Processing and Study of Optical and Electrical Properties of (Mg, Al) Co-Doped ZnO Thin Films Prepared by RF Magnetron Sputtering for Photovoltaic Application

In this study, high transparent thin films were prepared by radio frequency (RF) magnetron sputtering from a conventional solid state target based on ZnO:MgO:Al2O3 (10:2 wt %) material. The films were deposited on glass and silicon substrates at the different working pressures of 0.21, 0.61, 0.83 and 1 Pa, 300 °C and 250 W of power. X-ray diffraction patterns (XRD), atomic force microscopy (AFM), UV-vis absorption and Hall effect measurements were used to evaluate the structural, optical, morphological and electrical properties of thin films as a function of the working pressure. The optical properties of the films, such as the refractive index, the extinction coefficient and the band gap energy were systematically studied. The optical band gap of thin films was estimated from the calculated absorption coefficient. That parameter, ranged from 3.921 to 3.655 eV, was hardly influenced by the working pressure. On the other hand, the lowest resistivity of 8.8 × 10−2 Ω cm−1 was achieved by the sample deposited at the lowest working pressure of 0.21 Pa. This film exhibited the best optoelectronic properties. All these data revealed that the prepared thin layers would offer a good capability to be used in photovoltaic applications.


Introduction
Recently, transparent conducting oxide (TCO) materials are gaining much attention due to their physical properties. They are very promising for commercial applications such as displays, photovoltaic cells and light emitting diodes [1][2][3][4]. Among TCO materials, zinc oxide (ZnO) is one of the most used because of its relatively large band gap energy of 3.3 eV at room temperature, large exciton binding energy of 60 meV [5], high optical transmittance of ≥80% and Hall mobility at around 200 cm 2 /Vs at The preferential orientation was (101), and the (002) reflection can be fitted onto three Gaussian peaks, as shown in the inset of Figure 1. It can be observed three peaks with maxima at 2θ = 34.31°; 34.45° and 34.59°. The main peak at 34.45° was related to the (002) reflection, while the other two were assigned to ZnMgO phase, as demonstrated previous works with samples elaborated by sol-gel method [12,21]. Extra peaks related to Al2O3 or MgAl2O4 phases were not detected.
The crystallite size was estimated using the FWHM of the preferred orientation, (101) peak, using the Scherrer formula [21]: where K is the Scherrer constant with a value of 0.9, λ is the wavelength of incident radiation, β is the full width at half maximum (FWHM) and θ is Bragg's angle. The average lattice strain ε of the prepared target is obtained from the following expression: ε = β 4 tan θ (2) Finally, the dislocation density is calculated using [22]: Both parameters notify about the magnitude of defects in the crystal. Table 1 shows the comparison of the structural parameters of the ZnO target doped with different amount of MgO, specifically with 2 wt % and 10 wt %, and the ZnO target co-doped with MgO and Al2O3(10 wt % and 2 wt %, respectively). As it can be appreciated, when Al2O3 powder was incorporated in the mixture, a shift of the (101) peak position to lower 2-theta value, smaller nanoparticle size, higher lattice distortion and a wider FHWM were observed. Those phenomena may be related with the elongation suffered in the crystal structure due to the incorporation of Al 3+ with higher ionic radii than Mg 2+ ; and also by the introduction of the important amount of MgO of 10 wt % and formation of ZnMgO phase that could lead to an increase of defects and disorder into the ZnO lattice [23,24].
In this work, the ZnO-MgO:Al2O3 (10:2 wt %) target was used to deposit thin films to take advantage of the ZnMgO phase appearance for solar cells applications. The preferential orientation was (101), and the (002) reflection can be fitted onto three Gaussian peaks, as shown in the inset of Figure 1. It can be observed three peaks with maxima at 2θ = 34.31 • ; 34.45 • and 34.59 • . The main peak at 34.45 • was related to the (002) reflection, while the other two were assigned to ZnMgO phase, as demonstrated previous works with samples elaborated by sol-gel method [12,21]. Extra peaks related to Al 2 O 3 or MgAl 2 O 4 phases were not detected.
The crystallite size was estimated using the FWHM of the preferred orientation, (101) peak, using the Scherrer formula [21]: where K is the Scherrer constant with a value of 0.9, λ is the wavelength of incident radiation, β is the full width at half maximum (FWHM) and θ is Bragg's angle. The average lattice strain ε of the prepared target is obtained from the following expression: Finally, the dislocation density is calculated using [22]: Both parameters notify about the magnitude of defects in the crystal. Table 1 shows the comparison of the structural parameters of the ZnO target doped with different amount of MgO, specifically with 2 wt % and 10 wt %, and the ZnO target co-doped with MgO and Al 2 O 3 (10 wt % and 2 wt %, respectively). As it can be appreciated, when Al 2 O 3 powder was incorporated in the mixture, a shift of the (101) peak position to lower 2-theta value, smaller nanoparticle size, higher lattice distortion and a wider FHWM were observed. Those phenomena may be related with the elongation suffered in the crystal structure due to the incorporation of Al 3+ with higher ionic radii than Mg 2+ ; and also by the introduction of the important amount of MgO of 10 wt % and formation of ZnMgO phase that could lead to an increase of defects and disorder into the ZnO lattice [23,24].
In this work, the ZnO-MgO:Al 2 O 3 (10:2 wt %) target was used to deposit thin films to take advantage of the ZnMgO phase appearance for solar cells applications.

