International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
The Effect of Annealing Temperature on the Optical
Properties of CdS and CdS:Al Thin Films
IQBAL S.NAJI 1 , IMAN H.KHDAYER 2, *HANAA I. MOHAMMED 3
2
1
Assistant Professor, Physics Dept., Science College, University of Baghdad
Assistant Professor, Physics Dept. ,College of Education for pure Scince_Ibn Al-Haitham, University of Baghdad
3
Lecturer, Physics Dept. ,College of Education for pure Scince_Ibn Al-Haitham, University of Baghdad
Baghdad / IRAQ
Abstract
Cadmium sulfide and Aluminum doped CdS thin films were prepared by thermal evaporation technique in vacuum on a heated
glass substrates at 373K. A comparison between the optical properties of the pure and doped films was made through
measuring and analyzing the transmittance curves, and the effect of the annealing temperature on these properties were
estimated. All the films were found to exhibit high transmittance in the visible/ near infrared region from 500nm to
1100nm.The optical band gap energy was found to be in the range 2.68-2.60 eV and 2.65-2.44 eV for CdS and CdS:Al
respectively , with changing the annealing temperature from room temperature to 423K.Optical constants such as refractive
index, extinction coefficient, and complex dielectric constant were calculated.
Keywords: : CdS thin films, Al doped CdS thin films, effect of annealing temperature, thermal evaporation technique.
Introduction:
Cadmium sulfide (CdS) thin film has attracted increased attention in recent years because of its wide direct band gap
energy, optical and electrical properties, and stability, which is suitable for many applications[1],such as an excellent
window layer for CdTe [2],or CuInS2 [3] or CuInGaS2 [4] thin film based on heterojunction solar cells.
It has also been used in other applications including optoelectronics and photonics for photocells light emitting diodes
(LEDs), lasers, optical filters, and optical switches, transistors, as biological labels, and has been investigated as
photoconducting cells in sensors for ultraviolet radiation [5]-[7].
Regardless on the deposition technique, the post deposited films characterization and deposition processes optimization
is still an open subject. A large number of studies are carried in order to produce CdS thin films with good
optoelectronic properties suitable for photovoltaic applications. For this purpose several properties are required:
relatively high transparency and not too thick to avoid absorption in the buffer layer and favors the absorption in the
base layer, not too thin to avoid the short circuiting, relatively large conductivity to reduce the electrical solar cells
losses and higher photoconductivity to not alter the solar cell spectral response [8].
However, the usefulness of CdS for futuristic devices resides in the ability to dope them with impurities so as to achieve
the desired properties and to make them multifunctional[9].
In the past few decades, several techniques such as thermal evaporation [10], spray pyrolysis [11], chemical bath
deposition[12], gradient recrystallization and growth (GREG) [13], spin coating [14], pulsed laser deposition [15], and
close spaced sublimation [16] have been used in the deposition of CdS thin films.
Among these methods thermal evaporation is one of the suitable methods for depositing large area thin films for solar
cell application[17]. This physical method compared with chemical methods has possessed the advantages of
convenience, high growth rate, and there is no wastewater discharge [18].
In this work , we present the influence of post-deposition annealing treatment on the optical properties of CdS and CdS
doped with Aluminum films deposited by thermal evaporation method.
Experimental Procedure:
Thin films of CdS and CdS doped with 1% aluminum were fabricated by vacuum evaporation of CdS powder (99.999%
pure) in a residual pressure of 10-6 mbar on to a heated glass substrates maintained at 373 K.
Molybdenum was used as boat source. The deposition rate was 0.33 nm/sec and the film thickness in the range of 200
nm was measured by interference method. The films were annealed in air at different annealing temperatures
(323,373,423 K) for an hour.
After the formation of the film, the transmission (T) and the absorption (A) spectra of the prepared samples were
measured by normal incidence of light using a spectrophotometer model (UV-Visible 2601) double-beam
spectrophotometer, in the wavelength range 300-1100 nm, using a blank substrate as the reference position .
The absorption coefficient (α) was calculated using the formula:
A
(1)
  2.303
t
Volume 2, Issue 6, June 2013
Page 556
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
Where t is the film thickness and A is the optical absorbance.
