CuAlS2 thin films Dip coating deposition and characterization 2017 Journal of Science Advanced Materials and Devices

Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

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Original Article

CuAlS2 thin films e Dip coating deposition and characterization
Sunil H. Chaki*, Kanchan S. Mahato, Tasmira J. Malek, M.P. Deshpande
P. G. Department of Physics, Sardar Patel University, Vallabh Vidyanagar, Gujarat 388 120, India

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 6 October 2016
Received in revised form
12 April 2017
Accepted 14 April 2017
Available online 24 April 2017

CuAlS2 thin films were deposited by a dip coating technique at room temperature. The X-ray energy
dispersive (EDAX) and X-ray diffraction (XRD) analysis showed that the deposited CuAlS2 thin film is
nearly stoichiometric and possesses a tetragonal unit cell structure. The crystallite sizes determined from
the XRD data employing Scherrer's formula and modified forms of HalleWilliamson relation like the
uniform deformation model (UDM), uniform stress deformation model (USDM), uniform deformation
energy density model (UDEDM), and the sizeestrain plot method (SSP) were in good agreement with
each other. The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) studies

of the thin film revealed that the deposited film is uniform without any cracks and the film covers the
whole of the substrates. The atomic force microscopy (AFM) of the as-synthesized thin film surfaces
showed spherical grains having coalescences between them. The optical absorbance spectrum analysis
showed that the thin films possess both direct and indirect band gaps. The semiconducting and p-type
nature of the thin films was confirmed from dc e electrical resistivity versus temperature, room temperature Hall effect, and Seebeck coefficient versus temperature measurements. The effect of the
different illuminations on the CuAlS2 thin film showed that it can be used as a material for absorption of
ultra-violet radiation. All the obtained characterization results are deliberated in detail.
© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
CuAlS2
Thin film
Dip coating
XRD
Microscopy
Electrical transport properties

1. Introduction
I
The ternary chalcopyrites belonging to MI-MIII-CVI
2 (M e Cu, Ag;
VI
M e Al, Ga, In; C e S, Se, Te) compound semiconductor family
have received wide interest because of convenient band structures
suitable for optically active devices [1]. They have been synthesized
in single crystal form [2,3], but more recently experimental investigators have focused on thin films due to high potential for
large area photovoltaic modules. The CuAlS2 is one of the members
of the ternary chalcopyrite family having the direct optical band
gap of 3.5 eV [1,3]. The optical band gap of this compound is the
highest among those of all the chalcopyrite compound semiconductors making it an interesting material for applications. Due

to the wide optical band gap, the CuAlS2 has found potential applications in solar cells [4], in photovoltaic [5], as light emitting
devices in the blue region of the spectrum [6], as window layers of
solar cells [7] and in laser diodes operating in a short wavelength
region [8]. The CuAlS2 thin films have been used as oxygen gas
sensor operating at room temperature showing an enhanced
III

* Corresponding author.
E-mail address: (S.H. Chaki).
Peer review under responsibility of Vietnam National University, Hanoi.

sensitivity with the aging of the film [9]. Nanocrystals of CuAlS2
have been employed in targeted “in-vitro” imaging of cancer cells
after nano-engineering their surface [10]. The CuAlS2 micro- and
nano-particles have been used as the catalyst in cellulose pyrolysis
[11]. An additional major advantage of CuAlS2 is that its constituent
elements are copious in nature and are non-toxic. Inspired by the
importance and potential applications of CuAlS2 [12] a study on this
material in the thin film form has been undertaken in this
investigation.
Till now, a number of methods have been employed to deposit
CuAlS2 thin films. These methods include iodine transport [13],
metal organic decomposition (MOD) [14], single source thermal
evaporation [9], sulfurization of precursors in H2S flow [15], sulfurization of sputtered metallic precursors by sulphur vapours in
hermetically sealed ampoules [16], thermal evaporation of
elemental mixture [17], spray pyrolysis [18,19], pulsed plasma
deposition [20], horizontal Bridgman method [21], chemical bath
deposition (CBD) [22,23], two stage thermal evaporation [24] and
electron beam evaporation [25]. The literature shows no report of
deposition or study of CuAlS2 thin films by dip coating technique.

The advantage of dip coating deposition is that it is a low cost solution deposition technique mainly used for uniform coating of
large areas [26] and to synthesize thin films of high quality [27,28].

