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Fs-pulsed laser deposition of PbTe and PbTe/Ag thermoelectricthin films
A. Bellucci • E. Cappelli • S. Orlando •
L. Medici • A. Mezzi • S. Kaciulis •
R. Polini • D. M. Trucchi
Received: 14 November 2013 / Accepted: 16 May 2014 / Published online: 30 May 2014
� Springer-Verlag Berlin Heidelberg 2014
Abstract For the first time, thermoelectric thin films
were fabricated by femtosecond pulsed laser deposition
(fs-PLD) that represents a challenging technological solu-
tion for this application since it provides a correct film
stoichiometry compared to the starting target, capability of
native nanostructuring and a high deposition rate. In par-
ticular, this paper shows a preliminary work on PbTe and
PbTe/Ag thin films deposited at different substrate tem-
peratures by fs-PLD from a microcrystalline PbTe target.
Structural, morphological and compositional characteriza-
tions of the deposited films were performed to demonstrate
the formation of films composed by crystalline nanograins
(about 35 nm size) and characterized by a correct stoichi-
ometry. A remarkable deposition rate of 1.5 nm/s was
evaluated. The electrical conductivity and the Seebeck
coefficient (thermopower) were measured as a function of
operating temperature to derive the thermoelectric power
factor that was found to be less than a factor 2 with respect
to the bulk materials. Finally, a discussion about the
influence of compositional and structural properties of the
deposited films on the related thermoelectric performances
was presented.
1 Introduction
The possibility to exploit the waste heat represents an
important key in all the applications of thermal energy.
Thermoelectric (TE) devices play a very interesting role in
this field. Commercial bismuth telluride-based TE modules
are presently available, although they show low efficiency
and limited maximum operating temperature T B 500 K.
In the recent years, many research groups have been trying
to enhance the thermoelectric ZT figure-of-merit of specific
materials, equal to ða2r=jÞT , where a is the Seebeck
coefficient, r and j the electric and thermal conductivity,
respectively [1–3]. Obviously, it is important to decrease
the thermal conductivity j and to increase the power factor
P equal to a2r; these conditions can be achieved by
applying different shrewdnesses, such as material nano-
structuring or doping. But the optimization of all these
parameters that are strictly intercorrelated represents a
difficult challenge. The major interest has been directed to
bulk materials, and an improvement of their TE perfor-
mance has been already reported [4–6]. On the other hand,
TE thin films did not achieve yet a large diffusion, but the
possibility to obtain TE coatings represents an interesting
alternative also for future miniaturized thermal-to-electri-
cal conversion devices. Many deposition techniques have
A. Bellucci (&) � E. Cappelli � D. M. Trucchi
CNR-ISM, Via Salaria km 29.3,
00015 Monterotondo Stazione, Rome, Italy
e-mail: alessandro.bellucci@ism.cnr.it
A. Bellucci
Dipartimento di Fisica, Universita di Roma Sapienza, Piazzale
Aldo Moro 2, 00185 Rome, Italy
S. Orlando
CNR-ISM, U.O.S. Tito Scalo Zona Industriale,
85050 Tito Scalo, PZ, Italy
L. Medici
CNR–IMAA, Zona Industriale, 85050 Tito Scalo, PZ, Italy
A. Mezzi � S. Kaciulis
CNR –ISMN, Via Salaria km 29.3,
00015 Monterotondo Stazione, Rome, Italy
R. Polini
Dip. Scienze Tecnologie Chimiche, Universita di Roma Tor
Vergata, Via della Ricerca Scientifica, 1, 00133 Rome, Italy
123
Appl. Phys. A (2014) 117:401–407
DOI 10.1007/s00339-014-8526-9
been applied to obtain TE thin films, including molecular
beam epitaxy [7], thermal evaporation [8] and pulsed laser
deposition (PLD) [9]. PLD represents a very attractive
technique, since it allows the deposition of complex native
nanostructured films with good stoichiometry. However,
PLD assisted by ns-pulse laser results in low deposition
rates. The use of ultrashort-pulse laser (i.e. in the fs range)
and the possibility to exploit a high-pulse repetition rate
should overcome this limit and significantly increase the
deposition rate, owing to a far higher-pulse power density.
A preliminary study of lead telluride (PbTe) thin films
deposited by fs Ti:Sapphire laser is presented here. Our
attention has been focused on PbTe, since in the bulk
structure it shows one of the highest ZT in the temperature
range 300–800 K [2] that represents an useful range for
exploiting the heat waste in solar conversion and especially
in concentrated solar systems [10]. Nominally PbTe films
were used to understand the effect of deposition parame-
ters. The deposition of PbTe/Ag was attempted in order to
enhance the thermoelectric properties by improving the
electron transport properties [11–13].
