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Mayandi, Jeyanthinath; GOVINDARAJ, Baby Sri Pratha; SOMU, Allwin Rajesh; Venkatesan, Ragavendran; JAMESPANDI, Annaraj & Finstad, Terje Gunnar
(2023).
Facile Hydrothermal Synthesis and Characterization of Vanadium Pentoxide Hollow Spheres on Activated Carbon Template as an Electrode Material for Hybrid Supercapacitor.
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Finstad, Terje Gunnar; Mayandi, Jeyanthinath & Karlsen, Ole Bjørn
(2023).
Thermoelectric Multi-pnictogens Semiconductors: Cu Doping of ZnSbAs Alloys.
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Finstad, Terje Gunnar; Mayandi, Jeyanthinath; Soltani, Nayereh; Pokle, Anuj; Løvvik, Ole Martin & Almeida Carvalho, Patricia
(2023).
TiFe2Si Thin Films.
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Mayandi, Jeyanthinath; Soltani, Nayereh; Finstad, Terje Gunnar; Karlsen, Ole Bjørn & Flage-Larsen, Espen
(2022).
Zinc Multi-Pnictogens Semiconductors: Cu Doping and Thermoelectric Characteristics.
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Vijayan, Athira; Ragavendran, V.; Mayandi, Jeyanthinath & Karazhanov, Smagul
(2021).
Analysis of different concentration of TiCl4 treated with TiO2 as photoanode for dye-sensitized solar cell application.
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Almeida Carvalho, Patricia; Mayandi, Jeyanthinath; Rahman, Jamil Ur; Soltani, Nayereh; Schrade, Matthias & Gunnæs, Anette Eleonora
[Vis alle 9 forfattere av denne artikkelen]
(2021).
Exploring the structure of high entropy oxides, silicides and antimonides for functional applications.
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Mayandi, Jeyanthinath; Finstad, Terje; Stange, Marit Synnøve Sæverud; Vasque, Giovanni; Sunding, Martin Fleissner & Løvvik, Ole Martin
[Vis alle 8 forfattere av denne artikkelen]
(2021).
Engineering Electrical Properties of Reactively Sputtered High Entropy Alloy CrFeNiCoCu Films.
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Oxide containing films were deposited by reactive sputtering using a high entropy alloy (HEA) target made of equal concentrations of the five 3d-transion metals: Cr, Fe, Co, Ni, and Cu. We report on controlled alterations to the electrical properties made by post-deposition heat treatments in oxidizing and reducing atmospheres respectively. The temperature coefficient of resistivity of the films could be varied between -1.2×10-3 K-1 through 0 and to +0.7×10-3 K-1, while the measured effective resistivity could be varied between 1.3×10-4 Ωcm and 1.2×10-3 Ωcm by post deposition processing. To study the transport mechanisms, we performed temperature dependent Hall effect measurements down to 10 K. We correlated the electrical behavior with structural measurements by a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) for elemental mapping at various length scales and secondary ions mass spectroscopy (SIMS). The key to engineering the resistivity is controlling the topology of the film. The topology of the as- deposited thin films can be controlled by the oxygen flow during reactive sputtering as the most important parameter. For zero oxygen flow we have obtained a homogenous HEA film having a FCC structure with the CrFeNiCoCu target. For high oxygen flow we have observed a high entropy oxide with a rock salt crystal structure. Here we report on the case of intermediate flow of oxygen. The as-deposited structure is then complex. There are nano sized regions with oxides and regions with FCC HEA alloys with different compositions of all the 3d-metals but also supersaturated with oxygen. The topology of these regions, and also their concentration, can be altered by post-deposition heat treatment. The transport in the HEA regions are dominated by alloy scattering. The potential in the FCC HEA has disorder due to the random mixing of the elements, which also gives distortion of the individual atom sites. This gives resistivities above the Mott- Ioffe-Regel limit while still having a positive TCR. These disorder effects will be further enhanced by the uptake of oxygen atoms. For these cases one gets weak localization, yielding a negative TCR. The oxides may be insulating but may also conduct by variable range hopping of the Efros–Shklovskii-type or the Mott- type yielding characteristic negative TCR. The post-deposition yields oxide segregation, of for example of spinel type containing Cr. This reduce the entropy and stability of the metal alloy, creating FCC and BCC metal alloys of different compositions. In the case of reducing the heat treatment in a reducing atmosphere, the oxides are reduced. The experimental resistivities are in excellent agreement with the model for the conduction.
