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CURRENT -VOLTAGE CHARACTERISTICS OF CdO
NANOSTRUCTURE ULTRAVIOLET PHOTOCONDUCTIVE
DETECTOR
Asama N. Naje, Lamia K. abbas, Ghaida Salman and Estabraq T. Abdullah
Physics department, college of science, University of Baghdad
E-mail: Naje.as75@gmail.com , dr.Lamia@yahoo.com
Abstract: The Properties of photoconductive ultraviolet detector fabricated on CdO
nanofilms were presented. The Cadmium Oxide (CdO) semiconducting transparent
nanostructure film is deposited on glass and porous silicon substrates by spray pyrolysis. The
structural and optical properties of the grown films are presented. The crystalline structure
was studied by X-ray diffraction. The direct band gap of CdO nanofilm was found to be
3.4eV, comparing with that of the bulk CdO. The deposited CdO film was coated by
nanosheet of polyamind polymer to improve the photoresponsivity of the detector.
Keywords: CdO nanostructure, Spray pyrolysis, XRD, Optical properties.
1. Introduction
Cadmium oxide (CdO) attracts a great attention due to its electrical and optical
properties. CdO is an n-type semiconductor with a ranging direct band gap at approximately
2.2-2.7 eV [1-5]. CdO has many attractive properties such large energy bandgap, high
transmission coefficient in visible spectral domain, remarkable luminescence characteristics
etc.
Thin films of CdO have been prepared by employing various physical and chemical
deposition techniques, such as evaporation, spray pyrolysis, solution growth, Langmuir-
Boldgett deposition, sputtering, etc [6-10].
This materials have been widely studied for optoelectronic applications in transparent
conducting oxides (TCO) [11], solar cells[12], photovoltaic device [13], photodiodes [14] as
well as other types of applications like IR heat mirror, gas sensors [15], low-emissive
windows, thin-film resistors, etc [16-17].
In the present study, synthesis and characterization of CdO nanostructure ultraviolet detector
has been studied by depositing the CdO nanofilm on nanospikes silicon layer.
International Journal of Science, Environment ISSN 2278-3687 (O)
and Technology, Vol. 3, No 2, 2014, 684 – 691
Received Mar 4, 2014 * Published April 2, 2014 * www.ijset.net
685
Asama N. Naje, Lamia K. abbas, Ghaida Salman and Estabraq T. Abdullah
2. Experimental Works
N-type Si wafer of 0.05 .cm resistivity was used as a starting material in the
photochemical etching. The samples of 2 x 2 cm
2
dimensions were cut from the wafer and
rinsed with acetone and methanol to remove dirt. In order to remove the native oxide layer on
the samples, they were etched in diluted (10 %) HF acid. After cleaning the samples they
were immersed in HF acid of 50 % concentration in a Teflon beaker. The samples were
mounted in the beaker on two Teflon tablets in such a way that the current required for the
etching process could complete the circuit between the irradiated surface and the bottom
surface of the Si sample.
Tungsten halogen lamp of 250 Watts integrated with diacnamic ellipsoidal mirror was used
as the photon beam source. The photoetching irradiation time was chosen to be 10 minutes.
At the end of the photochemical etching process, the samples were rinsed with ethanol and
stored in a glass containers filled with methanol to avoid the formation of oxide layer above
the nanospikes film.
The CdO nanofilms were prepared by chemical spray pyrolysis technique. The films were
deposited on porous silicon layer heated to (250ºC). A 0.1M Spray solution is prepared by
dissolving cadmium acetate (Cd(CH
3
COO)
2
.2H
2
O )
of molecular weight equal to
266.527gm / mole in
a mixture of methanol and deionized water (1:1).
The above mixture
solution was placed in the flask of the atomizer and spread by controllable pressurized
nitrogen gas flow on the heated substrates. The spraying time was 4 seconds, which is
controlled by adjustable solenoid valve. The heated substrate was left for 12 sec after each
spraying run to give time for the deposited (CdO) layer to be dry. The optimum
experimental conditions for obtaining homogeneous CdO thin film at (250 ºC) were
determined by the spraying time, the drying time and the flashing gas pressure.
The thickness of the prepared films was measured by laser interferometer technique. The
thickness of the films
was found to be in the range between (800-1000m). The micro
ma
sk of (0.4mm) electrode spacing was used to deposit the Aluminum (Al) electrical
electrodes on the film surface.
