A Study on Structural and Optical Properties of Nanostructure Mg

For this paper, films have been grown under various deposition conditions in order to understand the effect of processing on the film properties and to specify the optimum condition, namely substrate at temperatures of 400 ° C, oxygen pressure (2×10 -1 ) mbar, laser fluence 400 mJ, and with different Mg doping ( x =0, 0.02, 0.04, 0.06), using double frequency Q-switching Nd:YAG laser beam (wavelength 532nm), repetition rate (1-6) Hz and the pulse duration of (10 ns), to deposit Mg x Zn 1-x O films on glass substrates with thickness of about 200±10 nm for all Mg x Zn 1-x O films at different deposition condition and the number of laser pulses was 100 pulses. The X-rays spectra revealed that the presence of diffraction peaks indicates that the polycrystalline of the films depended strongly on the Mg-content in the layers. All the grown films is (101) as predominant reflection . The Scanning Electron Microscopy (SEM) images, the average grain size less than 50 nm. From the study of atomic force microscopy (AFM), we can determine the root mean square (RMS) surface roughness of Mg doped ZnO films . The optical properties were characterized by the transmittance and absorption spectroscopy at room temperature, measured in the range from (300 - 900) nm. For all the films, the average transmittance in the visible wavelength region λ = (400 - 800) nm is greater than (70%). The maximum value of the transmittance is greater than (95%) was obtained for these films. (E g ) values of Mg x Zn 1-x O thin films are (3.37, 3.59, 3.82, and 4)eV corresponding to the Mg-content (x = 0, 0.02, 0.04 and 0.06) respectively. In other word, the optical band gap of Mg x Zn 1-x O thin films become wider as Mg-content increases and can be precisely controlled between 3.37 and 4eV.


INTRODUCTION
n recent years, studies on synthesis, characterization, and applications of ZnO nanostructures [1] have been studied extensively, because ZnO nanostructures have been regarded as a promising nanomaterial in a wide range of applications like solar cells, sensors, light-emitting diodes, piezoelectric nanogenerators, fieldeffect transistors, and transparent electrodes [2]. Zinc oxide is II-VI compound semiconductor with a wide direct band gap (3.37eV at room temperature) and a hexagonal wurtzite structure (space group P63mc with cell parameters (a= 3.25 Å, c = 5.207 Å) [3]. ZnO films can be grown by many deposition techniques, such as magnetron sputtering [4], chemical vapor deposition (CVD) [5], spray pyrolysis [6], pulsed laser deposition (PLD) [7], molecular beam epitaxy (MBE) [8], sol-gel [9], and so forth. ZnO thin films and nanostructures are widely used in various applications which include light emitting diodes (LED), UV photo detectors, transparent conducting oxides (TCOs), transparent thin film transistors (TTFTs), solar cells windows, piezoelectric transducers, Gas Sensors etc.. The large exciton binding energy makes ZnO a promising material for optical devices that are based on exciton effects. Due to a strong luminescence in the green-white region of the spectrum, ZnO is also a suitable material for phosphor applications. [10] The aims of this work to reveal specific properties of Mg x Zn 1-x O thin films nanocrystalline materials. Initially the series of samples have been prepared by pulsed laser deposition technique at different technological conditions on glass substrates. That supposed to result in the different structural properties, different surface morphology of the nanostructures to be obtained, also the optical properties.

Experiment
The system, power supply, computer controlling and cooling systems. The light route system is installed into the hand piece, while the power supply, controlling and cooling system are installed into the machine box of power supply. X-ray power diffraction (XRD) is one of the most powerful techniques for qualitative and quantitative analysis of crystalline compounds. This experimental technique has long been used to determine the overall structure of bulk solids, including lattice constants, identification of unknown materials, orientation of single crystals, orientation of polycrystals, defects, stresses, etc. In this study X-ray diffractrometer type SHIMADZU, power diffraction system with Cu-Kα X-ray tube (λ = 1.54056 Ǻ) is used. The X-ray scans are performed between 2θ values of 20° and 70°.
The SEM study carried out by (FEL Quanta 200, Netherlands) scanning electron microscope equipped with Energy dispersive X-ray (EDAX). The operation principle of an AFM the consists of a cantilever and a sharp tip at its end. The surface of the specimen is scanned with the tip. The distance between the specimen surface and the tip is short enough, to allow the van der Waals forces between them to cause deflection of the cantilever. The deflection follows Hooke's law and the spring constant of the cantilever is known, thus the amount of deflection and further, the topographical profile of the specimen, can be determined. Typically, the deflection is measured using a laser spot reflected from the back surface of the cantilever into an array of photodiodes.
The optical transmittance of Mg x Zn 1-x O thin films with different doping concentrations (x = 0, 0.02, 0.04, 0.06) on glass substrates with different deposition condition, by using spectrophotometer (SHIMADZU UV-1650 PC), for the wavelength range from 300 nm to 900 nm. The optical properties are calculated from these optical measurements.

