Crystallographic disorders depending on monovalent cations addition and their effects on ZnO's characteristics

Ceramics International xxx (xxxx) xxx–xxx

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Crystallographic disorders depending on monovalent cations addition and their effects on ZnO's characteristics Sakir Aydogana,b,c, Mehmet Yilmaza,d,∗ a

Advanced Materials Research Laboratory, Department of Nanoscience and Nanoengineering, Graduate School of Natural and Applied Sciences, Ataturk University, 25240, Erzurum, Turkey b Department of Physics, Science Faculty , Ataturk University, 25240, Erzurum, Turkey c Department of Electrical and Electronics Engineering, Ardahan University, 75000, Ardahan, Turkey d Department of Science Teaching, K. K. Education Faculty , Ataturk University, 25240, Erzurum, Turkey

ARTICLE INFO

ABSTRACT

Keywords: ZnO Impurities Films

Pure and lithium (Li) doped ZnO films have been grown on soda lime glass substrates by chemical spray pyrolysis technique. Zinc chloride (ZnCl2) and lithium chloride (LiCl), resoluble in deionized water, were used in the process. The growth mechanism of the films as a function of Li content, resulting texture and topography were discussed in detail by X-ray diffraction method (XRD) and scanning electron microscopy (SEM). Obtained films exhibited single phase with polycyrstalline nature. At the same time, changes in crystal quality due to lithium addition were observed and these changes were explained as the presence of lithium atoms in the ZnO's crystal structure as interstitial atoms and the replacement of Li+ and Zn2+ions. Topographical features of the films were confirmed by SEM images. In SEM photographs, it was observed that the crystallite size of the films varied as a result of an increase in the lithium doping concentration. Thus, a significant correlation was found between the structural and morphological properties of the films due to Li contribution. Optical properties of the films were evaluated by UV-VIS-NIR spectroscopy and found that the optical band gap of ZnO films exhibit increasing tendency up to 4 at.%Li content and decrease for further Li content. Elemental composition and presence of Li were approved by X-ray photoelectron spectroscopy (XPS) and Li 1s peak were observed in 55.6 eV in 5 at.% Li doped ZnO films.

1. Introduction Thin film semiconductors with high transparency and conductivity are important topics for researchers due to their wide application fields ranging from industrial purposes to scientific researches. Until recently, indium doped tin oxide films have generally been used in a very wide perspective from smart windows to solar cells [1,2] due to their unique features like wide-direct band gap and high conductivity [3]. In addition to the increasing energy demand with the developing technology, listing indium as a critical raw material category by the European Union makes the use of ITO in these applications risky. In this case, it is necessary to improve the efficiency of the more abundant materials in the nature or to increase the efforts to find alternative materials. Among these effords, use of metal-oxide semiconductors films instead of indium doped tin oxide (ITO) film in those applications have attracted great attention as being potential alternatives. In this context, materials that can be used instead of ITO must have similar physical properties

with ITO in order to be used in similar applications. This is the main reason why metal oxide-based semiconductors are shown as an alternative to ITO. Among the metal-oxide semiconductors, ZnO is evaluated as an important alternative material for optoelectronic applications like solar cell or photodiode [4,5]. Its superior electrical and optical features, such as high electron mobility and optical transparency make it suitable in above aforementioned applications [6,7]. In addition, its low cost, abundance in nature and non toxicity make ZnO an industrially attractive material [8,9]. Although these properties seem sufficient, developing technology requires an improvement in the quality of materials used in optoelectronic application. In this context, researchers have tried to improve ZnO's electrical and optical properties by different ways. In order to improve the electronic properties of ZnO, various methods are used by researchers, including controlled strain, application of an external electric field or doping [10,11]. Considering application based studies in literature, different heteroatoms can be used to make ZnO suitable for application. Namely, it can be said that

∗ Corresponding author. Advanced Materials Research Laboratory, Department of Nanoscience and Nanoengineering, Graduate School of Natural and Applied Sciences, Ataturk University, 25240, Erzurum, Turkey. E-mail addresses: [email protected] (S. Aydogan), [email protected], [email protected] (M. Yilmaz).

https://doi.org/10.1016/j.ceramint.2019.12.076 Received 29 October 2019; Received in revised form 28 November 2019; Accepted 6 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Sakir Aydogan and Mehmet Yilmaz, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.076

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doping is seen as the most popular and easiest way to control ZnO characteristics. Some of the studies in the literature [12,13] have focused on group (III) doped ZnO films and have shown that in addition to the optical transparency of ZnO, its electrical properties are improved by the addition of group (III) element to ZnO matrix. Furthermore, the electrical conductivity of ZnO films depends largely on the dopant type. That is, the substitution of the impurity atom with the zinc or oxygen atoms within the crystal structure of ZnO affects its electrical conductivity. So, both electrical properties and optical properties of ZnO films can be replaced as a function of dopant content. For example, Kuprenaite at al [14]. have focused on In, Ga and Al doped ZnO films in their study and revealed that In- doped ZnO films have showed lower carrier mobility and concentration and therefore higher resistivity has occurred compared to Ga and Al doped ZnO films. Also, Khuili et al. [15] have investigated Al, Ga and In doped ZnO films by computational method and concluded that the group (III) elements doped ZnO films have exhibited n-type electrical conductivity and they have also found that group (III) elements are the most suitable material for photovoltaic applications. On the contrary to above mentioned studies, it is necessary to add (I) and (V) group elements for p-type electrical conductivity of ZnO [16]. Among these, lithium is considered to be more advantageous than the other elements of groups (I) and (V) due to its benefits in some applications such as:

Fig. 1. XRD patterns of ZnO films with different Li content.

to 0, 1, 2, 4 and 5 at.%. After completing all aforementioned steps before spraying, the spray solutions were sprayed on the glass substrates at 400 °C substrate temperature in the experimental apparatus where oxygen was used as carrier gas and the distance between nozzle substrate was kept 35 cm. After spraying, all films were not removed from the hot surface until they reached ambient temperature. Structural, topographical and optical changes of the films produced under these conditions were investigated depending on the amount of lithium additive. For this aim, X-ray diffraction method (XRD) using CuKα as radiation source, scanning electron microscopy (SEM), UV–Vis spectrometer and X-ray photoelectron spectroscopy (XPS) were used to determine structural, topographical, optical and elemental composition of the films. So, data gathered from each measurement were analyzed in detail and researchers have tried to find out whether there has been a correlation between them.

✓ The effect of Piezoelectric potential screen can be reduced by Li content in natural n-type ZnO if Li acts as a shallow level acceptor [17]. ✓ In case of substitution of Zn2+ (0.074 nm) ions with Li+ (0.06 nm) ions in ZnO matrix, piezoelectric constant of ZnO increase as a result of the smaller ionic radius of Li+ ions, leading to an increase in stress in crystal structures of ZnO [18]. ✓ If ZnO films with natural n-type conductivity are doped with p-type lithium, the electrical resistance of ZnO increases so, leakage currents in device applications reduce [19]. Many studies can be find about the Li doped ZnO films obtained by different growing techniques in the literature. However, there are limited studies explaining whole features of thin films such as structural, topographic and optical variation depending on the Li content. In this context, the main motivation of this study is to find out that there is a meaningful correlation between changes in ZnO's characteristics and Li doping. Also, the low atomic number of lithium makes analysing lithium based defects and structural variations difficult via spectroscopic ways. The difference of this study from other studies is to show compatibility of spectroscopic changes with calculations made to show lithium-dependent change in characteristic properties of ZnO.

