Study of Physical Properties of CdO Thin Film Prepared by Chemical Spray Pyrolysis Technique

In this research, cadmium oxide thin films were deposited by chemical spray pyrolysis technique on the glass substrate at 350°C with thickness about (185) μm which calculated by interference fringe. The (XRD) measurements exhibit that the CdO films are polycrystalline with cubic crystal structure and many peaks (200), (202), (311), (222) and with preferential orientation along (111) plane. The surface morphology and roughness of the samples was studied by atomic force microscopy (AFM). The optical properties measure in the (UV –VIS), wavelength range (400-800) nm shows that the energy Gap is (2.3) eV and the optical transmittance of CdO films is about 88%. . The carrier's concentrations, Mobility Of cadmium oxide Thin film was calculated using Hall Effect measurements, Hall mobility display that cadmium oxide thin film is n-type semiconductor films. The D.C conductivity results shows that the CdO films have two values of activation energy as a result of polycrystalline structure of CdO film


Introduction
CdO thin films has be vastly use as transparent conducting oxides thin films because of low resistivity and high optical transmittance [1,2] , its optical bands gab lay between ( 2.2-2.7)eV at room temperature depend on the type an method and preparations conditions [3] very useful for many application such like solar cell , phototransistor , diodes , gas sensors , heat mirror and antireflection coatings [4.5] .In the last decade, various techniques used to prepared CdO thin films such as thermal evaporation, reactive sputtering, RF magnetron sputtering, pulsed laser deposition and chemical spray pyrolysis [6].The optoelectronic semiconductor material CdO, has been extensively studied as epitaxial and polycrystalline thin films prepared by different techniques because of its unique optoelectronic and other properties with a hope of exploring potentialities for fabrication of new scientific and technological devices.[7] .Many workers were prepared polycrystalline CdO thin films and characterized their structural, electrical and optical properties.Cadmium oxide thin films with enhanced electrical property were prepared on glass substrates at 300 °C using spray pyrolysis methods [8], X-Ray diffractions study reveal that the film exhibits cubic crystals structures.
From the transmittances Spectra, it's found that CdO thins films deposited with lower precursor concentrations have high transparent.Hall measurement confirm that CdO semiconducting behaviors as n-type electrical conductivity.The result authenticated that the precursor concentrations influenced the structural, optical and electrical properties of the deposited film.The composition and optical properties of cadmium oxides thins film be studied by different methods, CdO were prepare by chemical Spray pyrolysis deposition method and study the effect of Zinc as a dopants [9].CdO:Sn thin film deposit on glasses substrate use thermal evaporation at lower Vacuum.[10].
In this work we deposited CdO by chemical spray pyrolysis and then study some structural, optical and electrical characteristics of the prepared thin films.

Experimental details
CdO thin film was prepared by chemical spray pyrolysis method utilize a laboratory design atomizer.Cadmium acetate (Cd (CH3COO) 2⋅2H2O) were use a sources of Cd ions with concentration 0.1M, it is a very small crystalline grains with white color and its molecular weight 128.41 gm/mol, 1.332 gm of the acetate was dissolved in 50 ml of distiller water this solution was put on a magnetic stirrer to ensure the perfect solubility of the acetate.The result  1).This method relies on the interference of the laser beam reflected from the surface of a thin layer and then substrate, the films thickness was calculated using the following formula:   The structural parameter like the diffractions angles (2) , Full widths half maximums and phase specified belong (hkl) plane are estimated from those spectra as listed in table (1) The crystallite size of all peak was calculated using Scherrer's formula [12].
Where ƛ it's the X-Ray wavelength, β must bein radians and θComplies to the positions peak the larger of G.S and smaller  value indicated better crystallizations of the material

2-Atomic Force microscopy
To study the surface morphology of films materials it is important to used atomic force microscopy (AFM), it is one of the effective ways for the surface analysis due to its high resolution and powerful analysis software.as well as analysis of the AFM can calculate the thickness of film, roughness and grain size and gives an illustrative picture of the distribution of the particle size of the crystal on the surface rate .it seen from the fig ( 3: A, B, C) the analytical CdO thin film topographic image of the 2D, 3D, granularity normal distribution and appearance .It absorbed that the average roughness equal (11) nm, the values of Root Mean Square equal (12.9) nm its illustrate (total high surfaces and low square divide by the summation of these numbers are all unders there Square Root).The average diameter equal( 97.46 ) nm .theCdO film exhibit a lowers surfaces Roughness with regular orient this is maybe agree with that vertical structures which is associate with these (111) CdO textures growth.

