When semiconducting nanowires are
grown by vapor–liquid–solid
mechanism using gold as catalyst, the first stages, i.e., gold deposition
and subsequent annealing, are of prime importance as they determine
nanowire size and repartition. In this paper, we aim at identifying
key factors which drive the first stages when growing nanowires, viz.,
surface preparation of the silicon wafer, gold deposition, and subsequent
annealing. As silicon wafer surface preparation may or may not include
the etching of the silica protecting overlayer, we have investigated
the surface composition (a) after in situ gold deposition
on Si(001) substrates that are either clean or “epiready”
(i.e., SiO2-covered) and (b) during the subsequent in situ annealing from room temperature up to 600 °C,
using soft X-ray photoelectron spectroscopy at the TEMPO beamline
(SOLEIL synchrotron radiation facility). When Au is deposited directly
on clean Si(001), our results reveal the formation of a AuSi alloy
at surface, even when Au deposition is performed at room temperature.
Postdeposition annealing prompts a complex dewetting/demixing of this
initial AuSi alloy. When gold is deposited at room temperature on
SiO2-covered Si(001) substrate, it slightly sinks into
silica. During postdeposition annealing, a complex dynamic process
takes place: As the temperature increases, gold gradually sinks into
the silica and catalyzes its decomposition, which starts at 365 °C
thus lower than usual (840 °C). This etching process is concomitant
with Au dewetting on silica, which acts as a barrier against alloying.
This gold dewetting implies local changes in surface potential and
therefore local shifts of the Au–Si region toward higher binding
energies. The result is a superposition of spectra, which can be used
to monitor Au dewetting and therefore substrate coverage by Au. When
Au finally reaches the substrate after completing the catalytic desorption
of silica, it undergoes mixing with Si as on a clean Si substrate.
All these results give new insights in the VLS mechanism, which is
widely used for growing nanowires.