20A1 BM - (H-SGM) XAS, XPS
Adsorption and Thermal Reactions of H2O and H2S on Ge(100)
Wei-Lin Lee (李威霖), Pei-Yu Chunag (莊培祐), and Wei-Hsiu Hung (洪偉修)
Department of Chemsitry, National Taiwan Normal University, Taipei, Taiwan
The oxide layer is commonly considered to act as an
insulator or passivation layer in the electrical devices.
H2O can serve as a source of oxygen for the deposition of
an oxide layer on the semiconductor surface. Thus, the
interaction of H2O with the semiconductors has been an
important issue because of the fundamental and
technological interests. The passivation of the Ge surface
is required before depositing the gate dielectric, in order
to eliminate the intersurface states. The treatment of H2S
of the Ge surface is predicted to remove the surface states,
resulting in an electrically passivation surface which is a
critical requirement for the potential use of Ge in the
electronic devices.We report here an investigation of the
mechanism of adsorption and thermal reactions of H2O
and H2S on a Ge(100) surface, utilizing X-ray
photoelectron spectroscopy (XPS); such spectral
examination elucidates the reaction pathways of H2O and
H2S on the Ge surface. Finally, we compare the
intermediates and products following the adsorption and
decomposition of H2O and H2S on Ge(100) and discuss
their differences.
The thermal evolution of XPS spectra is used to
characterize the variation of surface composition during
thermal decomposition of H2O on Ge and correlates with
TPD results to elucidate the reaction intermediates.
Figure 1 shows the O 1s spectra for the Ge surface
saturated with H2O at 100 K and subsequently annealed
to various temperatures. Upon annealing the sample to
170 K, the O 1s component at 533.8 eV due to condensed
H2O disappears because of molecular desorption..
However, the intensity of the O 1s signal at 532.1 eV
attributed to surface OH increases. At 270 K, only the
dissociative OH is present on the surface without the
presence of molecular H2O. On further annealing to 500
K, the new O 1s binding energy appears at 530.5 eV and
is attributed to surface atomic O which is formed by
dehydrogenation of surface OH. Assuming that the
integrated intensity is proportional to the amount of
oxygen on the surface, 65-70 % of surface OH undergoes
the recombinative reaction. On annealing the sample to
700 K, the intensity of O 1s begins to decreases because
surface oxygen desorbs from the surface with a form of
GeO. The surface oxygen can be completely removed
from the surface at 770 K.
Based on the TPD and XPS data, the adsorption and
decomposition of H2O is summarized as the following
reactions.
1. H2O(g) → H2O(ad) or OH(ad) + H(ad) 100 K
2. H2O(ad) → OH(ad) + H(ad) 170 K
3. OH(ad) + H(ad) → H2O(g) 470-600 K
OH(ad) → O(ad) + H(ad) 470-600 K
4. 2H(ad) → H2(ad) 600-650 K
5. O(ad) → GeO(g) 650-750 K
Our TPD and XPS data elucidate the thermal
reaction of H2O and H2S. The adsorption and
decomposition of H2O and H2S undergo with a similar
mechanism although their corresponding reactions occurs
at temperatures with slight differences. At 100 K, H2O
absorbs molecularly and dissociatively on the Ge(100)
surface, whereas H2S preferentially dissociates to form
surface hydrogen and SH. The chemisorbed H2O
molecules completely dissociate to form OH by
annealing to 270 K. The resulting OH mainly recombines
surface hydrogen to desorb H2O. A small fraction of OH
further decomposes to form surface oxygen. Contrarily,
the surface SH mainly dissociates to form surface sulfur
and hydrogen and a small fraction of SH undergo the
recombination reaction to desorb H2S. By annealing the
sample to 770 K, the surface oxygen and sulfur atoms can
be completely removed by desorption of GeO and GeS,
respectively. Different from surface oxygen, a small
portion of the surface sulfur can react with hydrogen to
desorb H2S at 620 K.
Fig. 1:. XPS spectra of O 1s for a Ge(100) surface
exposed to H2O for 50 s at 100 K and subsequently
heated to indicated temperatures.
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