12 results on '"S R Sarath Kumar"'
Search Results
2. Doping site dependent thermoelectric properties of epitaxial strontium titanate thin films
- Author
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Husam N. Alshareef, Terry M. Tritt, Anas Abutaha, S. R. Sarath Kumar, and Arash Mehdizadeh Dehkordi
- Subjects
Materials science ,business.industry ,Doping ,General Chemistry ,Epitaxy ,chemistry.chemical_compound ,Thermal conductivity ,chemistry ,Electrical resistivity and conductivity ,Seebeck coefficient ,Thermoelectric effect ,Materials Chemistry ,Strontium titanate ,Optoelectronics ,Thin film ,business - Abstract
We demonstrate that the thermoelectric properties of epitaxial strontium titanate (STO) thin films can be improved by additional B-site doping of A-site doped ABO3 type perovskite STO. The additional B-site doping of A-site doped STO results in increased electrical conductivity, but at the expense of Seebeck coefficient. However, doping on both sites of the STO lattice significantly reduces the lattice thermal conductivity of STO by adding more densely and strategically distributed phononic scattering centers that attack wider phonon spectra. The additional B-site doping limits the trade-off relationship between the electrical conductivity and total thermal conductivity of A-site doped STO, leading to an improvement in the room-temperature thermoelectric figure of merit, ZT. The 5% Pr3+ and 20% Nb5+ double-doped STO film exhibits the best ZT of 0.016 at room temperature.
- Published
- 2014
3. Thermoelectric Properties of Strontium Titanate Superlattices Incorporating Niobium Oxide Nanolayers
- Author
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Terry M. Tritt, S. R. Sarath Kumar, Husam N. Alshareef, Mohamed N. Hedhili, and Dongkyu Cha
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Materials science ,General Chemical Engineering ,Superlattice ,Analytical chemistry ,General Chemistry ,chemistry.chemical_compound ,chemistry ,X-ray photoelectron spectroscopy ,Electrical resistivity and conductivity ,Seebeck coefficient ,Thermoelectric effect ,Materials Chemistry ,Strontium titanate ,Niobium oxide ,Orthorhombic crystal system - Abstract
A novel superlattice structure based on epitaxial nanoscale layers of NbOx and Nb-doped SrTiO3 is fabricated using a layer-by-layer approach on lattice matched LAO substrates. The absolute Seebeck coefficient and electrical conductivity of the [(NbOx)a/(Nb-doped SrTiO3)b]20 superlattices (SLs) were found to increase with decreasing layer thickness ratio (a/b ratio), reaching, at high temperatures, a power factor that is comparable to epitaxial Nb-doped SrTiO3 (STNO) films (∼0.7 W m–1 K–1). High temperature studies reveal that the SLs behave as n-type semiconductors and undergo an irreversible change at a varying crossover temperature that depends on the a/b ratio. By use of high resolution X-ray photoelectron spectroscopy and X-ray diffraction, the irreversible changes are identified to be due to a phase transformation from cubic NbO to orthorhombic Nb2O5, which limits the highest temperature of stable operation of the superlattice to 950 K.
- Published
- 2014
4. Orthorhombic Ti2O3: A Polymorph-Dependent Narrow-Bandgap Ferromagnetic Oxide
- Author
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Stephen J. Pennycook, Tom Wu, S. R. Sarath Kumar, Husam N. Alshareef, Haijun Wu, Shuai Dong, Mark B. H. Breese, Jingsheng Chen, Nimer Wehbe, Xiaojiang Yu, Xinmao Yin, Yangyang Li, and Yakui Weng
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Materials science ,Band gap ,Foundation (engineering) ,Oxide ,02 engineering and technology ,021001 nanoscience & nanotechnology ,Condensed Matter Physics ,01 natural sciences ,Engineering physics ,Electronic, Optical and Magnetic Materials ,Biomaterials ,chemistry.chemical_compound ,chemistry ,Ferromagnetism ,0103 physical sciences ,Electrochemistry ,Natural science ,Orthorhombic crystal system ,010306 general physics ,0210 nano-technology - Abstract
This work was supported by the King Abdullah University of Science and Technology (KAUST). Y.K.W. and S.D. were supported by National Natural Science Foundation of China (Grant No. 11674055). J.S.C. was supported by the Singapore National Research Foundation under CRP Award No. NRF-CRP10-2012-02 and IIP award No. NRF-IIP001-001.
