# Structural, optical and electrical properties of single-phase wurtzite ZnMgAlO thin films deposited by ultrasonic spray pyrolysis

Sourav Bose, Christy Fadel, Sidi Ould Saad Hamady, Olivier Perroud, Julien Guyon, Laurent Weiss, Nicolas Fressengeas

► To cite this version:

Sourav Bose, Christy Fadel, Sidi Ould Saad Hamady, Olivier Perroud, Julien Guyon, et al.. Structural, optical and electrical properties of single-phase wurtzite ZnMgAlO thin films deposited by ultrasonic spray pyrolysis. Micro and Nanostructures, 2023, 177, pp.207543. 10.1016/j.micrna.2023.207543. hal-04448096

HAL Id: hal-04448096

https://hal.univ-lorraine.fr/hal-04448096v1

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# Structural, optical and electrical properties of single-phase wurtzite ZnMgAlO thin films deposited by ultrasonic spray pyrolysis

Sourav Bose<sup>a</sup>, Christy Fadel<sup>a</sup>, Sidi Ould Saad Hamady<sup>a,*</sup>, Olivier Perroud<sup>b</sup>, Julien Guyon<sup>b</sup>,
Laurent Weiss<sup>b</sup> and Nicolas Fressengeas<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Ultrasonic Spray Pyrolysis
Magnesium Aluminum Zinc Oxide
Thin-Film
Crystal structure
Optical properties
Electrical properties

## ABSTRACT

Incorporation of various elements in ZnO to modulate its optical and electrical properties is crucial for applications of the resulting alloys in optoelectronics. In particular, the co-incorporation of magnesium Mg and aluminum Al in ZnO to form the quaternary Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O makes it possible to modulate the bandgap by Mg incorporation, and to control the electrical conductivity by Al incorporation. In this article, are presented the results of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O deposition by ultrasonic spray pyrolysis, using nontoxic and easily available precursors, and the study of the thin films structural, optical, and electrical properties. The deposition parameters are optimized in order to obtain wurtzite single-phase thin films with bandgap increasing with composition and transparency, higher than 90 %, in a wide wavelength range. The electrical properties variation exhibits two competing regimes due to the co-incorporation of Al and Mg. On the one hand, the Al incorporation tends to increase the free carrier concentration, and, on the other hand, the Mg incorporation induces a bandgap increase and the inhibition of shallow donors by a compensation mechanism decreasing the carrier concentration. The high transparency of the wurtzite single-phase films along with the modulation of the bandgap and electrical properties can be interesting for optoelectronic applications, in particular in all-oxide solar cells and detectors.

## 1. Introduction

To modulate the optical and electrical properties of ZnO, mainly for use as a transparent conducting oxide (TCO) or active thin film in optoelectronic devices, various elements were incorporated into it to develop a vast family of alloys [1–10]. In particular, the incorporation of aluminum and magnesium to obtain the quaternary ZnMgAlO has potential applications in solar cells as a window or buffer layer but also in display devices or as an active layer in some optoelectronic devices. For instance, Thapa et al. [8] developed by DC sputtering a luminescent device based on Zn<sub>0.83</sub>Mg<sub>0.17</sub>O after a post-annealing at a high temperature of 900 °C while Zhang et al. [6] used pulsed laser deposition to deposit ZnAlO and optimized its electrical properties after annealing in H<sub>2</sub>/N<sub>2</sub> for use as TCO in, e.g., solar cells. Indeed, incorporating Al in ZnO, where Al<sup>3+</sup> ions replace Zn<sup>2+</sup> ions, can induce an increase in the free carrier concentration, while incorporating Mg, with the substitution of Zn<sup>2+</sup> by Mg<sup>2+</sup>, will increase the bandgap without generating additional free carriers. It is therefore potentially possible to jointly modulate the electrical and optical properties of ZnO with the co-incorporation of Mg

