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Significant Improvement of Thermoelectric Efficiency in SiGe Nanowires
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Functional Materials, FNM.
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
(Department of Physics and Measurement Technology, Linköping University, SE-581 83 Linköping, Sweden)
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
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(English)Article in journal (Refereed) Submitted
Abstract [en]

The thermoelectric (TE) properties of SiGe nanowires (NWs) with width of 60 nm in a back-gate configuration have been studied experimentally and theoretically. The carrier transport in NWs was modified by biasing voltage to the gate for different temperatures. The original wafers were SiGe-on-oxide (SGOI), which were formed through condensation of SiGe on Si-on-oxide wafers (SOI).  The power factor of SiGe NWs was enhanced by a factor of >2 in comparison with SiGe bulk material over a temperature range of 273 K to 450 K. This enhancement is mainly attributed to the energy filtering of carriers in SiGe NWs which were introduced by the roughness in the shape of NWs, non-uniform SiGe composition and the induced defects during the manufacturing of SGOI wafers or processing of NWs. These defects create potential barriers which may significantly enhance the Seebeck coefficient, while the conductivity can be boosted by tuning the back-gate bias.

National Category
Nano Technology
Identifiers
URN: urn:nbn:se:kth:diva-192108OAI: oai:DiVA.org:kth-192108DiVA: diva2:958046
Note

QC 20160907

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2016-09-07Bibliographically approved
In thesis
1. Growth, processing and characterization of group IV materials for thermoelectric applications
Open this publication in new window or tab >>Growth, processing and characterization of group IV materials for thermoelectric applications
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Discover of new energy sources and solutions are one of the important global issues nowadays, which has a big impact on economy as well as environment. One of the methods to help to mitigate this issue is to recover wasted heat, which is produced in large quantities by the industry, through vehicle exhausts and in many other situations where we consume energy. One way to do this would be using thermoelectric (TE) materials, which enable direct interconversion between heat and electrical energy. This thesis investigates how the novel material combinations and nanotechnology could be used for fabricating more efficient TE materials and devices.

The work presents synthesis, processing, and electrical characterization of group IV materials for TE applications. The starting point is epitaxial growth of alloys of group IV elements, silicon (Si), germanium (Ge) and tin (Sn), with a focus on SiGe and GeSn(Si) alloys. The material development is performed using chemical vapor deposition (CVD) technique. Strained and strain-relaxed Ge1-x Snx (0.01≤x≤0.15) has been successfully grown on Ge buffer and Si substrate, respectively. It is demonstrated that a precise control of temperature, growth rate, Sn flow and buffer layer quality is necessary to overcome Sn segregation and achieve a high quality GeSn layer. The incorporation of Si and n- and p-type dopant atoms is also investigated and it was found that the strain can be compensated in the presence of Si and dopant atoms. 

Si1-xGexlayers are grown on Si-on-insulator wafers and condensed by oxidation at 1050 ᵒC to manufacture SiGe-on-insulator (SGOI) wafers. Nanowires (NWs) are processed, either by sidewall transfer lithography (STL), or by using conventional lithography, and subsequently manufactured into nanoscale dimensions by focused ion beam (FIB) technique. The NWs are formed in an array, where one side is heated by a resistive heater made of Ti/Pt. The power factor of NWs is measured and the results are compared for NWs manufactured by different methods. It is found that the electrical properties of NWs fabricated with FIB technique can be influenced due to Ga doping during ion milling.

Finally, the carrier transport in SiGe NWs formed on SGOI samples is tailored by applying a back-gate voltage on the Si substrate. In this way, the power factor is improved by a factor of 4. This improvement is related to the presence of defects and/or small fluctuation of nanowire shape and Ge content along the NWs, generated during processing and condensation of SiGe layers. The SiGe results open a new window for operation of SiGe NWs-based TE devices in the new temperature range of 250 to 450 K.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 53 p.
Series
TRITA-ICT, 2016:20
Keyword
Thermoelectric, SiGe, GeSn(Si), Chemical vapor deposition, Nanowires
National Category
Nano Technology Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-192142 (URN)978-91-7729-076-6 (ISBN)
External cooperation:
Public defence
2016-09-30, Sal B, Kistagången 16, Kista, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EM11-0002
Note

QC 20160907

Available from: 2016-09-07 Created: 2016-09-06 Last updated: 2016-09-09Bibliographically approved

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