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Applied Surface Science 257 (2011) 4901–4905

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

reparation and properties of SnS film grown by two-stage process

eng Jiang, Honglie Shen ∗, Chao Gao, Bing Liu, Long Lin, Zhou Shenollege of Materials Science & Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China

r t i c l e i n f o

rticle history:eceived 8 November 2010eceived in revised form7 December 2010

a b s t r a c t

SnS films have been prepared by a novel two-stage process. It involved sputtering of Sn film on glass sub-strate and sulfurization of the thin metallic tin precursor layers in a vacuum furnace. The X-ray diffractionresults showed that the SnS layers had orthorhombic structure and (0 4 0) preferential growth is moreand more obvious with the increase of sulfurization time. The SnS film obtained by this work shows high

ccepted 28 December 2010vailable online 4 January 2011

eywords:nS filmsin precursor layer

optical absorption efficiency, and the film has a direct optical band gap of about 1.3 eV. The films showp-type conductivity and the resistivity of SnS film decreased obviously under illumination.

© 2011 Elsevier B.V. All rights reserved.

ulfurizationhotoconductivity

. Introduction

Over the past several years, the most promise materials for mak-ng thin film solar cells are those based on copper indium galliumiselenide, cadmium telluride because of the high conversion effi-iency. But there are several problems remain, for example theack of Ga and In and the toxicity of Cd. So people endeavor toevelop new non-toxic, high-efficiency, and inexpensive materialso replace these materials [1,2]. Interestingly, SnS is a promise pho-ovoltaic material which has high absorption efficiency and nearptimum optical band gap of 1.5 eV for solar cell [3]. Further, SnSs nontoxic and the constituent elements are abundant in nature.esides, its electrical conductivity can be easily controlled by dop-

ng with Ag, Al, N, and Cl [4,5] and it exhibits both p-type and-type conduction depending on the concentration of tin [6]. Lofer-ki proved that the theoretical efficiency of SnS based solar celleached 25% [7]. From the above, SnS has been highlighted as theost promising new material for solar cells.In recent years, a variety of methods have been employed to

repare SnS thin film, such as chemical bath deposition [8], sprayyrolysis [9], pulse electro-deposition [10], sulfurization [11], etc.nd from a technological viewpoint, sulfurization, which is a reac-

ive solid-phase growth method using chalcogenide vapor, is mostesirable because of its simplicity and capability of large areareparation. The crystal growth of SnS should be studied for itspplication in solar cells. Sulfurization growth of SnS films has great

∗ Corresponding author. Tel.: +86 25 52112626; fax: +86 25 52112626.E-mail address: hlshen@nuaa.edu.cn (H. Shen).

169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.12.143

advantages in the fabrication of SnS solar cells. So, preparation ofSnS films by sulfurization should be investigated systematically.But the sulfurization conditions such as temperature and sulfur-ization time have not yet been clarified, and the impurity phase ofSnS2, Sn2S3 are easy to be existed.

In this paper, we report the preparation of the orthorhom-bic structure SnS film using two-stage process. The sulfurizationmechanism is discussed, and how the sulfurization time affects onstructural, electrical and optical properties of SnS films are investi-gated systemically.

2. Experiment details

2.1. SnS films preparation

Tin films with a thickness of 200 nm were used as the precur-sor materials for the preparation of SnS films. The tin precursorswere deposited on glass substrate by magnetron sputtering at roomtemperature. Then the tin precursors were sulfurized under sulfurvapor atmosphere at 220 ◦C for 15–60 min in MTI-OTF1200 furnace,while the vapor pressure in the furnace was maintained at one stan-dard atmospheric of pressure in the process of sulfurization. Thepurity of the tin sputtering target and sulfur powder is higher than99.999%.

