Effect of Nitrogen Doping On The Structural Surface And Optical Properties Of ZnO Thin Film Prepared By Spraypyrolysis

: Pure and Nitrogen doped ZnO thin films were fabricated using spray pyrolysis method. The influence of doping on the structural, optical and morphological properties of prepared films were studied. The prepared samples were characterized through XRD (X-Ray Diffraction), FTIR (Fourier Transform Infrared Spectroscopy), FT-RAMAN (Fourier Transform Raman), optical transmittances of prepared films were studied as a function of wavelength using UV-VIS-NIR spectrophotometer and SEM (Scanning Electron Microscope). Absorption coefficient and Extinction coefficient values were calculated. Thickness of the films were found increasing with increasing doping concentrations. Structural studies confirmed that the prepared films were highly crystalline and predominantly orientation along (0 0 2) direction. On varying the doping concentration of N, a shift in the (0 0 2) peak was observed. FTIR and Raman spectral investigations revealed bands at specified wavenumber regions corresponding to stretching and bending vibrations of Zn and O. High resolution SEM images proved the presence of regular distribution of grains and the surface was found continuous, free from crack and holes. EDAX spectrum shows the attained films contain of Zinc, Oxygen and Nitrogen elements. Present investigations confirmed the fusion of nitrogen into ZnO lattice and modified the structural, optical and morphological properties of ZnO films.


Introduction:
ZnO is a direct, wide bandgap semiconducting material with many properties for optoelectronics, transparent electronics and sensor applications.It has been used in wide range of applications such as, piezoelectric transducers, varistors and transparent conducting electrodes.
ZnO has several attractive characteristics for electronic and optoelectronic devices.It has direct bandgap energy of 3.37 eV and a large exciton binding energy of 60 meV at room temperature.
Few drops of HCl was added to the obtained mixture to avoid the formation of hydroxides.
Prepared precursor was sprayed on glass substrates preheated at a constant temperature of 350 °C.The structural properties were studied by X-Ray diffraction (PANANALYTICAL XPERT-PRO).Phase identification was confirmed by FTIR (Thermo Nicolet Avtar) and FT-Raman.The optical properties of the prepared ZnO thin films were studied by UV-VIS Spectrometer.The surface morphology of the prepared films were recorded using SEM (SEM JEOL JSM-6390LV) and Atomic Force Microscope respectively.Elemental composition of the prepared films were studied by energy dispersive X-Ray (EDX) instrument.

Structural Studies
Fig .1(a) shows the XRD pattern of pure and N doped ZnO thin films deposited at different solution concentrations (x= 0 ml, 0.1 ml, 0.2 ml, 0.3 ml and 0.4 ml).The diffracted peaks in the pattern can be indexed for its hexagonal wurtzite structure by COD card No: 96-900-4180.No diffraction peaks are identified at lower doping concentration of nitrogen.When the doping concentration is increased, intensity of (002) peak begins to arise and its intensity increases with doping.ZnO:N films exhibit high crystallinity and are preferentially orientation along the (002) direction.No other impurity peaks due to nitrogen was detected in the XRD.This shows the purity of the prepared films and N is well incorporated at the O lattice site in the hexagonal wurtzite structure of ZnO.Fig. 1b, shows the shift in (0 0 2) peak position and its widening with doping.The shift in the (0 0 2) peak is prominent with varying doping concentration, which may be due to the stress in the films i.e, tensile or compressive strain, the peak position shifts respectively towards higher or lower angle [18].Shift is clearly realized in highly doped film and this is the sign of N doping into ZnO lattice [19].
Lattice parameters a and c were calculated using the relation: Crystallite size of prepared pure and N doped ZnO thin films were calculated using Scherrer's formula [20].
Dislocation density (δ) was calculated using the formula: where, D, is the grain size.
Microstrain (ε) of ZnO films are calculated using the relation: where, β, the FWHM in radians, θ, is the diffracting angle.
The variation of thickness, crystallite size, dislocation density, and microstrain with doping concentration are given in Table 1.

