Ammonia
Photodissociation Promoted by Si(100)
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Abstract
Using
in situ X-ray photoelectron spectroscopy measurements after
reaction, we show that hydrogen-terminated Si(100) perturbs the bonding
of physisorbed NH<sub>3</sub> enabling a photochemical decomposition
pathway at wavelengths different from those characteristic of either
the molecule in the gas phase or the semiconductor bandgap. UV illumination
only of gas phase NH<sub>3</sub> at partial pressures from 0.1 to
100 Torr produced a maximum at 10 Torr in the N surface coverage.
This is in good agreement with a model of the radical production rate
showing that at this pressure the gas density balances the flux of
photons at the surface with energies sufficient to dissociate NH<sub>3</sub>. UV illumination of both the gas phase and the surface produced
a monotonic increase in the N coverage with pressure as well as coverages
that were 3–10 times higher than when only the gas phase was
illuminated. The amine saturation coverage scaled with the UV fluence
at 10 Torr and 75 °C, reaching 6.9 × 10<sup>14</sup> atoms/cm<sup>2</sup> (∼1 N atom per Si surface atom) at 19 mW/cm<sup>2</sup> and 12 × 10<sup>14</sup> atoms/cm<sup>2</sup> (∼1.8
N per Si) at 35 mW/cm<sup>2</sup>. Monochromatic illumination showed
that the wavelengths driving deposition were not correlated with the
Si bandgap, but instead were roughly the same as gas phase photodissociation
(λ < 220 nm). The primary driving force to replace the hydrogen
termination with amine groups was direct photodissociation of NH<sub>3</sub> molecules whose electronic structure was perturbed by interaction
with the surface. Amine groups enhanced the surface reaction of water
present as a contaminant in the source gas. These results show that
molecules in weakly bound surface states can have a dramatic impact
on the photochemistry