Angle Dependent XPS (ADXPS)
Study of ITO
The degree
of surface sensitivity of XPS may be varied by
collecting photoelectrons emitted at different take-off
angles (TOA) to the surface plane. Figure 1 illustrated that the surface sensitivity was
significantly enhanced using a low TOA. Hence, ADXPS
was a powerful tool for performing nondestructive surface depth
profile.
In this example,
Angular-dependent X-ray photoelectron spectroscopy (ADXPS)
and ultraviolet photoelectron spectroscopy (UPS) were used
to determine the chemical composition, the chemical states and
the work function of Indium tin oxide
(ITO) surfaces after each treatment of solvent
cleaning, by plasma of oxygen, argon, nitrogen
and by argon ion (Ar+)
sputtering.
Figures 2 and 3 showed one of
curve-fitting results for O (1s) envelope
and C (1s) envelope measured at
90o take-off angle.
Figure 2 Curve-fit results of O 1s ADXPS spectra
at TOA : 90 o after solvent cleaning treatment.
Figure 3 Curve-fit
results of C 1s ADXPS spectra at TOA:
90o after solvent cleaning treatment.
Table 1 Summary of
Carbon Atomic Concentration, Chemistry
and Work Function of ITO after Various Plasma Treatment.
It was found that oxygen plasma and nitrogen plasma
chemically reacted with the ITO surfaces to reduce the hydrocarbon
and C-O contamination at the surface. Yet little etching of the
surface can be observed after plasma treatments. Among all treatments,
oxygen-plasma-treated ITO achieved the highest work function of
4.40 eV, whereas Ar+-sputtered ITO surface had the
lowest work function of 3.90 eV. The stoichiometry of the
ITO surface is shown to be the major controlling factor of the ITO
work function.
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Selected Publications
on XPS/UPS Studies on Indium-Tin-Oxide Surface
- S.K. So, W.K. Choi, C.H. Cheng, L.M.
Leung, C.F. Kwong, "Surface
Preparation and Characterization of Indium Tin Oxide Substrates for
Organic Electroluminescent Devices", Appl.
Phys. A 68, 447-450 (1999).
- W.Song, S.K. So, L. Cao, "Angular-Dependent
Photoemission Studies of Indium Tin Oxide Surfaces", Appl. Phys.
A 72, 361-365 (2001).
- Weijie Song, S.K. So, Daoyuan Wang, Yong Qiu, Lili Cao, "Angle Dependent
X-ray Photoemission Study on UV-Ozone Treatments of Indium Tin Oxide",
Applied Surface Science 177 (2001) 158-164.
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