Association

5.2. Thermodynamic analysis of the EuO/Si interface 103
do not allow a clear prevention of hydroxide formation, two practical methods circumvent
Eu(OH) 3 : using bare Si instead of H-Si obviates the constituent H for Eu(OH) 3 formation.
Any further hydroxide formation can be prevented by depriving any traces of H 2 or H 2 Ogas
in the UHV system (Oxide-MBE).
For europium hydroxide, we conclude that neither hydrogen passivation of Si nor different
EuO growth regimes are capable to prevent hydroxide formation. Nevertheless, the energy
gains on formation are comparably small with respect to SiO x or higher Eu oxides. The
disappearance of Eu(OH) 3 is thermodynamically favored, yielding preferably trivalent Eu
oxides. To practically prevent the Eu hydroxide, a H-Si surface as well as a H 2 atmosphere in
the system of synthesis are to be avoided.
We exclude the following compounds from discussion due to their negligible formation probabilities:
mineralic contaminations on top of the Si surface, like silicates, will form only at
process temperatures of T S 800 ◦ C (Si(OH) 4 ) or 1500 ◦ C(Eu (II)
3 SiO 5). 197,198 Moreover, complicated
silicide phases Eu x Si y are predicted 199 or experimentally investigated 200,201 in previous
studies; here, we limit the discussion to the most probable native silicide, EuSi 2 . Finally,
silane phases (Si n H 2n+2 ) are considered to be negligible due to their thermal instability. 196
Nucleation probability and surface kinetics at the EuO/Si heterointerface
Figure 5.14.: Free energy of nucleation ΔG(j)for
different saturations Δμ.
At negative ΔG(j), nucleation of the deposit
is thermodynamically favored to proceed.
A maximum of the curvature in a
positive energy regime constitutes a nucleation
barrier. The curves are to scale for
the surface free energy term X = 4, and
for Δμ = −1, 0, 1, or 2. ΔG(j), Δμ, and X
are chosen to be in the unit k B T in agreement
with Weeks and Gilmer (1979). 202
Adapted from J. A. Venables (1999). 203
Reaction balances are thermodynamic in nature and ignore reaction kinetics. Thus, processes
that we have predicted to be favorable by the Gibbs free energy balances in Ellingham diagrams
can still be slow – or be enhanced by molecular kinetics. Therefore, we expand our
picture of the EuO/Si interface by means of the classical growth kinetics on surfaces: the nucleation
theory. 203 For the substrate–deposit heterosystems, we revisit the concept of surface
and interface energies. The predictions of the classical nucleation theory are dependent on
the knowledge of the saturation, the surface energies γ, and the geometry for each deposit. A
measure for the thermodynamic probability of a cluster nucleation containing j atoms is the
See section “MBE growth” in chapter 3.1.