7.3 Interface experiments
7.3.2 Interface to NiO
Using NiO as contact material in an interface experiment was tried as nickel oxide has been used as hole selective contact in photovoltaic applications with oxides before and can potentially lower the Fermi level even further than RuO2.[76, 110] For hematite it was shown that a thin layer of NiO can improve the photoelectrochemical performance by exhibiting the charge separation, lower the recombination, and improve the kinetics through catalytic effects.[293–295]
Li et al. reported a band alignment between the two materials with the valence band maximum of hematite being positioned about 0.2 eV below the valence band maximum of NiO.[293] It was determined from the PES signal of a bare hematite thin film compared to a hematite film partially covered with NiO nanoparticles. The experiment is, therefore, different from the approach which is discussed here but the results will be taken into ac-count in the discussion of the present results.
The experiment was carried out as discussed before. Again, two hematite thin films were used which were both deposited at 400◦C. One was nominally undoped and the other doped with zirconium. The detailed deposition parameters can be found in table 3.3. For the NiO films a room temperature deposition was chosen in both cases. The
5 Please also see Figure A20 and the associated discussion
162 7 Fermi level manipulation of the surface
oxygen content was 20 % in the sputter gas. More details of the depositions can be found in Table 3.4.
In contrast to the interface experiment with RuO2 where the Fe2p3/2 core-level was used to follow the shift of the Fermi level, for the experiment with NiO the position of the Fe2p1/2core-level had to be used. The reason for this different procedure is the superpo-sition of Fe2p3/2-emission with the LMM-Auger line from NiO. The core-level and valence band spectra of the two materials can be found in the appendix in Figure A16. Here, only the Fermi level behavior and band alignment shall be discussed which can be found in Figure 7.8.
The thickness of the NiO has been calculated assuming a constant rate of 1.4 nm min−1 as has been determined on thicker films before.[110] From the attenuation of the hematite spectra in Figure A16 it can be stated that by using this rate the film thick-ness is probably under-estimated. This, however, has no influence on the interpretation of the Fermi level behavior during the experiment.
Before the first deposition the Fermi level of the two samples do not differ much. De-spite being nominally undoped the sample without zirconium shows a high Fermi level of 1.55 eV. This might be related to the deposition procedure where four samples where deposited at once and then removed from the system. Then, these samples were re-introduced to perform the interface experiment. Possible surface contamination from e.g.
carbon hydroxides or water was removed by elongated heating in an oxygen atmosphere.
The heating most likely does have an influence on the Fermi level position but the high Fermi level is rather unexpected. A sample from the same deposition that was used in another experiment and heated in a similar way showed a Fermi level of 1.1 eV.
On the other hand, it cannot be completely disregarded that charging might occur due to poor contact between sample and sample holder. Especially after heating the different thermal expansion can cause such a problem. Also, the ITO sublayer may have a lower conductivity due to the heating in oxygen.
In the present case the binding energy shift due to charging is presumably about 0.5 eV.
It could, therefore, not be detected by the regularly performed check during the align-ment procedure where the O1s emission was being measured with and without electron flood gun. The Fermi level position from an UPS measurement on this sample was 2.03 eV which is too high and gives another argument for the charging.
A clear determination of the reason for the high Fermi level is not possible but there is a strong tendency to assign it to a charging effect. As a consequence, this value should
7.3 Interface experiments 163
be dealt with carefully. The same applies for the band bending and work function in Fig-ure 7.8b)6. The values which are affected from charging are labeled by an asterisk.
From the position of the Fermi level in the NiO film it can be concluded that after the second deposition step this film is not charging. This is then also the case for the hematite substrate. As a consequence, the Fermi level position at the interface can be deduced from this experiment.
The Fermi level in the Zr-doped sample is at 1.46 eV before the first deposition. This value is expected for a sample doped with zirconium and a similar value was extracted from the UPS measurement of the valence band. Please note that this sample was not removed from the system prior to the experiment. Also, it was deposited on platinized quartz.
For this sample it was possible to use the position of the Zr3d5/2 emission in addition to the Fe2p1/2core-level. As can be observed in Figure 7.8 the Fermi level positions from both core-levels are almost identical for this sample. There is no contribution from NiO in the Zr3d5/2core-level. Hence, by using this peak not only the Fermi level position from the Fe2p1/2emission is confirmed for this sample but the usability of this emission to fol-low the Fermi level in an interface experiment with NiO is verified.
In Figure 7.8a) the Fermi level positions in the two hematite samples are being shown in large scale. For sake of observability it was decided not to include the Fermi level positions in NiO. These are, instead, presented in the inset. The deposition time and, therefore, the NiO thicknesses for the two experiments were not the same.
