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Enhancement of electrical transport through the anisotropic nanostructure performance of heavily Yb-doped PbSe0.2Te0.8 thin films
Materials Chemistry and Physics 136 (2012) 1148e1155
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Enhancement of electrical transport through the anisotropic nanostructure performance of heavily Yb-doped PbSe0.2Te0.8 thin films A. Hmood*, A. Kadhim, H. Abu Hassan School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Polycrystalline Pb1xYbxSe0.2Te0.8 ingots were prepared by solid-state microwave synthesis. < The microstructure of the ingots was revealed by SEM. < Effect of Yb doping on morphology of films was formation of large grains. < The power factor increased with increasing Yb content up to x ¼ 0.075.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2012 Received in revised form 7 June 2012 Accepted 25 August 2012
Solid-state microwave-assisted plasma technique was used to synthesize high-purity Pb1xYbxSe0.2Te0.8 from initial components to obtain polycrystalline ingots with large grains. Pb1xYbxSe0.2Te0.8 thin films were then deposited onto glass substrates using thermal evaporation in vacuum. The films have the polycrystalline rock salt-type (NaCl) structure. The crystal lattice parameters for the powders and thin films obtained from X-ray diffraction (XRD) patterns showed that the values of the lattice parameters increased with increasing Yb content. The field emission scanning electron microscopy images reveal that the Pb1xYbxSe0.2Te0.8 thin films have uniform crystal grain sizes and dense nanostructures. The electrical transport properties of the thin films were measured in the temperature range of 298 Ke523 K. The Seebeck coefficient of the films increased with x when x was in the range of 0.015e0.045, whereas it decreased for x between the range of 0.06 and 0.105. The carrier concentration was 4.3 1017 cm3 for x ¼ 0.015, and the maximum value was 6.34 1017 cm3 at x ¼ 0.075. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: A. Semiconductors A. Thin films D. Electrical properties D. Microstructure
1. Introduction The quaternary compounds Pb1xYbxSeyTe1y form a large group of semiconducting materials with diverse optical, electrical, and structural properties [1]. These quaternary compounds are of particular interest because of their applicability as the basic material for developing heterostructure devices, such as laser diode, Schottky barriers, hydrogen detection, trace gas detection,
* Corresponding author. Tel.: þ60 142441397; fax: þ60 46579150. E-mail address: [email protected] (A. Hmood). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.08.069
and thermoelectric device [2,3]. Most of these compounds are grown by molecular beam epitaxy (MBE), liquid phase epitaxy, traveling heater zone, and solid-state reaction techniques [4]. G. Springholz et al. reported the realization of mid-infrared quantumdot lasers based on self-organized PbSe/PbEuTe quantum dots grown by MBE [5]. D.I. Pratun et al. reported high-temperature continuous wave operation (>100 K) and improved laser performance of the lattice mismatch PbTe/PbEuSeTe system grown by MBE [6]. A giant negative magneto-resistance effect has been studied in PbTe (Yb, Mn) bulk crystal grown using the Bridgman technique [7]. Recently, the thermoelectric materials of Pb0.9xSn0.1GexTe, prepared by heating the mixtures above the
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
at 300 K, which was performed by applying a vertical constant magnetic field of 1 T to the thin films using Lake Shore-637. 3. Results and discussion 3.1. Rietveld analysis of X-ray diffraction profiles
(422)
(420
(400)
x=0.105
x=0.09
Intensity (a.u.)
x=0.075 x=0.06 x=0.045 x=0.03
x=0.015 x=0.0 20
25
30
35
40
45
50
55
60
65
70
75
80
2 (degree)
(420)
(400)
(311) (222)
(422)
x=0.105
(220)
(111)
b
x=0.09 x=0.075
Intensity (a.u.)
Quaternary semiconductor compounds of Pb1xYbxSeyTe1y were synthesized from initial elements of high purity (Pb, Se, Te 99.999% 100 meshes, and Yb 99.9% 157 mm) weighing 2 g according to Pb1xYbxSeyTe1y (x ¼ 0.0e0.105), and y ¼ 0.2. A typical element ratio for the preparation of PbSe0.2Te0.8 is as follows: (1.2748 g) Pb, (0.0972 g) Se, and (0.6280 g) Te. The mixture was transferred into an agate mortar and pestle to create a homogenous mixture for 20 min, and was placed inside a quartz ampoule and then sealed under vacuum pressure of 105 mbar. The ampoule was exposed to microwave energy inside a microwave oven (MS2147C 800 W) operating at 2.54 GHz, with maximum power of 800 W for about 25 min. Solidification of the compound was achieved when the heat generated by the irradiation plasma inside the ampoule diffused the constitutional elements. Thin films of quaternary Pb1xYbxSe0.2Te0.8 were then deposited onto clean glass substrates using the thermal evaporation technique in vacuum at 106 mbar by an Alcatel-101 with tantalum boat to obtain thin films with a thickness of 164 nm that was measured using an optical reflectometer (Filmetrics F20, USA). Highresolution X-ray diffusion (XRD) analysis was used to confirm the crystal structure of the Pb1xYbxSe0.2Te0.8 powder and thin films using PANalytical X’Pert PRO MRD PW3040 with Cuka radiation for 2q values between 20 and 80 . The quantities of second phase percentage in the sublattices were examined using Rietveld refinement. The surface morphology of the ingots and thin films was examined through scanning electron microscopy, energy dispersive X-ray spectra (EDX, JSM-6460 LV), and field emission scanning electron microscopy (FESEM, Leo-Supra 50VP). The thermoelectric power was measured for all films at a temperature range of 298 Ke523 K between the two ends of each sample. One of the ends was heated using a designed electrical heater that supplies 100 W inside a brass block, and the other end was cooled by ice water supplied by a chiller inside a brass block cold sink. Thermal gradients of approximately 10 K at both ends were measured using two separate thermocouples (type-K EÓSun ECS820C) and were fixed to be in direct contact with the films. The electrical conductivity was measured using a four-point probe method in vacuum at 103 mbar. The carrier concentration was determined from the Hall voltage measurement using the Vander Paw method
YbTe
(311) (222)
2. Experimental procedure
(220)
(111)
a
(200)
The XRD patterns of the quaternary semiconductor compound Pb1xYbxSe0.2Te0.8 powder, which was synthesized by microwaveassisted plasma diffusion of initial components in stoichiometric form, are shown in Fig. 1 (a). The compounds have major polycrystalline phases of Pb1xYbxSe0.2Te0.8 compound in a rock salttype (NaCl) structure, and the diffraction peak (200) has a high intensity. The weak shoulder on the right-hand side of the (200) peak represents the second phase of YbTe in Pb1xYbxSe0.2Te0.8. These weak peaks disappear in the respective XRD spectra of Pb1xYbxSe0.2Te0.8 thin films, confirming the analytical results of the standard database (JCPDS No. 38-1435, and 18-1468), as shown in Fig. 1 (b). The average crystalline (grain) size D can be obtained
(200)
melting point of the constituent elements followed through the quenching method, have been examined [8]. Y. Gelbstein et al. investigated the development of p-type Bi2Te3-doped Pb1xGexTe and Sn1xGexTe alloys for thermoelectric applications [9,10]. The current paper reports the preparation of ternary and quaternary chalcogenide compounds, which may possess improved physical properties compared with compounds synthesized through other techniques [11,12], using solid-state microwave synthesis for the first time. This paper includes the study on the synthesis and characterization of Pb1xYbxSe0.2Te0.8 semiconductor compounds produced from the mixed stoichiometric ternary compounds of PbSe0.2Te0.8 with Yb-doping. The PbSe0.2Te0.8 was initially prepared. Then, Pb1xYbxSe0.2Te0.2-doped ingots were synthesized by exposure to microwave irradiation inside an evacuated quartz tube to obtain polycrystalline ingots with a large grain size, and were thermally evaporated to obtain thin films. The crystal structure of the quaternary semiconductor compounds of Pb1xYbxSe0.2Te0.8 (x ¼ 0.0, 0.015. 0.03, 0.045, 0.06, 0.075, 0.09, and 0.105) is that of a rock salt-type (NaCl) structure system with relative anomalies in the cubic unit cells, and thus in the lattice constants. The electrical transport properties of the quaternary Pb1xYbxSe0.2Te0.8 thin films were studied in the temperature range of 298 Ke523 K.
