Terje Gunnar Finstad

• Rahman, Jamil Ur; Soltani, Nayereh; Sunding, Martin Fleissner; Løvvik, Ole Martin; Gunnæs, Anette Eleonora & Finstad, Terje Gunnar [Vis alle 7 forfattere av denne artikkelen] (2026). Deciphering the structure of a high entropy Heusler antimonide. Acta Materialia. ISSN 1359-6454. Fulltekst i vitenarkiv
• Saravanan, C.; Anitha, K.; Finstad, Terje Gunnar & Mayandi, J. (2025). Study of diphenylamine scintillator crystals: Growth, structure characterization, analysis of optical properties, thermal behavior, lifetime measurements, DFT calculations, Hirshfeld surface analysis. Optical materials (Amsterdam). ISSN 0925-3467. 168. doi: 10.1016/j.optmat.2025.117377. Fulltekst i vitenarkiv
• Heng, C.L.; Li, X.L.; Wang, X.; Su, W.Y. & Finstad, Terje Gunnar (2024). Effects of ytterbium doping on the ultraviolet emissions of MgZnO films prepared by magnetron sputtering. Journal of Alloys and Compounds. ISSN 0925-8388. 981. doi: 10.1016/j.jallcom.2024.173704. Fulltekst i vitenarkiv
• Soltani, Nayereh; Rahman, Jamil Ur; Carvalho, Patricia Almeida & Finstad, Terje Gunnar (2023). A Highly Active and Reusable Multicomponent high Entropy Metal Oxide Catalyst for Nitroarenes Hydrogenation. Topics in catalysis. ISSN 1022-5528. doi: 10.1007/s11244-023-01846-z. Fulltekst i vitenarkiv Vis sammendrag

  It is well known that multicomponent catalysts can improve catalytic performance, however their rational design and precise control of catalytic activities by varying the composition of the elements is challenging. Herein, we present a facile and scalable synthetic strategy for production of senary and septenary metal oxide nanoparticles with a chemical composition of MSbOx and MSb2Ox (M: Fe, Ni, Co, Cu, and Zn). All samples were formed in a tetragonal crystal structure with space group 136 and crystallographic symmetry P42/mnm while the details of their constituent unit cells are different, belonging to rutile or trirutile structures. These nanocomposites have an oxygen vacancy-rich construction with uniform elemental distributions that produce various surface functionalities. They showed instantaneous hydrogenation catalytic performance for reduction of 4-nitrophenol to 4-aminophenol at room temperature. Among our samples, the senary catalyst with MSbOx chemical composition showed better durability and reusability owning to its morphological and microstructural properties. It showed 100% conversion of 4-nitrophenol to 4-aminophenol within 4 min at the 18th run without any by-products.

• Heng, Chenglin; Wang, Xuan; Zhao, Chaonan; Wu, Gang; Lv, Yanhui & Wu, Han-Chun [Vis alle 8 forfattere av denne artikkelen] (2023). Ultrathin Rare-Earth-Doped MoS2 Crystalline Films Prepared with Magnetron Sputtering and Ar + H2 Post-Annealing. Crystals. 13(2). doi: 10.3390/cryst13020308. Fulltekst i vitenarkiv
• Soltani, Nayereh; Rahman, Jamil Ur; Carvalho, Patricia Almeida & Finstad, Terje Gunnar (2023). Towards multicomponent intermetallic nanoparticles as efficient and low-cost catalysts. Journal of Alloys and Compounds. ISSN 0925-8388. 956, s. 1–11. doi: 10.1016/j.jallcom.2023.170254. Fulltekst i vitenarkiv
• Mayandi, Jeyanthinath; Schrade, Matthias; Ponniah, Vajeeston; Stange, Marit Synnøve Sæverud; Lind, Anna Maria & Sunding, Martin Fleissner [Vis alle 13 forfattere av denne artikkelen] (2022). High entropy alloy CrFeNiCoCu sputter deposited films: Structure, electrical properties, and oxidation. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 40(2). doi: 10.1116/6.0001394. Fulltekst i vitenarkiv Vis sammendrag

  High entropy alloy (HEA) films of CrFeCoNiCu were deposited by sputtering. Their structure was characterized and their electric transport properties were studied by temperature-dependent Hall and Seebeck measurements. The HEA films show a solid solution with an fcc structure. The residual electrical resistivity of the films is around 130 μΩ cm, which is higher than the Mott limit for a metal while the temperature dependence of the resistivity above 30 K is metal-like but with a small temperature coefficient of resistivity (2 ppm/K). The dominant scattering mechanism of charge carriers is alloy scattering due to chemical disorder in the HEA. The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure where the Fermi level passes through band states having both holes and electrons as indicated by band structure calculations. Below 30 K, the conduction is discussed in terms of weak localization and Kondo effects. The HEA structure appears stable for annealing in vacuum, while annealing in an oxygen-containing atmosphere causes the surface to oxidize and grow a Cr-rich oxide on the surface. This is then accompanied by demixing of the HEA solid solution and a decrease in the effective resistance of the film.

• Rahman, Jamil Ur; Carvalho, Patricia Almeida; Soltani, Nayereh; Schrade, Matthias; Gunnæs, Anette Eleonora & Finstad, Terje Gunnar (2022). Synthesis, microstructure, and thermoelectric properties of Sb-Based high entropy alloys. Intermetallics (Barking). ISSN 0966-9795. 143. doi: 10.1016/j.intermet.2022.107495. Fulltekst i vitenarkiv Vis sammendrag

  In this paper, we explore the synthesis of new antimonide compounds based upon concepts of high entropy alloys (HEA). HEAs are usually multi-element (⪆5) alloys with near the equiatomic composition. They have been given very large attention due to their vast potential within many fields. We investigated the thermoelectric transport properties of antimony-based high entropy alloys. The observed electrical conductivity, Seebeck coefficient, and thermal conductivity were comparable to those of cutting-edge half-Heusler thermoelectric alloys. Alloys with different valence electron counts (VEC) were synthesized and correlated with measured electrical properties.

• Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Dahl, Øystein; Ponniah, Vajeeston; Schrade, Matthias & Løvvik, Ole Martin [Vis alle 8 forfattere av denne artikkelen] (2022). Thin films made by reactive sputtering of high entropy alloy FeCoNiCuGe: Optical, electrical and structural properties. Thin Solid Films. ISSN 0040-6090. 744(139083). doi: 10.1016/j.tsf.2022.139083. Fulltekst i vitenarkiv Vis sammendrag

  Films were reactively sputtered from a high entropy alloy (HEA) FeCoNiCuGe target in an Ar/O2 plasma. The case of zero O2 gas flow yielded HEA films of FeCoNiCuGe. These as-deposited HEA films have a face centered cubic (FCC) structure, showing a texture with columnar grains containing many planar defects. The residual electrical resistivity of the films is around 225 μΩcm and the temperature dependence of the resistivity is metal-like. The temperature coefficient of resistivity is small (4.5 ppm/K). The Hall coefficient is positive while the Seebeck coefficient is negative. This is interpreted as arising from an electronic structure having both holes and electrons at the Fermi level as indicated by band structure calculations. The HEA FCC structure is unstable upon annealing in forming gas and showed demixing. Annealing in O2 also yielded inhomogeneous oxides, with a thick layer of CuO growing on the surface. The cases of reactive sputtering with an oxygen flow yielded oxides that are either nanocrystalline or amorphous dependent upon the sputter conditions. These can be classified as high entropy oxides (HEO) and have an optical bandgap around 1.9 eV and high transmission in the infrared region. The amorphous HEO has an electrical conduction interpreted as due to variable range hopping described by the Efros–Shklovskii theory. The HEO films were reduced in forming gas. For the amorphous HEO film, the reduction at 300–500 °C yielded hexagonal Ni5Ge2 and an FCC phase. For the nanocrystalline HEO, reduction resulted in creation of FCC and body centered cubic HEA metal phases.

• Zang, L.; Heng, C. L.; Wang, X.; Su, W. Y. & Finstad, Terje Gunnar (2022). Synergistically enhanced ultraviolet emission of Yb doped ZnO films by using a capping of ultrathin Al and SiO<inf>2</inf> microspheres. Optics Express. 30(21), s. 38167–38177. doi: 10.1364/OE.472497. Fulltekst i vitenarkiv Vis sammendrag

  We studied the enhancement effects of ultraviolet (UV) emission from rare earth ytterbium (Yb) doped ZnO films, by using capping layers of Al and SiO 2 micro-spheres. The films were deposited on Si substrates with magnetron sputtering followed by high temperature (∼1000°C) heat treatment, and then capped with a nanoscale ultrathin aluminum (Al) layer and/or SiO 2 micro-spheres on the surface of the films. The photoluminescence (PL) results indicate that compared to the case without any capping, the UV emission is enhanced by a factor ranging from several to dozens times, the films capped with 2.0 nm Al layer and 5.0 µm SiO 2 microspheres have the longest highest PL intensity among the samples. The PL enhancements are discussed in terms of increased optical (or electrical) fields around the surface of the films combined with defect passivation after the capping. Our work has proposed a strategy to enhance the UV emissions of ZnO, which will broaden the application potential of ZnO in UV photonics.

• Davis, Nithya; R, Althaf; Muraleedharan, Sreepriya; Thiruvenkatam, Vijayaraghavan; Mayandi, Jeyanthinath & Finstad, Terje Gunnar [Vis alle 9 forfattere av denne artikkelen] (2022). Grain boundary engineered, multilayer graphene incorporated LaCoO<inf>3</inf> composites with enhanced thermoelectric properties. Ceramics International. ISSN 0272-8842. 48(17), s. 24454–24461. doi: 10.1016/j.ceramint.2022.05.054. Fulltekst i vitenarkiv
• Soltani, Nayereh; Rahman, Jamil Ur; Carvalho, Patricia Almeida; Bazioti, Kalliopi & Finstad, Terje Gunnar (2022). From quaternary to senary high entropy antimonide nanoparticles by a facile and scalable thermal treatment method. Materials research bulletin. ISSN 0025-5408. 153. doi: 10.1016/j.materresbull.2022.111873. Fulltekst i vitenarkiv Vis sammendrag

  Transition metal antimonides form a class of intermetallic compounds, which has drawn considerable attention due to their potential applications in various fields. However, the formation of nanostructures containing multiple transition metal elements has been a challenge. Here, a new class of multicomponent antimonide nanoparticles with chemical composition ranging from quaternary to senary were synthesized via a simple, reproducible, and scalable thermal treatment method. This method allows uniform elemental distributions in a single nanoparticle, demonstrating the ability to obtain medium- and high-entropy antimonide nanostructures. The mechanism of formation was proposed and the characteristics of obtained nanoparticles were investigated by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. Under the achieved optimal synthesis conditions, the formation of a single-phase with a hexagonal structure (P63/mmc) was observed for the quaternary and quinary samples, whereas in the case of the senary samples, a trace of minor impurity can be observed.

