Low temperature DC conductivity, impedance spectroscopy and dielectric properties of Na doped Cd0.8Zn0.2S semiconductor compounds

Journal of Alloys and Compounds 609 (2014) 192–200

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Low temperature DC conductivity, impedance spectroscopy and dielectric properties of Na doped Cd0.8Zn0.2S semiconductor compounds G. Yellaiah ⇑, T. Shekharam, K. Hadasa, M. Nagabhushanam Department of Physics, University College of Science, Osmania University, Hyderabad 500007, India

a r t i c l e

i n f o

Article history: Received 18 February 2014 Received in revised form 15 April 2014 Accepted 18 April 2014 Available online 26 April 2014 Keywords: Co-precipitation technique DC conductivity Impedance spectroscopy Dielectric properties

a b s t r a c t Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2 and 0.3 mol%) samples have been synthesized by controlled co-precipitation technique. The samples are characterized by X-ray diffraction (XRD). DC conductivity studies were performed in the temperature region 77–300 K by using two probe method. The DC conductivity plots show the Arrhenius behavior and they exhibited VRH conduction mechanism. From the impedance, and dielectric measurements parameters like bulk resistance (Rg), capacitance (Cg), relaxation time (s) and dielectric constant (e0 ) were calculated. The relaxation time of Na doped Cd0.8Zn0.2S samples were found to increase with the increase in temperature and varied non linearly with the increase in Na content. AC conductivity of the samples was found to increase with the increase in frequency whereas dielectric constant was observed to decrease with the increase in frequency. The results were explained based on the defects and their complexes. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction II-VI compound semiconductors with wide energy gap and have been extensively used in optoelectronic devices, photo voltaic cells and solid state devices [1–5]. Among a variety of binary metal chalcogenides (CdTe, CdSe, ZnS, ZnSe, and CdS) the CdS – ZnS mixed system has been extensively studied. Moreover, with the compositional variation of Cd1xZnxS it is possible to alter the electronic behavior of the mixed system and can be widely used in electronic devices. Ternary semiconductors like CdZnS, CdZnSe and CdZnTe provide a possibility of tailoring their properties as per requirements and hence project themselves as important semiconducting materials for applications in the field of device fabrication [6–9].In recent years, due to the number of practical applications in the field of optoelectronics and electro-optics, a great deal of interest has been shown in the study of dielectric and conduction behavior of various semiconducting materials [10–13]. Cd1xZnxS ternary compound is also potentially used as a window material for the fabrication of p–n junctions without lattice mismatch in devices based on quaternary materials like CuInxGa1xSe2 [14]/CuIn(SxSe1x)2 [15]. Further, doping in binary and ternary semiconductors will alter the electrical properties due to defect creation in the lattice. The ⇑ Corresponding author. Tel.: +91 9908284675; fax: +91 40 7019020. E-mail addresses: [email protected], [email protected] (G. Yellaiah), [email protected] (M. Nagabhushanam). http://dx.doi.org/10.1016/j.jallcom.2014.04.124 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

most likely, the defect species in II–VI and their mixed compounds doped with Group I elements are interesting due to the photo characters of the defects. The incorporation of alkali metals like Na into interstitial sites of Cd0.8Zn0.2S leads to the formation of Na+i shallow donor centers [16]. These impurity centers make major contribution for the alteration in the electrical conductivity. Klyuev et al. [17] and Altosaar and Kukk [18] investigated electrical and luminescent properties on CdS and CdTe films and mono grain powders grown from aqueous solutions of cadmium complexes with thiourea and also on films of Cd1xZnxS solid solutions. In particular, defect structure, dielectric behavior and electrical properties of the films and nano compounds both pure and doped with Na alkali metal were studied [19]. The dielectric constant of a semiconductor is one among its most important properties. Its magnitude and temperature dependence are important for both fundamental and technological considerations. A thorough knowledge of this parameter is essential for solving many problems in semiconductor physics, particularly in the electric field stimulated emission studies, presence of impurities, voids, structural defects, polarization mechanisms, relaxation mechanisms [20,21]. Such studies in bulk Cd0.8Zn0.2S samples doped with alkali metal ion like Na is lacking. It is also estimated that Cd1xZnxS with x = 0.2 compound shows interesting optical and electrical properties [22,23]. Therefore, it was felt that the study of electrical, impedance analysis and dielectric properties of Cd0.8Zn0.2S semiconductor powders doped with Na would be interesting and useful.

