Electrical and dielectric properties of In2S3 synthesized by solid state reaction

Accepted Manuscript Electrical and dielectric properties of In2S3 synthesized by solid state reaction A. Timoumi, N. Bouguila, M. Chaari, M. Kraini, A. Matoussi, H. Bouzouita PII:

S0925-8388(16)30901-X

DOI:

10.1016/j.jallcom.2016.03.290

Reference:

JALCOM 37178

To appear in:

Journal of Alloys and Compounds

Received Date: 12 September 2015 Revised Date:

19 March 2016

Accepted Date: 30 March 2016

Please cite this article as: A. Timoumi, N. Bouguila, M. Chaari, M. Kraini, A. Matoussi, H. Bouzouita, Electrical and dielectric properties of In2S3 synthesized by solid state reaction, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.290. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Electrical and dielectric properties of In2S3 synthesized by solid state reaction A. Timoumi1,2, N. Bouguila3,*, M. Chaari4, M. Kraini3, A. Matoussi4, H. Bouzouita1

1

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Photovoltaic and semiconductor materials laboratory, National Engineering School of Tunis, Belvedere PO Box 37, 1002 Tunis, Tunisia. 2 Department of Physics, College of Applied Science, Umm AL-Qura University, Makkah, Saudi Arabia KSA. 3 Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à l’environnement, Université de Gabès, Faculté des Sciences de Gabès, Cité Erriadh, Zrig 6072 Gabès, Tunisia. 4

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*Corresponding author: [email protected]

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Laboratory of Composite Ceramic and Polymer Materials (LaMaCoP), Sfax Faculty. of Science, Soukra Road Km 4 Sfax 3038 Tunisia.

Abstract

Indium sulfide (In2S3) material was synthesized by solid state reaction technique. The obtained powder was characterized by means of X-ray diffraction (XRD) and impedance

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spectroscopy. X-ray analysis shows that the obtained phase is β-In2S3 with cubic structure. The average grain size is about 42.6 nm, indicating the nanocrystalline nature of In2S3 material. The ac conductivity and dielectric properties of In2S3 have been investigated in a

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wide frequency (10Hz-1MHz) and temperature (0°C-100°C) ranges. We found that the electrical conductance of In2S3 material is frequency and temperature

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dependent. The frequency dependence of ac conductivity follows the Jonscher’s universal dynamic law. The variation of ac conductivity and the frequency exponent ‘s’ are interpreted by the correlated barrier hopping (CBH) model. Activation energy value inferred from conductance spectrum matches very well with the value estimated from relaxation time, indicating that the relaxation process and conductivity have the same origin. In addition, the electronic conduction appears to be dominated by thermally activated hopping of small polaron (SPH) at high temperatures and by variable range hopping (VRH) at low

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ACCEPTED MANUSCRIPT temperatures. Values of dielectric constants ε′ and ε″ were found to decrease with frequency and increase with temperature. Such an observation is interpreted by Maxwell-Wagner-Sillars model (MWS).

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Keywords: In2S3; Solid state reaction; XRD; Electrical properties; Dielectric properties 1. Introduction

In2S3 thin films appear to be promising candidates for many technological applications

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due to their stability, large band gap and photoconduction [1, 2]. In2S3 is an important material

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for optoelectronic and photovoltaic applications [3] and photoelectrochemical solar cell devices [4].

In2S3 which is a III-VI material, is known to crystallize in three polymorphic forms as a function of temperature [5] e.i. a defect cubic structure called α-In2S3 (stable up to about 420°C, a defect spinel structure called β-In2S3 (stable up to about 754°C) and a higher

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temperature layered structure, the γ-form (above 754°C). Among these structures, β-In2S3 is the most stable phase at room temperature. In2S3 is a direct band gap material with an energy

[6,7].