• Zhang, L.; Heng, C. L.; Zhao, C. N.; Su, W. Y.; Gao, Y. K. & Yin, P. G. [Vis alle 7 forfattere av denne artikkelen] (2022). On the structure and ultraviolet emission of terbium doped zinc oxide thin films on silicon after high temperature treatment. Results in Physics. doi: 10.1016/j.rinp.2021.105121. Fulltekst i vitenarkiv Vis sammendrag

  Terbium (Tb) doped zinc oxide (ZnO) thin films were deposited on Si substrates by magnetron sputtering in an oxygen containing plasma. The Tb doping concentration was varied from 0 to 0.38 at.%. The effect of heat treatment at high temperature on the ultraviolet (UV) emission was studied along with the structure of the films. Photoluminescence (PL) spectra show that the UV intensity increases much by heat treatment and after 1000 °C annealing the doped films have stronger UV emission than that of an un-doped film. The enhanced UV emission is related to crystallinity improvements of the films. After heat treatment at 1000 °C Zn (Tb) silicates had formed. The film prepared with a Tb concentration of 0.029 at.% showed the best crystallinity and highest UV intensity. Finally, it was shown that the UV PL intensity can be increased further with TiN nanoparticle capping on the surface of the films.

• Mayandi, Jeyanthinath; Dias, M.; Stange, Marit Synnøve Sæverud; Lind, Anna Maria; Sunding, M.F. & Cerdeira, A.C. [Vis alle 12 forfattere av denne artikkelen] (2021). Partial oxidation of high entropy alloys: A route toward nanostructured ferromagnets. Materialia. 20. doi: 10.1016/j.mtla.2021.101250. Fulltekst i vitenarkiv Vis sammendrag

  The present work proposes a new approach to the development of nanostructured ferromagnetic composites based on progressive oxidation of high entropy alloys. Sequential ‘subtraction’ of the metals with the lowest oxidation energy destabilizes the remaining solid solution and induces phase separation at the nanoscale. By exploring immiscibility routes in the multidimensional phase diagrams, the strategy can be used to promote intimate contact between mixed oxides and metallic solid solutions with desired magnetic properties. To demonstrate the concept, the relation between oxidation energy, phase separation, and magnetic behavior has been investigated across the (CoCrCuFeNi)Ox composition space, for 0.1≤ x ≤1.3, in materials produced by reactive sputtering. The results point out directions for tuning the composition when aiming for phases with specific magnetic properties arranged in complex microstructures.

• Mayandi, Jeyanthinath; Finstad, Terje Gunnar; Stange, Marit Synnøve Sæverud; Vasquez, Geraldo Cristian; Sunding, Martin Fleissner & Løvvik, Ole Martin [Vis alle 8 forfattere av denne artikkelen] (2021). Controlling the Electrical Properties of Reactively Sputtered High Entropy Alloy CrFeNiCoCu Films. Journal of Electronic Materials. ISSN 0361-5235. 51, s. 803–812. doi: 10.1007/s11664-021-09343-3. Fulltekst i vitenarkiv Vis sammendrag

  Oxide-containing films were made by reactively sputtering a high-entropy alloy target of CrFeCoNiCu. We report on a wide range of changes to the electrical properties made by different heat treatments in oxidizing and reducing atmospheres, respectively. We combine temperature-dependent Hall effect measurements down to 10 K to study the transport mechanisms and correlate that with structural measurements by x-ray diffraction and scanning electron microscopy. The measured/effective resistivity could be varied between 1.3 × 10−4 Ω cm and 1.2 × 10−3 Ω cm by post-deposition processing. The temperature coefficient of resistivity could be varied between − 1.2 × 10−3 K−1 through 0 and to + 0.7 × 10−3 K−1. The key to the variation is controlling the morphology and topology of the film. The conduction of charge carriers is dominated by the relative contribution of weak localization and alloy scattering by varying the degree of disorder in the metallic high-entropy alloy and its topology.

• Song, Xin; Riis, Henrik; Prytz, Øystein & Finstad, Terje (2021). Metallization of ZnSb and contact resistance. Journal of Applied Physics. ISSN 0021-8979. 130(2). doi: 10.1063/5.0043958. Fulltekst i vitenarkiv Vis sammendrag

  We present results on electrical resistance of metal contacts to ZnSb. We synthesized the thermoelectric semiconductor ZnSb with specific doping concentrations by adding Cu as an acceptor to the melt, followed by solidification, crushing, ball-milling, hot-pressing, sawing, and polishing yielding wafers suitable for substrates for further processing. Many batches were made yielding different doping concentrations. We defined transmission line geometries in deposited metal films for specific contact resistance measurements. We prepared sets of Cu, Ti, and Ni films, respectively. We measured the contact resistance vs annealing temperatures. For Cu/ZnSb samples, we observed a specific contact resistance from 5 × 10−7 to 4 × 10−5 Ω cm2. We also measured the carrier concentration of ZnSb. The measurement data of the specific contact resistance had systematic dependence on doping concentration and annealing temperature and were analyzed by a model incorporating different transport mechanisms across the energy barrier at the metal–semiconductor interface. The data were discussed in terms of systematic variation in barrier height and density of states effective mass. We proposed these arising as a consequence of interactions at the interface and a nonparabolic valence band. We have also monitored the interface of the ZnSb substrate and metal films with transmission electron microscopy.

• Mayandi, Jeyanthinath; Madathil, Reshma Krishnan; Abinaya, C; Bethke, Kevin; Venkatachalapathy, Vishnukanthan & Rademann, Klaus [Vis alle 8 forfattere av denne artikkelen] (2021). Al-doped ZnO prepared by co-precipitation method and its thermoelectric characteristics. Materials Letters. ISSN 0167-577X. 288. doi: 10.1016/j.matlet.2021.129352. Fulltekst i vitenarkiv Vis sammendrag

  We report on structural and electrical characterization of zinc oxide material containing aluminum (Al:ZnO) prepared by a co-precipitation technique using different zinc and aluminum precursors. The as-prepared powders were calcinated and pre-annealed at 600 °C and 1200 °C. These were cold-pressed to pellets and sintered at 1000 °C. We also prepared ZnO material with no Al for comparisons. The electrical and thermal conductivity, as well as the Seebeck coefficient, were measured. These properties were correlated with structural analysis by X-ray diffraction characterization and scanning electron microscopy. The process yields a nanostructured material with much reduced thermal conductivity.

• Heng, C.L.; Zhao, C.N.; Zhang, L; Xiang, W; Su, W.Y. & Yin, H.X. [Vis alle 9 forfattere av denne artikkelen] (2020). Effects of Yb doping on the structure and near band-edge emission of ZnO thin films on Si after high temperature annealing. Journal of Luminescence. ISSN 0022-2313. doi: 10.1016/j.jlumin.2020.117153. Fulltekst i vitenarkiv Vis sammendrag

  n this work, we have investigated the effects of ytterbium (Yb) doping concentration on the structure and near band-edge (NBE) photoluminescence (PL) of ZnO thin films on Si after high temperature annealing. The films were made by magnetron sputtering in an Ar:O2 atmosphere. The structure of the films have been studied by Rutherford backscattering spectrometry, X-ray diffraction (XRD), scanning electron microscopy, and X-ray photoelectron spectroscopy. XRD indicates that the crystallinity of the ZnO improves with annealing temperature in the range 700 -1000 °C; and after the 1000 °C annealing, the crystallinity of the films show overall an improvement for increasing Yb concentration up to 1.35 at.%. At the higher temperatures the films react with Si substrate to form silicates with Zn and Yb, and the Yb has redistributed in the film and piles up towards the Si substrate. The PL measurements show that the NBE PL intensity of the films is correlated with the crystallinity for variations in annealing temperature and Yb concentration. The PL excitation measurements suggest that the films prepared with Yb addition have higher NBE PL efficiency than the un-doped ZnO, while the energy transfer between the host ZnO and Yb ions are not efficient

• Schrade, Matthias; Berland, Kristian; Kosinskiy, Andrey; Heremans, Joseph P. & Finstad, Terje (2020). Shallow Impurity Band in ZrNiSn. Journal of Applied Physics. ISSN 0021-8979. 127(4). doi: 10.1063/1.5112820. Fulltekst i vitenarkiv Vis sammendrag

  ABSTRACT ZrNiSn and related half Heusler compounds are candidate materials for efficient thermoelectric energy conversion with a reported thermoelectric figure-of-merit of n-type ZrNiSn exceeding unity. Progress on p-type materials has been more limited, which has been attributed to the presence of an impurity band, possibly related to Ni interstitials in a nominally vacant 4d position. The specific energetic position of this band, however, has not been resolved. Here, we report the results of a concerted theory-experiment investigation for a nominally undoped ZrNiSn, based on the electrical resistivity, the Hall coefficient, the Seebeck coefficient, and the Nernst coefficient, measured in a temperature range from 80 to 420 K. The results are analyzed with a semianalytical model combining a density functional theory (DFT) description for ideal ZrNiSn, with a simple analytical correction for the impurity band. The model provides a good quantitative agreement with experiment, describing all salient features in the full temperature span for the Hall, conductivity, and Seebeck measurements, while also reproducing key trends in the Nernst results. This comparison pinpoints the impurity band edge to 40 meV below the conduction band edge, which agrees well with a separate DFT study of a supercell containing Ni interstitials. Moreover, we corroborate our result with a separate study of the ZrNiSn0.9Pb0.1 sample showing similar agreement with an impurity band edge shifted to 32 meV below the conduction band.