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In the present paper the effect of temperature and Na content on dc conductivity, ac conductivity, dielectric constant and loss tangent (tan d) of undoped and Na doped Cd0.8Zn0.2S samples, prepared by co-precipitation have been studied and the results obtained are explained based on the photo defects formed due to the incorporation of Na in Cd0.8Zn0.2S matrix. 2. Experimental details Semiconductor powders of Cd0.8Zn0.2S doped with different amounts of sodium have been synthesized by controlled co-precipitation technique; the details of the preparation technique were given in our earlier articles [24–27]. Analytical reagent grade chemicals: cadmium acetate (Cd (CH3COO)22H2O), zinc acetate (Zn(CH3COO)22H2O), sodium chloride (NaCl) and thiourea (NH2CSNH2) have been used for the preparation. 1 M solution of cadmium acetate, zinc acetate, thiourea and 0.1 M solution of NaCl were prepared separately using double distilled water as a solvent, and were mixed in the stoichiometric proportion under vigorous stirring process. 1 M solution of triethylamine (TEA), a complexing agent, was added to the total volume of reaction mixture. This solution was made alkaline by adding 30% of liquid ammonia to maintain pH of the mixture about 10 and heated at 80(±2) °C with a constant stirring process for one hour. The final precipitate was dried at room temperature for 24 h and heated in nitrogen atmosphere for 2 h at 300 °C. The heat treated precipitate, after slow cooling (2 °C/min), was ground to fine powder and the process was repeated in order to obtain uniform particle size. The powder was pelletized to10 mm diameter and 2 mm thickness under the pressure of 5 ton/cm2 .The pellets were then sintered at 800 °C for 2 h in nitrogen gas. The pressure of the gas was maintained uniform (0.2 kg/cm2)throughout the heating process. Cd0.8Zn0.2S compounds containing different amounts of Nax (x = 0, 0.03, 0.1, 0.2, 0.3 mol%) were also prepared by this method. The X-ray diffraction (XRD) studies have been carried out using PANalytical’s X-ray diffractometer with Cu Ka radiation (k = 1.5418 Å). Spectra were recorded at room temperature in the angular range of 20° 6 2h 6 80° at a scan speed of 0.02°/s. The DC electrical conductivity measurements were performed using two-probe method by using high grade Eltecks 1228C silver paste as electrodes. A Keithley 182 Sensitive digital Voltmeter was used to measure the potential drop across the sample and Keithelymultimeter (Model 2000) was used to measure the output of the temperature sensor. A constant current source (Model Keithley 6220) was used to pass constant current across the sample. On the whole the error estimated in the conductivity measurements was about 3%. Impedance and AC conductivity measurements of samples were carried out using LCR meter (Model Hioki 3531 Z Hi Tester) at different constant temperatures from 383 K, 443 K to 513 Kin the frequency range 100 Hz–5 MHz with silver as electrode.

3. Results and discussion 3.1. Structural properties Fig. 1 shows the XRD patterns of Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2 and 0.3 mol%) compounds. The presence of X-ray diffraction

Fig. 1. X-ray diffraction pattern of Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2 and 0.3 mol%) compounds.

Fig. 2A. A Plot of lnr versus 1000/T in the temperature region (300–77 K) for Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2 and 0.3 mol%) compounds.

Fig. 2B. A plot of lnr versus 1000/T in the temperature region (300–77 K) for Cd0.8Zn0.2S: Nax (x = 0.2 mol%) compound with three activation energies. Table 1 Activation energies (Ea) and T0 values of Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2, and 0.3 mol%). Sample

Activation energy (Ea) (meV)

T0  103 K

Cd0.8Zn0.2S: Nax (x in mol%)

Ea1 (300– 250 K)

Ea2 (250– 166 K)

Ea3 (166– 100 K)

VRH (100– 77 K)

Undoped x = 0.03 x = 0.1 x = 0.2 x = 0.3

215.79 443.09 332.61 233.66 218.52

149.01 156.57 194.1 184.34 193.93

101.9 110.85 126.77 114.96 89.9

1.31 1.43 1.47 1.49 1.51

Fig. 2C. Plot of ln(r. T0.5) versus T0.25 for Na doped Cd0.8Zn0.2S compounds.

