an investigation on silar deposited cu s thin...
TRANSCRIPT
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 [email protected]
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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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