• Venkatesan, Ragavendran; Mayandi, Jeyanthinath; Søndenå, Rune; Finstad, Terje & Venkatachalapathy, Vishnukanthan (2020). Investigating antireflection properties of hybrid silicon nanostructures comprising rod-like nanopores and nano-textured surface. Materials Letters. ISSN 0167-577X. 275. doi: 10.1016/j.matlet.2020.128087. Fulltekst i vitenarkiv Vis sammendrag

  In the present work, we have fabricated hybrid silicon (Si) nanostructures comprising vertical rod-like nanopores and nano-textured surface by metal assisted chemical etching (MACE) method at room temperature. The as-received p-type Upgraded Metallurgical grade (UMG) Si wafers were chemical polished, prior to investigating the etching effects at the metal nanoparticle semiconductor interface. The influence of metal silver nanoparticle (AgNPs) concentration on the formation of hybrid nanostructures were studied systematically. Depending on the surface morphology, the hybrid structures exhibited constant 10% average reflectance in the UV–Visible spectral region or average 7.5% reflectance in range of 200–400 nm

• Mayandi, Jeyanthinath; Finstad, Terje; Venkatesan, Ragavendran; Vajeeston, Ponniah; Karazhanov, Smagul & Venkatachalapathy, Vishnukanthan (2020). Carbon-dioxide as annealing atmosphere to retain the electrical properties of indium-tin oxide. Materials Letters. ISSN 0167-577X. 276. doi: 10.1016/j.matlet.2020.128195. Fulltekst i vitenarkiv Vis sammendrag

  In practical applications of indium-tin-oxide (ITO) annealing at temperatures ~400 °C without degrading its electrical and optical properties is an important challenge. In the present work, commercial Indium-tin oxide (ITO) coated on glass was subjected to post-annealing treatment in the range of 200-400 °C at different annealing atmospheres; oxygen, nitrogen and carbon-dioxide. The annealed samples were characterized by X-ray diffraction, UV-visible spectroscopy and Hall measurements to evaluate the structural, optical and electrical properties. Both oxygen and nitrogen treated samples degrades the structural, optical and conducting properties of ITO, while carbon-dioxide atmosphere inhibits the degradation of ITO at 400 °C. The obtained results suggest that carbon-dioxide can be well utilized as annealing ambient to retain opto-electronic, structural and electrical properties of ITO and thereby improve the efficiency of ITO based solar cells.

• Chandrasekaran, Abinaya; Bethke, Kevin; Andrei, Virgil; Baumann, Jonas; Pollakowski-Herrmann, Beatrix & Kanngießer, Birgit [Vis alle 11 forfattere av denne artikkelen] (2020). The effect of post-deposition annealing conditions on structural and thermoelectric properties of sputtered copper oxide films. RSC Advances. 10(49), s. 29394–29401. doi: 10.1039/D0RA03906C. Fulltekst i vitenarkiv Vis sammendrag

  The development of thin-film thermoelectric applications in sensing and energy harvesting can benefit largely from suitable deposition methods for earth-abundant materials. In this study, p-type copper oxide thin films have been prepared on soda lime silicate glass by direct current (DC) magnetron sputtering at room temperature from a pure copper metallic target in an argon atmosphere, followed by subsequent annealing steps at 300 °C under various atmospheres, namely air (CuO:air), nitrogen (CuO:N) and oxygen (CuO:O). The resultant films have been studied to understand the influence of various annealing atmospheres on the structural, spectroscopic and thermoelectric properties. X-ray diffraction (XRD) patterns of the films showed reflexes that could be assigned to those of crystalline CuO with a thin mixed Cu(I)Cu(II) oxide, which was also observed by near edge X-ray absorption fine structure spectroscopy (NEXAFS). The positive Seebeck coefficient (S) reached values of up to 204 μV K−1, confirming the p-type behavior of the films. Annealing under oxygen provided a significant improvement in the electrical conductivity up to 50 S m−1, resulting in a power factor of 2 μW m−1 K−2. The results reveal the interplay between the intrinsic composition and the thermoelectric performance of mixed copper oxide thin films, which can be finely adjusted by simply varying the annealing atmosphere.

• Heng, C.L.; Xiang, W.; Su, W.Y.; Gao, Y.K.; Yin, P.G. & Finstad, Terje (2019). Effect of Eu doping on the near band edge emission of Eu doped ZnO thin films after high temperature annealin. Journal of Luminescence. ISSN 0022-2313. 210C, s. 363–370. doi: 10.1016/j.jlumin.2019.02.062. Fulltekst i vitenarkiv Vis sammendrag

  We studied the effect of europium (Eu) doping on the near band edge emission of ZnO thin films, fabricated by sputtering and high temperatures annealing. The doping concentration of Eu in the ZnO films was varied from 0.01 to 0.62 at.%. Photoluminescence (PL) spectra showed that the films ultra-violet (UV) emission was enhanced very much by high temperature treatment and was stronger for the Eu doped ZnO than that for the un-doped ZnO films after 1100 °C annealing. X-ray diffraction showed the films had hexagonal wurtzite structure, the width of the ZnO (002) plane diffraction peak was sensitive to the anneal temperature and also to the Eu doping concentration, and Zn silicates had formed after the 1100 °C annealing. Scanning electron microscopy showed that the surface morphology of the films became more uneven with increasing Eu concentration. The PL decay spectra inferred that the lifetime of the UV PL was correlated with the size of ZnO nanocrystals in the films. The results support that the intensity of the UV PL is primarily influenced by the crystalline perfection of the films which is also influenced by the Eu doping concentration.

• Schrade, Matthias & Finstad, Terje (2018). Using the Callaway Model to Deduce Relevant Phonon Scattering Processes: The Importance of Phonon Dispersion. Physica status solidi (b). ISSN 0370-1972. 255(12), s. 1–6. doi: 10.1002/pssb.201800208. Fulltekst i vitenarkiv
• Taklo, Maaike M. Visser; Schjølberg-Henriksen, Kari; Malik, Nishant; Poppe, Erik Utne; Moe, Sigurd T. & Finstad, Terje (2017). Al-Al Wafer-Level Thermocompression Bonding applied for MEMS, 2017 5th International Workshop on Low Temperature Bonding for 3D Integration (LTB-3D), Tokyo, 16-18 May 2017. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9784904743034. s. 11–11. doi: 10.23919/LTB-3D.2017.7947407. Fulltekst i vitenarkiv Vis sammendrag

  Wafer-level thermocompression bonding (TCB) using aluminum (Al) is presented as a hermetic sealing method for MEMS. The process is a CMOS compatible alternative to TCB using metals like gold (Au) and copper (Cu), which are problematic with respect to cross contamination in labs. Au and Cu are commonly used for TCB and the oxidation of these metals is limited (Au) or easily controlled (Cu). However, despite Al oxidation, our experimental results and theoretical considerations show that TCB using Al is feasible even at temperatures down to 300−350 °C using a commercial bonder without in-situ surface treatment capability.

• Heng, Chenglin; Xiang, W.; Su, W.Y.; Wu, H.C.; Gao, Y.K. & Yin, P.G. [Vis alle 7 forfattere av denne artikkelen] (2017). Strong near band edge emission of (Ce, Yb) co-doped ZnO thin films after high temperature annealing. Optical Materials Express. 7(8), s. 3041–3050. doi: 10.1364/OME.7.003041. Fulltekst i vitenarkiv Vis sammendrag

  We studied the photoluminescence (PL) properties of (Ce + Yb) co-doped ZnO thin films as a function of high temperature annealing. The films were fabricated by magnetron sputtering. After 1000-1100°C annealing, the near band edge (NBE) emissions of the films were dozens to a hundred times stronger than that of undoped ZnO, while the Yb3+ emission (~980 nm) was quite weak, indicating that energy transfers from the ZnO host to Yb3+ ions in the films were not efficient. X-ray diffraction analysis and scanning electron microscopy observations demonstrated that the (Ce + Yb) co-doping had a large effect on the morphology and crystallinity of the films. The crystallinity enhancement of the films is considered to be the main reason for the strong NBE enhancements of the co-doped ZnO films. © 2017 Optical Society of America

• Song, Xin; Schrade, Matthias; Carcases, Nahum Maso & Finstad, Terje (2017). Zn vacancy formation, Zn evaporation and decomposition of ZnSb at elevated temperatures: Influence on the microstructure and the electrical properties. Journal of Alloys and Compounds. ISSN 0925-8388. 710, s. 762–770. doi: 10.1016/j.jallcom.2017.03.339. Fulltekst i vitenarkiv Vis sammendrag

  The influence of annealing on the microstructure and electrical properties of undoped polycrystalline ZnSb samples has been investigated by different experimental techniques. In situ XRD in an argon at- mosphere showed that ZnSb powders decompose at 300 C, which is attributed to Zn evaporation. In situ SEM in a moist atmosphere showed fast surface deterioration at 450 C and above, reflecting decom- position of ZnSb and the formation of metallic Sb precipitates. The rate of Zn loss in a reducing atmo- sphere was determined by thermogravimetry and related to the Zn partial vapor pressure. The increase of the hole carrier concentration of ZnSb measured at room temperature after heat treatment was correlated with Zn evaporation at elevated temperature. The carrier concentration after annealing at 400 C is consistent with an activation energy for Zn vacancy formation of 0.4 eV and a maximum Zn deficiency x of Zn1-xSb of 1 10 3.

• Heng, Chenglin; Wang, T.; Su, W.Y.; Wu, H.C.; Yang, M.C. & Deng, L.G. [Vis alle 8 forfattere av denne artikkelen] (2017). Intense ultraviolet photoluminescent emission from Yb doped ZnO thin films on Si after high temperature annealing. Journal of Alloys and Compounds. ISSN 0925-8388. 695, s. 2232–2237. doi: 10.1016/j.jallcom.2016.11.072. Fulltekst i vitenarkiv
• Song, Xin & Finstad, Terje (2017). A study of S doped ZnSb. Materials Science in Semiconductor Processing. ISSN 1369-8001. 71, s. 421–426. doi: 10.1016/j.mssp.2017.09.007. Fulltekst i vitenarkiv Vis sammendrag

  We report on S-doping of ZnSb for S concentrations ranging from 0.02 at% to 2.5 at%. There are no previous reports on S-doping. ZnSb is a thermoelectric material with some advantages for the temperature range 400 K - 600 K. The solid solubility of S in ZnSb was estimated to be lower than 0.1% from observations of precipitates by scanning microscopy. Hall and Seebeck measurements were performed as a function of temperature from 6K to 623 K. The temperature dependence of the electrical properties suggests that S introduces neutral scattering centers for holes in the p-type material. An increase in hole concentration by S is argued by defect reactions involving Zn vacancies.

