5.4 Thermal characterization methods
Time resolved X-ray diffraction (TRXRD)
TRXRD is a pump-probe experiment and can be used to investigate the temperature evolution in a sample. An ultrashort laser pulse heats the sample and the following relaxation with the corresponding change of the lattice constant can be measured as time dependent peak shift by XRD. Further discussions of the measurement principle and the achieved results are shown in chapter 11.
The measurements were performed at the beamline ID09B of the European Syn-chrotron Radiation Facility (ESRF) in Grenoble together with Dr. A. Plech from the Institute for Photon Science and Synchrotron Radiation(IPS)/Karlsruher Institut f¨ur Technologien (KIT). The laser was a fs-regenerative Ti:sapphire amplifier with 1 kHz repetition rate and a pulse length of 2 ps. The spot size was 450µm and the pulse energy 10 mJ - 500 mJ. The used synchrotron radiation was pulsed by a chopper and the energy of 15 keV was filtered by a Si(111) monochromator. The X-ray pulses were synchronized with the laser pulses. The diffracted X-rays were detected by a scintillator together with a CCD camera (Frelon). It was measured in θ - 2θ geometry with an incident angle of 10◦. Further information can be found elsewhere [141].
Asynchronous optical sampling (ASOPS)
ASOPS is an optical pump and probe method for characterizing the acoustic modes in a material and can be used to extract the speed of sound. Furthermore reflection of phonons at the interfaces can be investigated and information about phonon scat-tering can be extracted. The underlying principle of ASOPS is explained in more detail in chapter 11 and elsewhere [142, 143]. The measurements were performed by Chuan He in cooperation with the group ”Moderne Optik und Photonik” of Prof. T.
Dekorsy from the Universit¨at Konstanz. Two mode-locked Ti:sapphire femtosecond lasers were used, which are operating at a repetition rate of ∼800 MHz. The fre-quency offset in the repetition rate of both laser was stabilized at 5 kHz and allows scans in a measurement window of ∼1.25 ns. The used wavelength is 790 nm for the pump and 820 nm for the probe laser. The nominal pulse length of both lasers is
∼50 fs. A collinear reflection geometry is chosen and it was measured in focus with a spot size of ∼2µm.
Raman spectroscopy
Raman spectroscopy is used to investigate the vibrational states of molecules or of solids. The sample is therefore irradiated with monochromatic laser light in the ul-traviolet or in the visible range. The light is scattered in the sample and the detected light coming from the sample has two contributions [144]. The first contribution oc-curs due to Rayleigh scattering and has the same frequency ν0 as the initial light. If a
vibrational mode with the eigenfrequency νm is excited or annihilated in the sample, light with a frequency ν0− νm or ν0+ νm can be detected. The second contribution are therefore lines with a frequency shift. The lines occurring at lower frequency are called Stoke lines and those ones with larger frequency anti-Stoke lines. Since the extractable information is equal for both, usually the Stoke lines are analyzed, since they exhibit the larger intensity [144]. Not all vibrational modes in a molecule or solid are Raman active. This is only the case if the vibration mode is polarizable [144].
How large the oscillation frequency is depends mainly on the bond (force constant) between the oscillating atoms and on their mass. Furthermore different vibrational modes exhibit different symmetries, which could be used to classify the modes with respect to their symmetry. The different classes (Eg, Fg and Ag modes) reveal dif-ferent response to difdif-ferent polarization of the incident light, which can be used to achieve further information from the measured Raman spectra about the occurring vibrational modes. An extended discussion about the basics of Raman spectroscopy can be found in ”Introductory Raman Spectroscopy” written by Ferraro, Nakamoto and Brown [144].
The measurements were performed together with Dr. O. D. Gordan and G. Tofighi in cooperation with the group for ”Halbleiterphysik” of Prof. D. R. T. Zahn of TU Chemnitz. In the performed experiment, a Horiba LabRam HR800 confocal micro Raman system was used with a green laser of 532 nm wavelength and a power of around 2 mW. It was measured in resonance (checked by ellipsometry) and with se-lection rules. The notation VV is thereby used for parallel and HV for perpendicular polarization.
6 Deposition of skutterudite thin films
6.1 Deposition chamber and deposition parameters
Most of the films were deposited in a ultra high vacuum (UHV) chamber for molecular beam epitaxy (MBE), equipped with a load lock, a transfer system to handle several samples and the main deposition chamber with a base pressure between 5 · 10−11 mbar and 5 · 10−10mbar.
The used substrates (4 ” wafer or pieces) were first introduced in the load lock and baked at 200◦C to remove water contamination. After reaching a pressure of 1 · 10−8 mbar the samples were transfered to the manipulator in the deposition chamber. The main chamber is equipped with an electron beam evaporator for the evaporation of Co and three effusion cells for Sb, Fe and Yb. The flux of Co can be controlled instantaneously by an optical detector (via electron induced emission spectroscopy, EIES, see details in [105]), which was calibrated by depositing a Co film with a specific nominal thickness. The real thickness was afterwards investigated by RBS and the tooling factor determined. The deposition rates of Sb, Fe and Yb can be controlled by the temperature of the effusion cells. For calibration, three films of each element were deposited at different temperatures for a deposition time of 10 min. Afterwards the thickness of each film was investigated by RBS allowing the calculation of the rate.
By plotting the rates as function of the cell temperature, an exponential calibration curve is achieved.
The film composition of alloys can be controlled by adjusting the individual fluxes for codeposition (as described in chapter 6.2 and 6.3) or by adjusting the individual layer thicknesses for the modulated elemental reactant method (ch. 6.2 and 6.3). The Sb flux, which is the largest one, was chosen always smaller than 1.2 ˚A/s (see ch. 7.1.3).
To ensure homogeneous films, the sample holder is rotated during the deposition, which was verified by RBS. There are no detectable differences in thickness for Co and Sb over the entire area of the 4 ” wafers, however Fe reveals an decrease of thickness towards the edge of the wafer. Therefore the composition of Fe containing films has to be verified for each individual piece broken from the wafer. Furthermore a chamber wall cooling system prevents the film from contamination due to warmed-up side walls. For depositions at elevated substrate temperatures, the manipulator provides the possibility to heat the substrate from the backside of the sample holder up to 1000◦C. The temperature is also measured at the backside, which yields a difference compared to the real temperature on the substrate surface.
Additionally the MBE chamber was used for annealing under UHV conditions by using the sample heater of the manipulator. In most cases, the films were therefore deposited on wafers and afterwards shortly exposed to air for breaking the initial samples into pieces. Then pieces of different samples were mounted on a sample holder and reintroduced to the load lock. After reaching a proper pressure without baking, the samples were transfered from the load lock to the main chamber. The annealing process is therefore also performed in UHV by heating from the backside using a heating rate of 10 K/min.