2.2. Characterization

The thickness of SnS films was measured by AMBIOS XP-1 stepprofiler. The surface morphologies of SnS films were analyzedby JSM-6300 scanning electron microscope (SEM). Structure and

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902 F. Jiang et al. / Applied Surfa

rystallographic orientations were evaluated using Bruker D8dvance X-ray diffraction (XRD) measurements. Optical transmit-

ance and reflection spectrum were measured by Varian Cary-5000pectrophotometer. The resistivity was measured at room tem-erature by the van der Pauw method. Photocurrent response andesistivity were measured by Keithley 2400 at room temperature.

. Results and discussion

The tin films which were sulfurized for 15 min, 30 min, 45 minnd 60 min are defined as S15, S30, S45 and S60, respectively. Evenhough the thickness of tin precursor films is 200 nm, while thehickness of S15, S30, S45 and S60 was founded to become 280 nm,85 nm, 409 nm and 420 nm after the sulfurization, respectively.

.1. Structural properties

Fig. 1 shows the X-ray diffraction patterns of SnS films whichere sulfurized at 220 ◦C for 15 min (S15), 30 min (S30), 45 min

S45) and 60 min (S60), respectively. The diffraction peak at� = 30.68 ◦ can be assigned to (2 0 0) diffraction peak of tin and allhe SnS peaks of S30, S45 and S60 are closely matched to thosef orthorhombic SnS corresponding to JCPDS card number 39-354. The diffraction peaks at 2� = 26.14◦, 27.48◦, 31.55◦, 32.00◦

nd 39.35◦ can be indexed to (1 2 0), (0 2 1), (1 1 1), (0 4 0) and (1 3 1)lanes of SnS, respectively. As it can be seen from Fig. 1 (S15), the tin2 0 0) peak is the dominate peak and the intensity of SnS (1 1 1) andnS (0 4 0) peaks are all very low. For the S30, S45 and S60, the SnS1 1 1) and SnS (0 4 0) peaks are the major peaks and SnS (1 2 0), SnS0 2 1) are also indicated in Fig. 1. The intensity of SnS (0 4 0) peakncreases with the sulfurization time, whereas, the intensity of tin2 0 0) peak decreases at the same time. From the result we con-lude that, as the sulfurization time increases, more and more tintoms react with sulfur to form the orthorhombic SnS. The reactionormula is given in the following:

n + S220 ◦C−→ SnS (1)

In Fig. 1, for sample S15, the intensity of SnS diffraction peaks isery low, but the SnS diffraction peak of sample S30, S45 and S60re obvious. With the increase of sulfurization time, more tin and

ulfur atoms reacted together to form SnS, so the thickness of SnSncreases with sulfurization time. So from Fig. 1, we can see thathe intensity of SnS (1 1 1) peak decreases with the increase of SnSlm thickness, whereas, the (0 4 0) preferential orientation is morend more obvious with the increase of SnS film’s thickness. These

ig. 1. The X-ray diffraction pattern of tin films as a function of sulfurization times:5 min, 30 min, 45 min and 60 min.

nce 257 (2011) 4901–4905

results indicate that the orientation of SnS films relates to the film’sthickness, Devika also reported this phenomenon [12]. Elangovanet al. [13] observed a similar behavior in sprayed SnO2: F films withthe change of film thickness. While Noguchi et al. [14] reportedthat the orientation of evaporated SnS films changed from (1 1 1)to (0 4 0) crystal plane with the increase of substrate temperature.