/>2468-2179/© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license
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216

S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

The high quality thin films by dip coating technique are achieved
due to the layer-by-layer growth during each dip of the substrate in
the aqueous solution. In each dip of the substrate, the individual
layer forms through ion-by-ion adsorption. Thus the dip coating
thin film formation is by ion-by-ion adsorption leading to layer-bylayer deposition in every dip, consequently to the possibility of
minimization or even elimination of defects and imperfections in
the synthesized thin films. The other advantages of dip coating are
the control over deposition rate and film thickness by means of
control on dipping time during each dip and by regulating the
number of dips, respectively. Other deposition parameters like dip
speed, withdrawal speed and dry duration; the time the substrate is
out of solution between consecutive dips can be a handle to control
the film deposition. In this study, CuAlS2 thin films have been
deposited on glass substrates by dip coating technique. The asdeposited dip coating thin films were comprehensively characterized for elemental composition, crystal structure, surface
morphology, optical and electrical properties.
2. Experimental
Cupric chloride (CuCl2$2H2O) [S D Fine Chem. Ltd., Mumbai,
India], triethanolamine (TEA) (C6H15NO3) [Sisco Chem. Pvt. Ltd.,
Mumbai, India], aluminium chloride (AlCl3$6H2O) [Oxford Laboratory, Mumbai, India], ammonia liquid (NH3) [Chiti-Chem Corporation, Vadodara, India] and thiourea (NH2CSNH2) [Chiti-Chem
Corporation, Vadodara, India] were used for the synthesis of

CuAlS2 thin films by dip coating technique. The chemicals were all
of AR grade and were used without any further purification or
processing.
In the synthesis of CuAlS2 thin films by using dip coating technique, firstly 10 ml of 0.5 M cupric (II) chloride (CuCl2$2H2O) solution was mixed with 5 ml of 3.7 M TEA solution in a 100 ml clean
dry glass beaker under continuous stirring for 5 min. The
CuCl2$2H2O acts as precursor for Cu and TEA acts as complexing
agent to slow down the release of the metal ions resulting in slow
precipitation of the compound by ioneion reaction and to prevent
the agglomeration of the desired metal ions. Then, in the above
solution, 16 ml of 10 M NH3 solution was added and stirred for
5 min. Here NH3 (liquid ammonia) is used as reagent to adjust the
pH of the solution. The pH of the solution was kept at 9.5, the
reason for this being if pH < 7 the solution becomes acidic due to
which it can corrode the deposited thin films. Under continuous
stirring, 10 ml of 0.7 M aluminium chloride (AlCl3$6H2O) solution
was added and stirred for 5 min. Finally, 10 ml of 1.0 M thiourea
solution was mixed and stirred for 5 min. At last, the final solution
was made to reach 100 ml by adding appropriate amount of
deionized water. The final solution of 100 ml volume was kept
under programmed dip coating unit apparatus [Dip Coating Unit,
Model No: HO-TH-02; Holmarc Opto-Mechatronics Pvt Ltd., Kochi,
Kerela, India] for thin films deposition. The dip coating parameters
were maintained for the CuAlS2 thin films depositions as: dipping
speed e 9 mm/s; withdrawal speed e 9 mm/s; dip duration e 10 s;
dry duration e 5 s and total number of dips e 600. In case of certain
characterizations the numbers of dips were increased to increase
the film thickness.
During the deposition of CuAlS2 thin films the following reaction
was expected to have occurred:
CuCl2$2H2O þ 2NH4OH þ TEA / [Cu (TEA)]2þ þ 2OH1À þ 2NH4Cl þ

2H2O
AlCl3$6H2O þ 3NH4OH / Al3þ þ 6H2O þ 3OH1À þ 3NH4Cl
2(NH2)2CS þ 2OH1À / 2C2H2N2 þ 2H2O þ 2HS1À

2HSÀ þ 2OH1À / 2S2À þ 2H2O
[Cu (TEA)]2þ þ Al3þ þ 2S2À / CuAlS2Y þ TEA
The average thicknesses of the dip coating deposited thin films
were determined by the gravimetric weight difference method
[27,28]. In the average thin film thickness calculation, the film
density was taken as 3.48 g/cm3 determined from the XRD data
analysis and will be discussed later in this paper.
3. Results and discussion
3.1. X-ray energy dispersive analysis
The chemical compositions of the as-deposited CuAlS2 thin films
were determined by the energy dispersive analysis of X-ray (EDAX)
technique. The EDAX analysis was done at five different spots of the
thin films. Fig. 1(a) shows the EDAX spectrum. The average weight %
of the elements from five different spots of the as-deposited thin
films with standard values are tabulated as inset of the Fig. 1(a).
The observed extra peaks of other elements like Si, Na, Mg, O, Ca
etc. in the EDAX spectrum are due to the glass substrate. The values
of Cu, Al and S are tabulated after deleting the glass substrate elements. The obtained data clearly states that the deposited CuAlS2
thin film under this analysis is nearly stoichiometric but slightly
rich in aluminium and deficient in sulphur.
3.2. Structural analysis
Fig. 1(b) shows the XRD patterns of CuAlS2 thin films taken by
the Philips X-pert-MPD X-ray diffractometer ðl ¼ 1:54056 ÅÞ. Here
CuKa (1.5405 Å) radiation without any filter was used as the X-ray
source. The step size (2q) employed was 0.050 with the default slit
setting and receiving slit height of 0.15 mm. The scan speed

