3.3 Surface characterization
3.3.1 X-Ray and UV photo-spectroscopy
3.2.4 In-situ plasma cleaning and hydrogen surface passivation
A hydrogen plasma source (Tectra Gen2 Hybrid Atom/Ion Source) was used for both surface cleaning and passivation of the Si/SiO2 samples as detailed in Chapter 11. Atomic hydrogen is obtained by cracking dihydrogen molecules in the plasma chamber set on atomic mode for which the current was set to 30 mA and the gas pressure to 0.2 mbar. The hydrogen cleaning step is realized at about 250°C for 10 min and allows to remove efficiently carbonated contaminants and possibly hydroxilated compounds, which are present on the surface because of air exposure of the Si/SiO2 samples. An additional 45 min hydrogen plasma exposure is necessary at higher temperature for the hydrogen passivation step according to reports found in the literature regarding Si/SiO2
passivation [123, 152, 153]. As the temperature range was limited in the setup, the temperature for hydrogen passivation was set to 350°C . To avoid possible hydrogen desorption from the sample, the plasma is maintained until the sample holder temperature reaches∼150°C.
3.2.5 Alumina deposition by ALD
Alumina layers deposited by Atomic Layer Deposition (ALD) have been performed on top of the hydrogen passivated Si/SiO2structure in the DAISY-MAT system at TU-Darmstadt [154]. The alumina layer is obtained by the alternative deposition of TMA (trimethylaluminum) and its oxidation by water.
One ALD cycle consisted in a TMA and water pulse lasting respectively 80 ms and 150 ms, which are separated by an pumping period of 5 min in vacuum.
As the alumina layers have been deposited on the hydrogen passivated Si/SiO2 samples, a low temperature for the alumina deposition was preferred.
The reason, even though this was not proven at this level, is that it would limit the risk of the depassivation of the Si/SiO2 structure. Thus, the temperature during the ALD process was constrained from room temperature to 200°C in this thesis.
composed of several electro-magnetic lenses, allows to filter the electrons having a specific kinetic energy and to count their quantity. Ultra High Vacuum (UHV) is required to reduce the electron scattering when the electron travels from the surface of the sample to the analyser. Thus, measurements in the Daisy-MAT system are carried out in the 10−8-10−9Torr range.
The technique allows to determine the electron binding energy (EB.E) of the electrons participating in the electronic structure of the material.
Basically, the technique measures the kinetic energy (Ek) of the electrons after photo-emission according to:
EB.E=hν−Ek−Φ (3.1)
with hν the photon energy emitted from the photon source (e.g.: X-Ray, UV light) and Φ the screening potential of the photo-emitted electrons with the local environment. Φ can be determined, for post-data correction, in measuring the energy levels (e.g.: Fermi energy) of reference samples such as Ag, Au and Cu.
Intensity (Arb. Units)
1200 800 400 0
Binding Energy (eV) O1s
C1s Ni2p
Ni LMM Ni2s
Ni3s
Ni3p
C KVV O KLL
In-situ XPS survey Ex-situ XPS survey
Figure 3.7: Left: schematic representation of XPS/UPS under operation. X-rays are directed towards the sample which in turns emits electrons (taken from Wikipedia 2).
The photoemitted electrons are quantified in the analyser. Right: Typical XP survey spectra of a NiO thin film; in red the sample has been produced in-situ in the Daisy-MAT system and in blue the NiO thin film has been exposed to air. Core level of NiO O 1s, Ni 2p are visible and carbon contamination (C 1s) is visible for the ex-situ sample. Also, Auger lines of Ni LMM, and O KLL are particularly visible on the in-situ sample. The in-situ XPS spectra is sharper and more intense than the ex-situ measurements as a thin contaminated layer is formed on the NiO surface of the ex-situ sample. The background increases to higher binding energy because of the number of scattered electrons increases.
Three types of photoemitted electrons can be identified: direct and indirect emitted electrons and scattered electrons. Direct and indirect emitted electrons are related to elastic processes (no energy is dissipated). The direct emission defines the emitted electrons directly originating from an electronic orbital under photo-excitation (e.g.: O 1s, C 1s, Ni 2p electrons...), while in indirect
emission the emitted electron acquires energy through another excited electron (e.g.: Auger emission). The scattered electrons arise from energy dissipation through inelastic collision of the emitted electrons. The scattered electrons are responsible for the background intensity particularly at high binding energy.
Finally, the contribution of the three type of photoemitted electrons provide typical X-ray photoelectron (XP) spectrum as represented in Figure 3.7 (right).
Photoemission from a material follows standard Beer-Lambert optical law, where the ratio of the emerging (photoemitted electrons) over the incident (the photons) flux at a certain depth in a material exponentially decays:
Iout
Iin = exp −t
λ
(3.2) where Iout and Iin are the emerging and the incident flux, respectively.
The parameter t is the the depth from the surface material and λ is the attenuation length, also referred as the inelastic mean free path (IMFP) which is a parameter quantifying the inelastic process between the incident photon and the material. The higher the IMFP, the deeper the information depth. The information depth can be determined as being tmax = 3λwhich corresponds to the depth where the intensity of the emerging flux is only 5 % of the photon intensity. The IMFP depends on the material but also on the photon source.
Thus, on platinum, for photons having an energy of 50 eV, the IMFP is 0.5 nm but can reach up 2.2 nm for photon having an energy of 2000 eV (Figure 3.8).