3.2. Morphological properties

The SEM images in Fig. 2 show different morphologies of the sur-face corresponding to different sulfurization time. The sulfurizationprocess includes two orientations: horizontal orientation and ver-tical orientation sulfurization. As the sulfurization time increasedfrom 15 min to 60 min, the sulfurization area of tin film expanded;at the same time more sulfur atoms diffused deeper into tin filmand chemically combined with tin atoms to form more SnS parti-cles, respectively. So, the surface area and thickness of SnS are allincreased with the sulfurization time. From SEM image of S15, wecan see that a few micro particles like islets which are SnS on thesurface of the grains, and the regular grains are still tin because thesulfurization time is very short. As the sulfurization time increasedto 30 min, we can see that there was a noticeable increase in thenumber of SnS particle and several SnS particles were gatheredon the surface of film. When the sulfurization time increased to45 min, more SnS particles were formed and gathered together,which results in the enlargement of SnS film’s area. As it can beseen from the SEM image of S45 in Fig. 2, there are a lot of SnS par-ticles gathered together and it is difficult to identify the tin grainseven though they exist indeed. As the sulfurization time increasedto 60 min, all of the tin atoms were sulfured to be SnS. The XRDresults also indicated that S60 is the pure SnS film. So from the SEMimage of S60, we know that all the particles on the surface are SnSgrains, and the surface of SnS film (S60) is compact.

3.3. Optical properties

The transmittance spectra of SnS films with different sulfuriza-tion times in the wavelength region of 200–2500 nm are shown inFig. 3. The transmittance of S15, S30, S45 and S60 are all almost0% in the wavelength region of 200–500nm. When the wavelengthis longer than 500 nm, the transmittance increases rapidly. As itcan be seen from Fig. 3 that the transmittance becomes more andmore higher as the sulfurization time increases. The transmittanceof S15 is relatively low, because the sulfurization times are shortand reaction of tin and sulfur is not complete. The S30, S45 andS60 have obviously absorption edges at about 1000 nm and havehigher transmittance than that of S15 in the wavelength region of500–2500 nm. The reason is that, with the increase of sulfurizationtime, more tin and sulfur atoms reacted together in the excess sul-fur environment leading to the formation of SnS. This result agreeswell with the result of XRD measurement in Fig. 1.

Since when a beam of light is perpendicularly incident on thesurface of the film, light reflection and transmission exist in the SnSfilm. Among transmittance (T), total reflectance (R) and absorptioncoefficient (˛) of the film, there is a relationship as the following[15]:

T = (1 − R) e−ad (2)

where d is the thickness of the film. If d, T and R are given, theabsorption coefficient ˛ can be calculated from the formula (2).

From the calculation based on formula (2), the absorption coeffi-cient of SnS films fabricated at different sulfurization times are allhigher than 5 × 104 cm−1 in the visible light region of 400–800 nm.

In order to determine the clear fundamental absorption edge, weneed to calculate the optical band gap Eg, the Eg can be determined

F. Jiang et al. / Applied Surface Science 257 (2011) 4901–4905 4903

sulfur

b

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Fig. 2. The SEM images of samples with different

y the following formula [15]:

˛h�)n = A(h� − Eg) (3)

here A is a constant and n characterizes the transition process.e know that n = 2 and 1/2 for direct band gap and indirect band

ap, respectively. This means that a plot of (˛h�)n vs. h� should betraight line with an intercept on the h� axis equal to Eg.

Fig. 4 shows the curves of (˛h�)2 vs. h� for the samples withifferent sulfurization times: 15 min, 30 min, 45 min and 60 min.inearity of the graphs confirmed that all the films have direct band

ap. Based on Fig. 4, the direct Eg of S15 was calculated to be about.9 eV because the large content of tin. The direct energy gap of S30,45 and S60 are about 1.16 eV, 1.22 eV and 1.3 eV, respectively.

ig. 3. The transmittance spectra of the as-prepared SnS films fabricated at differentulfurization time: 15 min, 30 min, 45 min and 60 min.

ization time: 15 min, 30 min, 45 min and 60 min.

3.4. Electrical properties

The S30, S45 and S60 were all founded to be p-type conduc-tivity in nature by thermoelectric probe test, but the conductivitytype of S15 cannot be confirmed because sulfurization time is shortand sample S15 contains a lot of tin metal. As it can be seen fromFig. 5, the dark and photo resistivity of S15 are almost the same. Butthe resistivity of S30, S45 and S60 all decreased obviously underillumination, which means S30, S45 and S60 all show obvious pho-toresponse.

