3.3 Sample Characterization
3.3.7 Conductivity measurements
Conductivity measurements were performed in a home-built setup as described in Chap-ter 2.3.5. For the measurements it was necessary to contact the samples with platinum contacts. These contacts had to cover the edges of the samples as well. In order to sputter deposit the contact onto the sample in a Quorum Q300T D sputter coater shadow masks on sample holders in a 45◦angle were being employed.
The sputter coater is being operated and maintained by the group of Prof. Dr. Ensinger
"Materials Analysis" of the Materials Science Department at TU Darmstadt. The opportu-nity to use this machine is gratefully acknowledged.
The conductivity measurements itself were being performed at elevated temperatures and in different atmospheres. These parameters are part of the discussion of the results of the measurements. Hence, they can be found there.
66 3 Experimental Procedure
4 Setting the baseline - Phase verification
The determination of any property of a material depends strongly on the quality of the samples that are available. A good knowledge about properties such as e.g. composition and structure is crucial in order to extract usable information from any sample. In order to prepare samples with the right structure and composition, the work was started with a study on the dependency of the iron oxide phase on deposition parameters such as oxygen partial pressure and temperature. The results can be seen as the fundament of any further characterization and modification. It is, hence, straight forward to start with thi part of the work.
Important aspects of this chapter
... more than 6 % oxygen in the sputter gas gives Fe2O3
... for sample temperatures of 300◦C and higher the samples crystallize ... epitaxial growth is possible on sapphire with different orientations ... shape of spectra depends strongly on the crystallinity
... the optical band gap is about 2.2 eV
... the Fermi level is within 0.8 eV to 1.6 eV above the valence band
... the work function is between 5.2 eV and 5.8 eV and the ionization potential
∼6.6 eV
... the distance of the valence band to the Fe2p3/2core level is 709.5 eV ... the distance of the valence band to the O1s core level is 528.4 eV
67
4.1 Oxygen partial pressure dependencies
The phase of an oxide material deposited by means of reactive sputtering depends among other parameters strongly on the oxygen partial pressure in the sputter gas. The two pos-sible choices for a target were either metallic iron or iron oxide of different composition.
For the latter case the oxygen partial pressure in the chamber depends on the sputter rate as well as on the composition of the added gas. This complicates the precise adjustment of this crucial parameter. Using a metallic target, however, solves this issue as oxygen has to be added as gas in any case. Hence, the control of the oxygen content is facilitated.
It was, therefore, decided to only use metallic targets over the course of this work.
Please note, that the oxygen needed for fully oxidized samples is depending on the sput-ter rate and, therefore, on the power that is applied to the target. It was decided to use the same power of 60 W at the iron target for all depositions within this work.
In order to find an oxygen partial pressure, where hematite would be formed, samples were deposited with different gas composition. All samples were deposited at 400◦C. The results of the structural characterization by means of XRD and Raman Spectroscopy are shown in Figure 4.1.
For a very low oxygen content of 1 % the X-ray diffraction pattern in Figure 4.1a) shows reflections from both, cubic iron and magnetite Fe3O4. The former has been identified with the help of the pdf-card 6-696 while for the identification of the latter the pdf-card 1-1111 has been used. The intensity of the most intense reflection, originating from the iron phase and marked with an asterisk has been manually cut in order to make the other reflections visible on this scale. The corresponding Raman spectrum in Figure 4.1b) shows peaks that are characteristic for magnetite while for the cubic iron no first order Raman modes exists as all atoms sit on inversion centers.[143, 144, 153, 154]
Increasing the oxygen content to 5 % eliminates the metallic iron in both XRD pat-tern and Raman spectrum. Now, both techniques show a composition of magnetite and hematite. Please note that for the Raman spectrum only the peak at 660 cm−1is related to magnetite.
Later, it will be argued that this peak origins from a initially Raman forbidden phonon scattering process that becomes Raman active due to symmetry disorder1. Here, however, the XRD pattern clearly shows magnetite which allows for the assignment of this mode to originate from this phase. For the identification of magnetite the same pdf-card as for the 1 % sample was used, while the hematite phase was identified with the pdf-card 84-307.