The refractive index was obtained from the following relation [19]
1 R
4R
(2)
[
 k 2 ]1 / 2
2
1  R (1  R)
Where k is the extinction coefficient which is related to the absorption coefficient and the wavelength as:

(3)
k 
4
The real (εr) and imaginary (εi)parts of dielectric constant can be expressed by the following equations:
 r  n2  k 2
(4)
and
 i  2nk
(5)
n
Results and discussion
The variation of transmittance as a function of wavelength for undoped and aluminum doped CdS thin films were
shown in figure (1) a and b respectively.
It is clear from these figures that all films exhibit a high transmittance that exceeds 70% for the wavelengths longer
than 500 nm.
(a) CdS
(b) CdS:Al
100
100
90
90
70
at R . T
T a =3 23 K
60
T a =3 73 K
T a =4 23 K
50
80
60
50
40
40
30
30
20
20
10
a t R .T
T a= 3 23 K
T a= 3 73 K
T a= 4 23 K
70
T%
T%
80
10
0
300
500
700
900
0
300
1100
l (nm)
500
700
900
l (nm)
1100
Figure 1 Transmittance vs. wavelength at different annealing temperatures for (a) CdS films (b) CdS:Al films.
The transmittance spectra show a decrease and a sharp fall in the transmission near the fundamental absorption when
the annealing temperature increases.
A red shift in the absorption edge towards lower band gap is noticed to undoped and aluminum doped films with the
increase of the annealing temperature. This suggests the decrease in optical band gap energy. Similar notice was
observed by Rizwan et al [20] who studied the effect of annealing temperature on the optical properties of CdS films.
When the film doped with Al , the transmittance spectra showed less transparency with respect to undoped film at
different annealing temperature. This may be attributed to more scattering of photons by the introduction of dopant as
foreign atoms, which may reduce transmittance.
This decrease in transmittance with doping is expected, because doping increases the number of charge carriers which
increases the absorption in the film, and then decreases the transmission of light. This agrees well with our earlier
report [21].Our results agree with Ikhmayies et al [22].
Optical band gap of the films were estimated by recording the absorbance spectra and obtained by using the following
equation [23] for a semiconductors.
αhυ= B (hυ-Eg)r
where α is the absorption coefficient, B is a constant which is related to the effective masses, r is a constant equals to
0.5 for direct transition and 2 for indirect transition.
Figure (2) a and b show the plot of (αhυ)2 versus photon energy (hυ) for CdS and CdS:Al at different annealing
temperature respectively. Linearity of the plots indicates that the material is of direct band gap nature. The
extrapolation of the straight line to (αhυ)2=0 axis gives the energy band gap of the films.
(a) CdS
(b) CdS:Al
1.0E+11
6.E+10
T a =3 73 K
2
T a =4 23 K
2.E+10
0.E+00
8.0E+10
(ahu) 2 (cm-1.eV)2
T a =3 23 K
-1
(ahu) (cm .eV)
2
a t R.T
4.E+10
a t R .T
T a= 3 23 K
T a= 3 73 K
6.0E+10
T a= 4 23 K
4.0E+10
2.0E+10
0.0E+00
2
2.5
3
hu (eV)
3.5
2
2.5
3
3.5
hu (eV)
Figure 2 (αhυ)2 vs. (hυ) plots at different annealing temperatures for (a) CdS films (b) CdS:Al films.
Volume 2, Issue 6, June 2013
Page 557
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
It is also observed that the value of Eg for annealed films decreased as compared to as-deposited films, it decreased from
2.68 eV to 2.60 eV for undoped and from 2.65 eV to 2.44 eV for Al doped films. This indicates that the crystallinity is
enhanced with annealing [20]. This result coincides with Sahay et al [24], Haider et al [25], and Desale et al [26].
It is clear that the absorption edge of Al doped CdS thin films is shifted towards lower energies with respect to undoped
films, it decreases from 2.68 eV to 2.65 eV for as deposited films and from 2.60 eV to 2.44 eV for films annealed at
423 K.
This is mainly due to the formation of band tails in doped semiconductors, which causes a strong modification of the
joint density of states and, consequently the absorption spectrum [22]. This is in agreement with khallaf et al
[27],[28],and Mahdavi et al [29].
Figure (3) a and b show the variations in refractive index as a function of wavelength. It is observed that for undoped
films , there is one well defined maximum, and it decreases with the increase of annealing temperature. This
decreasing in the refractive index is associated with the fundamental band gap absorption, while for doped films the
refractive index firstly increases and then decreases with the annealing temperatures. These observations are accounted
to the particular structure of films [24].
