Effect of thickness on the optical and dispersion parameters of Cd0.4Se0.6 thin films

Abstract

Cd0.4Se0.6 thin films have been prepared by chemical bath deposition technique.AFM images confirm the appearance of
nanostructure. The optical constants represented by absorbance, reflectance, extinction coefficient , refractive index, real
and imaginary parts of dielectric constant were studied as a function of thickness variation .It was found that all these
constants were affected by thickness. Dispersion relation was applied in order to show the effect of thickness on these
parameters which were discussed in details through this work.

Full text

JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 18, No. 9-10, September - October 2016, p. 822 - 826
Effect of thickness on the optical and dispersion parameters of Cd0.4Se0.6 thin films
S. S. CHIADa, N. F. HABUBIa*, W. H. ABASSb, M. H. ABDUL-ALLAHc
aAl_Mustansiriyah University, College of Education, Physics Department, Baghdad- Iraq
bAL- Mustansyriah University, College of Basic Education , Science Department,Baghdad- Iraq
cUniversity of Diyala, College of Science, Physics Department,Diyala- Iraq
Cd0.4Se0.6 thin films have been prepared by chemical bath deposition technique.AFM images confirm the appearance of
nanostructure. The optical constants represented by absorbance, reflectance, extinction coefficient , refractive index, real
and imaginary parts of dielectric constant were studied as a function of thickness variation .It was found that all these
constants were affected by thickness. Dispersion relation was applied in order to show the effect of thickness on these
parameters which were discussed in details through this work.
(Received October 23, 2015; accepted September 29, 2016)
Keywords: Nanostructure, Cadmium compound, Inorganic compound, Semiconducting II-VI material
1. Introduction
A huge interested was employed for the physical
characterizations of CdSe semiconductor due its wide
applications [1-7]. Cd0.4Se0.6 thin films can be either n-
type as well as p-type semiconductor depends on the kind
of dopant used during the deposition process. The
reported room temperature direct band gap of
Cd0.4Se0.6was 1.74 eV [8]. It is suitable for many
optoelectronic applications, laser diodes and solar cells
with excellent efficiency. Many methods were adopted for
the preparation of CdSe thin films like, Thermal
evaporation [9], Spray Pyrolysis [10], mechanical
alloying [11], chemical bath deposition [12], hot -
injection method [13] and the solvothermal technique
[14]. In a previous study of our group, Hazim et al. [15]
had shown that the deposited films of the same thickness
were contained nanostructure according to the data of
XRD and AFM.
In the present work, the preparation Cd0.4Se0.6 thin
film by chemical bath deposition method was performed
in order to study some optical and dispersion parameters
for different thicknesses of this compound.
2. Experimental procedure
Different thicknesses of Cd0.4Se0.6 thin films were
prepared utilizing chemical bath deposition technique.
Cadmium chloride and sodium selenosulphate were used
as a source of Cd2+ and Se2- respectively. Malonic acid
was used as a complex agent to obtain cadmium malonate
in order to be used for slowing down the reaction and to
get a uniform CdSe thin film. The PH value was fixed
during the operation via the use of ammonia and sodium
hydroxide, its value was 7.5.The substrates were subjected
to a cleaning process, including boiling in chromic acid for
2 hours, washed by flowing water, rinsed in absolute
ethanol, and finally cleaned in an ultrasonic bath filled
with re-distilled water.
The substrates were put vertically in the reaction bath
and were kept for (21,24,26,28and 30) hours at room
temperature in order to obtain the desired thickness. For
every period of time ,the substrate was removed from the
bath, washed with re-distilled water several times and
dried at ambient temperature ,and was kept in dark glass
desiccators . The color of the obtained films was orange
red. Gravimetric method was used to estimate the film
thickness and their values were (300,350,400 and 450) nm.
Transmittance and absorbance spectra were recorded
using double beam (Shimadzu UV -Probe Japan) in the
wavelength range from 350 to700 nm.
3. Result and discussions
In order to study the surface topography, AFM images
with (4µm x 4µm) was used. Fig. 1 shows the AFM
micrograph for 300 nm and 450 nm thickness. It can be
seen from the graphs sharp peaks appeared in the domain
and display densely packed columnor crystalline. No trace
of pores and large interface defects was observed. The
surface roughness increases from 0.93 nm to 3.16nm by
increasing the thickness from 300 nm to 450 nm. The
average diameter of the deposited films were 24 nm and
78 nm for 300 nm and 450 nm respectively, which falls in
the category of nanostructure by using the UV-Visible
spectrophotometer, the absorbance spectra in the range of
350-700 nm of Cd0.4Se0.6 thin films for various thicknesses
had been recorded as shown in Fig. 2.

