INFLUENCE OPTICAL, STRUCTURAL AND EXTERNAL MORPHOLOGICAL PROPERTY OF FE SUBSTITUTED IN ZNO THIN FILMS.

structure. UV-Vis spectra of show the decrement the band gap up to 3.18 to 2.92 eVafter the enhancing the Fe concentration. higher wavenumber side, is due change in binding energy of Zn-O bond as a result of replacing Zn 2+ by Fe 3+ and due to the induced strain, phonon confinement byboundaries and force constant changes that occurred by the incorporationof Fe-impurity into ZnO lattice. The enhancingthe intensity of E 2 (high) mode with increase of Fe-content in the films was attributed tothe change in lattice compression arisen due to the substitution ofFe 3+ ions with smaller ionic radius (0.64 Å) in place of Zn 2+ (0.74 Å) in ZnO host. The broadening of the E 2 peak with theincrease of Fe-doping shows that the oxygen vacancies, defects anddislocations were increased in the films. The vibrational mode presentat 538. 76 cm -1 essentially corresponds to E 1 (LO) (at 583 cm -1 ) and A 1 (LO)(at 574 cm -1 ) Raman modes of ZnO. The occurrence of these fundamentalRaman modes corresponds to the hexagonal wurtzite crystalphase of ZnO. The additional vibrational mode present at 608.04 cm -1 isattributed to oxygen vacancies (VO), zinc interstitials (IZn), anti-siteoxygen etc. in ZnO. The observed phonon modes are related to thewurtzite crystal structure of ZnO with a space group of P 63 mc (187). cm which are the belongsto vibrational modes of ZnO. The scanning electron microscopic (SEM) images showed irregular shaped grains grown over the substrate surface for pure ZnO. After Fe doped ZnO thin films exhibits the spherical and hexagonal structure. UV-Vis spectra of show the decrement the band gap up to 3.18 to 2.92 eV after the enhancing the Fe concentration.


ISSN: 2320-5407
Int. J. Adv. Res. 6(10), 1293-1299 1294 et al. [20] also described an increase, of 340 meV, in optical band gap of iron doped nanocrystals prepared by chemical route. n-type semiconducting metal oxides can also be used for gas sensing as they were able to show variation in some electrical parameters, viz. resistance and capacitance of the film upon adsorption of gases. Also, metal oxides are stable at elevated temperatures in air [21][22][23][24][25].In this paper we have study Fe doping in to ZnO up to 5% and improve the optical properties of ZnO material. In fact, Fe ion can be easily substituted on the Zn-site, resulting in two more free electrons to contribute to the electrical conduction. We have also study the structural property and morphological investigation of Fe doped ZnO thin films.

Experimental Details
All the reagents used in the present work for the chemical preparation were of analytical grade. For pure zno thin films, zinc acetate dehydrate (Zn(CH 3 COO) 2 ·2H 2 O) was dissolved in a 2-methoxyethanol ((CH 3 ) 2 CHOH) with monoethanolamine (MEA: H 2 NCH 2 CH 2 OH) which was used as a capping agent. The molar ratio of MEA to zinc acetate was kept to 1.0 and concentration of zinc acetate was 0.80 mol/l. The resultant solution was stirred at 70°C for 60 minute to produce a clear and homogeneous solution ready for coating. The coating was performed with recently prepared solution.The films on ultrasonically cleaned quartz substrates were prepared using spin-coating unit which was rotated at 3500 rpm for 40 s. The films were heated at temperature 300°C for 10 min in a furnace to evaporate the solvent and remove organic residuals. The spincoating to heating procedure was repeated 15 times. The films were then annealed in air at 400°C for three hour.For 3% and 5% Fe doped zno same procedure were used. Structural properties were studied using Bruker D8 Advance diffractometer in θ-2θ geometry (Cu Kα radiation, λ = 1.5406 Å) at UGC DAE CSR, Indore, India. Optical transmittance (band gap also) and surface morphology were studied using UV-Vis-NIR spectrophotometer (V-550) from Ms JASCO and Digital Instrument Nanoscope III (Si 3 N 4 ) in contact mode (force constant = 0.58 N/m), respectively at UGC DAE CSR, Indore, India. Surface morphology was obtained using SEM (Model-ZEISS) at UGC DAE CSR, Indore, India. Raman spectra of pure and Mn doped zno samples were recorded by using of Ar ion laser with 514.5 nm wavelength and 50 mw power was employed as the excitation source at UGC DAE CSR, Indore, India.