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varying in the range of 2.0-2.75 eV, depending on the composition and deposition parameters

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Using Cu(In,Ga)Se2 (CIGS) as an absorber film, photovoltaic conversion efficiencies higher than 10% have been achieved [8]. These higher efficiency values are tied to the use of a CdS interfacial buffer layer. Therefore, for environment protection and for industrial production, it would be preferable to replace CdS by a less toxic material. The constituent elements of In2S3 (indium and sulfur) are nontoxic and environment friendly. In2S3 seems to be an ideal candidate to substitute the highly toxic CdS as the buffer layer in CuInSe2 and CuInS2 based solar cells. The motivation of searching for an alternative buffer layer is not only to eliminate toxic cadmium but also to improve light transmission in the blue wavelength region, by using 2

ACCEPTED MANUSCRIPT a material having a wider band gap, than CdS [9]. A conversion efficiency of 11% was obtained for a CuInS2 based solar cell, in which CdS was replaced by Inx(OH,S)y [10]. In2S3 has been synthesized by various techniques such as co-evaporation [11-13], RF sputtering [14], atomic layer deposition (ALD) [15], sol gel [16], spray pyrolysis [17] and

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chemical bath deposition (CBD) [18].

Despite the abundant literature on In2S3, only few investigations about their a.c. conductivity have been published. For instance, Sankir et al. [19] have reported the electrical

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properties of silver doped In2S3 films prepared by spray pyrolysis. With the same technique, Bouguila et al. [20] have reported the ac conductivity spectra of annealed In2S3 thin films.

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Timoumi et al [21] and Seyam et al. [22] have investigated the ac conductivity of In2S3 films synthesized by thermal evaporation technique. To the best of our knowledge, this is the first report on the ac conductivity and dielectric properties of powder material In2S3 in pellet. To get a better understanding of the nature of electrical processes in In2S3 films, we

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have studied their electrical and dielectric properties by admittance spectroscopy technique. The latter is extremely useful, particularly in differentiating the transport characteristics in

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grains and grains boundaries. 2. Experimental details

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2.1. Powder preparation

The In2S3 powder was synthesized in vacuum from a solid solution of a stoichiometric mixture of pure indium and sulfur. These elements are sealed in quartz tube under a high vacuum of about 10-5 Torr. The tube was introduced into a programmable furnace with a maximum temperature of 1200 °C allowing great flexibility in the development of thermal profiles [23].

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ACCEPTED MANUSCRIPT The thermal profile adopted for In2S3 powder is shown in Fig. 1. This profile is achieved according to In2S3 phase diagram. To avoid possible explosion of the tube due to the high sulfur vapor pressure (10 atm at 493 °C and 20 atm at 740 °C) we have adopted a low heating

We have kept this temperature constant for 16 hours.

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rate of 10 °C/hour up to 460 °C in order to promote the reactivity of the sulfur with indium.

Then, a rise of 20 °C/hour has programmed to reach 880 °C. After that, the elements have been annealed for 6 hours at this temperature in order to have a good interdiffusion of these

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elements. Finally, we have programmed a descent of 10 °C/hour to 300 °C before stopping the heating and let the whole cool down naturally to room temperature. This experiment is carried

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out along seven days. The obtained powder constituted by fine particles with brick red color. The obtained powder was then pressed into pellet disk (of about 1mm in thickness and 6mm in diameter) and sintered at 300°C for one hour with a heating rate of 10 °C/min. Afterwards, the pellet was cooled slowly to room temperature.

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2.2. Characterization techniques

The crystalline structure and phase purity of the powder was analyzed by X-ray

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diffraction (XRD) using Cu-Kα radiation (1.5406 Å) of a Bruker D8 Advance diffractometer

0.02°.

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operating at 40 kV, 40 mA. The scan was performed in the range 2θ = 25°-50° with a step of

The dielectric analysis was performed by means of NOVOCONTROL system for broadband dielectric spectroscopy (BDS). A nitrogen flow was used for temperature adjustment of the pellet which was inserted between two parallel plate electrodes (sandwich geometry). Besides, a sinusoidal voltage was applied, creating an alternating electric field perpendicular to the pellet. The induced polarization in the pellet oscillates at the same frequency but with a phase angle shift δ, which was measured by comparing the applied voltage to the measured current.