1149
x=0.06 x=0.045 x=0.03 x=0.015 x=0.0
20
25
30
35
40
45
50
55
60
65
70
75
80
2 (degree) Fig. 1. XRD patterns of Pb1xYbxSe0.2Te0.8 as (a) powders and (b) thin films.
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A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
using Scherrer’s formula based on the XRD patterns of the films [13]:
D ¼
0:9l bcosqB
(1)
where l is the X-ray wavelength, b is the full width at half maximum of XRD peaks, and qB is the Bragg angle. The grain size is almost constant at 416 Å, with an increase in Yb content, except for three different compositions in which it decreases to 334 Å for both x ¼ 0.015 and 0.03, and to 278 Å for x ¼ 0.045. The cubic lattice constant, a, is calculated using the following equation [13]:
1 h2 þ k2 þ l2 ¼ 2 a2 d
(2)
where d is the inter planer distance, and h, k, and l are the lattice planes. Fig. 2 shows the respective lattice constants of Pb1xYbxSe0.2Te0.8 powders and thin films for different values of x. The lattice constants for both increase polynomial as the concentration x increases. The Yb substitution induces a continuous change in the nature of the chemical bonding, which is responsible for a linear xdependence of the lattice constant [14]. For x ¼ 0.015e0.105, the main phase of Pb1xYbxSe0.2Te0.8 averages 59% obtained from the Rietveld refinement of the XRD patterns of various Yb contents. As shown in Fig. 3, the XRD patterns fitted using the Rietveld refinement method for x ¼ 0.015, 0.075, and 0.105, and the corresponding results are listed in Table 1. At the same time, YbTe has the same NaCl-type structure (space group Fm3m) as the host PbSe0.2Te0.8, with a slightly smaller lattice parameter (aYbTe ¼ 6.365 Å, A) [15,16]. The atomic mass of Yb is 80% aPbSe0:2 Te0:8 ¼ 6:395 compared with the atomic mass of Pb, and the substitution of Yb for Pb induces an increase in the lattice constant a (Å) of the host lattice material PbTe [17]. However, the nature of bonding in Pb1xYbxSe0.2Te0.8 is rather complicated and comprises a mixture of ionic, covalent, and metallic bonds. A simple guess based on bond valence sums implies that all the bonds in Pb1xYbxSe0.2Te0.8 (metale chalcogenide, metalemetal, and chalcogenideechalcogenide) are more ionic than in pure PbTe. Hence, the most elongated of all, Ybe Te distance is z3% longer than the corresponding distance in YbTe (3.18 Å), and z1.5% longer than PbeTe distance in pure PbTe (3.23 Å) [18]. On the other hand, the larger atomic radius of tellurium (1.4 Å), compared with that of selenium atom (1.15 Å), presumably verifies that the tellurium atoms successfully enter into
-0.82 Powders Thin films Strain of the lattice
Lattice constant (Å)
6.45
-0.84 -0.86
6.44
-0.88
6.43
-0.90 -0.92
6.42
-0.94
Strain of the lattice (%)
6.46
6.41
-0.96 6.40
-0.98
6.39
-1.00 0.000
0.015
0.030
0.045
0.060
0.075
0.090
0.105
Yb content Fig. 2. Variations of the lattice constant for powder and thin films and the strain of the lattice with Yb content for thin films.
Fig. 3. X-ray diffraction patterns fitted using the Rietveld refinement method of Pb1xYbxSe0.2Te0.8 powders: (a) x ¼ 0.015, (b) x ¼ 0.075, and (c) x ¼ 0.105.
the lattice structures of PbSe0.2Te0.8. Three atoms are placed within this distance, that is, one from the basic components of the PbSe and PbTe (Pb or Se and Te), and the other is an impurity atom. The reduction in the lattice constant is the first indicator of the insertion of Yb into the cages of the Pb1xYbxSe0.2Te0.8 structure, thus changing Pbþ2 ions (radius ¼ 1.2 Å) into smaller Ybþ3 ions (radius ¼ 1.008 Å). A small fraction of the Yb ions (x ¼ 0.015) in the thin film decreased because of the transfer from a magnetic Yb3þ (4f13) charge state to a non-magnetically active charge state Yb2þ (4f14), which is usually present in x ¼ 0.03e0.105 [19,20]. The strain of the lattice exhibited the same behavior (Fig. 2), depending on the
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155 Table 1 The values of Rietveld refinement factors, weighted profile R factors (Rwp), profile R factors (Rp), expected R factors (Rexp), structure R factors (RF), goodness of fit (c), and D-statistics of Pb1xYbxSe0.2Te0.8 (x ¼ 0.015e0.105). x
0.015
0.03
0.045
0.06
0.075
0.09
0.105
Rp Rwp Rexp RI RF c ¼ Rwp/Rexp D-statistics
0.26667 0.2180 0.2844 0.5478 0.5211 0.766 0.1381
0.3743 0.3743 0.3265 0.6206 0.6134 1.146 0.1489
0.2806 0.2294 0.2610 0.6665 0.6598 0.879 0.1028
0.2446 0.2381 0.2650 0.7030 0.5418 0.898 0.0989
0.3176 0.2864 0.2828 0.6577 0.6349 1.013 0.0960
0.3141 0.2570 0.3209 0.6698 0.6472 0.801 0.1049
0.3572 0.3497 0.3047 0.6032 0.5864 1.148 0.0851
lattice constant of the as-grown thin films. This behavior can be calculated using the following equation [21]:
3 zz
¼
a ao 100% ao
(3)
where a is the lattice constant of the strained Pb1xYbxSe0.2Te0.8 asgrown films, and ao is the unstrained standard lattice constant of the bulk PbTe, i.e., 6.459 Å. The strains of the lattice for thin films for
a x=0.0
all x fractions have negative values, and are thus associated with compressive strains. 3.2. SEM and FESEM observation The microwave irradiation technique successfully formed Pb1xYbxSe0.2Te0.8 compounds, as clearly seen in the SEM images of the ingot shown in Fig. 4 (a), which reveals the formation of a microstructure. At a higher resolution, the ingots consist of closely irregular plates for doping levels x ¼ 0.0 and 0.015. The white grains that appear on the surface of the ingots for the doping levels of x ¼ 0.03e0.075 lead to the appearance of the second phase (as indicated in the XRD data) of Fig. 1 (a), which then disappear at the doping level x ¼ 0.09e0.105. Using EDX analysis, the amount of Yb substituted for Pb stoichiometric ratio of the ingots Pb1xYbxSe0.2Te0.8 was approximately equal to the standard elemental weight (Table 2). Deciding if such small aggregates are YbTe phase is not possible; the reason is that the YbeTe hinders the movement of the evaporation steps and slows down the evaporation. As the segregated phase has lower vapor pressure than PbTe, it can be formed only by the appearance of white grains on the surface [22].
x=0.015
1µm
1µm
x=0.045
x=0.03
YbTe
YbTe 1µm
1µm x=0.06
x=0.075 YbTe
YbTe 1µm x=0.09
1µm
1151
1µm x=0.105
1µm
Fig. 4. (a) SEM images of Pb1xYbxSe0.2Te0.8 ingots and (b) FESEM images of Pb1xYbxSe0.2Te0.8 thin films.
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A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
b x=0.0
x=0.015
100 nm
100 nm
x=0.03
x=0.045
100 nm
100 nm
x=0.06
x=0.075
100 nm
100 nm
x=0.09
x=0.105
100 nm
100 nm Fig. 4. (continued).