3. Results and discussion In this study, the structural changes in ZnO films as a result of Li incorporation have been evaluated by X-ray diffraction technique. XRD patterns of undoped and Li doped ZnO films have been shown in Fig. 1. As it can be observed in Fig. 1, the most intense peak is (002) for all films with a polycrystalline nature. This preferred orientation can be seen in all studies regardless of growing technique [22,23]. The fact that ZnO can occur with lower energy in (002) orientation can be shown as the cause for that. Additionally, no separation phase was observed since there was no diffraction peak related to Li or Li oxide in XRD diffractograms, indicating that Li ions are distributed as homogenous into ZnO crystal structure in the detecting limit of XRD. Similar expectation have been made by other researchers for cobalt doped ZnO films [24]. The interplanar distance (d) values of (002) peak for all samples are given in Table 1. As it can be observed in Table 1, “d” values initially increase with Li incorporation till 4 at. % level, then it decreases with more Li content. A research study investigated the effect of hydrogen flux on AZO films [25] mentioned that interplanar distance values can increase as a result of the placement of hydrogen atoms in the Zn–O bond center. Similar to this study, taking studies on doped TCO materials [26,27] into consideration, increase in “d (002)” values in this study can be correlated with the presence of dopant atoms in the interstitial lattice position in ZnO crystal structure. That has also been approved by XPS measurement. Otherwise, this case may imply that there has been a shift to lower Bragg angle as a result of the lattice strain induced by external defects occurred in the study or in the growing procedure [28]. Also, decrease in “d (002)” value for further

2. Experimental Highly transparent Li doped ZnO films were synthesised on soda lime glass substrates by home-made chemical spray pyrolysis method, consisting horizontal configuration type experimental setup. The experimental stages were arranged in three sections, including substrates cleaning, solutions preparation and spraying procedure. In the first step of the substrate cleaning process, the glass substrates were cleaned with detergent to remove dirties and residuals. These glass substrates were then exposed to acetone, methanol and distilled water for 25 min in the ultrasonic cleaner. After that they put into the UV-ozone cleaner for 25 min to complete the glass cleaning process. Similar glass substrate cleaning procedures were mentioned in previously published articles [20,21]. The spraying solution was prepared by using deionized water and ZnCl2. Totally, 0.1 M zinc chloride solution was prepared and obtained transparent solution. After the obtained transparent sol, 3–5 drops HCI added into the solution to hinder the formation of Zn(OH)2. Besides, Li addition was carried out by dissolving LiCl in the formerly obtained solution. The ratio of lithium to zinc (x = LiCl/ZnCl2) was set 2

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Table 1 Some structural parameters for ZnO:Li films. Samples

d

(002)

(Å)

Lattice constants (Å)

D (nm)

(x 10 4)

(x 1015)

(standard deviations)

ac ZnO:Li ZnO:Li ZnO:Li ZnO:Li ZnO:Li

0% 1% 2% 4% 5%

2.6028 2.6039 2.6041 2.6050 2.6043

3.258 3.255 3.255 3.251 3.256

5.208 5.209 5.211 5.228 5.202

25.40 25.06 24.82 23.46 27.98

4.46 4.52 3.14 2.86 9.32

1.55 1.59 1.62 1.81 1.27

37.23 38.88 41.18 43.24 38.37

width in radians at FWHM of the peak line and diffraction angle, respectively. From eq. (2), it may be calculated that linear fit to the data drawn between 1 hkl and cos hkl . From the interaction between eqs. (1) and (2), eq. (3) is obtained:

cos

k

=

hkl D

(3)

Considering that the FWHM of XRD peak also indicates the effect coming from lattice strain, strain can be calculated from Stokes-Wilson equation [34]. str

hkl

doping ratio may be related to the replacement of Zn ions (0.074 nm) lattice position by Li ions (0.06 nm) which has lower ionic radius compared with Zn ions. Widening lattice planes also leads to shift of XRD peak toward a higher angle. That is also clearly seen in Fig. 2. Similar variations in interplanar distance and 2 theta values have been observed for Mn doped ZnO thin films in Nagaraja et al.'s study [29]. The lattice constant values of Li doped ZnO samples have been determined by using well known formula [30] and results are given in Table 1. It is observed that doping of Li into ZnO crystal structure make a non-systematic change in “c” lattice constants compared with the standard sample's. This change in the “c” lattice parameters is consistent with the changes in 2 theta variations as shown in Fig. 2. Additionally, decrease in (002) peak intensity observed in Fig. 2 can be attributed to the degradation of the crystal structure as a result of increase in density of Li atoms in the interstitial regions [31]. The increase and decrease in peak intensity leads peak broadening or narrowing in XRD measurement. The peak broadening in the XRD patterns generally occurs according to two different effects. These are instrumental and sample related factors [32]. Generally, it can be handled via instrumental related factor by using optimum resolution of XRD and diffraction pattern of standard material. The residual peak broadening at the end of the process is known as sample related factor [33]. So, broadening in the peak line is observed as a result of sample related effects like grain size or strain. Instrumental corrected line broadening (βhkl) can be determined by using following equation: 2 (hkl) measured

=

2 (hkl)instrumental

D

cos

4 tan

(4)

=

D

+

S

(5)

That is, hkl

cos

( )

= k D + 4 sin

(6)

According to the uniform deformation model (UDM), strain accepted as uniform in all crystallographic dimension in the crystal structure [35]. Due to the fact that it is known that all features of materials are independent regardless of its direction due to isotropic nature of the crystals, UDMs for pure and Li doped ZnO films are represented in Fig. 3. Fig. 3 shows βcosθ vs.4sinθ plot for all samples. Also, y-intercept and plot slope allow the calculation of estimated crystallite size and strain resulting from crystal deterioration due to Li content. Calculated crystallite size values obtained from W–H approximation have been illustrated in Table 1. Taking the results into consideration, it is concluded that average crystallite size exhibit decreasing tendency till 4 at. % Li content and then increase again with further doping. Actually, considering the results given in Table 1, it can be said that there is a slight change in structural properties of ZnO at low contribution rate. This is mainly associated with the atomic number of lithium [36]. Also, It has been indicated in some studies [37,38] that Li atoms tend to be present in the interstitial region of thin films at low doping concentration. Therefore, it is predicted that estimated crystallite size reduced by a relatively small doping ratio up to 5 at. % due to the excessive amount of Li interstitial atoms. Namely, increasing Li atoms in ZnO crystal structure may cause new nucleation centers, which intercepts further crystal growth [39]. On the other hand, it has been observed that there is an increase in the (002) peak intensity for 5 at. % Li doped samples. This observation is also compatible with the variation in 2- theta shown in Fig. 2. Similar tendency in the intensity of (002) peak has also been observed in some studies [40,41] on Li doped ZnO films. The increase in intensity of (002) peak given in Fig. 2 also means improving crystal quality and reorientation redistribution of grains. In brief, increasing (002) peak

(1)

Average crystallite size of the samples have been evaluated using Scherrer's equation:

D= k

S

Eqs. (3) and (4) are showing size and strain related peak broadening at different Bragg angles. Additionally, these formulas allow separate size and strain depended broadening of X-Ray peak line. Depending on the equations above, peculiar effect on X-ray peak broadening can be calculated by using Williamson- Hall (W–H) approximation given as eq. (5).

