Synthesis and characterization of semimagnetic semiconductor Pb1−xSmxSe

Journal of Alloys and Compounds 429 (2007) 19–24

Synthesis and characterization of semimagnetic semiconductor Pb1−xSmxSe M.M. Ibrahim, E.M.M. Ibrahim ∗ , S.A. Saleh, A.M. Abdel Hakeem Physics Department, Faculty of Science (Sohag), South Valley University, Sohag 82524, Egypt Received 1 March 2006; received in revised form 25 March 2006; accepted 27 March 2006 Available online 3 May 2006

Abstract The microstructure, crystal structure, electrical conductivity (σ), and thermoelectric power (TEP) of Pb1−x Smx Se (x = 0.00, 0.03, 0.06, and 0.09 at.%) are investigated. Pb1−x Smx Se crystallizes in a single-phase rock salt structure. The electrical conductivity and Seebeck coefficient (S) of the system Pb1−x Smx Se were measured in the ambient temperature range 100 ≤ T ≤ 450 K. The temperature dependence of both σ and S shows metal–insulator transition (MIT) at Tm , which depends on the composition itself. The TEP measurements verified the domination of n-type semiconducting behavior. We also find that the power factor S2 σ reaches a maximum value. © 2006 Elsevier B.V. All rights reserved. Keywords: PbSe; Thermoelectric power; Figure of merit; Semiconductor semimagnetic

1. Introduction Lead chalcogenides and their solid solutions have attracted special attention due to the technological importance of these materials, in crystalline and polycrystalline forms, as thermoelectric devices, infrared detectors, photoresistors, laser, and other electronic devices [1–9]. Thermoelectric devices can be used for electricity generation directly from a heat source. For high-temperature applications, more efficient thermoelectric materials are needed. The efficiency of a thermoelectric power generator in converting heat energy into electrical energy depends upon the figure of merit (Z) which is a combination of three transport quantities: Z=

S2σ κ

(1)

where S is the Seebeck coefficient, σ the electrical conductivity, and κ is the thermal conductivity which contains both electronic (κe ) and thermal lattice vibrations (κph ) contributions (κ = κe + κph ; in degenerate condition κph ∼ = κe therefore, κ∼ = 2κe ). The higher the Z of a material, the more useful it is as a thermoelectric material.

One way to increase Z is to minimize κph and maximizing the power factor S2 σ. The power factor depends substantially on the electronic structure (band gap, band shape, and band degeneracy near the Fermi level) and scattering of charge carriers (electrons or holes) and they are not independently controllable parameters. PbTe is one of the best materials used in construction of thermoelectric generators operating at intermediate temperature (450–800 K) [10]. PbSe is also a promising material for the same applications. Although, the physical properties of this material either undoped or doped with (Mn, Cr, Yb, Eu, Gd, and Ce) have been studied [11–17], there is no information about thermoelectric properties of Sm doped PbSe in litreature. Doping with these elements transforms lead salt (IV–VI) semiconductors into semimagnetic-semiconductors (diluted magnetic semiconductors, DMS). In these DMSs, characterized by a small gap, the nearst-neighbours (NN) exchange interaction is the dominant one. The present work concerns studies on the microstructure, crystal structure, electrical, and thermoelectric power (TEP) properties of Pb1−x Smx Se over a wide temperature range (100–450 K). 2. Experimental

∗

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0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.03.085

The sample synthesis and preparation are described in detail elsewhere [18]. Briefly, the samples are made using conventional solid-state reaction. Stiochio-

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metric amounts of the constituent materials are reacted at high temperatures (1373 K) in a closed silica tube. The ingots are ground into a fine powder and the structural properties are measured by X-ray diffraction. For transport measurements, the powder is cold pressed inside a stainless steel die at 5 tonnes cm−2 for 1 min. All samples prepared in this study were firstly examined by X-ray powder diffraction analysis (XRD). The XRD analysis was performed using Brucker Axs-D8 Advance diffractometer at room temperature with Cu (K␣) ˚ radiation (λ = 1.5406 A). Microstructural investigations were performed using (JEOL JSM-5300) scanning electron microscope (SEM) combined with an energy dispersive analysis of X-ray (EDAX). The electrical property of the samples was carried out as published in [19]. Thermoelectric power measurements of the samples are measured using differential technique [18]. It may be noted that the same piece of sample has been used for the measurements of both the electrical conductivity and Seebeck coefficient.

