Synthesis and characterisation of tin sulphide thin films

Abstract

Stannous sulphide (SnS) thin films have been synthesised using two different solutions. The solutions were prepared by the dilution of SnCl₂ and thiourea in distilled water and methanol separately. The effects of solvents and deposition time on the properties of the prepared thin films are investigated. X-ray diffraction analysis revealed that the variation of the solvent and the deposition durations alters the structure of the prepared films. The films analysis by scanning electron microscope showed that the SnS films prepared with methanol are more dense and smooth than the SnS films prepared with distilled water. The resistivity depends on the used solvent and their growth duration. The elemental composition of the films deposited during 30 min with methanol indicate that SnS is quasi-stoichiometric, while those prepared with distilled water are slightly Sn rich. We have also found that the films deposited with distilled water are n-type, whereas those prepared with methanol are p-type.

Keywords

Semiconductors thin films spray pyrolysis solvent effect deposition time effect

Introduction

Metal chalcogenides semiconductors are very useful in different technological fields, such as polarisers, sensor and thermoelectric cooling materials.¹ Among these materials, tin sulphides have attracted considerable interest in recent years,^1,2 particularly in photovoltaic energy conversions.^3,4 These materials have the ability to form several binary sulphides, such as SnS, SnS₂, Sn₂S₃ and Sn₃S₄. This is due to the coordination features of polyvalent tin and sulphur. Tin monosulphide, SnS, seems to be one of the most important and the most studied compounds.⁵ Indeed, it can be used in many technological devices.⁶ It is specifically used as absorber in the fabrication of solar cells based on low-cost materials.^6-8 It is also used in holographique recording medium, light-emitting diodes, electrical switching, lithium ion battery, gas-sensor and optical material.⁹ Stannous sulphide (SnS) is a binary semiconductor with an orthorhombic structure,^10,6 and a very useful direct band gap for the photovoltaic conversion, with an almost optimum value varying in the range 1.3–1.7 eV.¹¹ In addition, it has a very high absorption coefficient (≈10⁵ cm⁻¹).¹² This compound was found to exhibit both n- and p-types conductivities.^7,13 Several deposition techniques have been used to prepare absorbers thin films, including metal organic chemical vapour deposition,¹⁴ precipitation method,¹⁵ one-pot synthesis method,¹⁶ aerosol-assisted chemical vapour deposition,¹⁷ spray pyrolysis,¹⁸ CSVT technique¹⁹ and plasma-enhanced chemical vapour deposition.⁵ In this paper, SnS thin films have been synthesised on glass substrates using the ultrasonic spray pyrolysis method. This technique was used owing to its numerous advantages, such as better stoichiometry control, ability of deposition of large uniform homogeneous layers area and low processing temperature. Our approach consists on the study of the effect of the solvents and the deposition time herein, the structural, morphological, optical and electrical properties of SnS thin films have been investigated.

Experimental details

Films preparation

The samples used in this study were grown by ultrasonic spray pyrolysis technique. This method is mainly composed of an ultrasonic spraying system and a substrate holder with heater. The ultrasonic vibrator frequency was 40 kHz. During the deposition, the nozzle–substrate distance was kept constant to 4.5 cm. The substrate temperature was fixed at 350°C, as it was found to be the optimum temperature value to obtain uniform and well adherent SnS thin films.²⁰

Two different solutions having identical molarity (0.07 M) were prepared by mixing tin chloride SnCl₂ and thiourea (SC(NH₂)₂) as sources of Sn and S, respectively. In the first solution which will be referred to as solution A, the precursors were dissolved in distilled water, whereas in the second solution, the precursors were dissolved in methanol and this solution which will be referred to as solution B. In both cases, different deposition durations were used. The films that were obtained from solutions A and B will be referred to as films A and films B , respectively. The experimental conditions are summarised in Table 1.

Table 1

Experimental conditions used for the preparation of SnS thin films

Deposition time (min)		Volume (mL)	Substrate temperature(°C)	Molarity (mol/L)
Solution A (Distilled water)	Solution B (Methanol)	Volume (mL)	Substrate temperature(°C)	Molarity (mol/L)
15	15	80	350	0.07
25	25
30	30

Characterisation

The crystalline structure of the deposited SnS thin films were investigated by X-ray diffraction (XRD) using a BRUKER-AXS type D8 diffractometer with CuK_α radiation (λ = 1.541838 Å). The morphology and the chemical composition of the prepared thin films were carried out using a scanning electron microscope (SEM), (JEOL JSM 6400 model), equipped with an energy dispersive X-rays (EDXs) microanalysis system. The optical absorption of the films was studied using a Cary 5000 UV–Vis–NIR spectrophotometer. The electrical characteristics were measured using a Hall Effect Measurement system (HMS-3000).