- Published
- 2017
5. Transparent ITO-Mn:ITO Thin-Film Thermocouples
- Author
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R.B Rakhi, S R Sarath Kumar, and Subbiah Kasiviswanathan
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Materials science ,business.industry ,Doping ,Thin film thermocouples ,Optical transparency ,chemistry.chemical_element ,Manganese ,Indium tin oxide ,chemistry ,Thermocouple ,Electrical resistivity and conductivity ,Thermoelectric effect ,Electronic engineering ,Optoelectronics ,Electrical and Electronic Engineering ,business ,Instrumentation - Abstract
Transparent thin-film thermocouples have been fabricated with indium tin oxide (ITO) and manganese doped indium tin oxide (Mn:ITO) as the positive and negative legs, respectively. Optical transparency of the thermocouples was above 85% in the visible region of the spectrum. Presence of Mn has been found to enhance the thermoelectric response, without significantly increasing the resistivity. Seebeck voltages of ~ 20 mV and ~ 31 mV have been obtained at 625 K, respectively, for the thermocouples with Mn:ITO legs having 1.6 at.% and 4.3 at.% Mn. The response has been found to be reproducible with an error of plusmn2%.
- Published
- 2009
6. High temperature thermoelectric properties of strontium titanate thin films with oxygen vacancy and niobium doping
- Author
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Husam N. Alshareef, S. R. Sarath Kumar, and Abeer Z. Barasheed
- Subjects
Materials science ,Dopant ,Annealing (metallurgy) ,Inorganic chemistry ,Doping ,Analytical chemistry ,Thermoelectric materials ,chemistry.chemical_compound ,Lattice constant ,chemistry ,Seebeck coefficient ,Thermoelectric effect ,Strontium titanate ,General Materials Science - Abstract
We report the evolution of high temperature thermoelectric properties of SrTiO3 thin films doped with Nb and oxygen vacancies. Structure-property relations in this important thermoelectric oxide are elucidated and the variation of transport properties with dopant concentrations is discussed. Oxygen vacancies are incorporated during growth or annealing in Ar/H2 above 800 K. An increase in lattice constant due to the inclusion of Nb and oxygen vacancies is found to result in an increase in carrier density and electrical conductivity with simultaneous decrease in carrier effective mass and Seebeck coefficient. The lattice thermal conductivity at 300 K is found to be 2.22 W m(-1) K(-1), and the estimated figure of merit is 0.29 at 1000 K.
- Published
- 2013
7. Correlation of Mn charge state with the electrical resistivity of Mn doped indium tin oxide thin films
- Author
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R.B Rakhi, Husam Niman Alshareef, S R Sarath Kumar, Mohamed Nejib Hedhili, and Subbiah Kasiviswanathan
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Manganese compounds ,Photolithography ,Materials science ,Physics and Astronomy (miscellaneous) ,Binding energy ,Analytical chemistry ,chemistry.chemical_element ,Manganese ,Indium ,Electric conductivity ,Electrical resistivity and conductivity ,Indium compounds ,Charge state ,Core levels ,Mn doped ,Thin film ,Oxide films ,Three component ,Tin oxides ,Electrical resistivity ,Charge (physics) ,Charge compensation ,Satellite peaks ,Indium tin oxide ,Indium Tin Oxide films ,chemistry ,Indium tin oxide thin films ,Tin ,Core level ,Mn-doped ,Mn concentrations ,Titanium compounds ,Bonding analysis - Abstract
Correlation of charge state of Mn with the increase in resistivity with Mn concentration is demonstrated in Mn-doped indium tin oxide films. Bonding analysis shows that Mn 2p3/2 core level can be deconvoluted into three components corresponding to Mn2+ and Mn4+ with binding energies 640.8 eV and 642.7 eV, respectively, and a Mn2+ satellite at ?5.4 eV away from the Mn2+ peak. The presence of the satellite peak unambiguously proves that Mn exists in the +2 charge state. The ratio of concentration of Mn2+ to Mn4+ of ?4:1 suggests that charge compensation occurs in the n-type films causing the resistivity increase. � 2010 American Institute of Physics.
