2+ ions in ZnS lattice and the particle size were confirmed by XRD and TEM along with selected area electron diffraction analysis. UV–Vis measurement showed reduction in the bandgap energy with the increase in Co doping. Maximum magnetization was observed for samples with x = 0.04. From photoluminescence, spectra luminescence efficiency was found to get enhanced on Co doping. Magnetization behavior can be understood to be due to defect-induced ferromagnetism; however, for higher doping concentration, the antiferromagnetic coupling of Co–Co interaction in the close proximity results in the decrease of the overall magnetization of the samples. Moreover, magneto-electronic study also showed a maximum negative magneto-resistance of ~43 % for x = 0.04. © 2015, Springer Science+Business Media New York.
Bound magnetic polaron driven room-temperature ferromagnetism in Ni doped ZnS nanoparticles
(Elsevier Ltd, 2018) Prayas Chandra Patel; Surajit Ghosh; P.C. Srivastava
Magnetism in dilute magnetic semiconductors (DMSs) has been a controversial topic since its discovery. There are many models which predict the origin of room temperature ferromagnetism (RTFM) in TM doped wide band gap semiconductors. Here, we report RTFM in chemically synthesized cubic Zn1-xNixS (0 ≤ x ≤ 0.08) DMS nanoparticles of ∼3–5 nm size. Ferromagnetic behavior (at 300 K and 5 K) was found to increase with the increase in Ni doping concentration and was understood due to defect induced ferromagnetism. The low temperature magnetization measurement (ZFC-FC) shows that the nanoparticles are strongly coupled by magnetic interactions. Optical studies showed decrease in the energy bandgap along with the presence of sulfur and zinc vacancies and surface defects. Low temperature resistivity measurement depicted the semiconducting nature of the synthesized samples. With increase in doping concentration, an increase in the resistive behavior was observed which was explained in the realm of defects states created due to doping of the Ni ions. © 2018 Elsevier B.V.
Contact potential difference measurement using a single-junction breakdown field method
(1975) A. Roy Bardhan; P.C. Srivastava; D.L. Bhattacharya
A technique for measuring contact potential differences using single-junction breakdown has been developed from evidence that recovery of a broken-down junction takes place after breakdown has occurred. Experiments have been performed with Al-Al2O3-metal thin film sandwiches. The aluminium oxide film was grown thermally on vacuum-deposited thin aluminium films. The present method appears to be superior to the usual two-junction breakdown method of Simmons. © 1975.
Core level X-ray photoelectron spectroscopy study of exchange coupled Fe/NiO bilayer interfaced with Si substrate (Fe/NiO-nSi structure)
(2013) Neelabh Srivastava; T. Shripathi; P.C. Srivastava
The chemical interactions and surface chemical states of exchange coupled structures of Fe/NiO bilayer interfaced with silicon substrate has been studied from X-ray photoelectron spectroscopy (XPS) technique. The structure has been irradiated from swift (∼100 MeV) heavy ions (of Fe7+) with a fluence of ∼5 × 1012 ions/cm2. The surface of the samples has also been cleaned from ion beam etching for various durations to record the XPS spectra. On the irradiation, signals due to Fe2p and Ni2p are emerging along with the signal of Si2p whereas no signal of Si2p has been observed, prior to irradiation. The observation shows that the Si atoms are present within the probed surface layer due to irradiation induced interfacial intermixing. We also observed a shift in the binding energy position of Si2p which could be due to the formation of silicide phases as a result of irradiation induced interfacial intermixing. XPS results have indicated the presence of ionic Fe (Fe2+ or Fe3+) and metallic Ni at the Fe/NiO interface, which could be due to the different chemical reactions at the Fe/NiO interface. © 2013 Elsevier B.V.
Deep states in hydrogenated Pd/Si diodes
(1988) P.C. Srivastava; D. Tripathi; S. Chandra
C-V characteristics of Pd/c-Si(n) diodes have been studied before and after hydrogenation. It has been found that there is a substantial deviation from theoretical calculations in the C-V plots in forward bias. It has been explained in terms of hydrogen-related acceptor-like states at 0.57 eV, spatially located at 0.15 mu m from the interface.
Dependence of charge carrier mobility of 4,4′,4″-tris(N-3- methylphenyl-N-phenylamino)triphenylamine on doping concentration of tetrafluoro-tetracyano-quinodimethane
(Elsevier B.V., 2012) Gayatri Chauhan; Ritu Srivastava; Arunandan Kumar; Omwati Rana; P.C. Srivastava; M.N. Kamalasanan
Electrical transport of pure and tetrafluoro-tetracyano-quinodimethane doped 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) films have been studied at various temperatures and doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J-V characteristics of doped m-MTDATA were ohmic at low voltages due to thermally released carriers from dopant states. At higher voltages the current density increases nonlinearly due to field dependent mobility and carrier concentration thereby filling of tail states of HOMO of the host. The conductivity of doped films were analysed using the Unified Gaussian Disorder Model (UGDM). The carrier concentration obtained from the fitting show a non-linear dependence on doping concentration which may be due to a combined effect of thermally activated carrier generation and increased carrier mobility. © 2011 Elsevier B.V. All rights reserved.