• Schrade, Matthias; Berland, Kristian; Eliassen, Simen Nut Hansen; Guzik, Matylda Natalia; Echevarria-Bonet, Cristina & Sørby, Magnus Helgerud [Vis alle 13 forfattere av denne artikkelen] (2017). The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X=Hf, Zr, Ti) half-Heusler alloys. Scientific Reports. 7. doi: 10.1038/s41598-017-14013-8. Fulltekst i vitenarkiv Vis sammendrag

  Thermoelectric application of half-Heusler compounds suffers from their fairly high thermal conductivities. Insight into how effective various scattering mechanisms are in reducing the thermal conductivity of fabricated XNiSn compounds (X = Hf, Zr, Ti, and mixtures thereof) is therefore crucial. Here, we show that such insight can be obtained through a concerted theory-experiment comparison of how the lattice thermal conductivity κLat(T) depends on temperature and crystallite size. Comparing theory and experiment for a range of Hf0.5Zr0.5NiSn and ZrNiSn samples reported in the literature and in the present paper revealed that grain boundary scattering plays the most important role in bringing down κLat, in particular so for unmixed compounds. Our concerted analysis approach was corroborated by a good qualitative agreement between the measured and calculated κLat of polycrystalline samples, where the experimental average crystallite size was used as an input parameter for the calculations. The calculations were based on the Boltzmann transport equation and ab initio density functional theory. Our analysis explains the significant variation of reported κLat of nominally identical XNiSn samples, and is expected to provide valuable insights into the dominant scattering mechanisms even for other materials.

• Malik, Nishant; Venkatachalapathy, Vishnukanthan; Dall, Wilhelm; Schjølberg-Henriksen, Kari; Poppe, Erik Utne & Taklo, Maaike M. Visser [Vis alle 7 forfattere av denne artikkelen] (2017). Texture of Al films for wafer-level thermocompression bonding. Superlattices and Microstructures. ISSN 0749-6036. 106, s. 216–233. doi: 10.1016/j.spmi.2017.03.053. Fulltekst i vitenarkiv Vis sammendrag

  Properties of aluminum thin films for thermocompression bonding have been studied in terms of surface roughness, grain size, and grain orientation by AFM, SEM, XRD and EBSD for thermocompression bonding. Al films were sputter deposited directly on Si and thermally oxidized Si wafers, respectively. The resulting Si/Al and Si/SiO2/Al sample types were compared after annealing (300–550 °C) in vacuum. The Si/SiO2/Al film samples showed higher surface roughness than the Si/Al samples. The as-deposited films had (111) preferred orientation, while (100) and (110) oriented Al grains were also present in Si/SiO2/Al samples. The Si/SiO2/Al samples and Si/Al sample annealed at 550 °C had a conical <111> texture. The observed evolution of the grain structure with annealing temperature is discussed in terms of native oxide, surface roughness, diffusivity and grain orientation dependent mechanical properties in order to shine light on previously observed differences in Alsingle bondAl thermocompression wafer-level bonding with Si/SiO2/Al and Si/Al wafers.

• Heng, Chenglin; Wang, T.; Li, H.; Liu, J.J.; Zhu, J.W. & Ablimit, A. [Vis alle 10 forfattere av denne artikkelen] (2016). Strong enhancement of ultra-violet emission by Ce doping of ZnO sputtered films. Materials Letters. ISSN 0167-577X. 162, s. 53–55. doi: 10.1016/j.matlet.2015.09.125. Fulltekst i vitenarkiv Vis sammendrag

  We report that ultra-violet (UV) emission of ZnO can be made much more intense by incorporating Ce in the ZnO films. After annealing at 1100 °C in N2 gas, the UV peak (≈3.34 eV, width 133 meV) – intensity for the film doped with Ce is two order of magnitude stronger than that of un-doped ZnO films. The large enhancement is correlated with improved crystallinity and appears due to fewer non-radiative recombination pathways. The improved crystallinity can be related to Ce affecting the self-texturing of ZnO during sputter deposition and annealing.

• Berland, Kristian; Song, Xin; Carvalho, Patricia; Persson, Clas; Finstad, Terje & Løvvik, Ole Martin (2016). Enhancement of thermoelectric properties by energy filtering: Theoretical potential and experimental reality in nanostructured ZnSb. Journal of Applied Physics. ISSN 0021-8979. 119(12). doi: 10.1063/1.4944716. Fulltekst i vitenarkiv Vis sammendrag

  Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by an order of magnitude when the filter barrier height was 0.5 eV. Measured values of the Hall carrier concentration in bulk ZnSb were then used to calibrate the transport calculations, and nanostructured ZnSb with average grain size around 70 nm was processed to achieve filtering as suggested previously in the literature. Various scattering mechanisms were employed in the transport calculations and compared with the measured transport properties in nanostructured ZnSb as a function of temperature. Reasonable correspondence between theory and experiment could be achieved when a combination of constant lifetime scattering and energy filtering with a 0.25 eV barrier was employed. However, the difference between bulk and nanostructured samples was not sufficient to justify the introduction of an energy filtering mechanism. The reasons for this and possibilities to achieve filtering were discussed in the paper.

• Malik, Nishant; Carvalho, Patricia; Poppe, Erik & Finstad, Terje (2016). Interfacial characterization of Al-Al thermocompression bonds. Journal of Applied Physics. ISSN 0021-8979. 119(20). doi: 10.1063/1.4952709. Fulltekst i vitenarkiv Vis sammendrag

  Interfaces formed by Al-Al thermocompression bonding were studied by the transmission electron microscopy. Si wafer pairs having patterned bonding frames were bonded using Al films deposited on Si or SiO2 as intermediate bonding media. A bond force of 36 or 60 kN at bonding temperatures ranging from 400–550 °C was applied for a duration of 60 min. Differences in the bonded interfaces of 200 μm wide sealing frames were investigated. It was observed that the interface had voids for bonding with 36 kN at 400 °C for Al deposited both on Si and on SiO2. However, the dicing yield was 33% for Al on Si and 98% for Al on SiO2, attesting for the higher quality of the latter bonds. Both a bond force of 60 kN applied at 400 °C and a bond force of 36 kN applied at 550 °C resulted in completely bonded frames with dicing yields of, respectively, 100% and 96%. A high density of long dislocations in the Al grains was observed for the 60 kN case, while the higher temperature resulted in grain boundary rotation away from the original Al-Al interface towards more stable configurations. Possible bonding mechanisms and reasons for the large difference in bonding quality of the Al films deposited on Si or SiO2 are discussed.

• Schrade, Matthias; Magraso, Anna; Galeckas, Augustinas; Finstad, Terje & Norby, Truls (2016). The Band Gap of BaPrO3 Studied by Optical and Electrical Methods. Journal of The American Ceramic Society. ISSN 0002-7820. 99(2), s. 492–498. doi: 10.1111/jace.13961. Fulltekst i vitenarkiv Vis sammendrag

  We report on measurements of the electrical and optical properties of BaPrO3. The temperature dependences of the electrical conductivity σ and the Seebeck coefficient α of polycrystalline samples were studied over a wide temperature range (300°C–1050°C). At lower temperatures, the observed charge transport can be described as thermally activated hopping of electron‐based small polarons with an activation energy of 0.37 eV. An observed change in temperature dependence of both σ and α around 700°C was observed and interpreted as a transition from extrinsic to intrinsic carrier transport. The intrinsic conduction can be modeled with an apparent electrical band gap of ~2 eV. Optical absorption and emission spectroscopy in the UV–VIS–NIR range revealed a series of characteristic absorption thresholds and the type of optical transitions was identified by combining transmittance and diffuse‐reflectance spectroscopy methods. An absorption edge of indirect type with onset at 0.6 eV is attributed to small polaron effects. The higher lying absorption thresholds of direct origin positioned at around 1.8 and 3.8 eV are correlated with thermal activation parameters from electrical measurements and discussed in terms of the band gap of BaPrO3.

• Heng, Chenglin L.; Wang, T.; Su, W.Y.; Wu, H.C.; Yin, P.G. & Finstad, Terje (2016). Down-conversion luminescence from (Ce, Yb) co-doped oxygen-rich silicon oxides. Journal of Applied Physics. ISSN 0021-8979. 119(12). doi: 10.1063/1.4945111. Fulltekst i vitenarkiv Vis sammendrag

  We have studied down-conversion photoluminescence (PL) from (Ce, Yb) co-doped “oxygen rich” silicon oxide films prepared by sputtering and annealing. The Ce3þ 􏲀510 nm PL is sensitive to the Ce concentration of the films and is much stronger for 3 at. % Ce than for 2 at. % Ce after annealing at 1200 􏲱C. The PL emission and excitation spectroscopy results indicate that the excitation of Yb3þ is mainly through an energy transfer from Ce3þ to Yb3þ, oxide defects also play a role in the excita- tion of Yb3þ after lower temperature (􏲀800 􏲱C) annealing. The Ce3þ 510 nm photon excites mostly only one Yb3þ 980nm photon. Temperature-dependent PL measurements suggest that the energy transfer from Ce3þ to Yb3þ is partly thermally activated. VC 2016 AIP Publishing LLC.

• Song, Xin & Finstad, Terje (2016). Review of research on the thermoelectric material ZnSb. I Skipidarov, Sergey & Nikitiin, Mikhail (Red.), Thermoelectrics for Power Generation - A Look at Trends in the Technology. IntechOpen. ISSN 9789535128465. s. 117–145. doi: 10.5772/65661. Fulltekst i vitenarkiv Vis sammendrag

  The thermoelectric material ZnSb has been studied intensively in recent years and has shown promising features. The other zinc-antimonide compound, Zn4Sb3 has remarkable low thermal conductivity, but it is accompanied with phase transitions at moderate temperature and has inherent stability problems. Compared to that, ZnSb is relatively phase stable and has a relative high charge carrier mobility and Seebeck coefficient, thus yielding a decent power factor. Meanwhile, its thermal conductivity can be reduced by means of nanostructuring, thus giving a good figure of merit at moderate temperatures, 400–600K. Many researchers have dedicated their efforts to study and improve ZnSb properties, and the figure of merit has been reported to be above one. Still, ZnSb as a thermoelectric material has features and behaviours that are not well-understood. The behaviour and properties of its intrinsic defects are not understood, but have interested researchers in recent years. This chapter intends to offer a comprehensive review on ZnSb to the readers. By combining own experiences from research on thermoelectric materials, the authors address the prospect for improving the thermoelectric properties of ZnSb and the concerns of transferring lab results to manufacturing.