(b) CdS:Al
(a) CdS
3
2.8
a t R .T
T a =3 2 3 K
T a =3 7 3 K
T a =4 2 3 K
2.2
2
1.8
2.5
n
n
2.6
2.4
1.6
1.4
a t R .T
T a = 32 3 k
T a = 37 3 K
T a = 42 3 K
2
1.5
1.2
1
300
500
700
900
1
400
1100
l (nm)
600
800
1000
1200
l(nm)
Figure 3The refractive index vs. the wavelength at different annealing temperatures for (a) CdS films
(b) CdS:Al films.
The variation of extinction coefficient as a function of wavelength are shown in figure (4) a and b for undoped and Al
doped CdS thin films at different annealing temperatures respectively.
(b) CdS:Al
(a) CdS
0.25
0.3
0.25
0.2
K
0.15
a t R .T
T a =3 2 3K
0.15
T a =3 7 3K
T a =4 2 3K
K
at R .T
T a =3 2 3K
T a =3 7 3K
T a =4 2 3K
0.2
0.1
0.1
0.05
0.05
0
300
500
700
l (nm)
900
0
450
1100
650
850
1050
l (nm)
Figure 4 The extinction coefficient vs. the wavelength at different annealing temperatures for (a) CdS films
(b) CdS:Al films.
The rise and fall in the extinction coefficient is directly related to the absorption of light. In the case of polycrystalline
films, extra absorption of light occurs at the grain boundaries. This leads to non-zero value of k for photon energies
smaller than the fundamental absorption edge.
The real part which represents the normal dielectric constant and imaginary part represents the absorption associated
with free carriers are illustrated in figures (5 a,b) and (6 a,b) as a function of wavelength for undoped and Al doped
CdS films annealed at different annealing temperature respectively.
(b) CdS:Al
(a) CdS
8
8
7
7
er
5
5
4
4
3
3
2
2
1
300
500
700
l (nm)
900
1100
at R .T
T a =323K
T a =373K
T a =423K
6
er
a t R .T
T a =3 2 3 K
T a =3 7 3 K
T a =4 2 3 K
6
1
400
600
800
1000
1200
l (nm)
Figure 5 Єr vs. the wavelength at different annealing temperatures for (a) CdS films (b) CdS:Al films.
Volume 2, Issue 6, June 2013
Page 558
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
The real and imaginary part of dielectric constant have similar trend of variation as for the refractive index and
extinction coefficient respectively.
(a) CdS
1.2
1
1
a t R .T
T a=3 23K
T a=3 73K
T a=4 23K
0.8
0.6
at R .T
T a =3 23 K
T a =3 73 K
T a =4 23 K
0.8
0.6
ei
ei
(b) CdS:Al
1.2
0.4
0.4
0.2
0.2
0
300
500
700
l (nm)
900
0
450
1100
650
850
l (nm)
1050
Figure 6 Єi vs. the wavelength at different annealing temperatures for (a) CdS films (b) CdS:Al films.
The optical properties parameters including , absorption coefficient , optical energy gap, and optical constants
(refractive index, extinction coefficient , real and imaginary part of the dielectric constant) at wavelength equals to 450
nm for CdS and CdS:Al thin films treated at different annealing temperature are listed in table (1).
Table 1: The optical properties parameters of CdS and CdS:Al thin films at different annealing temperatures.
at λ= 400 nm
CdS
films
CdS films
doped with
Al
Ta(K)
Egopt (eV)
α(cm-1)
*104
n
k
εr
εi
303
2.68
4.400
2.631
0.140
6.905
0.737
323
2.65
4.51
2.627
0.143
6.881
0.754
373
2.63
4.87
2.603
0.154
6.754
0.806
423
2.60
5.57
2.516
0.177
6.300
0.891
303
2.65
3.14
2.566
0.099
6.576
0.512
323
2.62
4.39
2.632
0.139
6.908
0.735
373
2.58
4.78
2.610
0.152
6.792
0.794
423
2.44
7.78
1.972
0.247
3.827
0.977
Conclusions:
The effect of post deposition annealing treatment on the optical properties of CdS and CdS:Al thin films deposited by
thermal evaporation technique were studied. All films exhibit high transmittance, low absorbance in the visible/ near
infrared region from 500 nm to 1100 nm, thus these films are suitable for optoelectronic devices as window layers in
solar cells. The films show a direct transition, where the optical band edge shift towards lower energies when the CdS
film annealed and doped with aluminum. Near the fundamental absorption the refractive index decreases while the
extinction coefficient increases with the increase of the annealing temperature.
