Effect of Annealing Temperature on Optical and Electrical Properties of ZnO / Ag / ZnO Multilayer Films for Photosensor

In this study, transparent thin films of ZnO/Ag/ZnO were deposited on Corning glass substrates by radio-frequency (RF) magnetron sputtering, and the effect of different annealing temperatures on the transmittance, electrical, and crystalline properties of the grown films was investigated. The results showed that the multilayered films exhibited a transmittance of approximately 80% at an annealing temperature of 300 °C. The resistivity (3.95 × 10−5 Ω-cm) was also the lowest at 450 °C, the mobility was 14.7 cm2/V-s, and the carrier concentration was 5.75 × 1021 cm−3. The experiments showed that as the annealing temperature was raised, the transmittance increased up to 450 °C. After this, the transmittance decreased. However, resistivity continued to drop with further increase in temperature, accompanied by an increase in crystal grain size.

The conductivity of ZnO thin films can be greatly improved by doping ZnO with another element, and multilayered films using metals such as Ag, Cu, Ti, and Al have received much attention.It has been found that Mo-doped zinc oxide is very useful for the fabrication of a diversified range of devices.(19) The element Ag is another choice since it offers the highest electrical conductivity.At the same time, the inclusion of a Ag layer within ZnO layers may lead to the enhancement of optical characteristics. (20,21)adio-frequency (RF) magnetron sputtering has many advantages for the deposition of thin films, including low substrate temperature, good surface roughness, and low cost.However, the properties of the deposited films are largely dependent on the sputtering parameters.In this study, a Ag metal layer was deposited between two ZnO thin-film layers.Sputtering was done at room temperature and resulted in a transparent ZnO/Ag/ZnO conductive multilayer film.The effect of annealing temperature on the microstructural, electrical, and optical characteristics of ZnO/Ag/ZnO multilayer films deposited on the glass substrates were investigated.

Materials and Methods
An RF magnetron sputtering system (I-Shien) was used with ceramic ZnO targets and a pure Ag (99.99%) target at room temperature at a base pressure of less than 6 mTorr.The Corning glass substrate used was cut into suitable sizes and chemically cleaned using acetone before use.The RF sputtering was carried out without heating the substrate; there was no oxygen present and the parameters were carefully controlled.The ZnO/Ag/ZnO multilayer films on the glass substrate were then annealed for 40 min at temperatures of 200, 300, and 450 °C.The specimens were examined using an X-ray diffractometer (XRD), and the XRD patterns were recorded.The photoelectric properties such as light transmittance, resistivity, and carrier concentration were measured using a spectrophotometer (UV Solution 2900) and a Hall measuring instrument.Finally, the surface morphology of the films was examined by scanning electron microscopy (SEM; JEOL 7001) and the grain size was measured using the SEM measuring tool.

XRD structural analysis
The XRD patterns of ZnO/Ag/ZnO thin films deposited on glass substrates at different annealing temperatures are shown in Fig. 1.It can be clearly observed from the figure that all the multilayer samples have diffraction peaks at (002) that correspond to a ZnO plane.As the annealing temperature increases to 450 °C, another peak (103) appears also corresponding to a ZnO crystalline structure.Furthermore, we can also see the (111) peak of the Ag element.The increased annealing temperature provides energy that affects the sputtered particles and enhances surface mobility, which, in turn, improves the crystalline structure.

Electrical analysis
Figure 2 shows the resistivity, mobility, and carrier concentration of the ZnO/Ag/ZnO multilayer films annealed at different temperatures.It can be seen that the resistivity of the films decreases with an increase in annealing temperature, and the films have the lowest resistivity at an annealing temperature of 450 °C.This may be due to a decrease in the oxygen vacancy concentration.The resistivity is 3.95 × 10 −5 Ω-cm for an annealing temperature of 450 °C.This high annealing temperature also improves the carrier concentration and mobility.

Transmittance analysis
Figure 3 shows the optical properties of ZnO/Ag/ZnO multilayer films annealed at different annealing temperatures.The transmittance over a range of wavelengths from 200 to 800 nm gradually increased with an increase in annealing temperature, and the average was slightly higher than that of the unannealed film.When these thin films are annealed, a phenomenon in which the grains are reformed occurs, which changes the energy gap slightly.With more energy being applied because of the higher temperature, the rate and extent of reforming change.450 °C seems to be the ideal temperature for the optimization of the material properties.The optical band gap (E g ) of the multilayer films can be calculated by considering the electron transitions from the valance band to the conduction band caused by the absorption of one photon of energy hv by the thin film.For semiconductors with a direct band gap, the absorption coefficient is given by Shan et al. as (22)   ) , where a is the absorption coefficient, E g is the optical band gap energy, h is the Planck constant, and m is the photon frequency.Figure 4 shows the calculated optical band gaps for annealed ZnO/Ag/ZnO multilayer films.The band gap increased from 3.21 to 3.31 eV as annealing temperature increased.The increase in the band gap of ZnO/Ag/ZnO thin films may be due to the defects introduced after Ag + is substituted for Zn 2+ and to the differences in the electronegativity and annealing temperature. (14,23)

Surface morphology analysis
Figures 5(a)-5(d) show SEM images of the surfaces of four different ZnO/Ag/ZnO thin films.Figure 5(a) is the image of an unannealed film surface, and Figs.5(b)-5(d) are images of the surfaces of films annealed at 200, 300, and 450 °C, respectively.It can be seen that the grain size increases with rising annealing temperature.We also investigated the growth of sputtered particles using the XRD patterns.Equation ( 2) below enables us to determine the grain size of the material.D is the grain size, β is the XRD peak full width at half-maximum (FWHM), λ is the wavelength of the incident X-ray, cosθ is the diffraction angle of incident light, and λ and cosθ are constant values.Equation (2) implies that the decrease in FWHM indicates the increase in grain size, so β becomes the main cause of grain size.As shown in Fig. 6, it can be seen that the grain size increases with annealing temperature, in agreement with the SEM analysis.

Conclusions
ZnO/Ag/ZnO multilayered thin films can be easily deposited as transparent conducting coatings by RF magnetron sputtering.Transmittance is relatively high in the visible range and increases slightly with annealing temperature.These multilayer films can be optimized to have a low resistance (3.95 × 10 −5 Ω-cm) by annealing at a temperature of 450 °C.The results indicated that ZnO/Ag/ZnO multilayer films have good transmittance and the highest responsivity with a narrow band gap and low resistivity.The multilayer film may be used as an optical sensor in optoelectronic applications.