- Published
- 2010
8. Role of oxygen vacancies in the high-temperature thermopower of indium oxide and indium tin oxide films
- Author
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R.B Rakhi, S R Sarath Kumar, and Subbiah Kasiviswanathan
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Electric resistance ,Photolithography ,Materials science ,Optical absorptions ,Absorption spectroscopy ,Inorganic chemistry ,Oxide ,Analytical chemistry ,chemistry.chemical_element ,High temperatures ,Indium oxides ,Indium ,Electrical resistivities ,Absorption ,chemistry.chemical_compound ,Sputtering ,Electrical resistivity and conductivity ,Ito films ,Materials Chemistry ,Light absorption ,Electrical and Electronic Engineering ,Thin film ,Oxide films ,Thermopower measurements ,Condensed Matter Physics ,Tin oxide ,Room temperatures ,Electronic, Optical and Magnetic Materials ,Indium tin oxide ,Oxygen ,Oxygen loss ,chemistry ,Indium tin oxide thin films ,Oxygen vacancies ,Tin ,Thermopower ,DC sputtering ,Titanium compounds ,As-grown ,Indium tin oxide films ,Electric network analysis - Abstract
Thermopower measurements in a range of 300-650 K along with room temperature optical absorption and electrical resistivity studies were performed on indium oxide (IO) and indium tin oxide (ITO) thin films grown by reactive dc sputtering. The thermopower of as-grown and oxygen-annealed IO and ITO films measured in Ar ambient displayed characteristics attributable to oxygen loss. The observations were substantiated with optical absorption and electrical resistivity results. � 2009 IOP Publishing Ltd.
- Published
- 2009
9. Ion beam studies on reactive DC sputtered manganese doped indium tin oxide thin films
- Author
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Thomas Osipowicz, P. Malar, S. Kasiviswanathan, Swarnadeep Banerjee, and S. R. Sarath Kumar
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Nuclear and High Energy Physics ,Materials science ,Silicon ,Analytical chemistry ,chemistry.chemical_element ,Manganese ,Rutherford backscattering spectrometry ,Indium tin oxide ,chemistry ,Sputtering ,Indium compounds ,Ion beams ,Magnetoelectronics ,Rutherford backscattering spectroscopy ,Saturation magnetization ,X ray diffraction ,Magnetic elements ,Spin-based electronics ,Thin films ,Thin film ,Tin ,Instrumentation ,Indium - Abstract
Indium based transparent conducting oxides doped with magnetic elements have been studied intensively in recent years with a view to develop novel ferromagnetic semiconductors for spin-based electronics. In the present work, we have grown manganese doped indium tin oxide (Mn:ITO) thin films, onto Si and Si/SiO2 substrates by DC reactive sputtering of a composite target containing indium-tin alloy and manganese, in a gas mixture of oxygen and argon. Glancing angle X-ray diffraction (GXRD) studies reveal the polycrystalline nature of the films. Magnetic measurements carried out using vibrating sample magnetometer (VSM) suggest that the films are ferromagnetic at room temperature, with a saturation magnetization of ?22.8 emu/cm3. The atomic percentages of In, Sn, Mn and O, as estimated using Rutherford backscattering spectrometry (RBS) are 37.0, 4.0, 1.6 and 57.4, respectively. RBS measurements reveal that the interface of the films with Si substrate has a ?30 nm thick intermediate layer. This layer consists of oxygen, silicon, indium, tin and manganese, in the ratio 1:0.56:0.21:0.07:0.03, indicative of diffusion of elements across the interface. The films on Si/SiO2, on the other hand, have a sharp interface. � 2007 Elsevier B.V. All rights reserved.