Results and Discussion
The results and discusses the effect of doping, upon the characterization such as structural and optical properties of the films grown by PLD, also the structural measurements such as, morphological features by Scanning Electron Microscopy (SEM), Atomic Force Microscope (AFM).
Structural Properties X-ray Diffraction: Figures (1), shows the X-ray diffraction profiles of Mg x Zn 1-x O thin films deposited at a temperature of (400 C 0 ) for different magnesium content, x. The presence of diffraction peaks indicates that the film is polycrystalline with a hexagonal wurtzite type crystal structure and no amorphous phase is detected. It is revealed that the sprayed film has peaks corresponding to (100), . 67 respectively. The (d) value, that is the interplanar spacing of (101) plane of the film was evaluated from the position of (101) peak from the XRD data. The observed (d) value is 2.471 which is in excellent agreement with the standard (d) value  The X-ray diffraction data revealed that the crystallinity of the films depended strongly on the Mg-content in the layers. All the grown films of ZnO pure and doping (101) as predominant reflection were polycrystalline with reflection along with the other (100), (002), (102), (110) and (112), reflections that corresponding to the hexagonal wuartzite-type nanostructure of ZnO films. [11]. For all the Mg x Zn 1-x O films , the angle position of the (101) peak moves toward greater values with increasing Mg-content, which indicates that Zn 2+ ions are successfully substituted by Mg 2+ in the ZnO lattice , which is in agreement with another reports [11].
On the other hand we can observe that the intensity of (101) peaks increases with increasing Mg-content whereas (002) peaks also become more intense as the Mgcontent increases, which is consistent with the another report [12].
The lattice constant a and c for the prepared ZnO thin film are (3.256Ǻ) and (5.204Ǻ) respectively , that is in a good agreement with the standard values (3.249Ǻ) and (5.205Ǻ) taken from (ASTM) card file data. This indicates that the degree of nanocrystalinity increase with increasing Mg-content in the film. The lattice constants and the relative intensity ratio, in the diffraction pattern of Mg x Zn 1-x O films are given in table (2). The lattice constants obtained are found to be in good agreement with (ASTM). [13]  The values of full width at half maximum (FWHM) of the preferred orientation ((101) for ZnO pure and ZnMgO) are increase with increasing Mg-content in the film as shown in table (3). This indicates that the grain size decreases with increasing Mgcontent. [14] The values of the structural parameters from XRD data for The values of the full width at half maximum (FWHM) increases from 0.1622 о for x=0 (pure ZnO) to 0.281 о for (Mg 0.06 Zn 0.94 O) as Mg content increased. The average grain size of Mg x Zn 1-x O films prepared at different Mg-content ( x = 0 , 0.02 , 0.04, 0.06 ) was calculated using the fringe width at half maximum (FWHM) of the films using Scherrer formula. The film prepared at ZnO pure, (x = 0) showed the highest crystallite size of (52 nm) and its value decreases with increasing Mg-content as shown in table (3). Also the XRD peaks can be widened (FWHM increase) by internal stress and defect when increasing Mg-content in the films, so the grain size decreases with increasing Mg-content. [ 15] The micro strain depends directly on the lattice constant (c) and its value related to the shift from the (ASTM) standard value. The values of micro strain increase with increasing Mg-content in the films as shown in table (3). The calculation of the film stress is based on the strain model, as shown in table (3). This strain and stress can be calculated from the formulas: [13] The residual stress (S s ) in ZnO films can be expressed as [13]  Here c ij is the elastic stiffiness constant for single crystal ZnO (where c 11 =208.8 GPa, c 33 =213.8 GPa, c 12 =119.7 GPa and c 13 =104.2 GPa [13]. To describe the preferential orientation, the texture coefficient, TC (hkl) is calculated using the expression [ Table (3) The Size -strain data of investigated thin films

Scanning Electron Microscopy (SEM)
SEM is a promising technique for the topography study of samples, as it provides valuable information regarding the growth mechanism, shape and size of particles and/or grains [16].
Figures (2) to (5) show the SEM micrographs of Mg x Zn 1-x O thin films (x = 0, 0.02, 0.04, 0.06), with a average magnifications of (500-50000 X). From these pictures, it can be observed that Mg x Zn 1-x O thin films consist of spherically nanosized grains of nearly regular size and covered the entire surface of the substrates. All films compactness is high and the surface's uniformity (homogeneity) is good without defects or cracks. the particle size is quite fine. The grain size is measured by keeping the SEM photograph under traveling microscope. The average grain size in the micrograph calculated to be approximately less than 50 nm.
It is observed that with increase Zn 2+ content from x=0.04 to x=0.06 the grain size decreases as shown in figure (4) and (5), while at x=0 and x=0.02 the differences in grain sizes are very small, as shown in figure (2) and (3). On the deposited films, bigger size of particles seems bright may be due to agglomeration of nanoparticles were scattered electrons in the same phase. The crystallite size obtained from XRD Scherrer's equation is lower than the observed value from SEM. The differences between them may be due to the average size of crystallites obtained from XRD whereas from Atomic force microscopy (AFM), the obtained sizes are from the surface of the grains, this agrees with, M. Salina et al. [17]. However, these characteristics are in good agreement with the films high transparency.    a) x=0, b) x=0.02, c) x=0.04 and d) x=0.06.

