Structural, optical, and electrical properties of tin ...

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Thin Solid Films 517 (2009) 2497–2499

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Structural, optical, and electrical properties of tin sulfide thin films grown by spray pyrolysis M. Calixto-Rodriguez a,⁎, H. Martinez a, A. Sanchez-Juarez b, J. Campos-Alvarez b, A. Tiburcio-Silver c, M.E. Calixto d a

Instituto de Ciencias Físicas-Universidad Nacional Autónoma de México, Apartado Postal 48-3, 62210, Cuernavaca, Morelos, México Centro de Investigación en Energía-Universidad Nacional Autónoma de México, 62580, Temixco, Morelos, México Instituto Tecnológico de Toluca-SEP, Apartado Postal 20, 52176, Metepec 3, Estado de México, México d Consultant, Cuernavaca, Morelos, México b c

a r t i c l e

i n f o

Available online 8 November 2008 Keywords: Tin sulfide Spray pyrolysis Thin films

a b s t r a c t Tin sulfide (SnS) thin films have been prepared by spray pyrolysis (SP) technique using tin chloride and N, N-dimethylthiourea as precursor compounds. Thin films prepared at different temperatures have been characterized using several techniques. X-ray diffraction studies have shown that substrate temperature (Ts) affects the crystalline structure of the deposited material as well as the optoelectronic properties. The calculated optical band gap (Eg) value for films deposited at Ts = 320–396 °C was 1.70 eV (SnS). Additional phases of SnS2 at 455 °C and SnO2 at 488 °C were formed. The measured electrical resistivity value for SnS films was ∼1 × 104 Ω-cm. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Tin sulfide compounds have recently attracted considerable attention because of their physical properties, which are suitable for optoelectronic device fabrication. The SnS thin films have a great potential for photovoltaic applications as absorber material, due to their band gap of 1.3 eV [1]. The SnS2 thin films can be used as window layers, due to its wide energy band gap of 2.2 eV [2,3]. In addition, by using tin sulfide compounds in photovoltaic structures would decrease the production costs of solar cells, because the materials involved are cheap, nonstrategic, and abundant in nature. SnS compound in thin film form is a p-type semiconductor [4] with an electrical resistivity of 32.9 Ω-cm, and an absorption coefficient greater than 104 cm− 1 [5]. The carrier density is in the order of ∼1015 cm− 3, and the Hall mobility of 139 cm2/Vs [5]. Its electrical properties can be modified by doping it with elements like Ag, Al, N, and Cl [6–8]. This compound crystallizes in the orthorhombic structure with the following lattice parameters: a=4.3291 Å, b=11.1923 Å and c =3.9838 Å [9]. SnS compound in thin film form has been prepared by several techniques, such as plasma enhanced chemical vapour deposition (PECVD) [10], vacuum evaporation [11], chemical bath deposition [12], electrodeposition [13], and spray pyrolysis [1]. The spray pyrolysis is a low cost technique, simple, and has the advantage of allowing deposition

⁎ Corresponding author. Postal address: Instituto de Ciencias Físicas-UNAM, Apartado postal 48-3, 62210, Cuernavaca, Morelos, Mexico. Tel.: +52 55 56227756; fax: +52 55 56227775. E-mail address: manuela@fis.unam.mx (M. Calixto-Rodriguez). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.026

over large areas. In this work the characterization of the physical properties of the SnS thin films deposited by spray pyrolysis is presented. 2. Experimental details SnS thin films were deposited by the spray pyrolysis technique on borosilicate microscope glass substrates from Corning (1.25 cm× 2.5 cm in size), using a solution of SnCl2 2H2O (0.1 M) and N, N-dimethyl thiourea (0.1 M), dissolved in a mixture of 369 ml of deionized water, 6 ml of hydrochloric acid, and 125 ml of isopropyl alcohol to adjust a total volume of 500 ml, in order to obtain a ratio of Sn/S =1. Previously, we have reported the deposition of the SnS2 thin films obtained using the same precursors and different deposition conditions [14]. In this work, SnS thin films were prepared at different substrate temperatures (320–488 °C); using compressed air at a pressure of 4 bar as the carrier gas. The gas and solution flow rates were kept constant at 10 l/min and 5 ml/min, respectively. The nozzle-to-substrate distance was 30 cm. The structural properties of the tin sulfide films were studied by X-ray diffraction (XRD) using a Rigaku D-Max diffractometer with CuKα radiation (λ = 1.5406 Å). The average size of the crystallites was estimated by the Scherrer formula. The composition of the tin sulfide thin films was determined by Energy Dispersive X-ray (EDS) analysis using a JEOL JSM 6400 SEM apparatus equipped with an Inca Oxford EDS analyzer. The optical transmission at normal incidence, T(λ), and specular reflection, R(λ), of the deposited thin films were measured with a Shimadzu model 3101PC double-beam spectrophotometer. The absorption coefficient (α) was calculated from T(λ) and R(λ) measurements, and from its dependence on the photon energy (hν), the optical band gap