employed was 0.2 /sec.
All peaks observed on the XRD patterns could be indexed as
those of CuAlS2 with tetragonal unit cell structure. The lattice parameters determined using the Powder e X software from the
recorded XRD patterns are: a ¼ b ¼ 5.33 Å and c ¼ 10.40 Å. They are
in good agreement with the reported values of a ¼ b ¼ 5.325 Å and
c ¼ 10.390 Å; according to JCPDS Card No. 25-0014. Other parameters like the Miller indices, 2q angle, interplanar spacing (d) and %
d errors for prominent XRD peaks are tabulated in Table 1.
The error of 1.57% for %d may be due to the presence of defects
arising owing to grain size. The X-ray density ‘r’ of the as-deposited
CuAlS2 thin film was calculated to be 3.48 g/cm3. This calculated
value is in good agreement with the reported value of 3.43 g/cm3
for bulk CuAlS2 [29].
The peak broadening in the XRD pattern occurs due to the
decrease of crystallite size arising as a result of the dislocation
generated lattice strains [25]. The crystallite size in the asdeposited CuAlS2 thin films was determined from the XRD peak
broadening employing Scherrer's formula [30], given by:

D ¼ K l=bhkl cos q

(1)

where D is the crystallite size, K is shape depending parameter and
is taken here as 1 considering the particles to be spherical in shape,
l is the X-ray wavelength (1.5405 Å), b is the angular line width at
half maximum intensity, and q is the Bragg angle in degree.
The value of the crystallite size D was evaluated from the slope
q
1
of the Scherrer's plot of cos
l versus b for as-deposited CuAlS2 thin

hkl

films and results are shown in Fig. 2(a). The graphically and
analytically determined crystallite sizes are tabulated in Table 2.


S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

217

Fig. 1. (a) e EDAX spectrum along with inset table of chemical composition; (b) e XRD pattern of a CuAlS2 thin film.

Table 1
Miller indices, 2q angle, inter-planar spacing (d), and %d error.

bhkl cos q=l ¼ 1=t þ ε4 sin q=l

Thin films

(hkl)

2q

d (Å)

%d error

CuAlS2

112

200
220
204
312
116

29.32
33.64
47.57
49.73
57.56
58.52

3.04
2.66
1.91
1.83
1.60
1.58

0.26
0.12
1.34
1.57
0.23
0.07

The source of strains in the thin films is due to the crystalline
imperfections, distortions and dimensional constraints. The
dependence of the full width at half maxima (FWHM) on the strain

and grain size is related by the HalleWilliamson relation [31],
which represents the Uniform Deformation Model (UDM). The
materials strain properties are independent of the crystallographic
direction because the strain was assumed to be uniform in all
crystallographic directions.







(2)


4sinq versus bhkl cosq
was plotted for the promiThe graph of
l
l
nent XRD peaks of CuAlS2 thin films, and is shown in Fig. 2(b). The
slope and the ordinate intercept of the fitted line give the strain and
the crystallite size, respectively. The positive slope value reveals the
presence of tensile strain produced due to the tensile stress. This
external tensile force tends to increase the inter-atomic distance as
observed from the values of the lattice parameters derived from
XRD data. The origin of the extrinsic stress in a thin film comes
mainly from the adhesion to the substrate, while the intrinsic stress
comes from the defects, such as dislocations in the film. The results
of the UDM analysis for the CuAlS2 thin films are tabulated in
Table 2.

The Hooke's law gives the linear proportionality relation between the stress ðsÞ and the strain ðεÞ as

s ¼ Yhkl ε;

(3)

Fig. 2. (a) e Scherrer's plot, (b) e Plot of the modified form of HalleWilliamson analysis representing UDM, (c) e Plot of the modified form of HalleWilliamson analysis using
USDM; (d): Plot of the modified form of HalleWilliamson analysis using UDEDM, and (e) e The SSP plot of CuAlS2 thin films.