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ACCEPTED MANUSCRIPT The real (ε′) and the imaginary (ε″) parts of the complex permittivity were calculated through the measurements of capacitance and conductance. The ac conductivity and the dielectric properties have been investigated in a wide frequency range (10 Hz-1 MHz) and for

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temperatures between 0°C and 100°C. 3. Results and discussion 3.1. Structural study

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Fig. 2 shows X-ray diffraction pattern of the synthesized In2S3 powder. All the XRD peaks correspond to the cubic structure of polycrystalline In2S3 with diffractions originating

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from (311), (222), (400), (331), (422), (511) and (440) planes, according to JCPDS card no. 65-0459. No impurity phase was detected. The results are shown in Table 1. At a relative error lower than 1% between the theoretical and experimental Bragg angle, the obtained results indicate that β-In2S3 is the dominant phase.

D=

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The average grain size D of In2S3 sample was determined by using Scherrer formula [24]: 0 .9 λ β cos θ

(1)

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where β is the full width at half maximum (in radians), λ is the X-ray wavelength and θ is the Bragg angle. The calculated average grain size is about 42.6 nm. Such value proves the

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nanocrystalline nature of In2S3. 3.2. Dielectric properties

The complex dielectric constant of materials is given by:

ε * = ε ' − jε "

(2)

where ε′ and ε″ are the real and imaginary parts of the complex dielectric constant.

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ACCEPTED MANUSCRIPT Fig. 3 shows the frequency dependence of ε′ of the sample at different temperatures. It is observed that ε′ decreases with the applied field frequency and increases with temperature. For polar materials, the decrease of ε′ with frequency can be explained by the contribution of different polarizability origins, deformational (electronic and ionic) and relaxation

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(orientational and interfacial) polarization [25]. The decrease of ε′ with frequency was also reported by Sankir et al. [19].

On the other hand, the increase of dielectric constant ε′ with temperature, as shown in Fig. 3,

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can be ascribed to the fact that the orientation polarization is associated with the thermal

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motion of molecules, so dipoles cannot orient themselves at low temperatures. When the temperature is increased, the orientation of dipoles is facilitated and tends to increase the value of orientation polarization, which conducts to an increase in the dielectric constant [22]. The imaginary part (ε″) of the dielectric constant is a measure of the dissipated energy in the dielectric due to the presence of an applied electric field. The evolution of ε″ as a function of

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frequency for different temperatures is shown in Fig.4. At low frequency, ε″ decreases speedily with the increase in frequency then, it attains a constant value for higher frequencies. Furthermore, the low frequency dispersion of ε″ increases with the increase in temperature.

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The sudden decrease of ε″ at lower frequencies is attributed to the existence of space charge

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polarization in the pellet. This type of polarisation may be explained on the basis of the MWS theory of dielectric dispersion [26,27]. The In2S3 material exhibits space charge polarization due to its structural heterogeneity and the surface defects at the grain boundaries [28]. As the temperature increases, the ε″ peak position shifts into the region of higher frequencies. This behaviour is due to the existence of the broad spectrum of the time relaxation tin the material [29]. An additional information that can be deduced from the measure of ε″ is the dielectric loss (tanδ) given by tanδ = ε"/ε', which is proportional to the amount of energy dissipated in a dielectric material. The variation of (tanδ) with frequency for different 6

ACCEPTED MANUSCRIPT temperatures is plotted in Fig. 5. At high temperature, the increase of tan δ is predominantly due to the dc conductivity contribution. tanδ values decrease with the increase of frequency which confirms that Maxwell-Wagner relaxation is responsible for the enhancement of the dielectric permittivity observed at low frequency. Then, it can be concluded that the

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investigated sample can be used for high frequency device applications. 3.3. Electrical properties

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The ac electrical properties of In2S3 material were also carried out in a wide range of temperature (0°C-100°C) and frequency (10Hz-1MHz). It is shown in Fig. 6 that the

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conductivity increases with increasing both frequency and temperature. The ac conductivity σac dependency on temperature and frequency can be given by [30]:

σ ac (ω , T ) = σ dc (T ) + A(T )ω s

(3)

where σdc is the dc conductivity, A is a constant which slightly depends on temperature and ‘s’

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is the frequency exponent factor that measures the interaction between the mobile ions. As seen in Fig. 6, the ac conductivity is nearly constant at low frequencies and is

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approximately equal to dc conductivity. On the other hand, σac depends strongly on temperature. Above a certain range of frequencies, it increases according to the power law

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σac= Aωs.

