henrrii

• 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.

• Torstensen, Jonathan Ø; Johnsen, Per-Olav; Riis, Henrik; Spontak, Richard J; Deng, Liyuan & Gregersen, Øyvind Weiby [Show all 7 contributors for this article] (2018). Preparation of cellulose nanofibrils for imaging purposes: comparison of liquid cryogens for rapid vitrification. Cellulose. ISSN 0969-0239. 25(8), p. 4269–4274. doi: 10.1007/s10570-018-1854-8. Full text in Research Archive Show summary

  Artifact-free imaging of cellulose nanofibrils (CNFs) from aqueous nanocellulose suspensions is nontrivial due to frequent irreversible agglomeration and structure damage during the course of sample preparation, especially as water is solidified prior to freeze-drying. In this study, we have examined the morphologies of CNF suspensions prepared by rapid vitrification in two different liquid cryogens—nitrogen and ethane—followed by freeze-drying. Results obtained by scanning electron microscopy confirm that vitrification in liquid ethane not only greatly improves the survivability of fine-scale CNF structural elements but also significantly reduces the propensity for CNF to agglomerate.

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• 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.

• 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.

• Riis, Henrik; Finstad, Terje & Prytz, Øystein (2017). Metallization of ZnSb. Full text in Research Archive
• Riis, Henrik; Schrade, Matthias & Song, Xin (2016). Termoelektrisk strøm av varme overflater. Full text in Research Archive
• Riis, Henrik; Gregersen, Øyvind Weiby & Torstensen, Jonathan (2016). Development of Structural Characterization Methods for Polyvinyl Alcohol (PVA)/Nanocellulose Composite Membranes. Norges teknisk-naturvitenskapelige universitet. Full text in Research Archive Show summary

  Structural characterization of polyvinyl alcohol (PVA)/nanocellulose films by scanning electron microscopy (SEM) generally only provides an qualitative estimate of how well the nanocellulose is distributed, and is not capable of giving detailed information on individual nanocellulose particles. Nor do the existing techniques preserve the native, porous state of the water-rich film or allow for uncoated imaging of the polymer films. In an effort to address this, new characterization methods have been developed. This has been done by image analysis of field-emission SEM images which revealed a segregation effect in both cellulose nanocrystal and cellulose nanofibril containing PVA films. This was believed to be caused by nanocellulose preferring the water phase over the solid matrix and receding with the water during drying. Further, methods using focused ion beam-SEM (FIB-SEM) and reactive ion etching (RIE) have been developed. FIB-SEM cross-sections of stained and unstained samples yielded no direct observation of nanocellulose, but voids believed to be at the PVA/nanocellulose interface were possibly seen. Preservation of the porous structure was done by liquid ethane and nitrogen freeze-drying. Samples frozen in liquid ethane observed in FIB-SEM showed a significant increase in cross-section features believed to be interfacial voids. This may indicate a partial preservation of the porous structure. RIE of the films resulted in surface roughening and a suitable etching recipe was not determined. However, several observations were made indicating the problem originated from surface heating during etching. High-energy particles bombarding the surface and exothermic surface reactions may be the cause of heating combine with poor heat conduction of PVA. Low-voltage SEM (LVSEM) successfully imaged uncoated surfaces with acceleration voltages around 1.4 kV because this range resulted in nearly zero surface charging. Much of the work was also supported by Monte Carlo simulations to determine the viability and likely starting parameters of experiments.

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