The Effect of Silver Oxide on the Structural and Optical Properties of ZnO: AgO Thin Films

s Compounds from ZnO doped with AgO in different ratio (0,3,5,7, and 9)wt.% were prepared.Thin films from the prepared compounds were deposited on a glass substrate using the pulsed laser deposition method. The XRD pattern confirmed the presence of a single-phase hexagonal wurtzite ZnO structure, without the presence of a secondary phase. AFM measurements showed an increase in both average grain size and average surface roughness with increasing concentration content of (AgO).The crystallite size of ZnO of the main peak corresponding to the preferred plane of crystal growth named (100) increases from 17.8 to 22.5nm by increasing of AgO doping ratio from 0 to 9%. The absorbance and transmittance in the wavelength range (350-1100 nm) were used to determine the optical properties. The results showed that the optical energy gap for direct allowed transition (r =0.5 )are (3.21, 3.2,3.15, 3.15, 2.95 ) Ev, respectively. The optical constants were also measured as a function of wavelength. This optical measurements showed the prepared thin film have many applications for solar cell fabrication and sensing devices .


Introduction
Scientists and industries all over the world are interested in zinc oxide (ZnO) nanoparticles, which are a potential photoluminescence material with strong oxidation capability, nontoxicity, chemical stability, and a phenomenally low cost. It was found that luminescence effects of ZnO nano-particles at primary blue and red changes on their Near Band Edge (NBE) emissions would be affected by doping and visible emission improvements which were made to be long-lasting. [1] Several experimental and theoretical aspects studies in different morphologies like nanoparticles, nanowires, and thin films with nanostructure were carried out due to their special optoelectronic characteristics, electric transducer transducers, instruments optoelectronic, space sensors, sensors, windows of the solar cell, and other applications. Pure ZnO nanoparticles do not meet the practical needs as a photocatalytic material, as activation of ZnO catalyst necessitates high-energy UV light, leading to low efficiency in the visible and near infrared regions. The rapid recombination of the photogenerated e -/h• pairs is another limiting factor that impedes the photocatalytic activity of ZnO nanoparticle [2]. ZnO is a compound semiconductor with a large range bandgap at room temperature (3.36 eV) and a high energy for the excitonic binding process (60eV) [3]. Morphology, development technique, synthesis, conditions of synthesis, and defects of these nanostructures influence ZnO nanoparticles luminescence properties. On semiconductor nanostructures, Ag-doped ZnO particles photocatalyst effect has a distinct effect. Additionally, Ag-doped ZnO particles are bioconsistent, biosafe, and of intracellular phototoxic content [4]. Metal ions doped in Ag-doped ZnO particles are utilized for a variety of applications at room temperature, including n-type doped nanolasers and p-type doping as semiconductor ferromagnetic material. Transition metals with lower levels of Fermi energy are highly absorbed optically, stimulating the mechanism of interfacial electron transfer [5]. The AgO :ZnO thin films can be prepared by a variety of methods [6], such as chemical vapor deposition(CVD) [7], magnetron sputtering [8], pulsed laser deposition(PLD) [9], spray pyrolysis [10] , molecular beam epitaxy (MBE) [11], and sol-gel technique [12]. In this work, the preparation of undoped and AgO-doped ZnO composites deposited on a glass substrate by pulsed laser deposition technique was reported. The structural and optical properties of these films were studied.

Experimental part:
The starting materials in this work: ZnO and AgO (of high purity, three nines after digit)were provided by Aldrich. The compounds with different doping ratios were weighted according to the atomic weight using an electronic balance with four digits after the point (10 -4 gm)company. The mixtures of the two oxides were sintered by putting the mixture in quartz ampoule with length ~ 25 cm and internal diameter ~ 8 mm, evacuated to ~10 -3 Torr, sealed and put in an oven and heated to 1273K for 5 hours,after which they were left to cool at room temperature. The obtained materials were grounded to obtain a homogeneous powder and pressed into pellets, with a diameter of 1 cm and 0.5 cm thick, with hydraulic piston( type (SPECIAL), under a pressure of 7 tons for 10 min. AgO doped zinc oxide thin films with various doping ratios (0, 0.03, 0.05,0.07, and 0.09) were deposited using the pulsed laser deposition technique. The deposition was done under vacuum of 2x10 -2 Torr with Nd: YAG laser beam ( of energy, pulsed, and frequency of 500 mJ,500, and 6Hz, respectively)which was focused through a window to be incident on the target. The ablated atoms were incident on the glass substrates and get gathered to create the thin films.

