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CVD Chemistry of Organoborons for Boron-Carbon Thin Film Depositions
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
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. , p. 47
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1849
National Category
Inorganic Chemistry Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-141548DOI: 10.3384/diss.diva-141548ISBN: 9789176855270 (print)OAI: oai:DiVA.org:liu-141548DiVA, id: diva2:1145622
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
List of papers
1. Gas phase chemical vapor deposition chemistry of triethylboron probed by boron-carbon thin film deposition and quantum chemical calculations
Open this publication in new window or tab >>Gas phase chemical vapor deposition chemistry of triethylboron probed by boron-carbon thin film deposition and quantum chemical calculations
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2015 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 3, no 41, p. 10898-10906Article 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
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-122673 (URN)10.1039/c5tc02293b (DOI)000363252200030 ()
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
2. Trimethylboron as Single-Source Precursor for Boron-Carbon Thin Film Synthesis by Plasma Chemical Vapor Deposition
Open this publication in new window or tab >>Trimethylboron as Single-Source Precursor for Boron-Carbon Thin Film Synthesis by Plasma Chemical Vapor Deposition
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2016 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 38, p. 21990-21997Article 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
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:liu:diva-132478 (URN)10.1021/acs.jpcc.6b06529 (DOI)000384626800097 ()
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: 2017-11-29Bibliographically approved

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