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Effect of Al incorporation in ZnO deposited films through Chemical Reactive Spray technique M

Effect of Al incorporation in ZnO deposited films through Chemical Reactive Spray technique

M.Sahal a * and B. Marib
a Department of Physics Chemistry, Polydisciplinary Faculty of Ouarzazate, Ibn-Zohr University, Ouarzazate (Morocco)
[email protected]
b Department of Applied Physics-IDF, Polytechnic University of Valencia, Valencia (Spain)
[email protected]

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Abstract
Undoped and Al-doped zinc oxide (AZO) layers, greatly transparent with a high mobility were prepared at a bas substrate temperature 573.15 K using chemical reactive spray technique on the glass substrate of Pyrex. Effect of incorporation of aluminum in the zinc oxide films lattice was achieved by diffraction of X-ray, spectroscopy of Raman, electrical measurement and optical analysis. All deposited layers reveal a hexagonal wurtzite structure. In fact, high amount of Al (> 2%) in deposited ZnO films produces to a reduction of the quality of the wurtzite crystal structure, and the films tend to get disorderly with a random crystallites orientation. Furthermore, the spectra of X-ray result study show a diminution of “c” lattice parameters with the raise of aluminum amount in zinc oxide layers, this trend, which follows the VEGARD’S law, so denote a substitution of zinc atoms by aluminum.
Low resistivity was found for AZO film at 3 % aluminum. A blue shift of optical gap was noted with the rise in aluminum content introduced in ZnO films. This enlargement of the bandgap is interpreted by the effect of Burstein-Moss revealed the rise in the concentration of carrier charges due to the incorporation of aluminum in the zinc oxide lattice. A density of free electron upper than 2.0×1019 cm-3 is obtained for the AZO thin films at 5% of Al.
This result encourages the employ of AZO deposited films at bas temperature like good electrodes and optic window in photovoltaic appliances.
Keywords-component; AZO thin films; Chemical Reactive Spray; XRD; Spectroscopy of Raman; Electric properties; Bandgap.?
Introduction
ZnO is a semiconductor of n-type with a large prohibited band commonly used in various industrial fields due to their interesting physicochemical properties.
ZnO is an II-VI binary material, non-toxic, abundant in nature, photoconductive and piezoelectric. It crystallizes in a hexagonal Wurtzite structure material belonging to (6mm) class, and exhibit native “n” type conductivity due to residual donors. It owns a direct gap around 3.2 eV 1, characterized by a great transmittance coefficient of about 85% in the visible spectrum with a large energy of exciton binding (60 meV) 1-3 compared to the gallium nitride (25meV). However, the electrical resistance of ZnO can be controlled in the scale of 10-3-105 ? • cm 4 via introduce the vacancies of oxygen acting as contributors otherwise by doping them with impurities, usually the group elements III, such as Ga, In, B, or Al 3, 5-8.
Diverse deposition methods were used to prepare this material in different morphologies (thin layers, clusters, wires, whiskers, nanocolumns, etc.) such as magnetron sputtering (SP) 9, RF sputtering 10, chemical vapor deposition (CVD) 11, pulsed laser deposition (PLD) 12,13, molecular beam epitaxy (MBE) 14, electrodeposition, sol-gel 15-19 and spray pyrolysis 20-23. This last method is mainly attractive due to its ease, able and none contaminate production. It is inexpensive to manipulate, no complicated elements or tools are necessary, well adapted for the production of mass and it simple to incorporate in an industrialized production line and to generate big surface coatings without the must of an ultra-high void. It is a superb technique to deposit the metallic oxides thin layers.
It’s for that reason, our article is not only dedicated to the elaboration and characterization of intrinsic and AZO layers by the chemical reactive spray technique at bas substrate temperature although to aims the involve of the deepen the understanding of the incorporation of Al in the ZnO lattice. To achieve this purpose, the role of the introduction of aluminum on structural, electrical, and optic properties of deposited layers has been investigated.
Experimental details
Undoped and AZO thin layers with a thickness of about 520 nm were elaborated by a chemical reactive spray technique on substrates of Pyrex glass at 573.15 K. The substrates Glass Pyrex were mop by detergent solution, ethanol, diluted HNO3 acid, and completely cleaned in bidistilled water
Acetate f zinc dehydrates Zn (CH3COO)2•2H2O and chloride of aluminum hexahydrate was employed AlCl3•6H2O like a sources of Zn and Al atoms, respectively.
The solution molarity was set at a value of 15×10-3mol/L and the percent of Al dopant (Al) was changed ranging 0 to 5% with respect to the concentration of zinc.
The precursor solution of zinc was dissolved in a mixed solution of bidistilled water and ethanol at the fraction (4:1). A few droplet of HCl acid were added in the initial solution to fix the pH of the sprayed solution.
The solution flux rate and the pressure of air gas were preserved constant at 4ml/min and 75×103 Pa, respectively. The substrate-beak distance was fixed at 28cm.
The structural properties of the as-grown AZO thin films were performed by X-ray diffraction using a Rigaku Ultima IV diffractometer in the standard configuration ?-2? via the radiation of CuK? (1.5418 Å). Structural properties were as well measured using a Raman scattering with a LabRAM HR UV spectrometer using a laser excitation line of about 532 nm. The resolution of spectra is better than 3 cm-1. The electrical resistivity was performed using the four-point probe method and optical characterization of the films was measured using a transmittance by means a lamp of Deuterium-Halogen (DT-MINI-2-GS Micropark) in associated to a spectrometer of HR-4000 Ocean Optics USB optimized for the UV-VIS spectrum.
Results and discussion
The X-ray spectra of AZO thin layers at diverse content of aluminum (0, 1, 2, 3, 4 and 5 %) prepared at 573.15 K is shown in Figure 1. All measurements were registered in a standard configuration (?-2?) under a scale angles 2? among 20 and 80 degrees.
The X-ray spectra of all specimens of AZO thin films indicate that are of polycrystalline nature of a hexagonal wurtzite structure with a preferential direction 002 normal to the plane of the substrate. This result is in good accordance with the results obtained by many authors 1, 24.
Moreover, we observe that as the Al quantity augments the intensity of 002 preferred orientation peak .is diminished, with the outward appearance of three extra peaks respectively at 2? = 31.65°, 36.19°, and 67.82° corresponding respectively to the family of planes (100), (101) and (112) of the ZnO hexagonal wurtzite structure. Furthermore, we can conclude the absence of any second crystalline phase, such as free aluminum or aluminum oxide, in particular Al2O3.
Increasing the content of Al in deposited thin layers generate a peaks displacement to the great angles 2? (Inset figure 1). In effect, a variation of lattice parameters of the structure implicates a change in the ? values, we can tell that these shift of peaks affect lattice size and the lattice parameters structure. The change of the lattice size of ZnO implies than the aluminum atoms can have two sites to occupy in ZnO lattice, whether in substitution place or in interstice site.

