STUDIA UNIVERSITATIS BABES – BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
SPECTROSCOPIC CHARACTERISATION OF CADMIUM SULPHIDE THIN
LAYERS DEPOSITED ON ITO COATED GLASS
Maria LĂDAR1, Elisabeth-Jeanne POPOVICI1, Ligia PASCU1,
Rodica GRECU1, Ionel Cǎtǎlin POPESCU2 and EMIL INDREA3
“Raluca Ripan“ Institute of Research in Chemistry, 30
Fantanele, 3400 Cluj-Napoca, Romania;
2
Faculty of Chemistry and Chemical Engineering
“Babes-Bolyai” University 3400 Cluj-Napoca,
3
National Institute for R&D of Isotopic & Molecular
Technology 3400 – Cluj-Napoca, Romania
1
Abstract
By chemical bath deposition, CdS thin films were grown onto ITO coated
glass. Crystalline structure and optical properties were investigated by
XRD and UV-VIS spectroscopy. The main CdS/ITO/glass characteristics
were correlated with the conditions of growing and post-growth treatment
Key words: cadmium sulphide; thin films; ITO coated glass; chemical
bath deposition
1. INTRODUCTION
Cadmium sulphide (CdS) is commonly used as n-type semiconducting layer
in different kind of “sandwich” heterostructures such as CdTe/CdS/ITO/glass with
interesting optoelectronic properties. Different techniques could be used for CdS
thin films preparation such as: chemical bath deposition, sputtering, chemical
vapour deposition or electrodeposition [1]. Among them, chemical bath deposition
(CBD) is a simple and low-cost method and produces uniform, adherent and
reproducible films. Moreover, CBD is a low temperature technique and can be used
for CdS deposition onto a wide range of substrates.
Thin films of cadmium sulphide were grown by chemical bath deposition
on indium tin oxide (ITO) coated glass platelets, using the multilayer technique
already mentioned in our previous works [2,3]. Cadmium sulphide films properties
are extremely sensitive to preparation conditions [4,5]. The aim of this work is to
study the influence of different preparative parameters on film quality and,
consequently, on some of the structural and optical characteristics of
CdS/ITO/glass structures.
2. EXPERIMENTAL PART
Cadmium sulphide layers were chemical bath deposited onto ITO coated
glass pieces from Optical Filters Ltd. Prior the deposition, the platelets (50mm x
25mm x 1mm) were ultrasonically cleaned with acetone/ethanol mixture and dried.
M. LĂDAR, E-J. POPOVICI, L. PASCU, R. GRECU , I.C. POPESCU AND E.INDREA
CdS films were formed in many successively deposition steps that always
were performed in renewed chemical bath (CB) prepared from cadmium acetate,
NH3 aqueous solution, sodium citrate and distilled water. The deposition bath was
continuously stirred and heated at 70ºC. The glass platelets were immersed into
the deposition bath, by vertically suspending them around the stirrer. After the
thermal equilibrium was reached, thiourea solution was added under stirring.
The multi-layer deposition procedure was as follows. The wet glass
platelets were immersed into the hot chemical bath. The platelets were taken out
after 1.0 hr, washed and reintroduced into a renewed hot CB. Deposition
parameters were: [Cd2+] = 3x10-3M; [C6H5O73-] = 1.2x10-1 M; [NM3] = 3x10-1 M;
[thioureea] = 3.10-2 M; pH = 10.511.0;
All samples were washed, dried and annealed in air, at 350ºC.
Cadmium sulphide thin films were characterised by thickness, crystalline
structure and UV-Vis transmittance spectra. XRD patterns were obtained by
means of a standard DRON-3M Diffractometer. Optical investigations were
performed with UNICAM Spectrometer UV4. The film thickness was evaluated by
the micro-weighing method [3].
3.RESULTS AND DISCUSSION
CdS/ITO/glass samples were prepared by chemical bath deposition
method, from cadmium acetate and thiourea solutions. The chemical bath
deposition of films involves the decomposition of alkaline solutions containing
thiourea in the presence of a metal salt, as follows:
Cd (CH3COO)2 + (NH2)2CS + 2OH- → CdS + H2CN2 + 2H2O + 2CH3COO –
Chemical bath contains also some chelating agents such as ammonia or
sodium citrate, whose role is to control the CdS film-growing rate.
CBD method was adapted for multilayer CdS film formation onto ITO
coated glass platelets. CdS/ITO/glass samples, with 1, 2 or 4 consecutively
deposited CdS layers, were prepared and characterised. Composition of the as
prepared heterostructures, packing density, growing rate and thickness of CdS
films are presented in table 1.
