PhD thesis defense of Duc-Quang Hoang
Nov 21, 2017 - 10.00 uur
Agoralaan Gebouw D
Lokaal Room C105
Duc-Quang Hoang invites you to the public defense of his doctoral thesis entitled: "Nucleation and growth mechanism of sputtered hexagonal boron nitride nanowalls."
Promoter is Prof. Dr. Ken Haenen.
Co-promoters are Prof. Dr. Hans-Gerd Boyen and Dr. Paulius Pobedinskas.
Hexagonal boron nitride (hBN) thin films were deposited on Si(100) substrates using a Ar (51%) / N2 (44%) / H2 (5%) gas mixture by the home-built unbalanced radio frequency (RF) sputtering system. The effects of various target-to-substrate distances (d = 3 cm < d < 6 cm), substrate temperatures (Tsub = 78 °C < Tsub 500 °C), and substrate tilting angles (a = 0° or 90°) were investigated. Physical properties of hBN films were characterized by X-ray diffraction, scanning/transmission electron microscopy, Raman and Fourier transform infrared spectroscopy techniques. The experimental results showed that hydrogen etching dominantly affects the films deposited at d = 3 cm and Tsub > 250 °C, while the effect is negligible for the films deposited at d = 6 cm. Moreover, the results also demonstrate that the hydrogen does not only act as a synthesizing factor of the hBN nanowall structures, but also as an etching agent to remove material during the deposition, i.e. wall branches, side-walls, inter-spaces/voids between neighboring walls. The degree of defects in deposited films was identified by tailoring the amount of hydrogen in the deposited films via H-N bonds of the given films. Furthermore, the hBN nanowalls are vertically oriented to their substrate surfaces, independent on the tilting of the substrate. This implies that chemical processes rather than physical ones govern the growth at the initial stage. The observed data reveal that hBN crystallinity is tunable with variations of Tsub, d, α and/or film thickness. Furthermore, the evidence of hydrogen absorption/desorption is given based on the results of relative measurements of infrared stretching (E1u), bending (A2u) and Raman (E2g) modes of the hBN optical phonon, and N-H vibration mode.
Besides, the growth behaviour of hBN nanowall films deposited at d = 3 cm on various substrate materials, i.e. Si(100), Si3N4 (amorphous), Cr / Au (polycrystalline catalyst metals) and CVD diamond (hydrogen terminated surface), were investigated by transmission electron microscopy (TEM). The obtained results demonstrate that the hBN crystallization is highly enhanced when a hBN thin film is deposited on a diamond substrate. In particular, we assigned that the hydrogen is an indispensable agent to form hBN nanowalls and plays a key role at facets of nanocrystalline diamond (NCD) grains to aid hBN nanowalls nucleate instantly when ionized B and N atoms arrive the NCD substrate. Diamond films were produced by a microwave plasma enhanced chemical vapor deposition (MW PE CVD) technique. The hBN film was then deposited on the NCD film after its growth. Structural properties of the NCD / hBN interface were examined with a high resolution S/TEM. An enhancement of hBN crystallinity at the NCD/hBN interface was found in comparison to the case of the structure on other substrates. This enhancement is mainly attributed to the hydrogen termination that appears on the diamond surface. In this way hBN nanosheets are directly grown on NCD facets with less amorphous/turbostratic BN (a/t BN) phase presents at the interface. This observation demonstrates that our NCD/hBN heterostructure is a promising candidate for future industrial applications, i.e. cold field emission, water cleaning, etc., where NEA properties of hydrogen terminated surface play a crucial role in such devices. In addition, an attempt was taken with a deposition of hBN thin film on Cr (10 nm) / Au (100 nm) buffer layer at Tsub = 450 °C. An enhancement of hBN crystallinity at the interface of the Au / hBN layers was also found. The possible catalyst working of the Au layer at Tsub = 450 °C is an important factor supporting the condensation processes that might occur faster at the interface of both layers.
The results of this thesis bring a better understanding on the deposition process of hBN films using the unbalanced RF sputtering technique. Moreover, the results of well-defined hBN nanowalls and porous BN nanosheets are useful for fundamental and applied studies of hBN nanowalls. Ultimately, based on the given results an outlook is given in the final chapter of this thesis.