APPLIED PHYSICS LETTERS 92, 101910 共2008兲
Diffusion of nickel and tin in p-type „Bi, Sb…2Te3 and n-type Bi2„Te, Se…3
thermoelectric materials
Y. C. Lan,1 D. Z. Wang,1 G. Chen,2 and Z. F. Ren1,a兲
1
Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
Department of Mechanical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139, USA
2
共Received 22 January 2008; accepted 20 February 2008; published online 11 March 2008兲
The diffusion and spatial distribution of tin from solder, and nickel from diffusion barrier in p-type
共Bi, Sb兲2Te3 and n-type Bi2共Te, Se兲3 thermoelectric materials were investigated using electron
microscopy. The results indicate that nickel is a suitable diffusion-barrier material for tin in both
共Bi, Sb兲2Te3 and Bi2共Te, Se兲3. However, even though it is not an issue in the 共Bi, Sb兲2Te3, the nickel
diffuses several microns into the Bi2共Te, Se兲3 during the soldering processing and degrades its
performance. Diffusion coefficients of nickel in p-type 共Bi, Sb兲2Te3 and in n-type Bi2共Te, Se兲3 were
also quantitatively studied. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2896310兴
The p-type 共Bi, Sb兲2Te3 and n-type Bi2共Te, Se兲3 are important semiconductors for thermoelectric cooling 共Peltier兲
devices.1–3 For simplicity, hereafter, the two semiconductors
will be referred to as p-type Bi2Te3 and n-type Bi2Te3, respectively. In Peltier devices, copper 共Cu兲 is usually chosen
as the metallic electrodes and a eutectic Sn42Bi58 alloy as
solder. Electrode materials and solders can diffuse into the
devices and degrade the device performance. For example,
Cu diffuses very rapidly in stoichiometric Bi2Te3 at room
temperature and changes the carrier concentration of the
material.4,5 The diffused Sn from Sn42Bi58 reduces the thermoelectric power of Bi2共Te, Se兲3.6 In order to avoid the diffusion of Sn from the solder and Cu from the electrode into
both p- and n-type Bi2Te3, a nickel 共Ni兲 layer of 3 – 5 ␮m
thick is always used as the diffusion barrier in commercial
Peltier devices.7 While the effectiveness of a Ni diffusion
barrier for Cu in undoped Bi2Te3 was reported,8 the effectiveness of a Ni barrier for Sn42Bi58 solder in Bi2Te3 is not
clear. As expected, our reported experiment shows that a Ni
diffusion-barrier layer can effectively block the diffusion of
Sn42Bi58 solder. However, the barrier material 共Ni兲 itself
diffuses into both p- and n-type, especially into n-type
Bi2共Te, Se兲3. This rapid diffusion of Ni in n-type
Bi2共Te, Se兲3 degrades the thermoelectric properties of Peltier
devices. The phenomenon reported here enhances our understanding on the function of Ni barrier layers and can help us
improve the Peltier devices.
The crystalline specimens of both p- and n-type Bi2Te3
legs with a dimension of 1.4⫻ 1.4⫻ 1.6 mm3 were obtained
from an unused Peltier device 共Fuxin, China兲. Both p- and
n-type Bi2Te3 were coated with Ni diffusion-barrier layers
and soldered with Sn42Bi58 to the Cu electrodes. The legs
were taken off the Peltier device, ground, polished, and Ar+
ion milled at 3.2 KV. They were then examined on a JEM2010F transmission electron microscope 共TEM兲 and JEOL
6340F scanning electron microscope 共SEM兲.
Figure 1共a兲 shows the cross-sectional TEM image of
p-type 共Bi, Sb兲2Te3 legs. An interface of the Ni barrier layer
and the 共Bi, Sb兲2Te3 is clearly observed. Figure 1共b兲 is the
TEM image of the interface region at a higher magnification.
The circles in Fig. 1 illustrate the energy dispersive x-ray
spectroscopy 共EDS兲 test regions. The EDS measurement indicates that the interface consists of a 共Bi, Sb兲2Te3 compound
and a Ni layer. Sn42Bi58 solder is detected beyond the Ni
layer 共regions A and B兲.
Figure 2 shows that 11 EDS spectra were acquired
across the Sn42Bi58 – Ni– 共Bi, Sb兲2Te3 interface. Each spectrum marked by a letter is collected from the corresponding
region in Fig. 1. The numbers in nanometers indicate the
distance of the EDS measurement region to the
Ni– 共Bi, Sb兲2Te3 interface. Bi and Sn are detected inside the
Ni diffusion-barrier layer, indicating that the solder elements
Bi and Sn diffuse into the Ni diffusion-barrier layer. As the
distance from the Ni to the Ni– 共Bi, Sb兲2Te3 interface
decreases, the strength of the x-ray signal of Bi decreases
while Ni increases. Only Ni was observed near the
Ni– 共Bi, Sb兲2Te3 interface, as shown by spectrum F. The
thickness of the pure Ni layer is about 15 nm, which is much
less than the previously reported. Obviously, the Ni diffusion
barrier layer effectively prevented the diffusion of Sn into
共Bi, Sb兲2Te3.