and Al. In a recent published work, Tsai et al. [11] prepared the quaternary ZnMgAlO thin films by radio frequency (RF) magnetron sputtering and presented a comprehensive study of their structural, electrical, and optical properties for use in ultraviolet optoelectronic applications. Yang et al. in [12] used pulsed laser deposition to deposit ZnMgAlO and demonstrated the potential of this material in particular for use as TCO in various optoelectronic devices. Another deposition approach, based on a sol-gel technique, was used by Cakiroglu et al. [13] to develop ZnMgAlO nanoparticles with 2 % of aluminum and with magnesium composition varying from 5 % to 20 % and studied their morphology and structural properties. Recently, Karzazi et al. [14] used standard spray pyrolysis to fabricate ZnMgAlO with a fixed aluminum composition of 2 % and a variable magnesium composition from 0 % to 6 %. They used anhydrous zinc acetate, hexahydrated aluminum chloride and magnesium sulfate as precursors. We propose here the development of ZnMgAlO by using ultrasonic spray pyrolysis (USP) deposition technique with aluminum compositions of 0 %, 1 % and 2 % and magnesium composition from 0 % to 7 %, with zinc acetate dihydrate, aluminum acetylacetonate and magnesium acetate tetrahydrate as precursors. The main advantages of using USP combined to the use of nontoxic and easily available precursors are the flexibility to deposit thin films with tuneable parameters, its cost effectiveness and the possibility of easily transferring the developed processes to industry. A comprehensive study of the structural, optical

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<footnote><sup>a</sup>Université de Lorraine, CentraleSupélec, LMOPS, F-57000 Metz, France</footnote>

<footnote><sup>b</sup>Université de Lorraine, CNRS, LEM3, F-57000 Metz, France</footnote>

<footnote>*Corresponding author</footnote>

<footnote>sidi.hamady@univ-lorraine.fr (S. Ould Saad Hamady)
ORCID(s): 0000-0002-6449-3321 (S. Bose); 0000-0002-9662-7216 (C. Fadel); 0000-0002-0480-6381 (S. Ould Saad Hamady); 0000-0001-5078-9626 (J. Guyon); 0000-0001-9438-5279 (L. Weiss); 0000-0002-5534-712X (N. Fressengeas)</footnote>
<table><caption>**Table 1**
High purity chemicals used for ultrasonic spray pyrolysis deposition of ZnMgAlO.</caption><thead><tr><td>Name</td><td>Formula</td><td>Supplier</td></tr></thead><tbody><tr><td>Zinc acetate dihydrate</td><td>Cu(CH<sub>3</sub>COO)<sub>2</sub>·2H<sub>2</sub>O</td><td>Merck 108802</td></tr><tr><td>Aluminum acetylacetonate</td><td>C<sub>15</sub>H<sub>21</sub>AlO<sub>6</sub></td><td>Alfa Aesar 44538</td></tr><tr><td>Magnesium acetate tetrahydrate</td><td>Mg(CH<sub>3</sub>COO)<sub>2</sub>·4H<sub>2</sub>O</td><td>Sigma Aldrich M5661</td></tr><tr><td>Acetic acid</td><td>CH<sub>3</sub>COOH</td><td>Alfa Aesar 10994-AE</td></tr><tr><td>Ethanol anhydrous</td><td>CH<sub>3</sub>CH<sub>2</sub>OH</td><td>Merck-Sigma Aldrich 186414</td></tr><tr><td>Deionized water</td><td>H<sub>2</sub>O</td><td>Avantor-VWR</td></tr></tbody></table>

and electrical properties of the USP deposited ZnMgAlO thin films is presented. The deposition conditions are detailed in section 2.1 alongside with the various characterization techniques in section 2.2. In section 3 are discussed the effect of aluminum and magnesium compositions on the structural, optical and electrical properties of the deposited thin films.