2 Experimental
In this work, an ultrashort Spectra Physics Spitfire Pro XP
Ti:Sapphire pulsed laser source (wavelength k = 800 nm,
pulse duration of 100 fs; repetition rate of 1,000 Hz;
energy of 3.7 ± 0.1 mJ/pulse) for the deposition of PbTe
and PbTe/Ag samples was used. Using a multi-target sys-
tem, the laser beam was focused at an angle of 45�, on
1-inch diameter targets of PbTe (99.999 % purity) and Ag
(99.999 % purity). During the deposition, the targets were
continuously rotated to ensure uniform erosion over the
surface. Technical-grade alumina arranged in small plates
(10 9 10 9 1 mm3) and rods (1 mm diameter, 5 mm
length) was used as deposition substrate. All the substrates
were ultrasonically cleaned in n-hexane and mounted on a
sample holder heated to deposition temperatures Tdep from
RT to 523 K, at a distance of *50 mm from the target.
The temperature range has been chosen to not exceed
523 K that represents the maximum temperature achiev-
able by the chamber heating system. Before starting
deposition, the chamber was evacuated to values
\3.0 9 10-7 mbar. The deposition rate has been evalu-
ated to be *1.5 nm/s; this is a high value that allows to
obtain thickness of 1 lm in only 10 min of deposition.
Table 1 shows all the deposition parameters in detail.
For the doping process, a PbTe/Ag deposition time
ratio of 3:1 was considered as an acceptable attempt
to reach a heavy doping condition. A doping sequence
A(t1)B(t2)/[N*(A(t3)B(t2))]/A(t1), with A = PbTe, B = Ag,
t1 = 70 s, t2 = 20 s, t3 = 50 s and N = 8 was used to
realize a deposition that should promote the Ag diffusion
into the film.
X-ray microdiffraction (lXRD) data were acquired
using a Rigaku D/MAX RAPID diffraction system, oper-
ated at 40 kV and 30 mA. This instrument and collection
mode are described in [14]. The data were collected in
reflection mode using various sample-to-beam geometries
and operating conditions, obtaining results mutually con-
sistent with different acquisition parameters. The unit cell
parameters of the PbTe samples were refined from the
lXRD data using the UnitCell software [15].
A Zeiss field emission gun-scanning electron micro-
scope (FEG-SEM) Leo Supra-35 has been used for study-
ing the surface morphology. The images were obtained
under an acceleration voltage of 15 kV, with a magnifi-
cation until 100 k9.
XPS depth profiling was carried out in an ESCALAB
250Xi spectrometer (Thermo Fisher Scientific, UK),
equipped with monochromatic Al Ka excitation source and
a 6-channeltron spectroscopic detection system. XPS
measurements were carried out at 90� take-off angle, at a
large spot of X-ray source (0.9 mm) and electromagnetic
lens mode resulting in 0.5 mm diameter of analysed sample
area. The peak fitting of registered spectra was carried out
by using Shirley background and a mixture of Gaussian
and Lorentzian functions. For the depth profiling condi-
tions, an Ar? beam of 2.0 keV energy and a sample current
of 3 9 10-6 A were employed, which was rastered over a
sample area of 2 9 2 mm2. At these experimental condi-
tions, the sputter rate of the films determined by using
reference samples [16] was about 0.5 nm/s.
The temperature-dependent electrical characterization
was performed in a vacuum chamber using the ‘‘four-
contact-in-line-points probe’’ method. This method allows
the measurement of the sheet resistance and, once known
the thickness, of the resistivity. Measurements have been
performed varying the temperature T, from room temper-
ature (RT) up to 520 K (the maximum temperature that is
possible to reach in this set-up), under vacuum conditions,
to avoid film structural modifications and oxidation. The
set-up scheme and the details of the method are reported in
a previous paper [17].
Table 1 The main deposition parameters for the fs-PLD
Name Tsubstrate
(K)
Ratio
PbTe/Ag
Deposition time
(min)
Thickness
(lm)
PbTe_01 RT – 20 1.72 ± 0.05
PbTe_02 448 – 20 1.76 ± 0.05
PbTe_03 523 – 20 1.89 ± 0.05
PbTe_04 523 3:1 12 0.95 ± 0.05
402 A. Bellucci et al.
123
The temperature-dependent Seebeck coefficient mea-
surements were performed using a commercial set-up,
MMR technologies K-20 and SB-100 with a high-imped-
ance amplifier (30 gain), according to the description
method reported in [18]. The temperature range was
300–600 K.
3 Results and discussion
All the XRD spectra performed on deposited films show a
defined pattern where the major peaks are attributable to
the PbTe compound in the cubic structure (Fig. 1), coher-
ently with other works present in the literature [19]. Minor
peaks, mostly visible in the PbTe_04 spectrum, have to be
assigned to the Al2O3 substrate. The presence of silver in
PbTe_04 does not change significantly the diffraction
patterns, apart a decrease in the peak intensities.
