Enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films

Abstract

The present study focuses on the enhancement of thermoelectric power of PbTe:Ag nanocomposite thin films, synthesized by the thermal evaporation technique. Thermoelectric measurements were carried out from room temperature to 400 K. It is observed that Ag addition improves the thermoelectric power and crystalline nature of the PbTe thin films. Synchrotron based X-ray diffraction was performed to confirm the phases of the Pb–Ag–Te alloy. This was further reconfirmed by X-ray photoelectron spectroscopy (XPS) and showed the precipitation of Pb on the surface of the PbTe:Ag films. The enhancement of thermoelectric power is thus attributed to the formation of Ag_2−xTe alloy and the precipitation of Pb nanostructures on the surface. The origin of such enhancement is understood based on the phenomenon of energy dependent filtering of charge carriers.

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1. Introduction

PbTe is a semiconductor with a narrow bandgap (0.25 eV at 0 K). It is one of the most favorable thermoelectric materials for converting waste heat into electricity over a wide range of temperatures (500–900 K).¹ PbTe contains a high concentration of charge carriers due to the presence of electrically active lattice defects as a consequence of its non-stoichiometric nature. Samples containing excess of Pb with respect to the stoichiometric composition results in n-type, while those with excess of Te in p-type.² In general, the efficiency of a thermoelectric material is described in terms of a dimensionless parameter known as thermoelectric figure of merit (zT). It is expressed as, zT = S²T/ρ(κ_E + κ_L), where S is the thermoelectric power (Seebeck coefficient), ρ is the electrical resistivity, κ_E and κ_L are the electronic and lattice components of the thermal conductivity (κ = κ_E + κ_L), respectively and T is the absolute temperature.³ Thus for better thermoelectric performance, a large value for S, and small values of ρ and κ are required. It is a major challenge to increase zT by optimizing the parameters like S, ρ and κ_E because these are interrelated through the carrier concentration (n).⁴ In low dimensional systems, these parameters can be tuned independently. Since, last few decades many efforts have been made to enhance the thermoelectric properties of PbTe using various approaches. One of the approaches is the incorporation of nano-inclusions. These nano-inclusions enhance the thermoelectric power due to the low energy carrier filtering and/or act as phonon scattering centers and hence reduce the thermal conductivity.^5,6 Recently, there has been extensive research on PbTe, for the purpose of improving its thermoelectric energy conversion efficiency. Lalonde et al.⁷ achieved significant improvement in zT (1.4 at ∼700 K) of p-type PbTe by carefully controlling the carrier density through iodine doping. Joseph Heremans et al. demonstrated that using Tl impurity in p-type PbTe, one can attain zT ∼ 1.5 at 773 K.⁸ Similarly, Snyder et al. reported zT ∼ 1.4 at 750 K in Na doped PbTe,⁹ and zT ∼ 1.8 at 850 K in Na doped PbTe_1−xSe_x alloy.¹⁰ Tavrina et al. studied Pb_1−xBi_xTe system and observed the effect of the Bi element on thermoelectric properties of PbTe and reported the maximum values of thermoelectric power factor F = 37 μW cm⁻¹ K⁻² for x ∼ 0.0025.¹¹ Beside all these elements, Ag has also been considered as one of the potential elements to enhance the thermoelectric properties of PbTe. It has been reported that Ag in PbTe at low concentrations (<a few 10¹⁹ cm⁻³) acts as a p-type dopant but at higher concentrations, i.e., in the range of 10¹⁹ cm⁻³, results in n-type.¹² Noda et al. showed that Ag addition in PbTe results in z ∼ 1.38 × 10⁻³ K⁻¹ with hole concentration of 1.34 × 10¹⁹ cm⁻³.¹³ Martin et al. also demonstrated enhancement in thermoelectric power through the energy barrier scattering in Ag incorporated PbTe.⁵ Dow et al. studied the effect of Ag and Sb addition on the thermoelectric properties of PbTe and estimated the maximum zT = 0.27 at 723 K for Pb_1−xAg_xTe alloys for x = 0.1.¹⁴ It may be noted that, so far most of the interest for PbTe composite was in the form of bulk nanostructured materials. PbTe thin films have also been known to enhance thermoelectric properties as compared to bulk PbTe due to quantum confinement effects.¹⁵ Thermoelectric materials in the form of thin films are suitable for specific applications and there are only limited studies on thin films. Therefore, the purpose of the present study is to synthesize, characterize and investigate the effect of Ag addition on the thermoelectric power of PbTe thin films. The present work is motivated by earlier work of Dow et al., who showed that PbTe in the bulk form gives best thermoelectric results for 10% Ag, therefore it would be interesting to study the effect of Ag using 10% and one higher concentration (20%) in PbTe thin films.