• Hansen, Per-Anders; Fjellvåg, Helmer; Finstad, Terje & Nilsen, Ola (2016). Luminescence properties of lanthanide and ytterbium lanthanide titanate thin films grown by atomic layer deposition. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 34:01A130. doi: 10.1116/1.4936389. Fulltekst i vitenarkiv
• Heng, Chenglin; Li, J.T.; Su, W.U.; Han, Z.; Yin, P.G. & Finstad, Terje (2015). The formation of Yb silicates and its luminescence in Yb heavily doped silicon oxides after high temperature annealing. Optical materials (Amsterdam). ISSN 0925-3467. 42, s. 17–23. doi: 10.1016/j.optmat.2014.12.013. Fulltekst i vitenarkiv Vis sammendrag

  We report on the formation of ytterbium (Yb) silicates and its photoluminescence (PL) properties for heavily Yb doped Si oxide films after various annealings. X-ray diffraction patterns and transmission electron microscopy indicate that different Yb silicates have formed in the oxides upon 1100 and 1200°C annealing. The Yb PL intensities after the high temperature annealings are much stronger than those after lower temperatures, which indicates that the Yb silicates have higher emission efficiency than the Yb configurations found for lower temperature annealing. The PL intensities of the films can be altered considerably by secondary oxidizing or annealing in forming gas (N2+7% H2) ambience.

• Ruoho, Mikko; Valset, Kjetil; Finstad, Terje & Tittonen, Ilkka (2015). Measurement of thin film thermal conductivity using the laser flash method. Nanotechnology. ISSN 0957-4484. 26(19). doi: 10.1088/0957-4484/26/19/195706. Fulltekst i vitenarkiv
• Valset, Kjetil; Song, Xin & Finstad, Terje (2015). A Study of Transport Properties in Cu and P doped ZnSb. Journal of Applied Physics. ISSN 0021-8979. 117(045709). doi: 10.1063/1.4906404. Fulltekst i vitenarkiv Vis sammendrag

  ZnSb samples have been doped with copper and phosphorus and sintered at 798 K. Electronic transport properties are interpreted as being influenced by an impurity band close to the valence band. At low Cu dopant concentrations, this impurity band degrades the thermoelectric properties as the Seebeck coefficient and effective mass are reduced. At carrier concentrations above 1 × 1019 cm−3, the Seebeck coefficient in Cu doped samples can be described by a single parabolic band.

• Heng, Chenglin; Li, J.T.; Su, W.Y.; Yin, P.G. & Finstad, Terje (2015). The photoluminescence and structural properties of (Ce, Yb) co-doped silicon oxides after high temperature annealing. Journal of Applied Physics. ISSN 0021-8979. 117. doi: 10.1063/1.4906444. Fulltekst i vitenarkiv
• Song, Xin; Valset, Kjetil; Graff, Joachim Seland; Thøgersen, Annett; Gunnæs, Anette Eleonora & Luxsacumar, Sivakanesar [Vis alle 9 forfattere av denne artikkelen] (2015). Nanostructuring of undoped ZnSb by Cryo-Milling. Journal of Electronic Materials. ISSN 0361-5235. 44(8), s. 2578–2584. doi: 10.1007/s11664-015-3708-6. Fulltekst i vitenarkiv Vis sammendrag

  We report the preparation of nanosized ZnSb powder by cryo-milling. The effect of cryo-milling then hot-pressing of undoped ZnSb was investigated and compared with that of room temperature ball-milling and hot-pressing under different temperature conditions. ZnSb is a semiconductor with favorable thermoelectric properties when doped. We used undoped ZnSb to study the effect of nanostructuring on lattice thermal conductivity, and with little contribution at room temperature from electronic thermal conductivity. Grain growth was observed to occur during hot-pressing, as observed by transmission electron microscopy and x-ray diffraction. The thermal conductivity was lower for cryo-milled samples than for room-temperature ball-milled samples. The thermal conductivity also depended on hot-pressing conditions. The thermal conductivity could be varied by a factor of two by adjusting the process conditions and could be less than a third that of single-crystal ZnSb.

• Malik, Nishant; Poppe, Erik; Schjølberg-Henriksen, Kari; Taklo, Maaike Margrete Visser & Finstad, Terje (2015). Environmental Stress Testing of Wafer-Level Al-Al Thermocompression Bonds: Strength and Hermeticity. ECS Journal of Solid State Science and Technology. ISSN 2162-8769. 4(7), s. P251–P257. doi: 10.1149/2.0181507jss. Fulltekst i vitenarkiv Vis sammendrag

  Hermeticity, reliability and strength of Al-Al thermocompression bonds realized by applying different bonding parameters have been investigated. Laminates of diameter 150 mm were realized by bonding wafers containing membrane structures to wafers with patterned bonding frames. The laminates were bonded applying a bond force of 36 or 60 kN at temperatures ranging from 300 to 400°C for 15, 30 or 60 minutes. The hermetic properties were estimated by membrane deflection measurements with white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was measured by shear test and pull tests. Laminates bonded applying a bond force of 60 kN at temperatures of 350 or 400°C resulted in hermetic bonds. No significant change in membrane deflection was observed after the steady-state life test or the thermal shock test. However, a gross leakage was observed in 1–11% of the dies after exposure to the moisture resistance test. The maximum leakage rate (MLR) estimated from membrane deflection measurements was below 10−11 mbar·l·s−1 for all laminates. The measured average bond strength of dies from selected laminates ranged from 28 to 190 MPa.

• Schrade, Matthias; Norby, Truls & Finstad, Terje (2015). Hall effect measurements on thermoelectric Ca3Co4O9: On how to determine the charge carrier concentration in strongly correlated misfit cobaltites. Journal of Applied Physics. ISSN 0021-8979. 117(20). doi: 10.1063/1.4921861. Fulltekst i vitenarkiv Vis sammendrag

  The Hall coefficient RH and electrical conductivity of misfit calcium cobalt oxide (Ca2CoO3−δ)q(CoO2) (CCO) were measured at room temperature for different oxygen vacancy concentrations δ. Based on these and numerous previous results, it is shown that the charge carrier concentrations n obtained by the classical formula RH = 1/ne are between 3 and 6 × 1020 cm−3 and thereby much lower than those derived by other experimental techniques and fail to explain the observed electric properties of CCO. We show that the experimental results are well described using an earlier proposed t–J-model for strongly correlated electrons on a triangular lattice. The hopping parameter t for CCO was found to be ≈ −20 K and the charge carrier concentration of fully oxidized CCO to be 5.7 × 1021 cm−3 (0.41 hole type carriers per formula unit), in agreement with other experimental techniques. This research was originally published in the Journal of Applied Physics. © AIP Publishing

• Malik, Nishant; Tofteberg, Hannah Rosquist; Poppe, Erik; Finstad, Terje & Schjølberg-Henriksen, Kari (2015). Environmental Stress Testing of Wafer-Level Au-Au Thermocompression Bonds Realized at Low Temperature: Strength and Hermeticity. ECS Journal of Solid State Science and Technology. ISSN 2162-8769. 4(7), s. P236–P241. doi: 10.1149/2.0201507jss. Fulltekst i vitenarkiv Vis sammendrag

  Hermeticity, reliability and strength of four laminates bonded at different temperatures by Au-Au thermocompression bonding have been investigated. Laminates with a diameter of 150 mm were realized by bonding a wafer containing membrane structures to a Si wafer with patterned bond frames. A bond tool pressure of 2266 mbar was applied for 15 minutes at temperatures ranging from 150–300°C. The hermetic properties were estimated by membrane deflection measurements applying white-light interferometry after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Bond strength was estimated by pull test measurements. A dicing yield above 90% was obtained for all laminates. Laminates bonded at 200°C and above had significantly higher hermetic yield than the laminate bonded at 150°C. No degradation in hermeticity was observed after the reliability tests. The maximum leakage rate (MLR) was estimated from two measurements of membrane deflection executed at two different times and was below 10−11 mbar ⋅ l ⋅ s−1. The average bond strength ranged from 44 to 175 MPa.

• Malik, Nishant; Schjølberg-Henriksen, Kari; Poppe, Erik; Taklo, Maaike Margrete Visser & Finstad, Terje (2015). Impact of SiO2 on Al–Al thermocompression wafer bonding. Journal of Micromechanics and Microengineering (JMM). ISSN 0960-1317. 25:035025(3). doi: 10.1088/0960-1317/25/3/035025. Fulltekst i vitenarkiv Vis sammendrag

  Al–Al thermocompression bonding suitable for wafer level sealing of MEMS devices has been investigated. This paper presents a comparison of thermocompression bonding of Al films deposited on Si with and without a thermal oxide (SiO2 film). Laminates of diameter 150 mm containing device sealing frames of width 200 µm were realized. The wafers were bonded by applying a bond force of 36 or 60 kN at bonding temperatures ranging from 300–550 °C for bonding times of 15, 30 or 60 min. The effects of these process variations on the quality of the bonded laminates have been studied. The bond quality was estimated by measurements of dicing yield, tensile strength, amount of cohesive fracture in Si and interfacial characterization. The mean bond strength of the tested structures ranged from 18–61 MPa. The laminates with an SiO2 film had higher dicing yield and bond strength than the laminates without SiO2 for a 400 °C bonding temperature. The bond strength increased with increasing bonding temperature and bond force. The laminates bonded for 30 and 60 min at 400 °C and 60 kN had similar bond strength and amount of cohesive fracture in the bulk silicon, while the laminates bonded for 15 min had significantly lower bond strength and amount of cohesive fracture in the bulk silicon.

