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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