Association

104 5. Results II: EuO integration directly on silicon
Figure 5.15.: Surface processes for a magnetic oxide (MO) on Si deposition. First, MO atoms are supplied
and arrive on the Si surface. Either desired processes may proceed (b,c,d) which form a
stoichiometric and heteroepitaxial MO layer; or undesired chemisorption or contaminations may
form with the Si surface (a,e). Adapted from Venables (1994). 204
nucleation energy,
ΔG(j)=−j Δμ + j 2 /3
X, X =
∑
C k γ k
k
}{{}
+ C SF (γ ∗ − γ F )
}{{}
interface energy
surface energy of k between substrate S
and film F
faces of the deposited cluster
. (5.3)
Herein, C k and C SF are geometrical constants depending on the shape of the cluster of size j.
The saturation factor Δμ of the deposit is discriminated between
⎧
⎪⎨ < 0, undersaturation, and
Δμ ⎪⎩ > 0, supersaturation of the deposit.
Typical shapes of ΔG(j) curves for different Δμ are shown in Fig. 5.14. In case of an undersaturation
of the deposit, nucleation is unfavored (red lines). If, however, a supersaturation
is eminent (Δμ ≫ 0), the thermodynamic behavior of ΔG(j) predicts growth of clusters of the
deposit. Thereby, the cluster size j allowing for a nucleation reduces to a few atoms.
Now, we apply the classical nucleation theory to obtain comparisons of cluster formation
probabilities at the EuO/Si interface. In the EuO/Si heterosystem, however, for EuO and
the interfacial contaminants EuSi 2 , Eu(OH) 3 , and SiO 2 the nanostructure of nucleation sites
directly on Si (001) is unknown to date. This hampers a numerical evaluation of ΔG(j) by
eq. (5.3), and thus predictions have to be taken with a grain of salt.
First, we consider the initial stage of EuO growth which is a Eu seed layer in the monolayer
regime. This means supersaturation and a large Δμ. Moreover, Eu has an extremely low surface
energy (γ Eu = 0.46 J/m 2 < 1 2 ·γ Si). Thus, for Eu on Si (001), the factor X in eq. (5.3) is
small and allows for negative energies ΔG(j) of the nucleation, independent from the cluster
size j: Eu wets the Si surface.
see Table 3.1 on p. 37.