## 2. Experimental

In this section, details of the deposition process and characterization techniques are presented. All the films were deposited at least three times to ensure the reproducibility of the process.

### 2.1. Preparation procedure

The Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films were prepared on soda lime glass (SLG) substrates using a Sono-Tek ExactaCoat USP system equipped with an Impact nozzle operating at 120 kHz. The SLG substrates were systematically cleaned three times using an ultrasonic system successively with acetone, isopropanol and distilled water for 15 minutes at each cleaning. The substrate holder was mounted on a heating plate with temperature regulated by a high-stability PID controller. The substrate temperature and the heating plate temperature were measured using two independent thermocouples. Compressed air was used as carrier gas with a controlled shaping air pressure. The precursors solution was prepared using zinc acetate dihydrate Cu(CH<sub>3</sub>COO)<sub>2</sub>·2H<sub>2</sub>O, magnesium acetate tetrahydrate Mg(CH<sub>3</sub>COO)<sub>2</sub>·4H<sub>2</sub>O and aluminum acetylacetonate C<sub>15</sub>H<sub>21</sub>AlO<sub>6</sub>. Zinc acetate dihydrate and magnesium acetate tetrahydrate were dissolved in deionized water and aluminum acetylacetonate was dissolved in ethanol. All used chemicals are shown in Table 1. Acetic acid was added to the precursor solution to reduce the formation and precipitation of hydroxides. Based on previous work [15], the pH was maintained at a value of 4 by adding the necessary amount of acetic acid to the solution. To optimize the preparation process in order to obtain high quality and transparent thin films, a design of experiments was implemented by varying the main deposition parameters as shown in Table 2.

The solution parameters used to control the Mg and Al composition were set as follows:

1. In order to change the magnesium composition x in Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O, the magnesium acetate tetrahydrate concentration in the precursor solution varied between 0.0015 mol/L and 0.0214 mol/L.

2. In order to change the aluminum composition y in Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O, the aluminum acetylacetonate concentration in the precursor solution varied between 0.0005 mol/L and 0.0026 mol/L.

3. The zinc acetate dihydrate concentration was fixed to a value of 0.05 mol/L.

Ethanol was used to dissolve aluminum acetylacetonate since it is not soluble in water. As soon as it is mixed with water-based solution (used for zinc acetate dihydrate and magnesium acetate tetrahydrate) the solubility is hindered when the concentration reaches a given limit. It was therefore mandatory to limit the aluminum acetylacetonate concentration to relatively low levels, below 5 % corresponding to volume percentages of ethanol in the range from 1 % to 3 %.

### 2.2. Characterization methods

The thicknesses of the deposited films were measured using a Bruker Dektak XT profilometer and a Filmetrics Profilm3D optical profilometer with a vertical resolution of 0.1 nm. The measurements were systematically performed at different regions on the sample surface in order to ensure reproducibility within a 10 nm range.

The morphology of the films was characterized using A FIB-SEM system of the Auriga series from Zeiss with an In-lens detector at an acceleration voltage of 20 kV with a current of 150 pA.

The structural properties of the thin films were analyzed using a Bruker D8 X-ray diffraction (XRD) equipment with cobalt Kα1 radiation. The texture quality of the deposited thin films was estimated using the Lotgering factor (LF). This method is a simple method to determine the preferred orientation by using a powder XRD spectrum as a reference, although it does not give a direct measure of the crystallite orientation distribution [16]. For a given (hkl) peak, the Lotgering factor is given by:
<table><caption>**Table 2**</caption><thead><tr><td>Deposition parameter</td><td>Range</td><td>Optimal value</td></tr></thead><tbody><tr><td>Zinc acetate dihydrate concentration</td><td>[ 0.03 mol/L ; 0.1 mol/L ]</td><td>0.05 mol/L</td></tr><tr><td>Number of spray cycles</td><td>[ 10 ; 100 ]</td><td>20</td></tr><tr><td>Dwell times between each spray cycle</td><td>[ 0.5 s ; 2 s ]</td><td>0.5 s</td></tr><tr><td>Heating plate temperature</td><td>[ 400 °C ; 480 °C ]</td><td>470 °C</td></tr><tr><td>Shaping air relative pressure</td><td>[ 6.8 kPa ; 20.6 kPa ]</td><td>20.6 kPa</td></tr><tr><td>Nozzle to substrate distance</td><td>[ 90 mm ; 110 mm ]</td><td>100 mm</td></tr></tbody></table>