218

S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

Table 2
Crystallite size, strain, stress, energy and dislocation density in CuAlS2 thin films.
Methods

Results

Scherrer's formula
HalleWilliamson relation

Size-Strain Plot method

Graphical
Analytical
UDM
USDM
UDEDM

Graphical

Crystallite size D (nm)

Strain ε Â 10À3

Stress s (MPa)

Energy density u (kJ/m3)

Dislocation density d  10À4 (nm)À2

20.86
17.27
18.32
17.98
18.19
16.68

e
e
0.23
0.21
0.28
0.22

e
e
e
21.99

23.03
e

e
e
e
e
4.11
e

27.56
33.52
29.79
30.94
30.23
23.27

where Yhkl is the modulus of elasticity or the Young's modulus.
Eq. (3) is valid for a significantly small strain. Assuming a small
strain to be present in the deposited CuAlS2 thin films, Hooke's law
can be employed. Applying the Hooke's law approximation to
HalleWilliamson relation, the equation is:

 

bhkl cosq
K
¼
D
l

þs




4sinq
lYhkl

(4)

Eq. (4) is known as the Uniform Stress Deformation Model
(USDM). For a tetragonal unit cell structure, Young's modulus [32] is
evaluated by the following Eq. (5),

Yhkl ¼

þ

pffiffiffi 4sinq
u

l

sffiffiffiffiffiffiffiffiffi !
2
:
Yhkl

(7)

qffiffiffiffiffiffi 

4sinq
2
of Eq. (7) is shown in
The plot of bhkl lcosq versus
Yhkl
l
Fig. 2(d). The square of the slope of the fitted line gives the energy
density u and the reciprocal of the y-intercept indicates the crystallite size D. Then stress and strain were calculated using Eqs. (3)
and (6), respectively. All the obtained values are tabulated in
Table 2. The value of the crystallite size determined using UDEDM is
in good agreement with the values determined using other models.

À 2
Á2
h þ k2 þ L2

À
À
Á
Á
s11 h4 þ k4 þ ð2s12 þ s66 Þh2 k2 þ ð2s13 þ s44 Þ h2 þ k2 L2 þ s33 L4

where L ¼ al/c, a and c are the lattice parameters; h, k and l are
Miller indices taken from XRD analysis. The elastic compliance
constants Sij (m2/N) of CuAlS2 were taken from the reported values
[32] and are presented in Table 3.
The determined value of the Young's modulus, Yhkl, for the

CuAlS2 thin films having tetragonal unit cell turned out to be
102.52 GPa, which is nearly equal to the reported value 102.13 GPa




q
[32] and 106.91 GPa [33]. An USDM plot of bhkllcosq versus 4sin
lY
hkl

for the CuAlS2 thin films is shown in Fig. 2(c). The parameters like,
stress calculated from the slope of the fitted line, the strain calculated using Eq. (3) and crystallite size determined from the intercept are tabulated in Table 2. They are in good agreement with the
values obtained from UDM.
Another model known as Uniform Deformation Energy Density
Model (UDEDM) was used to determine the crystallite size, strain
and stress. The energy density can also be determined by this
model. For an elastic system that follows Hooke's law, the energy
density (u) can be given as [32],

u¼

 

bhkl cosq
K
¼
D
l

ε2 Yhkl
:
2

(6)

The equation of HalleWilliamson relation [25], can be rewritten
using Eq. (6) as:

(5)

The grain size and the strain can also be evaluated using the
SizeeStrain Plot (SSP) method. In this estimation, it was assumed
that the crystallite size profile is described by a Lorentzian function
and the strain profile by a Gaussian function [32]. Hence,
2

ðdhkl bhkl cosqÞ ¼

  ε 2
K 2
d b cosq þ
;
D hkl hkl
2

(8)

where K is a constant that depends on the shape of the particles; for
spherical particles it is taken, e.g. as 1. In Fig. 2(e), the graph of

ðdhkl bhkl cosqÞ2 versus ðd2hkl bhkl cosqÞ is plotted by using Eq. (8) for
the prominent XRD peaks taken on the CuAlS2 thin films. In this
case, the crystallite size is derived from the slope of the line and the
square root of the y-intercept will give the value of the strain. The
obtained values are tabulated in Table 2.
The grain size (D) and the dislocation density (d) of the films
were calculated for the preferential orientations to have information about their crystallinity levels. The dislocation density (d),
defined as the length of dislocation lines per unit volume of the
film, was evaluated by Eq. (9) [34],

d¼

1

(9)