Results and discussion:-
Structural property X-ray (XRD)diffraction patterns used to investigate the crystallographic structure of the as-synthesized samples. The unit cell of the crystal was found to be hexagonal wurtzite with the presence of dominant peaks at (100), (002), (101), (102), (110), (103) and (112) as shown inFigure 1. It is in good agreement with the standard JCPDS card no. 89-0510. Irrespective of doping concentration, all the films showed peaks similar to pure zno, which indicates that no structural deformation arisen in zno lattice upon Fe-doping. This confirms the effective substitutional replacement of Fe ions in Zn lattice sites in the zno matrix. T. Srinivasulu [55] reported that all have same intensity but in our case, it was observed that the increase in intensity accounted for the influence of ions on the scattering factors of the unit cell. Enhancement in the intensity also may be due to change in the electronic density in the crystallographic position and increase in peak intensity is an indication of the improvement of crystallinity of the samples. The lattice parameters were evaluated by using the appropriate formulae mentioned in relation given below as available in the literature on hexagonal crystal structure [26].

(1)
Where λ is wavelength of X-rays (1.540 Å), θ is diffraction angle and h, k, l are miller indices, 'a' and 'c' are the lattice parameter and d is the interplanar spacing. The crystallite size of pure and Fe doped ZnO pristine and irradiated thin films was calculated using Debye Scherrer's formula.

(2)
Where D is the crystallite size,  is the wavelength with cuk α is the radiation (1.5406 Å), β is the full width at half maxima and θ is the Bragg's angle of diffraction.The evaluated D values varied in the range, 30 -45 nm. It is also observed that the crystallite size increased initially with the introduction of Fe into zno films, thereafter, it gets decreased with increase of doping concentration. A large crystallite size of 45 nm was obtained for Fe = 5 at.% doped zno film.
Where (hkl) is the measured relative intensity of ideal orientation plane (hkl), I 0 (hkl) is the standard intensity of the preferred orientation (hkl) taken from JCPDS data and n is number of diffraction peak considered.It is observed that TC values were decreased with increasing the Fe content, attributed to the crystal reorientation influence. Further, it is found that the ZnO film with at 5 % Fe-doping showed large crystallite size, low texture coefficient and high lattice defects than the otherpure ZnO films. Here, high texture coefficient of the samples indicate that large number ofcrystallites are oriented along the (002) orientation parallel to the substrate surface. Therefore, Fe-doping concentration of 5 % waschosen to optimize ZnO layers for better structural characteristics [28].The S/V ratio of pure and Fe doped ZnOsamples is decreases after doping of Fe concentration it is due to the enhancing the crystallite size.  1296

Morphological study
The morphology of Fe doped thin films were observed by SEM. Figure 2 shows the SEM micrographs of the pure and Fe doped ZnOfilms. In pure ZnOthe SEM results seemsthe spherical and agglomeration like morphology observed but after the 3 at% doping of Fe ions its shows the large spherical and randomly oriented morphology was observed but after high doping of i .e. 5% of Fe in ZnOsamples exhibits the big grain and hexagonal like morphology. The average crystallite was calculated with the help of imageJsoftware. The average crystallite size of pure and Fe doped thin films obtained 23 to 46 nm it is well match with XRD result After the Fe doped the crystallite size was enhanced due to the effect of Fe ions goes on interstitial site in the ZnO atom.

