Optical and electrical properties of Sn2S3 thin films grown by spray pyrolysis

Solid State Communications 150 (2010) 297–300

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Optical and electrical properties of Sn2 S3 thin films grown by spray pyrolysis M. Khadraoui a , N. Benramdane a , C. Mathieu b , A. Bouzidi a , R. Miloua a,∗ , Z. Kebbab a , K. Sahraoui a , R. Desfeux b a

Laboratoire d’Elaboration et de Caractérisation des Matériaux, Département d’Electronique, Université Djillali Liabes, BP89, Sidi Bel Abbés 22000, Algeria

b

Centre de calcul et de modélisation de Lens, Université d’Artois, Faculté Jean Perrin, Rue Jean Souvraz, Sp18, 62307 Lens Cedex, France

article

info

Article history: Received 25 April 2009 Received in revised form 9 August 2009 Accepted 24 October 2009 by M. Wang Available online 30 October 2009 Keywords: A. Sn2 S3 thin films D. Structural properties D. Optical properties E. Spray pyrolysis

abstract Polycrystalline films of Sn2 S3 compound were prepared on glass substrates by the spray pyrolysis technique at a substrate temperature of 270 ◦ C using tin chloride (SnCl2 ) and thiourea (Cs(NH2 )2 ) solutions with a concentration of 0.1 M. The X-ray diffraction pattern reveals an orthorhombic structure and an average grain size equal to 130 Å. The optical properties of the films were studied using optical transmittance and reflectance measurements over the wavelength range 200–2500 nm. The variation of refractive index n and extinction coefficient k with photon energy are reported. The dispersion of the refractive index in Sn2 S3 is analysed using the concept of a single oscillator. The values of oscillator energy E0 and dispersion energy Ed are determined to be 3.98 eV and 13.5 eV, respectively. The optical constants confirm that the Sn2 S3 thin films have a direct band gap of 2 eV. A value of 4.35 × 10−3 ( cm)−1 for the room temperature conductivity is found using the four-probe method. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction

2. Experimental description

In recent years, semiconducting chalcogenide thin films have received much attention due to their potential use in many applications such as absorbers in thin film solar cells [1,2] and optical fibers [3]. Sn2 S3 is one of the group IV–VI chalcogenides, having a layered structure in group IV–VI. It crystallizes with an orthorhombic crystal structure with lattice parameters a = 8.884 Å, b = 14.02 Å and c = 3.747 Å, and the band gap varies from 0.95 eV [4] to 2.2 eV [5]. This material appears to be suitable for preparing near-lattice-matched heterojunctions such as Sn2 S3 /CdTe, Sn2 S3 /GaSb, Sn2 S3 /AlSb, etc., which find applications in the detection and generation of infrared radiation. Sn2 S3 has been prepared using different techniques such as plasma-enhanced chemical vapour deposition (PECVD) [6] and spray pyrolysis [5,7,8]. In the present work, we report a systematic study of structural, optical and electrical properties of Sn2 S3 thin films obtained for the first time from SnCl2 −2H2 O and thiourea CS(NH2 )2 using the spray pyrolysis method.

Sn2 S3 thin films were deposited by the spray pyrolysis technique [9], using tin chloride (SnCl2 , H2 O) (purity: 99.98%) and thiourea CS(NH2 )2 (purity: 99.98%) on microscope glasses of (75 × 25) mm2 . The molarity of the prepared solution is 0.1 M. The SnCl2 was dissolved in a mixture of methanol and deionised water in the ratio of 1:1, while the thiourea was dissolved in deionised water. To enhance the solubility of SnCl2 , a few drops of HCl were also added. The prepared solutions of tin chloride and thiourea were appropriately mixed to obtain an Sn:S proportion of 2:3. The solutions obtained were pulverised on glass substrates with compressed air (2 bars) at a flow rate of 8 ml/min. The substrate temperature was maintained at 270 ◦ C. The distance from the spray nozzle to the heater was kept at approximately at 29 cm. Under these deposit conditions, good films are obtained. They are uniform and very adherent to the substrates. The samples were weighed before and after the spraying operation to determine the mass of the films [10]. Knowing the dimensions of the substrates used, the thicknesses can be determined considering the following equation [11]:

∗

Corresponding author. Tel.: +213 50 71 61 56. E-mail address: [email protected] (R. Miloua).

