Turk J Phys

(2014) 38: 245 – 252

c⃝ TUBITAK

doi:10.3906/fiz-1312-2

Turkish Journal of Physics

http :// journa l s . tub i tak .gov . t r/phys i c s/

Research Article

An investigation on SILAR deposited CuxS thin films

Aykut ASTAM1,∗, Yunus AKALTUN2, Muhammet YILDIRIM3

1Department of Physics, Science and Arts Faculty, Erzincan University, Erzincan, Turkey2Department of Electrical and Electronic, Engineering Faculty, Erzincan University, Erzincan, Turkey

3Department of Physics, Science Faculty, Ataturk University, Erzurum, Turkey

Received: 03.12.2013 • Accepted: 19.02.2014 • Published Online: 11.06.2014 • Printed: 10.07.2014

Abstract:A copper sulfide thin film was deposited on glass substrate by successive ionic layer adsorption and reaction

method at room temperature, using cupric sulfate and sodium sulfide aqueous solutions as precursors. The structural,

surface morphological, optical, and electrical properties of the as-deposited film were investigated via X-ray diffraction,

scanning electron microscopy, energy dispersive X-ray analysis, optical absorption, and d.c. 2-point probe methods. The

film was found to be amorphous, dense, and uniform. Average atomic percentage of Cu:S in the as-deposited film was

calculated as 63:37. The band gap energy of copper sulfide thin film was found to be 2.14 eV and the resistivity was

10−2Ω cm at room temperature. The effect of annealing on crystal structure was also studied and reported. X-ray

diffraction patterns revealed that annealing the film at various temperatures slightly improved the crystallinity. By

using thermoelectric power measurement, p-type electrical conductivity was determined for as-deposited and annealed

samples.

Key words: SILAR, copper sulfide, thin films, structural properties

1. Introduction

Metal chalcogenide compounds are of particular interest because of their potential applications in the field of

electronics and electro-optical devices [1]. Copper sulfide (CuxS) is an important p-type metal chalcogenide

semiconductor due to its potential uses in many photovoltaic and photothermal applications such as selective

solar control coatings, transparent and conductive coatings, and thin film Cu sensor electrodes [2–4]. CuxS thin

films are also used in room temperature solid state gas sensors for ammonia and other molecules containing the

NHn moiety as well as acetone and ethanol, while the most sensor materials are based on oxides that typically

demand temperatures of at least a few hundred degrees to achieve good gas sensing properties [5–7]. CuxS thin

films may be combined with other semiconductor thin films to form ternary compounds that have significant

optical and electrical properties for photovoltaic applications [8,9]. At room temperature, copper sulfides exist

in a wide variety of stable compositions: chalcocite (Cu2S), djurleite (Cu1.95S), anilite (Cu1.75S), covellite

(CuS), and villamaninite (CuS2) [10]. Besides these stable compositions, copper and sulfur also form a number

of mixed phases. Generally, the resistivity of CuxS films varies depending on the stoichiometry and fabrication

method and p-type conductivity is attributed to free holes from acceptor levels of copper vacancies [11]. CuxS

thin films have been prepared by several techniques such as atomic layer deposition [7], vacuum evaporation [12],

RF reactive sputtering [13], chemical bath deposition [3,14], spray pyrolysis [15,16], and successive ionic layer

∗Correspondence: a astam@yahoo.com

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ASTAM et al./Turk J Phys

adsorption and reaction (SILAR) [17–20]. The stoichiometry of CuxS films often depends on the fabrication

method used.

Chemical deposition methods are widely used for deposition of materials in thin film forms because they

are nonpolluting, simple, economical, and suitable for large deposition. In addition to these advantages, usually

chemical deposited thin films are either amorphous or poorly crystallized. Therefore, annealing is needed to

increase the crystallinity of the films [21]. SILAR is also a known modified chemical deposition method, first

reported in 1985 [22]. The name SILAR was ascribed to this method in the same year [23] and discussed in

subsequent papers [24,25] that dealt with ZnS, CdZnS, and CdS thin films. In the SILAR method, deposition

of thin films takes place from aqueous precursor solutions. Cationic and anionic precursor solutions are placed

in different vessels and the substrate is immersed into the precursor solutions and rinsed with double distilled

water after every immersion. This rinsing process enables excess absorbed ions to wash away from the substrate

surface and avoids homogeneous precipitation in the solutions. Thin film deposition can be achieved by repeating

these cycles. Moreover, anionic and cationic precursors in different vessels offer good control over the deposition

parameters such as concentration and pH of the solutions, adsorption, and reaction and rinsing time durations.