Fig. 6 can be divided into two frequency (f) regions. For f < 104 Hz, it can be seen that the frequency exponent ‘s’ decreases from 0.58 to zero, indicating a CBH model. For f > 104 Hz, ‘s’ increases from zero to 0.56 with the temperature increase . Such behavior indicates that small polaron tunnelling (SPT) is the dominant conduction mechanism in the In2S3 material for high frequency [31, 32]. The linear dependence of the dc conductivity σdc as a function of reciprocal temperature (Fig. 7), indicates that the dc conductivity obeys the Arrhenius relation:

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ACCEPTED MANUSCRIPT σ dc = σ 0 exp(−

E dc ) kT

(4)

where Edc is the activation energy, σ0 is the pre-exponential factor, k is the Boltzmann constant and T is the absolute temperature [33-36]. The dc activation energies Edc have been evaluated

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to be (0.73 eV, 1.06 eV). These values can be attributed to deep traps in the gap responsible of the low value of conductivity and therefore the intrinsic behaviour of In2S3.

There exists a critical frequency ωc for each temperature beyond which a power law is

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followed. Kilbride et al. [37] proposed an experimental definition of the critical frequency following the formula:

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σ (ω c ) = 1.1σ dc

(5)

The typical variation of ωc as a function of temperature is shown in Fig. 8. Experimental data can be fitted by a linear law. Activation energy values estimated from Arrhenius plots are 0.69 eV and 1.08 eV. These values are in good agreement with those deduced from the dc

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conductivity variation. Fig. 9 shows the logarithmic plot of the critical frequency ωc as a function of dc conductivity σdc. It is clear that σdc increases linearly with ωc, indicating a linear

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relationship. A well-known proportionality of σdc versus ωc is observed in systems characterized by hopping transport. The scaling frequency for a number of systems obeys to

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ω c ∝ T qσ dc law [38]. This equation is also known as BNN (Barton, Nakijama and Namikawa [39,40]) relation. The value of q depends on material nature. However, the observed linearity in the Fig. 9 implies that q = 0 for In2S3. The plot of (σdcT) in a logarithmic scale as a function of reciprocal temperature is depicted in Fig. 10. At high temperatures, a linear variation was observed, which proves that conductivity is dominated by thermally activated SPH and can be described by Mott and Davis law [41]:

σ dcT = A exp(

Ep k BT

)

(6)

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ACCEPTED MANUSCRIPT where A is the pre-exponential factor and Ep is the activation energy. The activation energy is found to be 1.08 eV, similar to earlier obtained values (1.06 eV, 1.08 eV). The variation of the conductivity versus T −1 / 4 is shown in Fig. 11. At lower temperature region, the observed linearity indicates that the 3D-VRH mechanism may be appropriate to

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describe the temperature dependence of the material conductivity [42]. The VRH description is considered as equally applicable to charge carriers as electrons, holes, polarons or bipolarons provided that the suitable wave function is employed. In this model, when the

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interaction between charge carriers is neglected, the dc conductivity is given by the law:

(7)

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σ dc = σ 0 exp(−T0 / T )1 / 4

where σ0 is a pre-exponential factor and T0 is the characteristic temperature which determines the hopping thermal activation. In the conventional VRH model, the parameters σ0 and T0 are function of localization length and density of states.

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4. Conclusion

In2S3 powder was synthesized by solid state reaction technique. X-ray diffraction

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measurements show that the obtained phase is β-In2S3 with cubic structure. The average grain size is about 42 nm, indicating the nanocrystalline nature of the films.