Results and discussion

Structural and morphological properties

XRD patterns of deposited SnS thin films using distilled water at different deposition time are shown in Fig. 1. The films were found to be polycrystalline and have an orthorhombic structure. The broad hump situated in the small diffraction angles (20–35°) is due to the amorphous glass substrates network as was observed by other researchers.^21,22 On the other hand, we note that when the deposition time increases, a peak located at ∼29.89° and assigned to the (101) plane of the SnS orthorhombic structure emerges.

XRD patterns of SnS thin films prepared with varied deposition times using distilled water

When the deposition time was further increased (from 25 to 30 min), the strong peak located at ∼26.93° and related to (021) plane of the SnS film deposited at 30 min became narrower and more intense. The small peaks situated at ∼22.03° and ∼50.35° were indexed with (110) and (221) planes, respectively. The spectrum of the film deposited during 25 min shows diffraction peaks related the SnS orthorhombic phase, and also some small diffraction peaks corresponding to the SnO₂ phases. The peaks of the XRD patterns were indexed using the standard card of orthorhombic SnS (PDF card no: 33-1375).

Fig. 2 shows the XRD diffraction patterns of deposited SnS thin films using methanol at different deposition time. The films were found to be polycrystalline with a relatively strong (120) peak. In addition, the diffraction pattern contained peaks that correspond to (110), (021), (111), (140), (210), (221), (151), (122) and (231) planes. The presence of these peaks indicates that the prepared SnS films B have the orthorhombic crystal structure (PDF Card No. 033-1375). The additional peaks appearing at 28.73°, 32.72° and 49.78° can been attributed to the SnS₂ phase (PDF Card No. 031-1399) which was found to accompany the preparation of SnS.²³ The films deposited with distilled water exhibit a broader base, contrary to the films prepared with methanol, which present a better crystallinity. We also noted the presence of small diffraction peaks corresponding to SnS₂ and SnO₂ phases. Figure 3 shows the variation of most intense peaks as a function of deposition time for films A and B . From this figure, we can note that, when the deposition time increases, films composition and the preferred crystal orientation are changed from tin disulphide <100> to monosulphide <021> for films A , and <120> for films B . This variation can be obtained when the sulphur is evacuated from the interior of the film towards its surface during the growth process.²⁴ For both solutions, the best films were obtained at the deposition time of 30 min. The values of grain sizes and internal strains of the prepared films have been from the XRD data using the standard Debye–Scherer formula, and are summarised in Table 2.

XRD patterns of SnS thin films deposited at different deposition times with methanol.

Variation of the intensities of most intense peak of different present phases of thin films A and B as a function of the deposition time

Table 2

Grain sizes and internal strain of films A and B

Deposition time (min)		D (nm)		Internal strain (ε × 10⁻³)
Solution A	Solution B	Films A	Films B	Films A	Films B
15	15	16.75	08.99	2.07	3.86
25	25	14.26	16.54	2.43	2.09
30	30	16.31	19.07	2.12	1.82

The grain size calculated from the XRD diffraction spectrum varied from 16.75 to 16.31 nm when the deposition time increased from 15 to 30 min for the films prepared with distilled water. These sizes values indicate the nanocrystalline nature of films A . For the samples prepared with methanol, the grain size varied from 8.99 to 19.07 nm when the deposition time was increased from 15 to 30 min. This confirms that the crystallinity of the films grown using methanol was improved when the deposition time increased. This can be attributed to the methanol viscosity that influences the processes of structural relaxation and crystallisation. Other investigators^25,26 have found similar values of the grain size.

Films morphology and chemical composition was carried out using SEM and EDX technique. The typical SEM images of SnS thin films A and B deposited during 30 min are shown in Fig. 4(a,b), respectively. Film (a) has some agglomerated particles on its surface as shown in Fig. 4(a). The film of the Fig. 4(b) is homogeneous, devoid of cracks and has a continuous and dense structure with a very smooth surface morphology. The diversity in films morphology (which is consistent with the XRD) can be related to the difference in the viscosity and the surface tension of water and methanol. Indeed, the surface tension and viscosity of methanol are much lower than those of the water. Therefore, when using methanol as solvent, the droplets are more easily spread on the substrate surface. This indicates that the used solvent can have an effect on the sprayed SnS thin film microstructure and morphology.