Structural Properties of AMZO Thin Films
The XRD spectra of AMZO thin films deposited at 300 • C at different working pressures are shown in Figure 2.

Structural Properties of AMZO Thin Films
The XRD spectra of AMZO thin films deposited at 300 °C at different working pressures are shown in Figure 2. All AMZO thin films presented hexagonal wurtzite structure with (101) as a preferred orientation. However, the increase of the sputtering pressure evolved the polycrystalline structure of ZnO. The (002) reflection peak appears as a preferred orientation at the highest working pressure of 1 Pa. This explains the tendency of a change in the grain orientation towards an ordered structure at those deposition conditions. Several studies about Mg and Al co-doping ZnO thin films fabricated by RF magnetron sputtering consider the (100) direction as the preferred orientation [24], but a All AMZO thin films presented hexagonal wurtzite structure with (101) as a preferred orientation. However, the increase of the sputtering pressure evolved the polycrystalline structure of ZnO. The (002) reflection peak appears as a preferred orientation at the highest working pressure of 1 Pa. This explains the tendency of a change in the grain orientation towards an ordered structure at those deposition conditions. Several studies about Mg and Al co-doping ZnO thin films fabricated by RF magnetron sputtering consider the (100) direction as the preferred orientation [24], but a change can occur due to the large density of target impurities [25]. In addition, a small extra (200) peak related to MgO phase was observed. The appearance of this peak was explained because of the high concentration of MgO in the target. No phases corresponding to ZnAl 2 O 3 and Al 2 O 3 indicating the effective substitution of Zn 2+ with Al 3+ ions. It seems that the increase of the working pressure did not affect significantly the position of diffraction peaks but rather their intensity. The influence of working pressure was also investigated by the evaluation of the average crystallite size calculated from (002) and (101) reflection peaks using the Scherrer Formula (1). The results show that crystallite size varied from 26 to 28 nm as function of the working pressure, as it is summarized in Table 1. The strain ε was kept nearly to 0.004 for all samples and hence, it was not influenced by the increase of the working pressure. In addition, Table 1 listed the calculation of the residual stress σ in AMZO thin films, by using the following formula [26]: where C ij are elastic stiffness constants, C 13 = 1.05 × 10 11 Pa, C 33 = 2.1 × 10 11 Pa, C 11 = 2.1 × 10 11 Pa, C 12 = 1.2 × 10 11 Pa [27], C is the lattice parameter and C 0 is the bulk value, 5.206 Å. The numerical calculation leads to this summary expression: The obtained values did not depend on the working pressure and they were in the order of 4.495 10 11 Pa for all samples, as it was expected. The films deposited at working pressures below 0.61 Pa exhibited the (101) as a preferred orientation, identical to what was shown in the spectrum of the nano powders (see Figure 1). In addition, the intensity of (101) peak was higher than (002) peak, almost negligible at that pressure range, suggesting that the surface energy of (101) was the lowest at those sputtering conditions. However, at working pressures of 0.83 Pa, the (002) peak began to appear with the same intensity that the (101) peak. At the highest working pressure of 1 Pa, the (002) crystallographic direction became the preferential orientation, indicating that the preferential orientation of the crystallites was perpendicular to the film surface. It is believed that this change in the preferred orientation was a consequence of a self-ordering caused by the minimization of the crystal surface energy [28], and by the change in diffusion rate of atoms at the surface during the deposition process resulting on the increase of the working pressure [29].