Figure 1. X-ray spectra of AZO layers at different Al content varying from 0 to 5 %. Inset: Detail of the (002) peak diffraction.
However, to determinate the nature of aluminum atom incorporation site in ZnO lattice, we studied the evolution of the crystallites sizes versus the aluminum concentration in deposit AZO thin layers.
As the ionic radius of the Al3+ (0.54Å) is minus than the radius ionic of Zn2 + (0.74Å), it’s provided by the law of VEGARD’s 25, 26 that if the process of the incorporation occurs by substitution site, the plus of aluminum ought to reduce appreciably the lattice of ZnO. On another way, if the integration process of Al in ZnO takes place through interstitial sites, this effect should enlarge the lattice.
Al (%)
in starting solution FWHM (Degrees) Grain size (nm)
0 0.1480 56.21
1 0.1430 58.18
2 0.1200 69.35
3 0.1420 58.61
4 0.2160 38.53
5 0.2162 38.52
Table 1. The evolution of crystallite size vs the aluminum amount in the sprayed solution.
The result grouped in Table 1 illustrates the variation of the crystallite size versus the aluminum quantity in the initial solution.
A marked rise in grain size can be observed with the rise of aluminum content in the initial solution. We can interpret that rise in grain sizes according to the VEGARD’s law by the occupancy of the interstitial spaces by the atoms of aluminum. By against, past Al-doped ZnO at 2% in the preliminary solution, the grain sizes diminish (Table.1), i.e. a decrease of lattice parameters “c” and “a”, which in follows VEGARD’s law, tend to indicate a substitution of the zinc atoms by the aluminum atoms into ZnO lattice. This result is in good accord with the obtained results by many authors 27, 28.
However, the decrease of diffraction intensities with the augment of Al concentration in the starting solution denotes that the overdone addition of Al (over than 4%) produce the disorientation of the crystallites, which reach out to a crystal structure distortion of the material. Therefore, we can interpret that the extra insertion of aluminum in the interstitial positions will produce an important defects number. Thereby generate a deformation of the structure of the crystal of the material.
ZnO hexagonal wurtzite structure behooves to the P63mc group space. The primal cell f ZnO possess four atoms, where each atom occupying C3v sites, leading 12 branches of phonon, three acoustic modes and nine optical modes 29. At the Brillouin zone center ”?”, the optic phonons enclose the inveterate representation ? “?” ?_opt=A_1+2B_1+E_1+2E_2 30, 31, while the modes of E are double degenerate.
The E2 (low) mode, low-frequency mode is assimilated to the zinc atoms vibrations of the subcells 32, while the high-frequency mode E2 (high) is attributed to oxygen atoms vibrations sublattice 33-36. In addition, the E1 and A1 are polar modes, together Infrared and Raman active and therefore both split into longitudinal optic modes (E1-LO and A1-LO) resultant from the beat alongside the c-axis, with various frequencies owing to the macroscopic electrical domain of the LO phonons, and transverse optic modes (E1-TO and A1-TO) arise from the beat in the basic plane. The E2 modes are non-polar and Raman active and the B1 are silent Raman modes 37-39. Towards lattice vibrations with E1 and A1 symmetry, the atoms displace collateral and normal to the axis of “c”, respectively.
Figure 2 shows the room temperature spectra of Raman scattering of intrinsic and AZO layers at diverse content of Al among 1 to 5 % range in the 50-1400 cm-1. We can distinguish several Raman peaks at 97 cm-1, 326 cm-1, 438 cm-1, 572 cm-1, 788 cm-1, 951 cm-1 and 1093 cm-1. Those peaks can be allotted to the E2 (low), TO -TA, E2 (high), A1 (LO), B1+LO, 2TO and 2LO modes of ZnO wurtzite hexagonal structure respectively 5, 40, 41.

