Simplified chemical deposition technique for good quality SnS thin films

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1991 Semicond. Sci. Technol. 6 132

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Semicond. Sci. Technol. 6 (1991) 132-134. Printed in t h e UK

LETTER TO THE EDITOR

S i m pl if ied c hem ica I deposition tech n iq ue for good quality SnS thin films

M T S Nair and P K Nail Photovoltaic Systems Group, Laboratorio de Energia Solar, IiM Universidad Nacional Autonoma d e Mexico, 62580 Temixco. Moreios. Mexico

Received 7 September 1990

Abstract. A chemical deposition technique, much simpler and more versatile than previously reported and capable of yielding good quality SnS films of thickness up to 1 1 . 2 p under a choice of deposition conditions, is presented. The as-prepared films are polycrystalline with p-type dark conductivity in the range 10 - 5-1 0 - 4 Q - 1 cm-' for the thicker ( - 1 pm) films and showing a photocurrent to dark current ratio of 510 under 500 W m-' tungsten halogen illumination. The optical transmittance and reflectance spectra and the photocurrent response curves of a series of SnS samples are explicitly presented to provide insight into possible applications of these films.

In recent articles we have described the chemical deposi- tion techniques capable of yielding good quality thin films ofCdS [I, 21, Cu,S [3,4]. PbS [S, 61, Bi,S, [7] and thin film combinations CdS-Cu,S [SI, PbS-Cu,S [SI and Bi,S,-Cu,S [IO] and have illustrated or suggested their application in the area of energy conservation and solar energy utilization [I 1,121. In this respect our atten- tion has been drawn to SnS, a material with an optical band gap of 1.3 eV [I31 and hence with the potential to become a versatile solar energy material. The high con- version efficiency of 1 2 5 % obtainable from it in photo- voltaic devices, according to Prince-Loferski diagrams [14,15], and its acceptability from the points of view of cost, availability, toxicity and stability ascribe to SnS a unique position among the metal sulphides, proba- bly to be shared only by FeS, [16, 171.

Among the chemically deposited metal sulphide thin films, SnS is a recent entry [18, 191. The methods re- ported earlier either employed propionic acid to dissolve elemental sulphur and anhydrous SnCI, to constitute the chemical bath to yield thin films of tin sulphide of varying compositions (namely, SnS, Sn,S, and SnS, of thick- ness - 1 pm in < 1 h [lS]), or used glacial acetic acid to dissolve SnCI2.2H,O to constitute baths containing triethanolamine (TEA), thioacetamide (TA) and up to 16 ml of 14 M NH,(aq) per 100ml OF the bath, yielding SnS films of - 0.1 pm thickness in 20 h [19J

In this paper, we present a simple chemical deposition technique in which SnC1,.2H,O is readily dissolved in acetone. Good quality SnS thin films of up to 1 pm thickness are deposited in about 5 h at 75 "C or in around 40 h at 25 "C from baths constituted with TEA, TA

0268-1242/91/020132+03 $03.50 @ 1991 IOP Publishing Ltd

and lesser quantities of NH,(aq) (IO ml of 4 mole per dm-'NH, per 100ml of bath) than that reported in [19]. We consider that this method is appropriate for the production of SnS films in the SnS-Cu,S thin film combination for solar control coatings 1201 and all-glass tubular solar collectors [21] and for any forthcoming photovoltaic applications of SnS. The optical transmit- tance and reflectance spectra and the photocurrent response curves of the SnS films are explicitly presented to illustrate their quality and to help stimulate ideas towards other possible applications of these films.

The physico-chemical bases of chemical deposition of metal sulphide thin films are rather well understood [22]. In the present work, the chemical bath for deposition of SnS thin films was constituted of

1 g of SnC1,.2H20 dissolved in 5 ml of acetone, 8 ml of 1 M TA in distilled water, 10 ml of 4 M NH, (aq), and 12 ml Of 50 % TEA,

with the volume made up to 100 ml with distilled water. Depositions were made on glass slides of 76 mm x 26 mm x 1 mm dimensions, by supporting them vertic- ally on the walls of 50ml beakers containing the solutions. The deposition could be done at room temper- ature, but higher bath temperatures of up to 75 "C may he employed to accelerate the deposition. Similarly, at any specific temperature the rate of deposition of the films can be controlled by varying the quantity of the TA in the bath. In the present work, we are reporting the result on two different bath compositions: composition X (as described above) and composition Y (as described

Letter to t h e Editor

Table 1. Deposition conditions of SnS thin films

Appearance in daylight Bath Temp. Duration Thickness

Sample type ("c) (h) (pm) Reflection Transmission

A X 50 6 0.35 B X 50 8 0.45 C X 50 14 0.85 D X 50 20 1.20 E X 25 22 0.40 F X 60 5 0.55 G X 75 5 0.80 H Y 60 5 0.30 I Y 75 5 0.50

bluish purple greenish brown black black greenish yellow brownish green black golden purple brownish ween

brownish yellow brownish orange deep red obscure red brownish yellow brownish orange deep red brownish yellow brownish oreen

above but with 4 ml of TA per 100 ml of the deposition mixture). Table 1 shows that the above bath composi- tions can allow the deposition of good quality SnS thin films with a range of specifications suitable for various applications.

