Structural and optical properties of Cd 0 . 7 Zn 0 . 3 S : Pb Nanocrystalline thin films deposited by chemical bath technique

Nanocrystalline Cd0.7 Zn0.3S:Pb thin films were deposited on glass substrates by chemical bath technique (CBD) at 80±2oC from an aqueous bath containing cadmium chloride, zinc chloride , thiourea and doped lead chloride in the presence of ammonia solution . Structural properties of the obtained films were studied by X-ray diffraction analysis. The structural parameters such as crystallite size have been evaluated. .The absorption spectra are recorded in the wavelength range 20


Introduction
Nanocrystalline semiconductors of group II-VI have potential applications in many technical fields, including photoluminescence, solar cells and photovoltaic applications [1].The ternary compound Cadmium Zinc Sulphide (Cd x Zn 1-x S) have been mostly extensively investigated as important candidate for wide band gap material [2,3].Cd x Zn 1-x S alloy compounds have attracted technological interest because the energy gap can be tuned and the lattice parameters can be varied [4].Cd x Zn 1-x S compound has a band gap between CdS (2.42 eV) and ZnS (3.66 eV) and the value of the band gap depends onto Cd an Zn ratio [5].Addition of Zn to the most widely used CdS buffer layer material enhances the electronic and optical properties of optoelectronic devices [6].In solar cell systems, where CdS films have been demonstrated to be effective, the replacement of CdS with the higher band gap Cd x Zn 1-x S alloys has led to a decrease in window absorption loss and an increase in the short circuit current [7,8].The ternary n-type Cd x Zn 1-x S compounds have been used as a window layer to form heterojunction solar cell with different p-type materials such as Si, Cu x S, CuInSe 2 , CuGaSe 2 , Cu(In,Ga)Se 2 (CIGS) and CdTe [9].In this work, nanocrystalline Cd 0.7 Zn 0.3 S:Pb thin films were deposited on glass substrates by chemical bath technique (CBD) at 80±2ºC.

Experimental
Cd 0.7 Zn 0.3 S:Pb thin films were deposited on glass slides using the CBD technique.A bath containing 0.1 M solutions of lead nitrate, cadmium chloride , zinc chloride , and thiourea was developed, and the pH of the solution was adjusted to 10 by addition of NH 4 OH.Glass slides were cleaned using liquid detergent in an ultrasonic bath, dipped into a chromic acid bath for 2 h at room temperature, washed with distilled water and acetone, and then dried.The glass substrates were immersed vertically in the reaction vessel, and the bath was set to the desired temperature.Deposition was carried out at 70 o C for 120 min, and the slides were left in the bath for 24 h at room temperature.The deposited films were then washed with distilled water and dried in air.The crystallinity phase and orientation of the Cd 0.7 Zn 0.3 S:Pb films were determined by XRD using a Philips PW 1840 instrument with a Cu-Kα target.A UV-Vis spectrophotometer (Jenway 6800) was used to measure the absorbance and transmittance of the films in the wavelength (λ) range from 300 nm to1100 nm; optical parameters were calculated from these measurements.corresponding to (hkl) = (0 0 2) as given in table 1.The value of 2θ increase from 26.3826 without doping to 26.98327 with doping with 5%Pb .The grain size was calculated from (0 0 2) peaks of XRD pattern by using Scherrer formula [10].The grain size is varied in the range 3-7 nm.The variation of grain size is not linear with the composition.where k=0.94, is the wavelength of X-ray, b is the full width at half the peak maximum in radians and θ is Bragg's angle.

Optical Studies
The optical absorption of the films has been studied in the range (300-1100 nm).Absorption coefficient associated with the strong absorption region of the films was calculated from absorbance (A) and the film thickness (t) using the relation [11].
where t is the film thickness.
The variation of the absorption coefficient (α) as a function of wavelength (λ) for the different films is shown in Fig. 2. It is clear that the value of the absorption coefficient (α) decreases with increase in the wavelength .
The value absorption coefficient is of the order of 10 4 cm -1 , that supports the direct band gap nature of the semiconductors.The optical band gap E g of thin films prepared was calculated by using the following formula [11].

( )
where A is constant , is the incident photon energy , and m is a factor whose value dependent on the nature of band transition ,m=(1/2) or (3/2) for direct allowed and direct forbidden transition.From fig. 3 The extinction coefficient(k) have been calculated by using the following formula [11]; decreases with increase in the wavelength .The refractive index (n) of thin films can be calculated from their reflectance and transmittance spectra using simple approximations relation [12].The refractive index is one of the foundation properties of an optical material because it is closely related to the electronic polarization of ions and the local field inside materials.It can be seen that n increases as wavelength increases as shown in fig. 5. Real and imaginary parts of dielectric constants were determined using the following equations [11].
Where ε 1 and ε 2 is the real and imaginary dielectric constant respectively .The plots of (ε 1 ) and (ε 2 ) of thin films are illustrated in figures (6 and 7) respectively , from fig. 6 the variation of ε 1 is follow the refractive index.x S films were deposited on glass substrate at a temperature of 80±2°C and a pH=10 doping with Pb at different concentration by using chemical bath deposition technique (CBD).The prepared films were homogenous and good adherent to the substrate.CBD is useful for deposition of nanocrystalline thin films for solar cells application.It has been observed that good quality films can be prepared by CBD method.The structure of the thin films is hexagonal, which has very strong preferential orientation along the (002) plane.Optical energy bandgap characteristics have been studied.
XRD patterns of Cd 0.7 Zn 0.3 S thin films with doping by Pb are shown in Fig.1 .All the patterns of thin films prepared are polycrystalline with peak (2θ)=( 26.3826) 2 nd Conference For Pure Science -university of Anbar 20-22/11/2012