monitor the wires (1) as grown, (2) after 5 min immersion in a 5% HF (or DF)
solution, (3) upon exposure to air or water for different times, (4) after annealing to
different temperatures. We note that the FTIR signals originate mostly from the
large-diameter SiNWs (the surfaces of which are larger) and only a small fraction
of the signal represents the small-diameter SiNWs. This means that if the stability
of SiNW surfaces is size-dependent (as indicated by theoretical calculations, see
Section 10.8), this would not be revealed in the experiment.
The as-grown SiNWs clearly show (Figure 10.27) only SiaO vibrations at 1050
and 800 cm
À1 . Immersion in HF (Figure 10.27) removes the SiaO related lines
and SiaH x ðx ¼ 1; 2; 3Þ absorption modes appear. These modes can be attributed
to mono-hydrides and tri-hydrides on Si(111) and di-hydrides on Si(100) as already
Fig. 10.27. ATR-FTIR spectra of (a) as-prepared SiNWs and
(b) HF-etched SiNWs in the range 700–4000 cm
À1 . Note the
removal of the oxide absorption line and the formation of
SiaH x absorption lines in the etched SiNW [76].
10 Oxide-Assisted Growth of Silicon and Related Nanowires
342