Fig. 2. Changes in the intensity and the position of (002) as a function of Li content.

hkl

=

(2)

where k,λ,βD and θ are shape factor, wave length of incident X-ray, full 3

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Fig. 3. Williamson-Hall plot for pure and Li doped ZnO films.

intensity as well as decrease FWHM of (002) peak indicate that crystallite size become bigger reference sample compared with the reference sample (ZnO:Li 0 at.%). Similar variations have been encountered in the literature [42,43]. We have already discussed that monovalent dopants ions like Li+ and K+ [44] can settle into the crystal structure of ZnO both interstitial and its lattice site. In this context, it can be said that the shift to lower or higher 2 theta values of (002) peak with Li incorporation indicates there is a competition between tensile or compressive strain occurring due to the presences of Li interstitial or lattice site of the ZnO. The tensile and compressive strain are indicated with positive and negative sign, respectively [45]. Considering the results given in Table 1, it is seen that all calculated strain values obtained from W–H plot are exhibited positive sign, which indicates tensile strain caused by substrate in the films. The minimum value of strain obtained from 4 at. % doped sample indicating less defects between substrate and as-grown ZnO films. The further increase in strain value can be correlated by the shift in preferred orientation. These speculations are harmony with our results and some literature [46,47]. Dislocation density analysis was also conducted in this study taking = 1 D2 equations [48] into consideration to investigate variation in the quality of films. In general, the term of “dislocation” means crystallographic defects in the material science and there is a strong relationship between estimated crystallite size varied by redistribution of grains. According to the dislocation density results represented in Table 1, it can be inferred that lower strain and dislocation density enhance the stoichiometry of the films. Additionally, there is a reverse relationship between strain and dislocation values except 1 at. % Li doped sample. This relationship also indicates that strain is an important factor during the formation of dislocations. In order to get

information about nature of growth mechanism of ZnO films, the standard deviation has been calculated by using following eq [49].

=

(

2 Ihkl

Ihkl )2 N

N

(7)

Where N and Ihkl are the number of reflection peaks and relative peak intensity obtained from XRD measurement, respectively. Obtained standard deviation ( ) values of the samples and their variations versus crystallites size have been given in Table 1 and Fig. 4. Other studies [50,51]suggest that minimum value of standard deviations is the predominant formation mechanism of homogenous nucleation during film growth, whereas vice versa shows heterogeneous nucleation. In the light of this information, it can be said that reference sample (un-doped ZnO) has more homogenous nucleation than the others. In addition, the adsorption-desorption phenomenon begins to dominate with Li addition, and that applies to up to 4 at. % doping rates. Saturation in σ values with further increase in Li concentration (5 at. %) indicates that homogenous nucleation has started again. This variation in σ values can be correlated with the changes in estimated crystallite size as an inverse proportion. So, it may be concluded that standard deviation that's to say, the type of nucleation is very important factor for the shape and size of grains. Another way to get information about growing nature of the films is the analysis of texture of the films. In this approximation, the peaks observed in XRD have been assessed by means of their intensity by using eq. (8) [52].

TC(hkl) =

4

I(hkl)/ I0(hkl) 1 N

I(hkl)/ I0(hkl)

(8)

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Fig. 4. Variation of D and

undoped ZnO is in the c-axis direction. This observation can also be supported with TC(002) result for un doped ZnO film. With the increased Li content in ZnO crystal structures, small grains and the porosity around these grains start increasing. (Fig. 6(b–d)). Due to the increase in the density of Li atoms in the interstitial regions, insoluble Li atoms may begin to accumulate at the grain boundaries and may suppress the growth of ZnO: Li crystals. Fig. 6(c) can be seen as a strong evidence for this comment. As discussed previously, 5% Li doped films started to be more effectively packed with larger particles of nanocrystals due to the redistribution of the orientation of the grain, and denser structures characterized by lower porosity began to be observed (Fig. 6(e)). Similar surface topography and speculation have been encountered and done by other researchers for ZnO films grown under different conditions [55,56]. Actually, different surface topography of the thin films can be related with different nucleation types and growth mechanism [57]. These important features determining the nature of growth of ZnO films can be correlated with polarity of ZnO. Adachi et al. [58] have also showed that the polarity of ZnO can be controlled and inversed by Al doping. In our study, only variable among the samples is Li content. Therefore, the diversity of the surface topography of ZnO films can easily be attributed to reduce or increase in the polarity of ZnO and thus the surface of ZnO films exhibit different shapes of nanoparticle [59]. The film thicknesses of the samples have been evaluated by cross-sectional SEM images and found to be as 300 ± 3 nm. So, SEM images proved that there has been no significant impact of Li content on the thickness of the film. Electronic variation of the samples with different Li content have been evaluated by means of UV–Vis measurements. The room temperature optical transmittance spectra of the films have been given in Fig. 7. Fig. 7 has revealed that all samples exhibit more than 82% transparency (see insetted Fig. in Fig. 7) indicating low absorption losses within the visible range, which makes it a candidate for p-n junction applications. Also, it can be seen from Fig. 7, the transmittance spectra of the films have exhibited sinusoidal characteristic, which may be the result of surface morphology of the films or differences in film concentration [60]. At the same time, the films exhibit a sharp absorption edge at about 372 nm which means that photons at this wavelength have sufficient energy to transport the electron from the valence band of the semiconductors to the conductive band. Gu et al.'s [61] study revealed that this case has also been correlated with presence of intrinsic band gap of the films. High optical transparency of the thin films can be related with its structural quality and crystallinity [62]. In this context, the enhance of optical transmittance up to 4 at. % Li content of the films may be related surface morphology of the samples. In the light of Musaab and Abass's study [63], it can be concluded that increasing in the optical transmittance of doped ZnO films can be associated with the reduction of voids and presence of uniformly distributed particle in ZnO films' surface. Taking our SEM images indicating the particle distribution as a function of Li content into consideration, we assume it has been derived from the same reason. A little decrease in optical transmittance has also been observed for 5 at. % Li doped ZnO films. Such behavior in the optical transmittance spectra may be explained with Li defects within the forbidden bands which leads absorbing incoming photons. Same optical transmittance characteristics have also been speculated for sol-gel coated Cu doped ZnO films in the literature [64,65]. The reflectance spectrum of semiconductors is also affected by their surface nature, impurity, oxygen vacancy [66]. The reflection spectrum has exhibited decreasing tendency with Li incorporation. Also, very low reflection values have been observed in visible and infrared region for all samples. Additionally, sudden reductions in the spectrum are caused by changing radiation source that is a halogen lamp for the visible (VIS) and near infrared (NIR) range, or deuterium lamp for the UV range [67]. To sum up, these kinks have been observed in the reflection spectra due to the

depending on Li content.

I(hkl),I0 and N in eq. (8), are the peak intensity of (hkl) plane obtained from XRD measurement and JCPDS card no:36–1451 and the number of reflection peak observed in XRD, respectively. Calculated TC values and their changes can be observed in Fig. 5. If the texture coefficient value is equal to 1, it can be concluded that a crystal particle is randomly oriented [53]. In order to have a crystallite orientation abundance in a certain plane during the formation of the crystal structure, TC value must be greater than 1. In this context, the peak values (002) and (112) in Fig. 5 are seen as peaks with the highest TC values for all samples. In addition, it can be concluded that Li doping leads an inverse relationship between (002) and (112) variations as well as it causes re-orientation effect during the crystal formation. Variation in estimated crystallite size values depending on Li content also supports our TC values and their variations especially at (002) peak. In the light of our past study published [54], we can correlate variations in structural properties of ZnO with its topographical properties. For this aim, top and side view of scanning electron microscopy analysis have been conducted to evaluate ZnO's surface morphology as a function of Li content. Obtained SEM images of ZnO films have been represented in Fig. 6. SEM analysis points out the presence of a large number of hexagonal grains, leading compact and uniform surface of undoped ZnO thin films. Considering the XRD result of undoped ZnO films, Fig. 6 (a) supports that the dominant growth mechanism of

