Abstract:To
improve the performance of semiconductor photoelectrodes for
water splitting, the amount of band bending in the depletion layer
of a semiconductor should be accurately ascertained, since it determines
the splitting efficiency of photogenerated carriers. Band bending
has been determined by X-ray photoelectron spectroscopy (XPS) from
the valence band maximum (VBM), which has been calculated from the
Ga 3d peak using the energy difference between VBM and Ga 3d (ΔE
VBM‑3d). This work validates several
values fo… Show more
“…The chemisorption (2.22 eV) was more stable than physisorption (0.709 eV). Obtained adsorption energy in the +c GaN surface was comparable with those in the other report [ 16 ] where hydrogen termination was applied to the opposite surface. Figure 3.…”
Section: Resultssupporting
confidence: 85%
“…On the other hand, when the ALD Al 2 O 3 layer was deposited on the GaN layers, drastic variations in the valence band spectra were observed as shown in Figures 9(c,d) . The chemical oxidation states of GaN caused by H 2 O vapor were detected at binding energies in the range of 5–7 eV and 10–12 eV [ 19 ] surrounded by the dotted squares in Figure 9 . The oxidation was apparently enhanced for p -GaN.…”
Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H
2
O, O
2
, N
2
O, and NO gases. It was found that H
2
O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H
2
O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H
2
O molecules were present on the (0001) (
+c
) surface: non-dissociatively adsorbed H
2
O (physisorption), and dissociatively adsorbed H
2
O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H
2
O molecules attacked the back side of three-fold Ga atoms on the (0001̅) (−
c
) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (101̅0)
m
-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al
2
O
3
layer using H
2
O vapor, the reactions caused at the interface were more remarkable for
p
-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H
2
O and O
2
should be used to avoid uncontrollable oxidation on GaN surfaces.
“…The chemisorption (2.22 eV) was more stable than physisorption (0.709 eV). Obtained adsorption energy in the +c GaN surface was comparable with those in the other report [ 16 ] where hydrogen termination was applied to the opposite surface. Figure 3.…”
Section: Resultssupporting
confidence: 85%
“…On the other hand, when the ALD Al 2 O 3 layer was deposited on the GaN layers, drastic variations in the valence band spectra were observed as shown in Figures 9(c,d) . The chemical oxidation states of GaN caused by H 2 O vapor were detected at binding energies in the range of 5–7 eV and 10–12 eV [ 19 ] surrounded by the dotted squares in Figure 9 . The oxidation was apparently enhanced for p -GaN.…”
Understanding the process of oxidation on the surface of GaN is important for improving metal-oxide-semiconductor (MOS) devices. Real-time X-ray photoelectron spectroscopy was used to observe the dynamic adsorption behavior of GaN surfaces upon irradiation of H
2
O, O
2
, N
2
O, and NO gases. It was found that H
2
O vapor has the highest reactivity on the surface despite its lower oxidation power. The adsorption behavior of H
2
O was explained by the density functional molecular dynamic calculation including the spin state of the surfaces. Two types of adsorbed H
2
O molecules were present on the (0001) (
+c
) surface: non-dissociatively adsorbed H
2
O (physisorption), and dissociatively adsorbed H
2
O (chemisorption) molecules that were dissociated with OH and H adsorbed on Ga atoms. H
2
O molecules attacked the back side of three-fold Ga atoms on the (0001̅) (−
c
) GaN surface, and the bond length between the Ga and N was broken. The chemisorption on the (101̅0)
m
-plane of GaN, which is the channel of a trench-type GaN MOS power transistor, was dominant, and a stable Ga-O bond was formed due to the elongated bond length of Ga on the surface. In the atomic layer deposition process of the Al
2
O
3
layer using H
2
O vapor, the reactions caused at the interface were more remarkable for
p
-GaN. If unintentional oxidation can be resulted in the generation of the defects at the MOS interface, these results suggest that oxidant gases other than H
2
O and O
2
should be used to avoid uncontrollable oxidation on GaN surfaces.
“…The saturated band bending of the high H 2 O pressure sample does not reach the flat-band condition. As discussed in ref this is probably caused by the Fermi-level pinning at the subsurface states originating from nitrogen vacancy V N in GaN. Although it is not clear whether the flat-band condition was realized at a particular surface coverage, we speculate here that the Fermi level of pristine, H 2 O-exposed, and O 2 -exposed GaN surfaces is invariably pinned by one of the following energy levels: Ga-3d-originated surface states, N-2p-originated surface states, or subsurface states.…”
Section: Resultsmentioning
confidence: 51%
“…The coverage of O-containing species was calculated by comparing the O 1s intensity with that of the Cu(110)–(2 × 1)O surface whose O coverage is known (5.45 × 10 14 atoms/cm 2 ). , The definition of the coverage was adapted from our previous works, , i.e., 1 ML corresponds to one O atom per Ga atom (1.14 × 10 15 atoms/cm 2 ). The (2 × 1) LEED pattern was observed for this sample.…”
GaN
is an excellent candidate for photocatalytic, optoelectronic,
and high-power devices, and the interaction between the GaN surface
and ambient species, especially H2O and O2,
has drawn exceptional attention. In this study, the evolution of the
n-GaN(0001) surface geometric structure and the corresponding band
bending (a key parameter that describes the surface electronic structure
of a semiconductor) during H2O and O2 exposure
is predicted from first-principles calculations and confirmed by ambient
pressure X-ray photoemission spectroscopy (AP-XPS) measurements. Overall,
the AP-XPS results are in good agreement with the predictions, and
we discuss the possible origin of the difference in the band bending
of H2O- and O2-adsorbed surfaces. In the case
of the O2-exposed surface, upward band bending is observed
above the effective coverage of 3/4 ML (3/8 ML of O atoms) because
the Fermi level becomes pinned to the N-2p-originated surface states,
which is formed through Ga–N bond scission by O atom adsorption
and insertion into the slab. As for the H2O-exposed surface,
the saturated band bending depends on the H2O supply rate:
When the supply rate is high, half dissociation of H2O
is dominant and the band bending approaches the flat-band condition
due to the termination of surface Ga dangling bonds by H and OH; when
the supply rate is low, the saturated band bending matches that of
the O2-adsorbed surface, presumably due to the O atoms
that are formed by full dissociation of H2O.
“…S1† further confirmed the Ga incorporation. 29 X-ray diffraction (XRD) data (Fig. S2†) showed the microstructure of Ga-coordinated C 3 N 4 nanosheets.…”
Catalytic properties of single-atom catalysts are very sensitive to the geometric interaction between metal sites and their supports. Their catalytic behavior is closely related to the local coordination environment of...
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