Thin films of Pb1xYbxSe0.2Te0.8 deposited at room temperature have a shiny and very smooth mirror-like surface. FESEM images of the film surface reveal the formation of irregular grain crystallites. The effect of Yb content on the morphology of these polycrystalline films can be clearly seen in Fig. 4(b). Owing to the enhancement of surface diffusion and agglomeration of add-alloys with doping, the formation of large grains can be seen clearly. Grain boundaries and intervening voids are clearly visible for x ¼ 0.075. 3.3. Electrical properties Electrical conductivity, s, is an important factor and reliable information about the transport phenomenon of materials. The electrical conductivity of all Pb1xYbxSe0.2Te0.8 alloy thin films increased with increasing temperature, indicating the semiconductor behavior of all alloys. The electrical conductivity data were analyzed to distinguish the possible mechanisms involved in the thermally activated process. The variations of ln s versus 1/T of eight Pb1xYbxSe0.2Te0.8 films are shown in Fig. 5. Evidently, based on these curves, the conduction by thermal activation has two
different mechanisms with two different slopes, indicating the presence of two activation energies. The activation energy DEa can be estimated from the plot of ln s against 1/T according to equation [23]:
s ¼ so expð DEa =kTÞ
(4)
where so is a parameter depending on the semiconductor nature, and k is the Boltzmann constant. From these slopes of linear plots, activation energy for conduction was calculated for two temperature regions. The first electronic transition is in the lowtemperature region of 3.3e2.42 K1 (300 Ke413 K), whereas the other transition is in the high-temperature region of 2.36e 1.91 K1 (423 Ke523 K), indicating that a transition occurred near 423 K. The obtained values of DEa at different Yb content (x) for the Pb1xYbxSe0.2Te0.8 thin films are shown in Table 3. The linearity of ln s against 1/T for the low-temperature region indicates that ln s intrinsic conduction predominates, whereas in the middletemperature region of 2.42e2.36 K1 (413 Ke423 K), the slope of the curves continuously decreases with increasing temperature consistent with a degenerate semiconductor conduction caused
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
by the formation of density of states near the Fermi level [24,25]. These densities of state oppose and reduce the electrical conduction by capturing the charge carriers. The Yb atoms behave like donors when they substitute Pb atoms. Therefore, the Ybþ2 (Ybþ2 ¼ Ybþ3 þ e transition) ions lead to the formation of deep localized or resonance impurity states in the forbidden band gap [5]. With increasing Yb content, the Fermi energy and the free electron density grow until the Fermi level reaches the impurity state level, after which no further increase in the electron density in the conduction band is evident, and the Fermi energy is pinned at DE1 (low-temperature region) for x ¼ 0.015e0.105. On the other hand, with increasing temperature at 413 Ke423 K, the impurity states lead to sink into the band gap, whereas the narrow width of the density-of-states of resonance states ensures a very strict pinning of the Fermi level relative to the conduction band edge, despite the inevitably non-uniform distribution of impurities and electrically active defects throughout the sample [26,27]. At DE2 (high-temperature region), ln s intrinsic conduction dominates due to the increase in the mobility of the carriers and the reduction of the trapping state and potential barriers [28]. The carrier concentration at room temperature is obtained from the measured Hall coefficient using equation RH ¼ 1/ne, where RH is the Hall coefficient, and n is the carrier concentration. The carrier concentration increases with increasing Yb content up to x ¼ 0.075, whereas it decreases at the doping level x ¼ 0.015 and 0.09. The Hall mobility can be calculated from the carrier concentration and the electrical conductivity according to s ¼ nem (e ¼ electron charge). The mobility at 300 K increases to a maximum value at x ¼ 0.075 because of the relatively larger values of electrical conductivity and carrier concentration (Table 3). The thermoelectric power (Seebeck coefficient, S ¼ DV/DT), as determined from the slope of thermoelectric e.m.f. versus the temperature difference between the hot and cold ends of the films, is shown in Fig. 6. The temperature dependence of the thermoelectric power for all the Yb-doped films has a semiconductor behavior, in which the values of the thermoelectric power increase as the temperature increases. Thus, the observed sign of the thermoelectric power is negative, indicating typical n-type semiconductors within the temperature range of 300 Ke523 K; therefore, electrons are the majority carriers in the films. A temperature difference causes exciting electrons in the conduction band to diffuse from the hot side to the cold side because of the concentration gradient, causing electric potential and the buildup of charge carriers on the cold side at equilibrium. The relation between thermoelectric power and charge-carrier (electron) concentration can be expressed as [29]:
Table 2 EDX spectra of Pb1xYbxSe0.2Te0.8 ingots. x
Element
Weight %
Atomic %
0.0
Se Te Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb
3.93 34.79 61.28 7.54 26.51 0.74 65.21 6.21 36.1 1.03 56.66 5.60 27.08 1.31 66.01 5.64 33.11 1.70 59.55 5.13 31.24 1.98 61.64 4.95 31.28 4.67 59.10 6.46 20.59 7.08 65.88
8.04 44.11 47.85 15.35 33.38 0.69 50.57 12.27 44.14 0.93 42.66 11.64 34.83 1.24 52.28 11.38 41.31 1.56 45.75 10.51 39.56 1.85 48.08 10.10 39.54 4.35 46.01 13.58 26.81 6.79 52.82
0.015
0.03
0.045
0.06
0.075
0.09
0.105
6.4 x=0.0 x=0.015 x=0.03 x=0.045 x=0.06 x=0.075 x=0.09 x=0.105
6.0
Ln (S/cm)
5.6 5.2 4.8 4.4 4.0
k S ¼ e
3.6 1.8
2.0
2.2
2.4
2.6
2.8
1000/T (K-1)
3.0
3.2
1153
3.4
"
3 !# 2 2pm* kT 2 5 rþ þ ln 2 h3 n
(5)
where r is the scattering parameter, h is the Planck’s constant, and m* is the effective mass of the electron. This relationship shows that
Fig. 5. Plots of ln s versus 1000/T for eight Pb1xYbxSe0.2Te0.8 thin films.
Table 3 Electrical conductivity (s), Seebeck coefficient (S), Hall coefficient (RH), carrier concentration (n), mobility (m) at 300 K, activation energy (DEa) at 298 Ke523 K. x
s (S cm1)
S (mV K1)
S2s (mW m1 K2)
jRHj (C cm3)
n 1017 (cm3)
m (cm2 V1 s1)
DE1 (eV) (300413) K
DE2 (eV) (423523) K
0.0 0.015 0.03 0.045 0.06 0.075 0.09 0.105
43.9 46.89 44.26 49.73 113.22 126.09 67.99 95.15
125.66 161.33 183.66 186.42 129.66 123.05 133.66 123.66
69.32 122.04 149.29 172.82 190.3 190.7 121.46 145.5
13.01 14.48 13.83 13.15 10.47 9.86 11.65 10.25
4.8 4.3 4. 52 4.7 5.97 6.34 5.4 6.1
571.14 678.97 612.11 653.95 1185.41 1243.25 792.08 975.28
0.096 0.095 0.073 0.044 0.08 0.05 0.068 0.035
0.541 0.430 0.152 0.091 0.23 0.138 0.265 0.123
1154
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155 5000
-100 x=0.0 x=0.015 x=0.03 x=0.045 x=0.06 x=0.075 x=0.09 x=0.105
-150
S (µV/K)
-175 -200
x=0.0 x=0.015 x=0.03 x=0.045 x=0.06 x=0.075 x=0.09 x=0.105
4500 4000
S2 (µW/mK2)
-125
-225 -250
3500 3000 2500 2000
-275
1500
-300
1000 500
-325
0
-350 300
325
350
375
400
425
450
475
500
525
550
300
325
Fig. 6. Temperature dependence of the Seebeck coefficient of Pb1xYbxSe0.2Te0.8 thin films.