Fig. 5. TC value variations for all peaks according to Li content. 5

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Fig. 6. Top and side view SEM images of ZnO:Li films a) 0 at.% Li b) 1 at.% Li c) 2 at.% Li d) 4 at.% Li e) 5 at.% Li.

measurement technique. Reflectance spectrum of the samples have also revealed that the reflectance reduces with Li content till 4 at. % doping and then increase with further Li content. Similar behavior has been observed by the other researchers [66]. Sharp decrease has also been observed in reflection spectra of the films. This characteristic in the reflection spectra may be explained by resonant plasma frequency approximation. Namely, if the incident radiation matches the resonance plasma frequency, it causes a sharp decrease in the reflectance of the film due to the resonance condition [68]. Both optical transmittance

and reflectance spectrum can be used to determine thin films' optical band gap. For this aim, the optical absorption coefficient (α) should be determined from the optical transmittance spectra by using eq. (9);

=

1/ dln (T )

(9)

After the determination of “α” the optical band gap values of the films can be obtained by well-known Tauc approximation given in eq. (10) [69]. 6

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Fig. 9. The variation in optical band gap values obtained by different approximation for ZnO:Li.

same tendency with increasing Li content. According to these results, we have inferred that determined optical band gap values are in the acceptable range compared with the other studies given in Table 2. Additionally, Fig. 9 has revealed that the optical band gap values of the samples increased up to 4 at. % Li content and then decrease with 5 at. Li content. Same variations have already been observed in other studies represented in Table 2 and they explained increasing tendency in the optical band gap with Burstein-Moss effect. In Burstein Moss effect, Pauli's principle can be seen the main approximation. That is, in the Moss-Burstein theory, in heavily doped zinc oxide films, donor electrons are thought to occupy states just below the conduction band. Due to the fact that Pauli principle predicts (1)states occupied by two electrons with reverse spin, as well as (2) the optical transitions to be vertical, (3) extra energy is needed to stimulate the valence electrons to high energy states in the conduction band [75,79]. So, measured optical band gap is observed as broad compared with the standard one. On the other hand, decreasing tendency in the optical band gap value with 5 at. Li content can be related to the presence of Li impurities in ZnO crystal structure, which leads new recombination center formation with lower emission energy. Also, some researchers explain band gap narrowing by means of the shrinkage effect occurred as a result of the increasing carrier concentrations [76,77]. Though the same variation in the optical band gap values obtained by Tauc and Kubelka-Munk method has been observed, there has been a difference between these values. The difference in optical band gap values obtained by these two different techniques may depend on the difference between measurements. That is, "α", which is used to construct Tauc graph, requires

Fig. 7. Transmittance spectrum of ZnO:Li films.

( h ) 2 = A (h

Eg )

(10)

In equation (10), A, hv and Eg represent constant, photon energy and optical band gap values, respectively. A plot showing the functional relationship between ( h )2 and photon energy is presented in Fig. 8. The optical band gap energy of the films is calculated from the extrapolation of the linear portion of ( h )2 vs. hv graph. On the other hand, Kubelka-Munk method has also been used to calculate optical band gap of the semiconductor thin films [70]. Following equation is known as KM method.

F (R) = (1

R) 2

2R

(11)

In this expression, while F(R) is a variable proportional to the extinction coefficient, R is given as reflectance. After multiplying the function F(R) with hν, Kubelka-Munk function can be modified using the coefficient (n) corresponding to an electronic transition as follows so that the optical band gap can be calculated. Thus, drawing (F (R) hv )n vs. hv (given in Fig. 8 as insetted figure) functional graph, the optical band gap can be determined by extrapolation of the linear portion of this graph as mentioned previously. Calculated optical band gap values of the samples and their variations as a function of Li content have been given in Fig. 9. Obtained results from both Tauc and Kubelka-Munk approximations exhibit the

Fig. 8. Tauch and (F(R) hv)1/2 vs. E plot (a) and reflectance spectra (b) of ZnO:Li films. 7

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Table 2 Comparison of optical band gap values for ZnO thin films obtained with different growth techniques and doping materials in other studies. Sample

Growing technique

Doping ratio (vol%, at.% or mol.%)

Eg (eV)

Ref.

ZnO: Co ZnO: S ZnO: Cu ZnO: Mn ZnO: Al ZnO: In ZnO:F ZnO: Mg

Spray pyrolysis Sol-gel spin coating Sol-gel spin coating Spray pyrolysis Sol-gel Sol-gel spin coating Sol-gel spin coating PLD

0, 1, 3, 5, 7, 9, 11, 15 0, 4, 7 0, 1, 2, 3 0, 2 0, 0.5, 2 0, 1, 3, 5 5, 10, 15, 20 0, 0.5, 1, 1.5, 2.5, 3, 5

3.29, 3.30, 3.32, 3.35, 3.33, 3.00, 2.97, 2.92 3.22, 3.15, 3.11 3.17, …., 3.11 3.3, 3.1 3.26, 3.28, 3.30 3.19, 3.17, 3.12, 3.08 3.289, 3.287, 3.285, 3.281 3.274, 3.285, 3.304, 3.317, 3.324, 3.334, 3.430

[71] [72] [73] [74] [75] [76] [77] [78]

negligible light scattering in order to implement the method successfully while in Kubelka-Munk method it cannot be neglected [80]. Some studies also show that quantum confinement effect has an important effect on the optical features of thin films [81,82]. The quantum confinement effect is mainly related to particles in the order of several nanometers in ZnO thin films. Particles' restrictions in a confined semiconductor in the nano-range scale may lead to an increase in both the band gap energy and the exciton binding energy [83]. In order to reveal the effect of quantum confinement effect on optical properties of ZnO films with different Li content, effective mass modified formula given in eq. (12) has been used.

E = Eg +

2 2

(1.82e 2)

1 me mh

4

R

(12)

where E and Eg are the calculated and standard (3.2eV) optical band gap of ZnO films, respectively, R is the radius of the particle. Also, it is known in the equation that , and h are the dielectric constant (2.1) of the ZnO film and space permittivity and the Planck's constant, respectively. The particle size dependence (QC) optical band gap values of the samples have been given in Fig. 9. According to Fig. 9, it is seen that the calculated optical band gap values of the samples have been found so close that they indicate the size dependence optical band gap of ZnO films. The variation in the optical band gap of ZnO films depending on Li content also means that the redistribution of the states located forbidden area in the optical band gap of the semiconductors. Thus, the width of localized states presents in the optical band gap of semiconductors known as Urbach tail changes. This change in the Urbach tail affects the optical transitions of ZnO. Urbach tail of the samples can be calculated by using eq. (13)

=

0 exp

(E E )

Fig. 10. The Urbach plots of ZnO films by means of Li content.