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M. Calixto-Rodriguez et al. / Thin Solid Films 517 (2009) 2497–2499 Table 2 Chemical composition of the Sn–S based thin films prepared by spray pyrolysis as a function of Ts

Fig. 1. X-ray diffraction patterns for tin sulfide thin films deposited at Ts in the range of 320–488 °C.

(Eg) was obtained. The thickness of the deposited thin films was measured using an Alpha Step model 100 from Tencor Instruments. Four silver print electrodes were applied to the films in order to study the electrical properties. The resistivity values for these films were obtained by the Van der Paw technique. Hall effect measurements were performed in all samples to find out the electrical conductivity type. 3. Results and discussion

Ts (°C)

Sn (at.%)

S (at.%)

376 396 455 488

59.58 60.26 64.44 88.85

40.42 39.74 35.56 11.15

when Ts increases. This may be due to the re-evaporation of sulfur with the increase in Ts, which leads to the formation of the SnO2 phase. Fig. 2a shows the results of T(λ) and R(λ) for films deposited at Ts = 376–396 °C. In this figure, a small shift in the absorption edge towards lower wavelengths is observed as Ts increases. Fig. 2b shows the results of T(λ) and R(λ) for films deposited at Ts = 455–488 °C, from this figure, when Ts is increased a significant change in the absorption edge towards lower wavelengths is clearly observed. This effect may be due to the change of phases from the mixture of SnS–SnS2 at Ts = 455 °C to SnS– SnO2 at Ts = 488 °C, as revealed by XRD measurements. The absorption coefficient, α(λ) for the deposited material was calculated using the data obtained from the T(λ), R(λ), and thickness (d) measurements in the well-known relationship for indirect transitions, αhν, given by [15] r αhm = A hm−Eg  Eph ;

Fig. 1 shows the XRD patterns for films deposited at various Ts (320– 488 °C). In this figure we can observe that films deposited at Ts between 320 and 455 °C show one peak located at 2θ = 31.54° which corresponds to the preferential orientation (111) of the SnS phase with orthorhombic structure (JCPDS 39-0354). Thin films deposited at Ts between 376 and 396 °C show better recrystallization for the SnS phase than those deposited at 320 °C and 455 °C. In thin films deposited at Ts between 376 and 396 °C two small peaks are also observed, one located at 2θ = 15.24° which corresponds to the (001) direction of the SnS2 phase (JCPDS 831705) and the other at 2θ = 65.9° which correspond to the (560) direction of the Sn2S3 phase (JCPDS 30-1379). The crystallite size for the film deposited at Ts = 376 °C was 14.6 nm, which was calculated using the Scherrer formula. Films deposited at Ts = 455 °C showed a mixture of phases SnS (JCPDS 39-0354) and SnS2 (JCPDS 21-1231) with a preferential orientation along the (002) direction. When Ts is increased up to 488 °C the peak corresponding to the SnS2 disappears, and the material is almost completely converted to SnO2 (JCPDS 41-1445). This effect can be understood considering the carrier gas employed, as the oxygen from air oxidize the tin of the solution prior to react with sulfur, due to the higher energies involved in the process. The lattice parameters of the orthorhombic cell of the SnS phase were calculated using the XRD data for films deposited in the range of 320–396 °C, these values are shown in Table 1. According to this table, the lattice parameters exhibit a more pronounced change as Ts increases; due to this anisotropy we may then expect some effects on the electrical properties of the films. The results of the chemical composition in atomic percent (at.%) for films deposited in the range of 376–488 °C are presented in Table 2. According to this table, the Sn/S ratio in solid state goes from 1.47 to 7.97