the thermoelectric power is highly dependent on the carrier concentration, and that it decreases with increasing carrier concentration. Furthermore, the electrical conductivity varies as a function of Fermi level EF; hence, the change in thermoelectric power at room temperature is predominated by the variation of carrier concentration. Therefore, the electronic scattering is independent of energy, making the electrical conductivity proportional to the density of state near Fermi level, which indicates acoustic phonon scattering as a predominant carrier scattering mechanism in the thin films of Pb1xYbxSe0.2Te0.8 [30]. The value of the thermoelectric power increases with increasing temperature in the low-temperature region ranging from 300 K to 375 K, and then decreases due to the increase in thermally excited carriers. This further confirms that the electron concentration increases with temperature in Yb doped samples, because the slight decrease in thermoelectric power with temperature ranging from 385 K to 423 K results from the increase in n. This effect probably reflects the weaker donor activity of this impurity resonant states at extrinsic region, that indicates that the Fermi level is stabilized by the impurity band and that the filling of the latter with electrons. Increasing n shifts the impurity band upward, so that the initial concentration of electrons in the conduction band, which equals the number of electrons in the impurity band, increases [31]. For x ¼ 0.03 and 0.045 doping concentrations, the thermoelectric power of the films increases in sequence at low temperature, thereafter decreasing at a temperature of 423 K. For x ¼ 0.06, the thermoelectric power increases at the higher end of the temperature range, whereas from x ¼ 0.0 to 0.105, it increases for the whole temperature range. The power factor S2s of the Pb1xYbxSe0.2Te0.8 films was calculated in terms of the electrical conductivity and thermoelectric power; it is shown as a function of temperature in Fig. 7. The power factor strongly increases with temperature up to 523 K. The value of the power factor generally decreases with the increase in the amount of Yb-doping at x ¼ 0.015e0.045, and 0.105. At x ¼ 0.045, 0.06, and 0.075, the highest power factor at room temperature reaches 172.82, 190.3, and 190.7 mW m1 K2, respectively. Experimental results suggest that the electronic transport properties of these films are heavily influenced by the high density of states near the Fermi level, leading to the production of electrons with heavy effective mass [32]. These effective mass carriers lead to a large thermoelectric power at 300 K from 0.015 to 0.045, while obtaining high power factors of x ¼ 0.06 and 0.075 at 523 K. These results give
350
375
400
425
450
475
500
525
550
T (K)
T (K)
Fig. 7. Temperature dependence of the power factor of Pb1xYbxSe0.2Te0.8 thin films.
a clear idea of the application of these alloy compositions for thermoelectric devices. 4. Conclusion The quaternary alloys of the nanostructure Pb1xYbxSe0.2Te0.8 were successfully produced through solid-state microwave synthesis after 25 min of irradiation. The lattice parameter of the cubic unit cell increased with the increased concentration of Yb for the ground powders and thin films of Pb1xYbxSe0.2Te0.8. All the samples of Pb1xYbxSe0.2Te0.8 had a rock salt-type (NaCl) structure with a dominant peak representing the (200) plane. Electrical conductivity and activation energy were investigated. The conduction process in these films was done through a thermally activated process. Based on the measurements of the thermoelectric properties, the maximum S2s calculated was significantly improved to 4525.61 mW m1 K2 at 523 K for the Pb0.925Yb0.075Se0.2Te0.8 sample. Acknowledgments The authors are grateful for the funding provided by the Postgraduate Research Grant Scheme (No. 1001/PFIZIK/844134) of Universiti Sains Malaysia. References [1] R. Suryanarayanan, S.K. Das, Appl. Phys. Lett. 67 (1989) 1612e1614. [2] Z. Feit, R. Woods, D. Kostyk, W. Jalenak, Appl. Phys. Lett. 55 (1989) 16e18. [3] I.I. Zasavitski, E.V. Bushuev, E.A. Andrada-e-Silva, E. Abramof, JETP Lett. 75 (2002) 559e562. [4] J. Heremans, D.L. Partin, Phys. Rev. B 37 (1988) 6311. [5] G. Springholz, T. Schwarzl, W. Heiss, G. Bauer, M. Aigle, H. Pascher, I. Vavra, Appl. Phys. Lett. 79 (2001) 1225e1227. [6] D.L. Pratun, J. Electron. Mater. 13 (1984) 493. [7] I.I. Ivanchik, D.R. Khokhlov, A.V. Morozov, A.A. Terekhov, E.I. Slyn’ko, V.I. Slynko, A. de Visser, W.D. Dobrowolski, Phys. Rev. B 61 (2000) 889. [8] M. Sondergaard, M. Christensen, S. Johnsen, C. Stiewe, T. Dasgupta, E. Mueller, B.B. Iversen, J. Solid State Chem. 184 (2011) 1172e1175. [9] Y. Gelbstein, O. Ben-Yehuda, Z. Dashevsky, M.-P. Dariel, J. Cryst. Growth 311 (2009) 4289e4292. [10] D.L. Pratun, J.P. Heremans, C.M. Thursh, Superlattice Microstruct. 2 (1986) 459. [11] A. Hmood, A. Kadhim, H. Abu Hassan, Chalcogenide Lett. 8 (2011) 579e585. [12] A. Kadhim, A. Hmood, H. Abu Hassan, Mater. Lett. 65 (2011) 3105e3108. [13] B.D. Cullity, Element of X-ray Diffraction, third ed., Wesley Publishing Company, USA, 1967. [14] S. Merah, D. Ravot, G.A. Percheron, F.J. Olivier, J.C. Jumas, A. Mauger, E. Parent, J. Alloys Compd. 260 (1997) 17e22. [15] A. Iandelli, in: E.V. Kelber (Ed.), Rare Earth Research, Macmillan Co., New York, 1961.
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155 [16] A. Hmood, A. Kadhim, H. Abu Hassan, Superlattice Microstruct. 51 (2012) 825e833. [17] A. Hmood, A. Kadhim, H. Abu Hassan, J. Alloys Compd. 520 (2012) 1e6. [18] I. Radisavljevic, N. Novakovic, N. Romcevic, M. Manasijevic, H.E. Mahnke, N. Ivanovic, J. Alloys Compd. 501 (2010) 159e163. [19] P.M. Nikolic, D. Lukovic, S.S. Vujatovic, K.M. Paraskevopoulos, M.V. Nikolic, V. Blagojevic, T.T. Zorba, B. Stamenovic, W. Konig, J. Alloys Compd. 466 (2008) 319e322. [20] E.P. Skipetrov, N.A. Chernova, L.A. Skipetrova, E.I. Slynko, Mat. Sci. Eng. B 91e 92 (2002) 412e415. [21] C.Y. Tsay, K.-S. Fan, S.-H. Chen, C.-H. Tsai, J. Alloys Compd. 495 (2010) 126e130. [22] E. Kaldis, W. Peteler, J. Cryst. Growth 52 (1981) 125e130. [23] S. Augustine, E. Mathai, Semicond. Sci. Technol. 18 (2003) 745e754. [24] S.K. Das, R. Suryanarayanan, Thin Solid Films 175 (1989) 221e226.
