Effect of annealing temperature on the optical properties of Sb-ZnO thin films prepared using co-sputtering technique

Transparent conducting oxide thin films of Sb-ZnO were prepared on optically flat quartz by radiofrequency (RF) sputtering method. The scan electron microscope was used to characterize the topological morphology of the surface of the as-prepared and annealed films at (300, 400, 470, and 525°C) for 4 h in air. The optical properties of the films were deliberated using their reflectance and transmittance spectra at normal incident light. The optical energy band gap energy (Eop ) values were found to increase by elevating the annealing temperatures. The dispersion curves of the refractive index of Sb-ZnOthin films were found to follow the single oscillator model. Optical parameters such as refractive index, real and imaginary parts of the dielectric constant, and optical conductivity were investigated.


INTRODUCTION
Zinc oxide is an auspicious material for optoelectronic devices due to its big band gap (3.37 eV).The n-type zinc oxide materials can be acquired by doping with Aluminum, Gallium or Indium.Besides, p-type ZnO considered to be low resistivity and high mobility it is hard to be fabricated with good quality, where, it is related to the construction of native donor defects such as Oxygen vacancies and Zinc interstitials (Look et al., 1999).The most used acceptor dopants for p-type zinc oxide is antimony Sb, nitrogen, and phosphorous (Minegishi et al., 1997;Lu et al., 2004;Joseph et al., 1999;Chen et al., 2005;Limpijumnong et al., 2005;Zhao et al., 2003).Doping ZnO with Sb was supposed to substitute Zn atom (Limpijumnong et al., (2004).Xiu et al. (2005) have carried out p-type ZnO:Sb film by molecular beam epitaxy and pulsed laser deposition (Xiu et al., 2005;Pan et al., 2007;Zi-Wen et al., 2010;Liang et al., 2015) confirming that Sb is a promising dopant for realizing p-type zinc oxide.Doping zinc oxide with tin oxide reveal that, increasing the content of tin oxide, ZnO nanocrystal changed from near spherical to dumbbell-like (Duan et al., 2017).Thermal annealing processing is used to enhance the properties of semiconductor material.Electro-deposition of Sb 2 S 3 absorber on TiO 2 nanorod array as photocatalyst for water oxidation has been investigated (Hong et al., 2018).As far as the author E-mail: drhalkhammash@gmail.com, khamash.h@tu.edu.sa.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License know, the effects of thermal annealing on Sb-doped ZnO thin films are rarely reported.So, this work focused on the effect of thermal annealing on the optical properties of Sb-ZnO.

EXPERIMENTAL TECHNIQUE
Thin films of Sb-ZnO were deposited on pre cleaned quartz substrates using sputtering unit model UNIVEX 350.The targets of ZnO and Sb are from Cathay Advanced Materials Limited Company.The base pressure of about 10 -6 torr and sputtering pressure of about 2×10 -2 torr.The distance between the substrate and target was 10 cm with an angle 65°.The standers cubic centimeter per minute (sccm) was kept constant at 20 cm 3 /min with rotation of substrate 2 rpm.The power on ZnO and Sb targets was kept constant of 100 W and 20 W respectively.The rate of deposition was kept at 2 nm/min.The thickness of the films were determined using multiple-beam Fizeau fringes in reflection (Tolansky, 1949).The scanning electron microscope (SEM) (Hitachi S4700) was used for characterizing the surfaces of the films.The double beam spectrophotometer (JASCO model V-670 UV-Vis-NIR) was used for detecting the transmittance T(λ) and reflectance at R(λ) at nearly normal incidence in the range of wavelength 300 to 1800 nm.The absolute values of T(λ) and R(λ) are given by El-Nahass (1992).
since Ift is the light intensity passing through the film and substrate, Ig is the light intensity passing through the reference, and Rg is the substrate reflectance, and the reflectance R is as follows: Im is the light intensity reflected from the reference mirror, Ifr is the light intensity reflected from the sample and Rm is the reflectance of the mirror.
In order to estimate the optical energy gap in the absorption region of the spectra, the absorption coefficient α and the absorption index, K, of the films at different wavelengths can be calculated using the following equations (Giulio et al., 1993;El-Nahass et al., 2010a, b): where t is the film thickness.

