Xin Song

• Fatima, K.; Song, Xin; Maryam, A.; Rasheed, M.N.; Iqbal, F. & Monakhov, Eduard [Show all 7 contributors for this article] (2021). Influence of environment on thermoelectric properties of n-type silicon-germanium alloy. Digest Journal of Nanomaterials and Biostructures. 16(3), p. 907–913. Full text in Research Archive
• 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. Full text in Research Archive Show summary

  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.

• Graff, Joachim Seland; Schuler, Raphael; Song, Xin; Castillo-Hernandez, Gustavo; Skomedal, Gunstein & Enebakk, Erik [Show all 11 contributors for this article] (2021). Fabrication of a Silicide Thermoelectric Module Employing Fractional Factorial Design Principles. Journal of Electronic Materials. ISSN 0361-5235. 50, p. 4041–4049. doi: 10.1007/s11664-021-08902-y. Full text in Research Archive Show summary

  Thermoelectric modules can be used in waste heat harvesting, sensing, and cooling applications. Here, we report on the fabrication and performance of a four-leg module based on abundant silicide materials. While previously optimized Mg2Si0.3Sn0.675Bi0.025 is used as the n-type leg, we employ a fractional factorial design based on the Taguchi methods mapping out a four-dimensional parameter space among Mnx-εMoεSi1.75−δGeδ higher manganese silicide compositions for the p-type material. The module is assembled using a scalable fabrication process, using a Cu metallization layer and a Pb-based soldering paste. The maximum power output density of 53 μW cm–2 is achieved at a hot-side temperature of 250 °C and a temperature difference of 100 °C. This low thermoelectric output is related to the high contact resistance between the thermoelectric materials and the metallic contacts, underlining the importance of improved metallization schemes for thermoelectric module assembly.

• 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, p. 762–770. doi: 10.1016/j.jallcom.2017.03.339. Full text in Research Archive Show summary

  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.

• Cerretti, G; Schrade, Matthias; Song, Xin; Balke, B; Lu, H & Weidner, T [Show all 10 contributors for this article] (2017). Thermal stability and enhanced thermoelectric properties of the tetragonal tungsten bronzes Nb8-xW9+xO47 (0 < x < 5). Journal of Materials Chemistry A. ISSN 2050-7488. 5(20), p. 9768–9774. doi: 10.1039/c7ta01121k. Full text in Research Archive
• Song, Xin & Finstad, Terje (2017). A study of S doped ZnSb. Materials Science in Semiconductor Processing. ISSN 1369-8001. 71, p. 421–426. doi: 10.1016/j.mssp.2017.09.007. Full text in Research Archive Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• Song, Xin & Finstad, Terje (2016). Review of research on the thermoelectric material ZnSb. In Skipidarov, Sergey & Nikitiin, Mikhail (Ed.), Thermoelectrics for Power Generation - A Look at Trends in the Technology. IntechOpen. ISSN 9789535128465. p. 117–145. doi: 10.5772/65661. Full text in Research Archive Show summary

  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.

• Stagge, James Howard; Seagren, Eric A. & Song, Xin (2016). Sorption of Naphthalene onto Natural and Surfactant-Amended Soils. Journal of environmental engineering. ISSN 0733-9372. 142(4). doi: 10.1061/(ASCE)EE.1943-7870.0001076. Full text in Research Archive
• 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. Full text in Research Archive Show summary

  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.

• Song, Xin; Valset, Kjetil; Graff, Joachim Seland; Thøgersen, Annett; Gunnæs, Anette Eleonora & Luxsacumar, Sivakanesar [Show all 9 contributors for this article] (2015). Nanostructuring of undoped ZnSb by Cryo-Milling. Journal of Electronic Materials. ISSN 0361-5235. 44(8), p. 2578–2584. doi: 10.1007/s11664-015-3708-6. Full text in Research Archive Show summary

  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.

• Song, Xin; Böttger, Paul Heinrich Michael; Karlsen, Ole Bjørn; Finstad, Terje & Taftø, Johan (2012). Impurity band conduction in the thermoelectric material ZnSb. Physica Scripta. ISSN 0031-8949. T148. doi: 10.1088/0031-8949/2012/T148/014001. Full text in Research Archive

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• Løvvik, Ole Martin; Graff, Joachim Seland; Schrade, Matthias; Sunding, Martin Fleissner; Wright, Daniel Nilsen & Song, Xin [Show all 10 contributors for this article] (2020). Silicide module produced in the TESil project. Full text in Research Archive
• Song, Xin; Riis, Henrik; Prytz, Øystein & Finstad, Terje (2019). Metallization of selected transition metals on ZnSb by electron beam thermal evaporation. Full text in Research Archive Show summary

  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.

• 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. Full text in Research Archive
• Riis, Henrik; Song, Xin; Prytz, Øystein & Finstad, Terje (2018). Characterization of Cu/ZnSb interfaces. Full text in Research Archive
• Finstad, Terje; Song, Xin; Riis, Henrik & Prytz, Øystein (2018). Cu Films on Thermoelectric ZnSb. Full text in Research Archive Show summary

  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.

