Effect of lithium on linear and nonlinear optical properties of Sn-doped zinc oxide prepared by spray pyrolysis

Li and Sn codoped ZnO (LTZO) thin films have been successfully deposited on heated glass substrates at 450°C using the spray pyrolysis technique, the effect of lithium of Sn-doped zinc oxide on the structural, morphological, optical and nonlinear optical properties was investigated using X-ray diffraction, transmission, the RMS average surface roughness, and third harmonic generation (THG). The value of optical band gap Eg was found to be decreased from 3.24 eV to 3.16 eV when the concentration of Li from 0 to 7%, while the concentration of Sn is fixed at 2%.The doping of ZnO films improves the nonlinear response and the highest susceptibility value χ (3) =13.422 × 10-12 (esu) is found at concentration of lithium 7%.


Introduction
Zinc oxide (ZnO) thin films have been extensively studied and used for different domains such asmicroelectronic and optoelectronic applications [1], with direct band gap of 3.37 eV, high exciton binding energy of 60 meV at room temperature, is attractive for optoelectronic application in the blue and UV region, especially the data storage , which is the main work of the year , the blue emission generated to produce the new light by physics Nobel laureates [2], it can be used also at transparent conductive oxide thin films such as for application in solar cells [3]. Furthermore, the effect of ZnO doping on its nonlinear optical properties have been studied for several decades [4,5,6]. It was shown that the Sn doping of ZnO improves its electrical and structural properties [7],and the doping by Li was shown to be attractive for modifying the microstructure , ferroelectricity and optoelectronic properties [8] and enhance the rate of the crystal growth due to an increase of its diffusivity [9]. Furthermore, Yousefi et al [10] have reported that the crystallite and particle sizes can be increased by doping with group-I elements, reduce the oxygen vacancies in ZnO Lattice, Behera et al [11] reported that Li doping has always been in controversy, since Li ion can take up at interstitial sites, thus acting as shallow donor and compensate the acceptor action (self-compensation). Due to unequal ionic radius of lithium (0,68 Å) and zinc (0,74 Å) atoms, lithium is also expected to occupy substitution sites and thus can influencelately the structural and optical properties [11].Li-Mg [12],Li-Al [13], Li-N [14], Cu-Li [8] co-doping of ZnO has also been investigated. In this work, Li and Sn co-doped ZnO (LTZO) thin films were prepared using a spray pyrolysis technique, where the deposition temperature, the used precursors, concentrationsthe spray rate most be controlled [15],and its influence on the linear and non-linear optical properties is studied and discussed.

Experimental detail
LTZO thin films were grown on glass substrates and heated at 450°C by spray pyrolysis, experimental setup has been described elsewhere [5]. The spraying solution was 0.05M zinc chloride. The doping was achieved by adding SnCl 2 in a concentration is fixed at 2%. Different concentration of Li were used for co-doping the ZnO by adding LiClO 4 in a concentration of 3, 5 and 7 %. The spray time was 3 min with solution flow rate of 5 ml min −1 and the distance between the nozzle and substrate distance was fixed to be 40cm. The structural properties of thin films were investigated using X-ray diffractometry (X'Pert³ Powder PANalytical diffractometer) with Cu (Kα) radiation (λ=1054Å). The optical transmission measurements were performed with Jasco V-570 UV/Vis/NIR spectrophotometer. Nano-surf Flex FM digital instruments operated in the tapping mode was used for the AFM imagesand the Third order nonlinear susceptibility was performed using third harmonic generation (THG) method [6]. Figure 1 shows the XRD patterns of LTZO films with an atomic concentration of 2% Sn and different concentrations of Li (0, 3, 5,7%). From the results, we can see that all films are polycrystalline with a hexagonal wurtzite structure [16].   Table 1. Lattice parameters, FWHMs and average crystal size obtained from XRD patterns Figure 2(a). Depicts the optical transmission spectra of LTZO thin films. All films exhibit an average optical transparency exceeding 85% in the visible region, the average transmittance was increased after Li/Sn codoping, reaching the maximum for Li and Sn concentration of 7% and 2% respectively. In effect Fellahi et al [17] reported that, the light scattering decreases with the improvement of the crystallinity of the ITO films. In fact the crystallinity probably affects the light scattering on the transparency of the LTZO films. Figure 2(b) illustrates a plot of (αhν) 2 vs. photon energy hν for the LTZO films. The extrapolation of the slopes of the curves, gives the optical band gap Eg. The values of optical band gaps decreases from 3.24 to 3.16 eV with increasing Li concentration from 0 to 7% .It has been reported that the increased concentration of Li in a Li/Al co-doped ZnO can induce a resonance structure in the density of state, which slipt off theband [13].  The transparency of ZnO at 1064 nm, which is the laser excitation wavelength, makes it a good candidate for nonlinear optical applications [18]. Figure 3 shows the THG spectra of LTZO films. The third-order optical nonlinearities of Li-Sn codoped ZnO (LTZO) films were investigated by THG technique in order to study the effect of lithium on nonlinear optical properties of Sn-doped zinc oxide thin films, Figure 3 exhibit Maker-fringes obtained by rotating the samples through the range from ± 60° to the normal.

Results and discussion
The χ (3) can be expressed as [19] Where theI 3ω is the THG intensity, the suffix s refers to the reference material, as silica. Lc,s is the coherence length of microscope slide which is around 18μm. The value of the nonlinear susceptibility is about: (3) = 5. 7841×10 −12 (esu).
The values of χ (3) calculated using the formula (1) are summarized in the table (2) it can be seen that the Li-Sn co-doping of zinc oxide thin films improves the nonlinear response, and the highest susceptibility value χ (3) =13.422 ×10 −12 (esu) was found at concentration of lithium (7%),which can be explained by the fact that the surface smoothened by the Li doping as shown in the table (2),so the light diffusion is reduced, this agree with the increase of the transmission of the films when the Li concentration is increased as shown in the figure 2.a.Moreover,as we showed in previous work [16]the scattering of light is strongly affected by the roughness and the structural properties of the films, While can be deducted that the smoothening of the surface reduce the scattering of light and generates the highest nonlinear optical response .Same behaviour was reported by Addou et al [20].

Conclusion
LTZO thin films have been deposited by spray pyrolysis technique. the films were polycrystalline with an hexagonal wurtzite structure, with an average optical transparency exceeding 85%in the visible range .The optical band gap Eg was found to decrease from 3,24 to 3,16 eV with increasing Li concentration from 0 to 7% .It was found that the codoping of ZnO thin films improves the nonlinear response .The highest susceptibility value χ (3) =13,422 ×10 −12 (esu) was found for the highest concentration of lithium (7%),which indicates the correlation between the structural, optical and morphological properties of co-doped ZnO thin films with the nonlinear optical properties .

Acknowledgment
This work is supported by Hassan II Academy of Science and Technology, ERANETMED-NInFEE and PPR 2015/9 Morocco