Spray pyrolysis deposition of CuS thin films

August 1997

Materials Letters 32 (1997) 73-77

ELSEVIER

Spray pyrolysis deposition of CuS thin films Cristina Nagcu a,*, Ileana Pop a, Violeta Ionescu a, E. Indrea b, I. Bratu b a Institute of Chemistry, ’ ‘Raluca Ripan”, Cluj-Napoca, 30 Frintrinele Str., Cluj-Napoca, Romania b Institute of Isotopic and Molecular Technology, P.O. Box 700, R-3400 Cluj-Napoca, Romania Received 23 May 1996; revised 14 December

1996; accepted

16 December

1996

Abstract Spray pyrolysis deposition of CuS thin films using aqueous solutions containing CuCl, .2H,O, thiourea and cationic surfactant, is presented. The transmission and reflexion spectra in the VIS-NIR region of such films are recorded. The electrical resistances and photosensitivities, as well as optical energy band gap (2.2 eV> are also determined. The spray pyrolyzed CuS films show p-type conductivity. Keywords: Copper sulfide; Thin film; Spray pyrolysis;

Transmittance;

1. Introduction Cu, S (X = l-2)

thin films

have recently

received

due to numerous technological applications in achievement of solar cells [l-81, in photothermal conversion of solar energy (as solar absorber coatings) [6,9- 121, as selective radiation filters on architectural windows (for solar control in the warm climates) [ 1,13- 171, as electroconductive coatings deposited on organic polymers [ 1S-261. At room temperature, Cu, S in the bulk form is known to exist in five stable phases: chalcocite (orthorhombic Cu, S), djurleite (Cu , ,95S), digenite (Cu t ,sS), anilite (Cu ,.,5S) and covellite, (CuS) [1,16]. Mixed phases are also known in the intermediate compositions [21]. Various techniques have been employed to prepare Cu,S thin films such as vacuum evaporation [27-301, chemical bath deposition [l-7,12considerable

attention

* Corresponding

author.

00167-577X/97/$17.00 Copyright PI1 SOl67-577X(97)00015-3

Reflectance;

Electrical

resistance;

Photosensitivity;

Energy band gap

18,20-23,31-381, spray pyrolysis (for x higher than 1) [39-421, sputtering etc. The spray pyrolysis method has been used widely to deposit other sulfides films (predominantly CdS, but also PbS, Sn,S,, SnS) because it is relatively cheap and simple. The work reports, possibly for the first time to our knowledge, spray pyrolysis deposition of CuS thin films. It also presents the basic optical, electrical and photoelectrical properties of these films.

2. Experimental 2.1. Deposition

procedure of the films

The apparatus used to carry out the chemical spray process consists basically of a device to atomize the spray solution and some sort of substrate heater. A typical experimental setup presented by Chamberlin and Skarman [43] was used. Our setup

0 1997 Elsevier Science B.V. All rights reserved

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C. Nap

et al. /Materials

Letters 32 (19071 73-77

consists of a reaction chamber of Pyrex glass joined to its lower part with a plate heated by electrical resistance. On the plate is placed the substrate, whose temperature is measured with a thermocouple, located under the substrate. Above, at an established distance, is fixed a glass spraying nozzle, through which the adequate solution is sprayed (from a receiver) by means of the carrier gas. The gases are exhausted through a hole, situated at the lower part of the chamber. The spraying nozzle can move on a 10 cm distance. Standard microscope glass slides, 75 mm X 26 mm X 1 mm, were used as substrates. The spray solution consists of an aqueous solution of 0.016 M CuCl, .2H,O, 0.08 M thiourea SC(NH,), and 1.67 X lo-’ wt% cetyl pyrydinium bromide C,,H,,BrN. H,O (as a cationic surfactant for improvement of uniformity of CuS films), with the copper and sulfur ions in a molar ratio 1 : 5. The substrate temperature was in the range 150-210°C. The solution and gas flow rates kept constant at 36 cm3 mm-’ and 61 min-‘. Dry air was used as carrier gas. The nozzle-substrate distance and the spraying time were kept constant at 30 cm and lo-150 s, respectively. The solution is sprayed on the support for 10 s. After 2 min, the cycle is repeated until the desired thickness is achieved. 2.2. Characterization

X-ray diffraction spectra were obtained by means of a DRON-3 diffractometer using Co Ko X-ray radiation. The thickness of the films was estimated by microweghing of the material, assuming the same density with that of a bulk CuS (4.68 g cm-3>. The VIS transmission spectra have been recorded with a SPECORD UV-VIS spectrometer and those of the NIR transmission and reflexion with a UR-20 Carl Zeiss Jena spectrophotometer. The authors have realized the NIR extension of the UR-20 spectrophotometer by a calibration with known standards, using the LiF prism. Sheet electrical resistances have been measured at 25°C using an E 0302 multimeter, after application of silver paste contacts and terminals from silvered thin wires.