spectrum are corresponding to Zn 2p, O 1s which are the main components of ZnO films. Also, the peak intensity of Zn 2p3 has exhibited higher intensity compared with Zn 2p1 peak's intensity for all films. In the grown films, that clearly indicates that Zn ions are essential in the chemical state of Zn2+ and that's why Zn0 is relatively less [84]. Due to the low atomic number of lithium, the presence of lithium in the structure was carried out by narrow scanning XPS measurements. The obtained results have been tried to be made more meaningful by fitting method (see insetted Figure in Fig. 11). According to the insetted figures given in Fig. 11, Li 1s peaks have been found in the range of 54–54.2 eV for 0, 1, 2, 4 at. % Li doped ZnO films and 55.6 eV for 5 at. % Li doped ZnO films. In many studies dedicate the presence of Li 1s peak at approximately 54 eV (< 55.3 eV) to Li interstitial defects, while the presence of Li 1s peak at higher than 55.3 eV suggests that Li+ ion and Zn2+ ion are displaced within the ZnO lattice structure [85,86]. This result is harmony with our structural changes and previously speculation by means of Li content. Fig. 12 shows the fitted O 1s peak for ZnO:Li films. Similar to the literature [84] O 1s peak fit into three Gaussian peaks with various binding energy components. Among them, the peak with lower, medium and highest binding energy represent oxygen lattice in wurtzite crystal structure (O1), the oxygen vacancies (O2) and chemisorbed oxygen on the surface of ZnO (O3), respectively. In other words, it can be said that O1 peak in O 1s spectrum can be dedicated to Zn bound by oxygen species, while O2 can be attributed O−2 ions which are in oxygen vacancies in ZnO matrix. Fig. 12 has pointed out that XPS spectra of O 1s exhibit asymmetrical trend referring multi-component oxygen species located at the near surface area. Binding energy values for O1, O2 and O3 peaks and their variations are represented in Fig. 12. These values and variations are harmony with the literature [87,88]. Additionally, the variations in binding energy values of O1, O2 and O3

(13)

U

where E and 0 represent photon energy and a constant, respectively while EU is Urbach energy indicating the width of exponential absorption edge. Fig. 10 indicates ln(α) plot based photon energy for lithium doped ZnO films. This behavior refers optical transitions as a response for the occupied states in the tail of the valence band and the available states at the edge of the conduction band. So, Urbach energy determined by

(

)

1

taking EU = d (ln ) d (hv ) into consideration. In Fig. 10, calculated Urbach energy values of Li doped ZnO films have been given and a reverse relationship between optical band gap and Urbach energy of the samples has been found. The decrease or increase in the optical band gap values can be attributed that variation in the redistribution of states responsible for band to tail or tail to tail transition. Thus, it is possible to observe narrowing or broadening in the optical band gap of the samples. The presence of Li impurities in ZnO crystal structure and its valence state have been examined by using X-rap photoelectron spectroscopy and obtained results have been given in Fig. 11. The elementary components of ZnO films have been identified thanks to their wide scanning spectrum in the energy range from 0 to 1100 eV. Wide scanning spectrum have revealed that photoelectron peaks in the

8

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Fig. 11. (a) XPS spectra of ZnO:Li films; (b) narrow scan at Li 1s peak.

peaks can be attributed by defect concentration occurs depending on Li content in ZnO matrix. Also, O1 peak has the highest intensity indicating there is a strong bonding between Zn and O atoms in co-doped ZnO films.

change in the crystallite size of the samples. The effect of Li content on the optical characteristics like transmittance and reflectance behavior was also evaluated. According to UV-VIS-NIR spectroscopy results, it can be said that the optical transparency of the samples was higher than 80%, indicating the convenience in optoelectronic application such as solar cells. Also, very low reflection values were observed in visible and infrared region for all samples. The optical band gap of the samples’ values as a function of Li content were calculated by considering three different approximations and all approximations exhibited similar tendency. As a result, the optical band gap values obtained by Tauch and quantum confinement approximations found to be very similar. This result support that quantum confinement effect observed in the

4. Conclusion In this study, ZnO films were obtained by chemical spray pyrolysis technique as a function of Li content. XRD measurement revealed that (002) peak is the most intense peak indicating c-axis orientation for all films. SEM images showed that surface morphology of the samples consisted of granular nanostructures, as well as that lithium caused a

Fig. 12. XPS spectra of O 1s in ZnO:Li films a)0 at.% b)1 at. %, c)2 at. % d) 4 at. % and e)5 at. %. 9

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crystallite range in this study. The presence of Li content in the structure of ZnO, XPS analysis were also conducted. XPS spectroscopy pointed out that Li 1s peaks found to be in range of 54–54.2 eV for 0, 1, 2, 4 at. % Li doped ZnO films and 55.6 eV for 5 at. % Li doped ZnO films. This result supports the changes in the structural properties of ZnO films due to the addition of Li. XPS spectra of O 1s peak was also obtained and O1, O2 and O3 peaks, which indicates oxygen in hexagonal wurtzite crystal structure, the oxygen vacancies, chemisorbed oxygen on the surface of ZnO, respectively. Furthermore, differences in their binding energy were explained taking Li content into consideration in detail.

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Declaration of competing interest

[21]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement

[23]

This research was supported by Ardahan University Research Fund, Project no. 2018-026.

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References [25]

[1] A.W. Lang, Y. Li, M. De Keersmaecker, D.E. Shen, A.M. Österholm, L. Berglund, J.R. Reynolds, Transparent wood smart windows: polymer electrochromic devices based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) electrodes, ChemSusChem 11 (2018) 854–863, https://doi.org/10.1002/cssc.201702026. [2] M. Socol, G. Socol, N. Preda, A. Stanculescu, F. Stanculescu, Laser prepared thin films for optoelectronic applications, Nanoscaled Film. Layers, InTech, 2017, pp. 51–80, , https://doi.org/10.5772/67659. [3] S.D. Nehate, A. Prakash, P.D. Mani, K.B. Sundaram, Work function extraction of indium tin oxide films from MOSFET devices, ECS J. Solid State Sci. Technol. 7 (2018) P87–P90, https://doi.org/10.1149/2.0081803jss. [4] X. Ji, J. Song, T. Wu, Y. Tian, B. Han, X. Liu, H. Wang, Y. Gui, Y. Ding, Y. Wang, Fabrication of high-performance F and Al co-doped ZnO transparent conductive films for use in perovskite solar cells, Sol. Energy Mater. Sol. Cells 190 (2019) 6–11, https://doi.org/10.1016/j.solmat.2018.10.009. [5] S.J. Lee, S. Kim, D.C. Lim, D.H. Kim, S. Nahm, S.H. Han, Inverted bulk-heterojunction polymer solar cells using a sputter-deposited Al-doped ZnO electron transport layer, J. Alloy. Comp. 777 (2019) 717–722, https://doi.org/10.1016/j. jallcom.2018.11.040. [6] P. Balakrishna Pillai, M.M. De Souza, Nanoionics-based three-terminal synaptic device using zinc oxide, ACS Appl. Mater. Interfaces 9 (2017) 1609–1618, https:// doi.org/10.1021/acsami.6b13746. [7] S.W. Cho, Y.B. Kim, D.E. Kim, K.S. Kim, Y.D. Yoon, W.J. Kang, W. Lee, H.K. Cho, Y.H. Kim, The pH-dependent corrosion behavior of ternary oxide semiconductors and common metals and its application for solution-processed oxide thin film transistors circuit integration, J. Alloy. Comp. 714 (2017) 572–582, https://doi. org/10.1016/j.jallcom.2017.04.289. [8] D.J. Sharmila, N. Bachan, V. Chandrakala, P. Naveen Kumar, J. Madhavan, J. Merline Shyla, A correlative investigation of photoconductive behaviour of bare and Al-doped ZnO thin films, Mater. Today Proc. 8 (2019) 250–255, https://doi. org/10.1016/j.matpr.2019.02.108. [9] A. Kocyigit, M.O. Erdal, M. Yıldırım, Effect of indium doping on optical parameter properties of sol–gel-derived ZnO thin films, Z. Naturforschung A 74 (2019) 915–923, https://doi.org/10.1515/zna-2019-0070. [10] M.R. Wagner, G. Callsen, J.S. Reparaz, J.-H. Schulze, R. Kirste, M. Cobet, I.A. Ostapenko, S. Rodt, C. Nenstiel, M. Kaiser, A. Hoffmann, A.V. Rodina, M.R. Phillips, S. Lautenschläger, S. Eisermann, B.K. Meyer, Bound excitons in ZnO: structural defect complexes versus shallow impurity centers, Phys. Rev. B. 84 (2011) 035313, , https://doi.org/10.1103/PhysRevB.84.035313. [11] H.V. Thang, G. Pacchioni, Electronic structure of Al{,} Ga{,} in and Cu doped ZnO/ Cu(111) bilayer films, Phys. Chem. Chem. Phys. 21 (2019) 369–377, https://doi. org/10.1039/C8CP06717A. [12] Y. Ammaih, A. Lfakir, B. Hartiti, A. Ridah, P. Thevenin, M. Siadat, Structural, optical and electrical properties of ZnO:Al thin films for optoelectronic applications, Opt. Quant. Electron. 46 (2014) 229–234, https://doi.org/10.1007/s11082-0139757-2. [13] T. Minami, H. Sato, H. Nanto, S. Takata, Group III impurity doped zinc oxide thin films prepared by RF magnetron sputtering, Jpn. J. Appl. Phys. 24 (1985) L781–L784, https://doi.org/10.1143/JJAP.24.L781. [14] S. Kuprenaite, T. Murauskas, A. Abrutis, V. Kubilius, Z. Saltyte, V. Plausinaitiene, Properties of In-, Ga-, and Al-doped ZnO films grown by aerosol-assisted MOCVD: influence of deposition temperature, doping level and annealing, Surf. Coat. Technol. 271 (2015) 156–164, https://doi.org/10.1016/j.surfcoat.2014.12.052. [15] M. Khuili, N. Fazouan, H.A. El Makarim, G. El Halani, E.H. Atmani, Comparative