• Song, Xin (2017). Thermoelectric Material ZnSb -- its Defects and Impurity Band Conduction. Full text in Research Archive
• Riis, Henrik; Schrade, Matthias & Song, Xin (2016). Termoelektrisk strøm av varme overflater. Full text in Research Archive
• Løvvik, Ole Martin; Eliassen, Simen Nut Hans; Berland, Kristian; Song, Xin; Schrade, Matthias & Shulumba, N. [Show all 9 contributors for this article] (2016). Predicting the thermoelectric figure of merit from first principles. Full text in Research Archive
• Song, Xin; Valset, Kjetil; Graff, Joachim Seland; Thøgersen, Annett; Gunnæs, Anette Eleonora & Carbalho, Partricia [Show all 10 contributors for this article] (2015). Nano Improves Thermoelectric Performance. Full text in Research Archive Show summary

  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.

• Løvvik, Ole Martin; Eliassen, Simen Nut Hansen; Berland, Kristian; Peters, Thijs & Song, Xin (2015). Nanostructures for energy applications - a brief tour of activities in Oslo. Full text in Research Archive
• Valset, Kjetil; Song, Xin & Finstad, Terje (2014). Stability and thermoelectric properties of Cu doped ZnSb. Full text in Research Archive
• Løvvik, Ole Martin; Song, Xin; Valset, Kjetil; Flage-Larsen, Espen; Fjeld, Harald & Graff, Joachim Seland [Show all 15 contributors for this article] (2014). Thermoelectric materials from first principles to final applications: Basic and applied thermoelectrics in Oslo. Full text in Research Archive
• Song, Xin; Valset, Kjetil & Løvvik, Ole Martin (2014). Nanopiller lager strøm av varme. [Internet]. Intervju på forskning.no. Full text in Research Archive
• Song, Xin; Valset, Kjetil; Graff, Joachim Seland; Thøgersen, Annett; Gunnæs, Anette Eleonora & Luxsacumar, Sivakanesar [Show all 8 contributors for this article] (2014). Nanostructuring by Cryomilling of Undoped ZnSb. Full text in Research Archive
• Song, Xin; Persson, Clas; Norby, Truls; Flage-Larsen, Espen; Karlsen, Ole Bjørn & Hansen, Vidar [Show all 16 contributors for this article] (2014). Conversion of Heat and Energy: Thermoelectrics. Full text in Research Archive
• Song, Xin; Karlsen, Ole Bjørn; Valset, Kjetil; Løvvik, Ole Martin & Finstad, Terje (2013). STUDIES ON SULPHUR DOPING OF ZnSb. Full text in Research Archive
• Song, Xin; Valset, Kjetil; Karlsen, Ole Bjørn; Graff, Joachim Seland; Thøgersen, Annett & Luxsacumar, Sivakanesar [Show all 8 contributors for this article] (2013). Characterization methods for nanostructured thermoelectric materials ZnSb. Full text in Research Archive
• Song, Xin (2013). Thermoelectric: from Heat to Energy. Full text in Research Archive
• Løvvik, Ole Martin; Valset, Kjetil; Rauwel, Protima; Prytz, Øystein; Flage-Larsen, Espen & Song, Xin [Show all 12 contributors for this article] (2012). Thermoelectricity in Oslo. Full text in Research Archive
• Song, Xin (2012). Heat to Green Energy: Thermoelectrics. Full text in Research Archive
• Song, Xin; Valset, Kjetil; Böttger, Paul Heinrich Michael; Karlsen, Ole Bjørn; Finstad, Terje & Taftø, Johan (2011). Structure and Thermoelectric Properties of Cr and Mn Doped ZnSb. Full text in Research Archive
• Song, Xin; Karlsen, Ole Bjørn; Taftø, Johan; Böttger, Paul Heinrich Michael & Finstad, Terje (2011). Electrical and structural characterization of thermoelectric material ZnSb with additions of Mn and Cr. Full text in Research Archive
• Song, Xin (2016). Thermoelectric Transport and Microstructure of ZnSb. Universitetet i Oslo. Full text in Research Archive Show summary

  Thermoelectric device converts waste heat into electricity, or vice versa, thereby enhancing the total energy output and exploitation efficiency. Although it always competes against other means of waste heat harvesting, its advantages, such as flexibility (scalability and compactness), human friendliness (no noise and toxicity) and no moving parts (less wear), allow it plays an important role in global energy management. In order to implement thermoelectric devices in a wide utilization, material improvement based on cheap and abundant elements is essential. This PhD work focuses on the thermoelectric material ZnSb, which has the aforesaid advantages meanwhile the optimal output at moderate temperature for many of industry and transportation waste. This work was built upon an understanding of semiconductor transport. The methodological focus is on different synthesis techniques to produce nanostructured bulk material, microstructural characterization, as well as the investigation of thermoelectric transport properties. Doping was also introduced to modify the electronic structure and scattering mechanism. The property of the material was improved due to the reduced thermal conductivity by nanostructuring and the increased carrier concentrations by doping. Possible energy barrier at grain boundaries was verified. And many efforts have been made on the understanding of intrinsic defects and impurity band conduction, which led to a deeper knowledge of thermoelectrics.

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