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Potential-Seebeck-Microprobe PSM:
Measuring the Spatial Resolution of the Seebeck Coefficient and the Electric Potential
D. Platzek1,2, G. Karpinski2, C. Stiewe2, P. Ziolkowski2, C. Drasar2,3 and E. Müller2
1
Physics Technology – Development and Consulting (PANCO), D-56218 Mülheim-Kärlich, Germany
2
German Aerospace Center (DLR), Institute of Materials Research, D-51170 Köln, Germany
3
University of Pardubice, Faculty of Inorganic Chemistry, CZ-53210 Pardubice, Czech Republik
correspondence email: [email protected]
Abstract
Thermoelectric power generators are typically operating in
a large temperature difference; indeed the properties of
thermoelectric semiconductors vary with temperature. Thus
the overall conversion efficiency is strongly dependent on
spatial variations of the material properties according to the
temperature profile along the entire thermoelectric generator
element. Similarly, a functionally graded module is capable of
accomplishing thermal sensors with linearised characteristics
over a wide temperature range.
The Seebeck-coefficient S is a measure of the electrically
active components in a material. Different components in a
single unit become visible by measuring the local S with a
scanning thermoprobe. This applies accordingly for the
electrical conductivity and therefore the behaviour of the
material in a certain temperature gradient becomes
predictable.
A scanning Seebeck Microprobe has been combined with
the measurement of the electric potential along the surface of
semiconducting or metallic material. A heated probe tip is
placed onto the surface of the sample under investigation,
measuring the Seebeck coefficient. Using a specially designed
sample holder, an AC current can be applied to the specimen,
allowing for the detection of the voltage drop between one
current contact and the travelling probe tip. This voltage is
proportional to the electrical conductivity at the tip position.
With this technique a spatially resolved imaging of the
Seebeck coefficient as well as the electrical conductivity can
be performed. Furthermore the electrical contact resistance
between different materials becomes visible, e.g., in
segmented thermoelectric or other devices.
State of the art
The local resolution of the Seebeck coefficient is a
measure for different electrically active components in
materials. This becomes important especially investigating
functionally graded material. A scanning Seebeck microprobe
is a device for measuring the Seebeck coefficient on a sample
surface spatially resolved to achieve information on the
homogeneity or distribution of the components [1].
So far it was reported about a measurement equipment for
the spatial resolution of the Seebeck coefficient [1,2]. With
this equipment it is possible to detect inhomogeneities,
different phases, even different doping levels or anisotropies
[3], that is hardly possible by other surface analysis methods
Internet: www.panco.de
like SEM, EDX etc. For many materials also the homogeneity
of the electrical resistivity plays an important role in their
performance, especially for good quality of semiconductors.
The Potential-Seebeck-Microprobe PSM
The Potential-Seebeck-Microprobe PSM is an instrument
to measure the spatial resolution of the Seebeck coefficient
and of the electrical resistivity.
Seebeck Microprobe
A heated probe tip is positioned onto the surface of a sample.
The probe is connected with a thermocouple (in this case type
T, Cu-CuNi) measuring the temperature T1. The sample is in
good electrical and thermal contact to a heat sink and also
connected with a thermocouple measuring T0. The heat flow
from the probe tip to the sample causes a local temperature
gradient in the vicinity of the tip.
Combining the Cu-Cu and the CuNi-CuNi wires of the
thermocouples a voltage U0 and U1 is measured yielding the
Seebeck coefficient Ss of the sample at the position of the
probe tip according to equations
U 0  ( SS  SCu )  (T 1  T 0)
(1)
U 1  ( SS  SCuNi )  (T 1  T 0)
(2)
and
yielding in
Ss 
U0
( SCu  SCuNi )  SCu
U1 U 0
(3),
whereat SCu and SCuNi are the Seebeck coefficients of Cu and
CuNi, respectively. Mounting the probe to a three dimensional
micro-positioning system (see Fig. 1) allows the determination
of the thermopower of each single sample position for a
certain temperature range [2], in the easiest case at room
temperature.
The result is a two dimensional image of the Seebeck
coefficient.
Figure 1: Schematic of the Seebeck Microprobe. The
temperature of the sample and the probe tip as well as the
Seebeck voltage can be measured. The probe tip is positioned
by linear stages.
Figure 2: Schematic of the potential probe. An electrical AC
current is applied to the sample and a probe tip scans the
surface of the sample and measures the electric potential in
each point resulting in the electrical resistivity.