The photocurrent response of S60 was measured by two probemethod. Two Ag electrodes with area 0.04 cm2 were deposited onthe surface of SnS film by sputtering. The photocurrent response ofS60 was shown in Fig. 6. The sample was connected to the experi-mental setup and maintained in dark under a constant applied bias

of 10 V to stabilize the current. After the stabilization period, thecurrent was recorded in the following sequence: 30 s in dark, 30 sunder illumination and, finally, 40 s in dark. Under illumination,the current increased by approximately 85%. That is because a lot

Fig. 4. The curves of (˛h�)2 vs. h� for the samples with different sulfurization times:15 min, 30 min, 45 min and 60 min.

4904 F. Jiang et al. / Applied Surface Science 257 (2011) 4901–4905

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Fig. 7. The energy-band diagram of S60.

ig. 5. The photo and dark resistivities of the samples with different sulfurizationime.

f carriers were excited by light illumination. After the illumina-ion period, the persistent photoconductivity (PPC) effect of S60ccurred under the dark. The transient decay current under theark, which follows the illumination period, indicates the presencef charge carrier traps in the film [10].

Fig. 7 is the energy-band diagram of S60. The black dots and hol-ow dots are electrons and holes, while Eth and Ete are the trap levelsor hole and electron, respectively. A represents for the releasend transition process of carriers bounded on trap levels, B repre-ents for the direct recombination process of surplus carriers. Therigin of the PPC can be explained that the photo-excited carri-rs are trapped and spatially separated by local potential barriers,hich suppresses the recombination of carriers. Because the holes

aptured in Eth trap levels and the electrons captured on Etn trapevels cannot release immediately, the surplus carriers should stayn conduct and valence band in order to keep electric neutrality.he recombination of surplus carriers includes two steps: A is therst step and B is the second step. The recombination time of B isery short and the time of process A is comparatively long, so theirect recombination (B) must undergo a long time. The long timexistence of surplus carriers results in the PPC effect.

The relaxation of PPC in Fig. 8 can be described by a stretchedxponential function [16,17]:

(t) = Idark + (I0 − Idark) exp

(− t

�

ˇ)

(0 < ˇ < 1) (4)

Fig. 6. The photocurrent response of S60.

Fig. 8. Comparison of PPC decay kinetics with a stretched-exponential function.

where Idark is the current measured under dark, I0 is the PPC builduplevel near the moment of light excitation being removed, � is thePPC decay time constant, and ˇ is the decay exponent. In Fig. 8, thesolid dots are the measured data and the solid curve is the leastsquares fit of data by the stretched-exponential function of Eq. (4).According to the function of fitting curve in Fig. 8, we calculatedthat � = 16.99, and ˇ = 0.274.

The � and ˇ of GaN and ZnO were reported in many articles[16,17], but the values of SnS are firstly reported in our work. Andthe interpretation of the physical basis behind these parameters iswork reserved for future study. The investigation of the PPC effecthas led to an understanding of the carrier relaxation and metasta-bility of crystal defects, which are very important for academicinterest and technological application.

4. Conclusions

Polycrystalline SnS films with orthorhombic structure were syn-thesised by sulfurization method. The XRD results showed that the(0 4 0) SnS preference growth is more and more obvious with theincrease of sulfurization time which relates to the SnS thickness.With the increase of sulfurization time, more and more tin atomsreacted with sulfur atoms to form SnS and no SnS2 and Sn2S3 phases

were detected. When the sulfurization time increased to 60 min,the pure compact SnS film without any other phase was formed.The SnS film shows high optical absorption efficiency and has adirect band gap of about 1.3 eV, which is close to the optimal band

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ap 1.5 eV for solar cells. The SnS films prepared by sulfurizationethod show p-type conductivity and exhibit obvious photore-

ponse, at the same time, the persistent photoconductivity effectas detected in the SnS films we prepared.

cknowledgement

This work is performed with financial support from the Chineseational High Tech. “863” Program (2006AA03Z219).

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