[26]

[27]

[28] [29]

[30] [31] [32]

[33]

[34] [35] [36] [37] [38]

[39] [40]

10

first principles study of ZnO doped with group III elements, J. Alloy. Comp. 688 (2016) 368–375, https://doi.org/10.1016/j.jallcom.2016.06.294. L. Fadillah, B. Soegijono, S. Budiawanti, I. Mudzakir, Fabrication and characterization of ZnO and Li doped ZnO by a sol-gel method, AIP Conf. Proc. (2017) 030053, , https://doi.org/10.1063/1.4991157. S.-H. Shin, Y.-H. Kim, M.H. Lee, J.-Y. Jung, J.H. Seol, J. Nah, Lithium-doped zinc oxide nanowires–polymer composite for high performance flexible piezoelectric nanogenerator, ACS Nano 8 (2014) 10844–10850, https://doi.org/10.1021/ nn5046568. Y.C. Yang, C. Song, X.H. Wang, F. Zeng, F. Pan, Giant piezoelectric d[sub 33] coefficient in ferroelectric vanadium doped ZnO films, Appl. Phys. Lett. 92 (2008) 012907, , https://doi.org/10.1063/1.2830663. H.M.A. Hamid, Z. Çelik-Butler, Characterization and performance analysis of Lidoped ZnO nanowire as a nano-sensor and nano-energy harvesting element, Nano Energy 50 (2018) 159–168, https://doi.org/10.1016/j.nanoen.2018.05.023. A. Kocyigit, R. Topkaya, Structural, optical and magnetic properties of Ni-Co codoped ZnO thin films, Mater. Res. Express 6 (2019) 096116, , https://doi.org/10. 1088/2053-1591/ab120a. S. Kundu, R. Majumder, R. Ghosh, M. Pal Chowdhury, Superior positive relative humidity sensing properties of porous nanostructured Al:ZnO thin films deposited by jet-atomizer spray pyrolysis technique, J. Mater. Sci. Mater. Electron. 30 (2019) 4618–4625, https://doi.org/10.1007/s10854-019-00754-x. J.-L. Zhao, X.-M. Li, S. Zhang, C. Yang, X.-D. Gao, W.-D. Yu, Highly (002)-oriented ZnO film grown by ultrasonic spray pyrolysis on ZnO-seeded Si (100) substrate, J. Mater. Res. 21 (2006) 2185–2190, https://doi.org/10.1557/jmr.2006.0291. D.K. Murti, T.L. Bluhm, Preferred orientation of ZnO films controlled by r.f. sputtering, Thin Solid Films 87 (1982) 57–61, https://doi.org/10.1016/0040-6090(82) 90571-5. J.H. Park, Y.J. Lee, J.-S. Bae, B.-S. Kim, Y.C. Cho, C. Moriyoshi, Y. Kuroiwa, S. Lee, S.-Y. Jeong, Analysis of oxygen vacancy in Co-doped ZnO using the electron density distribution obtained using MEM, Nanoscale Res. Lett. 10 (2015) 186, https://doi. org/10.1186/s11671-015-0887-2. W.F. Liu, G.T. Du, Y.F. Sun, J.M. Bian, Y. Cheng, T.P. Yang, Y.C. Chang, Y.B. Xu, Effects of hydrogen flux on the properties of Al-doped ZnO films sputtered in Ar +H2 ambient at low temperature, Appl. Surf. Sci. 253 (2007) 2999–3003, https:// doi.org/10.1016/j.apsusc.2006.06.049. X. Wang, X. Wang, Q. Di, H. Zhao, B. Liang, J. Yang, Mutual effects of fluorine dopant and oxygen vacancies on structural and luminescence characteristics of F doped SnO2 nanoparticles, Materials (Basel) 10 (2017) 1398, https://doi.org/10. 3390/ma10121398. A. Samavati, Z. Samavati, A.F. Ismail, M.H.D. Othman, M.A. Rahman, A.K. Zulhairun, I.S. Amiri, Structural, optical and electrical evolution of Al and Ga co-doped ZnO/SiO 2/glass thin film: role of laser power density, RSC Adv. 7 (2017) 35858–35868, https://doi.org/10.1039/C7RA04963C. S. Kahraman, H.M. Çakmak, S. Çetinkaya, F. Bayansal, H.A. Çetinkara, H.S. Güder, Characteristics of ZnO thin films doped by various elements, J. Cryst. Growth 363 (2013) 86–92, https://doi.org/10.1016/j.jcrysgro.2012.10.018. K.K. Nagaraja, S. Pramodini, A. Santhosh Kumar, H.S. Nagaraja, P. Poornesh, D. Kekuda, Third-order nonlinear optical properties of Mn doped ZnO thin films under cw laser illumination, Opt. Mater. (Amst). 35 (2013) 431–439, https://doi. org/10.1016/j.optmat.2012.09.028. K. Meziane, A. Elhichou, A. Elhamidi, A. Almaggoussi, M. Chhiba, Synthesis of lithium doped zinc oxide by sol gel, J. Phys. Conf. Ser. 758 (2016) 012019, , https:// doi.org/10.1088/1742-6596/758/1/012019. M. Ardyanian, N. Sedigh, Heavy lithium-doped ZnO thin films prepared by spray pyrolysis method, Bull. Mater. Sci. 37 (2014) 1309–1314, https://doi.org/10.1007/ s12034-014-0076-4. G.S. Thool, A.K. Singh, R.S. Singh, A. Gupta, M.A.B.H. Susan, Facile synthesis of flat crystal ZnO thin films by solution growth method: a micro-structural investigation, J. Saudi Chem. Soc. 18 (2014) 712–721, https://doi.org/10.1016/j.jscs.2014.02. 005. T.M.K. Thandavan, S.M.A. Gani, C.S. Wong, R.M. Nor, Evaluation of Williamson–Hall strain and stress distribution in ZnO nanowires prepared using aliphatic alcohol, J. Nondestruct. Eval. 34 (2015) 14, https://doi.org/10.1007/ s10921-015-0286-8. A.R. Stokes, A.J.C. Wilson, The diffraction of X rays by distorted crystal aggregates I, Proc. Phys. Soc. 56 (1944) 174–181, https://doi.org/10.1088/0959-5309/56/3/ 303. B. Rajesh Kumar, B. Hymavathi, X-ray peak profile analysis of Sb 2 O 3 -doped ZnO nanocomposite semiconductor, Adv. Nat. Sci. Nanosci. Nanotechnol. 9 (2018) 035018, , https://doi.org/10.1088/2043-6254/aadc6b. H.-P. Hsu, D.-Y. Lin, C.-Y. Lu, T.-S. Ko, H.-Z. Chen, Effect of lithium doping on microstructural and optical properties of ZnO nanocrystalline films prepared by the sol-gel method, Crystals 8 (2018) 228, https://doi.org/10.3390/cryst8050228. S. Ullah Awan, S.K. Hasanain, M.F. Bertino, G. Hassnain Jaffari, Ferromagnetism in Li doped ZnO nanoparticles: the role of interstitial Li, J. Appl. Phys. 112 (2012) 103924, https://doi.org/10.1063/1.4767364. J.B. Yi, C.C. Lim, G.Z. Xing, H.M. Fan, L.H. Van, S.L. Huang, K.S. Yang, X.L. Huang, X.B. Qin, B.Y. Wang, T. Wu, L. Wang, H.T. Zhang, X.Y. Gao, T. Liu, A.T.S. Wee, Y.P. Feng, J. Ding, Ferromagnetism in dilute magnetic semiconductors through defect engineering: Li-doped ZnO, Phys. Rev. Lett. 104 (2010) 137201, https://doi. org/10.1103/PhysRevLett.104.137201. M. Hjiri, M.S. Aida, O.M. Lemine, L. El Mir, Study of defects in Li-doped ZnO thin films, Mater. Sci. Semicond. Process. 89 (2019) 149–153, https://doi.org/10.1016/ j.mssp.2018.09.010. K.H. Kim, Z. Jin, Y. Abe, M. Kawamura, Effects of Li and Cu dopants on structural