SEM images of SnS films prepared at 30 min with distilled water a and methanol b

Optical properties

Figure 5 shows the absorption spectra of SnS films prepared with distilled water and methanol at various deposition times.

Variation of the absorption coefficient α as a function of the wavelength: a distilled water; b methanol

The optical band gap is obtained from the plot of the following relation: (1) where α is the absorption coefficient, A is a constant, E_g is the optical band gap, v is the frequency of the incident photon and h is the Planck's constant. The plots of (αhν)² versus the photon energy hν for direct transition for both films A and B which were deposited during 30 min are shown in Fig. 6. The band gap energies of these films are determined from the intercept of the tangent to the plot with the abscissa axis as indicated in Fig. 6.

Plot of (αhv)² versus hν of SnS films A and B : a films A at 30 min; b films B at 30 min

The obtained optical band gap values of films A and B deposited at different durations are summarised in Table 3. We note that the optical band gap of the prepared films decreases when the deposition time increases. This decrease of the E_g can be due the grain size increase.²⁷

Table 3

Optical band gap of SnS films A and B and their deposition time

Deposition time (min)		Optical band gap (eV)
Solution A	Solution B	Films A	Films B
15	15	1.63	2.18
25	25	1.61	1.66
30	30	1.59	1.56

Electrical properties

Resistivity variation

The resistivity variation of films A and B as a function of the deposition time is shown in Fig. 7. We note that the resistivity of films A and B decreases when the deposition time increases. Indeed, the resistivity of films A increase from 2.01 × 10³ to 2.38 × 10 ³ Ω cm (low grain size), then it decreases towards its lowest value of 4.23 × 10 ² Ω cm. For films B , the resistivity decreases rapidly with deposition time up to 25 min, and then starts to decrease slowly. This variation of the resistivity of films A and B can be due to the grain size increase as indicated in Table 2. These results are consistent with the literature.^12,28 The slightly higher resistivity exhibited by the film B deposed during 15 min can be due to the presence of the binary phase SnS₂ which is known to be highly resistive.^21,29 The films formed at higher deposition time have shown lower resistivity.

Variation of the resistivity of SnS films A and B as a function of the deposition time

Conduction type variation

The conduction type is one of the most important parameters of the studied films and its determination is carried out using Hall Effect measurement.

Tin monosulphide, SnS, is usually known to be a p-type semiconductor.³⁰ The acceptor levels are created by ionised tin vacancies.³¹ The atomic ratio obtained from the EDX analysis and the conduction types are indicated in Table 4. We note that films A have an n-type conductivity. Table 4 indicate that the films prepared with distilled water are Sn-rich, while the films obtained from methanol are quasi-stoichiometric. Since the conductivity type of the SnS films are essentially controlled by the Sn concentration in the compound. The Sn atoms act as donor and lead to the n-type conduction. Similar behaviour of conduction type due to the excess of Sn amount was observed by Sajeesh et al.³² On the other hand, films B show a p-type conductivity. This difference in films composition originates from the discrepancy in the solvent properties that may alter the films growth mechanism. Therefore, p-type and n-type SnS films can be used to forma p–n junction structure by ultrasonic spray technique.

Table 4

Electrical conductivity of SnS films A and B and their deposition time

Deposition time (min)		Sn/S (at.-%)		Conduction type
Solution A	Solution B	Distilled water	Methanol	Distilled water	Methanol
15	15	61.41/38.59	44.62/55.38	n-type	p-type
25	25	64.21/35.80	46.53/53.47	n-type	p-type
30	30	65.15/34.85	53.10/46.90	n-type	p-type

Conclusion

The effect of distilled water and methanol solvents and deposition time on SnS thin films was investigated. XRD analysis revealed that both films are polycrystalline and have an orthorhombic structure. In addition, the films deposited using methanol have a better crystallinity than the films deposited with distilled water. Analysis by SEM showed that SnS film deposited during 30 min with methanol is more dense and smooth than the film deposited in 30 min using distilled water. The elemental composition shows that the film deposited during 30 min with methanol is quasi-stoichiometric, whereas the film prepared with distilled water is slightly Sn rich. The electrical study indicate that the resistivity of films A and B decreases with the increase of the deposition time and the lowest values are obtained for the deposition time of 30 min. The films deposited with distilled water are n-type, whereas those prepared with methanol are p-type which make it possible to sequentially deposit n- and p-type SnS films for the fabrication of SnS homojunction. In addition, the good quality of the deposited films and the low fabrication cost of the used method can lead to solar cells whose cost-quality ratio is better than the solar cells which are fabricated by the other standard process.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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