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A typical XPS survey spectrum of a tin-doped indium oxide (ITO) thin film deposited by rf-magnetron sputtering from the old target before and after 5-cycle of ALD-Al2O3

coating is shown in Fig. 5.1. Survey spectra are measured with comparatively low energy resolution, enabling a quick scan of the entire binding energy range accessible with the respective excitation energy. As photoemission lines are characteristic of the specific elements, the resulting spectrum is mainly used to check whether the detected elements are in general agreement with the expected qualitative atomic composition of the sample. In addition, surface contamination by carbohydrates can also be examined from existence or non-existence of the carbon C1s emission peak found around 285 eV section 5.3.

5.3 Photoemission

binding energy. Due to the in-situ processing and analysis, no such contamination is present in the spectrum.

1400 1200 1000 800 600 400 200 0 ITO / Al2O3 ITO 3d In Sn Al 2p 2s 4d 4p 4s 3d 3p 3s O 1s 2s 4d 4p 4s 3p 3s O Auger Sn Auger In Auger VB BE/eV

Intensity [arb. units]

Figure 5.1: XP (AlKα) survey spectrum of Sn-doped In₂O₃ before and after ALD-Al₂O₃ coating, annotated with the binding energy (BE) of In, O, Sn and Al.

For further sample characterization, selected areas of the photoemission spectra are routinely measured at high energy resolution and at elevated counting times to achieve reasonable signal-to-noise ratios according to the user’s needs. In the case of uncoated and ALD-Al2O3 coated ITO, the detailed measurement contain at least one core-level

emission for each of the elements contained in the sample and the valence band (VB) region. Compared to core levels, valence band emissions generally have poor intensity in XPS measurements. Consequently extended counting times are needed in order to obtain satisfactory signal-to-noise ratios [176]. In uncoated Sn-doped In2O3 excited by

Al Kα radiation, In3d5/2, Sn3d5/2, O1s, and VB are the emissions. For ALD-Al2O3

coated samples, Al2p and Al2s emissions are also included.

Core level spectra of the O1s, Sn3d5/2, In3d5/2, Al2p, and VB regions recorded from a

20 nm thick ITO films deposited from the old target are displayed in Fig. 5.2. The thin films were deposited at the substrate temperatures of RT-400 ◦C, and coated with 5

cycles of ALD-Al2O3. For comparison, spectra from a 20 nm ITO thin film deposited

at RT without Al2O3 coating are also presented in the figure. The displayed spectra

were selected since they are representative for showing different changes in shape and binding energies of the core levels and VB.

535 530 525 490 485 480 445 440

Binding Energy [eV]

85 80 75 70 10 5 0 In₂O₃ Al₂O₃ ITO 400 °C + Al2O3 ITO 200 °C + Al2O3 ITO RT + Al2O3 ITO RT

Intensity [arb. units]

a) _O1s b) _Sn3d5/2 c) _In3d5/2 d) Al2p e) VB

Figure 5.2: XP (AlKα) detail spectra of the O1s (a), Sn3d_5/2(b), In3d_5/2 (c), Al2p (d), and VB (e) regions for 20 nm thick ITO films deposited at substrate temperatures of RT, 200 ◦C and 400 ◦C and coated with 5 ALD cycles of Al₂O₃. For the room temperature (RT) films both before and after spectra are represented for comparison.

5.3 Photoemission

The O1s, Sn3d5/2, and In3d5/2core level spectra of uncoated films show a clear asymmetry,

which is caused by excitation of plasmons in the free electron gas of the highly doped material [35, 196]. The high doping level of the substrate is further evident from the high binding energy of the valence-band maximum (VBM) of EF–EV B = 3.0 ± 0.1 eV

(at least for the films prepared at elevated temperatures of 200◦C and 400 ◦C, RT films show lower Fermi energy values), which is comparable or a slightly larger than the band gap and hence above the conduction band edge, which is at 2.6-2.9 eV above the VBM [108]. For more information about the band gap of In2O3 see also section 3.1.

By deposition of Al2O3, the intensity of Sn3d and In3d core levels are attenuated due

to the coverage of substrate with Al2O3, while the O1s intensity remains approximately

constant due to the increase of the Al2O3-related emission at higher binding energy of

∼532 eV. For 5 cycles of ALD-Al2O3 coverage, the chemical components of ITO- and

Al2O3-related O1s emissions are overlapping in the spectra due to comparable binding

energies of the O1s in ITO and Al2O3. An interface experiment of these two materials

revealed the appearance of an additional O1s peak at ∼532 eV for ALD cycles of ≥ 10 [38]. The binding energy of Al2p emission corresponds well with the value expected for Al2O3 [197].

With Al2O3 coating, the binding energy of the core levels and valence levels shift

considerably to the higher binding energy. This corresponds to an increase of the carrier concentration near the surface, which is also indicated by the change of the Sn3d and the In3d line shapes. After alumina coating both core levels exhibit a further increase of asymmetry, indicating the presence of a high electron concentration [35, 196, 198, 199, 200, 201].

Figure5.3presents a comparison of Sn3d5/2and In3d5/2core levels and VB photoelectron

spectra for 20 nm ITO films with and without Al2O3 deposition. Al2O3 deposition

resulting reduction of the ITO, which is evident from the appearance of additional metallic Sn and In peaks at their respective lower binding energies. These features appear for all probed ITO substrates prepared from both targets and for all deposition conditions. The intensity of valence band is also attenuated after Al2O3 deposition.