2. Experimental procedure

PbTe, and Ag incorporated PbTe thin films were synthesized by thermal evaporation method on quartz substrates at a pressure of ∼2 × 10⁻⁵ mbar. For convenience hereafter, the pristine PbTe, 10% and 20% Ag added PbTe thin films will be referred as PbTe, PbTe:10Ag and PbTe:20Ag, respectively. Rutherford backscattering spectrometry (RBS) was performed using 3 MeV He⁺ ions at scattering angle of 165° at IUAC, New Delhi for compositional studies. Rutherford manipulation program (RUMP) simulation code was used to simulate the experimental RBS spectra. X-ray diffraction (XRD) measurements were performed at grazing incident angle of 2° to identify the crystalline phases in the films using a Bruker D8 advance diffractometer with Cu Kα (1.5406 Å) X-ray source at a scan speed of 0.5° min⁻¹. High resolution XRD measurement was also carried out at synchrotron radiation facility of KEK (Japan) using 13.9 keV energy. X-ray photoelectron spectroscopy (XPS) study was performed for the elemental composition on the surface of the sample using a VG instrument having system resolution ∼0.9 eV with a Mg source, at IOP, Bhubaneswar. The surface morphology of thin film was examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM) in tapping mode. The electrical resistivity (ρ) and thermoelectric power (S) of the films were measured in the temperature range of 300 to 400 K using a standard DC four probe technique and bridge method,¹⁶ respectively. The Hall effect measurements were carried out using a magnetic field of 0.57 Tesla at room temperature to evaluate carrier concentration and mobility.

3. Results and discussion

3.1 Compositional study

RBS spectra (with RUMP simulation) of the three thin films PbTe, PbTe:10Ag and PbTe:20Ag are shown in Fig. 1. The simulation suggests that the thickness of these films are ∼60–70 nm. The estimated atomic percentages of Pb, Te, Ag and O ions in these films are given in Table 1. As evident Pb and Te ions exhibit deviations from expected stoichiometry and excess of Te ions. The Ag contents are found to be ∼11.6 at% and 19.3 at% in PbTe:10Ag and PbTe:20Ag, respectively. The RUMP simulation also shows the presence of ∼17–19 at% of O in all these films. Such O contamination is known to occur during thermal evaporation process.¹⁸

Fig. 1 The dots show the experimental data and continuous line corresponds to simulation by RUMP for (a) PbTe (b) PbTe:10Ag and (c) PbTe:20Ag thin films.