Abbas and Hasan
Iraqi Journal of Science, 2022, Vol. 63, No. 4, pp: 1526-1539 The optical interferometer method was used to determine the film thickness (t) using the following equation [13]: Where Δx, x, and λ are the shift of the interference fringes, the distance between the interference fringes, and the wavelength of He: Ne (632.8nm), respectively. The XRD measurements were conducted at (the Iranian Republic laboratories) using a diffractometer (type XRD / 6000 Shimadzu) with Cu-Kα (λ= 1.5404 Å) radiation, operated at 30 kV, 20 mA. The average crystal size (D) was calculated using Scherrer formula [14] : where β and θ are the full widths at half maximum and the glancing angle, respectively. The dislocations density and the strain δ were determined by applying the following relations: respectively. The lattice constant a for (100) plane was determined by the following relation [15] , While, the lattice constant c for the (002) plane was determined by the relation, [15] c= λ/ sinθ - At normal incidence , a double-track spectrometer (A double -beam UV/Vis spectrophotometer (Metertech) SP8001) was used to obtain the optical transmission and absorption spectra for the pulsed deposited film samples from the deposited thin films with an absorbent computer data collection and wavelength transmission range between 200 and 1100 nm. The absorption coefficient of the thin films was determined by applying the given relation [16]: where t and A are the thickness and absorption of the thin film sample,respectively. The optical band gap for direct allowed transition was determined by applying Tauc relation [17]: where hν, E gopt , and B are the photon energy, the optical band gap, and band tailing parameter, respectively, Real and imaginary dielectric constants were computed by applying the following relations [16]: -----------(10) using the concepts of refraction index (n) and extinction coefficient (k), the optical properties of semiconductors can also be determined using the following relations. [18]: Where R and λ are the reflectance and the wavelength of incident photon, respectively Results and Discussion: X-ray diffraction analysis is known as a good criterion to know the phase of the prepared samples. Figure 1 shows the X-ray diffraction patterns of ZnO prepared with different AgO doping ratios which were compatible with the standard diffraction peaks of wurtzite structure ZnO (JCPDS 96-230-0114). The observed diffraction peaks matched the (100) (002), and (101) reflections of the ZnO wurtzite structure [19] [20]. No peaks corresponding to the AgO phase were found which demonstrates that single phase of AZO was formed at low doping ratio i.e up 7%. Tiny peaks related to AgO were observed at a 9% doping ratio. The observed diffraction coincides with the (20-2) and (31-1)of AgO monoclinic structure standard cards (96-900-8963). The X-ray diffraction data also showed that the reflection peaks intensity of the doped films depended strongly on the doping ratio. ZnO thin films and residual doped thin films exhibited preferred (100) orientation. As the doping ratio was increased the (100) and (101) reflections of the film increased. This phenomenon can be attributed to the diffusion of atoms absorbed on the substrate and are accelerated to the migration of atoms to more energy favorable positions, resulting in the enhancement of the polycrystallinity of film and initiating grain growth which is indicated by the increment of (100), (101) reflections. Indeed the crystal size had grown from 17.8 to 22.4 nm by increasing the doping ratio from 0 to 0.09%. It can be noticed that the diffraction peaks positions of doped thin films with 3%AgO as compared to that of undoped ZnO had shifted toward larger angle direction, indicating a decrease of the unit cell lattice parameters resulting from the incorporation of Ag (0.57Å), with smaller atomic size than Zn (0.60Å) [21] .The peak clearly shows that the incorporation of Ag dopant with low ratio leads to a clear reduction in crystallite size. In contrast , the diffraction peaks of the 0.05, and 0.07 doping ratio samples shifted to smaller diffraction angles indicating an increase of the unit cell lattice parameters [17]. In the literature review, Ag can integrate conformity into the ZnO, which replaces Zn 2+ or as an intermediated space atom [22], according to the findings of others. The Ag dopant as a replacement for Zn 2+ helps to bring the XRD closer to the maximum [23]. By increasing AgO concentration, peaks positions were shifted , as shown in Table 1.This suggests that adding AgO to zinc oxide at low doping ratio (3%) leads to an intermediated space atom and a reduction of crysttalite size, while doping of ratios (5, 7, and 9 %) suggests the partial substitution of Ag+ ions at the ZnO lattice and increase in the crysttalite size. This issue is presumably related to the ionic size difference between the Ag+ (0.126 nm) and Zn 2+ (0.074 nm) ions [24]. The dislocation density of the prepared thin films exhibits to change in opposite to that of crystal size. The dislocation density and the strain was reduced from 3.15 x1015 to 1.98 x1015 (line / m2) and from 0.0019 to 0.0015 by the continuous addition of silver oxide to the host oxide (ZnO).    The atomic force microscope 3D images, shown in Figure 2, shows pure zinc oxide films doped with silver oxide of different ratios ( 3,5,7,9) wt%. It was noticed that the increase in the ratio of silver oxide increased the surface roughness as well as the ( RMS).They increased to values of 9.25 nm and 10.7 nm,respectively at a doping ratio of 9wt%. These values and those of the other AgO doping ratios are shown in Table 2. This result is in agreement with that of Karyaoui et al. [25]. Figure 3 shows the transmission spectrum of ZnO: AgO thin films with different doping ratios(0, 3, 5, 7 and 9 wt.%). It is clearly shown that the addition of AgO to ZnO led to a redshift, i.e. the absorption edge shifted towards longer wavelengths which is accompanied by a substantial reduction of the transparency of the thin films. Optical transmission is a well-known complicated function that is strongly influenced by the absorption coefficient (α). Figure 4 shows the relation between (αhν) 2 and (hν). By extrapolating the best fit linear part of the curve, the optical band gap Eg opt can be estimated. Our results showed that the optical band gap was reduced by increasing the silver oxide concentration up to 7wt% and then the blue shift takes place which can be due to transitions from different defect levels of metal oxides to the O 2p band [26]. Sutanto et al. [27] pointed out that AgO-doped ZnO edge moved toward the visible light wavelengths. This optical observation showed a very good thin film for photocatalyst and other applications. While Ali et al. [28]refered that the energy band gap of the pure and doped ZnO thin films increased with the AgO doping. This increase of the energy band gap is known as the Moss-Burstein shift [29] . Because of this, at high doping levels of zinc oxide films, the donor electrons occupy the states at the lower edge of the