RESULTS AND DISCUSSION
As shown in Figure 1a, the scan electron micrograph of the as-deposited film contain big grains besides, the films annealed at 400 and 525°C show more tighter crystal grains and the grain volume became smaller as shown in Figure 1b, c.This change in grain size is due to annealing which gaining the atoms of the thin films extra energy, and enhance crystallinity of the films; also, annealing can activate the Sb-as an acceptor (Zhao et al., 2011).
The transmittance spectra of Sb-ZnO thin films are shown in Figure 2 which reveal an excellent surface quality and homogeneity of the films due to the appearance of interference fringes (Abd El-Raheem et al., 2009).It is observed that, sharp interference fringes appeared and indicated that the air/layer and layer/glass interfaces are flat and parallel (El-Nahass et al., (2010b).Figure 2 shows also that the transmittance increased with elevating the annealing temperature, this is attributed to the decrease of the size of the particle.
The optical energy E op was depicted from Figure 3 representing the plots of versus (hν) revealing that the direct optical gap widened with elevating the annealing temperature, this may be due to atomic rearrangement during the annealing process.Therefore, some defects will be removed leading to minimizing the density of dangling bonds causing the widening of optical gap (Mansour et al., 2010).Another interpretation of this widening may be due to an enhancement in the crystalline structure of the film, since, if the film becomes more polycrystalline, a decrease in the band gap defects leading to band gap band gap broadening (Atta et al., 2016).Using Swanepoel, (1983, 1984) and Manifacier et al. (1976) methods, the refractive index of refraction n can be calculated.The index of refractive n of the thin films can be calculated using the equations: where T s is the substrate transmission, T M the maximum of the transmittance curves, T m is the identical minimum determined at the same wavelength λ. Figure 4 displays the refractive index spectra for the Sb-ZnO films suggesting normal dispersion behavior.Furthermore, n decreases with raising the annealing temperature according to increasing the transparency of the films with increasing annealing temperature (Mohamed et al., 2006), which is affirmed by our results.factor.The dispersion and absorption spectra for antimony doped zinc oxide thin films prepared under different annealing temperatures are inspected in Figures 5 and 6 respectively.It is evident that ε 1 behaves as the n as seen in Figure 5 ε 2 fundamentally shows a decrease with wavelength and then increase with prolongating the , where the permittivity of frees pace is ε o .The spectra of ζ 1 are shown in Figure 8.It can be seen that ζ 1 increases by increasing photon energy as shown in Figure 8 which can be owed to the excitation of the electrons by photon energy (Shaaban et al., 2006).
The surface and volume energy loss functions (SELF and VELF) can be calculated by using the relations (El-Nahass et al., 2014): As seen in Figure 9 the VELF decrease with raising the photon energy at low range of energy and then increases with raising the energy of the photon e.Furthermore, Figure 10 indicate that SELF behaves as VELF.Using the theory of Wemple and DiDomenico (1971), the dispersion energy E d and the single oscillator energy E o can be calculated using the following formula: (10) Since, (hν) is the incident photon energy.
The dispersion and the single oscillator energies are obtained from the slope and intercept of the plot (n 2 -1) -1 versus (hν) 2 as seen in Figure 11 for the Sb-ZnO thin films.The values of E d and E o are 30.5, 68.4, 68.5, 70.9, 62.3 eV, and 29.3, 13.4, 13.6, 14.1, 12.4 eV for the asprepared and annealed film at 300, 400, 475, and 525°C respectively.It is obvious that the dispersion has a tendency to increase with raising the annealing temperature, whereas the single oscillator energy has a   tendency to lower with elevating the annealing temperature.

Conclusion
For preparing antimony doped zinc oxide thin films, the Co-sputtering technique was used.It was found that the optical gap increases with raising the annealing temperature.Normal dispersion describes the behavior of the refractive index, and optical conductivity increase with raising the incident photon energy.Dispersion energy has a tendency to increase with raising the annealing temperature, whereas single oscillator energy has a tendency to lower with raising annealing temperature.Surface and volume energy loss functions found to depend on photon energy.

CONFLICT OF INTERESTS
The author has not declared any conflict of interests.

Figure 1 .
Figure 1.Scan electro micrograph for Sb-ZnO thin films a-as deposited, b-annealed at 400°C, and c-at 525 °C respectively.

Figure 2 .
Figure 2. Tranmittance spectra for a deposited and annealed Sb-ZnO thin films at different temperatures.

Figure 4 .Figure 5 .
Figure 4. Spectra of the refractive index for as-prepared and annealed Sb-ZnO thin films.

Figure 6 .
Figure 6.Spectra of imaginary part of the dielectric constant for as-deposited and annealed Sb-ZnO thin films at different temperatures.

Figure 7 .
Fig. (5) Spectra of the real part of the dielectric constant for as-prepared and annealed sb-ZnO thin films

Figure 8 .
Fig.(7) Variations of tan  with changing the angular frequency for as-prepared and annealedSb-ZnO thin filrms

Figure 9 .Figure 10 .
Figure 9. Variations of VELF with photon energy for as-prepared and annealed Sb-ZnO thin films.