Optical Properties
The optical properties of the pure Mg x Zn 1-x O films deposited by pulsed laser deposition technique are measured by UV-VIS spectrophotometer on glass substrate at 400 °C temperature in the range from 300nm to 900nm. The laser fluence energy density is 0.4 J/cm 2 and the oxygen pressure is maintained at 2x10 -1 mbar various Mgcontent(x=0, x=0.02, x=0.04 and x=0.06) with film average thickness 200 nm. The absorptance and transmittance have been studied. Also the optical energy gap and optical constants have been determined. . &Tech.Journal, Vol. 32,Part (B), No.6 fig. (7), shows the UV/visible h different Mg-contents. The transmittance spectra of the films can be analyzed as follows: 1. For all the films, the average transmittance in the visible wavelength region λ=(400-800)nm is greater than (80%). The maximum value of the transmittance is greater than (95 %) was obtained for at concentration increasing of doping (x=0.06) these films. 2. The slope of the absorption edge are gone up and there is an obvious shift of the absorption edge to the shorter wavelength with increasing Mg-content The Observed shift in the absorption edge towards the blue region clearly reflects the incorporation of Mg in the ZnO lattice, indicating that the optical band gap was enlarged by Mg doping regardless of crystallinity which is in agreement with the report [19]. 3. The transmittance of the Mg x Zn 1-x O thin films increases with increasing Mgcontent in the films, and when the transmittance increases the grain size decrease,which is consistent with another report [20].

Refractive Index (n)
The refractive indices (n) of the Mg x Zn 1-x O thin films, As shown in Fig.(11), the refractive indices of the films are influenced by the Mg-content. The refractive indices decrease as the Mg-content increases in the range of (1.5 -2.6) respectively. And the refractive index decrease as the wavelength increases, in our research the decreases in grain size with the decreasing of refractive index is observed. The decrease of refractive index and absorption index may be due to the improvement of stoichiometry [20], the decrease in grain size and the increase in micro strain, which is consistent with other reports [28,29].  Fig. (12) shows the extinction coefficient (K 0 ) as a function of wavelength for different Mg-content. The extinction coefficient of the films is influenced by the Mgcontent. The extinction coefficient decrease as the Mg-content increases. And the extinction coefficient decrease as the wavelength increases, which is consistent with other reports [20,28,29] Eng. &Tech. Journal, Vol. 32,Part (B)


The films deposited from pure ZnO and 6 wt.% Mg doped ZnO targets at oxygen pressure of (2x10 -1 ) mbar, (400) o C substrate temperature and laser fluency (400) mJ are good candidates for structural, morphology and optical properties.  The structural of the ZnO films are found to be dependent on the films doped The increase of the dopant concentration into the target. The result of (X-ray) diffraction shows that all thin films (pure and doped) exhibit polycrystalline nature, and has the hexagonal wurtzite structure with preferential orientation in the direction [101] plane, The crystal structure of the nanostructure Mg x Zn 1-x O films is hexagonal wurtzite and the films are highly oriented. The SEM studiy show that the obtain nanocrystalline Mg x Zn 1-x O thin film prepared with (x=0.04 and 0.06) has better surfaces. The AFM results show the slow growth of nanocrystallite sizes for the asgrown films. The root mean square (RMS) is decreasing from 56.6 to 39.4 nm as the doped increased up to x= 0.06. XRD, AFM and SEM analysis shows that the prepared films are nanocrystalline thin films with estimated comparable grain sizes. Also, the grain size of the prepared nanocrystalline thin films in the range ~ (30-50 nm).  The optical properties of Mg x Zn 1-x O thin films show that the films have allowed direct transition. The average transmittance for all the films is over 90% in the wavelength range (300-900) nm and the transmittance in UV region increases with the increase of films doped. The optical band gap is dependent on the films doped, increasing in the doping percentage for Mg cause a increase in the optical band gap value, also the values of refractive index (n) of Mg x Zn 1-x O films lie in the range of (1.5 -2.6). This means that the film suitable for using as a antireflection coating and also suitable for solar cell applications.