Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 05 Issue 06
Study of Structural and Optical Properties of
Quaternary CuxAg1-xAlS2 Thin Films
Sabeeha M. Ahmad *
University of Basrah, college of science, physics department, Iraq
[email protected]
*
Abstract— CuxAg1-xAlS2 thin films with 0≤ x ≤1 were
successfully deposited on glass slides using chemical spray
pyrolysis technique at 633k. Polycrystalline structure of the films
was confirmed using X-ray diffraction (XRD) analysis also XRD
was utilized to compute the grain size, strain and dislocation.
Surface morphology was characterized by using atomic force
microscopy (AFM).From optical study, the film showed direct
transition with nonlinear change of energy gap as concentration
change from (x = 0-1) (2.8) eV for AgAlS2 – (3.4) eV for CuAlS2
respectively. The optical constant such as extinction coefficient
(k), refractive index (n), real and imaginary dielectric (Є1, Є2)
were discussed.
Index Terms—Optical
compounds
properties,
Thin
films,
Ternary
I. INTRODUCTION
I
N recent years, interest on the preparation and study of
physical properties of ternary chalcogenide compounds has
increased. This is because of their possible applications in
solar cells, light emitting diodes and non – linear optical
devices [1]. Ternary compounds are found to be promising
materials for optoelectronic device applications such as green
emitting devices and are also suggested to be possible
materials for window layer of solar cells [2].
Structurally these compounds are derived from the binary
sphalerite structure (II-VI and III-V) with slight
distortion.Despite the overall structural similarity between the
ternary I-III-VI2 compound and their analogues, the band gap
of the former compound are substantially smaller than of the
latter. This difference is explained in terms of a chemical
factor Egchem. and a structural factor Egs. In the former case,
the 3d or 4d electrons of the noble metal hybridize with the plike valance band (p-d hybridization ) due to proximity of
their levels resulting in a reduction in the value of band gap by
up to 1 eV [3]. Chalcopyrite semiconductors compound such
as I-III-VI2 (I=Ag, Cu, III=In, Al, VI=S, Se), have a band
gap in the range (3.5-1.0) eV covering the wide spectral
region from ultraviolet to near infrared and they have
different attractive linear and nonlinear optical pr0perties
[4]. copper chalcopyrite Cu-III-VI2 (III - Al, Ga and VI -S,
Se), have energy band gaps ranging from 1.70 to 3.49 eV and
are therefore a promising candidates for light emitting devices
operating in the visible and ultraviolet spectral range [5].
Energy band gap is the most important parameter which
dominates the main optical –absorption peak of a
semiconductor, band edge structure also determines the optical
December 2016
– absorption behavior in materials [6].Within chalcopyrite
family the sulphide based compound CuMS2(M=In, Ga, Al)
has attracted much attention because it showed a direct band
gap covering from Eg=1.5 eV for CuInS2[7], Eg=2.43 eV for
CuGaS2[8], Eg=3.45 eV for CuAlS2[9] ; therefore they're
particularly suitable for optoelectronic as well as photovoltaic
application CuAlS2 is one of these compounds which have
good luminescent properties making it suitable for use as
material for light emitting devices in the blue region of the
spectrum [10]. Although rather few studies on AgAlS2 were
reported, possible applications were made to use this
material in solar cell because it has a good absorption
coefficient in the visible region [11, 12]. Quaternary alloys
have a large degree of variation in their properties as a
function of composition which enables the tailoring of the
material properties such as Ag1-xCuxInSe2 [13], CuAlxIn1-xS2
[14] and pentenary semiconductor CuGa 1-xInx(SySe1-y)2
[15]. Several methods are used to prepare thin films of ternary
compounds. Among these methods, chemical spray pyrolysis
(CSP) technique, a chemical method at lower cost and a large
area for preparing of thin films, is easy to dope any amount in
a ratio of required properties through the solution medium.
This method is convenient for preparing pinhole free,
homogenous, smoother thin films [16]. CSP technique was
used for preparing ternary thin films such as CuInS2 [17],
CuInSe2 [18], AgInS2 [19], totally sprayed solar cell [20].