Table 1
General characteristics of some heterostructures containing ITO and CdS films
CdS
Total dep.
Packing
Film
Growing
Samples
Heterostructures
film
time
density
thickness
rate
code
type
n x m*
(mg/cm²)
(nm)
(nm/min)
ITO 0
ITO/glass
0
0
20**
0
MonoITO1.0
CdS/ITO/ glass
1x2h
0.088
15
0.1
layer
ITO 2.1
CdS/ITO/ glass
2 x 1h
0.480
100
0.8
MultiITO 2.3
CdS/ITO/ glass
4 x 1h
1.424
295
1.2
layer
ITO 2.5
CdS/ITO/ glass
4 x 1h
1.296
270
1.1
* where: n = number of layers, m = deposition time; **- manufacturer measurement
SPECTROSCOPIC CHARACTERISATION OF CADMIUM SULPHIDE THIN LAYER...
The increase of cadmium sulphide film thickness with the total deposition
time could be noticed. Moreover, for the same deposition time, the multilayer film
(ITO 2.1.) is almost 7 times thicker then the corresponding monolayer one
(ITO1.0). The use of a high number of successively deposited layers (coatings)
determines the increase of the film thickness. One can note that, for various
heterostructures, the growing rate is different, increasing with the number of
coatings.
UV-Vis spectroscopy (figure 1 and figure 2) and X-ray diffraction (figure
3 and figure 4) were used to investigate the optical and structural properties of
different CdS/ITO/glass heterostructures. The transmission spectra of the
heterostructures that contain multilayer CdS films, illustrate the decrease of film
transparency parallel with the film thickness increases (figure 1). As for the postgrowing thermal treatment, this one seems to increase the film transmittance
(figure 2).
One can be noted that the ITO coated glass substrate shows a high
transparency on the entire visible domain.
100
100
ITO 0
ITO 0
80
80
Transmitance %
Transmitance( %)
ITO 1.0
60
ITO 2.1
40
ITO 2.3
20
60
ITO 2_3
40
ITO 2_3TT
20
0
0
300
400
500
600
700
800
900
300
1000
400
500
Figure 1. Transmission spectra of
CdS/ITO/glass samples with multilayered
CdS film
ITO
35.9
51.45
45.8
43.95
38.2
ITO
ITO
(400)
80
CdS
(100)
60
CdS
40
20
58.3
500
28.2
24.85
CdS
1000
CdS ITO
ITO 2_1TT
ITO 2_3TT
100
53.05
CdS CdS
ITO
900
CdS
(101)
CdS
(002)
ITO
(411)
67.2
2000
1500
Intensity [a.u]
ITO
2500
800
ITO
(222)
120
ITO 2.1TT
26.5
Intensity [a.u]
3000
700
Figure 2. Transmission spectra of
CdS/ITO/glass samples before and after the
thermal treatment
31.05
3500
600
Wavelength (nm)
Wavelength (nm)
0
0
23
25
30
35
40
45
50
55
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Diffraction angle 2 [degree]
Diffraction angle 2 [degree]
Figure 3. XRD spectra for CdS/ITO/glass
sample after the thermal treatment
Figure 4. XRD spectra for CdS/ITO/glass
samples with multilayer CdS films
M. LĂDAR, E-J. POPOVICI, L. PASCU, R. GRECU , I.C. POPESCU AND E.INDREA
The crystalline structure of the thermally treated CdS/ITO/glass
heterostructures was investigated by X-ray diffraction. Beside the strong bands of
the cubic indium-tin oxide, the characteristic bands of the hexagonal crystalline
structure (greenokite type) of the cadmium sulphide could be noticed (figure 3).
The cristallinity of the films are rather high due to the post-growing thermal
treatment.
In order to compare de effect of multiple layers on crystalline structure of
CdS films, XRD spectra were normalised, in rapport with (222) peak of the
indium-tin oxide (figure 4). The spectra normalisation put in evidence the increase
of the three XRD characteristic bands of cadmium sulphide, i.e. (100), (002), (101).
This fact could be explained by the presence of a higher CdS amount on the surface
of ITO coated glass platelets.
4. CONCLUSIONS
The bath composition and the special CBD technique used in our
experiments proved to be convenient for the deposition of adherent and
homogeneous films with controllable thickness onto the ITO coated glass substrate.
UV-Vis and XRD investigation illustrated the quality of the as prepared
CdS/ITO/glass heterostructures.
Acknowledgements:
The work was supported by CERES National Research Program, under contract no.
28/2002.
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