Abbas and Hasan
Iraqi Journal of Science, 2022, Vol. 63, No. 4, pp: 1526-1539 The structure of the ZAO System may explain this variation in the optical bandgap. The relation between the refractive index n and the extinction coefficient k and wavelength are shown in Figures 5 and 6. It is observed that n and k grow up by the addition of AgO up to 5% then fall to lower values but then return to grow up again. The addition of AgO makes the prepared AZO more compact (increasing the packing density), which decreases the propagation velocity of light through the sample, thus increasing the n values, since n represents the ratio of light velocity through the vacuum to velocity through any medium.While, the behavior explanation of k is due to the growth of absorption coefficient, which represents the ratio of light velocity through the vacuum to velocity through any medium. The reduction of n and k with the further increase of AgO concentration i.e. to (7%) can be explained as follows :the addition of AgO makes ZnO less dense (or more transparent) which consequently decreases the packing density and in turn increases the light velocity through the sample and so reducing the refractive index.The reduction of k is attributed to the reduction of absorption coefficient .

Abbas and Hasan
Iraqi Journal of Science, 2022, Vol. 63, No. 4, pp: 1526-1539 7253 Figure 6: The plot diagram of K versus wavelength for ZnO:AgO thin films prepared by PLD technique. Figures 7 and 8 show the relation between ε r and ε i and λ. It is remarked that ε r and ε i have the same trend of n and k, respectively thus similar explanation can be given.

Conclusion:
Pure and AgO-doped ZnO thin films were successfully fabricated by pulsed laser deposition method. The XRD patterns showed wurtzite structure for all the prepared thin films samples and no impurity phase was noted. The maximum crystallite size obtained from XRD was 22.4 nm. The UV-Visible results revealed that absorption underwent a redshift with AgO into ZnO as compared to pure ZnO exhibiting strong quantum confinement effects. Optical band gap energy was found to decrease from 3.21 to 2.91 eV with AgO doping, resulting in the increment in their crystallite size as a result of Ag doping. It could be concluded that defects play an important role in manipulating properties of ZnO thin film