Figure 4. (a) High-resolution microscopy image of the
c-Si/SiO2/ZnTiO3/Al contact; (b)
HAADF–STEM image, EDX mapping and line scan of 300 °C-annealed
c-Si/SiO2/ZnTiO3/Al contact; (c, d)
comparison of HAADF–STEM microscopy images of an as-fabricated sample
and a 300 °C-annealed sample.
To identify the valence state of Al occurring in the
ZnTiO3 film, high-resolution XPS characterizations have
been performed on the ZnTiO3 film with and without
annealing. Note that the ZnTiO3 film is also capped by
an Al layer, just like in solar cells, but the Al layer is removed by a
wet chemical method prior to the XPS measurement. More details can be
found in experimental part. The
spectra
of
Al
2p core levels are presented in Figure 5a. For the film without
annealing, two Al peaks can be obtained. The strong peak at the binding
energy of 74.4 eV corresponds to the Al-O bond, which slightly shifts
towards lower binding energies compared to the 74.6 eV peak position of
Al 2p3/2 in stoichiometric
Al2O3,46,47 indicating
that Al occurred in the ZnTiO3 film plays a role as
dopant (ZnTiO3:Al). The peak centered at 72.3 eV is a
characteristic of metallic Al.48 For the sample
annealed at 300 ℃, no metallic Al peak is observed, confirming that
post-annealing further promotes the doping of Al in
ZnTiO3 films, and made Al-O bonds in zinc titanate
completely. The doping of Al changes the WF of the
ZnTiO3 film, which is verified by UPS. As shown in
Figure 5b, a WF value of 3.1 eV can be extracted for the
as-fabricated
Al-ZnTiO3film, which is lower compared to the pure ZnTiO3 film
(4.0 eV).40 After 300 ℃ post-annealing, the WFof Al-ZnTiO3 film further decreases to 2.07 eV.