PhD Thesis_RuiMSMartins.pdf - RUN UNL

In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization
oriented grains of the Ni-Ti B2 phase. This experiment has also shown that the B2(200)
diffraction peak did not assume an important relevance as in the case of the Ni-Ti deposition
on naturally oxidized Si(100) substrates. For depositions on TiN buffer layers (topmost layer
formed mainly by oriented grains), there is no Ni-Ti/SiO 2 interface and, thus, the
development of the (100) orientation parallel to the substrate surface is not observed.
Therefore, a preferential growth of oriented grains of the B2 phase is observed since
the beginning of Ni-Ti film deposition.
The Ni-Ti films deposited on top of TiN buffer layers, with thickness values ≈ 15 nm
and ≈ 80 nm, where a dominating orientation could not be identified (i.e., primarily
and oriented grains nucleate and grow) exhibit a different behaviour. In this case,
oriented grains of the Ni-Ti B2 phase dominate at the beginning of the deposition,
while oriented grains take over at a later stage of the deposition. In the case of these
two samples, their TiN buffer layer has a considerable role in the appearance of the
oriented grains of the Ni-Ti B2 phase. There is an initial competitive growth between the
and crystal orientations – competition between an orientation promoted by the
TiN layer (due to its surface morphology) and, the (110) orientation, which is the more
densely packed crystallographic plane. Most likely, for the thinner TiN film (≈ 15 nm) on the
thermally oxidized substrate, a rough or granular surface with different crystal facets plays an
important role on the initial competitive growth between the and crystal
orientations of the Ni-Ti B2 phase. The XRD results performed on TiN layers with such
thickness did not detect any family of atomic planes of the TiN structure parallel to the TiN
surface [see Fig. 3.29(b)].
The effect of V b applied during the deposition of Ni-Ti films on TiN buffer layers with
≈ 15 nm and ≈ 215 nm, previously deposited on thermally oxidized Si(100) substrates, was as
well analysed. The Ni-Ti deposition with a substrate bias of −45 V on a TiN film with
≈ 15 nm has shown more pronounced development of the (110) orientation of the B2 phase
when compared with the sample deposited without V b . The more energetic ion bombardment
during the film growth and consequential increase in adatom mobility, which opposes
shadowing – due to the V b – favoured the stacking of (110) planes of the B2 phase parallel to
the substrate since the beginning of the deposition.
As described before, the Ni-Ti deposition on a topmost layer of TiN mainly formed by
oriented grains (TiN film with thickness ≈ 215 nm) leads to the preferential growth of
(110) planes of the Ni-Ti B2 phase parallel to the substrate from the beginning of the
Chapter 4 – Discussion 177