Table 1 Elemental compositions of PbTe, PbTe:10Ag, and PbTe:20Ag thin films determined by RBS simulation

Sample	Pb (at%)	Te (at%)	Ag (at%)	O (at%)
PbTe	39.6	41.6	0	18.8
PbTe:10Ag	34.4	37.1	11.6	16.9
PbTe:20Ag	31.1	32.6	19.3	17.0

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3.2 Phase study

The XRD spectra of PbTe, PbTe:10Ag and PbTe:20Ag films are shown in Fig. 2(a). The characteristic peaks in the spectra indicate the crystal structure of face centred cube (fcc) NaCl-type. The highest intensity peak at 2θ = 27.57° suggests a preferential growth along the (200) planes. The XRD spectra of PbTe:10Ag and PbTe:20Ag are similar to that of PbTe. No peak corresponding to Ag or its alloys is observed in XRD spectra. An increase in peak intensity and decrease in full width half maxima (FWHM) was observed in the spectra (Fig. 2(b)) for PbTe:10Ag and PbTe:20Ag which imply an increase in the crystalline nature of PbTe on adding Ag. The average size of the crystallites in thin films were determined by using Scherrer equation, L = Kλ/β cos θ_B, where L is crystallite size, K is a constant, λ is the wavelength of the X-ray, β is the half width of a diffraction peak, and θ_B is the diffraction angle. It is observed (Fig. 3) that the grain size increases with Ag addition but not in regular manner. The estimated values of grain size are ∼18.6 nm, 22.2 nm and 21.4 nm, respectively, for PbTe, PbTe:10Ag and PbTe:20Ag corresponding to (2 0 0) plane, as given in Table 2. The increase in grain size in PbTe:10Ag and PbTe:20Ag films may be due to the diffusion of Ag-ion into PbTe during evaporation, which provide nucleation site for grain growth and hence increases the crystallinity.¹⁷ XRD spectra obtained using lab source (Cu Kα) for PbTe:Ag samples do not show any peak of Ag or its alloy. Therefore high resolution XRD measurement was carried out at synchrotron radiation facility at KEK (Japan) using 13.9 keV energy to detect the small contents of Ag and its alloy. Synchrotron XRD spectra of PbTe:20Ag film showed two extra peaks at 2θ = 5.84° and 7.02° (Fig. 4) which correspond to either Ag₇Te₄ (JCPDS75-1022), Ag_1.85Te (JCPDS108-1186) or Ag₅Te₃ (JCPDS86-1168). Thus, these spectra suggest the presence of Ag_2−xTe phase in PbTe:Ag samples. The Ag–Te alloy formation is further confirmed by XPS.

Fig. 2 XRD spectra of thin films of PbTe, PbTe:10Ag and PbTe:20Ag. (a) Spectra showing 2θ from 20 to 60° and (b) closer view of the peak corresponding to (2 0 0) plane.

Fig. 3 Variation of grain sizes of PbTe on adding Ag in PbTe thin films.

Table 2 Peak position, FWHM and grain size of PbTe, PbTe:10Ag and PbTe:20Ag thin films

Sample	Peak position 2θ (Deg.)	FWHM (Deg.)	Crystallite size (nm)
PbTe	27.58	0.43	∼18.62
PbTe:10Ag	27.58	0.36	∼22.20
PbTe:20Ag	27.58	0.38	∼21.38

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Fig. 4 Synchrotron XRD spectra of PbTe and PbTe:20Ag measured at KEK (Japan) using energy of 13.9 keV.