Technical Setup
Electrical Potential Probe
Similar to the Seebeck surface scan the electric potential
can be measured. Therefore a sample holder was constructed
not only to support the sample mechanically, but also to apply
an electric current to the sample. A probe tip for the voltage
pick-up was installed (Fig. 2). The tip´s movement by linear
stages allows for scanning the sample, and the change of the
electric potential can be measured along the sample (Fig. 4).
The specific resistivity can be calculated for each
measurement point according to Ohm´s law with the measured
voltage U, the current I applied to the sample and
l
R
(4).
A
with the resistance R, , the specific resistivity , the lenght l
and the cross section A of the sample.
With help of this tool not only the electric resistivity can
be measured, but also the ohmic contact resistance between
different materials (Fig. 9), e. g., in a stacked thermoelectric
device.
The measurement data for low resistance material are
usually in the range of several µV and superposed with
distortions. Therefore, and to avoid any thermoelectric effects
during the potential measurement, the current is a low frequent
AC and the data acquisition is optimised by Fast Fourier
(FFT) analysis.
Using a special sample holder whereat an electrical current
can be applied to the sample and that also serves as a heat
sink, the Seebeck coefficient and potential between one end
and the probe tip can be measured in one scan. Both
measurement methods have been combined in a computer
program with a special trigger arrangement for a digital
voltmeter. Thus in the same run a spatially resolved imaging
of the Seebeck coefficient as well as the electrical resistivity
can now be performed, the information of S and  results from
the same position of the sample. The S and  measurements
can also be used separately.
The measuring equipment consists of following
components:
three axis micro positioning stage with controller unit
heatable measuring thermoprobe
contact detection system
analogue multiplexer
digital voltmeter
current supply
sample holder
PC with GPIB interface and controlling programm
Specifications
Positioning accuracy: ± 1µm
Travel:
x-direction 150mm, y-50mm
Lateral resolution of S: up to10µm, depending on the
sample´s thermal conductivity
Lateral resolution of :
up to ± 1µm
Measuring time S and : < 8s per local data point
Reproducibility:
better than 3% of S and 
Seebeck accuracy:
better than 5%
Accuracy el. cond. : better than 5 % for highly doped
semiconductors
better than 8 % for metals
The positioning accuracy of the linear stages is 1µm, the
reproducibility (bidirectional) is 3µm. The digital voltmeter in
combination with the analog multiplexer has a resolution of
100nV. A system for contacting the probe tip onto the sample
has been developed, so that the contact pressure of the tip can
be controlled and systematically varied. The thermal
conductivity  of the sample has a significant influence on the
physically possible lateral resolution of S, a low  of < 1
W/mK results in highest resolution.
Applications
With the measurement of the spatially resolved Seebeck
coefficient not only the homogeneity or different phases can
be visualised, even the functional grading of material becomes
visible. In Fig. 3 the Seebeck coefficient of a Bi2Te2.85Se0.15
sample is shown in a grey scaled diagram as well as the
abundance distribution and the Gaussian fit below. The width
of the different peaks is proportional to the homogeneity.
Since the grading is caused by only slight variations in the
doping level during the growing process (Czochalski method
[4]) this is not visible by other methods like SEM or EDX.
Figure 3: Seebeck coefficient of a graded Bi2Te2.85Se0.15
based crystal. The grey scale indicates the different Seebeck
coefficients and clearly shows the grading in the sample.
The measurement of the electrical potential in a stacked
material (in this case graded FeSi2) yields in a different slope
of the measured voltage, depending on the distance to the
reference electrode (Fig. 4). From that slope the electrical
resitivity can be calculated via equation (4) and Ohm´s law.
Figure 4: The measured voltage across a sample indicates the
change of the electrical resistance of a stacked material. The
slope of the curve is proportional to the electrical resistance.
From each single point of the scans the electrical resistivity
was calculated yielding in a coloured plot (Fig. 5). Fig. 6
shows the distribution of the Seebeck coefficient in this
sample, which is the same miniaturised functionally graded
FeSi2 based thermal sensor as shown in fig 4. The Seebeck
coefficient is in semiconductors up to 10 times higher than in
conventional metals for thermocouples like Ni/NiCr. The
temperature dependence of the Seebeck coefficient leads to a
non-linear voltage as a function of temperature for any
temperature sensor. A functional grading of a semiconducting
sensor will improve the linearity of the output voltage by a
superposition of the individual functions for each material of
the stack [5].