Figure 2. Spectra of Raman scattering of deposit AZO layers at the various amount in sprayed solution.
The intense peaks at 438 cm-1 is allotted to the E2 (high) nonpolar optic phonon of zinc oxide, which confirms the hexagonal wurtzite phase of deposited thin films. The A1(LO), Raman mode at 572 cm-1 reveals a considerable red-displacement from 572 to 568 cm-1, which is assigned to the formation of defects in the lattice of ZnO, suchlike interstitials zinc, oxygen vacancy and free carrier lack 38, 39, 42. A significant displacement is noted to the blue of (B1 + LO) Raman mode centered at 794 cm-1. Analogous results were obtained for zinc oxide layers doped with the Al 28, 43. This displacement in the cited position of modes is regular with the reduction of “c” parameter of the lattice of the wurtzite phase 44 (Fig. 2).
The Raman spectra display two strong bands about 438 and 1092 cm-1 related to E2 (high) and 2LO modes. Furthermore, the augment in Al content generates a diminish in the greatness of the Raman mode E2 (high) that don’t have any effect on the intensity of 2LO mode but enlarge the broadness at half maximum (FWHM) of the peaks moving from 22 cm-1 to 38 cm-1 for E2 (high) mode and from 99 cm-1 to 108 cm-1 for a 2LO mode. These outcomes show that the rise of Al concentration in the deposit AZO layers yields a diminution in the goodness of the crystalline system, which decrease the symmetry of the crystal and insert voids and the substitutional defect. Alike results own been reported by diverse authors. 28, 43.
The adding of Al produces a decline in grain size changing from 69 nm for AZO sample at 2 % of Al to 38 nm for Al-doped ZnO films at 5%. Howsoever, the order of the short-scale is keeping as evidenced by the spectra of Raman scattering.
Electrical properties
The figure 3 shows the evolution of the electrical resistivity of intrinsic and AZO thins films versus the Al content ranging 0% to 5%. The measurements were carried out at ambient temperature.
The results show that the resistivity of AZO samples declines while the Al amount rises, attain its lowest value of 1.5 10-3 ?•cm at 3% of Al concentration, and the carelessly augments.