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Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sagvolden, Espen; Sunding, Martin Fleissner; Dahl, Øystein & Schrade, Matthias
[Vis alle 12 forfattere av denne artikkelen]
(2019).
Structural and Electrical Properties of Sputtered HEA Thin Films of CrFeCoNiCu and their Oxidation Studies.
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High-entropy alloys (HEAs) represent a class of materials that is intensively investigated for a range of possible applications. They generally show a high degree of phase stability by the high entropy while the structure is a random atom position disorder unlike other alloys which can influence physical properties differently than regular alloys. In general, there are few studies on oxidation of HEA and studies on adding oxygen during the fabrication of HEA. In this study we have sputtered thin films of CrFeCoNiCu onto insulating and optically transparent substrates in order to measure structural, electrical and optical functional properties. We have varied the oxygen pressure in the sputtering environment as well as oxidizing the samples at elevated temperatures after deposition. Optical and electrical characterization was performed on films sputter deposited on fused quartz wafers. The films were characterized by TEM, XRD and XPS. The films with no intentional oxygen had an FCC structure with a texture showing strong (111) preferred orientation as seen by XRD. TEM analysis showed columnar morphology with twins parallel to (111) planes. Samples sputtered under high oxygen content showed a simple NaCl structure(FeO). The samples were annealed in air and O2 ambient in the temperature range of 300 to 500 °C. This caused an oxide layer growing on top of the FCC structure. XPS was utilized to find the atomic compositions and chemical stated of the elements. Hall measurements and Seebeck measurements were performed on the as prepared and oxidized films from 10 K to 600K. For the FCC structure the resistivity was a factor 104 higher than the elemental metals while can be satisfactory described by electron phonon scattering by the Bloch-Grüneisen description and the low temperature negative temperature effect by the Kondo effect. The sign of the Hall coefficient was positive while the Seebeck coefficient was negative, indicating the Fermi surface containing pockets of electrons and holes and an energy dependent scattering time. A detailed comparison of the as prepared and the oxidized thin films will be discussed in terms of the structural chemical and electrical properties of the grown films. In addition the electric properties will be discussed in terms of a model considering electronic structure and scattering.
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Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sunding, Martin Fleissner; Schrade, Matthias; Deuermeier, J. & Fortunato, E.
[Vis alle 10 forfattere av denne artikkelen]
(2019).
Physical properties of sputtered CrFeCoNiCu thin films on quartz substrate.
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Equiatomic high entropy alloys (HEA) have attracted considerable interest due to their exceptional properties, which might be closely related to their extreme disorder induced by chemical complexity. We have sputtered thin films of CrFeCoNiCu onto insulating and optically transparent substrates in order to measure electrical and optical functional properties. After several optimizations desired sputtering conditions were obtained for the deposition of CrFeCoNiCu onto fused quartz wafers. All the samples were characterized by structural, chemical and electrical methods. The as prepared films had fcc-type structure. Hall measurements were performed from 10 K to 600K. The resistivity was a factor 104 higher than the elemental metals and the temperature dependence was different. The sign of the Hall coefficient indicates that the film was dominated by holes, while the Seebeck coefficient was negative and dominated by electrons. The electric properties will be discussed in terms of a model considering electronic structure and scattering.
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Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Dahl, Øystein; Sunding, Martin Fleissner; Sagvolden, Espen & Schrade, Matthias
[Vis alle 12 forfattere av denne artikkelen]
(2019).