The characteristic shape of the valence band of Sn-doped In2O3 is characterized by O2p

states close to the VBM2_{, but has also some mixing of other orbital character away from} the VBM [202]. These consisted of In4d-O2p, Sn5p-O2p, and hybridization between these states. For comparison, the valence band of uncoated and Al2O3 coated ITO

films is displayed inFig. 5.3(c). The changes and appearance of new features are briefly described in the following:

• (1) represents the shift of Fermi energy to a higher binding energy upon Al2O3

deposition. This indicates the presence of surface electron accumulation. Such accumulation layer have been frequently reported to be present at the In2O3

surfaces [202, 203, 204, 205]. This accumulation layer is not observed in as deposited surfaces, as the samples have not been exposed to air before XPS measurement.

• (2) indicates an increase of the band gap emission for Al2O3 coated films. These

features are usually observed for strongly reduced films and is generally accepted that this emission is caused by Sn5s electrons of Sn2+ _{cations [206]. These cations}

are preferentially present at low-symmetry lattice sites, especially at the surfaces and grain boundaries [207]. Here, it is also due to the presence of metallic Sn and In.

• (3) the shape of valence band changed from the characteristic shape of ITO towards the one observed for Al2O3 [38, 196, 208].

Intensity [a.u.]

490 485 480

Binding Energy [eV] metallic Sn Sn3d5/2

12 8 4 0

Binding Energy [eV] 2

3 1

VB

450 445 440

Binding Energy [eV] metallic In In3d_5/2

ITO_400 °C + Al2O3

ITO_RT

a) b) c)

Figure 5.3: Comparison for XP (AlKα) detail spectra of Sn3d_5/2(a), In3d_5/2 (b), and VB (c), regions for 20 nm thick ITO film deposited at RT and for the film with the same thickness but deposited at 400 ◦C and with Al2O3 coating.

The Fermi level at the surface

Photoemission is a highly surface sensitive technique [209]. Therefore, the binding energies obtained from photoemission reflect the Fermi level position at the surface of the film. The absolute position of the Fermi level with respect to the band edges can be directly determined from the valence band maxima, since the binding energies are

5.3 Photoemission

calibrated such that zero binding energy corresponds to the Fermi level position. The Fermi level position (EF-EVB) of ITO films with different thickness deposited at the

substrate temperature of RT, 200◦C, and 400 ◦C with and without Al2O3 coating are

displayed in Fig. 5.4. 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 EF - EVB [eV] 200 150 100 50 0 0 50 100 150 200 Thickness [nm] 200 150 100 50 0 RT 200 °C 400 °C ITO/Al2O3 ITO

Figure 5.4: Surface Fermi level position of ITO films anticipated to different thickness, different

substrate temperature and with and without 5 ALD cycles of Al₂O₃ coating. Each E_F-E_{V B} data points are extracted from different samples.

For uncoated room temperature ITO films, the Fermi energy of 20 nm thick film is higher than that of 200 nm thick film. This corresponds well with the higher carrier concentration of thinner films than thicker ones, as shown in subsection 5.4.1. For the films deposited at 400 ◦C, the Fermi energy does not change with film thickness. This also corresponds well with the carrier concentrations of films deposited at elevated temperature (see also Fig. 5.7(c)). In all cases, an upward shift of the Fermi energy is observed after Al2O3 deposition. This upward shift indicates, on one hand, the

presence of surface electron accumulation and, on the other hand, it can partially be assigned to a chemical reduction of ITO substrate as it has been discussed above, see

alsoFig. 5.3. The Fermi energy in the Al2O3 films deposited by the low-pressure process

in DAISY-MAT is reproducibly found at 4.5 eV above the valence band maximum, independent on substrate [8, 38, 94, 95]. Moreover, a very small valence band offset between ITO and Al2O3 [98] is expected in the absence of pinning [38]. Therefore, it

can be expected that the Fermi energy in the ITO also raises to EF - EV B ≥4 eV after

Al2O3 deposition. This is clearly not the case as the binding energies of the ITO films

correspond to only to EF - EVB ≈ 3.2 ± 0.2 eV. The discrepancy between the expected

and the measured Fermi energies might be explained by the formation of a very narrow space charge region at the surface of the highly doped ITO, which is schematically represented inFig. 5.5(b). If this is narrower than the depth probed by XPS, the Fermi level directly at the interface cannot be observed. This is equivalent to an effective modification of the band alignment [12,38, 210, 211].

For semiconductors, a Fermi level position, which is different at the surface and bulk, can be explained by the presence of charged surface states and development of a space charge layer [212]. The density of surface states (Dss) and their energetic position

leads to surface Fermi level modification. The width of space charge layer (δ), which compensates the negative surface charge, depends on the doping of the material. For undoped semiconductor, it is expected to have larger width of space charge region, see

(δ1) of Fig. 5.5. However, degenerate semiconductors such as ITO should develop very

thin space charge layer and also required a very high density of surface states, which give rise to the Fermi edge emissions observed for highly doped semiconductors like ITO [35]. Even with a sufficiently high density of surface states one has to expect a rather thin space charge layer of the order of 1 nm [213]. Since ITO contains mobile donors, which is indicated by Sn segregation during deposition at elevated temperatures of ≥ 300 ◦C or post annealing at higher temperature [21], an even thinner space charge layer would result due to pile up of donors near the surface [214].

Figure 5.5: Schematic representation for electronic structures of surfaces of undoped TCO

(a) and ITO / ALD-Al2O3 system. The density of surface states (Dss) and their energetic

position leads to surface Fermi level modification. The width of space charge layer (δ), which compensates the negative surface charge, depends on the doping of the material. Undoped TCO have larger width of space charge layer (δ1) compared to ITO/ ALD-Al2O3 system (δ2), which