Surface Morphological Analysis
AFM images of AMZO thin films deposited at 300 • C and at different working pressures are shown in Figure 3. Grains with a feature size of 22-30 nm were presented in the films, similar to those values calculated from XRD spectra (see Table 1). Spherical, uniform and dense grains were also produced throughout the surface.
The sample deposited at the lowest working pressure of 0.21 Pa presented some voids, which were reduced with the working pressure. Consequently, the root-mean-square (RMS) roughness was decreased from 2.1 to 1.3 nm as working pressure increased up to 0.6 Pa attributed to the slightly smaller crystallite size. For working pressures above this value, the RMS increased quickly up to 2.5 nm, in agreement with the observed enhancement of the c-axis orientation for the sample deposited at 1 Pa. The more the working pressure increased, the higher the number of Zn atoms were substituted by dopant elements. This fact favored the increase of the intensity of (002) orientation, leading to a better structural ordering. This hypothesis was also confirmed by other authors using different dopants that also observed an increase of the roughness [30,31]. In addition, the increase of surface roughness endorsed oxygen absorption on the crystallites surface's which created dangling bonds performing as electron traps. These electron traps would be the responsible on the reduction in carrier concentration [32] behavior that will be discussed later.

Optical Properties
The transmittance and reflectance spectra of the sputtered films are pictured in Figure 4a,b. As it can be appreciated, the films were highly transparent in the visible wavelength range. The transmittance T% was close to 80% and it was enhanced with the working pressure. The absorption edge in the ultraviolet range was investigated by the evaluation of the band gap of the films. As it can be observed in Figure 4, the UV absorption edge moved toward the shorter wavelengths, which may be related to the variation of residual stress and crystal grain size of the films. The band gap was estimated by the absorption coefficient with respect to the incident photon energy [33] as followed:

Optical Properties
The transmittance and reflectance spectra of the sputtered films are pictured in Figure 4a,b. As it can be appreciated, the films were highly transparent in the visible wavelength range. The transmittance T% was close to 80% and it was enhanced with the working pressure.