After the deposition, the films from the 'wall sids' of substrates were retained for the measurements. The opti- cal transmittance with air in the reference beam was recorded on a Shimadzu UV 365 spectrophotometer for $film-side inciddnce. The film side near normal (5 ' ) specular reflectance was measured with front aluminized mirrors in the reference beam. The approximate thicknesses of the films were estimated from the positions of the inter- ference peaks in the reflectanceltransmittance spectra. The photocurrent response was measured using coplanar silver print electrodes (length equal to separation) ap- plied to the film surface. The measurements were done using a computerized measurement system as described in [I] under a tungsten halogen lamp producing an intensity of 500 W m-' a t the plane of the film and with an applied bias of I V.

We have observed that the quantities of TEA and NH,(aq) are very critical in obtaining good quality SnS thin films. An initial cloudy appearance of the bath has been found to be essential for the film deposition; excess quantity of TEA can lead to clear solution from which the films do not grow. Similarly, the use of excess NH,(aq) can lead to the incorporation of the hydroxide phase which would give a greyish appearance to the film surface-similar to that observed in the case of PbS films [SI. The deposition of the film is thought to follow the reaction suggested in [19]:

[sn(TEA)]'+ + CH,CSNH, + 2 O H - -t SnS + TEA + CH,CONH, + H,O.

The as-prepared films are crystalline, as is clearly indi- cated by the presence of the x-ray powder diffraction line corresponding to the d-spacing of 2.84 A, in the case of all the samples listed in table 1. This line has the maximum intensity in the XPD spectra listing for SnS (JCPDS 1983 card 33-1375). In the case of thicker samples, e.g., sample C, other lines are also seen superimposed on the x-ray spectra of the (amorphous) glass substrate.

Figures 1 and 2 show the optical spectra of samples

!

T , R C % 1

88 -

. 5 L E I . 5 *.a 2 . 5

Y.Y.,.ngth cum,

Figure 1. Optical transmittance T ( O h ) and near normal specular reflectance R (Yo) spectra of SnS samples A-D of table 1

A-G listed in table I . For the X-bath at 50 "C, the film thickness increases continuously from < 0.1 pm at 2 h to > 1 pm at 20 h. The good quality of the films is illus- trated by the presence of the interference peaks in the transmittance and reflectance spectra of the films. The presence of an optical band gap, E,, of approximately

1 8 8 , I r. R

.5 I . 8 1.5 2 . 8 2.5

Yavelangth cum1

Figure 2. T (YO) and R (%) spectra of samples E 4 of table 1, illustrating that a range of deposition conditions employing baths X and Y can be used for t h e SnS thin film deposition.

133

Letter to t h e Editor

5.0.87 ,

c photo - I.BE.12

B 288 4BB 688 T,",. :,=<.,

Flgure 3. Photocurrent response of the SnS films, measured at 1 V bias and 500 W m-' tungsten halogen illumination.

1.3 eV [I31 corresponding to the wavelength, 2

0.95 pm is clearly observable in the transmission spectra. The sum of T (%) and R (%) values at any wavelength, 2, for 2. > is 90-95 %, indicating a high degree of specu- lar reflectance, which is a characteristic of good quality thin films. The reflectance spectra in figure 1 and figure 2 also show that the heights of the interference peaks in the reflectance and transmittancc spectra vary with the thickness of the film, which is in accordance with the theory of thin film interference and of relevance to the application of the films in optical devices.

The photocurrent response curves in figure 3 show a photo- to dark-current ratio of only I O or less under 500 W m-' tungsten halogen illumination. The dark conductivity shows a marked increase with increase in thickness, from lo-' 0-' cm-' for a film of 0.35 pm thickness (sample A) to 4 x W' cm- ' for a film of 1.2 pm in thickness (sample D): a 4 x increase in con- ductivity for a 400 x increase in thickness. This is a common feature of thin films, arising from charge carrier trapping at the surface and intergrain boundaries, which are thickness-dependent [23, 241. The 'hot probe' test indicatcd that the as-prepared films are p-type, which is in accordance with the results reported on chemically depo- sited SnS thin films [IS] and on SnS powder samples obtained by passing H,S through an acidic solution of Sn(n) chloride [251. The plot oflog U against 1/T gave an activation energy of 0.44 eV for sample C, suggesting the presence of deep acceptor levels, in accordance with the results in [IS].

The SnS thin films reported here are readily usuable for the fabrication of SnS-Cu,S thin films for solar control applications [20] and for all-glass tubular solar collector fabrication [21]. Detailed work on the depen- dence of the dark- and photoconductivities, photocur- rent to dark current ratio and conductivity type on the

deposition conditions, and the effect of annealing at elevated temperatures (-500 "C) on altering the conduc- tivity from p-type to n-type [25] etc, may lead to possible application of the films as a photovoltaic and optoelec- tronic material.

The authors are grateful to Jod Campos and Aaron Sanchez for the electrical measurements and to Laticia Baiios for the XPD measurements.

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