The grain size analysis, shown in Fig. 2, highlights that
the films are nanostructured. This finding represents an
optimal starting point, since it was demonstrated that the
nanostructuring is able to reduce the heat phonon transport,
by lowering the lattice thermal conductivity as necessary
[1, 2]. The size of the grains was determined by using the
Scherrer’s formula [20] and evaluating the full width at
half-maximum (FWHM) of the (200) cubic PbTe peak: the
grain size increases with increasing deposition tempera-
tures as well as the order of crystallinity. The highest value
is 39 nm, obtained at Tdep = 250 �C. This grain size is
�100 nm that was observed to be the desirable condition
to reduce the lattice thermal conductivity [3]. From this
point of view, the fs-PLD demonstrated to be a promising
technique for obtaining samples with enhanced TE per-
formance. From the analysis of the sample morphology,
performed by FEG-SEM, all the film surfaces have a
granular aspect. In Fig. 3, the PbTe sample deposited at RT
and the PbTe/Ag deposited at 550 K are shown.
The deposition temperature does not influence signifi-
cantly the surface morphology: in fact, the grown film
covers the irregular microstructure of the substrate
(Fig. 3d), whereas a cross-sectional image (Fig. 3c) reveals
a dense and compact film structure.
The XPS depth profile of the sample PbTe_02 (Fig. 4a)
shows a stoichiometric ratio Pb:Te = 1:1. The same results
were obtained for the samples PbTe_01 and PbTe_03.
Apart from enrichment in Pb, observed at the surface, the
Te/Pb concentration ratio is 1.0 ± 0.1. In the PbTe/Ag
samples, a high amount of silver is present. Evaluating the
ratios of Te/Pb, Pb/Ag and Te/Ag for the sample PbTe_04
(Fig. 4b) and considering a deposition time ratio 3:1 of
PbTe/Ag, we observe that the deposition yield of Ag is
higher than that of PbTe. If we do not consider the topmost
layer of the film, where Pb segregation is present, the
atomic concentration of Ag is higher than both of Pb and
Te. In particular, one atom of Pb corresponds to 2.5 atoms
of Te and 3.5 atoms of Ag. Considering the XRD spectrum
of the sample, where the PbTe cubic pattern is evident, we
can conclude that Ag atoms partially substitute Pb atoms in
the PbTe matrix, and in the majority case, they create
metallic aggregates or are localized in interstitial sites.
Fig. 1 XRD patterns of the samples. The shown data were acquired
using 50-lm collimator, 1 h of collection time, a fixed revolution
angle x (3�) and rotation angles U (60�, 75�, 80�, 90�). The peak
assignments were made using JCPDS database
Fig. 2 Grain size evaluation as a function of the deposition
temperature for all the samples
Fs-pulsed laser deposition 403
123
In order to evaluate the power factor, electrical con-
ductivity and thermopower were measured as a function of
temperature, as described in the experimental section. The
evaluation of the thermopower (Fig. 5) allows also the
determination of the influence of the majority carriers to
the transport mechanism. PbTe films show a positive
Seebeck coefficient that is an indication of a p-type con-
duction, while the PbTe/Ag sample has negative a value
that is a mark of a n-type conduction. The patterns for PbTe
films show a maximum at intermediate temperatures. In
terms of absolute values, the thermopower of PbTe samples
(apart from the one deposited at 523 K) is higher than that
of PbTe/Ag sample. The large Seebeck coefficient values
for PbTe films would indicate a hole concentration prob-
ably \1019 cm-3, whereas relatively low values for PbTe/
Ag film could be induced by a too high electron concen-
tration (probably [1019 cm-3) [2, 12].
The electrical analysis, reported in Fig. 6 as a function
of T-1, clearly points out two different behaviours: the
PbTe samples show a semiconductor trend, characterized
by an increasing conductivity as a function of T. The higher
is the deposition temperature, the higher is the electrical
conductivity at comparable temperatures (inset of Fig. 6
shows the data measured at RT). Conversely, PbTe/Ag
sample shows a metallic behaviour, probably due to a
degenerate doping and/or a percolation mechanism through
silver aggregates. In any case, r value is far lower than that
of pure Ag. Moreover, pure Ag shows a positive Seebeck
coefficient [21], characterized by an opposite sign
compared to the values obtained with PbTe_04 sample.
Therefore, it can be excluded the case of a conduction
assisted exclusively by aggregated Ag atoms at grain
boundaries. In case of a degenerate semiconductor, the
approximation ðEC � EFÞ � kBT (kBT is the thermal
energy where kB is the Boltzmann constant) is not more
valid. It means that the Fermi level EF is over the minimum
of the conduction band EC. When this condition is reached,
an enhancement of the electrical conductivity is active
[11].