Optical Properties
The transmittance and reflectance spectra of the sputtered films are pictured in Figure 4a,b. As it can be appreciated, the films were highly transparent in the visible wavelength range. The transmittance T% was close to 80% and it was enhanced with the working pressure. The absorption edge in the ultraviolet range was investigated by the evaluation of the band gap of the films. As it can be observed in Figure 4, the UV absorption edge moved toward the shorter wavelengths, which may be related to the variation of residual stress and crystal grain size of the films. The band gap was estimated by the absorption coefficient with respect to the incident photon energy [33] as followed: The absorption edge in the ultraviolet range was investigated by the evaluation of the band gap of the films. As it can be observed in Figure 4, the UV absorption edge moved toward the shorter wavelengths, which may be related to the variation of residual stress and crystal grain size of the films. The band gap was estimated by the absorption coefficient with respect to the incident photon energy [33] as followed: (αhυ) 2 = A hυ − E g (6) where, A is a constant, hυ is the photon energy, E g is the band gap energy and α is the absorption coefficient.
Materials 2020, 13, 2146 8 of 12 Figure 5 shows the Tauc plot as function of the working pressure. The band gap was obtained by extrapolating the linear part of the curves to the horizontal axis. A monotonic decline was very well seen as the working pressure rising from 0.21 to 0.83 Pa. A decrease from 3.921 to 3.655 eV with increasing the working pressure from 0.21 to 1 Pa was observed. where, A is a constant, hυ is the photon energy, Eg is the band gap energy and α is the absorption coefficient. Figure 5 shows the Tauc plot as function of the working pressure. The band gap was obtained by extrapolating the linear part of the curves to the horizontal axis. A monotonic decline was very well seen as the working pressure rising from 0.21 to 0.83 Pa. A decrease from 3.921 to 3.655 eV with increasing the working pressure from 0.21 to 1 Pa was observed. On one hand, it can be remarked that these values were larger than the band gap energy of bulk undoped-ZnO, estimated to be 3.37 eV, despite the absence of quantum confinement effect since the crystallite sizes were larger than 10 nm. This blue shift may be attributed to the enhancement of the carrier concentration in ZnO, known as the Burstein-Moss effect [34,35]. In fact, AlZn donors [36] can be created from the process of Al 3+ doping. By following the same analysis, the reduction of the band gap energy of AMZO film with the working pressure was attributed to a reduction of carrier concentration. More ionized impurity scattering, impurity clustering and grain boundary that would lead to a loss of free electrons, were certainly created by increasing the working pressure.
The following equation describes the Burstein-Moss effect [37]: where h is the Planck constant, N is the carrier concentration, and me is the effective mass of electrons. From the above equation, the optical band gap is proportional to the carrier concentration. Here, few electrons populating the states near the bottom of the conduction band would explain the reduction of the band gap energy [38]. Figure 6 shows the evolution of refractive index n and extinction coefficient K of AMZO thin films deposited at different working pressures.  On one hand, it can be remarked that these values were larger than the band gap energy of bulk undoped-ZnO, estimated to be 3.37 eV, despite the absence of quantum confinement effect since the crystallite sizes were larger than 10 nm. This blue shift may be attributed to the enhancement of the carrier concentration in ZnO, known as the Burstein-Moss effect [34,35]. In fact, Al Zn donors [36] can be created from the process of Al 3+ doping. By following the same analysis, the reduction of the band gap energy of AMZO film with the working pressure was attributed to a reduction of carrier concentration. More ionized impurity scattering, impurity clustering and grain boundary that would lead to a loss of free electrons, were certainly created by increasing the working pressure.
The following equation describes the Burstein-Moss effect [37]: where h is the Planck constant, N is the carrier concentration, and m e is the effective mass of electrons. From the above equation, the optical band gap is proportional to the carrier concentration.
Here, few electrons populating the states near the bottom of the conduction band would explain the reduction of the band gap energy [38]. Figure 6 shows the evolution of refractive index n and extinction coefficient K of AMZO thin films deposited at different working pressures. where, A is a constant, hυ is the photon energy, Eg is the band gap energy and α is the absorption coefficient. Figure 5 shows the Tauc plot as function of the working pressure. The band gap was obtained by extrapolating the linear part of the curves to the horizontal axis. A monotonic decline was very well seen as the working pressure rising from 0.21 to 0.83 Pa. A decrease from 3.921 to 3.655 eV with increasing the working pressure from 0.21 to 1 Pa was observed. On one hand, it can be remarked that these values were larger than the band gap energy of bulk undoped-ZnO, estimated to be 3.37 eV, despite the absence of quantum confinement effect since the crystallite sizes were larger than 10 nm. This blue shift may be attributed to the enhancement of the carrier concentration in ZnO, known as the Burstein-Moss effect [34,35]. In fact, AlZn donors [36] can be created from the process of Al 3+ doping. By following the same analysis, the reduction of the band gap energy of AMZO film with the working pressure was attributed to a reduction of carrier concentration. More ionized impurity scattering, impurity clustering and grain boundary that would lead to a loss of free electrons, were certainly created by increasing the working pressure.
The following equation describes the Burstein-Moss effect [37]: where h is the Planck constant, N is the carrier concentration, and me is the effective mass of electrons. From the above equation, the optical band gap is proportional to the carrier concentration. Here, few electrons populating the states near the bottom of the conduction band would explain the reduction of the band gap energy [38]. Figure 6 shows the evolution of refractive index n and extinction coefficient K of AMZO thin films deposited at different working pressures.  The refractive index is related to the transmittance, reflectance and extinction coefficient K by the following expression [39,40]: where the K values were very low, in the range of 10 −8 , indicating the low dielectric losing in these films. It may be considered a qualitative indication of the excellent smoothness of thin films [41]. The refractive index plotted in Figure 6a showed a decrease in the main peak intensity between 400 and 600 nm with increasing the working pressure.  [42]. These relatively high values are preferred for antireflection coating materials in many optoelectronic device applications. As Z.C. Tu et al. [43] reported, the energetic position of the maximum band of the refraction index was directly related with the band gap energy of ZnO. Here, by increasing the working pressure, the band gap energy and the refractive index reduce slightly, which may be due to the formation of impurities and defects.