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[25] M. Mobarak, H.T. Shaban, A.F. Elhady, Mater. Chem. Phys. 109 (2008) 287e290. [26] Yu. I. Ravich, S.A. Nemov, Semiconductors 36 (2002) 1e20. [27] M.V. Nikolic, K.M. Paraskevopoulos, T. Ivetic, T.T. Zorba, S.S. Vujatovic, E. Pavlidou, V. Blagojevic, A. Bojicic, O.S. Aleksic, N. Nikolic, W. Konig, P.M. Nikolic, J. Mater. Sci. 45 (2010) 5910e5914. [28] N.F. Mott, E.A. Davis, Electronic Processes in Non-crystalline Materials, Clarendon Press, Oxford, 1979. [29] Yu. I. Ravich, B.A. Efimova, I.A. Smirnov, Semiconducting Lead Chalcogenides, Plenum, New York, 1970. [30] K. Wojciechowski, J. Tobola, M. Schmidt, R. Zybala, J. Phys. Chem. Solids 69 (2008) 2748e2755. [31] G.T. Alekseeva, M.V. Vedernikov, E.A. Gurieva, P.P. Konstantinov, L.V. Prokofeva, Yu. I. Ravich, Semiconductors 32 (1998) 716e719. [32] Y. Pei, A.D. La Londe, S. Iwanaga, G.J. Snyder, Energy Environ. Sci. 4 (2011) 2085e2089.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Enhancement of electrical transport through the anisotropic nanostructure performance of heavily Yb-doped PbSe0.2Te0.8 thin films A. Hmood*, A. Kadhim, H. Abu Hassan School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Polycrystalline Pb1xYbxSe0.2Te0.8 ingots were prepared by solid-state microwave synthesis. < The microstructure of the ingots was revealed by SEM. < Effect of Yb doping on morphology of films was formation of large grains. < The power factor increased with increasing Yb content up to x ¼ 0.075.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 January 2012 Received in revised form 7 June 2012 Accepted 25 August 2012
Solid-state microwave-assisted plasma technique was used to synthesize high-purity Pb1xYbxSe0.2Te0.8 from initial components to obtain polycrystalline ingots with large grains. Pb1xYbxSe0.2Te0.8 thin films were then deposited onto glass substrates using thermal evaporation in vacuum. The films have the polycrystalline rock salt-type (NaCl) structure. The crystal lattice parameters for the powders and thin films obtained from X-ray diffraction (XRD) patterns showed that the values of the lattice parameters increased with increasing Yb content. The field emission scanning electron microscopy images reveal that the Pb1xYbxSe0.2Te0.8 thin films have uniform crystal grain sizes and dense nanostructures. The electrical transport properties of the thin films were measured in the temperature range of 298 Ke523 K. The Seebeck coefficient of the films increased with x when x was in the range of 0.015e0.045, whereas it decreased for x between the range of 0.06 and 0.105. The carrier concentration was 4.3 1017 cm3 for x ¼ 0.015, and the maximum value was 6.34 1017 cm3 at x ¼ 0.075. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: A. Semiconductors A. Thin films D. Electrical properties D. Microstructure
1. Introduction The quaternary compounds Pb1xYbxSeyTe1y form a large group of semiconducting materials with diverse optical, electrical, and structural properties [1]. These quaternary compounds are of particular interest because of their applicability as the basic material for developing heterostructure devices, such as laser diode, Schottky barriers, hydrogen detection, trace gas detection,
* Corresponding author. Tel.: þ60 142441397; fax: þ60 46579150. E-mail address: [email protected] (A. Hmood). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.08.069
and thermoelectric device [2,3]. Most of these compounds are grown by molecular beam epitaxy (MBE), liquid phase epitaxy, traveling heater zone, and solid-state reaction techniques [4]. G. Springholz et al. reported the realization of mid-infrared quantumdot lasers based on self-organized PbSe/PbEuTe quantum dots grown by MBE [5]. D.I. Pratun et al. reported high-temperature continuous wave operation (>100 K) and improved laser performance of the lattice mismatch PbTe/PbEuSeTe system grown by MBE [6]. A giant negative magneto-resistance effect has been studied in PbTe (Yb, Mn) bulk crystal grown using the Bridgman technique [7]. Recently, the thermoelectric materials of Pb0.9xSn0.1GexTe, prepared by heating the mixtures above the
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
at 300 K, which was performed by applying a vertical constant magnetic field of 1 T to the thin films using Lake Shore-637. 3. Results and discussion 3.1. Rietveld analysis of X-ray diffraction profiles
(422)
(420
(400)
x=0.105
x=0.09
Intensity (a.u.)
x=0.075 x=0.06 x=0.045 x=0.03
x=0.015 x=0.0 20
25
30
35
40
45
50
55
60
65
70
75
80
2 (degree)
(420)
(400)
(311) (222)
(422)
x=0.105
(220)
(111)
b
x=0.09 x=0.075
Intensity (a.u.)
Quaternary semiconductor compounds of Pb1xYbxSeyTe1y were synthesized from initial elements of high purity (Pb, Se, Te 99.999% 100 meshes, and Yb 99.9% 157 mm) weighing 2 g according to Pb1xYbxSeyTe1y (x ¼ 0.0e0.105), and y ¼ 0.2. A typical element ratio for the preparation of PbSe0.2Te0.8 is as follows: (1.2748 g) Pb, (0.0972 g) Se, and (0.6280 g) Te. The mixture was transferred into an agate mortar and pestle to create a homogenous mixture for 20 min, and was placed inside a quartz ampoule and then sealed under vacuum pressure of 105 mbar. The ampoule was exposed to microwave energy inside a microwave oven (MS2147C 800 W) operating at 2.54 GHz, with maximum power of 800 W for about 25 min. Solidification of the compound was achieved when the heat generated by the irradiation plasma inside the ampoule diffused the constitutional elements. Thin films of quaternary Pb1xYbxSe0.2Te0.8 were then deposited onto clean glass substrates using the thermal evaporation technique in vacuum at 106 mbar by an Alcatel-101 with tantalum boat to obtain thin films with a thickness of 164 nm that was measured using an optical reflectometer (Filmetrics F20, USA). Highresolution X-ray diffusion (XRD) analysis was used to confirm the crystal structure of the Pb1xYbxSe0.2Te0.8 powder and thin films using PANalytical X’Pert PRO MRD PW3040 with Cuka radiation for 2q values between 20 and 80 . The quantities of second phase percentage in the sublattices were examined using Rietveld refinement. The surface morphology of the ingots and thin films was examined through scanning electron microscopy, energy dispersive X-ray spectra (EDX, JSM-6460 LV), and field emission scanning electron microscopy (FESEM, Leo-Supra 50VP). The thermoelectric power was measured for all films at a temperature range of 298 Ke523 K between the two ends of each sample. One of the ends was heated using a designed electrical heater that supplies 100 W inside a brass block, and the other end was cooled by ice water supplied by a chiller inside a brass block cold sink. Thermal gradients of approximately 10 K at both ends were measured using two separate thermocouples (type-K EÓSun ECS820C) and were fixed to be in direct contact with the films. The electrical conductivity was measured using a four-point probe method in vacuum at 103 mbar. The carrier concentration was determined from the Hall voltage measurement using the Vander Paw method
YbTe
(311) (222)
2. Experimental procedure
(220)
(111)
a
(200)
The XRD patterns of the quaternary semiconductor compound Pb1xYbxSe0.2Te0.8 powder, which was synthesized by microwaveassisted plasma diffusion of initial components in stoichiometric form, are shown in Fig. 1 (a). The compounds have major polycrystalline phases of Pb1xYbxSe0.2Te0.8 compound in a rock salttype (NaCl) structure, and the diffraction peak (200) has a high intensity. The weak shoulder on the right-hand side of the (200) peak represents the second phase of YbTe in Pb1xYbxSe0.2Te0.8. These weak peaks disappear in the respective XRD spectra of Pb1xYbxSe0.2Te0.8 thin films, confirming the analytical results of the standard database (JCPDS No. 38-1435, and 18-1468), as shown in Fig. 1 (b). The average crystalline (grain) size D can be obtained
(200)
melting point of the constituent elements followed through the quenching method, have been examined [8]. Y. Gelbstein et al. investigated the development of p-type Bi2Te3-doped Pb1xGexTe and Sn1xGexTe alloys for thermoelectric applications [9,10]. The current paper reports the preparation of ternary and quaternary chalcogenide compounds, which may possess improved physical properties compared with compounds synthesized through other techniques [11,12], using solid-state microwave synthesis for the first time. This paper includes the study on the synthesis and characterization of Pb1xYbxSe0.2Te0.8 semiconductor compounds produced from the mixed stoichiometric ternary compounds of PbSe0.2Te0.8 with Yb-doping. The PbSe0.2Te0.8 was initially prepared. Then, Pb1xYbxSe0.2Te0.2-doped ingots were synthesized by exposure to microwave irradiation inside an evacuated quartz tube to obtain polycrystalline ingots with a large grain size, and were thermally evaporated to obtain thin films. The crystal structure of the quaternary semiconductor compounds of Pb1xYbxSe0.2Te0.8 (x ¼ 0.0, 0.015. 0.03, 0.045, 0.06, 0.075, 0.09, and 0.105) is that of a rock salt-type (NaCl) structure system with relative anomalies in the cubic unit cells, and thus in the lattice constants. The electrical transport properties of the quaternary Pb1xYbxSe0.2Te0.8 thin films were studied in the temperature range of 298 Ke523 K.
