Structural and Optical Properties of (CdO) 1-x (SnO 2 ) x Thin Films Prepared by Pulsed Laser Deposition

. CdO thin films have been deposited at different concentration of SnO 2 (x= (0.0, 0.05, 0.1, 0.15 and 0.2)) Wt. % onto glass substrates by pulsed laser deposition technique (PLD) using Nd-YAG laser with λ=1064nm, energy=600mJ and number of shots=500. X-ray diffraction (XRD) results reveal that the deposited (CdO) 1-x (SnO 2 ) x thin films cubic structure and the grain size increase with increasing annealing temperature and increasing concentration of SnO 2 . The optical transition in the (CdO) 1-x (SnO 2 ) x thin films are observed to be allowed direct transition. The value of the optical energy gap decreases with increasing of annealing temperatures and increase with increasing concentration of SnO 2 for all samples.


Cadmium Oxide CdO
The unique combination of cardio thin film properties which were represented by high electrical conductivity, high carrier concentrations and high transparency in the visible range of the electromagnetic spectrum, made it suitable for a wide range of applications in different fields [7,6]. The applications of CdO thin films can be summaries as follows: -Its application in photovoltaic solar cells for front contacts window layer, or as heterostructure such as CdO/CdTe or CdO/Cu2O solar cells [1,8,9].
-It has been used as heat mirrors, due to its high reflectance in the infrared region together with transparency in the visible region [4].
Stannic Oxide SnO 2 In 1942 Masters [10] succeeded in preparing conductive transparent tin oxide, for the first time. A substance with white color has a molecular weight of (150. 69 g/mol). Its density (6.95 g/cm 3 ), its melting point (1630°C) and its boiling point (1900°C) [11]. Stannic oxide is an n-type semiconducting material with a direct band gap of about 4.0 eV and an indirect band gap of about 2.6 eV [12]. The electron concentration in the conduction band arises primarily from the lack of stoichiometry produced by oxygen deficiency. The property of SnO 2 makes the material useful for many applications. There for increasing attention is begin paid to study this oxide especially on the method of operation, and its electrical and optical properties. SnO 2 thin films have been fabricated using different techniques including pulse laser deposition, electron beam evaporation [13], chemical vapor deposition [14], RF sputtering [15], evaporation and chemical spray pyrolysis [16]. SnO 2 as transparent conducting oxide is used extensively for a variety of applications such as transparent electrodes in solar cells, architectural windows and flat panel displays [17]. Recently SnO 2 has been integrated into micro chemical silicon devices as a sensing element of micro sensor.

EXPERIMENTAL 2.1 Preparation Pellets
High purity powders (99.999%) of CdO and SnO 2 supplied from Fluka were used to form the target as a disk of 2.5cm diameter and 0.4 cm thickness by pressing it under 4 ton force. The pellets which containing the elements were heated to 873K for 3 hours then cooled to room temperature. The temperature of the furnace was raised at a rate of 10 o C/min. The amount of elements content of pellets was evaluated by using the following equation.

PLD and Thin Film Preparation
The (CdO) 1-x (SnO 2 ) x films were deposited on glass slides substrates of (2.5×7.5 cm) were cleaned with dilated water using ultrasonic process for 15 minutes to deposit the films at room temperature by PLD technique using Nd:YAG with λ= 1064 nm SHG Q-switching laser beam at 600 mJ, repetition frequency (6Hz) for 500 laser pulse is incident on the target surface making an angle of 45°. The under vacuum of (10 −3 mbar) at room temperature and annealing temperatures 523 K were presented.