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Fig. 3. Variation of AC conductivity with frequency at different temperatures of Na doped Cd0.8Zn0.2S compounds.

peaks in all the samples shows that the compounds are polycrystalline and the position of diffraction peaks matched well with the JCPDS file (Cord No. #491302). This confirms the phase singularity with hexagonal phase structure of the synthesized material [22]. As no other peak corresponding to their binary system and Na impurity is observed, it confirms the formation of alloy nanocrystals rather than separate nucleation or phase formation. The observed identical diffraction pattern of all Na doped compounds confirms that the hexagonal structure of the compounds is not altered due to the inclusion of Na ion into Cd1xZnxS lattice. Broadness of diffraction peaks indicates the small crystallite size of the particles and the full width at half maximum (FWHM) of these peaks is used to calculate the average crystallite size (D) by Debye Scherrer equation [28]

D¼

0:94  k b Cos h

ð1Þ

Fig. 4. Exponents relating ac conductivity and frequency for x = 0.1 mol% Na doped Cd0.8Zn0.2S sample at 383 K.

G. Yellaiah et al. / Journal of Alloys and Compounds 609 (2014) 192–200 Table 2 Exponents (S1, S2 and S3) relating ac conductivity and frequency (r = AxS) of Nax doped (0.1 mol%) Cd0.8Zn0.2S compound. Cd0.8Zn0.2S: Nax (x = 0.1 mol%) compound Temperature (K)

S1

S2

S3

383 443 513

0.036 0.011 0.004

0.270 0.243 0.065

0.796 0.630 0.155

195

change in the cell parameters. This change could be due to the incorporation of Na in the Cd0.8Zn0.2S lattice. In our publication [22] we have estimated the presence of Na in the compound from EDAX analysis. Further, it was also estimated from the consideration of the radii of interstitial site and Na atom, that the occupation of Na+ ion into the interstitial site is possible. It is observed that there is no significant change in the value of ‘a’ and ‘c’ with the variation of Na content. 3.2. Low temperature conductivity

where k is the wavelength of X-rays, b is FWHM and h is the diffraction angle. The estimated crystallite size of the compounds, using Eq. (1), is in the range 29–55 nm. Lattice parameters were calculated using POWD software based on hexagonal phase and are found to be a = 0.405 nm, c = 0.675 nm. These values are compared with the structural parameters of Cd0.8Zn0.2S compounds as per JCPDS data base of hexagonal phase with cell parameters a = 0.4137 nm and c = 0.6715 nm and found that there is significant

The electrical conductivity measurements of undoped Cd0.8Zn0.2S and doped with different amounts of sodium (Na = 0, 0.03, 0.1, 0.2, 0.3 mol%) compounds were performed in the temperature region (77–300 K). Arrhenius plots of undoped and sodium doped Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2 and 0.3 mol%) samples are shown in Fig. 2A. From Fig 2A, it can be seen that the conductivity increases with the increase in temperature and Na content. The rise in electrical conductivity in all the samples is governed by

Fig. 5. Cole–Cole plot of Na doped Cd0.8Zn0.2S compounds.

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thermally activated process. The variation of electrical conductivity with temperature is according to the typical activation law [29]. A plot drawn between ln(r) and 103/T for Cd1xZnxS sample with x = 0.2 mol% is shown in Fig. 2B. The curve shows four regions I (300–250 K), II (250–166 K), III (166–100 K) and IV (100–77 K). These regions have been attributed to transitions from a sub band of the valence band to the conduction band. A similar conductivity variation was also observed by Szabo and Cocivera [30] and Thutupalli and Tomlin [31]. In sample with x = 0.2 mol% The plot shown in Fig. 2A exhibits Arrhenius behavior in three different temperature ranges 300–250 K, 250–166 K and 166–100 K. The activation energy for all the three regions is found by the relationship,

r ¼ r0 expðDEa =kTÞ

ð2Þ

where r0 is the pre exponential factor and dEa is the activation energy, k is the Boltzmann constant and T the temperature in Kelvin. The activation energy values of all the three regions calculated for all the samples are given in Table 1. In region I, the conductivity at a temperature decreases with increase in Na concentration

x = 0.03 mol% and then increases with the increase in Na concentration. This variation is also supported by the variation in activation energy with Na concentration (Table. 1). The conductivity in regions II & III increases with the increase in Na concentration. And the activation energy increases up to x = 0.01 mol% and then it decreases for further increase in Na content. The presence of various defects such as structural disorders, dislocations, surface imperfections, also plays a role in the variation of conductivity. Below 100 K, the conductivity is almost constant indicating the freezing of charge carriers. The electrical conductivity at low temperatures has contribution from four different types of conductions [32], i.e.: (a) ordinary conductivity (b) thermally assisted hopping conductivity, (c) hopping of charge carriers owing to the existence of localized states around EF and (d) variable range hopping (VRH) conduction. Gupta and Deh [33] have also reported a similar type of conduction mechanisms. The observed straight line variation of conductivity below 100 K indicates the existence of VRH conduction (shown in Fig. 2B). As is well known, the general form of the VRH conductivity is given by