The XPS spectra of PbTe, PbTe:10Ag and PbTe:20Ag films are shown in Fig. 5. For PbTe film two groups of peaks assigned to Pb (4f_7/2) and Pb (4f_5/2) are shown in the Fig. 5(a). Each group contains a shoulder peak on the lower binding energy side. Deconvoluting the Pb (4f_7/2) peak by curve fitting distinguishes two components, one at 137.9 and another at 139.1 eV which correspond to PbTe (Pb²⁺) and PbO (Pb²⁺)¹⁸ respectively. Te (3d_5/2) and Te (3d_3/2) peaks (Fig. 5(a)) centered at 576.1 eV and 586.5 eV respectively, corresponding to TeO₂ (Te⁴⁺). Two additional peaks at 572.4 eV (3d_5/2) and 582.7 eV (3d_3/2) are also observed corresponding to PbTe (Te²⁻). The XPS data show the existence of PbO, TeO₂ and PbTe on the surface of the pristine film. This assignment is in good agreement with the data of Wu et al.¹⁹ Fig. 5(b) shows the XPS spectra of PbTe:10Ag. The Pb peaks corresponding to PbTe and PbO are found to be nearly at the same position as in pristine film. A shoulder peak on the higher binding energy side of Te peak is observed. Deconvoluting the Te (3d_5/2) peak distinguishes two components at 571.9 eV and 573.0 eV, corresponding to PbTe (Te²⁻) and Ag_2−xTe (Te²⁻) respectively. Along with these peaks, the TeO₂ (Te⁴⁺) (3d_5/2) peak also appears at 575.9 eV. XPS data reveal the presence of Ag in the PbTe:10Ag (Fig. 5(b)). The peaks for Ag (3d_5/2) and Ag (3d_3/2) are observed. Deconvoluting the Ag (3d_5/2) peak gives two components, at 368.2 and 368.9 eV, which correspond to Ag element (Ag⁰) and Ag_2−xTe (Ag⁺).²⁰ Similar components for Pb, Te and Ag are observed in PbTe:20Ag (Fig. 5(c)) confirming the existence of Ag–Te alloy (Ag_2−xTe) along with PbO, PbTe, TeO₂ and Ag element on the surface. The peak intensities of Pb, Te and Ag for all the films are shown in Fig. 6. While the peak intensity of Pb is higher for Ag added PbTe films, the peak intensity of Te is lower compared to PbTe. This indicates that precipitation of Pb on the surface. A binding energy shift towards higher energy side for Pb in case of PbTe:10Ag can be seen in Fig. 6(a). This is probably a final state effect and may be caused by very small sized nanostructures along with larger sized nanostructures. These explanations are supported by the morphological studies using AFM and SEM with EDS.

Fig. 5 XPS spectra of (a) PbTe (b) PbTe:10Ag (c) PbTe:20Ag corresponding to 3d peaks of Te, 4f peaks of Pb, and 3d peaks of Ag.

Fig. 6 Comparison of peak intensities of PbTe, PbTe:10Ag and PbTe:20Ag films at (a) Pb 4f, (b) Te 3d, and (c) Ag 3d edges.

3.3 Morphological study

Fig. 7 shows the AFM images of PbTe, PbTe:10Ag and PbTe:20Ag films and it is evident that Ag addition results in significant change in the surface morphology. Nanosize islands like nanostructures of dimension ∼60 nm × 150 nm are seen in PbTe:10Ag thin films. For PbTe:20Ag thin film these islands become much more dense and their dimensions are found to be ∼50 nm × 120 nm (i.e. smaller than PbTe:10Ag thin film). Surface roughness of the PbTe thin film is ∼52.4 nm which reduced to 31.6 nm and 41.7 nm for PbTe:10Ag and PbTe:20Ag thin films, respectively.

Fig. 7 AFM images of (a) PbTe (b) PbTe:10Ag and (c) PbTe:20Ag films.

Surface morphology and the elemental composition of nanostructures formed in Ag added PbTe thin films were analysed using SEM equipped with an energy dispersive spectrometer (EDS). Fig. 8(a)–(c) shows the SEM images of PbTe, PbTe:10Ag, and PbTe:20Ag films respectively. Images acquired from SEM are consistent with AFM images. The composition values at two different points A (where nanostructures are present) and B (on the flat region) on each set of sample is given in Table 3. From EDS study, it is observed that the nano-size islands like nanostructures contain Pb, Te, Ag and there are Pb rich while other regions Pb and Te are present in 1:1 (within 11%) stoichiometry. This indicates that the nanostructures are formed due to the presence of metallic Pb inclusions, possibly because of Ag ions which bind preferentially with Te by liberating Pb atoms to precipitate out.²¹

Fig. 8 SEM images of (a) PbTe, (b) PbTe:10Ag, and (c) PbTe:20Ag thin films.