D2 ðnmÞÀ2

The crystallization levels of the as-deposited thin films are good
because of their small d values derived from the Scherrer's formula,

Table 3
Elastic constants of CuAlS2 thin films.
S11 (m2/N)
1.421 Â 10

S12 (m2/N)
À11

À4.938 Â 10

S13 (m2/N)
À12

À5.802 Â 10

S33 (m2/N)
À12

1.467 Â 10

S44 (m2/N)
À11

1.769 Â 10

S66 (m2/N)
À11

1.860 Â 10À11


S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

the HeW plot, and the SSP plot, and collected in Table 2, which
represents also the amount of defects in the films.
The TEM and SAED images of the as-deposited CuAlS2 thin films
are shown in Fig. 3(a) and (b), respectively. The TEM image and
SAED pattern of as-deposited CuAlS2 thin films were recorded using
the Philips, TECNAI 20 Transmission Electron Microscope. The TEM

and SAED samples were prepared by scratch removing the thin
film. The scratch removed films were allowed to float on the
distilled water in a petri dish. The floating thin films were then

219

swiftly taken on a copper grid. The wet copper grid with the film
samples was dried by keeping it on a piece of filtering paper. The
copper grid along with sample was then inserted into the electron
microscope for TEM and SAED analysis.
The TEM image shows that the deposited thin film is uniform
without any cracks. The selected area electron diffraction (SAED)
pattern for CuAlS2 thin film (Fig. 3(b)), shows a concentric ring
pattern along with spots, revealing that the deposited thin films are
polycrystalline with large grain size in nature. The rings were

Fig. 3. (a) e TEM image, (b) e SAED pattern, (c): SEM image of large area, (d) and (e) e SEM images of small selected areas, (f) e 2D AFM image; (g) e height profile, (h) e 3D (x-y-z)
AFM image, and (i) e 3D (y-x-z) AFM image of the as-deposited CuAlS2 films.


220

S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

indexed as (112), (200), (220), (312), (116) and (400) indices, which
are associated with the tetragonal structure. All indexed planes
except the (400) one are in agreement with the XRD data.
The Fig. 3(c, d and e) present the SEM images of CuAlS2 thin
films deposited on glass substrates at room temperature. Fig. 3(c)
clearly shows that the film covers the whole surface of the substrates having a pocketed morphology variation. Fig. 3(d and e)

show a magnified image of pocketed morphology of the thin films.
Fig. 3(d) clearly shows the presence of a rod like structure whereas
Fig. 3(e) shows beautiful bunches of rods originating from the film
surface.
The AFM images of the as-deposited CuAlS2 thin films were
recorded by the Nano Surf Easyscan-2 in the tapping mode. Fig. 3(f)
shows the two dimensional (2D) and Fig. 3(h and i) show the three
dimensional (3D) AFM images of the as-deposited CuAlS2 thin
films, respectively. The height profile variation is shown in Fig. 3(g).
The Fig. 3(f) shows the 2D image of a film area of 1.98 mm  1.98 mm.
This 2D image shows clearly the presence of spherical grains having
coalescences between them. The Fig. 3(h and i) present the 3D
image of a film area of 256 mm  256 mm. This 3D image shows
obvious structures like hills and mountains having valleys between
them. The height profile parameters, illustrated in Fig. 3(g), taken
along the horizontal line of the AFM images of CuAlS2 thin films are
tabulated in Table 4. The parameters such as peak p, valley z or v
(Rp-v), root mean square (rms), roughness (Rq) and the average
roughness (Ra) values indicate the roughness in the vertical direction. Fig. 3(g) shows the rise in heights at the two ends of the
viewed horizontal scale. These heights increase may be due to
presence of bunch of nanorod features at the sites as observed in
the SEM image.
3.3. Optical analysis
The optical absorption spectrum, shown in Fig. 4(a), of CuAlS2
thin films deposited by dip coating has been recorded in the
wavelength range 200 nme3200 nm. The spectrum shows high
absorption in the ultra violet range with the absorption edge lying
at 290 nm corresponding to an energy of 4.28 eV.