The thickness of the film is controlled by the number of deposition cycles. SILAR is a relatively simple, quick,

and economical method and it does not require sophisticated instruments. As a low-temperature process,

SILAR enables formation of thin films on plastic and other flexible substrates, which can potentially lead to

lightweight, flexible, and foldable photovoltaic devices. More details of SILAR deposition are well documented

in the literature [26,27].

In this study, CuxS thin film was deposited on glass substrate by the SILAR method at room temperature

and ambient pressure and annealed at various temperatures in nitrogen atmosphere. The as-deposited film was

investigated in terms of structural, optical, and electrical properties and the effect of thermal annealing on the

crystal structure was also studied and discussed.

2. Experimental

For the deposition of CuxS thin film by the SILAR method, analytical grade anhydrous cupric sulfate (CuSO4),

sodium sulfide nonahydrate (Na2S.9H2O), and aqueous solution of ammonia were used without further purifi-

cation. Aqueous solutions of 0.1 M CuSO4 (pH ∼10) were used as cationic and 0.05 M Na2S (pH ∼12) as

anionic precursor. To adjust the pH value of CuSO4 solution, aqueous solution of ammonia was slowly added

with constant stirring. A commercial microscope glass slide was cleaned ultrasonically for 15 min in acetone,

water:ethanol (1:1) solution, and double distilled water consecutively and dried under a nitrogen flow prior to

film deposition. For solutions and deionized water 50-mL capacity glass beakers were used. The deposition was

carried out at room temperature and ambient pressure. For the deposition of CuxS thin film, each SILAR cycle

involved the following steps:

1. Immersion of the substrate in cationic precursor solution for 20 s in which copper ions are adsorbed on

the surface of substrate.

2. Immersion of the substrate in deionized water for 40 s to remove excess unabsorbed copper ions on the

substrate surface.

3. Immersion of the substrate anionic precursor solution for 20 s in which sulfur ions are reacted with pre-

adsorbed copper ions on the substrate.

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ASTAM et al./Turk J Phys

4. Immersion of the substrate in deionized water for 40 s again to wash away unreacted and loosely bonded

ions.

Immersion and rinsing times were determined experimentally. The optimized preparative parameters

are summarized in Table 1. At the end of 25 SILAR deposition cycles the glass substrate was taken out and

washed in double distilled water. The deposited film was uniform over the entire substrate surface and greenish

brown. At above 25 deposition cycles the film peeled off from the substrate surface during rinsing. This can

be attributed to weak adhesion of the copper compounds to oxide surfaces or tensile stress that tends to cause

delamination when the film becomes thick [28,29]. Film from one side of the substrate was removed using cotton

swabs moistened in dilute HCl. The thickness of the deposited film was determined by the gravimetric weight

difference method using a sensitive microbalance. The density of deposited material is assumed to be the same

as that of its bulk form. After 25 deposition cycles, the thickness of the film and average deposition rate were

about 210 nm and 8.4 nm/cycle, respectively.

Table 1. Optimized preparative parameters for the deposition of CuxS thin film.

Precursors CuSO4 + ammonia Na2SConcentrations (M) 0.1 0.05pH ∼ 10 ∼ 12Temperature (K) 300 300Immersion times (s) 20 20Number of SILAR cycles 25 25

The characterization of the crystal structure of the film was carried out using a Rigaku D/Max-IIIC

diffractometer with Cu-Kα (λ = 1.5405 A) radiation. The surface morphology of the CuxS thin film was exam-

ined by ZEISS SUPRA 50VP scanning electron microscopy with energy dispersive X-ray analysis (EDAX) to

evaluate the average atomic percentage quantitatively. For the optical absorption measurements a PerkinElmer

UV/VIS Lambda 2S spectrometer was used in the wavelength range 350 to 800 nm. Electrical resistivity of the

CuxS thin film was measured by d.c. 2-point probe method at room temperature. For this aim, pairs of silver

paint electrodes of 5-mm length at 10-mm separation were printed on the surface of the film. The film was also

annealed in nitrogen atmosphere at various temperatures ranging 100 to 400 C for 45 min to investigate the

effect of annealing on the crystal structure. By using thermoelectric power measurement the type of electrical

conductivity was determined from the polarity of the thermal generated voltage.