Table 3 Elemental compositions of PbTe, PbTe:10Ag and PbTe:20Ag thin films determined by EDS

Sample	A				B
	Pb (at%)	Te (at%)	Ag (at%)	Pb/Te	Pb (at%)	Te (at%)	Ag (at%)	Pb/Te
PbTe	52.3	47.6	0	1.09	52.6	47.3	0	1.11
PbTe:10Ag	50.1	37.6	12.3	1.33	49.2	44.2	6.6	1.11
PbTe:20Ag	45.7	35.8	18.5	1.27	45.2	43	11.8	1.05

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3.4 Electrical transport measurements

The variation of the thermoelectric powers (S) of PbTe, PbTe:10Ag and PbTe:20Ag nanocomposite films in the temperature range of 300 to 400 K are shown in Fig. 9(a). The measured thermoelectric powers at 400 K for PbTe, PbTe:10Ag and PbTe:20Ag film are ∼288, ∼376 and ∼317 μV K⁻¹ respectively. The measured value of thermoelectric power of PbTe sample is in accordance with the previous reports.²² The thermoelectric powers of PbTe:10Ag and PbTe:20Ag nanocomposite films are found to be ∼30% and ∼10% higher than the PbTe thin film, respectively.

Fig. 9 (a) Thermoelectric power and (b) electrical resistivity behaviors of PbTe, PbTe:10Ag and PbTe:20Ag thin films.

The temperature dependence of the electrical resistivity (ρ) in the temperature range of 300 K to 400 K for PbTe, PbTe:10Ag and PbTe:20Ag films are shown in Fig. 9(b). For all these films, electrical resistivity decreases with increasing temperature showing the behavior of a semiconductor. Furthermore, the resistivity of Ag added PbTe is found to be greater than the PbTe. The power factors (F = S²/ρ) for PbTe, PbTe:10Ag and PbTe:20Ag films in the temperature of 300 K to 400 K are shown in Fig. 10(a). Although the thermoelectric power of the Ag added thin films is higher than the PbTe, the power factor of Ag added thin films is lower than that of the PbTe thin film due to its high electrical resistivity in the same temperature range. Carrier concentration and Hall mobility of the films measured at room temperature for all three thin films are shown in Fig. 10(b).

Fig. 10 (a) Power factor, (b) carrier mobility and concentration of PbTe, PbTe:10Ag and PbTe:20Ag thin films.

In case of PbTe:10Ag thin film, both carrier concentration and mobility decrease and leading to increase in the resistivity (ρ = 1/neμ). In PbTe:20Ag, an increase in carrier concentration as well as mobility have cooperatively decreased the electrical resistivity in comparison to PbTe:10Ag thin film. The random distribution of nanostructures formed at the surface for Ag added PbTe thin films is likely to cause the reduction in carrier mobility as compared to PbTe. The high electrical resistivity can be reduced by several approaches. One such approach is tuning the size of the nanostructures in PbTe:Ag thin films by thermal annealing in vacuum. This may increase the size of nanostructures and hence the scattering of charge carriers from the boundary of nanostructures decreases. This leads to the decrease in electrical resistivity. Similar mechanism may result with ion beam irradiation but it may also generate defects that can be annealed out during thermal annealing. Since electrical resistivity of PbTe films increase up to 2–3 order of magnitude after exposing in air,²³ capping the surface may also ensure reduction in absorption of O and thus reduction in resistivity. Apart from these approaches, depositing thick films of PbTe:Ag can lead to reduction in electrical resistivity. Patil et al.²⁴ showed that resistivity decreases sharply with film thickness from 50 nm up to 150 nm and then it saturates for higher thickness. This is because, with film thickness, the grain size increases which in turn enhances the carrier mobility and decreases the electrical resistivity.