Table 4

Surface and line roughness analysis of the AFM profiles.
Surface roughness
Sa (nm)
Sq (nm)
Sy (nm)
Sp (nm)
Sv (nm)
Sm (pm)
Area (pm2)

Line roughness
34.971
46.325
384.91
193.13
À191.79
223.57

Ra (nm)
Rq (nm)
Ry (nm)
Rp (nm)
Rv (nm)
Rm (pm)
3.959

26.767
34.339
172.68
104.88

À67.801
223.4

The energy band gap Eg was determined from the optical absorption data using the near-band edge absorption relation, given
by the Eq. (10) [31] below,

À
Á
ða$h$vÞn ¼ A h$v À Eg

(10)

where, n characterizes the transition. For allowed and forbidden
direct transitions, n ¼ 2 and 2/3 respectively, and n ¼ 1/2 and 1/3 for
allowed and forbidden indirect transitions, respectively. The absorption coefficient ‘a’ was calculated employing the BeereLambert
Eq. (11) [35e37].

a ¼ 2:303 A=t

(11)

where A is the absorbance of light passing through the sample, t is
the path length of light which travels through the CuAlS2 thin film
sample (average thickness of the thin film in the measurement was
260 nm).
The analysis of Eq. (10) shows that n ¼ 2 and ½ fits well for the
as-deposited CuAlS2 thin films stating that the as-deposited CuAlS2
thin films possess direct and indirect allowed optical band gaps.
The plots of (a$h$n)2 versus h$n and (a$h$n)1/2 versus h$n are shown
in Fig. 4(b). The value of the direct allowed optical band gap was

determined by extrapolating the straight line portions of (a$h$n)2
versus h$n. The obtained value of the direct optical band gap is
3.82 eV for the CuAlS2 thin films in the present investigation which
is greater than the reported value of 3.49 eV for bulk material [1].
This shows that the blue shift occurred due to film thickness. The
value of indirect allowed optical bandgap of 3.11 eV was evaluated
by extrapolating the straight line portions of (a$h$n)1/2 versus h$n
for the as-deposited CuAlS2 thin films.
The transmittance (T%) and the reflectance (R%) spectra of the
as-deposited CuAlS2 thin films are shown in Fig. 5(a). The drop in
the transmittance for wavelengths higher than 700 nm may presumably be due to the absorption by free carriers. After 1200 nm
wavelength, the transmittance is stable and so this material can be
utilized as an infrared window. The data from the spectra have been
used to determine the optical constants of the film. The refractive
index is an important parameter for materials to be used for optical
applications. In the region of the inter-band transition that has
strong absorption, the refractive index of the film can be determined by the Eq. (12) [38] below, only when the illuminations of
electromagnetic waves are perpendicular to the surface of the film,

h¼

pffiffiffi
R
pffiffiffi; where R is reflectance
1À R
1þ

(12)

The plots of the refractive index (h) and the extinction coefficient (k ¼ a$l/4p) versus wavelength (l) are shown in Fig. 5(b).

Fig. 4. (a) e Absorbance spectrum, (b) e Plot of direct and indirect band gap of CuAlS2 thin films.


S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

221

Fig. 5. (a) e Transmittance (T) and reflectance (R) spectra, (b) e Plots of the refractive index (h) and the extinction coefficient (k) versus wavelength, (c) e Variation of real and
imaginary part of the dielectric constant with wavelength of the as-deposited CuAlS2 thin films.

The plots show that the refractive index (h) and the extinction
coefficient (k) vary with the wavelength in the range 290e3200 nm
for the as-deposited thin films. The variation shows that the
refractive index decreases in the wavelength range of 290 nm to
nearly 700 nm. The static refractive index h(0) determined using
the optical dispersion relationship has been found to be h(0) ¼ 1.84.
This obtained value is less than the reported value of 2.12 [38]. This
variation may be due to surface dissimilarity of the as-deposited
thin films and the reported investigated thin films. The dielectric
constant dependence on frequency is defined by the Eq. (13) below,

εðuÞ ¼ εr ðuÞ þ iεi ðuÞ:

(13)

where εr and εi are the real and the imaginary parts of the dielectric
constants, respectively, and these values were calculated using the
formulas of Eq. (14) [39] below,

εr ðuÞ ¼ n2 ðuÞ À k2 ðuÞ and εi ¼ 2nðuÞkðuÞ:

(14)

The variations of the εr and εi values of the as-deposited CuAlS2
thin films with wavelength are shown in Fig. 5(c). The εr values are
higher than that of εi values.