We also examined the other TEM specimen without the
Ni diffusion-barrier layer. It was found that the diffusion
depth of Sn into 共Bi, Sb兲2Te3 was greater than 4 ␮m. Therefore, the Ni layer is an effective diffusion barrier and can
prevent the diffusion of Sn and Bi into 共Bi, Sb兲2Te3.
a兲
FIG. 1. Cross-sectional TEM images of p-type 共Bi, Sb兲2Te3 at low 共a兲 and
high 共b兲 magnifications.
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0003-6951/2008/92共10兲/101910/3/$23.00
92, 101910-1
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101910-2
Lan et al.
Appl. Phys. Lett. 92, 101910 共2008兲
FIG. 2. Typical EDS spectra across the Ni– 共Bi, Sb兲2Te3 interface.
Ni, however, also diffuses into 共Bi, Sb兲2Te3, which has
not been reported before. The x-ray signal of Ni was observed inside 共Bi, Sb兲2Te3, 35 nm away from the
Ni– 共Bi, Sb兲2Te3 interface in 共Bi, Sb兲2Te3, as shown by EDS
spectrum H in Fig. 2. Further analyses 共see inset兲 indicate
that Ni was detected at 120 nm inside 共Bi, Sb兲2Te3 away
from the Ni– 共Bi, Sb兲2Te3 interface. In order to confirm the
Ni diffusion into 共Bi, Sb兲2Te3, electron energy-loss spectroscopy 共EELS兲 measurements were carried out in the same
regions. Ni was also detected in 共Bi, Sb兲2Te3 120 nm away
from the interface, agreeing fairly well with the EDS analyses. Compared with the height 共1.6 mm兲 of the 共Bi, Sb兲2Te3
legs, the Ni diffusion depth of 120 nm should be too thin to
affect the performance of the device. From this point of view,
Ni is an effective diffusion-barrier material for 共Bi, Sb兲2Te3.
n-type Bi2共Te, Se兲3 specimens were also examined. Figure 3共a兲 is a SEM image of a cross-sectional Bi2共Te, Se兲3 leg
coated with the Ni layer. An EDS Sn L␣ line scan profile
shows that the element Sn from the solder diffuses into the
Ni barrier layer to form an NiSn alloy. It did not, however,
diffuse into Bi2共Te, Se兲3. The phenomenon of Sn diffusion in
the Ni layer was further confirmed by the chemical microanalyses on TEM. The Ni layer prevents the diffusion of
Sn into Bi2共Te, Se兲3, as observed in 共Bi, Sb兲2Te3. Without the
Ni diffusion barrier, Sn diffuses into Bi2共Te, Se兲3 ⬃4 ␮m,
similar to the diffusion length in 共Bi, Sb兲2Te3. Therefore, Ni
is also an effective diffusion barrier for Sn42Bi58 solder in
Bi2共Te, Se兲3.
Surprisingly, Ni diffuses deeply into n-type Bi2共Te, Se兲3
several microns, as shown by the EDS Ni K␣ profile in Fig.
3共a兲. On the contrary, Ni diffusion was hardly observed in
the p-type 共Bi, Sb兲2Te3 under SEM. In order to confirm the
diffusion depth, a cross-sectional n-type specimen with
⬃100 nm thickness 关perpendicular to the page in Fig. 3共a兲兴
was prepared and quantitative chemical microanalyses were
carried out on TEM. Figure 3共b兲 is a typical cross-sectional
TEM image with a Ni– Bi2共Te, Se兲3 interface shown as the
vertical white line on the left. Figure 3共c兲 illustrates some
typical EDS spectra in Bi2共Te, Se兲3 close to the interface.
Obviously, Ni diffuses into Bi2共Te, Se兲3 at least 3.5 ␮m.
The deep diffusion of Ni was also confirmed by EELS
measurements.
All the tested specimens were taken from an unused
Peltier device. We found that the deep diffusion of Ni in
FIG. 3. 共Color online兲 共a兲 SEM image of a cross-sectional n-type
Bi2共Te, Se兲3 leg. Ni and Sn EDS line scan profiles show the element distribution across the Ni barrier layer. 共b兲 TEM image of Ni– Bi2共Te, Se兲3 interface. 共c兲 EDS spectra in Bi2共Te, Se兲3. Each spectrum comes from the corresponding region marked as the same letter in 共b兲. The numbers indicate the
distances of EDS test regions from the interface.
n-type Bi2共Te, Se兲3 is caused by the soldering process which
occurs when the device is assembled. Usually, the soldering
is carried out at several tens of degrees above the solder
melting point 共about 138– 182 ° C depending on the manufacturer兲 for several minutes. In reality, n- and p-type legs
need to work for many years at a moderate temperature. Ni
would diffuse into n-type Bi2Te3 legs even deeper and degrade the performance of n-type Bi2Te3 legs. At the same
time, the Ni diffusion-barrier layer would disappear as more
Ni atoms diffuse into the Bi2共Te, Se兲3 materials. Once the Ni
is gone, the solder and Cu would diffuse into Bi2共Te, Se兲3
materials and disable the n-type legs in a very short time.