Design of experiment implemented to select the optimal Zn₁⁻ₓMgₓAlᵧO deposition parameters. The thicknesses of all deposited films were about 300 nm. During the deposition, the substrate surface temperature is significantly lower than the heating plate temperature due to (i) the relatively low thermal conductivity of the substrate and substrate holder; (ii) the effect of the spray aerosol. The difference between these two temperatures is about 150 °C.

$$LF = \frac{p - p^{ref}}{1 - p^{ref}}$$

$$p = \frac{I_{(hkl)}}{\sum I_{(hkl)}}$$

$$p^{ref} = \frac{I_{(hkl)}^{ref}}{\sum I_{(hkl)}^{ref}}$$

Where *p* is the considered (hkl) peak integrated intensity divided by the sum of all diffraction peaks intensities for the thin film; *p*<sup>ref</sup> is the equivalent of *p* for the powder used as a reference (RRUFF ID: R060027.1 [17]). For a randomly oriented film, *p* is equal to *p*<sup>ref</sup> and therefore *LF* = 0. If the film is textured and polycrystalline with the (hkl) preferred orientation, the Lotgering factor of the corresponding peak will increase (*LF* > 0) and the other Lotgering factors will decrease below zero. For a perfect monocrystalline thin film, the Lotgering factor will be equal to the unity for the peak corresponding to the unique orientation. The crystallite size was calculated using the standard Scherrer formula for a given peak: D = $\frac{K\lambda}{\beta\cos(\theta)}$, where $\lambda$ is the X-ray wavelength ($\lambda$ = 0.1789 nm in our case), $\beta$ the peak full width at half maximum, $\theta$ the peak angle and K the Scherrer constant taken to be equal to 0.9.

The optical properties were characterized using a UV-VIS-NIR Perkin-Elmer Lambda 900 photospectrometer in the wavelength range of 300-850 nm with a 1 nm resolution step at room temperature. The classical Beer-Lambert formula was used to extract the absorption coefficient $\alpha$ from the transmittance, and the optical bandgap energy is determined using the Tauc model for the direct bandgap: $\alpha \times h\nu = A\sqrt{h\nu - E_g}$, where $h\nu$ is the photon energy, A is a constant and $E_g$ is the optical bandgap. The Urbach tail energy $E_u$ was obtained from the formula: $\alpha(h\nu) = B \exp(\frac{h\nu}{E_u})$ where B is a constant, by taking the inverse of the slope of $\log(\alpha)$ versus photon energy ($h\nu$).

The electrical properties of the deposited thin films were measured using the van der Pauw / Hall effect technique using a home-made setup with a 0.55 T permanent magnet and Keithley 2602 and 2636B source-measure units.

### 3. Results and Analysis

The aluminum and magnesium compositions in the Zn₁⁻ₓMgₓAlᵧO alloy were varied using the procedure detailed in the previous section 2.1, and the structural, optical and electrical properties of the obtained thin films are here conducted for aluminum compositions of 0 %, 1 % and 2 % and magnesium compositions in the [0 - 7] % range.

#### 3.1. Morphology and Structural Properties

The thickness of all the deposited films was measured using the Dektak XT profilometer on a profile created during deposition with a nickel mask attached to the side of the substrate. The thickness of all samples was about 300 nm and the film maximum roughness varies between 20 nm and 30 nm with the magnesium and aluminum compositions.