• Heinrich, Christophe P.; Schrade, Matthias; Cerretti, Giacomo; Lieberwirth, Ingo; Leidich, Patrick & Schmitz, Andreas [Vis alle 11 forfattere av denne artikkelen] (2015). Tetragonal tungsten bronzes Nb8-xW9+xO47-d: optimization strategies and transport properties of a new n-type thermoelectric oxide. Materials Horizons. ISSN 2051-6347. 2(5), s. 519–427. doi: 10.1039/c5mh00033e. Fulltekst i vitenarkiv Vis sammendrag

  Engineering of nanoscaled structures may help controlling the electrical and thermal transport in solids, in particular for thermoelectric applications that require the combination of low thermal conductivity and low electrical resistivity. The tetragonal tungsten bronzes Nb8−xW9+xO47 (TTB) allow a continuous variation of the charge carrier concentration while fulfilling at the same time the concept of a “phonon-glass electron-crystal” through a layered nanostructure defined by intrinsic crystallographic shear planes. The thermoelectric properties of the tetragonal tungsten bronzes Nb8−xW9+xO47−δ (0 < x < 2) were studied in the temperature range from 373 to 973 K. Structural defects and the thermal stability under various oxygen partial pressure pO2 were investigated by means of thermogravimetry, HR-TEM, and XRD. Nb8W9O47−δ was found stable at 973 K and a pO2 of ≈10−15 atm. The oxygen nonstoichiometry δ can reach up to 0.3, depending on the applied atmosphere. By increasing the substitution level x, the electrical resistivity ρ and the Seebeck coefficient S decreased. For x = 2, ρ reached 20 mΩ cm at 973 K, combined with a Seebeck coefficient of approximately −120 μV K−1. The thermal conductivity was low for all samples, ranging from 1.6 to 2.0 W K−1 m−1, attributed to the complex crystal structure. The best thermoelectric figure of merit zT of the investigated samples was 0.043, obtained for x = 2 at 973 K, but it is expected to increase significantly upon a further increase of x. The control of the oxygen non-stoichiometry δ opens a second independent optimization strategy for tetragonal tungsten bronzes.

• Mayandi, Jeyanthinath; Finstad, Terje; Foss, Steinar; Galeckas, Augustinas & Stange, Marit Synnøve Sæverud (2014). A comparative study of 1.5 μm photoluminescence from (Er, Si) and (Er, Ge) co-sputtered with Al2O3 on Si. Journal of Alloys and Compounds. ISSN 0925-8388. 590, s. 5–8. doi: 10.1016/j.jallcom.2013.12.060. Fulltekst i vitenarkiv Vis sammendrag

  We present a comparative study of the 1.54lm photoluminescence (PL) from Al2O3:(Er, Si) and Al2O3:(Er, Ge) films co-sputtered on Si substrates using a composite target. The PL yield of Si- and Ge- doped films subjected to different thermal treatment in the range 500–1100 °C is referenced against that of a control Al2O3:(Er) structure. The Er-related emission reaches maximum upon annealing at 700 °C for the Al2O3:(Er, Si) and 500 °C for the Al2O3:(Er, Ge) films, in both cases decreasing at higher annealing tem- peratures. Ge co-doped films are found to be approximately 20 times less luminous than the reference Al2O3:(Er) structure, whereas Si co-doping leads to greatly enhanced emission at 1.54 lm. By adapting energy filtered transmission electron microscopy (EFTEM), the presence of ﰀ2 nm Si nanoclusters was revealed, whereas no indications of Ge clusters or crystals could be observed implying that mere presence of Ge in the Al2O3 matrix can dramatically reduce the luminescence efficiency. We discuss the possible reasons for not forming the Ge nanoclusters in the Al2O3 host and the apparent quenching of the Er- related emission.

• Malik, Nishant; Schjølberg-Henriksen, Kari; Poppe, Erik; Taklo, Maaike Margrete Visser & Finstad, Terje (2014). Al-Al thermocompression bonding for wafer-level MEMS sealing. Sensors and Actuators A-Physical. ISSN 0924-4247. 211, s. 115–120. doi: 10.1016/j.sna.2014.02.030. Fulltekst i vitenarkiv Vis sammendrag

  Al–Al thermocompression bonding has been studied using test structures relevant for wafer level sealing of MEMS devices. Si wafers with protruding frame structures were bonded to planar Si wafers, both covered with a sputtered Al film of 1 μm thickness. The varied bonding process variables were the bonding temperature (400, 450 and 550 °C) and the bonding force (18, 36 and 60 kN). Frame widths 100 μm, 200 μm, with rounded or sharp frame corners were used. After bonding, laminates were diced into single chips and pull tested. The effect of process and design parameters was studied systematically with respect to dicing yield, bond strength and resulting fractured surfaces. The test structures showed an average strength of 20–50 MPa for bonding at or above 450 °C for all three bonding forces or bonding at 400 °C with 60 kN bond force. The current study indicates that strong AlAl thermocompression bonds can be achieved either at or above 450 °C bonding temperature for low (18 kN) and medium (36 kN) bond force or by high bond force (60 kN) at 400 °C. The results show that an increased bond force is required to compensate for a reduced bonding temperature for AlAl thermocompression bonding in the studied temperature regime

• Evrin, Tugay; Ilday, Serim; Turan, Rasit & Finstad, Terje (2014). Influence of Ge content and annealing conditions on the PL properties of nc-Si1-xGex embedded in SiO2 matrix in weak quantum confined regime. Journal of Luminescence. ISSN 0022-2313. 155, s. 170–179. doi: 10.1016/j.jlumin.2014.06.012. Fulltekst i vitenarkiv Vis sammendrag

  Fabrication of Si (nc-Si), Ge (nc-Ge), and Si1-xGex (nc-Si1-xGex) nanocrystals embedded in SiO2 matrix is achieved by thermal annealing of magnetron-sputtered thin films. Effects of annealing conditions, namely duration and temperature, as well as Ge content on the photoluminescence properties are investigated. Origin and evolution of the photoluminescence signal in the weak quantum confinement regime are discussed. It is found that photoluminescence signals can be decomposed into four Gaussian peaks originating from Ge-related radiative defects located at the sub-oxide (GeOx), either inside the matrix or at the interface region (peak M), nc-Si1-xGex/SiO2 interface-related localized states (peak I), localized states in the amorphous Si1-xGex bandgap (peak A) and quantum confinement of excitons in small nanocrystals (peak Q). The role of small and large nanocrystals in the photoluminescence mechanism is investigated by varying the mean nanocrystal size from 3 nm to 23 nm (from strong to weak quantum confined regime). Our results demonstrate that the quantum confinement effect in Ge nanocrystals manifests though spectral blueshift due to increase in Ge content. We also propose that the decreasing photoluminescence signal intensity with an increase in Ge content may originate from Ge-related nonradiative Pb centers.

• Schrade, Matthias; Kabir, Rezaul; Li, Sean; Norby, Truls & Finstad, Terje (2014). High temperature transport properties of thermoelectric CaMnO3-d - Indication of strongly interacting small polarons. Journal of Applied Physics. ISSN 0021-8979. 115. doi: 10.1063/1.4868321. Fulltekst i vitenarkiv Vis sammendrag

  The conductivity and Seebeck coefficient of CaMnO3−δ have been studied at temperatures up to 1000 °C and in atmospheres with controlled oxygen partial pressure. Both transport coefficients were varied in situ by the reversible formation of oxygen vacancies up to δ = 0.15. The charge carrier concentration was calculated using a defect chemical model. The Seebeck coefficient could be approximated by Heikes' formula, while the conductivity shows a maximum at a molar charge carrier concentration of 0.25. These results were interpreted as a signature of strong electronic correlation effects, and it was concluded that charge transport in CaMnO3−δ occurs via strongly interacting small polarons. General prospects for strongly correlated materials as potential candidates for high temperature thermoelectric power generation were discussed. This research was originally published in the Journal of Applied Physics. © AIP Publishing

• Schrade, Matthias; Casolo, Simone; Graham, Paul J.; Ulrich, Clemens; Li, Sean & Løvvik, Ole Martin [Vis alle 8 forfattere av denne artikkelen] (2014). Oxygen nonstoichiometry in (Ca2CoO3)0.62(CoO2): a combined experimental and computational study. Journal of Physical Chemistry C. ISSN 1932-7447. 118(33), s. 18899–18907. doi: 10.1021/jp5048437. Fulltekst i vitenarkiv Vis sammendrag

  The oxygen nonstoichiometry in the misfit calcium cobaltite (Ca2CoO3)0.62(CoO2) has been studied experimentally and by density functional theory (DFT) calculations. The standard oxidation enthalpy ΔH0Ox of oxygen deficient (Ca2CoO3)0.62(CoO2) was measured directly using simultaneous thermogravimetry and differential scanning calorimetry. ΔH0Ox was found to be in agreement with the prediction from a previously published defect chemical model based on purely thermogravimetrical analysis. A series of samples with different oxygen vacancy concentration was prepared by annealing in air, followed by rapid quenching. Room-temperature Raman spectroscopy showed a sharp mode at 700 cm–1 decreasing in intensity with increasing vacancy concentration. We discuss this observation as evidence for oxygen vacancies being preferably formed within the central layer of the Ca2CoO3 subsystem. DFT calculations demonstrated that the calculated electronic structure is sensitive to the chosen model of the crystal structure. Still, for all investigated models, the standard formation enthalpy of oxygen vacancies within the Ca2CoO3 moiety was much lower than that for a site within the CoO2 layer, in agreement with the presented experimental data.

