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Growth, processing and characterization of group IV materials for thermoelectric applications
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Functional Materials, FNM.
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 [en]
Thermoelectric, SiGe, GeSn(Si), Chemical vapor deposition, Nanowires
National Category
Nano Technology Other Physics Topics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-192142ISBN: 978-91-7729-076-6OAI: oai:DiVA.org:kth-192142DiVA: diva2:958213
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
List of papers
1. CVD growth of GeSnSiC alloys using disilane, digermane, tin tetrachloride and methylsilane
Open this publication in new window or tab >>CVD growth of GeSnSiC alloys using disilane, digermane, tin tetrachloride and methylsilane
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2014 (English)In: ECS Transactions, 2014, Vol. 64, no 6, 703-710 p.Conference paper (Refereed)
Abstract [en]

In this study, Ge1-x-y-zSnxSiyCz layers (0.01≤x≤ 0.06, 0≤y≤0.02 and 0≤z≤0.01) have been successfully grown at 280-330 °C on Ge and Si by using RPCVD technique. It was demonstrated that the quality of epitaxial layers is dependent on the growth parameters, layer thickness and the quality of Ge virtual layer. It was found that a proper strain balance in the matrix during the epitaxy where the Si is adjusted carefully with the Sn flux improves the incorporation of Sn in Ge matrix. A similar improvement of Sn incorporation has been observed for phosphorous, boron and carbon doping in GeSn layers as well. This is explained by the compensation of the compressive strain caused by Snand the tensile strain induced by Si to obtain the minimum energy in Ge matrix. This behavior was not observed for relaxed GeSn layers and Sn incorporation could be controlled only by the growth parameters. The thermal stability of GeSn is an important integration issue for device fabrication. The thermal stability of P- and B-doped GeSn layers was studied by rapid thermal annealing (RTA) in range of 400-600 °C and compared with intrinsic layers. The GeSn layers were stable up to 550 °C while the B-doped layers showed strain relaxation readily at 500 °C. The epitaxial quality of epi-layers was evaluated in terms of oxygen and water vapor contamination. The level of oxygen during epitaxy was as low as 10 ppb and the contamination amount was found as low as 1017 cm-3.

Series
, ECS Transactions, ISSN 1938-5862 ; 6
Keyword
Epitaxial growth, Germanium, Oxygen, Quality control, Rapid thermal annealing, Silicon, Silicon alloys, Thermodynamic stability, Tin, Boron and carbons, Compressive strain, Device fabrications, Epitaxial quality, Growth parameters, Integration issues, Layer thickness, Rapid thermal annealing (RTA), Tensile strain
National Category
Materials Engineering Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-167544 (URN)10.1149/06406.0703ecst (DOI)2-s2.0-84921269093 (ScopusID)
Conference
6th SiGe, Ge, and Related Compounds: Materials, Processing and Devices Symposium - 2014 ECS and SMEQ Joint International Meeting, 5 October 2014 through 9 October 2014
Note

QC 20150609

Available from: 2015-06-09 Created: 2015-05-22 Last updated: 2016-09-06Bibliographically approved
2. GeSnSi CVD Epitaxy using Silane, Germane, Digermane, and Tin tetrachloride
Open this publication in new window or tab >>GeSnSi CVD Epitaxy using Silane, Germane, Digermane, and Tin tetrachloride
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(English)Article in journal (Refereed) Submitted
Abstract [en]

In this study, strain relaxed and compressive strained Ge1-x-ySnxSiy (0.015≤x≤0.15 and 0≤y≤0.15) layers were epitaxially grown on Si substrate in a chemical vapor deposition reactor at atmospheric pressure. Digermane (Ge2H6) and germane (GeH4) were used as Ge precursors and tin tetrachloride (SnCl4) was used as Sn precursor. The growth temperature was kept below 400ᵒC to suppress Sn out diffusion. The layers crystal quality and strain were characterized using XRD, high resolution reciprocal lattice mapping and transmission electron microscopy and the surface morphology was investigated by atomic force microscopy (AFM). Furthermore, the low temperature epitaxial growth up to 15% Si atoms incorporation in Ge0.94Sn0.06 was demonstrated by adding silane (SiH4) as Si precursor. Sn contents calculated from high resolution XRD patterns were confirmed by Rutherford backscattering spectroscopy which shows that Sn atoms are mostly positioned in substitutional sites. AFM analysis showed below 1nm surface roughness for both strained and strain relaxed GeSn layers which make the promising materials for photonics and electronics applications.