References:
[1.] A.Jafari, A.Zakaria, Z.Rizwan, and M.S.Ghazali," Effect of low concentration Sn doping on optical properties of
CdS films grown by CBD technique", Int. J. Mol. Sci., 12,pp.6320-6328,2011.
[2.] F.L.Alvarado, J.A.Chavez, O.V.Galan,E.S.Meza, E.L.Chavez, and G.C.Puente,"C-V calculations in the CdS/CdTe
thin films solar cells", Thin Solid Films, 518,pp.1796-1798, 2010.
[3.] H.Goto, Y.Hashimoto, and K.Ito," Efficient thin film solar cell consisting of TCO/CdS/CuInS2/CuGaS2 structure",
Thin Solid Films, 451-452,pp.552-555, 2004.
Volume 2, Issue 6, June 2013
Page 559
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
[4.] N.G.Dhere, A.A.Kadam, A.H.Jchagirdar, S.S.Kulkarni, L.Weinhardt,D.Grob, C.Heske and E.Umbach,"
Spectroscopic analysis of CIGS2/CdS thin film solar cell heterojunctions on stainless steel foil", Journal of Physics
and Chemistry of Solids, 66,pp.1872-1875, 2005.
[5.] K.P.Acharya, K.Mahalingam, and B.Ullrich," Structural,compostional,and optoelectronic properties of thin film
CdS on p-GaAs prepared by pulsed-laser deposition", Thin Solid Films, 518,1784-1787, 2010.
[6.] K.R.Murali, C.Kannan, P.K.Subramanian," Photo-electrochemical properties of flash evaporated cadmium
sulphide films", Chalcogenide Letters, 5,pp.195-199, 2008.
[7.] N.S.Kozhevnikova, A.A.Rempel, F.Hergert, and A.Mageri,"Structural study of the initial growth of
nanocrystalline CdS thin films in a chemical bath", Thin Solid Films, 517,pp. 2586-2589, 2009.
[8.] H.Moualkia, S.Hariech, and M.S.Aida,"Structural and optical properties of CdS thin films grown by chemical bath
deposition", Thin Solid Films, 518,pp.1259-1262, 2009.
[9.] S.Chandramohan, A.Kanjilal, T.Strache,J.K.Tripathi, S.N.Sarangi, R.Sathyamoorthy and T.Som," Modifications
in Structural and optical properties of Mn-ion implanted CdS thin films", Applied Surface Science, 256,pp.465468, 2009.
[10.] Zhibing He, G.Zhao, W.Weng,Piyi Du, Ge shen and G.Han," The difference between the transverse and in-plane
resistivity of Vacuum evaporated cadmium sulfide (CdS) thin films", Vacuum, 79,pp.14-18, 2005.
[11.] J.Hiie, T.Dedova, V.Valdna and K.Muska," Comparative study of nano-structured CdS thin films prepared by
CBD and spray pyrolysis:annealing effect", Thin Solid Films, 511-512,pp.443-447, 2006.
[12.] H.Khallaf, I.O.Oladeji, G.Chai and Lee Chow," Characterization of CdS thin films grown by chemical bath
deposition using four different cadmium sources", Thin Solid Films, 516,pp.7306-7312, 2008.
[13.] M.L.Albor-Aguilera, M.A.Trujillo, A.C.Orea and M.T.Velazquer,"Thermal and optical properties of
polycrystalline CdS thin films deposited by the gradient recrystallization and growth (GREG) technique using
photoacoustic methods", Thin Solid Films, 517,pp.2335-2339, 2009.
[14.] M.Mukherjee, N.Deshmukh and S.K.Kulkarni,"X-ray reflectivity and AFM studies of polystyrene-CdS
nanocomposite thin films", Applied Surface Science, 218,pp.324-329, 2003.
[15.] A.Rotaru, A.M.Kropidlowska, C.Constantirescu, N.Scarisoreanu, M.Dumitru, M.Strankowski, P.Rotaru, V.Ion,
C.Vasiliu, B.Becker, and M.Dinescu,"CdS thin films obtained by thermal treatment of cadmium complex precursor
deposited by MAPLE technique", Applied Surface Science,255,pp.6786-6789, 2009.
[16.] V.Krishnakumar, K.Ramamurthi, A.Klein, and W.Jaegermann,"Band alignment of differently treated TCO/CdS
interface", Thin Solid Films, 517,pp.2558-2561, 2009.