Exploring functional properties of high entropy oxides.
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The present work explores the concept of high-entropy alloys to design FeCoNi-based oxides suitable for functional applications. Fundamental changes in electronic behaviour produced by metal substitution and variable oxygen content were screened to investigate the potential of these materials for specific applications, such as transparent conductors and thermoelectric materials. Elements with atomic radius similar to the average value expected for FeCoNi have been selected for substitutional replacement: (i) Cr and Cu as relatively abundant commodities with intensive industrial application and (ii) Ge due its semiconductor nature.
We have sputtered CrFeCoNiCu and of GeFeCoNiCu thin films onto insulating and optically transparent substrates in order to measure electrical and optical properties. The deposition was done by reactive DC magnetron sputtering from targets with equimolar proportions of the metallic elements using atmospheres ranging from 0 to 20 at.% O2 /Ar. The films where characterized by structural, spectroscopic and electrical methods. Transmission electron microscopy showed that for oxygen content up to 8 at.% the materials adopted an fcc-type structure while the NaCl-structure was found for higher oxygen concentrations. X-ray-photoelectron spectroscopy was used to characterize the oxidation state of the metals. Standard absorption analysis by UV-VIS spectrometry revealed that the films had bandgaps ranging from 0.8 to 2.8 eV. The resistivity measured at RT ranged from 10-4 to values above 104 Ω.cm. Hall measurements and Seebeck measurements indicated that both electrons and holes contribute to conduction and at RT the Hall coefficient and Seebeck coefficients have different sign. This behaviour is discussed in terms of the structural analysis and suggested electronic model of the films.
In conclusion, the optical band gap and resistivity of the materials produced span over large ranges demonstrating potential for a variety of functional applications.
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Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sagvolden, Espen; Sunding, Martin Fleissner; Dahl, Øystein & Schrade, Matthias
[Vis alle 12 forfattere av denne artikkelen]
(2019).
Development of sputtered HEA thin films and their functional properties .
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Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sunding, Martin Fleissner; Diplas, Spyridon; Almeida Carvalho, Patricia & Finstad, Terje
(2019).
Oxidation of CrFeCoNiCu sputtered thin films .
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Marstein, Erik Stensrud; Mayandi, Jeyanthinath; Syre, Marie Vardenær; Olaisen, Birger Retterstøl; Vines, Lasse & Holt, Arve
(2011).
Limitations of P Gettering During Shallow Emitter Formation.
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Tang, Chi Kwong; Lund, Esben; Monakhov, Edouard; Mayandi, Jeyanthinath; Holt, Arve & Svensson, Bengt Gunnar
(2010).
Electrically active centers introduced in p-type Si by rapid thermal processing.
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Finstad, Terje; Mayandi, Jeyanthinath; Galeckas, Augustinas; Diplas, Spyridon; Petersson, Anna Malou & Balasundaraprabhu, R
(2008).
Visible luminescence from DC magnetron sputtered ZnO with and without Ge.
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Mayandi, Jeyanthinath; Finstad, Terje; Thøgersen, Annett; Galeckas, Augustinas; Foss, Steinar & Turan, R
(2008).
Embedded Si nanocrystals in SiO2: atomic force microscope determination of depth vs distribution.
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Mayandi, Jeyanthinath; Finstad, Terje; Heng, Chenglin; Foss, Steinar; Klette, Hallgeir & Li, Yanjun
(2007).
Photoluminescence at 1.5 μm from Er in an SiO2 layer with Ge nanoclusters: The local environments of Er atoms and comparison with he case of Si nanoclusters.
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Mayandi, Jeyanthinath; Galeckas, Augustinas; Petersson, A. M.; Finstad, Terje; Rangasamy, Balasundara Prabhu & Diplas, Spyridon
(2007).
Visible luminescence from DC magnetron sputtered ZnO with and without Germanium.