Ceramics International xxx (xxxx) xxx–xxx

S. Aydogan and M. Yilmaz

[41] [42] [43] [44] [45] [46] [47] [48] [49] [50]

[51] [52] [53]

[54] [55] [56]

[57] [58]

[59]

[60]

[61] [62] [63] [64]

[65] T. Saidani, M. Zaabat, M.S. Aida, B. Boudine, Effect of copper doping on the photocatalytic activity of ZnO thin films prepared by sol–gel method, Superlattice Microstruct. 88 (2015) 315–322, https://doi.org/10.1016/j.spmi.2015.09.029. [66] S. Demirozu Senol, Influence of Mg doping on the structural, optical, and electrical properties of Zn 0.95 Li 0.05 O Nanoparticles, Int. J. Appl. Ceram. Technol. 16 (2019) 138–145, https://doi.org/10.1111/ijac.13060. [67] J. Gitonga, et al., Effect of substrate temperature on the optical properties of SnxSey/ZnO:Al P-N junction solar cell, Am. J. Mater. Sci. 7 (2017) 250–257, https:// doi.org/10.5923/j.materials.20170706.04. [68] C.L. Rhodes, S. Lappi, D. Fischer, S. Sambasivan, J. Genzer, S. Franzen, Characterization of monolayer formation on aluminum-doped zinc oxide thin films, Langmuir 24 (2008) 433–440, https://doi.org/10.1021/la701741m. [69] R. Vettumperumal, S. Kalyanaraman, R. Thangavel, Effect of Er concentration on surface and optical properties of K doped ZnO sol–gel thin films, Superlattice Microstruct. 83 (2015) 237–250, https://doi.org/10.1016/j.spmi.2015.03.028. [70] L.A. Al-Hajji, A.A. Ismail, Photooxidation of acetaldehyde over TiO2 film: impact of the substrate glassy structure on the photonic efficiency, Superlattice Microstruct. 129 (2019) 259–267, https://doi.org/10.1016/j.spmi.2019.04.012. [71] R. Baghdad, N. Lemée, G. Lamura, A. Zeinert, N. Hadj-Zoubir, M. Bousmaha, M.A. Bezzerrouk, H. Bouyanfif, B. Allouche, K. Zellama, Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method, Superlattice Microstruct. 104 (2017) 553–569, https://doi.org/10.1016/j.spmi. 2016.11.069. [72] M. Zafar, J.-Y. Yun, D.-H. Kim, Improved inverted-organic-solar-cell performance via sulfur doping of ZnO films as electron buffer layer, Mater. Sci. Semicond. Process. 96 (2019) 66–72, https://doi.org/10.1016/j.mssp.2019.01.046. [73] A.R. Nimbalkar, M.G. Patil, Synthesis of highly selective and sensitive Cu-doped ZnO thin film sensor for detection of H 2 S gas, Mater. Sci. Semicond. Process. 71 (2017) 332–341, https://doi.org/10.1016/j.mssp.2017.08.022. [74] V.R. Shinde, T.P. Gujar, C.D. Lokhande, R.S. Mane, S.-H. Han, Mn doped and undoped ZnO films: a comparative structural, optical and electrical properties study, Mater. Chem. Phys. 96 (2006) 326–330, https://doi.org/10.1016/j.matchemphys. 2005.07.045. [75] M. Wang, K.E. Lee, S.H. Hahn, E.J. Kim, S. Kim, J.S. Chung, E.W. Shin, C. Park, Optical and photoluminescent properties of sol-gel Al-doped ZnO thin films, Mater. Lett. 61 (2007) 1118–1121, https://doi.org/10.1016/j.matlet.2006.06.065. [76] M. Caglar, S. Ilican, Y. Caglar, Influence of dopant concentration on the optical properties of ZnO: in films by sol–gel method, Thin Solid Films 517 (2009) 5023–5028, https://doi.org/10.1016/j.tsf.2009.03.037. [77] S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanoglu, Structural, optical and electrical properties of F-doped ZnO nanorod semiconductor thin films deposited by sol–gel process, Appl. Surf. Sci. 255 (2008) 2353–2359, https://doi.org/10.1016/j.apsusc. 2008.07.111. [78] F.K. Shan, B.I. Kim, G.X. Liu, Z.F. Liu, J.Y. Sohn, W.J. Lee, B.C. Shin, Y.S. Yu, Blueshift of near band edge emission in Mg doped ZnO thin films and aging, J. Appl. Phys. 95 (2004) 4772–4776, https://doi.org/10.1063/1.1690091. [79] A. Jain, P. Sagar, R.M. Mehra, Band gap widening and narrowing in moderately and heavily doped n-ZnO films, Solid State Electron. 50 (2006) 1420–1424, https://doi. org/10.1016/j.sse.2006.07.001. [80] A. Escobedo-Morales, I.I. Ruiz-López, M. deL. Ruiz-Peralta, L. Tepech-Carrillo, M. Sánchez-Cantú, J.E. Moreno-Orea, Automated method for the determination of the band gap energy of pure and mixed powder samples using diffuse reflectance spectroscopy, Heliyon 5 (2019) e01505, , https://doi.org/10.1016/j.heliyon.2019. e01505. [81] J.C. Nie, J.Y. Yang, Y. Piao, H. Li, Y. Sun, Q.M. Xue, C.M. Xiong, R.F. Dou, Q.Y. Tu, Quantum confinement effect in ZnO thin films grown by pulsed laser deposition, Appl. Phys. Lett. 93 (2008) 173104, https://doi.org/10.1063/1.3010376. [82] P.K. Samanta, Strong and weak quantum confinement and size dependent optoelectronic properties of zinc oxide, Ann. Univ. Craiova, Phys. 28 (2018) 17–23. [83] X.D. Li, T.P. Chen, P. Liu, Y. Liu, K.C. Leong, Effects of free electrons and quantum confinement in ultrathin ZnO films: a comparison between undoped and Al-doped ZnO, Opt. Express 21 (2013) 14131, https://doi.org/10.1364/OE.21.014131. [84] Li Zhao, Wen Ai, Resistive switching characteristics of Li-doped ZnO thin films based on magnetron sputtering, Materials (Basel) 12 (2019) 1282, https://doi.org/ 10.3390/ma12081282. [85] C. Ai, X. Zhao, S. Li, Y. Li, Y. Bai, D. Wen, Fabrication and characteristic of a double piezoelectric layer acceleration sensor based on Li-doped ZnO thin film, Micromachines 10 (2019) 331, https://doi.org/10.3390/mi10050331. [86] S. Ghosh, G.G. Khan, K. Mandal, S. Thapa, P.M.G. Nambissan, Positron annihilation studies of vacancy-type defects and room temperature ferromagnetism in chemically synthesized Li-doped ZnO nanocrystals, J. Alloy. Comp. 590 (2014) 396–405, https://doi.org/10.1016/j.jallcom.2013.12.149. [87] S. Karamat, R.S. Rawat, P. Lee, T.L. Tan, R.V. Ramanujan, Structural, elemental, optical and magnetic study of Fe doped ZnO and impurity phase formation, Prog. Nat. Sci. Mater. Int. 24 (2014) 142–149, https://doi.org/10.1016/j.pnsc.2014.03. 009. [88] S.U. Awan, S.K. Hasanain, M.S. Awan, S.A. Shah, Raman scattering and interstitial Li defects induced polarization in co-doped multiferroic Zn 0.96-y Co 0.04 Li y O (0.00 ≤ y ≤ 0.10) nanoparticles, RSC Adv. 5 (2015) 39828–39839, https://doi. org/10.1039/C5RA03691G.