3.4. Electrical analysis
The dc e electrical resistivity variation with temperature in the
temperature range from ambient to 423 K was studied on CuAlS2
thin films using a four-probe set-up of the Model DFP-02 (Scientific
Equipment & Services, Roorkee, India). Using the measured voltage
while keeping the current constant, the resistivity (r) at each
temperature value was evaluated by taking into consideration the
correction factor. The average thickness of the thin films was
327 nm. The plots of logr versus 1000/T for as-deposited CuAlS2
thin films are shown in Fig. 6(a). The resistivity decreases with
increasing temperature, implying the thin film material to be
semiconducting in nature. The activation energy determined for
the linear portion of the plot arrived at a value of 0.81 eV, which is
in reasonable agreement with the reported value of 0.70 eV [40].
Hall Effect analysis at room temperature was carried out on the
as-deposited CuAlS2 thin films by using the Hall Effect setup, model
DHE-22 (Scientific Equipment, Roorkee, India). Graphite conductive
adhesive alcohol-based (Alfa Aesar) paste was used for making the
Ohmic contacts in the van der Pauw geometry. The Ohmic nature of
the electrical contacts made on the sample were confirmed by
measuring IeV characteristics between R12,12, R23,23, R34,34 and
R41,41 contacts of the thin films (see Fig. 6(b)) for both polarities in

the current range from À5 mA to þ5 mA.
The sample under investigation was kept in an applied magnetic
field which modifies the path of the majority carriers which produce Hall voltage. Fig. 6(c) shows the graph of Hall voltage (VH)

versus magnetic field (B). The Hall coefficient (RH), the mobility of
charge carriers (mH) and the charge carrier concentration (h) were
evaluated employing the standard formulae using the value of the
slope of the plot in Fig. 6(c), the thickness of the samples and the
constant measuring current. The average thickness of the thin films
used for the Hall measurement was 318 nm. The values obtained
are tabulated in Table 5. The positive value of the Hall coefficient
implies that the deposited thin films are of p-type in nature which
was also confirmed by the hot probe method. The evaluated carrier
concentration of thin films turned out to be in the order of
1016 cmÀ3 also revealing the samples to be semiconductors. The
value for the hole mobility determined from the Hall Effect measurement was 4.39 cm2/Vs for the as-deposited CuAlS2 thin films.
This value is in good agreement with the reported one (<5 cm2/
V1s1) [40].
The variation of the thermoelectric power ‘S’ as a function of
temperature was measured on the as-deposited CuAlS2 thin films
using the experimental set up, TPSS-200, (Scientific Solution,
Mumbai, India). The average thickness of the thin films employed
for the thermoelectric power measurement was 298 nm. The
variation of the potential difference between the two probes at a
constant temperature difference (DT) of 7 K was measured in the
temperature range from 300 K to 423 K.
The determined Seebeck coefficient (S) as a function of the inverse of temperature (1000/T) is shown in Fig. 6(d) for the asdeposited CuAlS2 thin films. The plot shows that the values of the
Seebeck coefficient increase with temperature revealing the semiconducting nature of the samples [41]. The absolute values of the
Seebeck coefficient at all evaluated temperatures is positive
implying the sample to be p-type in nature, which further uphold

the results of the Hall Effect and hot probe methods. The p-type
nature of the dip coating as-deposited CuAlS2 thin films as
confirmed by Hall Effect, the Seebeck coefficient and the hot probe
measurements are due to intrinsic acceptor defects arising owing
to metal rich condition [42]. The metal rich CuAlS2 having more Al
compared to Cu as confirmed by EDAX data, leads to AleCu substitutional defect dominance than that of copper vacancy and Cu-Al
substitutional defects [42]. This substitutional imperfection gives
rise to acceptor defects leading to p-type behaviour of the asdeposited CuAlS2 thin films. The carrier concentration of
~1016 cmÀ3 as obtained in the present study and presented in
Table 5, matches the reported data [42] for Al rich thin films and is
less by a factor of ~1000 for the Cu-rich CuAlS2 material, thus
substantiates Al-Cu defects dominance leading to the p-type nature. Moreover, Tell et al. [43] stated that more atomic percentage
concentration of Al compared to Cu atoms in CuAlS2 leads to the ptype behaviour. The present metal rich and sulphur deficient asdeposited dip coating CuAlS2 thin films as confirmed by the
EDAX data also corroborate to its p-type behaviour. The values of
Fermi energy (EF) and constant (A) were evaluated from the slope


222

S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

Fig. 6. (a) e Logr versus 1000/T plot, (b) e The IeV characteristics between different pairs of contacts, (c) e Hall voltage induced as a function of applied magnetic field, (d) e Plot of
Seebeck coefficient (S) versus 1000/T of the CuAlS2 thin films.