Raman study
In figure 3 Shows the Raman spectra of Pure ZnO and Fe doped ZnO thin films. In pure ZnO the vibration modes observed at 333.53 cm -1 , 377.77 cm -1 , 574.98 cm -1 , and 676.92 cm -1 it isoriginated due to the second order spectral feature obtained from thezone boundary phonons of E 2 (high) and E 2 (low). In 3% Fe doped ZnO thin film the modes are observed at333.53 cm -1 , 439.14 cm -1 , 575.53 cm -1 , 677.01 cm -1 . The 439.14 cm -1 mode is commonly observed in Raman spectra of Fe-doped ZnOfilms and attributed to E 2 (high) mode. The high-frequency E 2 modeinvolves predominantly the movements of lighter oxygen atoms. Thevariation of E 2 (high) in response to variation of Fe content in the films.
It is observed that as the concentration of 5% FeZnO thin filmsthe phonon mode present at439.14 cm -1 is shifted to higher wavenumber side, it is due tothe change in binding energy of Zn-O bond as a result of replacing Zn 2+ by Fe 3+ and due to the induced strain, phonon confinement byboundaries and force constant changes that occurred by the incorporationof Fe-impurity into ZnO lattice. The enhancingthe intensity of E 2 (high) mode with increase of Fecontent in the films was attributed tothe change in lattice compression arisen due to the substitution ofFe 3+ ions with smaller ionic radius (0.64 Å) in place of Zn 2+ (0.74 Å) in ZnO host. The broadening of the E 2 peak with theincrease of Fe-doping shows that the oxygen vacancies, defects anddislocations were increased in the films. The vibrational mode presentat 538. 76 cm -1 essentially corresponds to E 1 (LO) (at 583 cm -1 ) and A 1 (LO)(at 574 cm -1 ) Raman modes of ZnO. The occurrence of these fundamentalRaman modes corresponds to the hexagonal wurtzite 1297 crystalphase of ZnO. The additional vibrational mode present at 608.04 cm -1 isattributed to oxygen vacancies (VO), zinc interstitials (IZn), anti-siteoxygen etc. in ZnO. The observed phonon modes are related to thewurtzite crystal structure of ZnO with a space group of P 63 mc (187). the higher wavelengths as increasing the Fe content. The energy band gap of pure and Fe doped ZnO samples was calculated considering absorption coefficient (α), which depends on the film thickness and absorbance by equation [29].

( ) (4)
Where A is the absorbance, and d is the thickness. The energy gap was estimated by assuming a direct and indirect allowed transition between valence and conduction bands using the Tauc equation [30][31].
Where B is the constant and E g is the energy band gap of the material. The plots of (αhυ) 2 versus hυ. The energy band gap of the pure ZnO sample was calculated 3.18 eV, but, after enhancing the 3% of Fe content, it was found to that 3.081 eV. It is confirmed that the band gap wasreduced due to the formation ofvacancies of oxygen in the sample. For 5% Fe doped ZnO sample it was 2.92 eV. The reduced band gap of pure and 3%, 5% Fe doped ZnO as compared to the pure ZnO thin films. It also may be due to the substitution of Fe ion intoZnO lattice and hence affecting the sp-d interaction between the d-electrons of Fe ions and bandelectrons of ZnO in its tetrahedral crystal field and the clustering. This property may be applicable inseveral optoelectronic devices and reaching improve photocatalytic efficiency.

Conclusion:-
Fe doped ZnO thin films with various Fe doping concentrations have been successfully deposited by Sol-gel technique on glass substrates. XRD spectra reveals that hexagonal wurtzite structure in all samples, Unit cell parameters, a and c, decreases with the increasing the Fe content up to 5%, which specified that Fe ions substitute into the lattice of ZnO. The Raman spectroscopy measurement exhibited the peaks at 377.33 cm -1 , 439.14 cm -1 and 538.76 cm -1 which are the belongsto vibrational modes of ZnO. The scanning electron microscopic (SEM) images showed irregular shaped grains grown over the substrate surface for pure ZnO. After Fe doped ZnO thin films exhibits the spherical and hexagonal structure. UV-Vis spectra of show the decrement the band gap up to 3.18 to 2.92 eV after the enhancing the Fe concentration.