0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.10.032

d=

1m ρm lL

(1)

298

M. Khadraoui et al. / Solid State Communications 150 (2010) 297–300

0.7

(211)

600

0.6

500 (310)

T, R [%]

0.5 400 300

0.4

0.2

(350)

(311)

(111) (121)

0.3

(130) (220)

100

(120)

200 (020)

Intensity(arbitrairy units)

Transmittance Reflectance

0.8

700

0.1

0

0.0 10

20

30

40

50

60

0

500

1000

2θ(degrees)

1500

2000

2500

Wavelength(nm)

where (1m) is the difference between the mass after and before spraying, ρm (4.74 g/cm3 ) is the density, l the width and L the length. Structural characterization was carried out at room temperature using a Philips 1830 X-ray diffractometer with a CuKα peak λ = 1.546 Å. The optical transmittance and reflectance were recorded in the wavelength range from 200 to 2500 nm using a UV (Ultra-Violet)–Visible JASCO type V-570 double-beam spectrophotometer.

Fig. 2. The spectral transmittance T and reflectance R as a function of the wavelength. 1.0

0.8 (αhν)2(105cm-1eV)2

Fig. 1. Experimental X-ray diffraction pattern of Sn2 S3 prepared by the spray pyrolysis method.

0.6

0.4

0.2

Eg = 2eV

0.0 0

3. Results and discussion

(2)

β cos(θ )

where k = 0.9 is the shape factor, λ is the X-ray wavelength in Å, θ is the Bragg angle, and β is defined as the full width at half maximum (FWHM) of the most intense diffraction peak. The grain size estimated from the main peak (211) is equal to 130 Å. This size seems reasonable due to the growth at the substrate temperature of 270 ◦ C without thermal treatment [9]. The estimated thickness is about 530 nm. 3.2. Optical properties The optical properties of Sn2 S3 were determined from the optical transmittance T and reflectance R measured at room temperature with unpolarized light at normal incidence. The measurements were recorded in the range 200–2500 nm using a JASCO V-570 spectrophotometer. The transmittance T and the reflectance R are depicted in Fig. 2. The absorption spectrum of the thin films was calculated using the relation [5]

α=

d

 ln

(1 − R)2 T

4

where d is the film thickness, T the transmittance and R the reflectance. The fundamental absorption edge of semiconductors corresponds to the threshold for charge transitions between the highest nearly filled band and the lowest nearly empty band. The absorption is very small for photon energy much less than the band-gap energy and increases significantly for higher photon energies. Inter-band absorption theory shows that the absorption coefficient near the threshold versus incident energy is given by the following relation [13]:

(α hν) = An (hν − Eg )n

 (3)

(4)

where An is the probability parameter for the transition and Eg the band-gap energy. For allowed direct transitions the coefficient n is equal to 1/2 and for indirect allowed transitions n = 2. The curves (α hν)1/2 for the allowed indirect transition does not present evident linearity; this seems to suggest that Sn2 S3 has a direct band gap. The value Eg corresponding to the direct band-gap transition was calculated from the curve of (α hν)2 versus hν , using the formula

(α hν)2 = A(hν − Eg ); A : a constant.

1

3

Fig. 3. Plot of (α hυ)2 versus hυ for Sn2 S3 thin films.

The X-ray diffraction (XRD) profile of the spray-pyrolysed Sn2 S3 thin films is shown in Fig. 1. These films were not treated after deposition. All the peaks can be indexed to orthorhombic Sn2 S3 with lattice constants a = 8.824 Å, b = 13.98 Å and c = 3.735 Å, which are in good agreement with ASTM X-ray powder data files (Card No. 72–0031). The XRD analysis detected no other crystalline phases such as SnS and Sn2 S; however it is possible to have amorphous tin sulphide phases which cannot be detected by XRD. For estimation of the crystallite size of our films, we used the Scherrer formula [12]: kλ

2 hν(eV)

3.1. Structural studies

G=

1

(5)

The extrapolation of the linear part of the curve (α hν)2 to the energy axis is shown in Fig. 3. The direct band-gap energy is equal to 2 eV; this value is in good agreement with the value obtained by Koteeswara Reddy et al. [14]. However, this value is larger than those reported by Lopez et al. [7] (1.16 eV) and Sanchez et al. [6] (1.05 eV). According to Ben Hadj Salah et al. [5], the observed disparity in the gap energy is explained by the different growth conditions of these films. The refractive index (n) and the extinction coefficient (k) were calculated with an iterative method using the following equations:

M. Khadraoui et al. / Solid State Communications 150 (2010) 297–300

a

b

2.5

a

0.6

299

b

6

3

1.5

0.4

2

0.2

ε2

4

0.3

ε1

Extinction coefficient

Refractive index

0.5 2.0

1

2

0.1 1.0

0.0 1

2

3

4

0

1

2

3

4

0

0 1

Energy(eV)

Energy(eV)

2

3

0

4

1

Fig. 4. Variation of the refractive index (a) and extinction coefficient (b) of Sn2 S3 thin films.

λ 64


2

.

n2 + k

.ns

0.285

!



