Investigations on the characterization of ion implanted hexagonal boron nitride

Boron nitride (BN) in its cubic form (cubic boron nitride (c-BN)) is one of the
known superhard materials with superior mechanical, chemical and electronic properties.
These properties have made it an excellent material in many modern industrial
and electronic applications and as such, extensive research grounds have been
developed for over half a decade now with the aim of finding alternative ways to
synthesize it.
The work presented in this thesis was inspired by the fact that defects introduced
into the hexagonal form of boron nitride (h-BN) under certain conditions
can lead to a change in its local structure and hence the formation of the cubic BN
symmetry.
The work focused on the introduction of different ions which included helium,
lithium, boron, nitrogen and argon into h-BN, by the ion implantation process, in
order to promote a defect-induced phase change to the cubic symmetry and possibly
to other BN polymorphs. We introduced these ions at different fluences (number
of ions per unit area) and energies so as to investigate the best conditions that will
influence the lowest activation energy that will in turn favour the c-BN formation.
The resulting thin hard layer could be an excellent sub-surface treatment.
All the samples used were high quality polycrystalline and single crystal h-BN,
obtained from various manufacturers. The fluence range used was from 1×1013
ions/cm2 to 5×1016 ions/cm2, with energy ranging from 40 keV to 150 keV. This
energy and fluence choice was inspired by previous research that had been done at
higher energies (MeV range) and recommended that low energy (keV range) and
fluence could induce similar change.
To investigate these effects, various analysis techniques were employed. The
major techniques involved optical vibrational methods using Raman Spectroscopy
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(RS) and Fourier Transform Infrared Spectroscopy (FTIR) carried out on the samples
before and after implantation. Other techniques used included Glancing Incidence
X-ray Diffraction (GIXRD), Transmission Electron Microscopy (TEM), and
Energy Dispersive X-ray Spectroscopy (EDS).
Raman and FTIR measurements showed the introduction of new phonon and vibrational
modes in the samples after implantation. The position, size and broadening
suggested that they originated from a symmetry attributed to nano-structured
cubic BN (nc-BN). The nature and extent of the nc-BN features was very dependent
on the implantation parameters with different atomic mass ions each having
an optimum fluence with regards to the intensities of the Raman and FTIR signal
associated with them. Glancing incidence X-ray diffraction showed new diffraction
patterns whose angles corresponded to the cubic and rhombohedral BN symmetries.
The linewidths of these peaks were used to estimate the crystal size, which
were in the nanoscale range, hence complementing the results obtained by optical
spectroscopy.
The High-Angle Annular Dark-field Scanning Transmission Electron microscopy
(HAADF-STEM) analyses showed regions with low contrast within the implanted
region, suggesting that there were regions within the implanted layer that contained
dense structures which were attributed to the cubic BN symmetry.
Computer simulations using the Stopping and Range of Ions in Matter (SRIM)
programme were performed to understand the events that take place during the
interaction of the ions with h-BN. Phonon confinement model calculations were
also performed to understand the nature of peaks forming after implantation with
an aim of support Raman measurements and to estimate the size of the nc-BN
domains.
With these complementary analyses, it was concluded that indeed implantation
is an effective method of creating nanocrystalline c-BN under less extreme
conditions of pressure and temperature.
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