XRD patterns of the ZnO layer deposited without annealing are shown in Fig. 2, with the use of the PDXL2 software provided with the database (PDF-2, release 2014, 01 -073-8765). As it can be seen from the identification bars superimposed on the peaks 31.7 °, 34.35 °, 36.19 °, 47.55 °, 56.57 °, 62.74 °, 67.73 ° which gives us a structure of ZnO zincite (hexagonal phase, space group P63mc (186)) with mesh parameters a = b = 3.254167 A ° c = 5.216132 A °; c = 5.216132 A °. The XRD patterns obtained show that the deposited ZnO layer tends to present a preferential orientation (002) with the c axis perpendicular to the substrate . These results are confirmed by several works [12, 24, 25].
The value of crystalline size D can be calculated using Scherer's equation (1), where K = 0.9 is the Scherer constant, λ = 0.15406 nm is the wavelength, θ is the peak position and the angle of Bragg diffraction in radians, and β full width at half maximum (FWHM) in radians .
The inter-planar spacing dhkl was calculated using the Bragg law relation (2), where n = 1 corresponds to the diffraction order, λ = 1.5406 A ° is the wavelength of the X-ray, and θ is the diffraction angle in radians .
The volume of the unit cell V of the ZnO layer without annealing temperature was estimated by using equation (3) , such that a = b = 3.254167 A °, c = 5.216132 A ° extracted directly from the PDXL2 software. The dislocation density Φ in nm-2 and micro strain ξ are also calculated using the relation (4) and (5) [18, 28].
In table 1 we summarize the mean crystalline size D, inter-planer spacing, unit cell volume, dislocation, and the micro strain.
The control of the annealing temperature after the deposition of ZnO is very important to obtain a good crystallinity. The XRD patterns of ZnO films grown at different annealing temperatures from 200 ° C to 600 ° C with steps of 100 ° C , compared to that the ZnO layer without annealing are shown in Fig 3 (a). XRD patterns exhibited three intense peaks with reflections from Bragg's angles at ~ 31.7 °, 34.3 °, 36.2 ° which correspond to the hexagonal phase of ZnO lattice planes of orientations (100), (002), (101), respectively. The diffraction reflections observed turned out to be well suited to the database (PDF-2, release 2014, 01-073-8765). In addition, these results show that the deposited ZnO layers are oriented towards the C (002) axis . As the annealing temperature increased to 500 ° C, the intensity of the peaks at 31.7 °, 34.3 °, 36.2 ° became higher and sharper, indicating a good growth and crystallinity of the ZnO thin films. The high peak intensity corresponds to the annealing temperature of 500 ° C especially for the peak at ~ 34 °, on the contrary, for a temperature of 600 ° C the intensity of the peaks at 31.7 °, 34.3 °, 36.2 ° is reduced, while the intensity of the peak at 69 ° is increased. The database indicates the absence of metallic Zn phases or impurity phases, which indicates the formation of pure crystalline ZnO films. Figure 3 (b) reveals that the peak positions for the (100), (002) and (101) orientations of the ZnO films annealed at 300 ° C to 600 ° C were slightly shifted to higher 2θ values, revealing that a better crystallinity is obtained, result in good agreement with that obtained by M. Shaban et M. Zayed .
The inter-planar spacing, the dislocation density and the unit cell volume of ZnO films at different annealing temperatures were calculated by using successively the relations (2), (4), (3), the obtained results are shown in Table 2. The unit cell volume value is found about the same for unannealed and annealed ZnO samples (see table 1 and 2), dislocation density is found 0.144 nm-2 for ZnO without annealing and even for films annealed at 200 ° C up to 500 ° C, on the other hand the dislocation density increases from 0.144 nm-1 for 200°C to 0.49 nm-1 when the temperature increases to 600 ° C. Moreover, the inter planer spacing is also fairly the same (2.62 ~ 2.63) for temperature up to 500 ° C but it changes at 600 ° C and takes a value of 1.41 A°.
The micro strain ξ and the crystalline size D were calculated by the relation (1) and (5), and represented in Figure 4. The micro strain is slightly reduced from 6,76 .10-3 at ambient temperature to 5,32 .10-3 at temperature of 600 ° C. The crystal size of ZnO grains is increased from 18 nm at ambient temperature to 20,17 nm at 400 ° C, and takes a low value at 600 ° C (11.97nm) which indicates that the best annealing temperature is obtained at 400 ° C.
Figs. 5 (a, b, c, d, e, f) show the SEM image of the surface morphology of ZnO layers electrochemically deposited without annealing and with annealing from 200 ° C to 600 ° C with steps of 100 ° C, respectively. Figs.5 (a) and (b) show that the surface of silicon substrate is densely and homogeneously covered with ZnO deposit. The latter appears as clusters in the form of sand roses containing micro-pillars of hexagonal shape oriented perpendicularly to the surface as shown in the magnification inset Fig.5a.This result was obtained by several authors [17, 9]. In addition, inset of Fig.5(a) shows the heads of the micro pillars are pointed, and have an average diameter of 0, 46 µm. Above the annealing temperature of 200 ° C, Fig.5(b), the diameter of the micro pillars increased to 0.84 µm, and the heads became porous, the average diameter of the pore is about 80,72 nm. With increasing the annealing temperature (T> 200 ° C) the structure changes completely from a sand rose form to a granular structure, at 300 ° C (Fig.5 (C)), the grains are aggregated with a random distribution in all directions. Switching to image of Fig.5(d), corresponding to an annealing temperature of 400 ° C, the aggregates become larger and randomly and sparsely distributed with different sizes. By tuning to 500 ° C, Fig.5 (e), the deposit appears densely distributed on the surface compared with that obtained for the substrate annealed at 400 ° C, that the deposited ZnO surface becomes rougher. The last image (Fig.5(f)) shows very sparse and sparse aggregate of ZnO grains. It is clear from these results that the uniformity, distribution, morphology and crystallinity of the deposits depends on the annealing temperatures.
Contact angle measurements are frequently used to study porosity, texturing, and treatment effects on the surface , in our case; we studied the effects of annealing temperature on the ZnO surface. A drop of 3μl of ultra-pure deionized water (DI) is used to perform these measurements at room temperature. First, we performed the measurement on the ZnO layer without annealing ,where a contact angle value of 122,1 ° (Fig.6a) is obtained, annealing of the deposit at 200 ° C gives a contact angle value of 100 ° (Fig.6b), a value greater than 90, which confirms the hydrophobicity of the ZnO surface .It is known that DI is a very polar medium with a very high surface energy, which could indicates that the zinc oxide surface is nonpolar with a very low surface energy leading to the formation of the beads in contact with the surface of ZnO (Fig. 6).
The variation of the contact angle upon different annealing temperature is represented in Figure 7. A decrease in the contact angle value from 122.1° without annealing to 29.5° is observed after successive annealing temperature from 200 ° C to 600 ° C indicating that the surface becomes hydrophilic. In addition, this confirms the transformation of ZnO bonds into polar bonds and the decrease in surface energy.