It turns out that the information depth is about 1.5 nm at 50 eV but is 6.6 eV at 2000 eV.
XPS and UPS are based on the same principles but use two different photon sources, ∼ 1000-2000 eV for X-Ray sources (XPS) and ∼ 5-50 eV for UV sources (UPS). XPS can provide an information depth of a few nanometers (max: 10 nm) and can eject electrons from the materials having high binding energy. Thus, XPS is more adequate for studying the core electronic structure or for interface experiments. On the contrary, UPS is much more surface sensitive. UPS can bring valuable information about the electronic structure in the valence band and the surface electronic structure dependence on the surface termination. In addition, along this thesis, UPS measurements were performed to also determine the workfunction.
DAISY-MAT system description
DArmstadt’s Integated SYstem for MATerial Science (DAISY-MAT) is an UHV integrated system where deposition chambers and XPS/UPS chambers are all interconnected (Figure 3.9). This means that the vacuum is not broken when samples are transferred from, e.g., the deposition chambers to the XPS/UPS. The system has the great advantage to be able to study the surface electronic structure of the material by photoemission without surface contamination (C, OH, H2O. . . ). Thus, surface properties are not hindered by
2https://en.wikipedia.org/wiki/Photoemission_spectroscopy/
2.5
2.0
1.5
1.0
0.5
0.0
IMFP (nm)
5 6 7 8
100
2 3 4 5 6 7 8
1000
2
Photon Energy (eV)
Figure 3.8: Inelastic Mean Free Path (λ) in platinum according to photon energy as determined by Powell et al. [156]. High energy photons possess a higher attenuation length than low energy photons. Note the minimum around 100 eV.
external compounds.
In the DAISY-MAT system, the Fermi energy can be determined either by in-situ XPS or in-situ UPS. The Fermi energy was estimated by extrapolating or fitting the edge defining the highest occupied electronic state (the valence band maximum) of the photo-emission spectra (Figure 3.10).
Another properties which can be measured accurately in the DAISY-MAT system is the workfunction φ. This was realized by in-situ UPS. The workfunction was determined with the sharp edge provided by the secondary emission electron cut off (Figure 3.10, right). The sharp cut off arises when the energy of scattered electrons interacting with the surface material is lower than the workfunction. Physically, in the vicinity of the surface of the material, the electrons with low energy are captured and cannot escape the surface.
Furthermore, because of the specific architecture of Daisy-MAT, it is possible to interface two materials of different nature and observe how bands align at the interface under the consequence of the electrostatic strength between the two materials. The experimental procedure rely on the use of XPS which has a deeper information depth. The idea being to deposit on one material a thin layer of another material (<1 nm) and observe how the electronic states shifts. In increasing sequentially the thickness of the deposited material, one can observe a gradual shift of the binding energy of the electronic states at the interface. From this experiments, it is possible to determine the band alignments at the interface [9, 100].
Figure 3.9: Schematic of the DAISY-MAT system: the deposition chambers are coloured in yellow, the distribution chamber in light grey and the XPS/UPS chamber in dark grey. All parts are maintained in UHV condition allowing to transfer samples from one part of the system to the other without breaking the vacuum.
XPS and UPS setups in DAISY-MAT
Photo-electron spectroscopy is performed in the DAISY-MAT system with an angle of 45°between the incident photon beam and the normal of the sample surface. The angle between the incident photon beam and the analyzer is fixed to 90°.
XPS was operated using Al K-α monochromatic X-Rays (hν=1486.6 eV).
For surveys, the whole spectral region is analysed (Figure 3.7, right). The pass energy and step width of the scans were respectively set to 187.85 eV and 0.185 eV. Insight in the core levels and valence region were obtained by restraining the binding energy range to a narrow window around the state of interest as shown in left plot in Figure 3.10. Thus, pass energy and step width of the scans were set to 5.85 eV and 0.05 eV, respectively. During the measurements, the XPS chamber pressure is maintained in the 10−9Torr range.
UPS uses a Helium discharge lamp emitting UV radiation of hν=21.22 eV. Pass energy is set to 2.95 eV but step width is kept at 0.05 eV. A bias of -4 V is applied to the sample in order to provide additional kinetic energy to the electrons escaping the sample surface, particularly for the electrons having a binding energy close to the secondary emission cut off. UP spectra are constantly measured at 4.5×10−8 Torr.
Intensity (Arb. Units)
12 10 8 6 4 2 0 -2
Binding Energy (eV) VBM X-ray photo-emission spectra (Valence region)
Intensity (Arb. Units)
20 15 10 5 0
Binding Energy (eV)
Int. (counts)
4 3 2 1 0
Binding Energy (eV) VBM
Second electron emission cut off
UV photo-emission spectra
Figure 3.10: Left: Typical in-situ XP spectra near the valence band maximum (VBM) of a NiO thin film. Extrapolation of the VBM value is realized by linear extrapolation of the VBM edge (doted line). Right: Typical in-situ UPS spectra. VBM is also determined by linear extrapolation of the VBM edge (subwindow) and workfunction is measured according to the value of the secondary electron emission cut-off.
Silver standard is measured daily for calibration of the XPS/UPS. After Argon plasma cleaning for 10 min, Fermi edge and Ag 3d peak position values are recorded with XPS/UPS. The values obtained with the Ag standard are used to correct experimental data with a dedicated home-made IGOR3 macro.