Preparation and characterization of NiO thin films by PLD

In this work, NiO thin films have synthesized by pulsed laser deposition on glass substrates with different substrate temperature (100, 200, 300) ° C, using Q-switching Nd:YAG laser. Structure and optical properties have carried out by using FTIR, AFM and UV- Vis spectroscopy. FTIR spectraconformed of NiO bonding and AFM images show the increase in grain size with temperature. The optical transmission results show that the transparency of the NiO films is greater than 85% in the visible region which increases with the increasing substrate temperature, While the energy band gap was decreased with increasing substrate temperature.


INTRODUCTION
ickel oxide (NiO) films has a cubic rock salt (NaCl)P structure with a lattice constants of (4.178 Å) and a wide band gap energy from (3.5 to 4) eV [1][2][3] . The p-type semiconducting behavior can be produced by the creation of native defects, nickel cation vacancies and or interstitial oxygen in NiO crystallites formed in non-stoichiometric NiO x . In spite of, this behavior NiO is an insulator at room temperature with resistivity of the order (10 13 Ωcm) [4][5][6] .
In this work, pulsed laser deposition (PLD) was used for the deposition of NiO thin films on glass substrates at different substrate temperatures. The effect of substrate heating during deposition on optical and structural properties was studied.

Experiments details
The focused Nd:YAG Q-switching laser beam at 532nm (pulse width 7nsec, repetition frequency 6 Hz) were used to prepared NiO thin films. The films were deposited at energy density (1.6 J/cm 2 ), and at oxygen pressure (2*10 -1 mbar). Each film was subjected to 40 laser pulse with different substrate temperature (100, 200, 300) C o . High purity (99.99%)NiO powder supplied from Fluka company, was pressed hydraulically with 5 ton pressure to form a target with 2.5 cm diameter and 0.4 cm thickness. Films thickness is measured by optical interferometer (Fizeau method). The method based on interference of the light beam reflected from thin film surface and substrate bottom. The thickness wasdetermined using the formula [13] : Where (x) is fringe width, ∆x is the displacement in fringe position, and λ is wavelength of using source light.
The structure of thin films was diagnostic with FTIR and AFM (Atomic Force Microscope). FTIR spectra were recorded with(8400S,Shimadzu) spectrometer. The surface morphology of thin films was investigated by AFM with (Digital Instruments Nanoscope II, AA3000). A double-beam (UV-VIS) spectrophotometer was used to measure optical transmittance of films in range (200-900) nm.
Surface morphology image of synthesise thin films with different temperature was studied by AFM spectroscopy as shown in figure (2). These figures demonstrate the homogeneity in grain distribution and will be more spherical shape at temperature (300 C o ). The relation between the grain size calculated from AFM image at the deposited temperatures was shown in table (1). which increased with increasing temperature due to the nucleation which enhanced with rising temperature, these result agreed withI. Fasaki et.al. [1] .
The optical transmission of NiO thin films on glass substrate is shown in figure (3) as a function of the wavelength for different substrate temperatures. The figure shows that the transmission decrease with increaseing substrate temperature, which notice that the films deposited at substrate temperature (300C o ) have transmittance about 60%, this is due to the absorptivity dependence on crystalline state, the absorption coefficient (α) determined from transmittance data using the relation [10] : Where (d) is the thickness of thin films(0.25µm), and (T)is transmittance. Figure (4) shows the variation of absorption coefficient as a function of wavelength for different substrate temperatures. The highest absorption coefficient was for the film prepared at (300C o ), this is due to the strong influence of optical properties by the structure of thin film.
The absorption coefficient (α) was used to calculate the band gap of NiO films using the equation [9,14] : Where (A) is a constant, E g is energy band gap of NiO film, hν is the incident photon energy, where (m=1/2) for direct allowed transitions and (m=2) for indirect allowed transition for NiO films. Figure (5) shows the variation of (αhν) 2 versus the photon energy (hν) for the films. The extrapolation of the linear part to α=0 gives the band gap width as shown in table (1).The band gap decrease with increasing substrate temperature ,and this result agreement with I. Fasaki et.al. [1] .
The attenuation value of electromagnetic wave was determined from theextinction coefficientkby the following relation [15] :

Conclusion
NiO thin films were deposited by PLD on glass substrates at different depositiontemperatures (100, 200, 300) C • . All the films deposited at energy density 1.6 J/ cm 2 . FTIR and AFM results shows that the formation of NiO, the surface structure and grain size depend on substrate deposition temperature ,while the optical properties reveal a high transmittance up to 85% in the visible region for film deposited at temperature =100 C o .