3. Results and discussion The microstructural features of Pb1−x Smx Se (x = 0.00–0.09 at.%) shown by the SEM image (Fig. 1) are nanocrystalline grains with variable size and shape. Although varying in shape, they are indeed of the same chemical composition as shown in Fig. 2. The results of EDAX compositional analysis for all samples are close to the nominal composition. X-ray diffractograms of the samples were analyzed to obtain information about various crystallographic aspects. The XRD of Pb1−x Smx Se (x = 0.00, 0.03, 0.06, and 0.09 at.%) compounds are shown in Fig. 3. All the samples are composed of single (rock salt) crystal phase with no second phase inclusions [20–22]. Our samples consist of few slightly disoriented monocrystalline grains. The average grain sizes (D) of all samples were within a range of (162.2–191.2) nm as they were calculated from the main peaks (2 0 0) by using the Scherrer formula. D(h k l) =

0.9λ 2δθ cos(θ)

(2)

Comparison of the particle size determined from SEM to that determined from XRD suggests that the difference between the grain sizes calculated from the XRD patterns and those observed in SEM may be due to the aggregation of the nanocrystals of PbSmSe. The data determined from the XRD results reflect the size of a “single” crystal, while the SEM photographs show the aggregates of PbSmSe [23]. The parameter (a) was determined from the d values of XRD peaks using the plane-spacing equation for cubic crystal 1 h2 + k2 + l2 = d2 a2 Fig. 4 shows the lattice parameter (a) and the cell volume V as a function of x (Pb1−x Smx Se). As Sm content increases from 0.00 to 0.009, the parameter (a) and the cell volume decrease as x changes from 0.00 to 0.09. This result can be well understood on the basis of the fact that the ionic radius of Sm3+ is smaller than that of Pb2+ [24]. In Fig. 5 the temperature dependence of the electrical conductivity of the investigated compounds are displayed. All the samples exhibit semiconducting behavior: The conductivity decreases with increasing temperature, consistent with a degen-

Fig. 1. SEM photomicrographs for: (a) 0.00 Sm, (b) 0.03, (c) 0.06, and (d) 0.09 Sm.

erate semiconductor [25]; reach its minimum value and then increases. At low temperature, the electrical conductivity is characterized with extrinsic conduction, while at higher temperature the number of carriers thermally excited across the semiconducting energy gap begins to overwhelm the number of carriers due to ionized impurities and the intrinsic conduction begins to predominate [26]. σ(T) shows metal–insulator transition, (MIT). This interesting electric property of it was observed by several groups [14,27,28]. Also these samples have a distinct metallic phase below the transition temperature Tm and above this temperature they become semiconducting but the transition is shifted to higher temperature. The temperature dependence of electrical conductivity in intrinsic region (above Tm ) can be fitted to a usual relation

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Fig. 4. Variation of Sm content (at.%) with (a) lattice parameter and (b) unit cell volume.

Fig. 2. EDAX for: (a) 0.03 Sm, (b) 0.06 Sm, and (c) 0.09 Sm.

 σ = σ0 exp

−Eσ KT



where σ 0 is the pre-experimental factor representing the temperature independent conductivity and Eσ is the activation energy for conduction. The plots of ln(σ) versus 1000/T for Pb1−x Smx Se

Fig. 5. The temperature dependence of the electrical conductivity for the asprepared Pb1−x Smx Se compositions.

samples are shown in Fig. 6. These plots are straight lines for all samples indicating that the conduction in the samples is attributed to thermal activation and the dc conductivity increases exponentially over the considered temperature range. The activation energy values Eσ of the materials were calculated by assuming thermal-activation-type conduction and were presented in Table 1. The results of other workers [29–31] on lead salts support the thermally activated conduction. On the other hand, the observed behaviour of the electrical conductivity of DMS with T was explained as follows. Accordingly [32], the decrease of the conductivity with T is due to the different Fermi surfaces of the ferromagnetic and Table 1 Values of Eσ (eV) and Tm (K) of Pb1−x Smx Se

Fig. 3. The XRD diffraction patterns for green samples of Pb1−x Smx Se.

X (at.%) Eσ (eV) Tm (K)

0.00 0.193 330

0.03 0.184 395

0.06 0.145 410

0.09 0.178 400

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Fig. 6. Plots of ln(σ) vs. 1000/T for different concentrations of Sm content.

paramagnetic state. In the ferromagnetic phase the wave vector transferred in a scattering event across the Fermi surface is larger than in the paramagnetic phase and therefore, the conductivity is higher in the ferromagnetic phase than in the paramagnetic phase. The metal–semiconducting transition (Tm ), moves to a higher temperatures with increment of the samarium concentration (Table 1) up to 0.06 at.%. Measurement of the TEP is a useful tool to know the electronic properties because it is less sensitive to intergrain impurities in comparing with conductivity. Fig. 7 shows the temperature dependence of the Seebeck coefficient S for the prepared samples. The negative signs of S indicate that these materials are of n-type behavior (carriers in these compounds are electrons). The absolute value of the Seebeck coefficient increases with increasing temperature in the extrinsic region and then decreases due to increasing number of thermally excited carriers. Temperature dependences of S for all samples indicate that metal–insulator transitions (MIT) takes place because the slope changes from negative (metallic similarly as expected for degenerate semiconductors [33]) to positive (semiconducting).