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Mayandi, Jeyanthinath; Finstad, Terje; Heng, Chenglin; Li, Yanjun; Thøgersen, Annett & Foss, Steinar
(2006).
Comparison of Er PL emission with Si and Ge ncs embedded in SiO2 on a Si substrate.
Vis sammendrag
We have studied the 1.5 µm photoluminescence (PL) from Er ions after annealing two different sample sets in the temperature range 500 °C to 1100 °C. The different sample sets were made by magnetron sputtering from composite targets of Si+SiO2+Er and Ge+SiO2+Er respectively for the different sample sets. The annealing induces Si – and Ge-nanoclusters respectively in the different film sets. The PL peak reaches its maximum intensity after annealing at 700 °C for samples with Ge nanoclusters and after annealing at 800 °C for samples with Si. No luminescence from nanoclusters was detected in neither sample sets. This is interpreted as an energy transfer from the nanocluster to Er atoms. Transmission electron microscopy shows that after annealing to the respective temperature yielding the maximum PL intensity both the Ge and Si clusters are non-crystalline. Here we mainly compare the spectral shape of Er luminescence emitted in these different nanostructured matrixes. The PL spectral shapes are clearly different and witness a different local environment for the Er ions.
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Mayandi, Jeyanthinath; Finstad, Terje; Foss, Steinar; Thøgersen, Annett; Turan, R & Serincan, U
(2006).
SPM approach to investigate luminescent nanoclusters in SiO2 Films.
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Mayandi, Jeyanthinath; Finstad, Terje; Foss, Steinar; Heng, Chenglin & Klette, Hallgeir
(2006).
Near Infrared Electroluminescence from RF Sputtered SiO2 Films Doped with Ge and Er Ions on Si.
Vis sammendrag
Room temperature infrared electroluminescence (EL) from metal-oxide–semiconductor devices on Si substrates will be discussed. Here we compare the luminescence from RF sputtered oxide films containing SiO2 with different amounts of Ge and Er by using a composite target. The films were annealed in the temperature range 600-900 °C. This densifies the films and is likely to reduce the concentration of defects. Ge clusters may also precipitate in the films after annealing. In most cases for the different heat treatments and concentrations a luminescence peak located around 1150-1170 nm is observed. The corresponding photon energy is close to that of the Si band gap. In addition we observe several broad luminescence bands in the 1000-1750 nm. The band near 1330 nm gets stronger with the presence of Ge in the SiO2 film. Some of these bands have previously been suggested are directly associated with Ge. The intensities of all bands are strongest for the as deposited film. The EL band in the 1550 nm region is not correlated with the presence of Er, which suggest that it is due to oxide defects or possibly interface states. The intensity is correlated with Ge concentration. The spectral shape of the EL depends upon the injection level. These dependencies will be presented together with structural analysis of the devices. Finally a comparison with device structures containing Si clusters will also be presented.
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Mayandi, Jeyanthinath; Finstad, Terje; Foss, Steinar; Heng, Chenglin; Klette, Hallgeir & Sagberg, H.
(2006).
On the Long-term Stability of Sputtered SiO2 Films with Ge-rich Nanoparticles: the Effect of Nanovoids on the Infrared Photo-Luminescence from Er ions.
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Embedded Ge-rich nanostructures in SiO2 have technological interest and can be prepared in different ways. Here we report on rf-magnetron sputtered films with varying deposition parameters having a large impact on the long-term stability of the physical properties of the films related to the particular microstructure of the films. We observe efficient room temperature infrared photoluminescence (PL) from a (Er, Ge) co-doped SiO2 films where the PL and its intensity is correlated with the microstructure containing Ge-rich nanoclusters. The Er PL is thus a probe of the films microstructure. The films with nanovoids are particularly interesting but are not beneficial for applications as they are correlated with long term instability. We have investigated the microstructure mainly with analytical transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) spectrometry. Samples have been annealed and measured systematically over a one-year period. The effect of annealing has been found to be dependant on the as-deposited structure. The films containing nanovoids show a peculiar dependence of PL intensity upon annealing time at 700 °C; It is observed a maximum after 30 min, then a minimum before the signal rises again. This annealing behaviour is changed by storage over a 6 months period.