Figure 6 reports the experimental curves r(T-1) in a
semi-logarithmic plot, from which it is evident that the
patterns for PbTe films can be fitted by an Arrhenius
equation rðTÞ / expðEA=ðkBTÞÞ, where EA is an activation
energy that can be interpreted as the energy distance of the
Fermi level from the valence band maximum EV for PbTe
samples, showing a p-type transport. The values of EA
range from 150 to 40 meV as a function of deposition
temperature. Figure 7 depicts the variation of the energy
band diagram for nominal PbTe samples as a function of
Tdep, where EG = 311 meV is the PbTe bandgap at RT
[22]. Considering the Fermi level position, it is interesting
to notice that the films are lightly p-doped for
Tdep \ 448 K, whereas they show a heavily p-doped, but
not degenerate, behaviour at Tdep = 523 K. This is the
reason for the highest electrical conductivity in PbTe
samples.
Conversely, r(T) patterns relating to PbTe/Ag sample
reported in Fig. 6 can be fitted by a linear dependence
Fig. 3 FEG_SEM images:
a PbTe_01 (scale bar 200 nm);
b PbTe_04 (scale bar 100 nm):
c cross-section PbTe_04 (an
average value of the thickness,
equal to 950 nm, is marked);
d alumina substrate (scale bar
1 lm)
404 A. Bellucci et al.
123
rðTÞ / T�1, typical for degenerate semiconductors. In
PbTe_04, the presence of silver atoms induces a decrease
in the thermopower due to a very high free-carrier con-
centration, probably to values [1019 cm-3, causing low
thermopower values and high conductivity.
Hall measurements on samples differing in the per-
centage of silver concentration are going to be performed
in order to confirm this analysis and to clarify the role of
dopant within the PbTe matrix, fabricated by fs-PLD.
In Fig. 8, it is observed that the power factor P as a
function of temperature assumes similar values for all the
PbTe films, while for PbTe/Ag, it shows values higher than
one order of magnitude. The deposition temperature in
PbTe films induces an increase in the electrical
conductivity, but a decrease in the absolute value of the
Seebeck coefficient. This results in a compensation of the
P(T) values.
Fig. 4 a Te/Pb atomic concentration ratio for PbTe_02, similar for
all the PbTe samples. The atomic concentration ratio is (1 ± 0.1)
throughout the thickness of the film, less of the value at the surface;
b different atomic concentration ratios obtained by XPS depth profile
for PbTe_04. Ag atoms partly substitute Pb atoms in the cubic PbTe
structure and partly create metallic aggregates
Fig. 5 Thermopower (Seebeck coefficient) evaluation; it shows a
p-type conduction for PbTe samples and n-type conduction for PbTe/
Ag sample
Fig. 6 Measurement of electrical conductivity r as a function of T-1;
the PbTe/Ag sample shows a metallic behaviour, while the PbTe
samples have a semiconductor trend. The continuous lines refer to the
best fit of Arrhenius equation for PbTe films and to a linear fit for
PbTe/Ag film
Fs-pulsed laser deposition 405
123
PbTe_04 takes benefit from the high conductivity, but
show limited Seebeck coefficients. The reduction of the
silver content in the future represents the keytool to induce
the proper carrier concentration and to avoid possible Ag
aggregates, which induce an increase in electrical con-
ductivity, but do not contribute to an improved Seebeck
coefficient. Moreover, it is necessary to find the correct
compromise between large thermopower and high electri-
cal conductivity without reaching degenerate conditions. In
any case, the values P(T) of PbTe/Ag are only half of those
reported for thermoelectric bulk materials [6, 23].
4 Conclusions
PbTe and PbTe/Ag thin films have been deposited by fs
Ti:Sapphire PLD. Fs-PLD represents a useful and inter-
esting technique for the fabrication of thermoelectric films,
also for its high deposition rate. XPS characterization
points out the achievement of a correct stoichiometry for
the PbTe samples. XRD data show a long-range nano-
crystalline structure, with a grain size from 25 to 39 nm.
The dependence of the TE performance of nominal PbTe
films on the deposition temperature seems to be compen-
sated, while the silver content enhances the power factor of
the films. Nevertheless, the carrier concentration and
mobility measurements have to be performed in a Hall set-
up in order to find a good compromise between electrical
conductivity and Seebeck coefficient and to make a deeper
explanation of the observed behaviour. Finally, the mea-
surements of thermal conductivity would be fundamental
to quantify the absolute values of ZT.
Acknowledgments This work was supported by the European
Project E2PHEST2US (Grant Agreement no. 241270), funded in the
context of the Seventh Framework Programme.
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