5.3.1 In-situ electrical measurement
As displayed in Figure 5.2, the in-situ sheet resistivity of the RT-NiO thin films prepared at TU-Darmstadt displays a clear dependency on the oxygen concentration during reactive sputtering. Thus, the conductivity increases by 3 orders of magnitude when the oxygen concentration varies from 5 % to 15
%. At 15 % of oxygen concentration, the conductivity is about 6.5 S.cm−1. The conductivity obtained for the samples prepared for the in-situ electrical measurements is in the same order of magnitude to what can be found in the literature [87, 95, 163].
5.3.2 In-situ photoemission
The in-situ photoemission spectra obtained in the DAISY-MAT system on the samples from chamber #2 at 5%, 10% and 15% of oxygen concentration
0.001 0.01 0.1 1 10
Conductivity (σ/cm)
16 14 12 10 8
6 4
Oxygen concentration (%)
Figure 5.2: In-situ conductivity measurements of the NiO thin films deposited at TU-Darmstadt on gold patterned fused silica substrates.
are displayed in Figure 5.3. Three regions have been measured: the valence band, the O 1s and the Ni 2p regions. More details about the XP spectra in the measured region are given in Chapter 4. Thus, the Ni 2p region is made of the the Ni 2p53d9Z, Ni 2p53d9L and the Ni 2p53d8 orbitals [81] (Figure 5.3). In the rest of this chapter, for simplification, the Ni 2p53d9L state will be referred to as the satellite peak Ni 2p(Sat.) and the Ni 2p53d9Z state as the main peak Ni 2p(Main) (Figure 5.3). The O 1s region is composed of the prominent photoemission from lattice oxygen (O2−) denoted O 1s(Main) and, at a higher binding energy, a shoulder can be observed which increases with increasing oxygen concentration. Following Chapter 4, the shoulder at higher binding energy to the main peak in the O 1s region has been labelled O 1s(Def.).
The position of the edge of the valence band is an indication of the Fermi energy [9]. The latter has been determined by the intersection of the linear extrapolation of the valence band edge with the background emission in the bandgap. It can be observed that the Fermi energy is relatively constant for the three samples and is consistently about 0.60-0.67 eV.
It should be mentioned that the Ni 2p(Sat.) peak cannot be spontaneously associated to the presence of Ni3+ in NiO contrary to numerous reports misleading the XPS spectra assignment. However, it can be noticed that, with increasing oxygen concentration, the valley between the Ni 2p(Main) peak and the Ni 2p(Sat.) peak becomes less pronounced (Figure 5.3).
Intensity (arb. unit)
870 865 860 855 850
Binding Energy (eV) Ni2p region
2p53d8 2p53d9L 2p53d9Z Valley Ni2p(Sat.)
Ni2p(Main)
Intensity (arb. unit)
534 532 530 528 526
Binding Energy (eV) O1s(Main)
O1s(Def.) O1s region
Intensity (arb. unit)
6 5 4 3 2 1 0
Binding Energy (eV) Valence band region
O2 (%) EF (eV) 0.67 0.60 0.60 5 10 15 Oxygen conc.
5 % 10 % 15 %
Figure 5.3: In-situ photoemission spectra of a) the Ni 2p, b) the O 1s and c) the valence band regions. The measurements are realized on the samples of chamber #2 after in-situ electrical measurements (see Figure 5.2). The Fermi energy is determined by linear extrapolation of the valence band edge to the X-axis. It can be seen that there is no obvious reduction of the Fermi energy (EF) with increasing oxygen concentration.
5.3.3 NiO thin films prepared on silicon
The thin films structure
As shown by the ACOM-TEM images in Figure 5.4, the RT-NiO thin films are about 50 nm thick and adopts a poly-cristalline structure. In line with literature [150, 178], the thin films include numerous grain boundaries at any oxygen concentration during the sample preparation. Also, the grains are more dense and noticeably elongated along the direction of growth at high oxygen concentration (17 % and 20 %).
Figure 5.4: ACOM cross-sectional image of the three studied RT-NiO thin films (2.5%, 17% and 20% of oxygen concentration) prepared on oxide free silicon sample at UCL.
