2.3 Methods
2.3.5 Conductivity measurements
The electrical properties of transition metal oxides are of high interest as their applica-tion always involves charge carrier transport phenomena. Hence, very early studies have already investigated the conductivity of hematite samples.[77] In this work the thin film samples were measured in a home-built setup which allows for the conductivity mea-surement at elevated temperatures and in an controlled atmosphere of two samples on a ceramic sample holder in a quartz tube oven at the same time. The setup had al-ready been installed at the beginning of this work and a detailed (german) descriptions can be found in Refs. [146] and [113]. In the following a brief introduction into the setup and into the chosen sample geometry is given. Physical concepts on the conductiv-ity and how it is influenced by e.g. the atmosphere is given in the discussion on the results.
The measurements are performed in a geometry proposed by L.J. van der Pauw.[147]
Here, a thin film can be measured regardless of its exact shape. It is, however, necessary for the film to have a homogeneous thickness without holes.
In addition, the contacts should be as small as possible and be positioned on the edge of the film to be measured. Figure 2.14 shows a schematic sample with substrate, film and contacts. The latter have to cover also parts of the edges of the substrate in order to be contacted.
In the present example a current is being applied between the contacts A and B. In order to measure a current-less voltage drop the potential between C and D is being measured.
This allows for a measurement of the conductivity which is independent on the contact resistance. For better statistics each possible contact pair is being used in both directions (forward/backward mode) with the respective potential measurement. This gives eight measurements of which the mean value is used. The specific resistance and thereby the conductivity of a single measurement is given by a geometry factor, the resistance that is being measured and the thickness of the filmd:
ρ= 1 σ = π
ln2·d·UDC IAB = π
ln2 ·d·RAB,CD (2.10) The current should be chosen as high as possible with two boundary conditions. First of all, it must be able to inject all carriers into the sample. If it is not possible to apply the
48 2 Fundamentals
current the sample is "in compliance" and the measurement cannot be used. Secondly, the potential measured in forward mode has to be the same as in backward mode. Hence, their ratio times 100 needs to be at 100. If deviations from this value are observed the current has to be potentially lowered. This, however, requires also the delay time between two measurements to be increased. For high resistive samples such as hematite often the measurement is only possible with very low currents at higher temperatures.
The temperature can be controlled by a
Eu-Figure 2.14: Sample with contacts for measuerments in van der Pauw geometry.
rotherm controller and is being applied ac-cording to a calibration measurement which has been performed before. There is no di-rect measurement of the temperature within the oven. By noting the times at which the temperature is being changed, however, an almost correct temperature program can be created.
For the control of the atmosphere different gases are available. On the oven that has
been used in this work these are pure argon and pure oxygen. By controlling the flow of the respective gases a range between 100 % argon to 100 % oxygen is possible. The oven also features a vacuum pumping system to lower the oxygen partial pressure even further. This feature, however, has not been used.
2.3 Methods 49
3 Experimental Procedure
3.1 The lab
All samples of this work were prepared in the DAISY-MAT system at TU Darmstadt. In this integrated system the XPS and UPS analysis of the samples could be performed as well. As an integrated system it allows for the sample transfer from the preparation chambers into the XPS chamber without leaving the ulta-high vacuum (UHV) conditions. This ensures clean sample surfaces without the contamination of carbon hydrates or water from the atmosphere.
In addition to the sputter chambers a few other possibilities to treat samples are avail-able. These are an ALD chamber in which the sample can be exposed to water or its surface can be modified by an alumina layer and a plasma source in which oxidizing or reducing plasmas can be ignited. A schematic representation of the lab can be found on the left hand side of Figure 3.1. Further descriptions of the lab can be found e.g. in Refs.
[148] and [149].
Photoelectron spectroscopy measurements on non-contaminated samples are essential to study the intrinsic electronic surface properties of the material. Any adsorbate on the surface might influence the electronic properties by a charge transfer and result in a shift of the Fermi Level. In addition, the oxygen emission that can give information on the chemical state of oxygen in oxide materials is being altered if oxygen-containing species are adsorbed.
Many PES measurements from literature are performed on samples, which have been prepared elsewhere and are introduced into the PES chamber in order to be measured.
Cleaning by heating or ion etching are effective sample treatments that will change the properties. Such measurements are referred to as ex-situ in order to identify them as be-ing performed on potentially contaminated samples.
The measurements on samples, which have never left the vacuum conditions, on the other hand, are referred to as in-situ1. This shall distinguish these measurements as being performed on clean, non-contaminated samples.
1 Strictly speaking an "in-situ" measurement would be performed during the sample prepara-tion/treatment. This has NOT been done. The forgiveness of the reader for this small flaw is ap-preciated.
51
Figure 3.1:The DAISY-MAT system from the Surface Science Division at TU Darmstadt. On the left hand side a schematic drawing of the system with the central distribution chamber around which the preparation chambers are located is shown. The integration of a Physical Electronics PHI5700 multitechnique surface analysis system allows for the characterization of the samples by XPS/UPS without breaking the UHV conditions. The right hand side shows a schematic drawing of the chamber (Ox0) in which most samples of this work have been prepared. The chamber had to be rebuilt into a co-sputtering chamber with rotating heater prior to the first sample depositions. Schematic drawing of DAISY-MAT adapted from Ref. [148].
On the right hand side of Figure 3.1 a sketch of the co-sputtering chamber in which the iron oxide samples have been prepared is shown. Prior to the first deposition the cham-bers top flanche had to be designed in order to allow for the co-sputtering of the doped samples. During the deposition the sample is positioned at the point where the normals of the two targets cross (focus point). This point is being reached by manipulation of the height of the sample. Each target has an angle of 33◦ to the normal of the top flanche.
To compensate for this angle a sample rotation was being established. As wiring makes it complex to achieve a 360◦ rotation it was decided to settle for a 270◦ back-and-forth rotation.
Each target can be manipulated in the distanced to the focus point and powerP. With these values the sample composition can be changed. In addition, the use of RF and DC signals for plasma generation is possible.
52 3 Experimental Procedure
Heating of the sample is achieved by a halogene light bulb below the sample. The temperature of the sample cannot be measured directly. Instead two thermo-couples are being installed on the heater itself. By using a calibration sample it is possible to corre-late the heating current to a certain sample temperature, which can be controlled by the temperature at the two thermo-couples.
In this context one important aspect has to be mentioned. For years it was the stan-dard procedure to use a silicon calibration sample for the temperature calibration. At one point, however, this sample was lost and was replaced with a quartz substrate. It was noticed that the temperature at the same current was much lower on a quartz substrate than compared to the silicon substrate. From this point on , calibrations were being done by using quartz and/or sapphire as calibration samples. The difference between the latter two can be up to 150◦C at the maximal heating current of 10 A. The best calibration can always be expected with the same substrate as calibration sample that is to be used dur-ing the deposition. For the chamber Ox0 which was used durdur-ing this thesis temperature calibrations were performed for quartz and sapphire substrates.