Effect of thickness on the optical and dispersion parameters of Cd0.4Se0.6 thin films 823
300 nm
450 nm
Fig. 1. AFM images for Cd0.4Se0.6 thin films
for 300 and 450 nm thickness
From this figure, it can notice that the absorbance
decreases with the increasing wavelength and increases
with the increasing of thickness for all prepared thin
films. Fig.3 shows the relationship between the reflectance
and wavelength for various thicknesses of Cd0.4Se0.6 thin
films. From this figure, it can be noticed that there was a
decrease in reflectance with the increasing of film
thickness until 570 nm . However, beyond this wavelength
the situation was reversed .
Fig. 2. Variation of absorbance with wavelength
for Cd0.4Se0.6 thin films.
Fig. 3. Variation of reflectance with wavelength
for Cd0.4Se0.6 thin films.
The refractive index (n) is related to the electronic
polarization of ions. Fig.4 represent the relationship
between the refractive index and wavelength. From this
figure, it can be noticed that, there was a decrease in
refractive index with increasing Cd0.4Se0.6 thin film
thicknesses until 570 nm, and then the refractive index
increases with increasing film thickness.
Fig. 4. Variation of refractive index with wavelength
for Cd0.4Se0.6 thin films.
From Fig. 5, it can be noticed that the behavior of
extinction coefficient (k) of Cd0.4Se0.6 thin film with
wavelength was the same as the behavior of refractive
index and reflectance .

824 S. S. Chiad, N. F. Habubi, W. H. Abass, M. H. Abdul-Allah
Fig. 5. Variation of extinction coefficient with
wavelength for Cd0.4Se0.6 thin films.
Real (εr) and imaginary (εi) dielectric constants are
represented in Figs. 6-7 which were depended on the
wavelength. From these figures, it can be noticed that
there was a shift in the peak maxima toward higher
wavelength for the real part while the shift in the peak
maxima for imaginary part was toward lower wavelength.
Fig. 6. Variation of Real part of dielectric constant with
wavelength for Cd0.4Se0.6 thin films.
Fig. 7. Variation of Imaginary part of dielectric constant with
wavelength for Cd0.4Se0.6 thin films.
Urbach energy was estimated from the known formula
utilizing Fig. 8 and their values are presented in Table (1)
,which shows an inverse relation between their values and
the optical energy gap , which confirms that there was an
enhancement in the order of crystallanity during the
increase of thickness.
Fig. 8. Variation of (ln

) with (hυ) for Cd0.4Se0.6 thin films.
Wemple and DiDomenico [15-16] used a single
oscillator description of the frequency-dependent dielectric
constant to define dispersion energy parameters Ed and Eo.
The model describes the dielectric response for transitions
below the optical gap. The single oscillator model can be
interpreted using the following relation [16]:
22
21EE
EE
n
o
do

 (1)
where n is the refractive index, Eo is the single-oscillator
energy for electronic transitions and Ed is the dispersion
energy which is a measure of the strength of inter band
optical transitions. From the plotting of (n2-1)-1vs. (hυ)2 ,
the oscillator parameters Eo and Ed values can be
determined from the slope and intercept on the vertical
axis of (n2-1)-1vs. (hυ)2 plot, as shown in Fig. 9. These
values are listed in Table (1). It can be noticed that Eo and
Ed were decreased with increasing thickness, and also Eg
that represent Eo/2.
Fig. 9. Variation of (n2-1)-1 with (hυ)2 for Cd0.4Se0.6 thin films.

Effect of thickness on the optical and dispersion parameters of Cd0.4Se0.6 thin films 825
The refractive index has been studied in order to
maintain static refractive index at infinite wavelength (no)
can be calculated by the relation:
o
d
oE
E
n 1 (2)
and the static dielectric constant represented by (ε∞ = n0
2)
[15-16]. The calculated values were shown in Table 1 .By
using the single oscillator at wavelength (λo) at high
frequency. The average oscillator wavelength (λo) and the
average oscillator strength So a can be calculated by
applying the following simple dispersion relation [17]:
2
o
2
oo
2
)(1
S
1n





(3)
where λ is the wavelength of the incident light. From Fig
(10) So and λo values were obtained from the slope of 1/So
and intercept of (So λ2o)-1 of the plotted curves. The values
of So and λo were listed in Table (1).
The interband transitions can be obtained with he help
of M-1 and M-3 moments, which can be expressed as
follows:
3
1
2
oM
M
E



