Effect of Annealing Temperature on Structural and Optical Properties of CdO Thin Films Prepared by pulsed laser deposited (PLD) technique

In this paper Cadmium oxide (CdO) thin films has attracted interest due to its potential applications in optoelectronics devices. thin films were grown on glass substrates by the pulsed laser deposition (PLD) technique at room temperature and different annealing temperatures Ta=(573,673 and 773) K. The structural studies (XRD) measurements for CdO thin film is polycrystalline with cubic structure and it is evident that the film is highly oriented in the (111) direction. Atomic force microscopy (AFM) was used to examine CdO surfaces, After annealing the films become more homogeneity and notice to increment the root mean square, surface roughness and average grain size. The optical properties of CdO thin films are studied as a function to wavelength in region (375 1100) nm. The optical transmittance of the prepared CdO films notice that the transmittance pattern of all deposited thin films increases with increasing the annealing temperature (Ta). The direct energy gap for CdO films was decreases with increasing of annealing temperature for all samples due to the growth of the crystallites .The optical constants such as refractive index, extinction coefficient and dielectric constant were also calculated.


Introduction
Transparent conducting oxides (TCOs) are potential candidates for energy efficient devices like solar cells, light emitting diodes, photo detectors because of their high transparency in the visible region and high electrical conductivity at room temperature [1]. A (II-VI) elements in the periodic table. CdO is an n-type semiconductor with band gap of approximately 2.5 eV, CdO is cubic structure with each ion surrounded by six ions of opposite electric charge, octahedrally arranged [2,3] with lattice constant equal to 4.69 Aº and a unit cell of face center cubic (FCC) [4]. Cadmium oxide is a reddish powder and its color varies from greenishyellow brown to nearly black depending on its thermal history and particle size [5]. CdO thin films are prepared by many physical and chemical techniques such as thermal evaporation, sputtering, solution growth and pulsed laser sputtering [6]. CdO films have been successfully used for many applications, including phototransistors [7], gas sensor [8], transparent electrodes, photovoltaic solar cells [9], liquid crystal displays, photodiodes, IR detectors and anti-reflection coatings and optoelectronic devices [1,9]. It is a promising candidate for a transparent conducting oxide material because it has a simple rock-salt crystal structure, high carrier mobility, and high conductivity [10]. Due to its low optical band gap, CdO is not widely used as transparent conducting electrodes, although CdO thin films show low resistivity due to defects of oxygen vacancies and cadmium interstitials [11].

2-Experimental Work
Cdo thin films are prepared by PLD technique. The pulsed laser deposition experiment is carried inside a vacuum chamber at vacuum (10-3 m bar) conditions, Pulsed laser deposition is a thin film growth method that consists of three steps (1) The interaction of the pulsed laser beam with a ceramic target usually in the form of a sintered pellet (ablation).
(2) The formation and expansion of the laser-induced plasma.

Target preparation
Cadmium oxide (CdO)High purity (99.999) powder by Redel-De Haen, German Company of these materials were mixed in gate mortar was used to form the target as a disk of 1.5cm diameter and 0.3cm thickness using hydraulic piston type (SPECAC), under pressure of 6 tons for 10 minute, as shown in Fig. (2). It should be as dense and homogenous as possible to ensure a good quality of the deposit. To study the structural and optical films properties, CdO films are deposited on glass substrates with (2.5x 2.5) cm 2 dimensions to obtain the most durable and adherent coating substrate, the support surface must be free from contamination films such as grease, absorber water etc…., the cleaning of the substrates was very necessary. The Film deposition on the Substrat. Thin film was achieved immediately after the laser beam hits the target resulting in the evaporation of the target material, which itself mounted on rotating holder with 45° orientation from the substrate in order to ensure that the plasma plume is right-angled with respect to the substrate. The following Fig. (3) show the target after ablation Process. The rotation of the target was employed to avoid the ablation of the same spot on the target. The distance between the target and the substrate is about 1.5 cm and it was found to be the optimum distance. All the samples were annealed in a furnace type England, S30 2AU at temperature (573,673 and 773) K for 2 hours were studied by X-ray diffraction (XRD) techniques using a( Philips PW) X-ray diffractometer system. This system recorded the intensity as a function of Bragg's angle. The film surface topography(AFM) of the samples were studied by taking image for the films surface with tapping mode using atomic force microscope (AA3000 Scanning Probe Microscope SPM. Angstrom Ad-Vance Inc, tip NSC35/AIBS). the optical measurements of thin films depend on, thickness, homogeneity, structure, materials used and the preparation conditions. were measured using UV/ Visible SP-8001 spectrophotometerover the range ( 375-1100) nm.