(1 + n)2 + k2 . (n + ns )2 + k2 . (1 + ns )2 (6)  1/2
R+1 (R + 1)2 2 n=− + − 1 + k R−1 (R − 1)2  1/2 where ns = T1 + T1 − 1 [15] is the refractive index of the s s T

substrate, Ts is the substrate transmittance, d is film thickness and λ the wavelength. The curves obtained are depicted in Fig. 4. The refractive index of the film varies between 1.65 and 2.08 with a change of energy in the range 1–4 eV. A similar variation of both optical constants n and k has been observed by Koteeswara Reddy et al. [14] on SnS films grown by the spray pyrolysis method at a substrate temperature equal to 275 ◦ C. The dielectric constant of Sn2 S3 films was calculated using the relation

ε = ε1 + i ε2

(7)

where ε1 and ε2 are the real and imaginary parts of the dielectric constant ε , respectively. The values of ε1 and ε2 were calculated using the following equations:

ε1 = n2 − k2 ε2 = 2nk

(8)

and they are depicted in Fig. 5. We notice the increase of ε1 from 2.72 to 4.38 with increasing photon energy from 1 to 1.5 eV. However, ε2 exhibits a minimum value of 0.25 at the photon energy of 1 eV. The dispersion of refractive index was analysed using the concept of a single-oscillator model and is expressed by the relationship [1,11] n2 − 1 =

4

0.290

4.π .d

× ln

3

Fig. 5. Variation of the real (a) and imaginary (b) parts of the dielectric constant of Sn2 S3 thin films.

E0 .Ed E02

− E2

(9)

where E is the photon energy, E0 is the oscillator energy and Ed is the dispersion energy. A plot of (n2 − 1)−1 versus E 2 is illustrated in Fig. 6. Eo and Ed can be determined from the slope (Eo Ed )−1 and intercept on the vertical axis, (Eo /Ed ). The values obtained are 3.98 and 13.5 eV, for Eo and Ed , respectively. As was shown by Tanaka [16], the oscillator energy Eo scales with the energy gap very well, according to the relationship Eo ≈ 2Eg . Hence, we found that Eg = 2 eV. The hot-probe method indicates that the films prepared in this work exhibit n-type electrical conductivity. The ambient temperature conductivity of the deposited thin films, prepared under the same conditions, was measured using a four-probe method. The conductivity of Sn2 S3 is 4.35 × 10−3 ( cm)−1 and the carrier concentration is 9.4 × 1014 cm−3 . Other values

0.280 (n2- -1)-1

k=

2 Energy(eV)

Energy(eV)

0.275 0.270 0.265 0.260 0.255 0.250 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

E2(eV)2

Fig. 6. Plot of (n2 − 1)−1 versus E 2 of Sn2 S3 thin films.

of the conductivity have been reported by Koteeswara Reddy et al. using spray pyrolysis (∼ 1.5 × 10−2 ( cm)−1 ) [17] and by Sanchez–Juarez et al. using the PECVD method (2.5 × 10−5 ( cm)−1 ) [6]. It is clear that the method we used gives different results depending on the growth conditions and precursors. However, the measured conductivity is better than that obtained using PECVD. Hence, we conclude that the spray pyrolysis technique is suitable for fabricating Sn2 S3 thin layers with good conductivity. 4. Conclusion This work reports the growth of polycrystalline thin films of Sn2 S3 from SnCl2 and thiourea, using the spray pyrolysis method at a low temperature, (270 ◦ C). The deposited material shows an optical band gap of 2 eV with a direct allowed transition. This result is confirmed using the single-oscillator model. The electrical conductivity of the films is about 4.35 × 10−3 ( cm)−1 and the electrons are the majority carriers. These interesting optical characteristics make the material suitable for photovoltaic applications. References [1] M.M. El-Nahaas, H.M. Zeyad, M.S Aziz, N.A. El-ghamaz, J. Opt. Mater. 20 (2002) 159. [2] B. Thangaraju, P. Kaliannan, Semicond. Sci. Technol. 15 (2000) 849. [3] P.M. Nikolic, S.S. Vujatovic, D.M. Todorvic, J. Phys. C: Solid State Phys. 19 (1986) L717. [4] U. Alpen, J. Fenner, E. Gmelin, Mater. Res. Bull. 10 (1975) 175. [5] H. Ben Hadj Salah, H. Bouzouita, B. Rezig, Thin Solid Films 480 (2005) 439. [6] A. Sanchez-Juarez, A. Ortiz, Semicond. Sci. Technol. 17 (2002) 931. [7] S. Lopez, S. Granadoz, A. Ortiz, Semicond. Sci. Technol. 11 (1996) 433. [8] S. Lopez, A. Ortiz, Semicond. Sci. Technol. 9 (1994) 2130. [9] N. Benramdane, M. Latrache, H. Tabet, M. Boukhalfa, Z. Kebbab, A. Bouzidi, Mater. Sci. Eng. B 64 (1999) 84.

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