Bhatia, Charanjit Singh
Email Address
elebcs@nus.edu.sg
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Publication Electrical detection of microwave assisted magnetization reversal by spin pumping(2014-03-24) Rao, S.; Subhra Mukherjee, S.; Elyasi, M.; Singh Bhatia, C.; Yang, H.; ELECTRICAL & COMPUTER ENGINEERINGMicrowave assisted magnetization reversal has been investigated in a bilayer system of Pt/ferromagnet by detecting a change in the polarity of the spin pumping signal. The reversal process is studied in two material systems, Pt/CoFeB and Pt/NiFe, for different aspect ratios. The onset of the switching behavior is indicated by a sharp transition in the spin pumping voltage. At a threshold value of the external field, the switching process changes from partial to full reversal with increasing microwave power. The proposed method provides a simple way to detect microwave assisted magnetization reversal. © 2014 AIP Publishing LLC.Publication Electric-field-induced magnetization changes in Co/Al2O 3 granular multilayers(2013-01-24) Sahadevan, A.M.; Kalitsov, A.; Kalon, G.; Bhatia, C.S.; Velev, J.; Yang, H.; ELECTRICAL & COMPUTER ENGINEERINGWe study experimentally the effect of electric field on the magnetization of Co/Al2O3 granular multilayers. We observe two distinct regimes: (a) low-field regime when the net magnetization of the system changes in a reversible way with the applied electric field and (b) high-field regime when the magnetization decreases irreversibly. The former is attributed to the changes in the relative 3d-orbital occupation of the minority and majority bands in the Co granules. A theoretical model has been developed to explain the electric-field-induced changes in the band structure of the granular system and hence the magnetic moment. The latter result may be understood assuming the electric field induces oxygen migration from Al2O3 to the Co granules, since an increase in oxidation state of the Co granules is shown, through ab initio calculations, to give rise to a reduced magnetization of the system. © 2013 American Physical Society.Publication Ambipolar bistable switching effect of graphene(2010-12-27) Shin, Y.J.; Kwon, J.H.; Kalon, G.; Lam, K.-T.; Bhatia, C.S.; Liang, G.; Yang, H.; ELECTRICAL & COMPUTER ENGINEERINGReproducible current hysteresis is observed in graphene with a back gate structure in a two-terminal configuration. An opposite sequence of switching with different charge carriers, holes, and electrons is found. The charging and discharging effect is proposed to explain this ambipolar bistable hysteretic switching. To confirm this hypothesis, one-level transport model simulations including charging effect are performed and the results are consistent with our experimental data. Methods of improving the on/off ratio of graphene resistive switching are suggested. © 2010 American Institute of Physics.Publication Bi-level surface modification of hard disk media by carbon using filtered cathodic vacuum arc: Reduced overcoat thickness without reduced corrosion performance(2014) Yeo, R.J.; Rismani, E.; Dwivedi, N.; Blackwood, D.J.; Tan, H.R.; Zhang, Z.; Tripathy, S.; Bhatia, C.S.; ELECTRICAL & COMPUTER ENGINEERING; MATERIALS SCIENCE AND ENGINEERINGThe corrosion performance of commercial hard disk media which was subjected to bi-level surface modification has been reported. The surface treatment was carried out by bombarding the surface of the magnetic media with C+ ions at 350 eV followed by 90 eV using filtered cathodic vacuum arc (FCVA). The energy and embedment depth of the impinging C+ ions were adjusted by applying an optimized bias to the substrate and simulated by a Stopping and Range of Ions in Matter (SRIM) code which predicted the formation of a graded atomically mixed layer at the carbon-media interface. Cross-section transmission electron microscopy (TEM) revealed the formation of a 1.8 nm dense nano-layered carbon overcoat structure on the surface of the media. Despite an ~ 33% reduction in the thickness, the bi-level surface modified disk showed corrosion performance similar to that of a commercially manufactured disk with a thicker carbon overcoat of 2.7 nm. This improvement in the corrosion/oxidation resistance per unit thickness can be attributed to the formation of a dense and highly sp3 bonded carbon layer, as revealed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. This study demonstrates the effectiveness of the bi-level surface modification technique in forming an ultra-thin yet protective overcoat for future hard disks with high areal densities. © 2014 Elsevier B.V.Publication Nanopatterned and self-cleaning glass substrates for solar cell packaging(2012) Sakhuja, M.; Son, J.; Le, H.V.; Baojuan, X.; Verma, L.K.; Zeng, H.C.; Yang, H.; Danner, A.J.; Bhatia, C.S.; ELECTRICAL & COMPUTER ENGINEERINGSolar energy is indispensable and needs to be developed to meet the future energy demands of the world. One of the impediments to this technology is the low efficiency of the solar module, the glass shield being one of the reasons. At present, solar modules suffer from 4% transmission loss at the air/glass interface (at normal incidence) which increases with an increase in the accumulation of dust particles. Hence, in this paper techniques have been presented to reduce the transmission loss in solar modules as well as to keep them clean. Nanopatterning of the glass shield surface exhibited an increase of ∼3% in optical transmission as well as an improvement in omnidirectional antireflective behaviour. A solar module was also fabricated which exhibited an improvement of ∼6% in the efficiency with the nanopatterning of packaging glass cover. Perfluorodecyltriethoxysilane (PFTS) and Titanium dioxide (TiO 2) coatings have been employed for self-cleaning applications. Setup and performance of a 10 kW