Fig. 6. Variation of imaginary part of impedance (Z00 ) with frequency at different temperatures of Na doped Cd0.8Zn0.2S compounds.

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rT 1=2 ¼ r0 exp ðT 0 =TÞS

ð3Þ

where r0 is preexponential factor and T0 is a characteristic temperature coefficient. The value of the exponent ’s’ depends critically on the nature of hopping process. In the case of VRH, if the density of states at the Fermi level is constant, the VRH conductivity model is expressed with s = 1/4 in Eq. (3) (Mott Law) [34]. T0 is a characteristic temperature coefficient which depends on the density of states N(EF) at the Fermi Level [35]. The first term represents the conduction in the conduction band, most likely due to singly ionized native Cd interstitial defect [36,37] having ionization energy EA1  0.014 eV and

T 0 ¼ 163a =kNðEF Þ

ð4Þ

where a1 is constant of the spatial extension of the wave function exp (ax) associated with the localized states. A plot of ln(r. T0.5) versus T0.25 is drawn for the undoped Cd0.8Zn0.2S and Na doped samples is shown in Fig. 2C. From the plot it can be seen that in the temperature range 100–77 K there is a straight line indicating the validity of hopping conduction mechanism. The slope of the curve gives the value of T0. For each curve, value of T0 was calculated and given in Table 1. These values are in good agreement with Mott’s VRH method. 3.3. AC conductivity Frequency dependence of the conductivity for undoped and Nax doped (x = 0.03, 0.1, 0.2 and 0.3 mol%) Cd0.8Zn0.2S samples at different temperatures are shown in Fig. 3. Each curve in the figure displays a low frequency region, plateau region, which corresponds to the dc conductivity of the sample and a dispersive region at high frequency which corresponds to the AC conductivity. The conductivity shows dispersion that shifts to higher frequencies with an increase in temperature. This behavior suggests that the electrical conduction in the material takes place via hopping mechanism governed by Jonschers universal power law. It is seen from Fig. 3 that AC conductivity decreases with decreasing frequency and tends to become independent of frequency after a certain frequency. Extrapolation of this part towards lower frequency gives DC conductivity. The conductivity spectra are investigated by the Jonscher universal power law;

rtot ¼ rdc þ Axn :

ð5Þ

rac ¼ A1 xn1 þ A2 xn2 þ A3 xn3

ð6Þ

where ‘n’ is the frequency exponent in the range 0 < n < 1 [38]. Both rdc and ‘A’ are thermally activated quantities. All the curves in Fig. 3 show three different frequency variations of conductivity (regions I, II and III). Three different linear regions corresponding to exponents, n1, n2, and n3, relating ac conductivity and frequency (Eq. (6)) are explicitly shown for the sample x = 0.1 mol% of Na at 383 K is shown in Fig. 4. From this plot the variation have seen explicitly. These regions are associated with pre exponential factors A1, A2 and A3 and n1, n2 and n3 are exponents. The values of n1, n2 and n3 are given Table 2. The values of n3 are always more than n2 and n1 in the temperature range investigated. With increase of temperature, n3 decreased and found to vary between 0 and 1. The values of n2 and n1 are found to lie between 0 and 1 in the temperature range and their temperature dependence indicates that the conduction mechanism in these materials is not due to the intrinsic polarization process, but is associated with the hopping of charged species across the charged defects. A charge carrier trapped in a lattice within the distribution produced by its presence in the localized site is known as polaron. Depending upon the spatial extent of influence in the lattice, in the vicinity of carrier, the term small and large polaron is used. Such type of behavior is shown

commonly by various other materials and was reported in the literature (Mardare and Rusu 2004) [39]. 3.4. Impedance spectroscopy Impedance analysis has been widely used to study the electrical properties of materials. The complex impedance is given by