The variation of photoresponsivity of CdO Photoconductive UV detector with the bias
voltage was carried out under the illumination with UV diode of 2.5 mWatt power and of 385
nm wavelength.
Current-Voltage Characteristics of CdO Nanostructure Ultraviolet...
3. Result and Discussion
3.1 Structural Characteristics
The X-ray diffraction (XRD) pattern of the CdO nanofilm deposited on nanospike layer
of n–type silicon substrate is illustrated in Figure 1.
The figure shows the (111), (200), and (220) peaks occurred at 2
values of 33
°
, 38
°
and
55.2
°
respectively, with full width at half maximum (FWHM) of (200) peak of about
0.658°. The CdO nanofilm are strongly crystallized with a preferred (200) orientation,
which has been observed by other authors [5,7,18]. Particle size was determined from
the width of XRD peaks using Scherer's formula [19]:
θβ
λ
cos
K
D=
Where
D is the grain size, K is the shape factor, being equal to 0.9, is the wavelength of X-
ray, is the full-width at half maximum FWHM (degree), and is the diffraction angle in
degree
. Figure 1 shows the grain size of CdO sample (24.4nm) obtained from the
FWHM of peak corresponding to 2=38.60
o
3.2 Optical properties
The absorption spectrum of the CdO nanofilms deposited on glass substrate is shown in
Fig 2.
The figure shows high absorption coefficient in the UV region, whereas it is transparent in
the visible region. Assuming direct transition, the dependence of (h)
2
on the photon energy
h is plotted following Taue relation [20] and the graph is illustrated in Fig.3.
2 theta
Intensity
111
200
220
Figure 1. XRD of CdO nanofilm.
686
687
Asama N. Naje, Lamia K. abbas, Ghaida Salman and Estabraq T. Abdullah
The extrapolation of the linear part of the above plot to ( h )
2
= 0 give the energy gap value
of the CdO nanofilm, which was found to be about 2.5eV, and 3.46 eV. The above two
values may be related to the nanostructured CdO film and to bulk CdO material.
This value
is in a good agreement with the values presented by other workers [3-21].
The photoluminescence spectrum of CdO nanofilm on glass substrate is plotted using SL 174
spectrofluorometer supplied by ELICO Company covering the 300–900 nm wavelength
range. The room temperature photoluminescence spectrum of CdO film deposited on glass
substrate excited by 300nm line is shown in figure 4.
(nm)
Absorbance
Figure 2. The absorpance spectrum of CdO
nano
film
deposite
on glass
substrate
.
E
g
(eV)
(h)
2
×1o
14
(
eV/m)
2
Figure3
.
(h)
2
versus E
g
plot of CdO nanofilm
200 300 400 500 600 700
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Current-Voltage Characteristics of CdO Nanostructure Ultraviolet...
The spectrum shows two peaks: the first peak at 386 nm which can be referred to the strong
direct band transition (or band to band transition). The second peak at 520nm is due to the
exciton emission.
The energy band gap from photoluminescence spectrum of the CdO film is calculated by
using the following equation
( )
nm
Eg
λ
1240
=
For the PL wavelength 386nm and 520nm the energy band gap found are to be
(3.2 and 2.38eV).
Similar peaks in spectrum of CdO have been reported by [22].
3.3 Electrical Properties
The variation of the photoconductive response of the fabricated photoconductive detector as
a function of the bias voltage at dark and under illumination with UV source of 2.50 mw
radiation power for etching time (10min) are illustrated in Fig .
5.
Figure 4: The Photoluminescence spectrum of CdO film on glass
substrate
PL(a.u)
(nm)
688
689
Asama N. Naje, Lamia K. abbas, Ghaida Salman and Estabraq T. Abdullah
I (mA)
Bias Voltage (volt)
Figure.5. I-V characteristics of CdO nanofilm with and
without polymer
It can be noticed from the figure that the dark current was about 68mA at 5 v bias whereas
the photoconductive current was 161mA. This result reflects a good UV radiation sensitivity
with photoconductive gain
(G)
of 2.36.
The functionalization of the CdO film surface by polyamide nylon improved the
photoconductive gain as shown in Fig.5.
Conclusion
The CdO UV detectors prepared by chemical spray pyrolysis technique were fabricated on
photochemical etched silicon substrates. The direct band gap of CdO nanofilm was found to
be
2.5eV, and 3.46 eV. The above two values may be related to the nanostructured CdO film
and to bulk CdO. The variation of the photoconductive response of the fabricated detector
was 161mA
and
photoconductive gain
(G)
was 2.36.
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