In both samples the Fermi level is being lowered after the first deposition of NiO. The nominally undoped sample shows a slightly higher Fermi level position in both, hematite substrate and NiO film. This might still be a charging effect. After the next deposition the Fermi level in the Zr-doped sample has reached it’s final position of 0.8 eV above the valence band. Any further deposition does not show an effect on the Fermi level anymore.
The second deposition step in the experiment of the nominally undoped sample was slightly longer and, therefore, lead to a slightly thicker film. The Fermi level position in hematite is now at about 0.53 eV which is below the position in the Zr-doped hematite sample. The Fermi level in the NiO film is now, however, very similar to the Fermi level position in NiO on the Zr-doped sample. This indicates that no charging is present.
6 The work function was derived by subtracting EVBM from the XPS measurement from the ionization potential from an UPS measurement. This procedure allows for an extraction of a charging independent work function in case of homogeneous, light charging in UPS (but not XPS). In the present case where a charging effect in XPS cannot be neglected, however, it does, unfortunately, not apply.
164 7 Fermi level manipulation of the surface
NiO thickness / nm EF-EVBM / eV
2.0 1.5 1.0 0.5
0.0
1 0
1.5 1.0 0.5 0.0
5 4 3 2 1 0
0.5
0.5
0.8 6* 0.5 0 17
0.8
0.5
Figure 7.8:Interface behavior of hematite to NiO. a) Fermi level dependency in nominally undoped (FN) and zirconium doped hematite (ZFN) on thickness of the NiO film and b) band alignment at the Hematite|NiO interfaces. The film thickness in a) was calculated based on the deposition times with an assumed deposition rate of 1.4 nm min−1.[110] The inset in a) shows the Fermi level dependency of the NiO films in dependency of the film thickness. All annotated energies are given in eV. Values labeled with an asterisks might suffer from charging effect.
After the next deposition the determination of the peak position becomes less precise due to the low intensity of the iron signal and the broad peak. Initially a value of 0.4 eV was being derived. It is, however, also possible to derive are value of 0.53 eV. In order to show this uncertainty it was decided to include both values in Figure 7.8a). As a final value of the Fermi level position at the interfaceEF−EVBM=0.5 eV is being assumed.
The band alignment derived from the two different experiments is being drawn in Fig-ure 7.8b). For the undoped hematite no valence band offset is derived. A value for band bending cannot be given as the initial Fermi level position of the bare substrate is ques-tionable.
In the doped case, a different band alignment is derived. Here, a valence band offset
∆EVBM of 0.17 eV could be calculated. A small band bending of 0.13 eV in NiO was as-sumed. To obtain this value the band bending in hematite of 0.66 eV was subtracted from the difference between first and last Fermi level position in NiO during this experiment of 0.79 eV. The valence band offset agrees very well with the results from Li et al. who determined a value of∆EVBMof 0.21 eV for Ti-doped n-type hematite and NiO.
The band bending of 0.66 eV in the Zr-doped thin film, however, needs further con-siderations. From the difference of the work functions of the two materials a maximal band bending of 0.27 eV should be expected. For NiO, however, the work function was
7.3 Interface experiments 165
deduced from the surface of the this film. It is most likely not the same at the interface even if band bending is neglected.[110] The reason for this can be found in the complex electronic structure of NiO that is only complete once a certain film thickness has been achieved. This has been calculated by van Veenendal et al. and corresponds to the shape of the Ni2p core-level.[289] From the results shown here, it can be assumed that a higher work function is present for thinner NiO film. This assumption is supported by the re-ported work function of NiO nano-particles of 6.55 eV from Li et al.[293]
Assuming that the sum of band bendingeVBBNiO+eVBBFe2O3 is the difference in work func-tion the following applies
φNiO=φFe2O3 +eVBBNiO+eVBBFe2O3=5.11 eV+0.13 eV+0.66 eV=5.9 (7.1)
From earlier interface experiments with NiO and RuO2 it is known that the valence band of NiO is about 0.5 eV below the Fermi edge of RuO2.[110] Jan Morasch stated in his thesis that this is a strong argument for a work function of RuO2 of 5.5 eV at the in-terface. The same argumentation holds for a work function of 6.0 eV at the interface for NiO. This is in very good agreement with the value estimated above.
The different band alignment in the two samples cannot conclusively be explained, yet.
The NiO phase should be identical for both experiments. Hence, it should be possible to shift the Fermi level in the Zr-doped samples as far down as in the undoped sample unless there is another mechanism that pins the Fermi level. An intrinsic mechanism within hematite can be omitted because lower Fermi level positions have been achieved in Mg-doped samples. The pinning level might, therefore, be related to the doping with zirconium and the introduced disorder in the sample. The interface experiment of un-doped hematite and NiO should be performed again and further evidence for a different final Fermi level position at the interface of hematite to NiO needs to be collected before a model can be proposed to explain these.