Variable methods have been used for preparing ternary
chalcopyrite CuAlS2 such as CSP [9], chemical bath
deposition [21, 22], spark plasma scattering [23].Few studies
on AgAlS2 were done such as single crystal [24]. A mixture
of ternaries was needed in the form of quaternary or pentenary
alloy system in order to match materials to CdS with band gap
which can be variable at will for solar cell application. This
cannot be done with a single ternaries compound as many
studies were done in this field [4, 14, 15, ].In this paper we
report the preparation of ternary CuAlS2, AgAlS2 and their
Quaternary alloy CuxAg1-xAlS2 by chemical spray pyrolysis
(CSP) technique, the effect of Cu, Ag concentration on the
structural and optical properties were investigated .The asprepared films were characterized by X-ray diffraction (XRD),
atomic force microscopy (AFM) and Ultraviolet – Visible
(UV-VIS) absorption spectrophotometer.
II. EXPERIMENTAL PROCESS
2.1 Sample Preparation
Ternary CuAlS2, AgAlS2, quaternary CuxAg1-xAlS2 thin
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Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 05 Issue 06
films were deposited onto heated glass substrates by chemical
spray pyrolysis technique, the films were deposited by taking
equimolar (0.05)M aqueous solution of AgNO3, (NH3)2SC,
Al(NO3)3 in appropriate volumes to obtain Cu:Al:S ratio
1:1:2, Ag:Al:S ratio 1:1:2 and Cu:Ag:Al:S ratio x:1-x:1:2
(x=1), the substrate temperature was set at 633 k, the adhesion
of the films onto the substrates was quite good, CuAlS2
exhibited deep greenish with a slight reddish color for
AgAlS2 as Ag concentration increased.To prevent the
substrate from excessively cooling, we sprayed for 20 s with
10 s intervals. Since layers formation is a function of substrate
temperature and other variables such as solution
concentration, having found a good set of conditions
mentioned elsewhere [26] we preferred not to vary them. By
using weighing method the film thickness 150 ± 5 nm.
2.2 Structural and optical measurement
The structural properties of the films were analyzed by X
Pentpro MPD.A filtered CuKα radiation (λ=1.548 Ao) was
used, an estimation of the grain size of the polycrystalline thin
films were obtained from broadening of the XRD peaks
according to the Scherer's formula [27]
D = 0.9λ / βcosθ
(1)
Where D is the grain size, β is the experimentally observed
diffraction peak width at full wave half maximum intensity
(FWHM) and θ is Bragg angle, the strain ξ of as deposited
films has been obtained from the following relation [28]
Βcosθ/λ = 1/D + ξ sinθ /λ
(2)
In order to determine the optical characterization of the thin
films deposited optic transition spectrum measurement was
performed at room temperature and parameters namely
refractive index (n), extinction coefficient (k) and dielectric
constants(Є1, Є2) have been determined from absorption
coefficient, the dielectric constants Є1, Є2 were estimated from
the relation [29]
Є1 =n2- k2 ,
Є2 =2nk
where k = αλ/4π , n = 1+R^1/2/1-R^1/2
III. RESULTS AND DISCUSSION
3.1 Structural Properties
Fig.(1) shows XRD pattern of CuxAg1-xAlS2 (0 ≤ x ≤ 1) in
the angular range (20-70).All films were found to show
similar behavior, the films possess small peaks (112, 103, 211
) which corresponding to(2θ =27.6, 31, 3, 39.2) respectively
for sample (fig. 1a) (x=1, CuAlS2) meaning that films were
polycrystalline with multiphase such CuAlO2, Cu2O. These
results are probably due to residual oxidant in precursor, the
calculated lattice constant a =5.4, c =10.1 for CuAlS2 are good
agreement with that reported [22], but since the XRD pattern
and miller indices for AgAlS2 were rare, we cannot calculate
the lattice constant for AgAlS2 and CuxAg1-xAlS2 series. From
XRD depicted in fig. 1.a, b, and c, the specimen broadening
intensity arises due to small crystalline grain size and strain.
The strain can be calculated by using equation (2) and from
the plot of βcosθ/λ varies sinθ/λ the value of the slope and the
intercept yield the strain and grain size respectively. We can
use the grain size in the relation δ=1/d2 to obtain the value of
dislocation as it appears in table 1.