[17.] S.Gupta, D.Patidar, N.S.Saxena, K.Sharma," Electrical study of thin film Al/n-CdS schottky junction",
Chalcogenide Letters, 6,pp.723-731, 2009.
[18.] Y.Dingyu, Z.Xinghua, W.Zhaorong, Y.Weiqing, L.Lezhong, Y.Jun, and G.Xiuying,"Structural and optical
properties of polycrystalline CdS thin films deposited by electron beam evaporation", Journal of Semiconductors,
32,pp.1-4, 2011.
[19.] S.Ilican, M.Zor, Y.Caglar, M.Caglar, "Optical characterization of CdZn(S1-xSex)2 thin films deposited by spray
pyrolysis method", Optica Applicata,7776(1),pp.29-37, 2006.
[20.] Z.rizwan, B.Azmi, M.Sabri, "Effect of annealing temperature on the optical properties of nanostructureed CdS
films prepared by chemical bath deposition technique", Optoelectronics and Advanced Materials-Rapid
Communications, 5(4),pp.393-397, 2011.
[21.] Iqbal S.Naji, "Study of annealing induced changes in structural and electrical properties of CdS and CdS:Al thin
films", Indian Journal of Applied Research, 3(2),pp.343-346, 2013.
[22.] S.Ikhmayies, R.Ahmad, "A comparison between the electrical and optical properties of CdS:In thin films for two
doping ratios", Jordan Journal of Mechanical and Industrial Engineering, 4(1),pp.111-116, 2010.
[23.] N.F.Mott, and E.A.Davis , Electronic Processes in Non-Crystalline Materials,2ndEd., Clarendon Press, oxford, 1979.
[24.] P.P.Sahay, R.Nath, and S.Tewari, "Optical properties of thermally evaporated CdS thin films", Crystal Research
Technology, 42(3),pp.275-280, 2007.
[25.] A.J.Haider, A.M.Mousa, and S.M.Al-Jawad, "Annealing effect on structural, electrical and optical properties of
CdS films prepared by CBD method", Journal of semiconductor technology and science, 8(4),pp.326-331, 2008.
[26.] D.J.Desale, S.Shaikh, F.Siddiqui, A.Ghosh, R.Birajdar, A.Ghule, R.Sharma, "Effect of annealing on structural
and optoelectronic properties of CdS thin film by SILAR method", Advance in Applied Science Research,
2(4),pp.111-115, 2011.
[27.] H. Khallaf, G.Chai, O.Lupan, L.Chow, S.Park,and A.Schults, "Investigation of aluminum and indium in situ
doping of chemical bath deposited CdS thin films", J.Phys.D:Appl.phys.,41,pp.1-10, 2008.
[28.] H. Khallaf, G.Chai, O.Lupan, L.Chow, S.Park,and A.Schults, "Characterization of gallium-doped CdS thin films grown
by chemical bath deposition", Applied Surface Science, 255,pp.4129-4134, 2009.
[29.] S.M.Mahdavi, A.Irajizad, A.Azarian and R.M.Tilaki,"Optical and structural properties of copper doped CdS thin
films prepared by pulsed laser deposition", Scientia Iranica, 15,pp.360-365, 2008.
Volume 2, Issue 6, June 2013
Page 560
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 2, Issue 6, June 2013
ISSN 2319 - 4847
AUTHORS
Iqbal S. Naji received a Physics B.Sc. degree from Baghdad University College of Science in 1995 ,
Master Degree from Baghdad University in 1998 and a Ph.D. in solid state/thin films from Baghdad
University in 2005. She works as Assistant professor in physics department , College of Science ,Baghdad
University, Baghdad, Iraq.
Iman H. Khudhir received the B.S. and M.S. and P.H.D in physics-solid state/thin films from Baghdad
university /department of physics in 1984,1995,and 2005 respectively. Now she is working as doctors in
Department of Physics College of Education Ibn-Al-Haithem, University of Baghdad, Baghdad, Iraq.
Hanaa I. Mohammed received B.Sc. degree in physics from Baghdad University College of Science in
1995, and M.S. in solid state/thin films from College of Education Ibn-Al-Haithem, University of Baghdad
in 2008. Now she is working as lecturer in Department of Physics College of Education Ibn-Al-Haithem,
University of Baghdad, Baghdad, Iraq.
Volume 2, Issue 6, June 2013
Page 561