Diffusion-limited aggregation in potato starch and hydrogen borate electrolyte system
(2013) Tuhina Tiwari; Manindra Kumar; Kamlesh Pandey; Neelam Srivastava; P.C. Srivastava
Natural growth of diffusion-limited aggregate (DLA), without any external stimuli, in boric acid doped starch system is reported here. Fractals grown were confirmed to be of diffusion-limited aggregate (DLA) pattern having fractal dimension 1.49. Effect of substrate and humidity on growth pattern has also been discussed. The existence of a different vibration band of H3BO 3 in FTIR confirmed that growth structures are related to boric acid. XRD pattern has shown broad peak along with some sharp peaks. Broad peak is related to starch's amorphous nature, as where intense sharp peaks are due to boric acid. © 2013 Tuhina Tiwari et al.
Donor states in Pd/p-GaAs devices and the effect of hydrogenation
(1995) U.P. Singh; P.C. Srivastava; S. Chandra
The effect of hydrogenation on a Pd/p-GaAs diode in hydrogen gaseous ambient has been studied by I-V and C-V measurements. Hydrogenation has been found to improve the ideality factor of the diode. Reverse C-V characteristics show that the number of shallow and deep acceptors is reduced on hydrogenation. The forward C-V characteristics has shown the presence of interfacial deep donors at Ec-1.05 eV and Ec-0.88 eV, the density of which decreased on hydrogenation. The observed results are discussed in terms of the interaction of the diffusing hydrogen with the interface and bulk states of the Pd/GaAs structure.
Effect of glutaraldehyde on electrical properties of arrowroot starch + NaI electrolyte system
(2011) Tuhina Tiwari; K. Pandey; Neelam Srivastava; P.C. Srivastava
A biopolymer based electrolyte system has been developed using NaI as dopant salt and Arrowroot starch as polymer matrix. Problem of fungal growth in such system has been removed by the addition of Glutaraldehyde (GA). The conductivity of the Arrowroot + NaI electrolyte system is of the order of 6.7 Ã - 10-4 S cm-1. Starch without crosslinker GA seems to be highly unstable and prone to fungal growth whereas the GA crosslinked electrolyte is quite stable. Color of the material as well as its conductivity data indicate that presence of GA affects the salt dissociation and multiplets formation. Ionic transference number of system is ≥ 0.95 indicating that prepared system is ionic conductor/electrolyte. © 2011 Wiley Periodicals, Inc.
Effect of hole transport layer doping on the organic light emitting diode performance
(2008) Gayatri Chauhan; Ritu Srivastava; P.C. Srivastava; M.N. Kamalasanan
Effects of doping on the hole conductivity of α-NPD has been analyzed by fabricating the hole-only device and organic light-emitting devices using pure and doped α-NPD. Doping increases the current densities by 2 orders of magnitude. OLED with 0.4 wt % F4-TCNQ doped α-NPD shows the maximum luminescence and efficiency.
Effect of impurity concentration on optical and magnetic properties in ZnS:Cu nanoparticles
(Elsevier B.V., 2017) Prayas Chandra Patel; Surajit Ghosh; P.C. Srivastava
To obtain enhanced room temperature ferromagnetism (RTFM) along with the increase in optical bandgap in the compound semiconductors has been an interesting topic. Here, we report RTFM along with increase in energy bandgap in chemically synthesized Zn1−xCuxS (0 ≤ x ≤ 0.04) DMS nanoparticles. Structural properties of the synthesized samples studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show the formation of cubic phase Cu doped ZnS nanoparticles of ~3–5 nm size. An intrinsic weak ferromagnetic behavior was observed in pure ZnS sample (at 300 K) which got increased in Cu doped samples and was understood due to defect induced ferromagnetism. UV–vis measurement showed increase in the energy bandgap with the increase in Cu doping. The PL study suggested the presence of sulfur and zinc vacancies and surface defects which were understood contributing to the intrinsic FM behavior. © 2017 Elsevier B.V.
Effect of interfacial modifications on magnetic, morphological and transport properties of CoFe/n-Si thin film structures using ion irradiation
(Elsevier B.V., 2019) Arvind Kumar; Neelabh Srivastava; P.C. Srivastava
Interfacial structures of ferromagnetic (FM) metal combined with semiconductor (SC) are key step for realizing spintronic devices where spin degree of freedom can also be utilized in addition to the charge of electron. Tailoring of such interfaces (of FM/SC) is known to tune the magnetic, optical, structural and transport properties. Swift heavy ion irradiation (SHI) can be used as a potential tool to tailor the interfaces. In our present work, effect of ion beam irradiation (100 meV Ni+7 ions) on the CoFe/n-Si interfacial structures had been studied. Thin films of CoFe was deposited by electron beam evaporation technique using CoFe metallic alloy on Si substrate. The realized interfacial structure was characterized from structural, morphological and magnetic point of view. The surface exhibits grainy structure with very fine domain (∼50 nm) on the irradiation. Electronic transport study across the interface shows an increase in current on irradiation with significant magnetic field sensitivity and a magnetoresistance of ∼7% has been observed. The observed results of increased magnetization and enhanced magnetic field sensitivity has been understood in the realm of irradiation induced interfacial intermixing across the interfaces. The chemical phases formed and nature of the interface as a result of irradiation was probed by X-ray photoelectron spectroscopy (XPS) study which confirms the formation of various silicide phases along with the metallic phase. The results were also compared with our earlier studies [15–16] on CoFe/n-Si system and found that fluence of 1 × 1011 ions/cm2 seems significant to result in interesting magnetic and transport properties without much damage to interface. © 2019 Elsevier B.V.