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Gas phase chemical vapor deposition chemistry of triethylboron probed by boron-carbon thin film deposition and quantum chemical calculations
Linköping University, Faculty of Science & Engineering. Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. European Spallat Source ESS AB, Sweden.
University of Marburg, Germany; University of Marburg, Germany.
University of Marburg, Germany; University of Marburg, Germany.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering. European Spallat Source ESS AB, Sweden.
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2015 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 3, no 41, 10898-10906 p.Article in journal (Refereed) Published
Abstract [en]

We present triethylboron (TEB) as a single-source precursor for chemical vapor deposition (CVD) of BxC thin films and study its gas phase chemistry under CVD conditions by quantum chemical calculations. A comprehensive thermochemical catalogue for the species of the gas phase chemistry of TEB is examined and found to be dominated by beta-hydride eliminations of C2H4 to yield BH3. A complementary bimolecular reaction path based on H-2 assisted C2H6 elimination to BH3 is also significant at lower temperatures in the presence of hydrogen. Furthermore, we find a temperature window of 600-1000 degrees C for the deposition of X-ray amorphous BxC films with 2.5 less than= x less than= 4.5 from TEB. Films grown at temperatures below 600 degrees C contain high amounts of H, while temperatures above 1000 degrees C result in C-rich films. The film density and hardness are determined to be in the range of 2.40-2.65 g cm(-3) and 29-39 GPa, respectively, within the determined temperature window.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2015. Vol. 3, no 41, 10898-10906 p.
National Category
Physical Sciences Chemical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-122673DOI: 10.1039/c5tc02293bISI: 000363252200030OAI: oai:DiVA.org:liu-122673DiVA: diva2:871688
Note

Funding Agencies|European Spallation Source ESS AB; Knut and Alice Wallenberg Foundation; German Science Foundation (Research Training Group 1782); Beilstein Foundation (Frankfurt/Germany)

Available from: 2015-11-16 Created: 2015-11-13 Last updated: 2017-09-29
In thesis
1. Chemical vapour deposition of boron-carbon thin films from organoboron precursors
Open this publication in new window or tab >>Chemical vapour deposition of boron-carbon thin films from organoboron precursors
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Boron-carbon (BxC) thin films enriched in 10B are potential neutron converting layers for 10Bbased solid-state neutron detectors given the good neutron absorption cross-section of 10B atoms in the thin film. Chemical Vapour Deposition (CVD) of such films faces the challenge that the maximum temperature tolerated by the aluminium substrate is 660 °C and low temperature CVD routes for BxC films are thus needed. This thesis presents the use of two different organoboron precursors, triethylboron –B(C2H5)3 (TEB) and trimethylboron – B(CH3)3 (TMB) as single-source precursors for CVD of BxC thin films.

The CVD behaviour of TEB in thermal CVD has been studied by both BxC thin film deposition and quantum chemical calculations of the gas phase chemistry at the corresponding CVD conditions. The calculations predict that the gas phase reactions are dominated by β-hydride eliminations of C2H4 to yield BH3. In addition, a complementary bimolecular reaction path based on H2-assisted C2H6 elimination to BH3 is also present at lower temperatures in the presence of hydrogen molecules. A temperature window of 600 – 1000 °C for deposition of X-ray amorphous BxC films with 2.5 ≤ x ≤ 4.5 is identified showing good film density (2.40 – 2.65 g/cm3) which is close to the bulk density of crystalline B4C, 2.52 g/cm3 and high hardness (29 – 39 GPa). The impurity level of H is lowered to < 1 at. % within the temperature window.