Presence of electronically active defects
STEM-EELS measurements provided information about the electronic inhomogeneities at the grain size level in the NiO thin films and emphasized the presence of a specific pre-peak at 529 eV in the O K edge, which is particularly observed at the grain boundaries of the sample prepared with 17
% and 20 % oxygen concentration and in the bulk of the grain if prepared with 20 % oxygen concentration (Figure 5.5). XAS-literature reports that spectra
Intensity (arb. unit)
550 545 540 535 530 525 520
Energy Loss (eV) Oxygen holes
HR-EELS over grain boundaries O2 during deposition:
2.5 % 17 % 20 %
Intensity (arb. unit)
550 545 540 535 530 525 520
Energy Loss (eV) HR-EELS in the bulk of the grains
Oxygen holes
Figure 5.5: EELS obtained on a) grain boundaries and b) in the bulk of the grains for the three NiO thin films deposited on silicon at room temperature at 2.5 %, 17 % and 20 % of oxygen concentration at UCL.
of extrinsically or intrinsically doped NiO materials display a similar electronic feature at 529 eV in the O K edge region and so could be the fingerprint of a higher conductivity [83, 84, 86]. Although the exact nature of this feature is controversial, its presence has been related to positive charges in nickel oxide [83, 84, 86–88] and might be more appropriately denoted as a hole in the Ni 3d8L orbital, with L the oxygen ligand [179]. Therefore, the feature observed in the O K edge region at 529 eV will be labelled as an oxygen hole in the rest of this chapter.
Measurements show that these oxygen holes are promoted at high oxygen concentration as the pre-peak at 529 eV is not visible for the sample prepared with 2.5 % (Figure 5.5). The oxygen hole feature tends to accumulate at the grain boundaries before also being observed in the bulk of the grain at higher Oxygen concentration. Indeed, the sample produced at 17%
oxygen concentration only provide the feature associated to holes if EELS is performed at grain boundaries. Taking the sample prepared at 20 % oxygen concentration, it can be seen that the feature is visible the boundaries and in the bulk of the grains (Figure 5.5).
Bicolor images have been realized from EELS measurements on the NiO/silicon samples (Figure 5.6) using the region corresponding to the oxygen holes in the O K edge spectra (energy window from 528 to 530 eV). These bicolor maps clearly evidence a chemical homogeneity of the sample prepared with 2.5 % of oxygen, whereas the samples prepared with 17 % and 20 % have a more pronounced pre-peak at 529 eV over the grain boundaries (Figure 5.6).
As the oxygen hole pre-peak has been reported in doped NiO but not in pure NiO (without dopant), its presence could be an indication of higher conductivity. Therefore, the heterogeneity revealed in the bicolor images of
2.5% 17% 20%
Figure 5.6: Top row: Bicolor images realized from the cross-sectional EELS measurements of the 2.5 %, 17% and 20% NiO thin film where the red color indicates where the pre-peak associated to oxygen holes is the strongest. The green color does not necessarily imply an absence of peak but only a reduction of the pre-peak intensity in comparison to the most intense region. Bottom row: Corresponding ADF images where grain boundaries are highlighted with a denser contrast. The yellow arrows indicate the position of the same grain boundary in the top and lower row for a given sample.
Figure 5.7: Schematic representation of the NiO thin film morphology deposited on silicon according to the oxygen partial pressure in the deposition chamber. The deeper the red, the more defects. At low oxygen concentration the thin film is considered to be homogeneous. When the oxygen concentration is increased the defects accumulate primarily at the grain boundaries before entering the whole crystal structure. As detailed in Appendix A.2, cationic inter-diffusion occurs at the interface which leads to the formation of a SiOx layer on top of a nickel rich region.
the samples prepared with 17% and 20% of oxygen concentration indicates that the grain boundaries would primarily support electrical conductivity of the RT-NiO thin films.
A schematic representation of the distribution of the chemical species responsible for the oxygen hole in the NiO thin film has been represented in Figure 5.7 according to the oxygen content in the deposition chamber. With regards to the STEM-EELS measurements, the oxygen hole can be considered as absent if the thin film is prepared at low oxygen concentration but gradually appears at grain boundaries before being present in the whole structure at higher oxygen concentration. The threshold when the oxygen holes appear in the bulk of the grain can be triggered when grain boundaries are saturated with oxygen holes.