Fig. 1. X-ray diffraction of as deposited CuxAg1-xAlS2 thin films
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Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 05 Issue 06
TABLE I
GRAIN SIZE, DISLOCATION AND STRAIN VALUES OF CUXAG1-XALS2
In order to achieve a more direct insight into surface
structural features of the films, atomic force microscopy
(AFM) imaging had been preformed, the surface image in the
area of 1μmx1μm of the films deposited at various
concentration of x is shown in Fig.2 a, b, c. AFM reveals the
granular nature of the particle and is an indication to the
presence of small crystalline grains which lightly varies its
sizes with Ag: Cu ratio and also we can observe that the films
have homogenous uniform surface and well cover the
substrates without any cracks and are continuous with very
well connected grains, the surfaces are comprised of numerous
nanoparticles with size ranging from (50-70) nm.
Fig. 2.
3.2 Optical Properties
Optical measurements have been carried out on several
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samples and good reproductively was observed, Fig.(3)
depicts the transmittance (%T) spectra of CuxAg1-xAlS2 for
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Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 05 Issue 06
different concentrations (x=0, 0.4, 1) in the wavelength (3001000) nm, it is clear that the transmission of samples (C5 and
C3) reaches a maximum value (80 %) NIR region comparable
with Co which extended to 25 %, the property of poor
transmittance in the UV region makes the film a good material
for screening off the UV portion of the electromagnetic
spectrum which is dangerous to human health [22]. Fig. (4)
revealed contra-behavior observed for the absorbance, a
gradual decreasing with increasing wavelength starting from
300 nm, the absorbance of sample C3 is found to be better
compared to that of samples C5, Co this may be due to the
better distribution of the grains
The absorption coefficient (α) was directly calculated using
the relation
α=2.303A/t
Where t is the thickness of the film, the variation of α vs. h‫ע‬
for as deposited films are shown in fig. (5). The absorption
data manipulated for the determination of the band gap
energy, fig.(6a, b.c) shows the expected values of energy gap
as determined from the following relation [30].
αh‫=ע‬A (h‫ – ע‬Eg)n
Fig. 3. spectral variation of the transmittance T% for
CuxAg1-xAlS2 thin film
Fig, 4. The absorbance spectrum of CuxAg1-xAlS2 thin films
Fig. 6. Direct band gap plot for CuxAg1-xAlS2 thin films
Fig. 5. The absorption coefficient of CuxAg1-xAlS2 Thin films
December 2016
Where A is the constant and n assumes 1/2 for allowed
direct transitions. Since the absorption coefficient (α) is in the
order of 104cm-1 for all as deposited films, the optical band
gap ternary AgAlS2 is 2.8 eV which is greater than 2.3 eV
reported elsewhere [11] but lower 3.1 eV for single crystal
[25], this result is in agreement with expectation when
allowance is made for blurring of the band gap edge due to
band tail state which is expected to reduce the observe energy
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Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 05 Issue 06
gap. On other hand, the energy gap of CuAlS2 3.4 eV, this
value is slightly lower than the values reported [9, 31, 32], this
due to the localized states in the band that can be evaluated
from urbach energy (∆E) from equation[21].
α= αo exp (h‫ע‬/∆E)
(3)
to anion displacement behavior (u) as there is a minimum
effect of anion radius on the displacement and because the
structure anomaly ∆Egs is mostly due to anion displacement
with a little contribution of tetrahedral distortion the anion
alloy have a little effect on the structural anomaly. On the
other hand, the cation radii has a great effect on the anion
displacement which is in turn effects the structural anomaly
∆Egs so mixing A or B cation have a great structural effect
especially if the difference in the added cations was big which
causes the variation of band gap to be nonlinear, the curve in
fig. (8) is to be true for the relation [2]
Eg(x) # x Eg (x=1) + (1-x) Eg (x)
(5)
When the left side indicates the experimental value of
energy gap (the nonlinear curve) while the right side shows
the linearity of band gap with parameter x as their values
tabled in table (2)
Fig. 7. Plot of Ln αVs photon energy for Cu xAg1-xAlS2 thin films
TABLE II
VALUES OF EXPERIMENTAL AND THEORETICAL ENERGY
GAP FOR CuxAg1-xAlS2
Fig. 8. Variation of Eg with composition parameter x for Cu xAg1xALS2 thin films
By plotting Lnα as a function of h‫ ע‬as shown in fig. (7) for
CuxAg1-xAlS2 thin films, the reciprocal slope of the linear part
gives the value of ∆E, the variation of ∆E and Eg is shown in
table (2). Fig.(8) shows the estimated experimental values of
energy gap as function of copper concentration , we can notice
the nonlinearly variation, due to atomic radii of anion (group
VI), cation (group III) and to the band mismatch between
anion and cation, the upward concave in the plot, can be
expressed by the following relation [2].
