Investigation of Structural, Optical and Electrical Properties of ZnO Prepared Thin Film by PLD

In this paper , Zinc Oxide (ZnO) films were grown on glass substrates by Pulsed Laser Deposition (PLD) technique at room temperature under the vacuum pressure of 3×10 −3 mbar. Employing a Nd:YAG pulses laser at wavelength 1064nm was used in this technique .The effect of number of laser pulses (200,500 and 800) at annealing temperature 450 C o on the structural, optical and electrical properties was studied. The structure of the ZnO thin films was examined by X-Ray diffraction (XRD), it was found that ZnO thin films are polycrystalline with many peaks, and the results of Atomic Force Microscopy (AFM) indicated that all films have grain size around 90 nm. The optical properties concerning the photoluminescence (PL) spectra were studied for the prepared thin film. From the PL, the optical gap of the ZnO thin film was determined. The Hall effect measurements confirmed that the ZnO thin films are n-type , While the number of laser pulses is increasing, the charge carriers concentration (n) increases, and Hall mobility ( µ R H R ) decreases.


INTRODUCTION
inc Oxide (ZnO) has been attracting much attention due to its unique electrical, and optical properties that together with its low cost, ease of preparation [1] and non toxic [2] .In recent years, nano ZnO has found wide ranging applications such as an electrode for dye-sensitized solar cell. [3,4] Almost all of the major deposition techniques such as Sputtering, Chemical Vapor Deposition, sol-gel, chemical spraying and electron plasma sputtering, have been employed for the growth of ZnO films.As the pulsed laser deposition (PLD) method has been recognized to offer the potential of growing high quality thin film at relatively lower substrate temperatures than other techniques [5] , its use in growing ZnO films .The high melting and boiling points of ZnO allow one to explore a variety of heat treatments required for alloying purposes and device formation.Being an oxide, ZnO also enjoys the extreme stability against the oxidation problem which can severely affect the device performance as in case of some of semiconductors such as GaAs, InAs etc [6] .Other advantages of ZnO are the resistance to high energy radiation, which makes it suitable for space applications, as well as the stability and amenability to wet chemical etching , which can be exploited for the fabrication of small size devices [7] In this work , the (ZnO) deposit by PLD method using Nd:YAG laser using pulsed Nd:YAG pulse laser at 1064nm wavelength and repetition rate 6Hz .The deposits were characterized by XRD and AFM to observe the surface structure ,PL to investigate and hall effect to examined the state the novelty ρ this work electrical properties of the films.

Experimental Work
Target preparation: ZnO powder with high purity 99.9 % powder was pressed under 5 Ton for 7 min.

Laser Source:-
Nd: YAG Laser work at wavelength 1064nm was used for the deposition of ZnO thin film at different number of laser pulses (200, 500, 800) and (energy, frequency,)constant for each pulse: 800mJ, 6Hz, respectively.During this work pulse laser deposition (PLD) is used to prepare ZnO thin film.The experimental setup of Pulse Laser Deposition (PLD) system consist of laser source and deposition chamber which include inside it the target, the substrate and the vacuum system.

Electrodes Deposition
Aluminum electrodes were evaporated to mare ohmic controls on the surface of ZnO films using thermal evaporation technique through a mask giving sensitive area as illustrated in figure

Characterization Measurements
The structure of the ZnO are studied by X-ray diffraction (XRD) (SHIMADZU JAPAN, XRD -6000) with Cu Kα radiation (λ = 1.542Aº) was used, and Atomic Force Microscope (AFM, model AA3000 scanning probe microscope) using tapping mode, the optical testing of the film were investigated by (PL) and electrical testing study by Hall effect.