1 Please see in 4.2.3 Figure 4.9 and in 6.1 Figure 6.1
68 4 Setting the baseline - Phase verification
Fe2O3 Fe3O4 Fe
Intensity / arb. units
2 / ° Raman shift / cm-1
Intensity / arb. units
80 70 60 50 40 30
20 400 800 1200 1600
*
1% O2 5% O2 10% O2
A1g Eg Eg A1g Eg PS
A1g
T2g
T2g
104 110 024
113 116 118 300214
a) b)
Figure 4.1:Structural characterization of iron oxides deposited with different oxygen par-tial pressures in the sputter gas by a) GIXRD and b) Raman Spectroscopy. XRD reflec-tions were identified with the help of the pdf-cards 6-696 (Fe), 1-1111 (Fe3O4), and 84-307 (Fe2O3).
For 10 % oxygen in the gas phase pure hematite samples are deposited. Both, XRD and Raman spectroscopy show only signals which correspond toα– Fe2O3. The XRD pattern shows an enhancement of the (110) reflection with respect to the (104) reflection which is most prominent in the powder file of the pdf-card. This indicates that a preferred (110)-orientation is associated with the sputtered hematite thin film.
With the knowledge of the structural characteristics of the samples the development of the electronic structure with increasing oxygen content in the sputter gas can be observed by XPS. For this purpose Figure 4.2 shows core-level and valence band spectra measured by XPS and UPS.
The Fe2p spectra clearly show the change from the metallic iron containing sample to the phase pure hematite. In the spectra of the 1 % sample there are three features observ-able, which can be attributed to the three different oxidation states of iron in this sample.
4.1 Oxygen partial pressure dependencies 69
Binding energy / eV Binding energy / eV
Intensity / arb. unitsIntensity / arb. units A
B C
735 725 715 705 532 530 528
6 4 2 0
10 8 6 4 2 0
a) Fe2p b) O1s
c) VB XPSVB XPS d) VB UPS
Fe3+
sat.
Figure 4.2:Characterization of the composition in dependence of the oxygen partial pres-sure. a) Fe2p-, b) O1s-, and c) valence band spectra measured by XPS and d) the valence band measured by UPS.
Namely, on the low binding energy side of the Fe2p3/2emission a shoulder is visible, which is labeled as A. This signal originates from the metallic iron in the sample.[125, 126] Be-sides the metallic iron there are two further emissions. Another shoulder at slightly higher binding energies labeled with B can be attributed to originate from Fe2+cations and the main emission C arises from Fe3+cations.[125, 126]
The existence of three different oxidation states in this sample can be explained by the two verified phases. Within one phase this situation would not be very likely as the Fermi level would need to be in the vicinity of two charge transition points2. In two phases, however, more oxidation states can occur.
As can be expected from the structural analysis the addition of more oxygen into the sputter gas results in the disappearance of the signal from metallic iron. In the 5 % sample
2 See section 2.2.3
70 4 Setting the baseline - Phase verification
only the signals B and C are present. This observation can be expected from a mixture of Fe2O3and Fe3O4.
By adding 10 % oxygen into the sputter gas the signal B from Fe2+ cations vanishes as well. Now, the signal does only originate from Fe3+cations. The strong satellite at about 718.5 eV is characteristic for Fe3+.[129, 130, 155]
The O1s-peaks in Figure 4.2b) show a broadening of the signal of the mixed-phase samples compared to the phase pure Fe2O3sample. The higher binding energies of these two emissions is expected for Fe3O4 containing samples compared to Fe2O3 as has been reported in e.g. Refs. [125, 126, 155] before.
The valence bands of the mixed-phase sample in Figure 4.2c) and d) both show a sig-nal at the Fermi energy. This is expected for metallic iron and the low band gap material magnetite. The valence band of the 10 % sample shows the features for hematite e.g.
the valence band maximum indicates a band gap. The electronic structure of the valence band of hematite will be discussed in Chapter 5.