∆Egalloy =b x (x -1)
(4)
Where ∆Egalloy is alloy band gap reduction, b is bowing
parameter (b>0). The large fraction of b in figure can be
understood assuming that such alloys do not have single cation
– anion bond length but instead , maybe thought of as having a
local chalcopyrite coordination with bond alternation
(δ=R2AC – R2BC) and anion displacement (U-1/2 # 0) as stated
by mikkelson and boyy[33] , the connection between anion
displacement and band gap anomaly may help to answer some
questions about alloys of chalcopyrite compound, for example
when one have solid solution of two different anion e.g.
CuInS2xSe2(1-x) [34] then the variation of band gaps almost
linear with parameter x, on other hand, when there is alloying
on the B cation e.g. CuAlxIn1-xS2 [4] or alloying on A cation
e.g. Ag1-xCuxInSe2[13] then the optical bowing is usually
found, the difference between two states is thought to be due
December 2016
The extinction coefficient (k) which is a measurement of the
fraction of light lost because of scattering and absorption per
unit distance in the participating medium is calculated using
the relation k=αλ/4π [35], the spectral dependence of k, n, Є1,
Є2 in fig. (9) which showed an increase in k with increasing
photon energy but decrement with Cu concentration in the
films . Refractive index (n) is one of the fundamental
properties of an optical material because it is close relationship
to the electronic polarization of ions and local field inside
material, evaluation of the refractive indices of optical
materials is considerably important for application in
integrated optic devices such as switches, filters, a key
parameter for the device design[36], as shown from fig.(9) n
increases as photon energy increases, the highest value is at
photon energy 2.5 eV for samples (C3, C5)while Co extended
to 3 eV , we also calculate real and imaginary parts of the
dielectric constant using the relation [29].
Є1 = n2 –k2
Є2 = 2nk
We can notice the behavior of Є1 is similar to refractive
index because of the smaller value of k2compartion to n2while
Є2 depends mainly on k values , fig.(10)shows variation of Є1,
Є2, n, k for as deposited films for photon energy (h‫=ע‬3 eV).
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Fig. 9. Variation of n , k , Є1 , Є2 with photon
energy for CuxAg1-xAlS2 thin films
The photoluminescence (PL) emission of chalcopyrite
compound depends on the molecularity of the material . The
deviation from the valence stoichiometry has also a
considerable impact on the photoluminescence.
The pL response of samples seen in fig.(11) shows mainly a
single peak at 1.64 eV , 1.63 eV , 1.96ev for AgAlS2 , CuxAg1xAlS2 , CuAlS2 respectively , the intensity of peaks is
Fig. 10. (a,b,c) Variation of n , k , Є1 , Є2 with
composition parameter x (0≤ x ≤ 1) CuxAg1-xAlS2
Thin films
increasing as x content increases reaching to x=1
(CuAlS2)which has the highest one , on other hand, the pL
spectrum has a blue shift as x content increase; this can be
attributed to donor–accepter pair recombination. This
observation correlates with studies indicating that CuAlS2 has
good luminescence properties.[5].
Fig. 11. Room temperature photoluminescence emission of Cu xAg1-xAlS2 thin film
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Asian Transactions on Science & Technology (ATST ISSN: 2221-4283) Volume 05 Issue 06
IV. CONCLUSION
We have shown that CuxAg1-xAlS2 thin films can be formed on
glass substrates by chemical spray pyrolysis technique. The
structural and optical dispersions were studied , XRD results
indicate that the CuxAg1-xAlS2 thin films are in the
polycrystalline form surface morphology that is evaluated by
using atomic force microscopy , and the energy band gap of
films obtained using optical absorption spectra varies non
linearly between (2.8) eV(AgAlS2) – (3.4) eV (CuAlS2) . The
non-linearity of the band gap change related to the difference
in chemical and structural effects between AgAlS2 and CuAlS2
moreover the optical constants such extinction coefficient
,refractive index , real and imaginary of dielectric constants
were computed .Such films are advantageous for solar cell
applications because of their wide optical band gap.
ACKNOWLEDGMENT
The author would to thank Dr. Abd-alla for his assistance
concerning the FAM measurement
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