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Mayandi, Jeyanthinath; Foss, Steinar; Finstad, Terje; Thøgersen, Annett; Serincan, U & Turan, R.
(2006).
Scanning Probe Measurements on Luminescent Si Nanoclusters in SiO2 Films.
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Mayandi, Jeyanthinath; Finstad, Terje; Heng, Chenglin; Foss, Steinar & Klette, Hallgeir
(2006).
Infrared Electroluminescence from a Si MOS Structure with Ge in the Oxide.
Vis sammendrag
We report on room-temperature infrared electroluminescence (EL) from metal-oxide–semiconductor devices made from Si. We compare the luminescence from RF sputtered oxide films containing SiO2 with different amounts of Ge by using a composite target. The films were annealed in the temperature range 700-900 °C. This densifies the films and is likely to reduce concentration of defects while in films containing Ge quantum dots are formed. In most cases for the different heat treatments and concentrations a luminescence peak located around 1150-1170 nm is observed. This photon energy is close to that of the Si band gap. In addition we observe several broad luminescence bands in the 1000-1750 nm. These bands get stronger with increasing Ge concentration in the SiO2 film. Some of these bands have previously been suggested are directly associated with Ge. Since we observe the intensity is correlated with the presence of Ge while the mere presence of the bands is not, we rather suggest the EL bands are due to defects which concentration is influenced by the presence of Ge in the oxide. The EL is observed for different bias polarities and different electrode contacts and shows very similar results are obtained in these cases. The spectral shape of the EL depends upon the injection level. These dependencies will be presented together with structural analysis of the device structures.
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Mayandi, Jeyanthinath; Heng, Chenglin; Foss, Steinar; Finstad, Terje & Klette, Hallgeir
(2005).
Efficient Infrared Photo-Luminescence from sputtered SiO2 films with Ge-rich nanoparticles and Er ions.
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Finstad, Terje; Mayandi, Jeyanthinath; Foss, Steinar; Serincan, U. & Turan, Rasit
(2005).
Luminescence in ion beam synthesized quantum dots correlated with nanocrystal depth and size distrubutions.
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Finstad, Terje; Heng, Chenglin; Li, Yanjun; Mayandi, Jeyanthinath; Olsen, Arne & Jørgensen, Sissel
(2005).
Strong luminescence from Er in an environment of Ge nanoclusters in SiO2.
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Heng, Chenglin; Finstad, Terje; Li, Yanjun; Mayandi, Jeyanthinath; Jørgensen, Sissel & Storås, Preben
[Vis alle 8 forfattere av denne artikkelen]
(2005).
The precipitation of Ge-rich nano particles in an (Er,Ge) co doped SiO2 matrix.
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Mayandi, Jeyanthinath; Finstad, Terje; Foss, Steinar; Thøgersen, Annett; Serincan, U. & Turan, Rasit
(2005).
Luminesce from ion beam synthesized Si nanocrystals embedded in SiO2 films and the effect of damage on nucleation.
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Mayandi, Jeyanthinath; Finstad, Terje; Foss, Steinar; Serincan, U. & Turan, Rasit
(2005).
Luminescence from Si nanocrystals in SiO2 films.
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Venkatesan, Ragavendran; Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Venkatachalapathy, Vishnukanthan & Pearce, Joshua
(2003).
Formation of 2D Hierarchical Si Nanostructure in Upgraded Metallurgical Grade (UMG) Wafers by Metal Assisted Chemical Etching.
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Mayandi, Jeyanthinath; Finstad, Terje & Kuznetsov, Andrej
(2007).
Embedded semiconductor nanocrystals/nanoclusters in oxides.
Unipub forlag.
ISSN 1501-7710.