The physical mechanism responsible for thermoelectric power enhancement is proposed to be due to grain-boundary potential barrier scattering. Fig. 11 schematically represents the mechanism of energy filtering of charge carriers by the potential barrier formed at the interfaces of nanostructures where W is the width of the potential barrier, which is equal to the size of the nanocomposite.¹⁵ This energy barrier formed at the interfaces of nanostructures (E_b) obstruct the low energy charge carriers to take part in the transport process and act as additional scattering centers, resulting in the high electrical resistivity for PbTe:10Ag thin film. The charge carriers having energy lower than E_b (red circle in Fig. 11) get trapped along the grain interfaces, whereas, the charge carriers with higher energy than E_b (green circle in Fig. 11) move freely. With the increases in temperature (from 300 K to 400 K), more charge carriers gain energy to overcome the energy barrier resulting in a decrease in electrical resistivity. Energy filtering of charge carriers increases the average energy of the carriers taking part in the transport process.

Fig. 11 Schematic representation of the mechanism of charge transport across the nanocomposite interfaces of the sample. Red and green circles represent the low and high energy charge carriers at the grain boundaries.

The value of thermoelectric power depends upon the average energy of charge carriers and is inversely related to the carrier concentration. Therefore the phenomenon of energy filtering of charge carrier leads to the enhancement in thermoelectric power of PbTe:10Ag thin film by 30% in comparison to the PbTe thin films. The above observation is in good agreement with the results obtained for Ag-incorporated PbTe bulk synthesized via hot-press method.¹⁵ The formation of Ag_2−xTe structures in Ag added samples are also responsible for the enhancement in thermoelectric power, as Ag₂Te is reported to be a better thermoelectric material.²⁵ The thermoelectric power and electrical resistivity of PbTe:20Ag films are lower than PbTe:10Ag thin film. Since, the nanostructures formed on the surface of PbTe:20Ag are smaller in size than PbTe:10Ag thin, therefore the width of the potential barrier is smaller. These nanostructures are connected to each other which results in a small increase in mobility. Besides this, the carrier concentration in PbTe:20Ag film is also increased, therefore as a result of the combined effect of increase in carrier concentration and carrier energy filtering by potential barrier, the electrical thermoelectric power and resistivity reduces in comparison to PbTe:10Ag thin films.

4. Conclusion

The PbTe, PbTe:10Ag and PbTe:20Ag nanocomposites thin films were synthesized by thermal evaporation method. AFM and SEM images showed a major change in surface morphology for Ag added PbTe thin films. Island like nanostructures are formed on the surface due to the presence of Pb inclusions, which precipitated out due to the formation of Ag–Te Alloy. The XRD results show increase in crystallinity in Ag added films and also indicate the presence of Ag_2−xTe alloy which was further confirmed by XPS. Thermoelectric power of PbTe:10Ag and PbTe:20Ag were found to be ∼30% and 10% higher than PbTe thin film respectively. Power factor is decreased in Ag added PbTe thin film due to increase in the electrical resistivity. The increase in thermoelectric power is attributed to increase in scattering centers which obstruct the low energy charge carriers to take part in the transport process. The phenomenon of carrier energy filtering leads to the enhancement of thermoelectric power in this system.

Acknowledgements

One of the authors (Manju Bala) is grateful to the Council for Scientific and Industrial Research (CSIR) for providing financial support in the form of fellowship. The authors are grateful to the Department of Science and Technology (New Delhi) for providing the SEM under the Nano Mission Project and XRD system at IUAC under the IRHPA project to boost the research activities. The authors would like to thank Mrs Indra Sulania, Dr Saif A Khan, Mr Sunil Ojha, and Dr Pawan K. Kulriya for their help in various analysis and discussion. The authors thank Mr Santosh Choudhury, IOP Bhubaneswar for XPS measurements and Ms Jagdish, Panjab University, Chandigarh in sample preparation.

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Footnote

† This manuscript was written based on contributions from all authors. All authors have given approval to the final version of the manuscript.

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