Figure 1 shows the SEM images of Zn₁⁻ₓMgₓAlᵧO for different Al and Mg compositions. For the films without Mg, for Al compositions 1 % and 2 %, the surfaces reveal polycrystalline nature with some roughness, a relatively uniform grain distribution and no voids. Some small clusters can be observed in the samples with Al 2 % while the incorporation of Mg induces a more uniform surface. This can be related to the decreasing lattice disorders with the increase of the Mg content in the films as deduced from the optical properties analysis which will be detailed in the next section. The morphology of the films shown here is comparable to that of films deposited by RF magnetron sputtering [3, 18].

The XRD diffractograms of the Zn₁⁻ₓMgₓAlᵧO thin films, with magnesium compositions up to 7 % and fixed aluminum compositions of 1 % and 2 %, are shown in Figure 2. The films exhibit a single hexagonal wurtzite crystal structure with peaks for the (100), (002), (101), (102), (110), (103) and (112) planes of the space group P6₃mc,
<figure><img src="image_3.png" /><figcaption>**Figure 1:** SEM images of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with different Al (y) and Mg (x) compositions: (a): y = 1 % and x = 0%; (b): y = 1 % and x = 3%; (c): y = 1 % and x = 7%; (d): y = 2 % and x = 0%; (e): y = 2 % and x = 3%; (f): y = 2 % and x = 7%.</figcaption></figure>

identified by using the ZnO powder diffractogram (RRUFF ID: R060027.1) [17] as a reference. It was observed in the literature that the presence of multiple orientations might be due to oxygen sub-stoichiometry of the ZnO based films [15, 19, 20]. Indeed, when a few nanometers of a stoichiometric ZnO are deposited on the substrate, they tend to crystallize preferentially in the (002) direction of the wurtzite structure. However, when a deviation of the stoichiometry appears in a ZnO based film, such as the case of oxygen sub-stoichiometry, multiple orientations are observed mainly in the (100) and (101) directions. In the USP process, the film orientation can be controlled by adjusting the parameters of the precursor solution which impact the deposited film stoichiometry, as shown in a previous work [15]. In the XRD diffractograms of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O, no peak related to formation of MgO or Al<sub>2</sub>O<sub>3</sub> has been observed. The diffractograms show a small variation with the Al and Mg compositions, which could indicate that most of the Al<sup>3+</sup> and Mg<sup>2+</sup> ions have replaced the Zn<sup>2+</sup> ions or occupied the interstitial sites without modifying the structure of the films, as also evidenced in films prepared by pulsed laser deposition [11].

Figure 3 (a) and Figure 3 (b) show the values of the crystallite sizes estimated using the Scherrer formula for (100), (002), (101), (110) peaks. The crystallite sizes estimated with the (002) peak are larger than those estimated using the other diffractogram peaks, in accordance with the Lotgering factor values, shown in Figure 3 (c) and Figure 3 (d), indicating that the most dominant peak is (002). This confirms the observation of a slightly more intense (002) peak when the Mg content is increased for a fixed value of Al content (1 or 2%) seen in the XRD results shown in Figure 2. But also, this validates the observation of a more

intense (002) peak when going from a 1 % Al content to 2 % Al content while comparing the XRD diffractograms with the same Mg content. This is true because the overall crystallite size, and Lotgering factor, of the (002) peak when incorporating 2 % Al was seen to be higher than that of the films with 1 % Al. Nevertheless, we see a decrease in this average size of the crystallites with the composition, as displayed in Figure 3 (a) and Figure 3 (b), showing an overall increase in the density of defects. The here obtained crystallite sizes, for as-grown films without post-annealing, are slightly higher than the values recently reported in films deposited using RF sputtering with a substrate temperature of 400 °C [11]. The Lotgering factor related to the (002) preferred orientation increased with the increase in the Mg composition. Similar results were already evidenced for ZnO deposited by USP, showing that films with c-axis orientation have the lowest surface energy, uniformly aligned crystallites, and less defects and trapping sites [21].