3. Results and discussion

Characterization of the crystal structure of the as-deposited and annealed CuxS thin film obtained from the

SILAR method was carried out by X-ray diffraction. The recorded XRD patterns of the film before and after

annealing at various temperatures ranging from 100 to 400 C are presented in Figure 1. XRD measurements

revealed that as-deposited CuxS thin film is amorphous in nature and 100 C annealing temperature did not

cause a discernible change in structure. Annealing the film at 200 C led to the appearance of 2 diffraction

peaks of cubic Cu2S along the (111) and (220) planes (JCPDS data card number 65-2980). At 300 C

annealing temperature, a new diffraction peak of monoclinic Cu2S along the (141) plane appeared when (111)

reflection disappeared (JCPDS data card number 65-3816). At this annealing temperature a diffraction peak of

hexagonal CuS (105) reflection was also observed (JCPDS data card number 65-3556). Annealing the film at

400 C did not affect the composition or structure, but slightly reduced the peaks’ intensities. With increasing

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ASTAM et al./Turk J Phys

annealing temperature, a small shift in the position of the (220) peak was also observed, indicating an increase

in interplanar distance towards its standard value (1.980 A). The broad hump seen either in as-deposited or

annealed samples in the 2θ range of 20–35 is due to the amorphous glass substrate. The as-deposited and

annealed film was characterized by random orientation and showed no sign of texturing. With the help of X-

ray diffraction measurements, interplanar distances (d) of the diffracting planes were calculated using Bragg’s

diffraction condition [30]:

200

300

400

500

20 30 40 50 60 70200

300

400

500

200

300

400

500

200

300

400

500

200

300

400

500 Δ

(141

)(1

41

)

as-deposited

2θ (degrees)

annealed at 200 °C

annealed at 100 °C

Δ

(220

)(2

20

)(2

20

)

(11

1)

CuS

Cu 2SΔ Δ annealed at 400 °C

annealed at 300 °C

(105

)(1

05

)

Δ

ΔΔ

Inte

nsi

ty (

a.u.)

Figure 1. XRD patterns of the as-deposited and annealed CuxS thin film.

2d sin θ = nλ, (1)

where n is the order of the diffracted beam, λ is the wavelength of the X-ray used, and θ is the Bragg’s angle

that corresponds to the peak analyzed. Crystallite grain sizes (D) of the film were determined from the full-

width at half maximum height (β) of the predominant peak at 2θ = 46 corresponding to (220) orientation.

This was done using the Debye–Scherrer formula [31]:

D =Kλ

βcosθ, (2)

where K is the shape factor, taken equal to 0.9. To gain additional information some structural parameters

such as dislocation density, strain, and number of crystallites per unit surface area of the CuxS thin film were

also calculated using the predominant peak. Dislocation density (δ) of the film was estimated from the following

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ASTAM et al./Turk J Phys

relation [32]:

δ =1

D2. (3)

Strain values (ε) were determined using the following equation [33]:

ε =βcosθ

4(4)

and the number of crystallites (N) per unit surface area was calculated using the relation [34]:

N =t

D3(5)

where t is the thickness of the film. Observed and standard interplanar distances with calculated structural

parameters are shown in Table 2. It is seen from the table that up to 300 C annealing temperature, dislocation

density and strain decrease, whereas grain size increases, which indicates improvement in the crystallinity. This

improvement can be attributed to the enhancement of clusters and removal of the defects formed during the

thin film deposition. At 400 C annealing temperature, dislocation density and strain increase, whereas grain

size decreases. This deterioration in crystal structure may be due to the glass substrate.

Table 2. Observed and standard interplanar distances with calculated crystallite grain size, dislocation density, strain,

and number of crystallites.

Sample2θ d (A) d (A)

hkl D (nm)δ × 10−3 ε× 10−2 N × 1015

(observed) (standard) (observed) (nm−2) (line−2 m−4) (m−2)

as-deposited - - - - - - - -

annealed at 100 C - - - - - - - -

annealed at 200 C27.60 3.233 3.229 111 - - - -46.00 1.980 1.971 220 14.33 4.87 13.86 71.36

annealed at 300 C

32.40 2.764 2.761 141 - - - -38.80 2.319 2.319 105 - - - -45.90 1.980 1.975 220 20.28 2.43 9.79 25.18

annealed at 400 C

32.40 2.764 2.761 141 - - - -38.80 2.319 2.319 105 - - - -48.80 1.980 1.979 220 13.34 5.62 14.89 88.46

The surface properties of the semiconducting thin films influence their optical and electrical properties.

