Thin films of Zn 100-x Cd x O with x = 0, 2, 4, 6 and 8 at% were deposited by electron beam evaporation technique on glass substrates. The structural, optical and electrical properties of Zn 100-x Cd x O films with x = 4 at% have been investigated as a function of annealing temperature. Only zinc and cadmium appeared in the as-deposited films, by annealing their oxides found to exist. It was observed that the optical properties, such as transmittance, reflectance, optical band gap, and refractive index of Zn 100-x Cd x , were strongly affected by annealing temperature. Other parameters named free carrier concentration , electrical resistivity of the films were studied as a function of annealing temperature for 4 at% CdO content. The figure indicates that the best value of annealing temperature is at 450℃.
Zinc and cadmium play an important role as alloying elements in many superconductor and semiconductor alloy properties [
Transparent conducting oxides (TCOs) are a class of materials that transmit visible radiation and conduct electricity. They find application as transparent electrodes in numerous applications such as photovoltaic, flat panel displays, heat reflective coatings on energy-efficient windows, and electrochromics such as smart mirrors [
Electrical transport properties of CdO-ZnO thin film heterostructure have been studied. ZnO thin film deposited on conducting CdO thin film forms an n-n isotype heterostructure. The ZnO layer achieved significantly high electrical conductivity. A thin conducting channel is created in the interface at the ZnO side. Band bending near the interface creates electron accumulation at the ZnO side giving rise to a highly conducting channel at the interface [
This work focuses on preparing Zn100−xCdxO thin films using e-beam evaporation method for CdO content at 0, 2, 4, 6 and 8 at%. The characterization, optical and electrical measurements will be carried out. The figure of merit will be studied as a function of annealing temperature.
For preparing Zn100−xCdxO (x = 0, 2, 4, 6 and 8 at%) bulk samples, proper quantities of ZnO (99.98% purity) and CdO (99.95% purity) were sealed in an evacuated quartz ampoule and heated at 500˚C. Continuous stirring of the melt was carried out to ensure good homogeneity. The melt was then rapidly quenched in ice water. Thin films of the considered ratios were prepared by electron beam evaporation in an Edward’s high vacuum coating unit model 306A under pressures of 5 × 10?6 and 8 × 10?5 Torr before and during film deposition, respectively. The films were deposited on an ultrasonically cleaned microscopic glasses held at room temperature. The thickness of the films (≈200 nm) was controlled using a digital film thickness monitor model TM 200 Maxtek. The deposition rate was ≈12.5 nm/s and then annealed in air for 1 h.
A Jasco model V-570 (UV-Visible-NIR) double beam Spectrophotometer (with photometric accuracy of ±0.002 - 0.004 Abs. and ±3% Trans.) was employed to record the transmission T and reflection R spectra over the wavelength range from 200 to 2500 nm at normal incidence. The absorption coefficient α of the films was determined directly from the spectrophotometer readings using the formula [
α = 2.303 d log 10 ( 1 − R T ) (1)
where d is the film thickness, T is the transmittance and R is the reflectance of the film.
The optical energy band gap Eg was estimated from the optical measurements by analyzing the optical data with the expression for the optical absorbance, and the photon energy, hυ using the following equation:
( α h υ ) 2 = A ( h υ − E g ) (2)
where α represents the absorption coefficient, h is the Planck s constant, and A is a constant, the values of Eg were obtained by extrapolating the linear portion of the plots of ( α h υ ) 2 versus h υ to α = 0.
The refractive index n was calculated from the following equation:
n = 1 + R 1 − R ± [ ( R + 1 R − 1 ) 2 − ( 1 + k 2 ) ] 1 / 2 (3)
where k = αl/4p is the extinction coefficient and l is the incident light wavelength. In the present work, more reasonable values for n may be determined by considering the plus sign of Equation (3).
The resistivity measurements were carried out using a two-terminal configuration where the measurements were done at room temperature. Electrical contacts were made by applying silver paste over the surface of the films with a separation of 2 mm.