## 3.2. Optical and Electrical Properties

The transmission spectra of the Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with aluminum compositions 1 % and 2 % and with varying magnesium compositions are shown in Figure 4 (a) and Figure 4 (b) respectively. The average transmission of the films in the wavelength range 500-800 nm is shown in Figure 5 while the bandgap and the Urbach energy variation are shown in Figure 8. The transparency of all the films with 1 % Al remains near 90 % which is comparable to the highest values usually obtained for ZnO based materials [5, 10]. When the aluminum composition increased to 2 %, the transparency decreased due to the increased density of defects partially related to the decreased solubility of
<figure><img src="image_4.png" /><figcaption>**Figure 2:** XRD diffractograms of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to the Mg and Al compositions. Are identified the (100), (002), (101), (102), (110), (103) and (112) peaks of the hexagonal wurtzite crystal structure (space group P6<sub>3</sub>mc).</figcaption></figure>

the aluminum precursor, thereby inducing an incomplete pyrolysis reaction, and to the increase of Al concentration in the deposited thin films.

The Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O bandgap increases with the increase in Mg composition. This variation can be attributed to the substitution of Zn<sup>2+</sup> ions by Mg<sup>2+</sup> ions in some of the sites of Zn<sup>2+</sup>, as already suggested by the XRD results. Indeed, the 4s-Zn electron states determine the bottom of the ZnO conduction band. The substitution of Zn<sup>2+</sup> by Mg<sup>2+</sup> can shift it to higher energy while the valence band remains steady, as suggested in [07, 22, 23] for the Zn<sub>1-x</sub>Mg<sub>x</sub>O alloy. The bandgap variation slightly loses linearity with Al incorporation, due to some extent to the increasing disorder introduced in the film, as also suggested by the variation of the Urbach energy with the Al composition. Indeed, the Urbach energy is directly related to the alloy disorder and the increase of the density of defects. At a fixed Al composition, the Urbach energy values remains almost constant when varying the Mg composition indicating that the incorporation of magnesium does not significantly degrade the crystalline quality. This result can be correlated with the XRD results showing, on the one hand, that the (002) preferred orientation increased when increasing the Mg composition even if, on the other hand, the average size of the crystallites decreases with the composition. It should be emphasized that the Urbach energy is only a qualitative indicator and does not quantify the disorder in the material, nor is it sensitive to all kind of disorder. It is one indicator among others and should be correlated to other characterization results.

The electrical properties of the Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films were analyzed with respect to the magnesium composition along with the fixed aluminum compositions. After deposition, and prior to the electrical measurements, all the films were annealed using the rapid thermal annealing (RTA) system at a temperature of 300 °C for 30 minutes. The annealing was performed to increase the grain size, activate the shallow donors and to improve the overall crystal quality. Figure 9 (a) shows that without Al incorporation, when the magnesium composition increases from 0 % to 7 %, the resistivity increases dramatically, corresponding to a decrease of the free carrier concentration from 2.7 × 10<sup>19</sup> cm<sup>-3</sup> down to 6.2 × 10<sup>15</sup> cm<sup>-3</sup>. As shown in Figure 9 (b), the carrier mobility slightly increases with the Mg content, and does not appear to be affected by the decrease of the average crystallite size with the composition, for a Mg content ≤ 5 %. For a Mg content > 5 %, the mobility decreased, showing that for higher compositions, the average crystallite size, which decreases with composition as shown by the XRD results, has an impact on the transport properties. It is admitted that interstitial Zn (Zn<sub>i</sub>) plays a dominant role in the background n-type doping in ZnO [24]. Indeed, Zn<sub>i</sub> behaves as a shallow donor in ZnO, which is significantly more prevalent than other intrinsic defects, including oxygen vacancy, according to theoretical calculations using the local density approximation [24] and comparison of theoretical results with experimental findings in proper conditions [25]. Furthermore, magnesium incorporation may result in a reduction in the formation of these Zn<sub>i</sub> sites, lowering the free
<figure><img src="image_5.png" /><figcaption>**Figure 3:** Crystallite size in Zn<sub>1-x</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to the Mg compositions and fixed Al compositions of 1 % (a) and 2 % (b), using the (100), (002), (101), (110) peaks, in addition to the average crystallite size. Lotgering texture factor of (100), (002), (101), (110) peaks for Al compositions of 1 % (c) and 2 % (d).</figcaption></figure>

