PhD Thesis Arne Lüker final version V4 - Cranfield University

Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications
Even at room temperature, the buckling pattern is dynamic, with the dimers executing
a kind of "rocking" motion or oscillation around the idealized structure of Figure 7.5(a).
As a result the surface dimers are chemically very reactive and can serve as receptors for
certain types of atoms or even molecules.
The most common atom in nature which docks onto this structure is O, forming the
native SiO2 layer, Fig.7.6. But Sr
ad-atoms will saturate the
dangling bonds of one silicon
dimer with its two valence
electrons as well. A saturated
dimer looses its buckling, since
all dangling bonds are filled and
both Si atoms prefer the
tetragonal sp 3 configuration. The
Sr atom first occupies the trough
between the dimer rows, in the
center of four dimers.
C.J. Först et al. [3, 4] have
simulated this adsorption process
of Sr ad-atoms onto Si in more
detail. The result is shown in Fig.
7.7. One of the Sr neighboring Si
dimers is unbuckled due to the
electron donation from the ad-
atom. The others three dimers are
orientated such that the upper,
and thus negatively charged Si
(a)
(b)
Fig. 7.5: The top surface of silicon; (a), the “dimer row”
reconstruction, and (b), the “buckled row” reconstruction [2].
Fig. 7.6: The Si-SiO2 interface. Black atoms are Oxygen
atom points towards the Sr 2+ ion. The energy penalty for placing a lower and thus
positively charged silicon atom next to a Sr ad-atom is roughly 0.4 eV. Therefore the ad-
atoms pin the dimer buckling as observed experimentally [5]. The filled dimer offers a
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