- Published
- 2008
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10. Temperature dependent thermoelectric properties of chemically derived gallium zinc oxide thin films
- Author
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Husam N. Alshareef, S. R. Sarath Kumar, and Abeer Z. Barasheed
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Materials science ,Dopant ,Annealing (metallurgy) ,chemistry.chemical_element ,Nanotechnology ,General Chemistry ,Zinc oxide thin films ,Oxygen ,chemistry ,Chemical engineering ,Thermoelectric effect ,Materials Chemistry ,Thin film ,Gallium - Abstract
In this study, the temperature dependent thermoelectric properties of sol–gel prepared ZnO and 3% Ga-doped ZnO (GZO) thin films have been explored. The power factor of GZO films, as compared to ZnO, is improved by nearly 17% at high temperature. A stabilization anneal, prior to thermoelectric measurements, in a strongly reducing Ar/H2 (95/5) atmosphere at 500 °C was found to effectively stabilize the chemically derived films, practically eliminating hysteresis during thermoelectric measurements. Subtle changes in the thermoelectric properties of stabilized films have been correlated to oxygen vacancies and excitonic levels that are known to exist in ZnO-based thin films. The role of Ga dopants and defects, formed upon annealing, in driving the observed complex temperature dependence of the thermoelectric properties is discussed.
- Published
- 2013
11. Enhanced carrier density in Nb-doped SrTiO3thermoelectrics
- Author
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Husam N. Alshareef, Kemal Özdoğan, S. R. Sarath Kumar, Udo Schwingenschlögl, and M. Upadhyay Kahaly
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Materials science ,Condensed matter physics ,chemistry ,Band gap ,Ab initio quantum chemistry methods ,Seebeck coefficient ,Thermoelectric effect ,Niobium ,General Physics and Astronomy ,chemistry.chemical_element ,Density functional theory ,Thermoelectric materials ,Epitaxy - Abstract
We study epitaxial SrTiO3 interfaced with Nb-doped SrTi1-xNbxO3 (x = 0, 0.125, 0.25, 0.375, and 0.5) by full-potential density functional theory. From the electronic band structures obtained by our ab-initio calculations we determine the dependence of the induced metallicity on the Nb concentration. We obtain a monotonous increase of the carrier density with the Nb concentration. The results are confirmed by experiments for SrTi0.88Nb0.12O3 and SrTi0.8Nb0.2O3, demonstrating the predictive power and limitations of our theoretical approach. We also show that the Seebeck coefficient decreases monotonously with increasing temperature.
- Published
- 2012
12. Modeling the transport properties of epitaxially grown thermoelectric oxide thin films using spectroscopic ellipsometry
- Author
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R.B Rakhi, Anas Abutaha, Husam Niman Alshareef, S R Sarath Kumar, and Mohamed Nejib Hedhili
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chemistry.chemical_compound ,Materials science ,Physics and Astronomy (miscellaneous) ,chemistry ,Ellipsometry ,Annealing (metallurgy) ,Seebeck coefficient ,Thermoelectric effect ,Oxide ,Analytical chemistry ,Electrical measurements ,Thin film ,Thermoelectric materials - Abstract
The influence of oxygen vacancies on the transport properties of epitaxial thermoelectric (Sr,La)TiO3 thin films is determined using electrical and spectroscopic ellipsometry (SE) measurements. Oxygen vacancy concentration was varied by ex-situ annealing in Ar and Ar/H2. All films exhibited degenerate semiconducting behavior, and electrical conductivity decreased (258–133 S cm−1) with increasing oxygen content. Similar decrease in the Seebeck coefficient is observed and attributed to a decrease in effective mass (7.8–3.2 me), as determined by SE. Excellent agreement between transport properties deduced from SE and direct electrical measurements suggests that SE is an effective tool for studying oxide thin film thermoelectrics.
- Published
- 2012
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