3
3
1
2
dM
M
E



(4)
Their values were listed in Table 1.
Fig. 10. Variation of (n2-1)-1 with 1/λ2 for Cd0.4Se0.6 thin films.
Table (1) the optical parameters of Cd0.4Se0.6 thin film.
Sample Ed
(eV) Eo
(eV) Eg
(eV) 

n(o) M-1
M-3
eV-2
So x1013
m-2
o

nm
300 nm 13.1 3.54 1.77 4.70 2.17 3.70 0.296 1.26 848
350 nm 19.3 3.48 1.74 6.56 2.56 5.56 0.460 1.55 803
400 nm 34.5 3.45 1.72 11.00 3.32 9.00 0.840 1.82 779
450 nm 48.8 3.41 1.71 15.29 3.91 14.28 1.220 2.17 758
4. Conclusion
In this work, the preparation of Cd0.4Se0.6 thin films
with various thicknesses by chemical bath deposition was
done , and the effects of thickness on optical and
dispersion parameters were analyzed. From this study, we
have shown that the increase of absorbance with
increasing thickness while the reflectance and refractive
index were decreasing with increasing thickness for
wavelength (λ≤ 570 nm). Dispersion parameters such as
Ed, Eo and λo are decreased with increasing thickness and
also the energy gap, while ε∞, n(0), and So are increased
with increasing thickness.
Acknowledegment
I would like to thank Al-Mustansiriyah
University (www,uomustansiriyah.edu.iq) Baghdad -Iraq
for their support in the present work
References
[1] C. Baban, G. I. Rusu, Applied Surface Science
211, 6 (2003).
[2] G. Perna, V. Capozzi, M. Ambrico,V. Augelli,
T. Ligenzo, A. Mirafra, L Schiavulli, M. Pallara,
Applied Surface Science 233, 366 (2004).
[3] P.P. Hankare, S. D. Delekar, M. R. Asabe,
P. A. Chate,V. M. Bhuse, A. S. Khomane,
K. M. Garadkar, B. D. Sarwade J. of Physics and
Chemistry of Solids 67, 2506 (2006).
[4] H. P. Jakobsen, J. W. Andreasen, K. F. C. Krebs,
Thin Solid Films 516, 2203 (2008).
[5] Y. Akatum, M. A. Yildirim, A. Ates, M. Yildirim,
Optics Communications 284, 2307 (2011).
[6] S. Ahmed, M.Ganaie, S. Islam, M. S. Khan,
K. Asokan, and M. Zulfequar, International Journal
of Physics and Astronomy 2, 79 (2014).
[7] H. K. Sadekar, A. V. Ghule, R. Sharma, International
Journal of innovations in Engineering and
Technology 5, 38 (2015).
[8] R. H.enriquez, P. Grez, E. Munoz, H. Gemez,
J. A. Badan, R. E. Marotti, E. A. Dalchele, Thin
Solid Films 518, 1774 (2010).
[9] H. Mahfoz, M. A. Dabban, A. Y. Abdel-Latif,
M. M. Hafiz, J. of Alloy and Compounds
512, 115 (2012).
[10] A. A. Yadav, M. A. Barote, E.U. Masumdar,
Materials Chemistry and Physics 121, 53 (2010).

826 S. S. Chiad, N. F. Habubi, W. H. Abass, M. H. Abdul-Allah
[11] G. L. Tan , J. H. Du, C. H. Bhosale, J. of alloys and
Compounds 468, 421 (2009).
[12] A. S. Khomane, Materials Research Bulletin,
46, 1600 (2011).
[13] U. Farava, C. Park ,Solar Energy Materials and Solar
Cells 94, 303 (2010).
[14] S. Kumar, N. Kumari, S. Singh, T. Singh, S. Jain,
Collids and Surface A: Physcochem. Eng. Aspects,
398, 1 (2011).
[15] Hazim L. Mansour, Khudheir A. Mishjil, Nadir F.
Habubi, Sami S. Chiad Int. J. Thin Fil. Sci. Tec.
3, 57 (2014).
[16] M. DidomenicoandS.H. Wemple, J. Appl. Phys.
40, 720 (1969).
[17] S.H. Wemple, M. Didomenico, Phys. Rev. B
3, 1338 (1971).
_________________________________
*Corresponding author: nadirfadhil@uomustansiriyah.edu.iq