Table 1 Calculated structural parameters of pure and N-doped ZnO thin films
As observed, the lattice parameter decreases with doping concentration.Also the crystallite size decreases from 40.274 to 24.14 nm then increases to 35.32 nm with doping.The increase in the crystallite size reveals the enhancement of crystallinity under c axis orientation of ZnO thin films.Ebru Senadim Tuzemen reported grain size first increases then decreases [8].
Dislocation density values increases with increasing doping concentration.Since dislocation density and microstrain values are inversely proportional, initially microstrain values decreases and then increases.Calculated lattice constant values of N zoped ZnO samples are smaller than undoped ZnO samples.

FTIR Analysis of ZnO Thin Films
Vibrational studies can be used to investigate the chemical bonding, defect structures, impurity content etc.The presence of various vibrational modes in pure and N doped ZnO thin films were analyzed using FTIR in the wave number range from 400-4000 cm -1 , which is shown in Fig. 2. The characteristic IR peaks below 1000 cm -1 is very important to study the presence or absence of Zn-O bonds and the functional groups.The IR bands at 407, 512 and 623 cm -1 corresponds to vibrational modes of Zn-O bond in the prepared ZnO thin films.The vibrational mode at 886 cm -1 is related to oxygen site bound to the lattice Zn site [28].Another sharp peak at 1640 cm -1 is attributed to H-O-H bending vibration which may due to presence water species in prepared thin films.This band is more prominent in doped samples, which signify the incorporation of nitrogen ions into ZnO lattice.Also FTIR spectrum reveals the presence of stretching vibrational bond of O-H around 3450 cm -1 for undoped and N doped samples.The gradual shifts in the absorption frequency with N-doping are caused by the difference in the bond lengths that occurs when N ions replace native ions [30].For undoped and N doped ZnO thin films modes observed from 1000 to1500 cm -1 and these modes are usually assigned to O-C=O (symmetric and asymmetric stretching) vibrations and C-O vibrations due to ambient atmosphere.[31].

FT-Raman Analysis
Fig. 3 shows the FT-Raman spectrum of pure and N-ZnO thin films.Raman shift can be observed in the wave number region 400-4000 cm -1 .The wurtzite structure of ZnO belongs to space group C6V (P63mc) [32].So one primitive cell includes two formula units, with all atoms occupying 2b sites of symmetry C3V.Hence group theory predicts the existence of optic modes: Γ = A1 + 2B1 + E1+ 2E2 at the th Brillouin zone.The optical absorption coefficient can be calculated using the Lambert law relation, where, 't' is the film thickness, 'T' is the transmission co-efficient respectively.The extinction coefficient can be related the absorption co-efficient by the following equation; where, k(λ) is extinction coefficient.The variation of absorption coefficient (α) with wavelength is reported for doped and undoped ZnO thin films.Higher α values are obtained for undoped ZnO thin films.Fig. 6b shows the extinction coefficient variations with increasing wavelength.The extinction coefficient is high in undoped samples when compared to N doped ZnO thin films.Initially K values increases in UV region due to the intrinsic absorption in the upper gap energies then decreases in the visible region which confirms the layers are transparent [33].
The band gap energy of thin films can be calculated using Tauc plots, drawn between photon energy (hγ) and square of the absorption coefficient (hγ) 2 .This plot yields a straight line in the high energy region of the spectrum.The intercept of the plot in energy axis gives the optical band gap energy 'Eg' in electron volt.Fig. 6 shows the Tauc plots drawn for ZnO films deposited at various precursor concentrations.Obtained results are in good agreement with the previous reported results.

3.4.1Scanning Electron Microscope
Fig. 8   It is clearly seen uniform surface morphology in all the prepared samples.The surface of the films looks to be smooth and without any crack and holes.In the pure sample, the grains are in sperical shape.In doped sample, the shape of the grains are non-spherical, it changes with doping concentrations.On increasing the doping concentrations, the shape of the grains changes through polygoanal, flower and ricelike structure.Estimated grain size decreases with increasing doping concentrations.Grain size decreases from 90 nm to 42 nm, when the N doping concentartion increases.Grain size decreasing with increasing film thickness and obtained grains sizes well match with grain size obtained from XRD spectra.Shape modifications in grains give significant property modifications which find applications in optoelectronic industry.The prominent peaks at 0.39 keV, 0.59 keV, 1.03 keV, 8.44 keV and 9.33 keV were strongly observed for all the N doped samples.