1149
x=0.06 x=0.045 x=0.03 x=0.015 x=0.0
20
25
30
35
40
45
50
55
60
65
70
75
80
2 (degree) Fig. 1. XRD patterns of Pb1xYbxSe0.2Te0.8 as (a) powders and (b) thin films.
1150
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
using Scherrer’s formula based on the XRD patterns of the films [13]:
D ¼
0:9l bcosqB
(1)
where l is the X-ray wavelength, b is the full width at half maximum of XRD peaks, and qB is the Bragg angle. The grain size is almost constant at 416 Å, with an increase in Yb content, except for three different compositions in which it decreases to 334 Å for both x ¼ 0.015 and 0.03, and to 278 Å for x ¼ 0.045. The cubic lattice constant, a, is calculated using the following equation [13]:
1 h2 þ k2 þ l2 ¼ 2 a2 d
(2)
where d is the inter planer distance, and h, k, and l are the lattice planes. Fig. 2 shows the respective lattice constants of Pb1xYbxSe0.2Te0.8 powders and thin films for different values of x. The lattice constants for both increase polynomial as the concentration x increases. The Yb substitution induces a continuous change in the nature of the chemical bonding, which is responsible for a linear xdependence of the lattice constant [14]. For x ¼ 0.015e0.105, the main phase of Pb1xYbxSe0.2Te0.8 averages 59% obtained from the Rietveld refinement of the XRD patterns of various Yb contents. As shown in Fig. 3, the XRD patterns fitted using the Rietveld refinement method for x ¼ 0.015, 0.075, and 0.105, and the corresponding results are listed in Table 1. At the same time, YbTe has the same NaCl-type structure (space group Fm3m) as the host PbSe0.2Te0.8, with a slightly smaller lattice parameter (aYbTe ¼ 6.365 Å, A) [15,16]. The atomic mass of Yb is 80% aPbSe0:2 Te0:8 ¼ 6:395 compared with the atomic mass of Pb, and the substitution of Yb for Pb induces an increase in the lattice constant a (Å) of the host lattice material PbTe [17]. However, the nature of bonding in Pb1xYbxSe0.2Te0.8 is rather complicated and comprises a mixture of ionic, covalent, and metallic bonds. A simple guess based on bond valence sums implies that all the bonds in Pb1xYbxSe0.2Te0.8 (metale chalcogenide, metalemetal, and chalcogenideechalcogenide) are more ionic than in pure PbTe. Hence, the most elongated of all, Ybe Te distance is z3% longer than the corresponding distance in YbTe (3.18 Å), and z1.5% longer than PbeTe distance in pure PbTe (3.23 Å) [18]. On the other hand, the larger atomic radius of tellurium (1.4 Å), compared with that of selenium atom (1.15 Å), presumably verifies that the tellurium atoms successfully enter into
-0.82 Powders Thin films Strain of the lattice
Lattice constant (Å)
6.45
-0.84 -0.86
6.44
-0.88
6.43
-0.90 -0.92
6.42
-0.94
Strain of the lattice (%)
6.46
6.41
-0.96 6.40
-0.98
6.39
-1.00 0.000
0.015
0.030
0.045
0.060
0.075
0.090
0.105
Yb content Fig. 2. Variations of the lattice constant for powder and thin films and the strain of the lattice with Yb content for thin films.
Fig. 3. X-ray diffraction patterns fitted using the Rietveld refinement method of Pb1xYbxSe0.2Te0.8 powders: (a) x ¼ 0.015, (b) x ¼ 0.075, and (c) x ¼ 0.105.
the lattice structures of PbSe0.2Te0.8. Three atoms are placed within this distance, that is, one from the basic components of the PbSe and PbTe (Pb or Se and Te), and the other is an impurity atom. The reduction in the lattice constant is the first indicator of the insertion of Yb into the cages of the Pb1xYbxSe0.2Te0.8 structure, thus changing Pbþ2 ions (radius ¼ 1.2 Å) into smaller Ybþ3 ions (radius ¼ 1.008 Å). A small fraction of the Yb ions (x ¼ 0.015) in the thin film decreased because of the transfer from a magnetic Yb3þ (4f13) charge state to a non-magnetically active charge state Yb2þ (4f14), which is usually present in x ¼ 0.03e0.105 [19,20]. The strain of the lattice exhibited the same behavior (Fig. 2), depending on the
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155 Table 1 The values of Rietveld refinement factors, weighted profile R factors (Rwp), profile R factors (Rp), expected R factors (Rexp), structure R factors (RF), goodness of fit (c), and D-statistics of Pb1xYbxSe0.2Te0.8 (x ¼ 0.015e0.105). x
0.015
0.03
0.045
0.06
0.075
0.09
0.105
Rp Rwp Rexp RI RF c ¼ Rwp/Rexp D-statistics
0.26667 0.2180 0.2844 0.5478 0.5211 0.766 0.1381
0.3743 0.3743 0.3265 0.6206 0.6134 1.146 0.1489
0.2806 0.2294 0.2610 0.6665 0.6598 0.879 0.1028
0.2446 0.2381 0.2650 0.7030 0.5418 0.898 0.0989
0.3176 0.2864 0.2828 0.6577 0.6349 1.013 0.0960
0.3141 0.2570 0.3209 0.6698 0.6472 0.801 0.1049
0.3572 0.3497 0.3047 0.6032 0.5864 1.148 0.0851
lattice constant of the as-grown thin films. This behavior can be calculated using the following equation [21]:
3 zz
¼
a ao 100% ao
(3)
where a is the lattice constant of the strained Pb1xYbxSe0.2Te0.8 asgrown films, and ao is the unstrained standard lattice constant of the bulk PbTe, i.e., 6.459 Å. The strains of the lattice for thin films for
a x=0.0
all x fractions have negative values, and are thus associated with compressive strains. 3.2. SEM and FESEM observation The microwave irradiation technique successfully formed Pb1xYbxSe0.2Te0.8 compounds, as clearly seen in the SEM images of the ingot shown in Fig. 4 (a), which reveals the formation of a microstructure. At a higher resolution, the ingots consist of closely irregular plates for doping levels x ¼ 0.0 and 0.015. The white grains that appear on the surface of the ingots for the doping levels of x ¼ 0.03e0.075 lead to the appearance of the second phase (as indicated in the XRD data) of Fig. 1 (a), which then disappear at the doping level x ¼ 0.09e0.105. Using EDX analysis, the amount of Yb substituted for Pb stoichiometric ratio of the ingots Pb1xYbxSe0.2Te0.8 was approximately equal to the standard elemental weight (Table 2). Deciding if such small aggregates are YbTe phase is not possible; the reason is that the YbeTe hinders the movement of the evaporation steps and slows down the evaporation. As the segregated phase has lower vapor pressure than PbTe, it can be formed only by the appearance of white grains on the surface [22].
x=0.015
1µm
1µm
x=0.045
x=0.03
YbTe
YbTe 1µm
1µm x=0.06
x=0.075 YbTe
YbTe 1µm x=0.09
1µm
1151
1µm x=0.105
1µm
Fig. 4. (a) SEM images of Pb1xYbxSe0.2Te0.8 ingots and (b) FESEM images of Pb1xYbxSe0.2Te0.8 thin films.
1152
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
b x=0.0
x=0.015
100 nm
100 nm
x=0.03
x=0.045
100 nm
100 nm
x=0.06
x=0.075
100 nm
100 nm
x=0.09
x=0.105
100 nm
100 nm Fig. 4. (continued).
