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Bala, Manju; Bhogra, Anuradha; Khan, Saif A; Tripathi, Tripurari S.; Tripathi, Surya K.;
Avasthi, Devesh K.; Asokan, Kandasami
Enhancement of thermoelectric power of PbTe thin films by Ag ion implantation
Published in:
Journal of Applied Physics
DOI:
10.1063/1.4984050
Published: 07/06/2017
Document Version
Publisher's PDF, also known as Version of record
Please cite the original version:
Bala, M., Bhogra, A., Khan, S. A., Tripathi, T. S., Tripathi, S. K., Avasthi, D. K., & Asokan, K. (2017).
Enhancement of thermoelectric power of PbTe thin films by Ag ion implantation. Journal of Applied Physics,
121(21), [215301]. https://doi.org/10.1063/1.4984050
Enhancement of thermoelectric power of PbTe thin films by Ag ion implantation
Manju Bala, Anuradha Bhogra, Saif A. Khan, Tripurari S. Tripathi, Surya K. Tripathi, Devesh K. Avasthi, and
Kandasami Asokan
Citation: Journal of Applied Physics 121, 215301 (2017); doi: 10.1063/1.4984050
View online: https://doi.org/10.1063/1.4984050
View Table of Contents: http://aip.scitation.org/toc/jap/121/21
Published by the American Institute of Physics
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Enhancement of thermoelectric power of PbTe thin films by Ag ion
implantation
Manju Bala,
1
Anuradha Bhogra,
1
Saif A. Khan,
1
Tr ipurari S. Tripathi,
2
Sur ya K. Tripathi,
3
Devesh K. Avasthi,
4
and Kandasami Asokan
1
1
Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India
2
Aalto University, V
€
armemansgr
€
anden 2, 02150 Esbo, Finland
3
Department of Physics, Panjab University, Chandigarh 160 014, India
4
Engineering and Technology, Amity University Campus, Noida 201313, India
(Received 20 January 2017; accepted 10 May 2017; published online 1 June 2017)
Enhancement of the figure of merit (ZT) of thermoelectric materials is the topic of current research
in energy studies. We report an enhancement in the thermoelectric power (TEP) of thermally
evaporated PbTe thin films by low energy Ag ion implantation. This implantation results in
PbTe:Ag nanocomposites. Implantations were carried out at a 130 keV Ag ion beam with ion
fluences of 3 10
15
, 1.5 10
16
,3 10
16
, and 4.5 10
16
ions/cm
2
. The atomic concentrations were
determined using Rutherford backscattering and found to be 1 at. %, 5 at. %, 10 at. %, and 14 at. %
in the implanted PbTe films. Scanning electron microscopy images show the presence of fine
cracks on the surface of as-deposited PbTe thin films that get shortened and suppressed and finally
disappear at higher fluences of Ag ion implantation. The TEP measurements, from 300 K to 400 K,
show 25% enhancement in the Seebeck coefficient of the Ag ion implanted films in comparison
to the pristine PbTe thin film. The synchrotron based high resolution X-ray diffraction and X-ray
photoelectron spectroscopy investigations reveal the formation of Ag
2
Te in the surface layer after
Ag ion implantation. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4984050]
I. INTRODUCTION
The advancement in nanotechnology relies on the down-
scaling of semiconductor devices towards nanometer dimen-
sions for better performance and lower costs. In recent years,
to achieve better device performance, implantation technology
is recognized as a key for introducing dopants, since diffusion
of impurities generally requires very high temperatures. Ion
implantation has been effectively used in semiconductor tech-
nology to modify the electrical transport properties.
1,2
Energetic ions penetrate the surface of th e target (host) mate-
rialandcometorestinanapproximatelyGaussiandepthdis-
tribution. In addition to the solute pro file so-introduced, the
incoming ions leave a tra il of damage as these ions penetrate
in the materi al.
3
The ions while passing through the material
lose their energy mainly by elastic collisions. Ion implantation
thus can modify the physical, chemical, or electrical properties
of the target material. The impla nted species in the semicon-
ductor can make solutions or compounds with the host mate-
rial and change the charge carrier type and density. For a
p-type dopant, a hole is created, and for an n-type dopant, an
electr on is created in the target material. This alters the local
conductivity of the material. Ion implantation can be employed
in thermoelectric materi als as a process to produce controlled
modifications to alter the lattice and charge carriers.
Thermoelectric materials convert heat (temperature differ-
ences) directly into electrical energy, and a good thermoelec-
tric material must have high electrical conductivity (r), high
Seebeck Coefficient (S), and low therm al conductivity (j). The
efficiency of a given material to produce a thermoelectric
power is governed by its “figure of merit” ZT ¼ S
2
rT/j.
4,5
For
many years, the main three semiconductors known to have
both low thermal conductivity and high power factor (S
2
r)
have been bismuth telluride (Bi
2
Te
3
), lead telluride (PbTe),
and silicon germanium (SiGe).