Structural properties
The X-ray diffraction patterns of CdO thin films grown on glass substrate were prepared

Atomic Force Microscopy
Atomic force microscopy (AFM) was used to examine CdOfilms surfaces, average pore diameter, depth and roughness surface for each sample. The increase in surface roughness of the films leads to an increase in the efficiency for sensing properties, therefore, it is very important to investigate the surface morphology of the films. and peak -peak are deduced from AFM images of these samples were listed in Table (2).
After annealing the film becomes more homogeneity and notice to increase the root mean square of surface roughness and average grain size. This result may be attributed to higher atom mobility with the increase in temperature which causes more effective recrystallization and grain growth of the films that result in larger grains. On the other hand, the larger grains or hillocks on the surfaces of the films reveal an improvement in the crystalline quality of the CdO films, which is in agreement with XRD results. absorbance spectrum in the wavelength range (375-1000) nm. The optical energy gap E g and optical constant which includes the extinction coefficient k ,which involves refractive index (n)such as real dielectric constant ( r) and imaginary dielectric constant ( i ) also were calculated.

The Transmission Spectrum (T)
CdOfilm deposited on glass substrate. The variation of optical transmittance of the film was studied in the wavelength range (375-1100) nm. Fig (6) shows plots of transmittance spectra versus wavelength for the CdOthin films annealed at different temperatures. It is observed that all films showed the light transmittance is low in UV region since thin film absorbs less energy at high wave length, while light transmittance is high in VIS region since thin film absorbs more energy at low wavelength (below 375nm) transparency depends on the growth temperature. We notice also that the transmittancepattern of all deposited thin films increases with increasing the annealing temperatures T a =(RT,573,673 and 773) K and shifts to longer wavelengths, which means a decrease in absorption that occur due to the increase of annealing temperature. It is observed that the film which has higher temperature at (773 K) displayed lower transmittance because of the improved structure of the film and large grain size which both cause increase of absorption. This increase and decrease in transparency is related to the structural properties of the film characteristics. The optical transmittance of the prepared films was typically higher than 80% at wavelengths beyond the absorption edge. Clear that the decreases or the shift towards longer wavelengths is not sharp. The parallel transmission shift however indicates that it is related to changes in film structure.
It can be seen also that the increase in transmittance in UV region is not sharp. This

The Optical Energy Gap
The optical energy gap in general depends on the arrangement and distribution of atoms in the crystal lattice and the films crystal structure. In order to determine the optical energy gap values (E g ) for CdO thin films have been determined by using Tauc relation equation which is used to find the type of the optical transition, graph is plotted with (αhν) 2

The Dielectric Constants for CdOfilms
The variation of the real ( r ) and imaginary ( i ) parts of the dielectric constant values respectively. The plots of real and imaginary (ɛ r and ɛ i ) parts of the dielectric constant with wavelength for CdO deposited at different annealing temperature. Also Fig. (11 a and b)

Conclusions
Thin films of CdO were deposited on glass substrates are prepared by means of simple and low cost by pulsed laser deposition (PLD) technique. Post-annealing process helps to improve the crystalline quality thin films. The effect of growth temperature on structural and optical properties was studied. All the films show polycrystalline nature containing cubic structure of pure CdO phase. The optical transmittance spectra show the NIR transmittance of 80% and direct band gap values decreases with increasing annealing temperatures about 2.5 eV for the film annealed at 773 K for 2 hour.