Spectroscopic and structural studies of cadmium oxide thin films prepared by D . C magnetron sputtering

Cadmium oxide thin films were prepared by D.C magnetron plasma sputtering using different voltages (700, 800, 900, 1000, 1100 and 1200) Volt. The Cadmium oxide structural properties using XRD analysis for just a voltage of 1200 volt at room temperature after annealing in different temperatures (523 and 623) K were studied .The results show that the films prepared at room temperature have some peaks belong to cadmium element along the directions (002), (100), (102) and (103) while the other peaks along the directions of (111), (200) and (222) belong to cadmium oxide. Annealed samples display only cadmium oxide peaks. Also, the spectroscopic properties of plasma diagnostic for CdO thin films were determined and the results show that the electron temperature and electron density increase with increasing of sputtered voltage.


Introduction
Cadmium oxide is (n-type) semiconductor material.Its thin films can be prepared by physical and chemical methods such as spray pyrolysis, DC sputtering, Sol-Gel and Chemical bath methods etc [1].It can also be obtained by heating cadmium with temperature less than its melting point [2].It is used in various DOI: 10.20723/ijp.16.37.7-14 industrial applications such as diodes, transistors, detectors, solar cells and photovoltaic cells [3].The wide band gap properties of CdO thin films are of interest particularly for applications such as solar cells and transparent electrodes [4].These applications of CdO are based on its optical and electrical properties.Such as CdO films show a high transparency in the visible region of the solar spectrum, as well as a high conductivity [5,6].
In this work pure CdO thin films were prepared on glass by D.C magnetron sputtering technique and the spectroscopic spectrum and structural properties at different sputtered voltage were investigated.

Experimental Preparation of CdO thin films
DC magnetron sputtering plasma system consists of glass chamber of 11 cm diameter and 35 cm height, vacuumed by two stage rotary pump type Edward, with two disc electrodes of 7 cm radius, the anode electrode made of aluminum (Al) while the cathode electrode from cadmium (Cd) target with ring magnet above it to enhance and increase the sputtering, DC-power supply high voltage, voltmeter and ammeter devices.The gases were delivered into the chamber using needle valve controlled by two flowmeter, Pirani gauge type Edward and mixer to control Oxygen: Argon ratio (20 %) and gas pressure ( 310 -1 mbar), sputtering time (3 min).The electrodes were polished before every run to clear it from deposited impurities.The CdO thin films were prepared on glass slides substrates dimensions (25.4 76.2) mm at different voltages from 700 to 1200 V at constant electrodes separation of 6 cm.The produced thin films annealed in oxygen at atmosphere pressure inside closed vessel at (523 and 623) K.
Plasma was diagnostic by optical emission spectroscopy (OES).The prepared films on glass slides substrates were examined by X-ray diffraction.

Measurements
The study of structural properties is very important "to know the type and nature of the thin film and crystalline development, used Scherrer equation to calculate rate of crystallite size [7,8]. ( where crystalline size, Ө angle of diffraction and λ the XRD wavelength equal=1.5406Ǻ, (β) the full width at half maximum.While, the distance between the adjacent atomic layers (d) using Bragg's Law [9].n λ=2d sinθ (2) where (n) rank reflection, (d) the distance interface between the levels of the crystal.The microstrains ϒ in lattice was calculated by [10].
where plasma diagnostic by Optical Emission Spectroscopy (OES) can used to calculate electron temperature (T e ) by using the intensity ratio between two emission lines in the following equation (depending on Boltzmann distribution) where I 1 and I 2 are the intensities of the two spectral lines, and A 1 and A 2 are the transition probabilities of the two spectral lines, g 1 and g 2 are the statistical weights of the upper level energy, λ 1 and λ 2 are the wavelength of the two emission lines, E 1 and E 2 are the upper level energies and (K B ) is the Boltzmann's constant [11].
The electron density (n e ) can be calculated by Star broadening effect using the following equation [12], = where ( ) the full width at half maximum for selected spectral line, Electron Impact Parameter (the value of the stark broadening), (N r ) constant =10 16 .And then extracted the Debye length (λ D ), plasma frequency (f p ), by the following equations.without any peaks for cadmium element due to annealing in oxygen, make the cadmium peaks hide [13], and regarded it one method to prepare cadmium oxide, and we are note through the data recorded in Table 2 the full width at half maximum for preferred orientation (111) direction increase, i.e. decrease the crystallite size (G.S), and microstrain ϒ increase with increase annealing temperature [14].Fig. 4 illustrates the calculation of full width at half maximum ( ) by Gaussian Fitting to argon atom (Ar1) for wavelength (750.38)nm.It can be noted that the ( ) increase with increase voltage from 800 to 1200 V as shown in Table 3.  Table 4 shows The temperature of the electron T e was increased by increasing the voltages between the cathode (Cd) and the anode up to (1.135) eV at (1200) volt, which is a large temperature, but it was considered a position (the thermal capacity was small) The increase in the numerical density of the electrons was caused by the increase in the ionization probability (the cross-section of ionization), which was related to the increase in the energy of the electrons (Fig. 5).The length of the debye decreases with increasing voltages and increasing the electronic temperature and electronic density according to Eq. ( 6), while the plasma frequency values are increased by increasing the voltages and numerical density of the electrons according to Eq. (7).

Conclusions
The cadmium oxide films prepared by D.C plasma magnetron sputtering with the voltage (1200) V, at the temperature of (523, 623) K, are polycrystalline and cubic type and the preferred orientation along (111) direction matching with standard card No 96-900-8610 (JCPDS).andalso the peaks for cadmium element hide due to annealing in oxygen and which regarded it one of method prepare cadmium oxide.
Also, the microstrain increases by increasing the temperature of the annealing and the reason was the growth of the film, and thus the expansion of the lattice, which leads to a change in the vertical distance between the atoms.
Plasma diagnosis in the spectral analysis showed that electronic temperature and electron density increase by increasing voltages, and that the increase in the electrons density is caused by the increase in ionization probability, (The crosscollision of ionization), which is related to the increase in the energy of the electrons, and that the length of the debye decreases while the frequency of the plasma becomes more directly proportional "with the electronic density.

Fig. 3
Fig.3 illustrates the optical emission spectrum (OES) of the plasma used during this research, shows different

Fig. 3 :
Fig. 3: OES to plasma diagnostic between intensity as a function of wave length for different voltage (800-1200) Volt.

Fig. 5 :
Fig. 5: The variation of electron temperature and electron density with DC voltages.