Study of Physical Properties of CdO Thin Film Prepared by Chemical Spray Pyrolysis Technique
Where A is constants,  the Absorption coefficients, ħ the incident photon energies it can be seen that transmittance value of cadmium oxide thin film in the range (340-1040) nm, is about 88%.As show in Fig ( 4) also the transmittance increasing with ƛ as is the case of transparent conducting oxides (TCOs).The refractive index (n) is the ratio between the speeds of light in vacuum to its speed in material that doesn't absorb this light.Figure (7) shows the variation of (n) as a function of The activations energy could be calculate from the plot of lnσ versuses 1000/T accord to equations [16] σ = σ ° exp ( Whereσ ° is the minimum electrical conductivity at 0K, T is the temperature,   is the Boltzmann's constant and Ea is the activations energies which correspond to (Eg/2) for intrinsics conductions, [17].
The activation energy was measured as show in the Fig ( 9) and table (2).We can notice that

Fig. 1 :
Fig. 1: Schematic of interference fringe patterns of CdO thin films XRD measurements were recorded using Philips PW 6000, Cu K target and Ni as filter, ƛ =1:5418A by corresponding diffraction angle 2 from 10 to 80.The surfaces morphology of (CdO) Thin film was carried out byAtomic force microscopy AFM (A100 SGS)Angstrom Ad − Vance Inc, tip NSC35/AIBS) .
Result and discussions 1-X-Ray diffractions studies: XRD patterns of CdO thin film deposit at 350Ċ are shown in (fig.2) It can be observed from the figure polycrystalline of cubic crystal structure of the CdO thin film , it can be clearly seen that the film are preferentially orientated along (111) crystallographic directions, as well as the emergence of four diffraction peaks { ( 200) , (202) , ( 311) ,( 222) } , When comparing these results with JCPDS Card ( No:05-0640) it was found its agrees with the card and with the previous study despite the different preparation methods [21].

Fig 2 :
Fig 2: The X-Ray diffraction (XRD) pattern of prepared CdO thin film prepared by chemical spray method Fig (3) AFM image: A-2D image B-3D image C-Distribution of granular3-Optical propertyOptical characteristics of a semiconductors can be known like property these include the interactions between lights and the semiconductors, involve absorptions, diffractions, polarizations, reflections, and scattered effect[21].Those properties were significant forever the understand of these technicality for the electrons transitions between energies band the measured of absorptions and transmissions of a S.C the optical features of semiconductors thin film were basic requirement suitable for various application on optoelectronics device.Thin films was measured by a computer-programmable, were a double beam spectrophotometer in the wavelength ranging from 200 to 2500nm in ordinary cases with the

Figure ( 4 )Fig ( 5 )Figure ( 6 )
Figure (4): transmittance spectra as a functions of ƛ for CdO thins films prepared by chemical spray methodThe optical absorption of CdO thin film seen from the Fig (5) it showed that the maximum absorption occurs at (3.5) eV.These spectra reveal that the absorption increase with the increasing of the photon energy for CdO thin film.From the value of absorption coefficient is equal ( ≥ 10 4 ) we can be eshmited that the transmission of CdO film is direct transmissionsThe optical energy gap   was predestined by presume a direct transition between valance and

Fig ( 7 )
photons.The variation of the extinction coefficient with photon energy show that ( ) increases at the range (1.5-3.5)eV of photon energy and then decreased at (4eV) with increasing of hv for CdO thin film as in Figure(8).The extinction coefficient is associated with the absorption coefficient according to the following relationship: the films have more than one value of activation energy as a result of polycrystalline structure of CdO films, the first  2 occurred in the temperature within (383-473) k and its obtained by transfers the carrier between extend state of V.B to C.B and the second one  1 that occurred in the low temperature within ( 393-383)k ,its obtained by the transfer the carrier between localized state below C.B and above V.B.

Fig ( 9 )
Fig (9): The variation of (ln) as a function of temperature for CdO thin film deposited by chemical spray taqnique