Figure 3. The evolution of electrical resistivity as a function of the aluminum content in the initial solution.
This reduces in resistivity with the increase of aluminum content can be explained by a modify of numbers of carrier concentration charges (electrons) and the mobility resulted from the move of the donor ions Al3+ to Zn2+ cations lattice locations and the adsorption of O2 from the surface of ZnO: Al, grain boundaries and the interstices 45, 46. Furthermore, increasing the Al amount produce an increase of the resistivity, which is perhaps owing to a decline in the mobility carrier charges consequential from Al surplus, such behavior were except like an outcome of the substitution the Zn2+ sites by the Al3+ dopant making a process of additional free carrier charges.
When the Al content is augmented, further aluminum atoms incorporate in lattice spaces of Zn atoms, which increase the carriers of charge. Though, past some critical level of dopant, we can reach the limit solubility of aluminum in the lattice of zinc oxide, and then no extra positions of zinc can be taken by Al dopant. Thereby, the excess aluminum leads to the deformation of the structure of the crystal, like it was revealed in the Raman and X-ray outcome. In this case, the atoms of aluminum do not operate like a dopant, they are dispersed in the boundary of grain, so that act as though center scattering and as a snare for the charge carriers.
Furthermore, Shen et al have attributed this rise of the resistivity, with the raise of the concentration of charge carrier to the diffusion mechanism of the ionized impurity in Al-doped zinc oxide layers 47.
Optical properties
To study the role of aluminum incorporation in zinc oxide lattice on optic properties of deposit layers, we have carried out the optic transmittance for undoped and AZO layers.
Figure 4 exhibit the Al effect insertion in ZnO layers at various content of Al on the behavior of absorption limit.

Figure 4. Transmittance of intrinsic and AZO thin layers at diverse content of Al.
All deposited films are highly transparent and exhibit a great transmittance upper than 82 % in the visible spectrum. Further, the spectra show a blue change of the edge of the absorption that takes place at diverse wavelengths for the Al content in the prepared specimens. Numerous authors have reported alike results for deposit AZO thin films by various methods 45, 48-51. However, as it’s clearly observed that the transition has been always direct regardless the aluminum proportion introduced in the ZnO lattice, then the absorption coefficient (?) has been indeed out from the spectra of transmittance via the equation (1) 52, 53:
?(h?)=1/d ln{(1-R)^2/2T+|R^2+(1-R)^2/2T^2 |^(1?2) } (1)
Where “d” is the width of the samples, “R” is the reflectance and “T” is the transmittance.
Consequently, the optical bandgap energy corresponding to the absorption abrupt edge AZO layers were carried out via the relation (2) 54-57:
(?h?)^2=A(h?-E_g ) (2)
Where ” A ” is a constant.
Figure 5 shows the optical bandgap evolution of AZO films as a function of the Al concentration in the initial solution.