Plasma chemical vapour deposition has been studied using TMB as single-source precursor in Ar plasma for investigating BxC thin film deposition at lower temperature than allowed by thermal CVD and further understanding of thin film deposition process. The effect of plasma power, total pressure, TMB and Ar gas flow on film composition and morphology are investigated. The highest B/C ratio of 1.9 is obtained at highest plasma power of 2400 W and TMB flow of 7 sccm. The H content in the films seems constant at 15±5 at. %. The B-C bond is dominant in the films with small amount of C-C and B-O bonds, which are likely due to the formation of amorphous carbon and surface oxidation, respectively. The film density is determined as 2.16±0.01 g/cm3 and the internal compressive stresses are measured to be <400 MPa.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 29 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1741
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-123909 (URN)10.3384/lic.diva-123909 (DOI)978-91-7685-858-5 (ISBN)
Presentation
2016-01-21, Jordan/ Fermi, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 13:15 (English)
Opponent
Supervisors
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2016-02-29Bibliographically approved
2. CVD Chemistry of Organoborons for Boron-Carbon Thin Film Depositions
Open this publication in new window or tab >>CVD Chemistry of Organoborons for Boron-Carbon Thin Film Depositions
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Boron-carbon thin films enriched with 10B are potential neutron converting layers for 10B-based solid state neutron detectors given the good neutron absorption cross section of 10B atoms in thin films. The common neutron-transparent base material, Al (melting point 660 °C), limits the deposition temperature and the use of chlorinated precursors forming corrosive by-products such as HCl. Therefore, the organoborons triethylboron B(C2H5)3 (TEB) and trimethylboron B(CH3)3 (TMB) are evaluated as precursors for CVD of BxC films. In order to get a complete understanding of the CVD behaviour of these precursors for deposition of boron containing films, both thermal CVD and plasma CVD of BxC films have been demonstrated. A gas phase chemical mechanism at the corresponding thermal CVD conditions was proposed by quantum chemical calculations while chemical mechanism in the plasma was suggested based on plasma composition obtained from Optical emission spectroscopy (OES).

The behaviours of TEB and TMB in thermal CVD are investigated by depositing BxC films in both H2 and Ar atmospheres, respectively. Films deposited using TEB within a temperature window of 600 – 1000 °C are X-ray amorphous with 2.5 ≤ x ≤ 4.5. The impurity level of H is less than 1 at. % above 600 °C. Calculations predict that the gas phase reactions are dominated by β-hydride eliminations of C2H4 to yield BH3. In addition, a complementary bimolecular reaction path based on H2 assisted C2H6 elimination to BH3 is also present at lower temperatures in the presence of hydrogen molecules. As for films deposited with TMB, dense, amorphous, boron rich (B/C = 1.5-3) films are obtained at 1000 °C in both H2 and Ar atmosphere.  The quantum chemical calculations suggest that the TMB molecule is mainly decomposed by unimolecular α- elimination of CH4 complemented by H2 assisted elimination of CH4.

Plasma CVD of BxC thin films has been studied using both TMB and TEB as single-source precursors in an Ar plasma at temperatures lower than that allowed by thermal CVD. The effect of plasma power, TMB/TEB and Ar gas flow on film composition and morphology are investigated. The highest B/C ratio of 1.9 is found for films deposited at highest plasma power (2400 W) and high TMB flow (7 sccm). The H content in the films stays almost constant at 15±5 at. %. The B-C bonding is dominant in the films while small amounts of C-C and B-O exist, likely due to formation of amorphous carbon and surface oxidation. Film density is determined as 2.16±0.01 g/cm3 and the internal compressive stresses are measured to be less than 400 MPa. OES shows that TMB is decomposed to mainly atomic H, C2, BH, and CH. A plasma chemical model for decomposition of the TMB is constructed using a combination of film and plasma composition. It is suggested that the decomposition of TMB starts with dehydrogenation of the methyl groups followed by breakage of the B-C bonds to form the CH radicals. This bond breaking is at least partly assisted by hydrogen in forming the BH radicals.

When films are deposited using TEB flow of 5 and 7 sccm, the B/C ratio is found to be plasma power dependent while the carbon content is almost not affected. The highest B/C ratio of 1.7 is obtained at the highest power applied (2400 W) and attributed to better dissociation of TEB at higher plasma power. The H content in the films is within 14-20 at. %. The density of films is increased to 2.20 g/cm3 with increasing plasma power and attributed to a higher energetic surface bombardment during deposition. The oxygen content in the film is reduced to less than 1 at. % with increasing plasma power due to the densification of  the films preventing surface oxidation upon air exposure. Plasma composition from OES shows that the TEB molecules are also dissociated mainly to BH, CH, C2 and H. A plasma chemical model where the first ethyl group is split off by β-hydrogen elimination to form C2H4, which is further dehydrogenated to C2H2 and  forms C2 and CH is suggested. The BH species is assumed to be formed by the dehydrogenation of remaining ethyl groups and breakage of the remaining B-C bonds to form BH.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017. 47 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1849
National Category
Inorganic Chemistry Physical Sciences
Identifiers
urn:nbn:se:liu:diva-141548 (URN)10.3384/diss.diva-141548 (DOI)9789176855270 (ISBN)
Public defence
2017-10-06, Planck, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-09-29 Created: 2017-09-29 Last updated: 2017-10-12Bibliographically approved

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Höglund, CarinaJensen, JensHultman, LarsBirch, JensPedersen, Henrik

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