In this study, surface morphological properties of the as-deposited CuxS thin film were investigated using SEM.

Figures 2a and b show the SEM images of the CuxS thin film at 15,000 and 50,000 magnifications, respectively.

It is observed that the surface of the film is dense, uniform, and smooth without any pinholes and cracks. It

is also seen there is overlapping of a large number of small spherical grains, which are aggregated together to

form relatively big islands. This type of surface is due to the formation of limited initial nucleons at particular

substrate places, which then grow with new nucleons until the critical size is reached [35].

The elemental composition of the as-deposited CuxS thin film was investigated using EDAX and the

pattern is shown in Figure 3. Peaks of Cu and S exhibit the presence of these elements in the as-deposited film.

The peaks of silicon and oxygen originate from the glass substrate. It is also possible that SILAR deposited

thin films may be contaminated with oxygen from the environment as the film is exposed to the environment

during deposition. The elemental analysis was carried out only for Cu and S and the average atomic percentage

of Cu:S in the as deposited film was calculated as 63:37.

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ASTAM et al./Turk J Phys

(a) (b)

Figure 2. The SEM images of CuxS thin film at 15,000 (a) and 50,000 (b) magnifications.

Inte

nsi

ty (

cou

nts

/s)

2.00 4.00 6.00 8.00 10.00 12.00Energy (keV)

Figure 3. EDAX spectrum of the as-deposited CuxS thin film.

The fundamental absorption, which corresponds to the transition from valance to conduction band, can

be used to determine the optical band gap energy of the material. It is well known that the relationship between

the absorption coefficient (α) and the optical band gap (Eg) is given by:

α =A(hν − Eg)

n

hν, (6)

where hν is the photon energy, A is a constant, and n assumes values of 1/2, 2, 3/2, and 3 for allowed direct,

allowed indirect, forbidden direct, and forbidden indirect transitions, respectively. This equation gives a band

gap of direct allowed transitions, when the straight portion of (αhν)2 versus hν plot is extrapolated to the point

α = 0. The optical band gap energy of the as-deposited CuxS thin film was determined by optical absorption

measurement. Figure 4 shows the plot of (αhν)2 versus hν with variation of optical absorbance versus the

wavelength of incident photons. The linear variation of (αhν)2 versus hν indicates that direct transition is

present. The band gap of the as-deposited CuxS thin film was found to be 2.14 eV, which is comparable with

the values reported previously [15,36].

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ASTAM et al./Turk J Phys

The sheet resistance of CuxS thin film was found to be 10−2 Ωcm by d.c. 2-point probe method at

room temperature, which is agreement with the values reported previously [18,19]. Since the film was of low

electrical resistance, sheet resistance was also directly measured using a digital multimeter.

By establishing a temperature gradient between the 2 ends of the film, thermoelectric power measurement

was carried out and p-type conductivity was determined for the as-deposited and annealed sample, which is

known to arise from copper deficiency.

400 500 600 700 800

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

3.25

3.50

1.5 2.0 2.5 3.0 3.50.0

2.0x1010

4.0x1010

6.0x1010

8.0x1010

1.0x1011

1.2x1011

(αhν)

2 (eV

/cm

)2

Photon energy (eV)

Eg =2.14eV

Ab

sorb

ance

(a.

u.)

Wavelength (nm)

Figure 4. Plot of (αhν)2 versus hν with variation of optical absorbance versus the wavelength of incident photons of

the as-deposited CuxS thin film.

4. Conclusions

In this study, CuxS thin film was deposited on glass substrates using the SILAR method at room temperature

and the results of structural, surface morphological, optical, and electrical studies of the film were discussed.

Postdeposition thermal annealing was also performed at various temperatures in nitrogen atmosphere to investi-

gate the effect of annealing on the crystal structure. The XRD results showed that the as-deposited film has an

amorphous structure but after being annealed it changes to slightly polycrystalline. With increasing annealing

temperature slight diffraction peaks of Cu2S as well as CuS phase were observed. SEM images indicated that

the surface of the as-deposited film is fairly smooth and continuous without any pinholes and cracks. The

quantitative elemental analyses of the CuxS thin film were carried out using EDAX and the results revealed

that the atomic percentage ratio of Cu:S in the as-deposited film was about 1.7:1. From the optical absorption

measurement the band gap value of the as-deposited film was determined as 2.14 eV. Resistivity of the film was

found to be 10−2Ω cm and p-type conductivity was determined.

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