Z  ¼ Z 0  jZ

00

ð7Þ

Z 0 ¼ ZCos u and Z 00 ¼ ZSin uÞ

ð8Þ

where Z0 is the real part of impedance, Z00 is the imaginary part of impedance, u is the phase angle. Fig. 5 shows the Cole–Cole plot, (a plot of the imaginary part versus real part of complex impedance) of undoped and Nax doped (x = 0.03, 0.1, 0.2, 0.3 mol%) Cd0.8Zn0.2S samples over the frequency range 100 Hz–5 MHz at different temperatures. The plots clearly show that there is an inclined straight line in the low frequency region, followed by a semicircular arc at the high frequency region. The Cole–Cole plots produced are depressed semicircles and the centre of the semicircle lies below the real impedance axis [40]. As the centre of the semicircle in the Fig. 6 is below the Z0 axis, the relaxation present in all the samples must be non–Debye type and follows Maxwell–Wagner relaxations. The bulk resistance is found to decrease with increase in the temperature and manifests as decrease in radius of the semicircle. Fig. 5 shows the decrease in impedance with increase in temperature and increase in Na content may be the reason for the decrease in bulk resistance with increase in temperature and Na content. The bulk resistances (Rg) of the samples at various temperatures have been calculated from the low frequency intercept of the real axis. The capacitance (Cg) were obtained from the frequency corresponding to the highest point in each semicircle, using the relation 2pfmaxRC = 1; where fmax is the relaxation frequency. These parameters are given in Table 3. The table shows a clear decrease in bulk resistance with the increase in Na content. Fig. 6 shows the variation of imaginary part of impedance (Z00 ) with frequency at different temperatures (383, 443 and 513 K). From this figure it is observed that, Z0 0 peak shifts towards higher frequencies with increase in temperature. All these curves merge into single curve at a frequency higher than 5 MHz. The magnitude of this peak decreased with increasing temperature, thereby indicating an increase in ionic migration loss in the samples. Z0 0 value decreased drastically and the broadness/diffuseness of these peaks Table 3 Resistance (Rg), capacitance (Cg), relaxation time (s) and dielectric constant (e0 ) values of Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2, and 0.3 mol%). Temperature (K) Undoped 383 K 443 K 513 K X = 0.03 mol% 383 K 443 K 513 K X = 0.1 mol% 383 K 443 K 513 K X = 0.2 mol% 383 K 443 K 513 K X = 0.3 mol% 383 K 443 K 563 K

Capacitance (pF)

fmax (kHz)

s1

e0 (at

(107 s)

100 Hz)

1.73 9.66 1.77

0.041 0.643 0.428

220 256 2100

7.23 6.21 0.75

594 1700 768

12.51 29.30 0.66

0.472 6.033 0.927

269 900 2600

5.91 1.76 0.61

362 828 1447

11.34 5.50 1.05

0.159 0.321 0.561

880 900 2700

1.80 1.76 0.58

351 428 570

9.57 2.67 0.61

0.554 0.396 0.965

300 1500 2700

5.30 1.06 0.58

353 435 501

9.41 9.31 3.01

0.758 0.341 0.230

223 500 2300

7.13 3.18 0.69

274 1628 4292

Resistance (kX)

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increase with increase of temperature. This behavior suggests the presence of temperature dependent relaxation process in the samples. This behavior was also observed in ferrite (CuxFe3xO4) and semiconducting (ZnS, ZnSe) materials [41–43]. The relaxation time (s), calculated using the relaxation frequency and the relation s = 1/2pfmax, are given in Table 3. From Table 3 it can be seen that the relaxation time (i) increases with increase in temperature (ii) varies non linearly with the increase in Na content, this may occur due to the various defects like structural inside the samples.

3.5. Dielectric studies The dielectric properties of any system can be characterized by frequency dependent parameters that can be defined by the complex permittivity e*. The complex permittivity or dielectric constant of a system is defined by

e ¼ e0  je00

ð9Þ

where e0 is the real part of dielectric permittivity, e00 is the imaginary part of dielectric permittivity. The relationship between the conductivity and dielectric loss factor is given by

e0 ¼ 

Z 00 2

2

xC 0 ðZ 0 þ Z 00 Þ

ð10Þ

and

e00 ¼

Z0 2

2

xC 0 ðZ 0 þ Z 00 Þ

ð11Þ

where x is the angular frequency, C0 is the vacuum capacitance of the measuring cell.The net polarization in any dielectric material is made up of four different components according to the nature of the

Fig. 7. Variation of dielectric constant (e0 ) with frequency at different temperatures of Na doped Cd0.8Zn0.2S compounds.

