Effect of Annealing Temperature on Structural and, Morphological Properties of Zno Thins Films

Moustafa Debbab (  mus.debbab@gmail.com ) Abou Bekr Belkaid University Tlemcen: Universite Abou Bekr Belkaid Tlemcen Nassera Ghellai Université Abou Baker Belkaid Meymoun Belaoui Université Abou Baker Belkaid Cha aa Yaddadene Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE), Malika Berouaken Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE), Noureddine Gabouze Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE),


Introduction
In recent years, much researches have been carried out on different semiconductors, including zinc oxide (ZnO) which has a band gap of 3.37ev, with a high exciton binding energy (60 meV) [1]. Zinc oxide has attracted a great attention of many scientists and researchers for its different advantages such as its structural and morphological properties, its non-toxicity and its abundance of its components, its physical and chemical stability, its biocompatibility, high photosensitivity, piezoelectric and pyroelectric properties [2][3][4]. Because of its exceptional properties the ZnO has been exploited in various applications, including light emitting diodes, eld effect transistors, gas sensors, and super capacitor, photovoltaic cells, etc [5][6].
There are many methods of elaborations of thin lm of ZnO, two main ways can be noted: the physical way, such as sputtering, deposition by evaporation, growth by molecular jets, laser ablations, the methods consist in making the lms from the material coming from a target [7]. However, these methods present several disadvantages such as they require very heavy equipment and very particular working conditions for example; vacuum, temperature, etc [8]. Unlike the chemical process, which consists in developing the lm by chemical reaction or decompositions of a molecule, such as chemical vapor deposition CVD, spreading by centrifugation (spin coating), sol-gel method, spray pyrolysis method, or the electrodeposition method [9] [10], which has been chosen in this work. Thin lms of ZnO can be deposited from the latter, using a rich solution of Zn 2 + ions in an oxygen-rich medium, in our case, we choose hydrated hexa zinc nitrate as the source of Zn 2 + ions, and potassium nitrate as supporting electrolyte [11][12]. Different researchers have studied the effect of the annealing temperature on the different properties of ZnO, such as morphology and structure [13][14]. Recently A. Al Zahrani and A.
Zianal have studied this effect [15] by increasing the temperature from 300 ° C to 450 ° C with a step of 50 ° C. The obtained results indicate a decrease in particle size from 108.5 nm to 107.5 nm with an increase in the crystallite size from 17.41 to 19.51 nm. In this work, the effect of the annealing temperature (from 200° C to 600 ° C) on different parameters such as inter-planar spacing, cell volume, dislocation density, the micro strain, and the crystallite size has been studied. The structure and morphology of ZnO thin lms upon annealing temperature was investigated using X-ray diffractometry (XRD) and SEM scanning electron microscopy technique, supported by surface characterization by measuring the contact angle.