carrier concentration and therefore explaining the resistivity increase observed in Figure 9 (a) for Zn<sub>1-x</sub>Mg<sub>x</sub>Al<sub>y</sub>O with Al 0 %. This type of compensation, with the inhibition of Zn<sub>i</sub> by Mg was already observed in previous work [7] and in ZnMgO deposited by sputtering [26]. When Al is introduced in the films, the resistivity still increased with the Mg content but for the higher Mg contents > 4%, the resistivity started to decrease with the Al content. Therefore, we identified two regimes, as shown in Figure 9 (a):

1. For a Mg content going from 0 % up to about 4 %, the resistivity significantly increases with Mg. In this

first regime, at a given Mg content > 0%, the resistivity increases when increasing Al content. Without Mg, the resistivity slightly decreases when increasing the Al content. The mobility increases up to about 16 cm²/Vs for a Mg content of 5 %.

2. For a Mg content above 4 %, the resistivity significantly decreases, at a given Mg content, when increasing the Al content from 0 % to 1 % and varies less markedly when Al increases from 1 % to 2 %. Here, the mobility decreases with the Mg content for an Al content > 0%.
<figure><img src="image_6.png" /><figcaption>**Figure 4:** Transmission spectra of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to Mg composition with fixed Al compositions of 1 % (a) and 2 % (b). The spectrum of the SLG substrate is also shown, with nearly constant transmission at 100 %. A zoom in the absorption edge spectral region is shown in the inset plots of each figure, showing a blue shift of the transmittance spectra when increasing Mg composition, related to an increase in the bandgap.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 5:** Average transmission of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to Mg composition with fixed Al compositions of 1 % (a) and 2 % (b) in the wavelength region 500 nm to 800 nm.</figcaption></figure>

The increase in the resistivity corresponds to a free carrier concentration decrease from 1.6 × 10<sup>19</sup> cm<sup>-3</sup> down to 1.1 × 10<sup>17</sup> cm<sup>-3</sup> for 1 % Al content and from 8.8 × 10<sup>18</sup> cm<sup>-3</sup> down to 8.3 × 10<sup>16</sup> cm<sup>-3</sup> for a 2 % Al content. Indeed, co-incorporation of Al and Mg in ZnO have different and relatively opposite effects, even if they are interdependent. On the one hand, the incorporation of Mg makes it possible

to widen the bandgap without introducing free carriers, given that Zn<sup>2+</sup> is substituted by Mg<sup>2+</sup>. The fact that the incorporation of Mg increases the bandgap, coupled with the fact that Mg does not act as a shallow dopant and, on the contrary, inhibits the Zn<sub>i</sub> sites, induces the increase in the resistivity observed here. On the other hand, the incorporation of Al, with the replacement of Zn<sup>2+</sup> sites with
<figure><img src="image_8.png" /><figcaption>**Figure 6:** Tauc plots used to extract the bandgap energy for Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to Mg composition and fixed Al compositions of 0 % (a), 1 % (b) and 2 % (c).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 7:** Absorption plots used to extract the Urbach energy for Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to Mg composition and fixed Al compositions of 0 % (a), 1 % (b) and 2 % (c).</figcaption></figure>