Electrical Study
The conductivity σ, the Hall mobility µ and the carrier concentration n of AMZO thin films are shown in Table 2. The carrier concentration n was severely reduced with the working pressure, which is in good agreement with the Burstein-Moss effect adopted to justify the large reduction of the band gap energy E g , estimated to be 0.2 eV. It has been reported [44] that the increase of the working pressure induces a reduction of the kinetic energy of the dopant atoms which would be limiting their energy of surface diffusion. Consequently, the activation amount of Al 3+ and Mg 2+ dopants would decrease. Thus, the carrier concentration also decreased with the working pressure.
To the contrary, the Hall mobility µ presented the opposite evolution of the carrier concentration n, while the product of µ and n decreased with the working pressure. As consequence, the decrease of the conductivity σ with the working pressure can be attributed since σ is proportional to µ and n. Finally, the high improvement of the mobility was due to the lower carrier concentration and hence the reduction of the number of collisions between the electrons into the grain boundaries. In addition, the loss of kinetic energy was lower, which offered both greater area and energy to the carriers to move freely.

Conclusions
AMZO thin films were successfully deposited on Corning glass by RF magnetron sputtering from a home-made fabricated 4-inch diameter ZnO-MgO:Al 2 O 3 (10:2 % wt) target using the conventional solid-state method. Prior to be used in the sputtering system, the mechanical stability and the structural quality of the target to fabricate AMZO thin films were evaluated and demonstrated.
With regards to the AMZO thin films, the effect of the working pressure on their structural, optical and electrical properties was studied. A progressive change of the axis preferred orientation of the growth from (101) to (002) was observed, but no significant change in the crystallite size and the lattice strain was detected. Hence, XRD results put into evidence that increasing the working pressure enhanced the crystalline quality of the films. Optical analysis showed larger values of the band gap energy of the AMZO films compared to that of un-doped ZnO, attributed to the Burstein-Moss effect. However, the reduction of the band gap energy with the working pressure was due to the decreased of carrier concentration, as it was revealed from the Hall measurements. These measurements showed that the increase of working pressure affected the electrical conductivity as a result of the inverse trend of carrier concentration and Hall mobility. Finally, the good optical and electrical performance of the AMZO films presented in this work make them suitable for optoelectronic and photovoltaic applications.