The Structure and Optical Properties of Ag doped CdO Thin Film Prepared by Pulse Laser Deposition (PLD)

At a temperature of 300 K, a prepared thin film of Ag doped with different ratios of CdO (0.1, 0.3, 0.5) % were observed using pulse laser deposition (PLD). The laser, an Nd:YAG in λ=1064 nm, used a pulse, constant energy of 600 mJ ,with a repetition rate of 6 Hz and 400 pulses. The effect of CdO on the structural and optical properties of these films was studied. The structural tests showed that these films are of a polycrystalline structure with a preferred orientation in the (002) direction for Ag. The grain size is positively correlated with the concentration of CdO. The optical properties of the Ag : CdO thin film we observed included transmittance, absorption coefficient, and the energy gap in the wavelength range of 300-1100 nm. The prepared films, direct energy gap is negatively correlated to concentration of CdO.


Introduction:
Thin film technology is on the leading edge of the study of semiconductors, primarily by giving a clear idea of many of their physical and chemical properties )1(. Pulsed laser deposition (PLD) is one versatile thin film deposition technique, where flashes of laser light are used to generate an atomic spray that produces a thin film )2(. The advantages of this technique are speed, flexibility in the choice of material, greater control of growth in any environment, variable growth rate )3(, epitaxy at low temperature and growth by varying laser parameters )4(. The disadvantages of technique include uneven coverage )5(, high defect or particulate concentration and mechanisms dependence on parameters not well understood )6(. Silver is nearly white lustrous )7( element with the highest electrical conductivity of all metals. Due to its great value as a precious metal, it has not been used as a semiconductor for optoelectronic applications. It is increasingly used in solar energy, where silver paste is used to convert solar energy into electrical energy )8(. The crystalline structure of Ag is a cubic face )9(. In addition to silver, the Cadmium oxide (CdO) cubic structure semiconductors are n-type )10( with a band gap of 2.18 eV at room temperature and have been utilized in optoelectronic devices, solar cells, and photodiodes )11(.

Material and Methods:
Ag and CdO were mixed together at (0.1, 0.3, 0.5) % wt using agate mortar for one hour under a five-ton hydraulic type SPE CAC to mask pellets at 1.5 and 3 cm diameters. Substrates were transferred to glass using deposition films. Then the glass bases were washed with water and washing powder and then distilled water. To ensure a clean surface, the rinse is placed in a beaker and placed inside the ultrasonic waves for 15 minutes. Pulse laser deposition at 600 mJ was used to achieve the desired, film thickness of 250 nm by using an optical interferometer with a repetition rate of 6 Hz. The laser used was an Nd:YAG in λ=1064 nm with 400 total pulses.

Results and Discussion:  x-ray diffraction measurements (XRD)
This analysis was used to identify thin film quality and crystalline structure by X-ray diffraction on the thin film. Fig. (1) shows the results of x-ray diffraction measurements (XRD) for Ag-doped CdO thin film at 300K and a thickness of 250 nm on the glass substrates. All thin films have a polycrystalline structure and Ag cubic phase with a preferred orientation along 200. The average crystallite size (D) was measured by Debye-Scherer formula )12(:

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Where θ is the degree of the diffraction peak, β is the peak width of the diffraction peak profile at half maximum. Table (    optical measurements The study of the optical properties are closely related to the composition of the energy levels and the crystalline structure of the material prepared for Ag-doping with different concentrations of CdO (0.1, 0.3, 0.5) % wt, using UV-Vis light in the range of 300-1100 nm. The optical properties were studied for thin film prepared by PLD, and in preparation for conditions that included a baseline temperature of 300 o C, an oxygen pressure of 0.1 mbar and a laser power of 600 mJ. Fig. (2) shows the transmittance spectrum of Agdoping with different concentrations of CdO on a glass substrate. We note that transmittance decreases slightly with increased vaccination rates and their values vary according to wavelength, most likely due to the fact that vaccination rates are low. This may lead to the formation of significant levels of impurities within the energy gap that lead to increased absorption and hence reduced permeability. The absorption coefficient behaves as absorption behavior, where increasing rates of vaccination to levels of added impurities within the energy gap. From fig. (3), we show the absorption coefficient changes as a function of wavelength that all films were fairly high values reached above 10 4 cm -1 . This indicates a high probability of direct electronic transitions and the high energies these values were calculated for are the energies of a direct energy gap. The absorption coefficient is low at the low photonic energies and the probability of electronic transmissions is low. The absorption coefficient values increases at the absorption edge its high energies.

Conclusion:
The pulse laser deposition (PLD) technique was used to grow the Ag:CdO thin film at 300K with different concentration of CdO at a thickness of 250 nm on glass. The results of x-ray diffraction indicated that all thin films have a polycrystalline structure and an Ag cubic phase, with a preferred orientation along (200) direction. The average crystallite size increased with the increase concentration of CdO. The optical transitions in CdS thin film is direct transition.