X-ray diffraction results
The main purpose of this section is to investigate the structural type of semiconductor material that is relevant to the work. Also, the effect of (CdO) 1-x (SnO 2 )x ratio at room temperature and annealing temperature 523 K on the thin films structure have been studied. X-ray diffraction pattern of (CdO) 1-x (SnO 2 ) x at different concentration of SnO2 (x= 0, 0.05, 0.1, 0.15 and 0.2) showed that all these samples have a crystalline structure except (x= 0.2 at R.T) also polycrystalline structure for it cubic phases (card No. 96-900-6688) with preferred orientation along (111) direction at 2θ around 32.9135°. As shown in Figure (1) to (2) and Table (1), which is in good agreement with the standard JCPDS (Joint Committee on Power Diffraction Standards). The grain size increase with increasing of concentration of SnO 2 , also In conducting the annealing process for films prepared were the results of X-ray diffraction showed that there is an increase in the height of the peaks and intensity decrease in (FWHM) any increase crystallized material membranes, this means that the thermal treatment caused the reduced crystalline defects caused due to the preparation and disadvantages of the interface by giving atoms material enough energy to rearrange themselves in a crystalline lattice and disposal of the resulting stresses due to thermal lattice [18]. The grain size of thin film calculated using the Scherer's equation [19].
International Letters of Chemistry, Physics and Astronomy Vol. 59

The Optical Properties of (CdO) 1-x (SnO 2 ) x thin Films
The optical properties of deposited (CdO) 1-x (SnO 2 ) x films on glass substrates for different concentration of SnO 2 at room temperature and annealing temperatures 523 K have been determined by using UV-visible transmittance spectrum in the region of (360-1100) nm. Also the energy gap and optical constants have been determined.

The Absorption Coefficient (α)
The absorption coefficient (α) of the (CdO) 1-(SnO 2 ) x thin films deposited with different concentration of SnO 2 (x=0, 0.05, 0.1, 0.15and 0.2) at room temperature and annealing temperatures 523 K are shown in Figure (4) .The absorption coefficient exhibits high values (α >10 4 ) which means that there is a large probability of the direct transition [20], and then (α) decreases with the increasing of wavelength. It is observed that the absorption coefficient (α) decrease with increasing the concentration of SnO 2 , and this is due to the increasing of energy gap with concentration of SnO 2 . Also, we can notice from this Figure (4) that (α) in general increases with the increasing of annealing temperatures and this is due to the decreasing of energy gap with annealing temperatures. The absorption coefficient (α) was calculated in the fundamental absorption region from the following Equation [21]: =2. 303A/t (3)

Optical Energy Gap
The values of optical energy gap (E g opt ) for (CdO) 1-x (SnO 2 ) x films with a different SnO 2 concentration (x=0, 0.05, 0.1, 0.15 and 0.2) deposited at room temperature and annealing temperatures 523 K have been determined using Tauc equation E g opt is determined by the extrapolation of the portion at (αhυ) 2 from the relations between (αhυ) 2 versus the photon energy (hυ), as shown in Figure (5) and Table (2). Thin films have been determined by using Tauc equation [22].
(αhν) = A(hν -Eg) 1/2 (4) In general, the values of direct optical energy gap increase with increasing concentration of SnO 2 (x) for all samples. The direct E g opt increases from (2.64 to 3.05) eV and from (2.4 to 2.7) eV for (R.T and 523) K respectively. This is due to the decrease of the density of state inside the optical gap, the increasing concentration of SnO 2 (x) leads to decreases from the secondary levels and structural defects, which lead to the contract tails region and this leads to expand in the optical energy gap, while the optical energy gap decrease with the increasing of annealing temperatures. Annealing causes a reduction in E g , this may be due to the dilate of the lattice which causes a shift in the position edge of V.B and C.B because of the temperature dependence of the electron-lattice interaction that leads to change in the lattice constant by growth of grain size and the decrease in the defect states near the bands [23].
International Letters of Chemistry, Physics and Astronomy Vol. 59

CONCLUSIONS
Cubic structure is the CdO phase for (CdO) 1-x (SnO 2 ) x and orientated along (111).The optical transition in the (CdO) 1-x (SnO 2 ) x thin films is observed to be allowed direct transition. The value of the optical energy gap decreases with increasing of annealing temperatures and increase with increasing concentration of SnO 2 for all samples.

T a (K) x E g (eV)
R