Association

5.2. Thermodynamic analysis of the EuO/Si interface 101
(a) Silicon oxide formation during O-rich EuO growth:
2(H-Si) + 2Eu + 3O 2 2SiO 2 + 2EuO + H 2
(g)
(b) Silicon oxide formation during EuO distillation growth:
2(H-Si) + 2(3Eu+O 2 ) 2SiO 2 + 6Eu + H 2
(g)
(c) Silicon oxide dissolution during EuO distillation growth:
2(3Eu+O 2 ) + SiO 2 2Eu 2 O 3 + Si + 2Eu (evap)
2(3Eu+O 2 ) + SiO 2 Eu 3 O 4 + 2EuO + Si + Eu (evap)
2(3Eu+O 2 ) + SiO 2 6EuO + Si (dummy)
(d) Silicon oxide formation when stable EuO on H-Si or Si:
2(H-Si) + 2EuO
(g)
SiO 2 + 2Eu + Si + H 2
Si + 2EuO SiO 2 + 2Eu
ΔG reaction (T) (kJ/mol)
1000
500
0
-500
-1000
-1500
-2000
-2500
0
0
500
temperature (°C)
500 1000
SiO 2
EuO formation (for orientation)
1000 1500
temperature (K)
Figure 5.12.: Resulting Gibbs free energies of EuO/Si interface reactions involving SiO 2 during EuO
synthesis, and for stable EuO on Si (001).
1500
2000
the divalent EuO phase.
In the next step, we proceed with reactions involving SiO 2 during EuO synthesis. First, the
analysis of oxygen-rich EuO (regime I in the Gibbs triangle) growth is depicted in Fig. 5.12a
and reveals in conjunction with divalent EuO a highly favorable formation of SiO 2 (red circle).
This denotes thus an exclusively oxidic result of the initial EuO layers without any
metallic silicide or Eu. We proceed with the complementary EuO growth mode, the Eudistillation
growth (regime III in the Gibbs triangle) of EuO on H-Si as depicted in Fig. 5.12b–
c. Herein, the resulting Gibbs free energy of the SiO 2 formation during Eu distillation growth
equals almost the bare EuO formation energy. Which one is the thermodynamically favored
result? If we analyze the energy gain of the EuO formation, we find it at GEuO f (300 K) =
−559 kJ/mol, while the SiO 2 formation reaction results in GSiO f 2
(300 K) = −546 kJ/mol, which
is only 2% less gain of Gibbs free energy and thus a weak argument for a possible dominance
of the EuO formation.
In order to elucidate the SiO 2 behavior from another viewpoint, we consider its disappearance
during Eu distillation growth in Fig. 5.12c. We find, that the SiO 2 disappearance is
very probable yielding all different EuO valencies, with an energy gain of GSiO dissol.
2
(300 K) ≈
−2500 kJ/mol. However, upon SiO 2 disappearance the most favorable products are the mixedvalent
Eu 3 O 4 and divalent EuO. This underlines the importance of minimization of any SiO x
at the EuO/Si interface, because SiO 2 may act as constituent for oxidation yielding antiferromagnetic
higher Eu oxides. We remark, that disappearance reactions of SiO 2 need an activation
energy and are more likely at elevated temperatures.
Finally, when layers of stoichiometric EuO have been grown on top of the Si surface, we
investigate the thermodynamic stability of this system. In Fig. 5.12d, stable EuO on either
bare Si (001) or hydrogen-passivated Si (001) are analyzed with respect to a possible
SiO 2 formation. In case of H-Si as the substrate, the resulting Gibbs free energy is
GSiO f 2
(300 K) = +900 kJ/mol ≫ 0 and thus extremely unfavored. Here, it is safe to state
that EuO is thermodynamically stable directly integrated with H-Si (001), this coinciding
with a previous thermodynamic prediction. 14 Furthermore, when we consider EuO on bare
This is evident from the large gain of Gibbs free energy upon SiO 2 formation as already summarized in the
introductory figure 5.7 on p. 96.