Controlling the diffusion zones between the layers is
essential for the long term stability of such a sensor. In this
case the scan step was 10 µm and a diffusion zone of < 50 µm
was measured. With the PSM exists an important tool to
control the function and the degradation or stability of those
sensors.
Figure 5: Electrical resistivity of a stacked material scanned
with the electrical potential probe. The grey scale indicates the
ohmic resistivity and the interface of different materials
becomes clearly visible.
Figure 6: Seebeck coefficient of a graded FeSi2 sensor with
different doping levels. The cut-out indicates the diffusion
zone with a high resolution of 10µm.
The next example shows the Seebeck coefficient and the
electrical resistivity of a segmented leg of FeSi2 with different
doping levels. The bright part in Fig. 7 at the right side (0 to
-1mm) shows a p-type material, at position -1mm a p-n
transition becomes visible. This is also clearly visible in the
0.012
0.010
Resistance ( Ω)
scan of the electrical resitivity (Fig. 8). The electrical scan
shows a change in the resistance close to the p-n transition in
the material, that is also clearly visible in the Seebeck scan.
The line in the upper part of Fig. 8 shows a more or less
parabolic increase of the measured voltage close to the p-n
transition. There is hardly a change in the electrical potential
observable in the part with negative Seebeck coefficient. Thus
the PSM is suitable to detect phenomena of p-n transitions,
even in semiconductor fabrication processes and especially for
quality control.
cross section = A cm2
contact resistance per unit area = ∆R x A
resistivity = m x A Ω cm
0.008
0.006
m = resistance per
unit length
0.004
0.002
R
0.000
-0.20 -0.15 -0.10 -0.05 0.00
0.05
0.10
0.15
0.20
Displacement length, x (cm)
Figure 9: The contact resistance of any ohmic contact can be
measured by the potential probe. The gap in the measured
resistance R is proportional to the contact resistance.
Figure 7: Seebeck coefficient of a segmented FeSi2 leg. On
the right part a p-n transition is observed. The insert shows the
abundance distribution of S for the different segments.
Figure 8: Electrical resistivity of the same sample as shown in
Fig. 7. The p-n transition is also observable in the electrical
properties. The insert shows the potential that hardly changes
after the p-n transition.
The contact resistance of any ohmic contact can be
measured by the potential probe as described before. Fig. 9
shows the contact resistance of a Zn4Sb3 pellet soldered on a
copper plate measured with the electrical potential probe. The
difference between the base line and the beginning of the
curve with a slope m, R indicates the contact resistance.
Multiplication with the samples cross section area A results in
the contact resistance per unit area, in this case 2,7 10-4 cm2,
which is in the range of commercial soldered standard
material.
Conclusions
The Potential Seebeck Microprobe and thus the
combination of the scanning Seebeck Microprobe and a
scanning electrical potential probe are an elegant and
important tool for controlling the degree of homogeneity in
materials, to visualise grading or to detect interdiffusion
between single layers of a stacked material to determine
degradation. Even for fuel cell contacts these measurement
methods can give information about the actual state that is not
possible to detect with other methods.
Due to the new trigger arrangement in combination with
the control system for the contact pressure of the probe tip the
measurement time for a single point can be decreased from
<8s to less than half.
This tool is now under commercialisation and will be
available in 2006.
References
1. P. Reinshaus et al., Proc. 2nd Europ. Symposium on
Thermoelectrics – Materials, Processing Techniques, and
Applications, Dresden, Germany 1994, 90.
2. D. Platzek et al. “An Automated Microprobe for
Temperature Dependent Spatial Scanning of the Seebeck
Coefficient“, Proc. 22nd Int. Conf. on Thermoelectrics
(ICT2003), La Grande Motte, France, IEEE, Piscataway
2004, p. 528-531.
3. D. Platzek et al. “Anisotropy of the Seebeck Coefficient
Detected by the Seebeck Scanning Microprobe“, Proc.
VIIIth European Workshop on Thermoelectrics. 2004.
Krakow Poland: University of Krakow.
4. T. E. Svechnikova et al. “The Influence of Tin on the
Electrophysical Properties of n-type Bi2Te2.85Se0.15 Single
Crystals”, Proc. VIIth European Workshop on
Thermoelectrics 2002, Pamplona, Spain.
5. E. Müller et al. „Functionally Graded Materials for Sensor
and Energy Applications”, Mat. Sci. Eng. A362 (2003) 1739.