G. Yellaiah et al. / Journal of Alloys and Compounds 609 (2014) 192–200

displaced charge. The average dipole moment per molecule is given by [44];

l ¼ ap E; and the electric polarization by,

P ¼ Nap E;

ð12Þ

where N is the number of molecules per unit volume, E is the electric field intensity acting on each molecule, and ap is the net polarization,

ap ¼ ae þ ai þ ao þ as

ð13Þ

where ae, aI, ao, and as are the electronic, ionic, orientational, and interfacial or space charge polarizabilities. Fig. 7 shows the plot of the real part of dielectric constant (e0 ) with logarithmic frequency

199

of undoped and Na doped (0.03, 0.1, 0.2, 0.3 mol%) Cd0.8Zn0.2S samples at different temperatures. From the plot it can be seen that the dielectric constant initially decreases exponentially with increase in frequency and then attains almost a constant value in the high frequency region. At low temperature and high frequency, we may take it as negligible. However, it is significant in the low frequency region. As the space charge effect increases, the contribution of the space charge effect towards polarization may have a tendency to increase. This also indicates that the value of the dielectric constant increases with the temperature. The contribution towards the decrease in the dielectric constant due to electronic polarization is quite less. The contribution of space charge depends on the purity of the sample. The decrease of e0 with frequency can be attributed to the fact that, at low frequencies e0 depends on both the deformational (electronic and ionic) and relaxation (orientational and space

Fig. 8. Variation of loss tangent (tan d) with frequency at different temperatures of Na doped Cd0.8Zn0.2S compounds.

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charge) polarization mechanisms. The deformational polarization depends on the displacement of electrons and ions while the relaxation polarization depends on the oriental or interfacial effects. The increase in frequency leads to a decrease in orientational polarization, since the molecular dipoles take more time to change their orientation in response to field. The decrease tends to reduce the value of e0 with increasing frequency. The increase in e0 with temperature is due to the fact that, when temperature is increased the orientation of dipole is facilitated and this increases the orientational polarization [45]. The increase in orientational polarization, results in the increase of dielectric constant (e0 ). The high value of the dielectric constant at low frequencies may be due to the presence of all the four polarizations; namely, electronic, ionic, orientational, and interfacial or space charge polarization and its low value at higher frequencies may be due to the gradual loss of significance of these polarizations. Fig. 8 shows the variation of loss tangent (tan d) with frequency for all the undoped and Na doped Cd0.8Zn0.2S samples at different temperatures. These curves suggest that the dielectric loss is strongly dependent on the frequency of the applied field, similar to that of the dielectric constant. The dielectric loss decreases at all the temperatures with an increase in the frequency at almost all temperatures, but appears to achieve saturation in the higher frequency range of 1 MHz and above. In the low frequency region, high energy loss is observed, this may be due to the dielectric polarization, space charge and rotation direction polarization occurring in the low frequency range. 4. Conclusions  Cd0.8Zn0.2S: Nax (x = 0, 0.03, 0.1, 0.2 and 0.3 mol%) samples have been synthesized by controlled co-precipitation technique.  XRD studies revealed that the samples have polycrystalline nature with hexagonal structure and the average crystallite size varied from 29 to 55 nm.  Low temperature (77–300 K) DC conductivity studies showed three types of conduction mechanisms namely (a) ordinary conductivity (b) thermally assisted hopping conductivity, (c) hopping of charge carriers owing to the existence of localized states around EF and (d) variable range hopping (VRH) conduction.  From the impedance, and dielectric measurements bulk resistance (Rg), capacitance (Cg), relaxation time (s) and dielectric constant (e0 ) values were calculated.  In Na doped Cd0.8Zn0.2S samples the relaxation time increases with the increase in temperature and varies non linearly with the increase in Na content.  Dielectric loss tangent (tan d) value is maximum at low frequency and minimum at high frequency and it decreased with temperature.

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Acknowledgements

[43] [44]

One of the authors (G. Yellaiah) thanks the UGC-BSR, New Delhi for providing research meritorious fellowship. Authors thank the Head department of physics for his constant encouragement and also thank Dr. T.L. Prakash, Director, C-MET, hyderabad and Dr. Y. Purushotham, Scientist – E, C-MET, Hyderabad for permitting to use the dielectric measurements at center.

[45]

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