2.2-Electrodeposition of ZnO thins lms
Zinc oxide layers were electrochemically deposited on n-type Si substrate using the chronoamperometry method. First, the silicon wafers are cut with a diamond point into small silicon substrates of 1.5 cm x 1.5 cm, and then rinsed in three ultrasonic baths. The rst one contains the tricloro-amine heated at 60 ° C during 20 minutes, after that the substrates are successively heated in acetone and ethanol at 55 ° C during 10 minutes, nally the wafers were rinsed in DI water and dried with nitrogen gas ow. The anodic electrodeposition of the ZnO layers was performed by using a simple electrochemical technique with a conventional three-electrode system, where the reference electrode was Ag/AgCl, the counter electrode was Platinum and n-Si substrates as the working electrode. Before each electrodeposition, the silicon substrates were dipped in hydro uoric acid (10%) in order to remove the native oxide layer from the surface. The solution used was containing 6.10 -3 M zinc nitrate hexahydrate Zn(NO 3 ) 2 -6H 2 O, and 1.10 -1 M potassium nitrate KNO 3 which was prepared by using deionized water (EDI). The temperature was kept at 68 ° C during the electrodeposition and the deposition time was 30 min. for all experiments. An Autolab 128N potentiostat-galvanostat was used with a NOVA 1.8 control software.
The electrochemical synthesis of ZnO was carried out mainly by the method of raising the local pH, at the level of the working electrode, as cited by Peulon et al. [16]. According to several articles, the electrochemical formation mechanism of ZnO is initiated by the reduction of nitrate ions that produces hydroxide ions, followed by the precipitation of Zn(OH)2 [11,17,18]. If the temperature is su ciently high (68 ° C in our study), this hydroxide dihydrates to form zinc II oxide (reaction 3) [12,18]. The sequence of the ZnO deposition can be summarized by the following equations [19,20]: In order to de ne the polarization voltage for the ZnO electrodeposition, a cyclic voltammetry has been performed. Figure 1 shows cyclic voltammetry in a potential range from + 0,5 to -1,6 V with respect to Ag / AgCl at a scan rate of 10 mV s -1 . During the return sweep we note the existence of a peak around -1,2V which corresponds to the formation of ZnO [21], during the reverse scan direction no peak has been detected suggesting the absence of metallic zinc [22]. The results indicate that the applied voltage for the ZnO plating should be around -1.2V according to several works [9,19, 21,].

2.3-Materials characterization
In order to study the effect of the annealing temperature on the structural properties of thin zinc oxide, we used an X-ray diffractometer (XRD, Ultima IV, Itm2036E302) with radiation λCuKa = 1.54059A °, the voltage generator and tube current were (40kV, 40mA). The slew rate was 5,000 deg / min with a step of 0.05 °, the samples were scanned from 30 to 80 °. The contact angle measurements were carried out using a DGIDROP (GBX), using 3 μl drops of EDI deionized water deposited on the substrate. The system has built-in digital camera controlled using standard Visiodrop software. The morphologies of the obtained lms have been investigated using an electron microscope JEOL JSM-6360 LV.

Results And Discussions
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  (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 [23]. These results are con rmed 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 [15].

……………………….. (1)
The inter-planar spacing d hkl 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 ………………………….. (2) The volume of the unit cell V of the ZnO layer without annealing temperature was estimated by using equation (3) [27], 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)  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 lms 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 re ections from Bragg's angles at ~ 31.7 °, 34.3 °, 36.2 °w hich correspond to the hexagonal phase of ZnO lattice planes of orientations (100), (002), (101), respectively. The diffraction re ections 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 [23]. 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 lms. 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 lms. Figure 3 (b) reveals that the peak positions for the (100), (002) and (101) orientations of the ZnO lms 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 [27].
The inter-planar spacing, the dislocation density and the unit cell volume of ZnO lms 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 lms 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 micropillars of hexagonal shape oriented perpendicularly to the surface as shown in the magni cation 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 [28], 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 con rms the hydrophobicity of the ZnO surface [29].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 con rms the transformation of ZnO bonds into polar bonds and the decrease in surface energy.

Conclusion
ZnO thin lms have been successfully elaborated by electrodeposition method on silicon substrates. In this present work, we have based our study on the effect of annealing temperature on the morphology and structure of ZnO layers. The results showed that by increasing the temperature up to 500 ° C the crystallinity is also increased and a maximum crystal size of ZnO grains of 20 nm is obtained, on the other hand the micro strain is reduced, also is seen that the inter planar spacing, the density of dislocation and cell volume are the same for all temperatures except for 600 ° C. The contact angle measurements revealed to us that the deposited ZnO layer is hydrophobic at temperatures below 200 ° C. SEM images showed that the ZnO deposit exhibited sand-rose structure at temperatures below 200 ° C, while at temperatures higher, the structure becomes granular and textured, we deduce that the annealing temperature provides an average of controlled surface, structure and morphology of zinc oxide lms.    XRD patterns of ZnO without annealing temperature.