photovoltaic system has also been discussed. © 2011 IEEE.Publication Electrical transport properties of polycrystalline CVD graphene on SiO 2/Si substrate(2014-05) Kumari, A.; Prasad, N.; Bhatnagar, P.K.; Mathur, P.C.; Yadav, A.K.; Tomy, C.V.; Bhatia, C.S.; ELECTRICAL & COMPUTER ENGINEERINGTransport behavior of single layer graphene (SLG) grown by chemical vapor deposition technique on copper foil and transferred to SiO2/Si substrate has been studied by measuring the dc conductivity and Hall mobility in the temperature range 2-460 K. The samples of size 1 × 1 cm2 have been found to be polycrystalline in nature. Raman spectrum has been studied at various locations of the sample and the formation of SLG has been confirmed. From dc conductivity and mobility measurements it has been concluded that the one dimensional grain boundary defects are mainly responsible for the deterioration of mobility and conductivity of charge carriers in the polycrystalline samples. © 2014 Elsevier B.V. All rights reserved.Publication Enhancement of optical transmission with random nanohole structures(2011-01-03) Son, J.; Kumar Verma, L.; Danner, A.J.; Singh Bhatia, C.; Yang, H.; ELECTRICAL & COMPUTER ENGINEERINGWe demonstrate an enhancement of optical transmission by creating randomly distributed nanoholes in a glass surface using a simple bottom-up fabrication process. V-shaped holes with sub-100 nm diameter are created by anodized aluminum oxide template and dry etching on glass substrates. The broadband and omnidirectional antireflective effect of the proposed nanostructures is confirmed by measuring the transmittance of the patterned glasses, leading to 3% better transmission. Subsequently, the short-circuit current and the open-circuit voltage of a solar cell with nanostructures are enhanced by 3-4%, improving the solar cell efficiency from 10.47% to 11.20% after two weeks of outdoor testing. ©2010 Optical Society of America.Publication Excellent c-Si surface passivation by thermal atomic layer deposited aluminum oxide after industrial firing activation(2013-09-25) Liao, B.; Stangl, R.; Ma, F.; Mueller, T.; Lin, F.; Aberle, A.G.; Bhatia, C.S.; Hoex, B.; ELECTRICAL & COMPUTER ENGINEERING; SOLAR ENERGY RESEARCH INST OF S'POREWe demonstrate that by using a water (H2O)-based thermal atomic layer deposited (ALD) aluminum oxide (Al2O3) film, excellent surface passivation can be attained on planar low-resistivity silicon wafers. Effective carrier lifetime values of up to 12 ms and surface recombination velocities as low as 0.33 cm s-1 are achieved on float-zone wafers after a post-deposition thermal activation of the Al 2O3 passivation layer. This post-deposition activation is achieved using an industrial high-temperature firing process which is commonly used for contact formation of standard screen-printed silicon solar cells. Neither a low-temperature post-deposition anneal nor a silicon nitride capping layer is required in this case. Deposition temperatures in the 100-400 °C range and peak firing temperatures of about 800°C (set temperature) are investigated. Photoluminescence imaging shows that the surface passivation is laterally uniform. Corona charging and capacitance-voltage measurements reveal that the negative fixed charge density near the AlOx/c-Si interface increases from 1.4 × 1012 to 3.3 × 1012 cm-2 due to firing, while the midgap interface defect density reduces from 3.3 × 1011 to 0.8 × 1011 cm-2 eV-1. This work demonstrates that direct firing activation of thermal ALD Al2O3 is feasible, which could be beneficial for solar cell manufacturing. © 2013 IOP Publishing Ltd.Publication Frictional characteristics of exfoliated and epitaxial graphene(2011-10) Shin, Y.J.; Stromberg, R.; Nay, R.; Huang, H.; Wee, A.T.S.; Yang, H.; Bhatia, C.S.; ELECTRICAL & COMPUTER ENGINEERING; PHYSICSTo determine the friction coefficient of graphene, micro-scale scratch tests are conducted on exfoliated and epitaxial graphene at ambient conditions. The experimental results show that the monolayer, bilayer, and trilayer graphene all yield friction coefficients of approximately 0.03. The friction coefficient of pristine graphene is less than that of disordered graphene, which is treated by oxygen plasma. Ramping force scratch tests are performed on graphene with various numbers of layers to determine the normal load required for the probe to penetrate graphene. A very low friction coefficient and also its high pressure resistance make graphene a promising material for antiwear coatings. © 2011 Elsevier Ltd. All rights reserved.Publication Single contact electron beam induced current technique for solar cell characterization(2013) Meng, L.; Street, A.G.; Phang, J.C.H.; Bhatia, C.S.; ELECTRICAL & COMPUTER ENGINEERINGThis paper reports the first demonstration of single contact electron beam induced current (SCEBIC) technique on multicrystalline silicon (mc-Si) solar cells. A lumped single-diode analytical model is also proposed to theoretically explain the SCEBIC phenomenon within solar cells as well as the current transient characteristics of the major model parameters, such as shunt resistance Rsh, junction capacitance Cj, and parasitic capacitance Cs. The accuracy of the analytical model is then verified using PSPICE simulations, which show a close match with the experimental results. It is found that a large value of parasitic capacitance Cs is necessary to achieve good SCEBIC signal strength with a relatively low signal-to-noise ratio (SNR), and this is realized experimentally by adopting a metal enclosure in the measurement setup. In addition, the advantage of SCEBIC over conventional double-contact method is also demonstrated by SCEBIC characterization of partially processed solar cells, which clearly illustrates the high degree of flexibility of SCEBIC in solar cell characterization. © 2013 IEEE.