6
Among them, PbTe has
proven to be a very important intermediate thermoelectric
material. It crystallizes in the NaCl crystal structure with Pb
atoms occupying the cation and Te forming the anionic lat-
tice. It is a narrow gap semiconductor with a bandgap of
0.32 eV.
7
The PbTe system can be optimized for power gen-
eration applications by improving the power factor via band
engineering. It can be made either n-type or p-type with
appropriate dopants. Halogens are often used as n-type dop-
ing agents. PbCl
2
, PbBr
2
, and PbI
2
are commonly used to
produce donor centers. The common p-type doping agents
are Na
2
Te, K
2
Te, and Ag
2
Te. Besides all these elements, Ag
has been found to be one of the potential dopants to enhance
the thermoelectric properties of PbTe. It has been reported
by Heremans et al. that Ag doping in PbTe acts as a p-type
dopant at low concentrations (<a few 10
19
cm
3
) but at
higher concentrations, (>10
19
cm
3
), it results in n-type.
8
Enhancement in thermoelectric power through the energy
barrier scattering in Ag doped PbTe was demonstrated by
Martin et al.
9
The effect of Ag and Sb doping on the thermo-
electric properties of PbTe was reported, and the maximum
ZT ¼ 0.27 at 723 K for Pb
1x
Ag
x
Te alloys when x ¼ 0.1 was
estimated.
10
Hence, Ag doping in PbTe has been proven to
improve thermoelectric properties by forming PbTe:Ag
nanocomposites.
11,12
Further, ZT of PbTe can also be
boosted by reducing its thermal conductivity by introducing
point defects, nanoscale precipitates, and mesoscale grain
boundaries.
13
These act as effective scattering centers for
phonons with different mean free paths, without affecting
0021-8979/2017/121(21)/215301/9/$30.00 Published by AIP Publishing.121, 215301-1
JOURNAL OF APPLIED PHYSICS 121, 215301 (2017)
charge carrier transport. By applying this method, the highest
value of ZT for PbTe that has been achieved in the Na doped
PbTe-SrTe system is approximately 2.2.
14
Ion implantation can be employed in thermoelectric
materials to modify their lattice thermal conductivity and
charge carriers. It is used to create an implanted buried thin
layer and change the properties of PbTe materials. Shen
et al. showed that Sn
þ
ion implanted PbTe can create a Pb
1-
x
Sn
x
Te thin layer which lowers the lattice thermal conductiv-
ity and hence improves the ZT of PbTe.
15
It was reported by
Kato et al. that n-type PbTe implanted with 120 keV Te
þ
ions shows p-type character at a fluence of 1 10
16
ions/
cm
2
.
16
Similarly, Donnelly et al. reported that Sb
þ
ion
implantation can convert layers of p-type PbTe into n-type.
17
However, there has been no report on the effect of these
implantations on the thermoelectric properties of PbTe films.
The present study aims to understand the thermoelectrical
properties of Ag ion implanted PbTe thin films and also the
morphological, structural, and electrical modifications at dif-
ferent doses.
II. EXPERIMENTAL
Thin films of PbTe were deposited on quartz substrates
at room temperature by the thermal evaporation method at a
base pressure of 2 10
5
mbar. The Ag ions were incorpo-
rated in PbTe thin films by Ag ion implantation using the
Negative Ion Implanter (NII) facility available at the Inter-
University Accelerator Centre (IUAC), New Delhi. The
implantation was carried out at room temperature and the
pressure was maintained at 10
4
Pa. The PbTe films were
implanted with 130 keV Ag
ions to 4 different fluences:
3 10
15
, 1.5 10
16
,3 10
16
, and 4.5 10
16
ions/cm
2
for
implanting 1, 5, 10, and 14 at. % of Ag in PbTe, and these
films will be referred to hereafter as PbTe-I:1Ag, PbT e-
I:5Ag, PbTe-I:10Ag, and PbTe-I:14Ag.
Rutherford backscattering (RBS) spectrometry was per-
formed using 2 MeV He
þ
ions at a scattering angle of 165
at IUAC, New Delhi, for compositional studies. The
Rutherford manipulation program (RUMP) simulation code
was used to simulate the experimental RBS spectra. The sur-
face morphology of the thin film was examined usin g scan-
ning electron microscopy (SEM) by means of a TESCAN
MIRA II LMH microscope. X-ray diffraction (XRD) meas-
urements were performed at a grazing incident angle of 2
to
identify the crystalline phases in the films using a Bruker D8
advance diffractometer with a Cu Ka (1.54 A
˚
) X-ray source
at a scan speed of 0.5
/min. The electrical resistivity (q) and
thermoelectric power (S) of the films were measured in the
temperature range of 300–400 K using a standard DC four
probe technique and bridge method,
18
respectively. The Hall
Effect measurements were carried out using a magnetic field
of 0.57 T at room temperature to evaluate carrier density and
mobility. The high resolution XRD measurement was carried
out at the synchrotron radiation facility of KEK, Japan, using
13.9 keV. The X-ray photoelectron spectroscopy (XPS) study
was performed for the elemental composition on the surface
and the electronic structure of the films using a PHI 5000
Verse Probe II system and Al radiation at 100 W power at
Indian Institutes of Technology, Kharagpur.