Table 5
The room temperature Hall parameters of as-deposited CuAlS2 thin films.
Carrier concentration h (cmÀ3)

Hall coefficient RH (cm3/C)

6.08 Â 10

103

16

and the intercepts of the Fig. 6(d), respectively. Using the value of
‘A’ and the carrier concentration (h) obtained from the Hall Effect
measurements, the scattering parameter (s), the effective density
of states (NA) and the effective mass of holes (mh*) were evaluated
by employing standard equations. The calculated values are tabulated in Table 6.
The IeV characteristics study on CuAlS2 thin films was carried
out in dark as well as under white and UV electromagnetic illuminations. The sample setup for IeV measurement was prepared by
taking as-deposited dip coating CuAlS2 thin films being deposited
on one side of a rectangular glass substrate of the dimension
35 mm  26 mm. The average thickness of the films was 314 nm.
The four contacts on the four vertices of the rectangular sample
were made using flexible thin copper wires bonded with graphite
conductive adhesive alcohol-based (Alfa Aesar) paste. The copper
wires used for the contacts were very thin and the silver paste

Hall mobility m (cm2/Vs)

Semiconductor type

4.39

p

contacts on the thin films were kept minimal with contacts at the

periphery of the set-up to avoid blocking of illumination. The IeV
measurements under white illumination (Philips) was made using
a 4 W lamp providing an illuminating intensity on the sample
surface of 6614 Lux, whereas the UV illuminated (Model: UVSL-14P,
Ultra-Violet Products Ltd. Cambridge CB4 1TG, UK) IeV measurement was carried out with a 4 W lamp providing illuminating intensity of 31 Lux on the sample surface. The intensity was measured
with a light Luxmeter (Model: MECO-930, MECO Meters Pvt. Ltd.,
Navi Mumbai, India). The recorded IeV characteristics in dark,
under white and UV illuminations are shown in Fig. 7. In case of
dark and white illumination, the sample just behaves as a simple
resistor unaffected by the external illumination, thus both IeV plots
nearly overlap. The results of the UV illumination of the thin films
show IeV characteristics deviations from those of the dark and
white illumination. The UV illuminated curve shows that a small

Table 6
Values of Fermi energy (EF), constant (A), scattering parameter (s), room temperature Seebeck coefficient (S), effective density of states (NA) and effective mass (m*) of asdeposited CuAlS2 thin films.
Fermi energy EF (eV)
0.007

A
0.383

Scattering constant (s)
2.117

S (mV/K)
10.96

NA (cmÀ3)
5.68 Â 10

17

m*h (kg)
7.3 Â 10À32

0.09 me


S.H. Chaki et al. / Journal of Science: Advanced Materials and Devices 2 (2017) 215e224

223

Fig. 7. The IeV plots of CuAlS2 thin film in dark as well as under white and UV illuminations.

increase in voltage induces a large increase in the current. This
indicates that the UV-illumination produces more charge carriers.
This relates to the optical energy direct band gap of the deposited
CuAlS2 thin film being 3.82 eV which matches to the wavelength of
the exciting UV radiation (~325 nm). The observed effect of the
illumination on the CuAlS2 thin film suggests that this film can be
used as a photovoltaic material for absorption of UV radiation.
4. Conclusion
Thin CuAlS2 films on glass substrates have been fabricated by
the room-temperature dip coating technique. Various characterization and analysis measurements, among others including also
optical absorbance, Hall effect, hot probe, dc e resistivity, Seebeck
coefficient and optical illumination responses, etc. have been carried out to study and determine the structure, morphology and
other intrinsic physical properties of the as-deposited thin films.
Results obtained have confirmed that the as-deposited thin films
are p-type semiconductors of tetragonal structure with activation

energy of 0.81 eV, a carrier concentration of about 1016 cmÀ3 and
the band gaps of 3.82 eV and 3.11 eV for the allowed direct and
indirect transitions, respectively. All measurements and analyses
have consistently revealed the p-type semiconducting nature of the
as-deposited thin films and suggested that these materials are
potentially applicable for absorption of the ultra-violet radiations.
Acknowledgements
Two of the authors (SHC and MPD) are thankful to the Gujarat
Council on Science and Technology (GUJCOST), Gandhinagar for
providing financial assistance through Research Project; vide letter
Nos. GUJCOST/MRP/2016-17/433 dated 27/06/2016 & GUJCOST/
MRP/16-17/300 dated 20/06/2016 for carrying out this research
work. One of the authors, TJM, is thankful to University Grants

Commission (UGC), New Delhi for the award of Maulana Azad
National Fellowship (MANF) to carry out this research work.
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