Figure 5. (?h?)2 vs the energy of incidents photon calculated at ambient temperature for undoped and AZO layers at diverse aluminium content .
It’s obvious that the optical bandgap of AZO layers increases with the rises of Al content, from 3.27eV on behalf of undoped zinc oxide to 3.38 eV for AZO films at 5%, which is in good agreement to the outcomes obtained by various authors using diverse methods 4, 50, 58, 59.
This shift in the bandgap of AZO thin layers can be contributed to the augment in the charge carrier’s numbers, which stops the lower positions in the band of conduction, recognized like the effect of Burstein-Moss 60, 61
The Burstein-Moss effect results from the Pauli Exclusion Principle and is observed in semiconductors as a change in the bandgap with the increase in dopant. So it is known as the difference of energy separating a maximum part of the band of valence and the vacant energy positions in the band of conduction. This variation arises for the reason that the Fermi level energy (Ef) is in the band of the conduction for high n-type doping or in the band f the valence for high p-type doping. Such padding state blocks thermal or optical excitation.
The effective optic gap (Eg) can be determinate like the summation of the optical bandgap for undoped matter (Eg0) and the increase of the bandgap energy due to the effect of BM (?EBM) (relation 3) 62:
E_g= E_g0+??E?_(BM ) (3)
In the n-type semiconductor, the (?EBM) energy shift with parabolic bands is determined by the relation (4) 62:
?E_BM=?^2/(2m^* ) (3?^2 n)^(2?3) (4)
Where ? is a reduced Planck’s constant, m * is electron effective mass and ” n ” is a free electron concentration.
The optical bandgap determined from the extrapolation of (?h?) 2 curves and the carrier charge numbers calculated from the relations (3) and (4) as functions of Al concentration are shown in figure 6. The free electron concentration has been determined from the relation (4) employing an effective mass of electron, m* = 0.29•me, and a bandgap for the undoped material, Eg0=3.27 eV.
It’s clearly in the spectra, that the carrier concentration reaches a maximum when the aluminum amount increase in the starting solution and achieve a carrier density of 2.582•1019 cm-3 at 5% of Al amount in starting solution. Furthermore, an except tendency toward saturation of resistivity was observed for Al doping samples at higher dopant concentrations, (at 4% and 5%), while the mobility tends to pursue to downfall as though more diffusion and the effect of the barrier of grain boundaries occur.
Such comportment was anticipated as a result of the substitution of Zn2+ place by Al3+ dopant, thus creating in the procedure an extra free carrier charges. Like the level of the dopant is increased, supplementary dopant elements take place of the lattice spaces of ZnO crystallite, ensuing in an additional density of charge. Although after a critic level of dopant, no most Zn spaces can be filled full by dopant elements owing to the limited of solubility of aluminum in the crystallites of zinc oxide, so the extra Al, lead to deformation of the crystalline structure like it was exposed in the diffractogramme results of X-ray.
As expected, the increase of the Al amount in the initial solution occurs a rise of free carrier charge density and a blue shift of bandgap due to the effect of Burstein-Moss. These results prove the effectiveness of the effect of the incorporation of the aluminum dopant in deposit ZnO films. The high density of carriers charges about 2.582•1019 cm-3 has been achieved for AZO thin films at 5% of Al in the initial solution.

Figure 6. Apparent bandgap and density of free electron agreed to the relations (3) and (4) as a function of Al concentration in the AZO layers. Red stars present the bandgap calculated from the extrapolation of (?h?)2 as a function of Al amount in the sprayed solution. Green triangles are a density of free electron determined for AZO layers using an effective mass of electron m* = 0.29•me and an optic bandgap for the undoped ZnO, Eg0 = 3.27 eV
?
Conclusion
The result of structural, electrical and optical characterization of intrinsic and AZO thin films clearly demonstrates the effect of the incorporation of aluminum in lattice of ZnO.
Structural analysis showed that all deposited layers are hexagonal wurtzite structure with a preferred orientation to the direction 002, furthermore as the aluminum content increase in deposited AZO thin films the preferential peak intensity are decreased. The Raman scattering spectroscopy analysis of the Al-doped ZnO layers proves the wurtzite structure nature of all films with a diminished quality when the Al quantity rises in the sprayed films. Analog results were achieved from XRD analysis.
A lowest resistivity of around a 1.5•10-3 ?•cm was achieved for AZO films at 3% of Al. Further, the apparent optical properties of AZO thin films show a direct gap, with a blue shift gap from a 3.27 eV for intrinsic ZnO to 3.38 eV for AZO films at 5%. This blue shift is attributed to the effect of Burstein-Moss and is obviousness of the rise of the density of carrier’s charges owing to the incorporation of Al in the lattice of ZnO. In addition, a free electron concentration of about 2.582•1019 cm-3 is obtained for AZO layers at 5% in starting solution.
The high conductivity (about 6.67•104 S/m) and the high transmission of deposited layers in the visible range (more than 82%) confirm the potential use of deposited AZO thin layers trough spray pyrolysis as a good optical and conductor window to the manufacture of solar cells.
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