3.4.2Atomic Force Microscopy
From these, peaks located at 0.59 keV and 1.03keV represents the K shell of oxygen and L shell of zinc.In detail,the L-shell emission at 1.03 keV,as observed here can be measured as the convolution of Zn 2p3/2 and Zn 2p1/2 photoelectron energies,which have been reported at 1.02 keV and 1.04 keV respectively.The X-ray energies at 8.44 keV and 9.33 keV are added emissions from Zn core levels .Over all the existence of these basic Zn and O emissions endorse the existence of Zn and O atoms in prepared nano particles.As evident through the presence of N1 score level emission around 0.39 keV.

4.Conclusion
Pure and N doped ZnO thin films were synthesized successfully by using spray pyrolysis method.Obtained results shows that the structural and surface morphology of the ZnO:N films depend strongly doping concentration.On doping, initially the crystallinity of the prepared films was poor.The average value of crystallite size was decreased from 40 to 24 nm when N concentration is increased, due to the suppression of nucleation and successive growth.

Fig. 1 (
Fig. 1 (a) XRD patterns of pure and N-doped ZnO films for different concentrations (x) (b) Shifts in (0 0 2) peak position and peak widening in pure and N doped ZnO thin films.

Fig. 2 FTIR
Fig. 2 FTIR spectra of pure and N doped ZnO thin films for different concentrations (x)

Fig. 3 FT
Fig. 3 FT-RAMAN spectra of pure and N doped ZnO thin films for different concentrations (x)

3 . 3 Fig. 4
Fig. 4 Optical absorption spectra of pure and N doped ZnO thin films for different concentrations (x).

Fig. 4 Fig. 5 Fig. 5
Fig.4show the UV-vis patterns of all the films near absorption edges, which are indicated with dashed lines.A great difference of the ZnO:N thin films from the undoped ZnO is the redshift.As we can see in fig.4, the absorption edge gradually shifts to long wavelength direction as nitrogen solution concentration increases.May be this red-shift result suggests a uniform substitution of N for O in the lattice[15].Fig. 5 Shows the transmittance spectra of pure and N doped ZnO thin films for different concentrations.
Fig. (6a) & (6b) Variation of optical absorption coefficient and extinction coefficients of pure and N doped ZnO thin films for different concentrations (x)

Fig. 7 TaucTable 3
Fig. 7 Tauc Plot of pure and N doped ZnO thin films for different concentrations (x)

Fig. 8
Fig. 8 SEM micrographs of pure and N doped thin films for different doping concentrations (x)

Fig . 9
Shows the Atomic force microscopy (AFM) image of the ZnO thin for undoped and N doped ZnO films.

Fig. 9
Fig. 9 AFM image of pure and N doped ZnO films for different concentrations (x)

Fig. 10
Fig. 10 shows the EDAX spectra of prepared Zno thin films.The atomic concentrations of N doped Zno thin films are shown in fig.

Fig. 10 EDAX
Fig. 10 EDAX Spectra of N doped ZnO thin films for different doping concentrations (x) Higher N-doping significantly improved the crystallite size by creation of distortion centers and Zn/N interstitials.Presence of chemical bonding between Zn-O was confirmed through FTIR and FT-Raman spectra.Observed shift in these bands confirmed the substitution of N into ZnO lattice.From SEM studies, high resolution images revealed smooth and crack free surfaces.Changes in the shape and size of the surface microstructures revealed dependency to doping concentrations.Rouhghness of films increases for N doped ZnO thin films.EDAX analysis confirms the presence of Zn, O and N element.

Table . 2 Trnamittance percentage of pure and N doped ZnO thin films for different concentrations
in the region of about 380 nm, which is characteristic of ZnO and indicated that the films were of good optical quality.Obtained ZnO samples are suitable for opto electronics devices because of its higher tranmittance percentage for the wavelength greater than 380 nm region.
(x)The transmittance percentage was calculated for wavelengths in the range from 400 nm to 800 nm.Transmittance percentage is high (>88 %) in N doped ZnO thin films while lower percentage (67 %) obtained in Pure ZnO thin films.The spectra reveal that the films have a shows the SEM micrographs of pure and N doped ZnO thin films for different