National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-192065 (URN)
External cooperation:
Note

QC 20160906

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2016-09-06Bibliographically approved
3. Effect of strain on Ni-(GeSn)x contact formation to GeSn nanowires
Open this publication in new window or tab >>Effect of strain on Ni-(GeSn)x contact formation to GeSn nanowires
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2014 (English)In: Materials Research Society Symposium Proceedings, 2014, Vol. 1707Conference paper (Refereed)
Abstract [en]

In this study, the formation of Ni-(GeSn)x on strained and relaxed Ge1-xSnx (0.01≤x≤ 0.03) nanowires in contact areas has been investigated. The epi-layers were grown at different temperatures (290 to 380°C) by RPCVD technique. The strain in GeSn layers tailored through carefully chosen of growth parameters and virtual substrate. The nanowires were fabricated through both I-line and dry-etching. 15 nm Ni was deposited either on the contact areas or whole length of nanowires. The wires went through rapid thermal annealing at intervals of 360 to 550°C for 30s in N2 ambient. The results show the thermal stability and amount of particular phases were strain-dependent. The formation of Ni-GeSn was eased when GeSn layers were strain-free. When the Sn content is high the epi-layers suffer from Sn segregation. The Sn-rich surface impedes remarkably the Ni diffusion. The electrical conductivity measurement of nanowires shows low resistivity and Ohmic contact are obtained for Ni-GeSn.

Series
, Materials Research Society Symposium Proceedings, ISSN 0272-9172
Keyword
Ge, Ni, Sn, Germanium, Nanowires, Ohmic contacts, Rapid thermal annealing, Tin, Contact formation, Effect of strain, Electrical conductivity measurements, Growth parameters, Low resistivity, Strain-dependent, Virtual substrates, Whole lengths, Nickel
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-167536 (URN)10.1557/opl.2014.559 (DOI)2-s2.0-84926314889 (ScopusID)
Conference
2014 MRS Spring Meeting, 21 April 2014 through 25 April 2014
Note

QC 20150610

Available from: 2015-06-10 Created: 2015-05-22 Last updated: 2016-09-06Bibliographically approved
4. Electrical properties of sub-100 nm SiGe nanowires
Open this publication in new window or tab >>Electrical properties of sub-100 nm SiGe nanowires
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2016 (English)In: Journal of semiconductors, Vol. 37, no 10Article in journal (Refereed) In press
Abstract [en]

In this study, the electrical properties of SiGe nanowires in terms of process and fabrication integrity, measurement reliability, width scaling, and doping levels were investigated. Nanowires were fabricated on SiGe-on oxide (SGOI) wafers with thickness of 52 nm and Ge content of 47%. The first group of SiGe wires was initially formed by using conventional I-line lithography and then their size was longitudinally reduced by cutting with a focused ion beam (FIB) to any desired nanometer range down to 60 nm. The other nanowires group was manufactured directly to a chosen nanometer level by using sidewall transfer lithography (STL). It has been shown that the FIB fabrication process allows manipulation of the line width and doping level of nanowires using Ga atoms. The resistance of wires thinned by FIB was 10 times lower than STL wires which shows the possible dependency of electrical behavior on fabrication method.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2016
National Category
Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-192072 (URN)
External cooperation:
Note

QC 20160907

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2016-09-07Bibliographically approved
5. A Comparison of Power Factor in N and P-Type SiGe Nanowires for Thermoelectric Applications
Open this publication in new window or tab >>A Comparison of Power Factor in N and P-Type SiGe Nanowires for Thermoelectric Applications
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2016 (English)In: Journal of Nanoscience and Nanotechnology, ISSN 1533-4880, E-ISSN 1533-4899Article in journal (Refereed) Accepted
Abstract [en]

This work presents the thermoelectric properties of n- and p-type doped SiGe nanowires and shows the potential to generate electricity from heat difference over nanowires. The Si0.74Ge0.26 layers were grown by reduced pressure chemical vapor deposition technique on silicon on insulator and were condensed to the final Si0.53Ge0.47 layer with thickness of 52 nm. The nanowires were formed by using sidewall transfer lithography (STL) technique at a targeted width of 60 nm. A high volume of NWs is produced per wafer in a time efficient manner and with high quality using this technique. The results demonstrate high Seebeck coefficient in both n- and p-types SiGe nanowires. N-type SiGe nanowires show significantly higher Seebeck coefficient and power factor compared to p-type SiGe nanowires near room temperature. These results are promising and the devised STL technique may pave the way to apply a Si compatible process for manufacturing SiGe-based TE modules for industrial applications.

National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-192090 (URN)10.1166/jnn.2016.13728 (DOI)
External cooperation:
Note

Qc 20160907

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2016-09-07Bibliographically approved
6. Significant Improvement of Thermoelectric Efficiency in SiGe Nanowires
Open this publication in new window or tab >>Significant Improvement of Thermoelectric Efficiency in SiGe Nanowires
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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:nbn:se:kth:diva-192108 (URN)
External cooperation:
Note

QC 20160907

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2016-09-07Bibliographically approved

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