JOURNAL OF MATERIALS SCIENCE LETTERS19, 2000,2135– 2137

SnS thin films grown by close-spaced vapor transport

YANUAR∗, F . GUASTAVINO, C. LLINARES‡C. E. M. 2 Universite Montpellier II (UMR–C. N. R. S–5507), Place Eugene Bataillon,34095 Montpellier, Cedex 05, FranceE-mail: llinares@cem2.univ-montp2.fr

K. DJESSAS, G. MASSECentre d’Etudes Fondamentales (Semiconducteurs), Universite de Perpignan,52, Av. de Villeneuve, 66860 Perpignan, France

Thin films of tin sulfide (SnS) have attracted consid-erable attention because of a wide variety of potentialapplications. SnS is one of the most promising materialsfor low-cost thin film solar cells and optoelectronics, asits direct band-gap (1.3–1.4 eV) [1] is near the optimumvalue of 1.5 eV, the band gap required for efficient lightabsorption.

Thin films of SnS have been prepared by differ-ent techniques such as vacuum-evaporation [2, 3], rfsputtering [4], cathodic electrodeposition [5] spraypyrolysis [6, 7], electrochemical deposition [8] andatmospheric pressure CVD [9]. However, close-spacedvapor transport (CSVT) has not yet been used for thismaterial. CSVT has been widely used to deposit othermaterials because it is a very low-cost simple method[10]. This paper summarizes the first results obtainedin the growth of polycrystalline SnS thin films by aclose-spaced vapor transport technique and their char-acterization.

The CSVT system consists basically of a vertical re-actor, made from a quartz tube whose diameter can bechosen according to the devices to be grown. The reac-tor has a inner diameter of about 20 mm and a lengthof 10–20 cm. The source of SnS (powder pressed at300 kg cm−2) and substrate are facing each other, andare separated by a quartz spacer whose thickness is be-tween 0.3 and a few mm. Solid iodine is placed in theupper part of the reactor, then this part is sealed undervacuum. Finally, the complete system is placed above aSiC heating element which is operated by a voltage reg-ulator. The thermal gradient between the source and thesubstrate is adjusted by using a heating coil, which alsoallows for preheating of the substrate before heating thesource [10].

The powder was obtained from bulk materials syn-thesized from the stoichiometric mixture of Sn and Selements (99.9999% purity). The deposited films werecharacterized for their physical properties using the ap-propriate techniques. The composition of the films wasdetermined by an energy dispersive X-ray spectroscopy(EDS) system attached to a scanning electron micro-scope (SEM). The structural properties of the films werestudied with an X-ray diffractometer using the CuKα ra-

∗Permanent Addresse: Physics Department, University of Riau, Pekanbaru, 29832 Indonesia.‡Author to whom all correspondence should be addressed.

diation (1.5406A). Optical measurements were carriedout using a Beckman UV 5240 double beam UV-VIS-NIR spectrophotometer and the electrical conductivitytype was determined with a hot thermal probe tech-nique. The other electrical properties of the sample weredetermined by Hall effect measurements at room tem-perature.

SnS thin films were deposited on soda lime glass sub-strates at source temperatures of the order of 500◦C forabout 10 min. Table I shows the deposition conditionsand the composition of a typical sample.

The uncertainty in the EDS measurements being ofthe order of 2 at.%, it can be deduced from Table I thatthe typical films are quasi-stoichiometric. The crystal-lographic structure of the films was studied by X-raydiffraction. The diffraction pattern of sample YP22 isgiven Fig. 1. Only the SnS phase is present. The peakswere indexed with the orthorhombic cell parameters :a = 0.432 nm,b = 1.128 nm andc = 0.399 nm whichare close to those reported for SnS (JCPDS 33-1375),the data are presented in Table II. The spectrum showsa strong peak oriented along the (111) direction. Thestructural data are in agreement with the results deducedfrom SnS films prepared by other techniques [3]. Thesurface morphology shows prismatic crystallites with

Figure 1 XRD spectrum of a typical SnS film on glass substrate.

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TABLE I Deposition conditions, composition and electrical conductivity type of a typical film

Sample Substrate Source Temperature Spacer Time EDS composition Cond.type

YP22 soda-lime 500◦C 0.5 mm 10 min Sn S pglass 53.2% 46.8%

TABLE I I XRD data corresponding to Fig. 1

Lattice spacing,d (nm) Peak intensity

Present PresentPics no. study Reporteda study Reporteda hkl

1 .40436 .4041 70 32 1102 .34132 .3427 80 67 1203 .3241 .3248 75 70 0214 .2927 .2934 29 53 1015 .2832 .2838 100 100 1116 .2326 .2352 25 1 1407 .2159 .2166 28 2 2008 .1868 .1876 31 30 1419 .1868 .1876 26 27 211

aTaken from the JCPDS 33-1375 for SnS (Herzenbergite).

sizes of approximately 1µm. Fig. 2 shows the SEMphotograph of a typical (YP22) SnS film.

The transmittance and reflectance of the SnS filmswere measured at normal incidence in the wavelengthrange 800–2500 nm and are shown in Fig. 3. FromFig. 3, it can be seen that the transmission edge is lo-cated at around 1000 nm. This value is similar to that re-ported by Ichimuraet al.[8]. The high absorption regionincreases from 5% to about 40% for a film thicknessof 3µm.

Fig. 4 shows the absorption coefficientα of the filmwas calculated from the transmittance and reflectancedata. A detailed analysis of the absorption coefficientdata revealed the presence of several optical transmis-sion mechanisms, as observed by several authors [3, 6].The film showed a high absorption coefficient,α about104 cm−1 above the fundamental absorption edge. Thisvalue agree well with ones reported in the literature [3].

Fig. 5 shows the energy band gap of a typical filmsample on glass, determined from the plot of (αhν)2 asa function ofhν, wherehν is the photon energy. The

Figure 2 SEM photograph of a typical SnS film on glass substrate.

Figure 3 Reflection and transmission of a typical SnS film on glasssubstrate.

Figure 4 Absorption coefficientα as a function ofhν.

straight line indicates that the absorption near the fun-damental absorption edge is a direct optical transition,and the evaluated energy band-gap is 1.32 eV. This de-duced value of 1.32 eV is very similar to those valuesreported in the literature [7].

Figure 5 Plot of (αhν)2 as a function ofhν for a typical SnS film onglass.

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The SnS films exhibited p-type conductivity, deter-mined by hot-probe technique. The resistivity, Hall mo-bility and carrier density of these films was measuredusing the Van der Pauw method. At room temperature,the resistivity of 14.5Ä cm was in good agreement withthe results in the literature [3, 4, 7]. The Hall mobilityof about 3.73 cm2/Vs obtained for the SnS sample isrelatively low when compared to the values reported inthe literature for evaporated films [3]. The SnS films ex-hibited a high carrier density of 1.16× 1017 cm−3, thisvalue being larger than the results of the order of 1014–1015 reported in the literature [3]. This decrease in mo-bility is a consequence of the increase in the hole con-centration, when our sample was quasi-stoichiometric(Sn-rich).

From these results we can conclude that a close-spaced vapor transport technique can be used to growSnS films. The films are p-type conducting and showa single phase and a good crystalline quality with pris-matic crystallites of about 1µm. The band-gap is direct,with an energy of the order of 1.32 eV and the absorp-tion coefficient reaches 104 cm−1. These results suggestthat SnS films could be used in photovoltaics.

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Received 13 Marchand accepted 3 April 2000

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