Thin films of Pb1xYbxSe0.2Te0.8 deposited at room temperature have a shiny and very smooth mirror-like surface. FESEM images of the film surface reveal the formation of irregular grain crystallites. The effect of Yb content on the morphology of these polycrystalline films can be clearly seen in Fig. 4(b). Owing to the enhancement of surface diffusion and agglomeration of add-alloys with doping, the formation of large grains can be seen clearly. Grain boundaries and intervening voids are clearly visible for x ¼ 0.075. 3.3. Electrical properties Electrical conductivity, s, is an important factor and reliable information about the transport phenomenon of materials. The electrical conductivity of all Pb1xYbxSe0.2Te0.8 alloy thin films increased with increasing temperature, indicating the semiconductor behavior of all alloys. The electrical conductivity data were analyzed to distinguish the possible mechanisms involved in the thermally activated process. The variations of ln s versus 1/T of eight Pb1xYbxSe0.2Te0.8 films are shown in Fig. 5. Evidently, based on these curves, the conduction by thermal activation has two
different mechanisms with two different slopes, indicating the presence of two activation energies. The activation energy DEa can be estimated from the plot of ln s against 1/T according to equation [23]:
s ¼ so expð DEa =kTÞ
(4)
where so is a parameter depending on the semiconductor nature, and k is the Boltzmann constant. From these slopes of linear plots, activation energy for conduction was calculated for two temperature regions. The first electronic transition is in the lowtemperature region of 3.3e2.42 K1 (300 Ke413 K), whereas the other transition is in the high-temperature region of 2.36e 1.91 K1 (423 Ke523 K), indicating that a transition occurred near 423 K. The obtained values of DEa at different Yb content (x) for the Pb1xYbxSe0.2Te0.8 thin films are shown in Table 3. The linearity of ln s against 1/T for the low-temperature region indicates that ln s intrinsic conduction predominates, whereas in the middletemperature region of 2.42e2.36 K1 (413 Ke423 K), the slope of the curves continuously decreases with increasing temperature consistent with a degenerate semiconductor conduction caused
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155
by the formation of density of states near the Fermi level [24,25]. These densities of state oppose and reduce the electrical conduction by capturing the charge carriers. The Yb atoms behave like donors when they substitute Pb atoms. Therefore, the Ybþ2 (Ybþ2 ¼ Ybþ3 þ e transition) ions lead to the formation of deep localized or resonance impurity states in the forbidden band gap [5]. With increasing Yb content, the Fermi energy and the free electron density grow until the Fermi level reaches the impurity state level, after which no further increase in the electron density in the conduction band is evident, and the Fermi energy is pinned at DE1 (low-temperature region) for x ¼ 0.015e0.105. On the other hand, with increasing temperature at 413 Ke423 K, the impurity states lead to sink into the band gap, whereas the narrow width of the density-of-states of resonance states ensures a very strict pinning of the Fermi level relative to the conduction band edge, despite the inevitably non-uniform distribution of impurities and electrically active defects throughout the sample [26,27]. At DE2 (high-temperature region), ln s intrinsic conduction dominates due to the increase in the mobility of the carriers and the reduction of the trapping state and potential barriers [28]. The carrier concentration at room temperature is obtained from the measured Hall coefficient using equation RH ¼ 1/ne, where RH is the Hall coefficient, and n is the carrier concentration. The carrier concentration increases with increasing Yb content up to x ¼ 0.075, whereas it decreases at the doping level x ¼ 0.015 and 0.09. The Hall mobility can be calculated from the carrier concentration and the electrical conductivity according to s ¼ nem (e ¼ electron charge). The mobility at 300 K increases to a maximum value at x ¼ 0.075 because of the relatively larger values of electrical conductivity and carrier concentration (Table 3). The thermoelectric power (Seebeck coefficient, S ¼ DV/DT), as determined from the slope of thermoelectric e.m.f. versus the temperature difference between the hot and cold ends of the films, is shown in Fig. 6. The temperature dependence of the thermoelectric power for all the Yb-doped films has a semiconductor behavior, in which the values of the thermoelectric power increase as the temperature increases. Thus, the observed sign of the thermoelectric power is negative, indicating typical n-type semiconductors within the temperature range of 300 Ke523 K; therefore, electrons are the majority carriers in the films. A temperature difference causes exciting electrons in the conduction band to diffuse from the hot side to the cold side because of the concentration gradient, causing electric potential and the buildup of charge carriers on the cold side at equilibrium. The relation between thermoelectric power and charge-carrier (electron) concentration can be expressed as [29]:
Table 2 EDX spectra of Pb1xYbxSe0.2Te0.8 ingots. x
Element
Weight %
Atomic %
0.0
Se Te Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb Se Te Yb Pb
3.93 34.79 61.28 7.54 26.51 0.74 65.21 6.21 36.1 1.03 56.66 5.60 27.08 1.31 66.01 5.64 33.11 1.70 59.55 5.13 31.24 1.98 61.64 4.95 31.28 4.67 59.10 6.46 20.59 7.08 65.88
8.04 44.11 47.85 15.35 33.38 0.69 50.57 12.27 44.14 0.93 42.66 11.64 34.83 1.24 52.28 11.38 41.31 1.56 45.75 10.51 39.56 1.85 48.08 10.10 39.54 4.35 46.01 13.58 26.81 6.79 52.82
0.015
0.03
0.045
0.06
0.075
0.09
0.105
6.4 x=0.0 x=0.015 x=0.03 x=0.045 x=0.06 x=0.075 x=0.09 x=0.105
6.0
Ln (S/cm)
5.6 5.2 4.8 4.4 4.0
k S ¼ e
3.6 1.8
2.0
2.2
2.4
2.6
2.8
1000/T (K-1)
3.0
3.2
1153
3.4
"
3 !# 2 2pm* kT 2 5 rþ þ ln 2 h3 n
(5)
where r is the scattering parameter, h is the Planck’s constant, and m* is the effective mass of the electron. This relationship shows that
Fig. 5. Plots of ln s versus 1000/T for eight Pb1xYbxSe0.2Te0.8 thin films.
Table 3 Electrical conductivity (s), Seebeck coefficient (S), Hall coefficient (RH), carrier concentration (n), mobility (m) at 300 K, activation energy (DEa) at 298 Ke523 K. x
s (S cm1)
S (mV K1)
S2s (mW m1 K2)
jRHj (C cm3)
n 1017 (cm3)
m (cm2 V1 s1)
DE1 (eV) (300413) K
DE2 (eV) (423523) K
0.0 0.015 0.03 0.045 0.06 0.075 0.09 0.105
43.9 46.89 44.26 49.73 113.22 126.09 67.99 95.15
125.66 161.33 183.66 186.42 129.66 123.05 133.66 123.66
69.32 122.04 149.29 172.82 190.3 190.7 121.46 145.5
13.01 14.48 13.83 13.15 10.47 9.86 11.65 10.25
4.8 4.3 4. 52 4.7 5.97 6.34 5.4 6.1
571.14 678.97 612.11 653.95 1185.41 1243.25 792.08 975.28
0.096 0.095 0.073 0.044 0.08 0.05 0.068 0.035
0.541 0.430 0.152 0.091 0.23 0.138 0.265 0.123
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-100 x=0.0 x=0.015 x=0.03 x=0.045 x=0.06 x=0.075 x=0.09 x=0.105
-150
S (µV/K)
-175 -200
x=0.0 x=0.015 x=0.03 x=0.045 x=0.06 x=0.075 x=0.09 x=0.105
4500 4000
S2 (µW/mK2)
-125
-225 -250
3500 3000 2500 2000
-275
1500
-300
1000 500
-325
0
-350 300
325
350
375
400
425
450
475
500
525
550
300
325
Fig. 6. Temperature dependence of the Seebeck coefficient of Pb1xYbxSe0.2Te0.8 thin films.