3. Results and discussion The color of spray deposited CuS films is dark green. The films show good adhesion to glass substrate. The XRD pattern of a 0.25 pm Cu,S film deposited by the spray pyrolysis technique is shown in Fig. 1. After comparison against the JCPDS-ICCD diffraction pattern 6-464, from PDF-2 Sets l-43 Database, the deposited material was identified as the covellite phase CuS (hfxagonal CuS,othe lattice parameters are: a = 3.792 A, c = 16.344 A and density = 4.68 g cmm3>.

of the films

Identification of the deposited material composition was done by X-ray diffractometry (XRD). The

2 Theta Fig. 1. X-ray diffraction

4s

( deg.)

pattern of a 0.25 p,rn thick CuS film.

C. Napzu et al. /Materials Letters 32 (1997) 73-77

The first series of experiments accomplished at 200°C refers to the investigation of the influence of the deposition time on the film thickness. The variation of the thickness with deposition time, shown in Fig. 2, is linear, implying that the growth conditions can be held stable and the experimental results are reproducible. Another series of films was deposited for substrate temperatures between 150 and 210°C. It is observed that the higher the substrate temperature, the greater optical clarity of the film is obtained. This range of temperature was chosen because CuS is reported to decompose at = 220°C [44]. The decomposition of the films at 220°C is shown by the conversion of the film to a nearly transparent one. The optical transmission spectra of CuS films obtained by spray pyrolysis are presented in Fig. 3. It can be seen that the films transmittance decreases with increasing thickness, mainly because of the increased abundance and/or size of crystallites growth. The transmission spectra of the films of different thicknesses vary in such a way that the transmission in the visible region (h = 0.4-0.7 pm) becomes more and more remarkably peaked around 0.6-0.7 p,m, while at the same time a substantial decrease in transmission throughout the NIR region (0.7-2.5 pm) is observed. In Fig. 3 the reflectances of the mentioned films are also presented. The NIR reflectance of CuS films increases with thickness. In

0

05

15

1 Wavelength

1.5

2

2.5

brn1

Fig. 3. Transmission and reflexion spectra of the CuS films of different thicknesses (161-0.015 km, 163-0.022 km, 165-0.027 pm, 168-0.048 pm, 170-0.062 km).

the case of chemical bath deposition of CuS films [ 16,17,2 1,381, same phenomenon concerning the changes in transmittance and reflectance of CuS films take place. Therefore this makes the CuS films suitable as solar control coatings in the warm climate regions, as already suggested by Nair et al. [l&17], Grozdanov et al. [21] and Nascu and Vomir [38]. Some electrical and photoelectrical properties of CuS films were also investigated. The electrical resistances in the dark (R q d) and under illumination with tungsten lamp of 100 W mm2 (R q i) as well as the photosensitivity S (defined as S = (R q ,, R 0 i)/R q i) [45I are summarized in Table 1. We have found that all films show p-type conductivity as was determined by the thermo-electric probe method.

Table 1 Sheet resistances and photosensitivities of CuS films obtained by spray pyrolysis (R,, - sheet electrical resistance in the dark, R _. n i - sheet resistance under illumination, S - photosensitivity)

20

60

LO Deposition

Fig. 2. Dependence tion time.

of the thickness

80

100

time (5)

of CuS films on the deposi-

Sample

Thickness(bm)

R,,(R/n) _”

R,,(fi/El) -.

S

161 163 165 168

0.015 0.022 0.027 0.048

.2x106 >2X106 33000 406

>2X106 >2X106 14000 221

1.35 0.78

D D D D

C. Nap

76

#x50

et al./Materiuls

i

LO -

,-

30.

‘E 0 >

/ ..- -// L*.

20.

10

2.5

-2

Photon

Fig. 4. Graphical CuS films.

determination

energy

3

3.5

ieV)

of the optical energy band gap for

The films present photoconducting properties even at these small thicknesses. The optical band gap energy, graphically determined from the plot of o2 (a is the absorption coefficient calculated from transmission data) versus photon energy in the visible region, is 2.2 eV (see Fig. 4).

4. Conclusions Spray pyrolysis deposition of CuS thin films on glass offers the possibility for large-area deposition at relatively low cost. The optical properties of such films make them suitable for solar control coatings. The CuS films are conductive and photosensitive even at thicknesses < 0.05 urn.

Acknowledgements This work was financially supported by the Ministry of Research and Technology, Romania. The authors wish to thank Miss Dana Ungurean for help in the preparation of the samples.

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