• Schrade, Matthias; Fjeld, Harald; Finstad, Terje & Norby, Truls (2014). Electronic Transport Properties of [Ca2CoO3-δ]q[CoO2]. Journal of Physical Chemistry C. ISSN 1932-7447. 118(6), s. 2908–2918. doi: 10.1021/jp409581n. Fulltekst i vitenarkiv
• Hansen, Per-Anders; Fjellvåg, Helmer; Finstad, Terje & Nilsen, Ola (2014). Luminescence properties of europium titanate thin films grown by atomic layer deposition. RSC Advances. 4(23), s. 11876–11883. doi: 10.1039/c3ra47469k. Fulltekst i vitenarkiv Vis sammendrag

  Luminescent europium titanium oxides (EuxTiyOz) have been deposited as thin films by atomic layer deposition (ALD) in the temperature range 225 to 375 °C with control of the stoichiometry from pure TiO2 to pure Eu2O3, and their luminescent properties have been characterized. The current work demonstrates the feasibility of tailoring the luminescent properties of these films by varying ALD deposition parameters. The highest luminescence efficiency was observed for the amorphous phase while it decreases steadily with increasing annealing temperature up to 700 °C due to the formation of crystalline Eu2Ti2O7. The limiting factor for luminescence at low Eu contents is the low concentration of emitting Eu3+ atoms, while the luminescence at high Eu contents is limited by a low UV absorption due to the low Ti4+ content in these samples. The precursors used for the Ti–O deposition were TiCl4 and H2O, while Eu(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) and ozone were used for the deposition of Eu–O. The films have been characterized by spectroscopic ellipsometry, photoluminescence, X-ray diffraction, X-ray fluorescence and atomic force microscopy. The depositions gave smooth (RMS roughness < 0.4 nm) and amorphous films for 30% and more Eu–O cycles (9.5 cationic% Eu), while 10 and 20% gave rough and partly crystalline films. The refractive index, extinction coefficient and luminescence efficiency are shown to vary continuously with Eu content for 30% Eu–O cycles or more, while the shape of the luminescence spectrum is independent of Eu content in the same range. For the 10 and 20% Eu–O depositions, we observed a gradual change in the luminescence spectra, coinciding with an increase of the proportion of anatase phase with decreasing Eu content.

• Schrade, Matthias; Fjeld, Harald; Norby, Truls & Finstad, Terje (2014). Versatile apparatus for thermoelectric characterization of oxides at high temperatures. Review of Scientific Instruments. ISSN 0034-6748. 85(10). doi: 10.1063/1.4897489. Fulltekst i vitenarkiv
• Malik, Nishant; Tofteberg, Hannah Rosquist; Poppe, Erik; Finstad, Terje & Schjølberg-Henriksen, Kari (2014). Hermeticity and Reliability of Au-Au Thermocompression Bonds, Realized at Low Temperature. I Moriceau, Hubert; Baumgart, H; Goorsky, M.S.; Hobart, K.D.; Knechtel, R.; Suga, T. & Tan, C.S. (Red.), 2014 ECS and SMEQ Joint International Meeting, October 5, 2014 - October 9, 2014 Semiconductor Wafer Bonding 13: Science, Technology, and Applications. The Electrochemical Society. ISSN 9781623321857. s. 167–176. doi: 10.1149/06405.0167ecst. Fulltekst i vitenarkiv Vis sammendrag

  Hermeticity and reliability of four laminates bonded at different temperatures by Au-Au thermocompression bonding have been investigated. Laminates of diameter 150 mm were realized by bonding a wafer containing membrane structures to Si wafers with patterned bond frames. The laminates were bonded applying a bonding pressure of 8.5 MPa at temperatures of 150–300 °C for 15 minutes. The hermetic properties were estimated by measurement of membrane deflection at two different times after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. A dicing yield above 90 % was obtained for laminates bonded at or above 150 °C. Laminates bonded at 200 °C and above had significantly higher hermetic yield than the laminate bonded at 150 °C. No degradation in hermeticity was observed after the reliability tests. A maximum leak rate in the range of 10-11 mbar×l×s-1 was estimated.

• Malik, Nishant; Poppe, Erik; Schjølberg-Henriksen, Kari; Taklo, Maaike Margrete Visser & Finstad, Terje (2014). Hermeticity and Reliability of Al-Al Thermocompression Wafer Bonding. I Moriceau, Hubert; Baumgart, H; Goorsky, M.S.; Hobart, K.D.; Knechtel, R.; Suga, T. & Tan, C.S. (Red.), 2014 ECS and SMEQ Joint International Meeting, October 5, 2014 - October 9, 2014 Semiconductor Wafer Bonding 13: Science, Technology, and Applications. The Electrochemical Society. ISSN 9781623321857. s. 149–160. doi: 10.1149/06405.0149ecst. Fulltekst i vitenarkiv Vis sammendrag

  Hermeticity and reliability of laminates bonded by Al-Al thermocompression bonding applying different bonding parameters have been investigated. Laminates of diameter 150 mm were realized by bonding a wafer containing membrane structures to wafers with patterned bonding frames. The laminates were bonded applying a bond force of 36 or 60 kN at temperatures ranging from 300 to 400 °C for 15, 30 or 60 minutes. The hermetic properties were estimated by optical measurement of membrane deflection after bonding. Reliability was tested by exposing the laminates to a steady-state life test, a thermal shock test, and a moisture resistance test. Laminates bonded applying a bond force of 60 kN at temperatures of 350 °C or higher resulted in hermetic and reliable bonds. All the dies on the laminates survived steady-state life test and thermal shock test. Leakage was observed in some dies after exposure to moisture resistance test. Measurements of membrane deflection estimated a maximum leak rate below 10-11 mbar×l×s-1 for all tested bonding parameters.

• Heng, Chenglin; Su, Wenyong; Zhang, Qiwei; Ren, Xiaoqian; Yin, Penggan & Pan, Huiping [Vis alle 8 forfattere av denne artikkelen] (2014). The photoluminescence from (Eu, Yb) co-doped silicon-rich Si oxides. Journal of Luminescence. ISSN 0022-2313. 154, s. 339–344. doi: 10.1016/j.jlumin.2014.04.035. Fulltekst i vitenarkiv
• Hansen, Per-Anders; Finstad, Terje & Nilsen, Ola (2014). Luminescent Properties of Multilayered Eu2O3 and TiO2 Grown by Atomic Layer Deposition. Chemical Vapor Deposition. ISSN 0948-1907. 20(7-8-9), s. 274–281. doi: 10.1002/cvde.201407113. Fulltekst i vitenarkiv

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• Rahman, Jamil Ur; Gunnæs, Anette Eleonora; Finstad, Terje Gunnar; Løvvik, Ole Martin & Carvalho, Patricia Almeida (2023). Structure of a high entropy Heusler antimonide. Fulltekst i vitenarkiv
• Soltani, Nayereh; Rahman, Jamil Ur; Almeida, Patricia & Finstad, Terje Gunnar (2023). Correction to: A Highly Active and Reusable Multicomponent high Entropy Metal Oxide Catalyst for Nitroarenes Hydrogenation (Topics in Catalysis, (2023), 10.1007/s11244-023-01846-z). Topics in catalysis. ISSN 1022-5528. doi: 10.1007/s11244-023-01852-1. Fulltekst i vitenarkiv
• Rahman, Jamil Ur; Sunding, Martin Fleissner; Løvvik, Ole Martin; Gunnæs, Anette Eleonora; Finstad, Terje Gunnar & Carvalho, Patricia Almeida (2023). Deciphering the structure of a high-entropy Heusler antimonide. Fulltekst i vitenarkiv
• Finstad, Terje Gunnar; Mayandi, Jeyanthinath & Karlsen, Ole Bjørn (2023). Thermoelectric Multi-pnictogens Semiconductors: Cu Doping of ZnSbAs Alloys. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv
• Finstad, Terje Gunnar; Mayandi, Jeyanthinath; Soltani, Nayereh; Pokle, Anuj; Løvvik, Ole Martin & Carvalho, Patricia Almeida (2023). TiFe2Si Thin Films. Fulltekst i vitenarkiv
• Løvvik, Ole Martin; Rahman, Jamil Ur; Finstad, Terje Gunnar; Gunnæs, Anette Eleonora & Carvalho, Patricia Almeida (2022). Thermoelectric properties of multiple component (high entropy) silicides and antimonides. Fulltekst i vitenarkiv
• Mayandi, Jeyanthinath; Soltani, Nayereh; Finstad, Terje Gunnar; Karlsen, Ole Bjørn & Flage-Larsen, Espen (2022). Zinc Multi-Pnictogens Semiconductors: Cu Doping and Thermoelectric Characteristics. Fulltekst i vitenarkiv
• Carvalho, Patricia Almeida; Rahman, Jamil Ur; Soltani, Nayereh; Schrade, Matthias; Stange, Marit Synnøve Sæverud & Gunnæs, Anette Eleonora [Vis alle 8 forfattere av denne artikkelen] (2021). Functional high entropy antimonides and silicides. Fulltekst i vitenarkiv
• Carvalho, Patricia Almeida; 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. Fulltekst i vitenarkiv
• Soltani, Nayereh; Rahman, Jamil Ur; Carvalho, Patricia Almeida & Finstad, Terje (2021). Multicomponent transition metal antimonide nanoparticles. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Rahman, Jamil Ur; Carvalho, Patricia Almeida; Guzik, Matylda N.; Soltani, Nayereh; Gunnæs, Anette Eleonora & Karlsena, Ole Bjørn [Vis alle 7 forfattere av denne artikkelen] (2021). Synthesis and crystal structure of Sb-based high entropy alloys. Fulltekst i vitenarkiv
• Rahman, Jamil Ur; Carvalho, Patricia Almeida; Soltani, Nayereh; Schrade, Matthias; Karlson, Ole Bjørn & Gunnæs, Anette Eleonora [Vis alle 7 forfattere av denne artikkelen] (2021). A study of new Sb-based high entropy alloys synthesized by non-conventional methods. Fulltekst i vitenarkiv
• Jeyanthinath, Mayandi; Stange, Marit Synnøve Sæverud; Dahl, Øystein; Sunding, Martin Fleissner; Sagvolden, Espen & Flage-Larsen, Espen [Vis alle 13 forfattere av denne artikkelen] (2020). Functional Properties of CrFeCoNiCu and GeFeCoNiCu oxides. Fulltekst i vitenarkiv Vis sammendrag

  The present work explores the concept of high-entropy alloys to design FeCoNi-based oxides suitable for functional applications. Fundamental changes in electronic behavior 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. CrFeCoNiCu and GeFeCoNiCu thin films with variable oxygen concentration were deposited by reactive DC magnetron sputtering onto optically transparent substrates and characterized by structural, spectroscopic and electrical methods. Transmission electron microscopy showed that for low oxygen content 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. The resistivity measured at room temperature ranged from 10-4 to values above 104 Ω.cm. Hall measurements and Seebeck measurements show that both electrons and holes contribute to conduction and that at room temperature the Hall coefficient and Seebeck coefficient have different sign. This behaviour is discussed in terms of the structural analysis and suggested electronic model of the films.

