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Trimethylboron as Single-Source Precursor for Boron-Carbon Thin Film Synthesis by Plasma Chemical Vapor Deposition
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. European Spallat Source ERIC, Sweden.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering. European Spallat Source ERIC, Sweden.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering. European Spallat Source ERIC, Sweden.
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2016 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 38, 21990-21997 p.Article in journal (Refereed) Published
Abstract [en]

Boron–carbon (BxC) thin films are potential neutron converting layers for 10B-based neutron detectors. However, as common material choices for such detectors do not tolerate temperatures above 500 °C, a low temperature deposition route is required. Here, we study trimethylboron B(CH3)3 (TMB) as a single-source precursor for the deposition of BxC thin films by plasma CVD using Ar plasma. The effect of plasma power, TMB/Ar flow ratio and total pressure, on the film composition, morphology, chemical bonding, and microstructures are investigated. Dense and boron-rich films (B/C = 1.9) are achieved at high TMB flow under a low total pressure and high plasma power, which rendered an approximate substrate temperature of ∼300 °C. Films mainly contain B–C bonds with the presence of B–O and C–C, which is attributed to be the origin of formed amorphous carbon in the films. The high H content (15 ± 5 at. %) is almost independent of deposition parameters and contributed to lower the film density (2.16 g/cm3). The plasma compositional analysis shows that the TMB molecule decomposes to mainly atomic H, C2, BH, and CH. A plasma chemical model for the decomposition of TMB with BH and CH as the plausible film depositing species in the plasma is proposed.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016. Vol. 120, no 38, 21990-21997 p.
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-132478DOI: 10.1021/acs.jpcc.6b06529ISI: 000384626800097OAI: oai:DiVA.org:liu-132478DiVA: diva2:1046268
Note

Funding Agencies|European Spallation Source ERIC; Knut and Alice Wallenberg Foundation; BrightnESS project (Horizon) [676548]; Carl Tryggers Foundation for Scientific Research [CTS 14:431]

Available from: 2016-11-13 Created: 2016-11-12 Last updated: 2017-09-29Bibliographically approved
In thesis
1. 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)
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Available from: 2017-09-29 Created: 2017-09-29 Last updated: 2017-10-12Bibliographically approved

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