As the desired material of this work was hematite, no efforts were undertaken to isolate the phases of magnetite Fe3O4 or wurtzite FeO. It should be possible to deposit these by choosing the right oxygen partial pressures. For Fe3O4this would be less than 5 % as here there is already some Fe2O3included in the films. For FeO the oxygen content should be even lower. Besides the control of the oxygen content, also the temperature at which the phase should be deposited has to be chosen accordingly.
The oxygen partial pressure, however, at which hematite is being formed was of interest for this thesis. The sputter rate of any deposition is drastically decreased by the addition of oxygen. It was, therefore, attempted to chose an oxygen partial pressure which results in a reasonable rate and phase pure samples.
Figure 4.3 shows Fe2p3/2and O1s core-level binding energies and Fermi level positions in dependence of the oxygen partial pressure. The colors indicate the present phases in the respective samples. A black dot represents a mixture of Fe and Fe3O4 , a red dot of Fe3O4 and Fe2O3, and a blue dot of pure Fe2O3 . The phase composition of the sample represented by the green dot is unknown. As it is found at low oxygen contents it might be possible that FeO is included in this sample but this was not further investigated. Until 5 % the Fermi level is at zero binding energy which indicates that the samples contain metallic iron, wurtzite or magnetite.
With 6 % or more oxygen in the sputter gas hematite is being formed as a single phase.
This was confirmed by Raman measurement on these samples. Additional XP and Raman spectra of samples deposited with higher oxygen contents in the sputter gas can be found
4.1 Oxygen partial pressure dependencies 71
in Figures A1 and A2. Only for the highest oxygen concentrations the Raman measure-ment did not correspond to crystalline hematite. However, the satellite structure of the Fe2p emission indicated pure Fe2O3 . It will be shown in part 4.2.3 that the crystallinity of the samples has a great influence on the Raman spectrum.
The development of the phase is di-711.5
711.0 710.5 710.0 709.5
2.0 1.5 1.0 0.5 0.0
20 15 10 5
0 530.5 530.0 529.5 529.0 528.5
EF-EVBM / eVBE O1s / eVBE Fe2p3/2 / eV
Oxygen content in sputter gas / % Figure 4.3:The dependency of the Fermi level position on the oxygen partial pressure.
rectly observed from the different behav-ior of the binding energies of the three samples that were deposited with less than 6 % oxygen. Increasing the oxygen partial pressures lead to changes of the binding energy of Fe2p3/2 and O1s spec-tra. These are different from each other for the two core-levels.
As a consequence the core-levels show a different distance to the valence band maximum than samples which have been deposited with higher amounts of oxy-gen. This is expected, if a phase change occurs. In the present case, where there is even a phase mixture present, no constant distant of the core-levels to the valence band maximum can be expected.[134, 135]
At 6 % oxygen, however, the situation changes. Now, the binding energy values mea-sured for Fe2p3/2, O1s, and valence band maximum shift parallel and their relative posi-tion indicates that, both, Fe2p3/2 and O1s have a constant distance to the valence band maximum of 709.5 eV and 528.4 eV, respectively.
After hematite is being formed the position of the Fermi level is almost independent on the oxygen content of the sputter gas. All values are between 0.9 eV to 1.2 eV. Consid-ering the optical band gap of hematite to be 2.2 eV this indicates an intrinsic Fermi level position at mid-gap.[61, 64] This is somewhat surprising as undoped hematite in litera-ture is described to be an n-type semiconductor. The reason for this misconception can be found in Chapter 8 and will be further discussed there.
The results from the structural and electronic characterization show that by a combined study of the three techniques the phase of the sample identified. Raman spectroscopy and XRD are both structural characterization techniques and it was shown that their results
72 4 Setting the baseline - Phase verification
show a very good agreement to each other. It was, therefore, decided that for most samples within this study a phase characterization by one of the techniques was adequate.
For the sake of time and simplicity Raman spectroscopy was chosen to give a structural reference.