• Løvvik, Ole Martin; Graff, Joachim Seland; Schrade, Matthias; Sunding, Martin Fleissner; Wright, Daniel Nilsen & Song, Xin [Vis alle 10 forfattere av denne artikkelen] (2020). Silicide module produced in the TESil project. Fulltekst i vitenarkiv
• Song, Xin; Riis, Henrik; Prytz, Øystein & Finstad, Terje (2019). Metallization of selected transition metals on ZnSb by electron beam thermal evaporation. Fulltekst i vitenarkiv Vis sammendrag

  The thermoelectric module made by ZnSb can be dated back to 1870, even if the performance was considerable low by today’s standard [1]. In recent decades, both the electrical properties and thermal properties of the material have been improved, attributed to introducing proper doping/co-doping and nanostructuring [2]. The figure of merit of ZnSb has been reported close to 1 or even above in a few studies [3], which is considered to be a material with a possible decent thermoelectric performance. In this study, we focus on the electrical contact resistance between ZnSb and metal contact, i.e the metallization of ZnSb. This is a step forward from optimized thermoelectric material to a useful thermoelectric device. For thermoelectric devices, ohmic contact is mostly wanted. In theory, ohmic contacts are derived from Schottly contacts and is dependent on the work function between semiconductor and metal, and doping concentration; while in reality, many other factors can have significant impacts on function and performance, for instance interface defects, adhesion, thermal expansion between semiconductor and metal, inter-diffusion and oxidation. The scope of this work is to test the method for metallization and understand the transport at the contact. In addition, we verified the methodology for measuring contact resistance and estimated the experimental uncertainties. We selected several transition metals that are often used as electrical contact for semiconductor devices. The substrates, i.e. ZnSb with various doping concentration, have been synthesized by the hot-pressing method and prepared by the standard procedure for wafer preparation. We deposited the metal with electron beam thermal evaporation, following post-annealing at different temperatures. The structure at interface was investigated by Transmission Electron Microscopy (TEM). The contact resistance and semiconductor resistivity were extrapolated from transmission line measurements of samples subjected to different post-annealing. We compared the measurements with the idealized models combining thermionic emission, thermionic filed emission and tunneling. Although this model is an idealization, it can provide a guide for further detailed study.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Mayandi, Jeyanthinath; Stange, Marit Synnøve Sæverud; Sunding, Martin Fleissner; Diplas, Spyridon; Carvalho, Patricia Almeida & Finstad, Terje (2019). Oxidation of CrFeCoNiCu sputtered thin films. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv
• Song, Xin; Graff, Joachim Seland; Götz, Adam; Skomedal, Gunstein & Finstad, Terje (2018). Investigating the influence of synthesis approach on the microstructure and thermoelectric properties for HMS. Fulltekst i vitenarkiv
• Riis, Henrik; Song, Xin; Prytz, Øystein & Finstad, Terje (2018). Characterization of Cu/ZnSb interfaces. Fulltekst i vitenarkiv
• Finstad, Terje; Song, Xin; Riis, Henrik & Prytz, Øystein (2018). Cu Films on Thermoelectric ZnSb. Fulltekst i vitenarkiv Vis sammendrag

  ZnSb is a semiconductor that is experiencing a renewed interest as a thermoelectric material (as well for other applications). For thermoelectric applications the high abundance of the elements and their low toxicity are favorable. Most of the reports have been on optimizing the material without explicitly addressing the integration into a thermoelectric module. The necessary physical and electrical contacts to the material are important, challenging efficient fabrication, durability, thermal stability, thermal stress etc. The detailed understanding of the thermoelectrical material ZnSb in intimate contacts with metals is lacking. This work is our first step towards studying metal contacts to ZnSb. We start by studying deposited Cu films on ZnSb because Cu may be one of the constituents of a metallization scheme. Some of the reasons for choosing Cu is that its thermal expansion matches that of ZnSb, Cu has low cost and the technology for bonding patterns to insulator substrates like alumina is well established. Further Cu is a p-type dopant for ZnSb yielding optimum thermoelectric characteristics at the solubility limit. The solubility of Cu in ZnSb should thus promote tunneling and low contact resistance. The Cu/ZnSb interface has been investigated after heat treatments in the temperature range 200 to 350°C. The ZnSb samples were made by hot pressing grains of ZnSb. A 100nm thick layer of Cu was e-beam deposited. The samples are characterized by SEM with EDS and several TEM techniques. The TEM specimens were made by Focused Ion Beam. The elemental distributions and phase formation will be presented. The contact resistance of the samples is also under investigation.

• Riis, Henrik; Finstad, Terje & Prytz, Øystein (2017). Metallization of ZnSb. Fulltekst i vitenarkiv
• Schrade, Matthias; Berland, Kristian; Guzik, Matylda Natalia; Løvvik, Ole Martin & Finstad, Terje (2017). Why does the thermal conductivity of XNiSn vary for nominally identical samples? Fulltekst i vitenarkiv
• Berland, Kristian; Eliassen, Simen Nut Hansen; Schrade, Matthias; Tofan, Raluca; Guzik, Matylda Natalia & Gunnæs, Anette Eleonora [Vis alle 11 forfattere av denne artikkelen] (2017). Using theory to understand thermoelectric materials. Fulltekst i vitenarkiv
• Taklo, Maaike M. Visser; Schjølberg-Henriksen, Kari; Malik, Nishant; Poppe, Erik Utne; Moe, Sigurd T. & Finstad, Terje (2017). Al-Al Wafer-Level Thermocompression Bonding applied for MEMS. Fulltekst i vitenarkiv
• Finstad, Terje (2017). Cryo-Milled Nanopowders of ZnSb for Reduction of Thermal Conductivity. Fulltekst i vitenarkiv
• Schrade, Matthias; Echevarria-Bonet, Cristina; Eliassen, Simen Nut Hansen; Berland, Kristian; Persson, Clas & Tofan, Raluca [Vis alle 9 forfattere av denne artikkelen] (2016). Thermal Properties of XNiSn(X = Hf, Zr, Ti) half Heusler Alloys. Fulltekst i vitenarkiv
• Schrade, Matthias; Echevarria-Bonet, Cristina; Guzik, Matylda Natalia; Tofan, Raluca; Gunnæs, Anette Eleonora & Eliassen, Simen Nut Hansen [Vis alle 10 forfattere av denne artikkelen] (2016). Thermal properties of XNiSN (X = Ti, Zr, Hf) half Heusler alloys. Fulltekst i vitenarkiv
• Song, Xin; Valset, Kjetil; Graff, Joachim Seland; Thøgersen, Annett; Gunnæs, Anette Eleonora & Carbalho, Partricia [Vis alle 10 forfattere av denne artikkelen] (2015). Nano Improves Thermoelectric Performance. Fulltekst i vitenarkiv Vis sammendrag

  The rule of thumb for improving the performance of thermoelectric materials is to reduce the thermal conduction, which attributes to electron transport and phonon transport. Phonons are characterized by different wavelength, Short wavelength phonons are scattered by impurity atoms in alloyed matels, Mid- and Long wavelength phonns are scattered by the iuinterfaces. Nanostructuring can engineer the phonon transport by introduce numerous grain boundaries, thus increasing phonon scattering, further improving thermal resistance. The aim of the present study is the nano-grains of the thermoelectric material ZnSb and the correlation with thermal transport measurements. We explored cryomilling (by ball-milling powders at Cryogenic temperature) as a technique to created nano—powders of ZnSb for pressing into pellets. The effect of cryomilling flowed by hot-pressing of undoped ZnSb was investigated.

• Malik, Nishant; Venkatachalapathy, Vishnukanthan; Poppe, Erik; Schjølberg-Henriksen, Kari; Taklo, Maaike Margrete Visser & Finstad, Terje (2015). Grain structure of Aluminium films for wafer-level thermocompression bonding. Fulltekst i vitenarkiv
• Heinrich, Christophe P.; Schrade, Matthias; Cerretti, Giacomo; Malerød-Fjeld, Harald; Finstad, Terje & Norby, Truls [Vis alle 7 forfattere av denne artikkelen] (2015). Tetragonal Tungsten Bronzes Nb8−xW9+xO47−δ: Optimization Strategies and Transport Properties of a New Group of n-Type Thermoelectric Oxides. Fulltekst i vitenarkiv
• Valset, Kjetil; Song, Xin & Finstad, Terje (2014). Stability and thermoelectric properties of Cu doped ZnSb. Fulltekst i vitenarkiv
• Løvvik, Ole Martin; Song, Xin; Valset, Kjetil; Flage-Larsen, Espen; Fjeld, Harald & Graff, Joachim Seland [Vis alle 15 forfattere av denne artikkelen] (2014). Thermoelectric materials from first principles to final applications: Basic and applied thermoelectrics in Oslo. Fulltekst i vitenarkiv
• Song, Xin; Valset, Kjetil; Graff, Joachim Seland; Thøgersen, Annett; Gunnæs, Anette Eleonora & Luxsacumar, Sivakanesar [Vis alle 8 forfattere av denne artikkelen] (2014). Nanostructuring by Cryomilling of Undoped ZnSb. Fulltekst i vitenarkiv
• Song, Xin; Persson, Clas; Norby, Truls; Flage-Larsen, Espen; Karlsen, Ole Bjørn & Hansen, Vidar [Vis alle 16 forfattere av denne artikkelen] (2014). Conversion of Heat and Energy: Thermoelectrics. Fulltekst i vitenarkiv
• Malik, Nishant; Tofteberg, Hannah Rosquist; Poppe, Erik; Finstad, Terje & Schjølberg-Henriksen, Kari (2014). Hermeticity and Reliability of Au-Au Thermocompression Bonds, Realized at Low Temperature. Fulltekst i vitenarkiv
• Malik, Nishant; Poppe, Erik; Schjølberg-Henriksen, Kari; Taklo, Maaike Margrete Visser & Finstad, Terje (2014). Hermeticity and Reliability of Al-Al Thermocompression Wafer Bonding. Fulltekst i vitenarkiv
• Libassie, Hailu Asmare & Finstad, Terje (2019). PV Installation System: Analysis of Performance and Degradation Mechanisms. Universitetet i Oslo. Fulltekst i vitenarkiv

Se alle arbeider i NVA