From this point of view, Ni is not a good diffusion-barrier
material for n-type Bi2共Te, Se兲3. This finding has not been
reported before.
According to a recent report on the reliability of Peltier
devices, the high failure rate of Bi-based Peltier devices9 was
attributed to the contamination of Cu 共electrode materials兲
and to the diffusion of solders. From this study, we believe
that the rapid diffusion of Ni in n-type Bi2共Te, Se兲3 instead of
Sn or Cu could be the source of the degradation of the ther-
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101910-3
Appl. Phys. Lett. 92, 101910 共2008兲
Lan et al.
FIG. 4. Ni concentration profile 共a兲 in p-type
共Bi, Sb兲2Te3 and 共b兲 in n-type Bi2共Te, Se兲3. Solid lines
are from Fick’s law. The filled squares are the experimental data.
moelectric properties in Peltier devices. We do not presently
know how the Ni diffusion affects the thermoelectric power
and electrical conductivity of Bi2共Te, Se兲3. However, it is
reasonable to presume that Ni behaves similar to other metals such as Pb in undoped Bi2Te3,10 Sn in Bi2共Te, Se兲3,6,11
and Cu in Bi2共Te, Se兲3.11 All of these dopants increase the
electrical conductivity, thus reducing the thermoelectric
power.
In order to compare the Ni diffusion in p-type
共Bi, Sb兲2Te3 and n-type Bi2共Te, Se兲3, Ni concentration is calculated from the EDS spectra and plotted in Fig. 4. Ni concentration monotonically decreases away from the
Ni– Bi2Te3 interface. It was clearly observed that Ni diffuses
much deeper in n- than in p-type Bi2Te3. According to Fick’s
law, the concentration of Ni within p- or n-type Bi2Te3 can
be expressed as ⳵␾共x , t兲 / ⳵t = D⳵2␾共x , t兲 / ⳵x2, where ␾共x , t兲 is
the concentration of Ni at the position x away from the
Ni– Bi2Te3 interface at time t, D = D0e−E/RT is the diffusion
coefficient of Ni in p- or n-type Bi2Te3, and E is the activation energy for diffusion. The solution of the diffusion equation in our cases is ␾共x , t兲 = ␾共x = 0兲erfc共x / 冑2Dt兲, where erfc
is the complementary error-function. In Figs. 4共a兲 and 4共b兲,
the solution was plotted as a solid line with diffusion length
冑2Dpt = 60.6 nm in p-type 共Bi, Sb兲2Te3 and 冑2Dnt = 2.5 ␮m
in n-type Bi2共Te, Se兲3, respectively. All of the experimental
data fell around the theoretical lines with good approximation. If we assume the soldering temperature T = 500 K
and soldering time t = 100 s and D0 = 1 ⫻ 10−3 – 5 ⫻ 10−3
m2 / s,5,12,13 the diffusion coefficients D p = 1.8⫻ 10−17 m2 / s
共E p ⬃ 1.3 eV兲 and Dn = 3.1⫻ 10−14 m2 / s 共En ⬃ 1.0 eV兲 with a
diffusion coefficient ratio of Dn / D p ⬇ 1700. The high diffusion coefficient Dn in n-type Bi2Te3 is comparable to that of
copper, gold, and silver in Bi2Te3.5,12,13
A possible explanation for the different diffusion coefficients is the conflicting activation energy in p- and n-type
Bi2Te3. The element doping in Bi2Te3 changes the chemical
bonds in the crystals, causing a different local distortion in
the crystal lattice. When Ni ions migrate in the p- and n-type
Bi2Te3, the local distortion of the crystal lattice affects the
activation energy E and induces a different diffusion coefficient D.
In summary, the element diffusion of Sn42Bi58 solder and
Ni barrier layer in p- and n-type Bi2Te3 was investigated
using electron microscopy and chemical microanalysis techniques. Nickel diffuses deeply into n-type Bi2Te3 while penetrating superficially into p-type Bi2Te3. The rapid diffusion
of Ni in n-type Bi2Te3 legs could be another important
source to degrade Peltier devices. The Ni diffusion-barrier
layer blocks the diffusion of Sn42Bi58 solder both in p- and
n-type Bi2Te3 effectively.
This work is sponsored by NSF NIRT 共No. 0506830兲
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