ð1Þ

where A is a constant, Eg is the optical band gap, and Eph is the energy of the absorbed (+) or emitted (−) phonons. In Eq. (1), r = 2 for allowed and r = 3 for forbidden, indirect transitions. Thus Eg can be obtained for allowed indirect transitions from a plot of (αhν)1/2 versus hν. The Eg and Eph values were calculated from the best fit of the plot (αhν)1/2 versus hν, and its extrapolation to (αhν)1/2 = 0. In Eq. (1), r = 1/2 for allowed direct transitions, so that Eg can be obtained from a plot of (αhν)2 versus hν. The Eg values were calculated from the best fit of the plot (αhν)2 versus hν, and its extrapolation to (αhν)2 = 0. Fig. 3a shows plots of (αhν)1/2 versus hν for indirect inter-band transitions assisted by phonons for films obtained at Ts = 376 and 396 °C. The calculated band gap values were Eg = 1.70 and 1.67 eV, and the phonons involved in the processes had energies of Eph = 0.05 and 0.02 eV, respectively. These values are larger than those reported by Lopez and Ortiz [16] (1.27 eV) and Reddy and Reddy [5] (1.32 eV). The observed discrepancies in the Eg values can be attributed to the film

Table 1 Comparison of the lattice parameter variations of the SnS phase as a function of Ts and the standard reference values given by Schnering [9] Ts (°C) 320 376 396

a (Å)

b (Å)

4.3291

11.1923

c (Å) 3.9838

4.3328 4.2097 4.4589

11.3649 10.8018 10.9063

3.9384 4.1076 3.9662

Fig. 2. Optical Transmission and Reflection spectra for: a) SnS films deposited at Ts = 376 and 396 °C, respectively and (b) films deposited at 455 and 488 °C (change of phases from SnS–SnS2 to SnS–SnO2).

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Different SnS thin film compounds can be obtained by changing the substrate temperature. In the range of Ts =320–396 °C, the SnS compound with orthorhombic structure and preferential orientation along the (111) direction was obtained. The calculated Eg values for tin sulfide thin films were ∼1.70 eV, and it increased to 2.0 eV with the increase of Ts, which may be due to the change of phases from a mixture of SnS–SnS2 to SnS– SnO2. The electrical resistivity (ρ) for the SnS thin films varied from 8.2×103 to 1.9×104 Ω-cm. The ρ values decreased dramatically when SnS2 and SnO2 were the predominant phases, ρ=7.2 and 0.02 Ω-cm for films deposited at 455 and 488 °C, respectively, which may be due to the Sn excess. In order to make the SnS thin films suitable for photovoltaic applications the electrical resistivity must be lowered by adequate doping.

Acknowledgments

Fig. 3. Optical absorption spectra for: (a) SnS samples (indirect optical transitions), (b) mixture of SnS–SnS2 and SnS–SnO2 (direct optical transitions). Eg is the optical band-gap and Eph is the energy of phonons involved in the process.

growth conditions used in this work, which led to different grain sizes and lattice parameters. Fig. 3b shows plots of (αhν)2 versus hν for direct inter-band transitions for films deposited at Ts =455 and 488 °C. It was observed that Eg increases as Ts increases due to the change of phases from a mixture of SnS–SnS2 (Eg =1.74 eV) to SnS–SnO2 (Eg =2.0 eV), as revealed by XRD studies (see Fig. 1). The measured electrical resistivity values for films deposited at Ts =320–396 °C varied from 8.2×103 to 1.9×104 Ω-cm. The ρ values decreased dramatically when SnS2 and SnO2 were the predominant phases, ρ=7.2 and 0.02 Ω-cm for films deposited at 455 and 488 °C, respectively. The low resistivity in SnS2 films can be due to a deviation in stoichiometry in this compound (excess of tin atoms) as shown by the EDS analysis. The electrical conductivity type could not be measured by Hall effect due to the high electrical resistivity of the films deposited at Ts =320–396 °C. Whereas for films deposited at Ts =455 and 488 °C, n-type electrical conductivity was obtained. 4. Conclusions Tin sulfide thin films have been prepared by the spray pyrolysis technique using SnCl2 and N, N-dimethyl thiourea as the starting materials.

The authors would like to thank Maria Luisa Ramón for XRD analysis, J. Ortega-Cruz for the electrical measurements, and Gildardo Casarrubias for technical support (CIE-UNAM), Dr. Osvaldo Flores for EDS analysis, and Mr. A. González for technical support (ICF-UNAM). Authors are grateful to DGAPA-UNAM for the financial support under contract PAPIIT1N111506.

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