Fig. 8. The temperature dependence of the Seebeck coefficient for different Sm concentration.

As it is seen in Fig. 8 each S versus 1000/T plot seems to consist of two linear segments satisfying the following relation:    Es Kb (3) S=− e Kb T + A  where the activation energy (Es ) represents the Fermi energy for n-type materials [34] and A is a dimensionless parameter concerning the carrier’s scattering mechanisms [35] or assumed to be a measure of the kinetic energy transported by carriers [36]. As mentioned above, all samples exhibited negative sign TEP, which is the behavior of n-type semiconductors. The electrons contributing to the observed TEP exhibited thermal activation and thus, the values of S increased continuously with increasing T. However, such thermal activation of electrons became weaker as the temperature exceeded a certain value depends on the composition itself. This might indicate that the behavior is not purely that of n-type but it is a mixed type and plus charge carriers (holes) become with significant contribution at higher temperatures. Besides, the value of the activation energies in the ranges below Es1 and above Es2 the transition temperature Tm , the values of Tm and those for the dimensionless parameter A1 and A2 corresponding to the extrapolations of the ranges below and above Tm changed with composition as shown in Table 2. The sign reversion of Es from plus to minus confirms the possibility of compensation of electrons by holes, and the domination of the contribution of the latter, despite the fact that the general behavior is still that of n-type, since the sign of TEP is Table 2 Values of Es (eV), Tm (K), and A (V/K) of Pb1−x Smx Se

Fig. 7. The temperature dependence of the Seebeck coefficient for different Sm concentration.

Tm (K)

A2 (V/K)

Es2 (eV)

A1 (V/K)

Es1 (eV)

Sm content

250 210 240 220

0.51770 −1.76691 0.19372 −1.58560

−0.05061 −0.01529 −0.05973 −0.01865

−2.44794 −4.07554 −3.75074 −3.99770

0.01774 0.022594 0.02325 0.02607

0.00 0.03 0.06 0.09

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applications. So the optimum operating temperature of this system could be shifted towards room temperature. However, the power factor temperature dependence shows that these materials have the potential for high temperature applications with promising thermoelectric performance. This improvement increases well the efficiency of the thermoelectric generators and modules fabricated from the very fine powder, which is of great value in the thermoelectric industry.

References

Fig. 9. The temperature dependence of the power factor for the as-prepared Pb1−x Smx Se compositions.

still minus. Also, in Table 2 it is obvious that the dimensionless parameter A possesses either a minus or plus sign depending on the extent of the contribution of electrons and/or holes respectively, as T extends to an infinite value. Like conductivity, the metallic behavior of S takes place but at different Tm , this may be due to that the conductivity measurements are strongly influenced by the grains boundaries. The smaller the grain size, the more abundance the grain boundary, and the lower the electrical conductivity [37]. In the other side, the TEP is more reliable to see intrinsic properties of materials inside the grains. Generally, when the doping increases up to 0.06 at.%, the samples have rather high σ and large S, which is required for thermoelectric applications. Due to the σ and |S| varying consistent way, optimizing the carrier concentration by modified compositions is needed to get the best TE device. The calculation of power factor (PF) can help us to evaluate the electrical components of the TE device. The higher the PF of a material, the more useful it is as a thermoelectric material. Fig. 9 shows the PF versus temperature for different Sm content. The power factor value for all compounds increases with raising temperature, reaches a maximum and then decreases. The maximum power factor values of 25 × 10−4 W/mK2 were observed for Pb0.94 Sm0.06 Se compound at 240 K. Since, the maximum power factor was achieved with some tradeoff, it is possible to get higher Z for real thermoelectric device. 4. Conclusion Structural formation, electrical conductivity, and thermoelectric power properties of Pb1−x Smx Se (x = 0.0–0.09 at.%) semimagnetic–semiconducting system have been studied. The results of this study confirm that the very fine-grained PbSmSe is very effective in improving the power factor (also the figure of merit) of the thermoelectric material. We also demonstrated that the power factor can be increased, suggesting that a thermoelectric device with higher efficiency depending on composition and temperature is possible for real

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