Result and desiccation:-Structure measurements:-X-Ray Diffraction Analysis:-
The XRD pattern of the ZnO powder is studied with the diffraction angle (10˚ -80˚).All the peaks are in 100% phase matching with the ZnO hexagonal phase of ASTM cared, as shown in Figure (2).There are no other characteristic impurities peaks were present which also confirm that the product obtained is in pure phase.Figure (3) shows XRD patterns of the ZnO thin film at different Number of pulses (200, 500 and 800) of Nd:YAG laser at annealing temperature by 450 °C for 2 h.XRD pattern of synthesized ZnO nanostructures by PLD present all peaks of the obtained product are corresponding to the hexagonal structure of ZnO.The diffraction peaks at scattering angle (2θ) of (31.7 o ), (34.4 o ), (36.2 o ), (47.4 o ) and (56.5 o ) correspond to the reflections from (100), (002), (101), ( 012) and (110) crystal planes respectively.These reflections correspond to the standard card ASTM.were observed from figure effect number of laser pulse on intensity suggests that as the increases the diffraction peaks are narrower and exhibit a higher intensity.The average grain sizes were found to be increasing with increasing number of laser pulse as shown in Figure (4), average grain sizes measurement by using Scherrer equation.

AFM Analysis
The surface morphology of the ZnO thin films was investigated using Atomic Force Microscopy (AFM) with different No. of pulses (200, 500 and 800) at annealing by 450 °C for 2 h as shown in figure (5).Average grain size and surface roughness values were listed in Table (1).The result of the AFM analysis illustrated the average grain size and roughness increase with increasing number of laser pulse because of increasing the thickness of the film.6) shows the PL spectrum of ZnO thin film deposited on glass substrata at different number of pulses (200, 500 and 800) at annealing temperature at 450ºC.Typical luminescence behavior with two emission peaks: a sharp peak of UV emission (a bandedge luminescence band and a broad peak of Deep Level Emissions (DLE).PL characteristics of ZnO thin films showed strong relation to the number of pulse.The intensity of the two peaks increases markedly with the increase of numbers of pulse reached to 5500 (a.u) at temperature 450 °C , due to the large exciton bending energy of thin film.Shorter wavelength (higher energy) excitation photons cause more phonons to be emitted before luminescence occurs.If the excitation energy is less than the energy difference between the ground state and the first excited state, then no optical absorption will occur, resulting in no PL.Table (2) shows the peak values and the intensity of the luminescence spectrum of ZnO.

Electrical testing Hall Effect
From the Hall Effect measurements, the resistivity (ρ), carrier concentration (N H ) and carrier mobility (μ H ) values were calculated and listed in table (3).The negative sign of Hall coefficient indicates the conductivity nature of the film is n-type this result agreement with reframes .From figure (7 and 8) can be a plot of the variations of n H , μ H with different number of pulse (200, 500 and 800).Hall measurement indicates that the as grown films are high resistance, which is usually attributed to poor crystal quality.The carrier concentration of ZnO thin films increased with increasing number of pulses.This is attributed to the improved crystallinity and increase grain sizes that weakens inter crystallite boundary scattering and increases carrier lifetime.The n H was calculated by using the equation Where q is the electron charge.

847
• Hall Effect measurement proves that ZnO is N-type semiconductor.
• As the number of Nd:YAG laser pulse increase the n H increase while the mobility µ H decrease.
• PL characteristics of ZnO thin films showed strong relation to the temperature and number of pulses.The intensity of the two peaks increases markedly with the increase of numbers of pulses.

Figure ( 1 )
Figure (1) Al electrodes deposited on the surface of ZnO.

of ZnO prepared Thin Film by PLD 846 Figure (7) The relation-ship between the n H of ZnO thin film and No. of pulses (200, 500 and 800) of Nd:YAG laser. Figure (8) The relation-ship between the μ H of ZnO thin film and No. of pulses (200, 500 and 800) of Nd:YAG laser. Conclusion•
XRD analysis indicated that the structure ZnO thin film polycrystalline of the annealed to 450 C o .And the intensity of peaks increased as No. of pulsed increasing from 200 to 800 pulse.•The grain size and roughness increase of ZnO thin film as the number of Nd:YAG laser pulse increase.