The X-ray diffraction patterns of annealed Zn100?xCdx (x = 0, 2, 4, and 8 at%) thin films at 300˚C for 1h are shown in Figures 2(a)-(d) indicate that, it possesses a polycrystalline hexagonal wurtzite structure with a preferred orientation along the ZnO2 (200) plane at 2θ = 36.96. For x = 0.0, the peaks appear at 2θ = 33.88˚, 35.02˚, 36.96˚, 39.71˚, 43.94˚ and 77.62˚, which correspond to Zn (002), (100), ZnO (200), Zn (100), ZnO2 (101) and (004) planes respectively as seen in
The X-ray diffraction patterns of annealed Zn100?xCdx (x = 0, 2, 4, and 8 at%)
films at 500˚C for 1h are shown in Figures 3(a)-(d) indicating that, it possesses a polycrystalline hexagonal wurtzite structure with a preferred orientation along the ZnO (002) direction. For
appear at 2θ = 31.50˚, 34.17˚, 35.95˚ corresponding to ZnO in the direction (100), (002) and (101) plane. For x = 2%
Doping Cd with x = 4% as seen in
From above, it is clear that Zn and Cd only appeared in the X-ray diffraction of the as-prepared films, whereas, ZnO and CdO are shown after annealing at 300˚C for 1 h, besides, ZnO and Cd exists in the X-ray diffraction patterns with annealing temperature 500˚ results in
The typical UV-VIS-NIR optical transmittance and reflectance spectra of annealed films (Zn100−xCdx with x = 4 at%) at different annealing temperature as a function of wavelength in the range from 300 to 2500 nm are shown in
From the transmittance data it is possible to infer the optical energy gap of the films by plotting (αhυ)2 vs hυ (where α is the absorption coefficient, and hυ the photon energy) and by extrapolating the straight line portion of this plot to the energy axis. The obtained values have been plotted in
3.31 eV, which can be explained as follows: the unsaturated defects are gradually annealed out producing a larger number of saturated bonds leading to decreases in the density of localized states and consequently the optical gap increased [
The variations of refractive index and the extinction coefficient in the visible wavelength range of (Zn100−xCdx with x = 4 at%) films annealed at different temperature are depicted in
The packing density P of the film can be estimated from the following equation [
n f 2 = ( 1 − p ) n y 4 + ( 1 + p ) n y n s 2 ( 1 + p ) n y 2 + ( 1 − p ) n s 2 ,
where nf is the refractive index of Zn100−xCdx with x = 4 at% films, ns is the refractive index of the solid part of the film, that for single crystal ny Is the refractive index of the voids (equals one for air) and P is the packing density. It is observable also that the extinction coefficient decreases with increasing the annealing temperature. This could be correlated to the decrease of absorption with increasing the temperature of annealing, where K = lα/4p is the absorption coefficient [
The dependence of electrical properties on the temperature of annealing is shown in
to 450˚C showing semiconductor behavior. The lowest resistivity value 5.3 × 10 − 5 Ω-cm has been obtained for annealed film at temperature of 450˚C. It is observable also that, the carriers mobility increased with increase in annealing temperature. This may be due to the increase of the grain size with increase in annealing temperature, and this leads to reduction of the grain boundary scattering due to charge carriers. The decrease of the electrical resistivity of annealed films is due the increase of the mobility carriers.
In order to predict the selective properties of transparent conductive coatings from the fundamental optical and electrical properties, the factor of merit can be employed φ = T m / ρ [
Thin films of Zn100?xCdxO with x = 0, 2, 4, 6 and 8 at% have been deposited by electron beam evaporation technique. The effect of heat treatment on the electrical, optical and structural properties of these films was carried out. For content of cadmium oxide at 4%, optical measurements indicated that, the optical energy gap, the mobility of the carriers, and the figure of merit increase with increasing the annealing temperature. On the other hand, our results indicate that, the refractive index, extinction coefficient, and the resistivity decrease with increasing the annealing temperature. The best value of the figure of merit found to be at annealing temperature 450˚C.
The authors declare no conflicts of interest regarding the publication of this paper.
Alkhammash, H., El-Ghanny, H.A. and El-Raheem, M.M.A. (2018) Structural, Electrical and Optical Characteristics of Zn100?xCdxO Thin Films. Open Access Library Journal, 5: e4687. https://doi.org/10.4236/oalib.1104687