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The electrical conductance of In2S3 is found to be dependent on both temperature and frequency. The ac conductivity was found to obey to Jonscher’s universal power law. Indeed, for low frequencies, the conduction mechanism is interpreted by CBH model but for high frequencies, the SPT conduction mechanism is dominated. Activation energy inferred from conductance spectrum matches very well with the value estimated from relaxation time indicating that relaxation process and conductivity have the same origin. The activation energies have been found to be around 0.73 eV and 1.06 eV. These deep levels are responsible to insulator behavior of the films. In addition, the electronic conduction appears to be 9

ACCEPTED MANUSCRIPT dominated by thermally activated SPH at high temperatures and by VRH at low temperatures. Values of dielectric constants ε′ and ε″ were found to decrease with frequency and increase with temperature. They are interpreted by MWS model. Then, it can be concluded that this sample can be applied for high frequency device applications such as design of varistors‚

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super-capacitors‚ MOSFET and GBT for power devices.

Acknowledgments

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The authors would like to thank S. Alaya (Faculté des Sciences de Gabès, Tunisia), (I.

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Halidou, Faculte´ des Sciences et Techniques, Universite´ Abdou Moumouni, Niamey, Niger) and H. Rahmouni (Institut Supérieur des Sciences appliquées et de Technologie de Kasserine,

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Tunisia), for the manuscript revision and the useful discussions.

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ACCEPTED MANUSCRIPT [32] M.H. Buraidah, L.P. Teo, S.R. Majid, A.K. Arof, Ionic conductivity by correlated barrier hopping in NH4I doped chitosan solid electrolyte, Physica B 404 (2009) 1373-1379. [33] P. Nunes, E. Fortunato, P. Vilarinho, R. Martins, Effect of different dopant elements on the properties of ZnO thin films, Vacuum 64 (3-4) (2002) 281-285.

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ACCEPTED MANUSCRIPT Figures and Tables captions Fig. 1. Thermal profile of synthesized In2S3 particles. Fig. 2. XRD spectra of In2S3 powder.

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Fig. 3. Variation of the real part of dielectric constant as a function of frequency at different temperature.

Fig. 4. Frequency and temperature dependence of imaginary part of dielectric constant of In2S3

Fig. 5. Dielectric loss spectrum of In2S3 material.

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material.

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Fig. 6. Variation of ac conductivity (σac) as a function of frequency at different temperature for In2S3 material.

Fig. 7. Arrhenius plots of the dc conductivity in In2S3 material.

Fig. 8. Plot of ωc versus reciprocal temperature for In2S3 material.

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Fig. 9. Logarithmic plot of dc conductivity (σdc) versus critical frequency ωc for In2S3 material. Fig. 10. Plot of log(σdcT) versus reciprocal temperature for In2S3 material. Fig. 11. Plot of log(σdc) versus T-1/4 for In2S3 material.

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spectrum.

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Table 1. Comparison between the theoretical and experimental Bragg angles of In2S3 X-ray

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ACCEPTED MANUSCRIPT Table I

I (u.a)

(hkl)

Dominant phase

JCPDS results [65-0459]

∆θ (°)

(∆θ/θ)

27.55

1815

(311)

β-In2S3

27.43

0.12

0.43%

28.75

219

(222)

β-In2S3

28.67

33.25

1096

(400)

β-In2S3

33.23

36.38

79

(331)

β-In2S3

36.31

41.09

130

(422)

β-In2S3

41.00

43.69

666

(511)

β-In2S3

47.80

894

(440)

β-In2S3

0.27%

0.02

0.06%

0.07

0.19%

0.09

0.21%

43.61

0.08

0.18%

47.71

0.09

0.18%

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0.08

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Angle θ(°)

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Figure 1

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Figure 2

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ACCEPTED MANUSCRIPT Highlights In2S3 synthesized via solid state reaction technique.

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At high-T the conduction process has been found to be suitable to SPH model.

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VRH mechanism has been observed at low temperature.

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The high relaxation is ascribed to be the MWS model.

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