Al<sup>3+</sup> ions, tends to increase the free carrier concentration. At a given Mg content > 0%, the resistivity increases with Al due to decrease of the free carrier concentration. This latter variation is tentatively explained by: (i) an increase in the Al donor activation energy induced by Mg, as also correlated to the increase in the bandgap shown in Figure 8, in the first regime where the Mg content is lower than 5%; (ii) fewer Zn<sub>i</sub> sites due to the Al incorporation and the change in stoichiometry. With a Mg content > 5%, the increase in the bandgap with Mg slows down and we can therefore assume that, from this Mg content value of 5%, the effect of Al on the conduction becomes dominant with an increasing free carrier concentration, hence the change in regime with now a decrease in the resistivity with Al content, at a given Mg content. There are therefore two competing mechanisms, the first is essentially linked to the incorporation of Mg and the second due to the incorporation of Al. The Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O material conductivity can then be controlled by the co-incorporation of magnesium and aluminum, allowing the material to be tuned for certain optoelectronic applications, such as ultraviolet detectors, as

active layer, and “all-oxide” heterojunction solar cells, as window or buffer layers.

## 4. Conclusion

The Al and Mg co-incorporation to prepare the ZnMgAlO thin films allows to modulate the optical and electrical properties by varying the Al and Mg compositions. With Al and Mg, the transparencies were higher, above 90%, with wider bandgap. The crystalline properties, including the grain size and the texture factor, of the deposited films with Mg were kept optimal and the wurtzite structure of the crystals was retained. These results demonstrate the quality of the control on the spray deposition parameters. The electrical and optical properties were tuned with Al and Mg co-incorporation with, in particular, controlled conductivity and relatively high mobilities for the films. The high transparency of the films in the visible region of the spectrum along with the modulations of the bandgap energies and the conductivities can be interesting for applications such as window or buffer layers in “all-oxide”
<figure><img src="image_10.png" /><figcaption>**Figure 8:** Variation of the bandgap and Urbach energy of Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to Mg composition with fixed Al compositions of 1 % (a,c) and 2 % (b,d).</figcaption></figure>

solar cells. The quaternary Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O can also have useful applications in ultraviolet photodetectors and other optoelectronic devices.

**Nature of the contribution of each author**

S. Bose prepared samples, conducted structural, electrical and optical measurements, analyzed the characterization results and wrote the initial draft; C. Fadel analyzed the characterization results, performed a literature review and revised the paper; S. Ould Saad Hamady analyzed the characterization results, wrote the initial draft and supervised the project; O. Perroud conducted the XRD measurements

and analysis and revised the paper; J. Guyon conducted the SEM measurements and analysis and revised the paper; L. Weiss analyzed the characterization results and revised the paper; N. Fressengeas analyzed the characterization results and revised the paper; All authors discussed the results and contributed to the final manuscript.

**Declaration of competing interest**

The authors declare that they have no known competing financial interests or personal relationships that could have
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 9:** Variation of resistivity (a), carrier concentration (b) and mobility (c) for Zn<sub>1-x-y</sub>Mg<sub>x</sub>Al<sub>y</sub>O thin films with respect to Mg composition and fixed Al compositions of 0 %, 1 % and 2 %.</figcaption></figure>

appeared to influence the work reported in this paper.

## Acknowledgments

This research was supported by “AAP mi-lourd – Centrale-Supélec”, “Université de Lorraine” and “Institut Carnot Icéel” in the framework of the “Hector” research project. The authors gratefully acknowledge Mrs. Maryne Crouzet (LMOPS, Metz, Université de Lorraine & CentraleSupélec), Mr. Queny Kieffer (Carel Platform, LMOPS, Metz, Université de Lorraine & CentraleSupélec) for their help in chemistry and instrumentation respectively, and the experimental facilities MicroMat from LEM3 (Metz, Université de Lorraine, CNRS UMR 7239) for SEM and XRD measurements.

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