III. RESULTS
A. Compositional analysis
Figure 1 shows RBS raw data of the as-deposited and
Ag-implanted PbTe films on the SiO
2
substrate. The peak
position of an element in RBS is governed by its atomic
number and its depth distribution. The as-deposited and
implanted thin films have high atomic number elements Pb
(82), Te (52), and Ag (47); therefore, their positions on the
RBS spectra are quite close and as the films are thick
enough, the peaks of Pb, Te, and Ag overlap each other and
form a broad peak; thus, it is difficult to resolve ind ividual
peak without fitting or simulation. As a consequence of over-
lapping, the signals from Ag in the spectra of implanted films
are not clearly noticeable. However, a close observation of
the tail at the inner edge of the broad peak shows a change in
the slope for implanted samples, indicating the presence of
Ag atoms. In Fig. 1, the surface energies for Pb, Te, and Ag
are represented by arrows, and the inset in Fig. 1 shows the
spectra of pristine PbTe and PbTe-I:5Ag films. All the films
were fitted using the RUMP simulation to determine the
thickness of the films, atomic percent, and the depth profiling
of Ag. There exist differences in thickness among the inves-
tigated samples as seen from the RBS spectra, i.e., the thick-
ness varies from 340 nm to 360 nm. However, since the
differences can hardly be correlated with the ion doses and
considering that the film was deposited by thermal evapora-
tion system in which the substrate is positioned perpendicu-
larly to the target without substrate rotation, it is assumed
that the difference in thickness exists originally in the film
before implantation. However, the measured thickness of the
PbTe-I:14Ag film is quite small 245 nm with respect to
other implanted films because of sputtering of few atomic
FIG. 1. RBS spectra of PbTe, PbTe-I:1Ag, PbTe-I:5Ag, PbTe-I:10Ag, and
PbTe-I:14Ag thin films, and the inset shows the magnified RBS spectra of
PbTe and PbTe-I:10Ag for a selected region.
215301-2 Bala et al. J. Appl. Phys. 121, 215301 (2017)
layers by ion irradiation at the fluence of 4.5 10
16
ions/
cm
2
. In RBS, it is difficult to present more than one spectrum
along with their simulated data. Either raw data can be plot-
ted in one graph or raw data with their simulated data.
Therefore, only the raw data of PbTe and PbTe-I:5Ag films
are shown in the inset of Fig. 1 and their simulated spectra
are shown separately in Figs. 2(a) and 2(b). The simulation
shows the presence of Ag in the PbTe-I:5Ag thin film. Using
the transport of ions in matter (TRIM) simulation,
19
it is
observed that the 130 keV Ag ion comes to rest at a depth of
50 nm (ion range) in the PbTe film and forms an approxi-
mately Gaussian distribution up to a depth of 100 nm
beneath the surface due to straggling (Fig. 3). Therefore, for
getting a fairly precise dopant concentrations and their distri-
bution, the RUMP simulation has been done by dividing the
film into several sublayers containing different Ag concen-
trations and simulating separately each layer to get the best
curve fitting. The depth distribution of Ag in the PbTe film
obtained from the RBS simulation for all the Ag implanted
PbTe films is shown in Fig. 4(a). The unit of the y axis has
been changed to the atomic fraction of Ag for a more clear
representation. The peak concentrations of Ag are 0, 1, 5,
10, and 14 at. % for the fluences 3 10
15
, 1.5 10
16
,
3 10
16
, and 4.5 10
16
ions/cm
2
, respectively. At higher
fluences, the sputtering is significant and hence these higher
fluences are not included for further studies. The atomic den-
sity assumed for the RBS simulation for finding the thickness
is Pb-3.29 10
22
atoms/cm
3
, Te-2.95 10
22
atoms/cm
3
, and
Ag-5.84 10
22
atoms/cm
3
. The estimate d atomic percen-
tages of Pb, Te, Ag, and O atoms for all the films are also
given in Table I. Figure 4(b) shows the theoretically
FIG. 2. RBS spectra of PbTe and PbTe-I:5Ag fitted using the RUMP simula-
tion (black line shows the raw data and red line shows its simulation).
FIG. 3. TRIM simulation for atomic distribution of Ag in implanted PbTe.
FIG. 4. Simulated depth profiles of Ag ions in PbTe, PbTe-I:1Ag, PbTe-
I:5Ag, PbTe-I:10Ag, and PbTe-I:14Ag as obtained from the (a) RUMP sim-
ulation and (b)TRIDYN code.
215301-3 Bala et al. J. Appl. Phys. 121, 215301 (2017)