the thermoelectric power is highly dependent on the carrier concentration, and that it decreases with increasing carrier concentration. Furthermore, the electrical conductivity varies as a function of Fermi level EF; hence, the change in thermoelectric power at room temperature is predominated by the variation of carrier concentration. Therefore, the electronic scattering is independent of energy, making the electrical conductivity proportional to the density of state near Fermi level, which indicates acoustic phonon scattering as a predominant carrier scattering mechanism in the thin films of Pb1xYbxSe0.2Te0.8 [30]. The value of the thermoelectric power increases with increasing temperature in the low-temperature region ranging from 300 K to 375 K, and then decreases due to the increase in thermally excited carriers. This further confirms that the electron concentration increases with temperature in Yb doped samples, because the slight decrease in thermoelectric power with temperature ranging from 385 K to 423 K results from the increase in n. This effect probably reflects the weaker donor activity of this impurity resonant states at extrinsic region, that indicates that the Fermi level is stabilized by the impurity band and that the filling of the latter with electrons. Increasing n shifts the impurity band upward, so that the initial concentration of electrons in the conduction band, which equals the number of electrons in the impurity band, increases [31]. For x ¼ 0.03 and 0.045 doping concentrations, the thermoelectric power of the films increases in sequence at low temperature, thereafter decreasing at a temperature of 423 K. For x ¼ 0.06, the thermoelectric power increases at the higher end of the temperature range, whereas from x ¼ 0.0 to 0.105, it increases for the whole temperature range. The power factor S2s of the Pb1xYbxSe0.2Te0.8 films was calculated in terms of the electrical conductivity and thermoelectric power; it is shown as a function of temperature in Fig. 7. The power factor strongly increases with temperature up to 523 K. The value of the power factor generally decreases with the increase in the amount of Yb-doping at x ¼ 0.015e0.045, and 0.105. At x ¼ 0.045, 0.06, and 0.075, the highest power factor at room temperature reaches 172.82, 190.3, and 190.7 mW m1 K2, respectively. Experimental results suggest that the electronic transport properties of these films are heavily influenced by the high density of states near the Fermi level, leading to the production of electrons with heavy effective mass [32]. These effective mass carriers lead to a large thermoelectric power at 300 K from 0.015 to 0.045, while obtaining high power factors of x ¼ 0.06 and 0.075 at 523 K. These results give
350
375
400
425
450
475
500
525
550
T (K)
T (K)
Fig. 7. Temperature dependence of the power factor of Pb1xYbxSe0.2Te0.8 thin films.
a clear idea of the application of these alloy compositions for thermoelectric devices. 4. Conclusion The quaternary alloys of the nanostructure Pb1xYbxSe0.2Te0.8 were successfully produced through solid-state microwave synthesis after 25 min of irradiation. The lattice parameter of the cubic unit cell increased with the increased concentration of Yb for the ground powders and thin films of Pb1xYbxSe0.2Te0.8. All the samples of Pb1xYbxSe0.2Te0.8 had a rock salt-type (NaCl) structure with a dominant peak representing the (200) plane. Electrical conductivity and activation energy were investigated. The conduction process in these films was done through a thermally activated process. Based on the measurements of the thermoelectric properties, the maximum S2s calculated was significantly improved to 4525.61 mW m1 K2 at 523 K for the Pb0.925Yb0.075Se0.2Te0.8 sample. Acknowledgments The authors are grateful for the funding provided by the Postgraduate Research Grant Scheme (No. 1001/PFIZIK/844134) of Universiti Sains Malaysia. References [1] R. Suryanarayanan, S.K. Das, Appl. Phys. Lett. 67 (1989) 1612e1614. [2] Z. Feit, R. Woods, D. Kostyk, W. Jalenak, Appl. Phys. Lett. 55 (1989) 16e18. [3] I.I. Zasavitski, E.V. Bushuev, E.A. Andrada-e-Silva, E. Abramof, JETP Lett. 75 (2002) 559e562. [4] J. Heremans, D.L. Partin, Phys. Rev. B 37 (1988) 6311. [5] G. Springholz, T. Schwarzl, W. Heiss, G. Bauer, M. Aigle, H. Pascher, I. Vavra, Appl. Phys. Lett. 79 (2001) 1225e1227. [6] D.L. Pratun, J. Electron. Mater. 13 (1984) 493. [7] I.I. Ivanchik, D.R. Khokhlov, A.V. Morozov, A.A. Terekhov, E.I. Slyn’ko, V.I. Slynko, A. de Visser, W.D. Dobrowolski, Phys. Rev. B 61 (2000) 889. [8] M. Sondergaard, M. Christensen, S. Johnsen, C. Stiewe, T. Dasgupta, E. Mueller, B.B. Iversen, J. Solid State Chem. 184 (2011) 1172e1175. [9] Y. Gelbstein, O. Ben-Yehuda, Z. Dashevsky, M.-P. Dariel, J. Cryst. Growth 311 (2009) 4289e4292. [10] D.L. Pratun, J.P. Heremans, C.M. Thursh, Superlattice Microstruct. 2 (1986) 459. [11] A. Hmood, A. Kadhim, H. Abu Hassan, Chalcogenide Lett. 8 (2011) 579e585. [12] A. Kadhim, A. Hmood, H. Abu Hassan, Mater. Lett. 65 (2011) 3105e3108. [13] B.D. Cullity, Element of X-ray Diffraction, third ed., Wesley Publishing Company, USA, 1967. [14] S. Merah, D. Ravot, G.A. Percheron, F.J. Olivier, J.C. Jumas, A. Mauger, E. Parent, J. Alloys Compd. 260 (1997) 17e22. [15] A. Iandelli, in: E.V. Kelber (Ed.), Rare Earth Research, Macmillan Co., New York, 1961.
A. Hmood et al. / Materials Chemistry and Physics 136 (2012) 1148e1155 [16] A. Hmood, A. Kadhim, H. Abu Hassan, Superlattice Microstruct. 51 (2012) 825e833. [17] A. Hmood, A. Kadhim, H. Abu Hassan, J. Alloys Compd. 520 (2012) 1e6. [18] I. Radisavljevic, N. Novakovic, N. Romcevic, M. Manasijevic, H.E. Mahnke, N. Ivanovic, J. Alloys Compd. 501 (2010) 159e163. [19] P.M. Nikolic, D. Lukovic, S.S. Vujatovic, K.M. Paraskevopoulos, M.V. Nikolic, V. Blagojevic, T.T. Zorba, B. Stamenovic, W. Konig, J. Alloys Compd. 466 (2008) 319e322. [20] E.P. Skipetrov, N.A. Chernova, L.A. Skipetrova, E.I. Slynko, Mat. Sci. Eng. B 91e 92 (2002) 412e415. [21] C.Y. Tsay, K.-S. Fan, S.-H. Chen, C.-H. Tsai, J. Alloys Compd. 495 (2010) 126e130. [22] E. Kaldis, W. Peteler, J. Cryst. Growth 52 (1981) 125e130. [23] S. Augustine, E. Mathai, Semicond. Sci. Technol. 18 (2003) 745e754. [24] S.K. Das, R. Suryanarayanan, Thin Solid Films 175 (1989) 221e226.
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[25] M. Mobarak, H.T. Shaban, A.F. Elhady, Mater. Chem. Phys. 109 (2008) 287e290. [26] Yu. I. Ravich, S.A. Nemov, Semiconductors 36 (2002) 1e20. [27] M.V. Nikolic, K.M. Paraskevopoulos, T. Ivetic, T.T. Zorba, S.S. Vujatovic, E. Pavlidou, V. Blagojevic, A. Bojicic, O.S. Aleksic, N. Nikolic, W. Konig, P.M. Nikolic, J. Mater. Sci. 45 (2010) 5910e5914. [28] N.F. Mott, E.A. Davis, Electronic Processes in Non-crystalline Materials, Clarendon Press, Oxford, 1979. [29] Yu. I. Ravich, B.A. Efimova, I.A. Smirnov, Semiconducting Lead Chalcogenides, Plenum, New York, 1970. [30] K. Wojciechowski, J. Tobola, M. Schmidt, R. Zybala, J. Phys. Chem. Solids 69 (2008) 2748e2755. [31] G.T. Alekseeva, M.V. Vedernikov, E.A. Gurieva, P.P. Konstantinov, L.V. Prokofeva, Yu. I. Ravich, Semiconductors 32 (1998) 716e719. [32] Y. Pei, A.D. La Londe, S. Iwanaga, G.J. Snyder, Energy Environ. Sci. 4 (2011) 2085e2089.