properties of zinc oxide nanorods, Superlattice Microstruct. 77 (2015) 101–107, https://doi.org/10.1016/j.spmi.2014.11.015. A.Y. Oral, Z.B. Bahşi, M.H. Aslan, Microstructure and optical properties of nanocrystalline ZnO and ZnO:(Li or Al) thin films, Appl. Surf. Sci. 253 (2007) 4593–4598, https://doi.org/10.1016/j.apsusc.2006.10.015. A.R. Khantoul, M. Sebais, B. Rahal, B. Boudine, O. Halimi, Structural and optical properties of ZnO and Co doped ZnO thin films prepared by sol-gel, Acta Phys. Pol., A 133 (2018) 114–117, https://doi.org/10.12693/APhysPolA.133.114. S. Sönmezoğlu, E. Akman, Improvement of physical properties of ZnO thin films by tellurium doping, Appl. Surf. Sci. 318 (2014) 319–323, https://doi.org/10.1016/j. apsusc.2014.06.187. S.Y. Wakhare, M.D. Deshpande, The electronic and optical properties of monovalent atom-doped ZnO monolayers: the density functional theory, Bull. Mater. Sci. 42 (2019) 206, https://doi.org/10.1007/s12034-019-1901-6. A. Srivastava, N. Kumar, S. Khare, Enhancement in UV emission and band gap by Fe doping in ZnO thin films, Opto-Electron. Rev. 22 (2014) 68–76, https://doi.org/10. 2478/s11772-014-0179-x. A. Ismail, M.J. Abdullah, The structural and optical properties of ZnO thin films prepared at different RF sputtering power, J. King Saud Univ. Sci. 25 (2013) 209–215, https://doi.org/10.1016/j.jksus.2012.12.004. T. Srinivasulu, K. Saritha, K.T.R. Reddy, Synthesis and characterization of Fe-doped ZnO thin films deposited by chemical spray pyrolysis, Mod. Electron. Mater. 3 (2017) 76–85, https://doi.org/10.1016/J.MOEM.2017.07.001. M.I. Khan, K.A. Bhatti, R. Qindeel, N. Alonizan, H.S. Althobaiti, Characterizations of multilayer ZnO thin films deposited by sol-gel spin coating technique, Res. Phys. 7 (2017) 651–655, https://doi.org/10.1016/j.rinp.2016.12.029. D.E. Sands, Handbook of X-rays, Microchem. J. 13 (1968) 514, https://doi.org/10. 1016/0026-265X(68)90121-5. A.R. Babar, S.S. Shinde, A.V. Moholkar, C.H. Bhosale, J.H. Kim, K.Y. Rajpure, Physical properties of sprayed antimony doped tin oxide thin films: the role of thickness, J. Semicond. 32 (2011) 053001, , https://doi.org/10.1088/1674-4926/ 32/5/053001. A.V. Moholkar, S.M. Pawar, K.Y. Rajpure, C.H. Bhosale, Effect of concentration of SnCl4 on sprayed fluorine doped tin oxide thin films, J. Alloy. Comp. 455 (2008) 440–446, https://doi.org/10.1016/j.jallcom.2007.01.160. M.H. Babu, B.C. Dev, J. Podder, Texture coefficient and band gap tailoring of Fedoped SnO2 nanoparticles via thermal spray pyrolysis, Rare Met. (2019) 1–10, https://doi.org/10.1007/s12598-019-01278-3. D. Fang, K. Lin, T. Xue, C. Cui, X. Chen, P. Yao, H. Li, Influence of Al doping on structural and optical properties of Mg–Al co-doped ZnO thin films prepared by sol–gel method, J. Alloy. Comp. 589 (2014) 346–352, https://doi.org/10.1016/j. jallcom.2013.11.061. M. Yilmaz, M.L. Grilli, The modification of the characteristics of nanocrystalline ZnO thin films by variation of Ta doping content, Philos. Mag. 96 (2016) 2125–2142, https://doi.org/10.1080/14786435.2016.1195023. S. Marouf, A. Beniaiche, H. Guessas, A. Azizi, Morphological, structural and optical properties of ZnO thin films deposited by dip coating method, Mater. Res. 20 (2016) 88–95, https://doi.org/10.1590/1980-5373-mr-2015-0751. M. Schuster, S. Groß, F. Roider, I. Maksimenko, P. Wellmann, Tuning electrical and optical properties of transparent conductive thin films using ITO and ZnO nanoparticles, sol-gel-ZnO and Ag nanowires, Int. J. Nanoparticles Nanotechnol. 3 (2017) 1–11, https://doi.org/10.35840/2631-5084/5513. Z. Baji, Z. Lábadi, Z.E. Horváth, G. Molnár, J. Volk, I. Bársony, P. Barna, Nucleation and growth modes of ALD ZnO, Cryst. Growth Des. 12 (2012) 5615–5620, https:// doi.org/10.1021/cg301129v. Y. Adachi, N. Ohashi, T. Ohgaki, T. Ohnishi, I. Sakaguchi, S. Ueda, H. Yoshikawa, K. Kobayashi, J.R. Williams, T. Ogino, H. Haneda, Polarity of heavily doped ZnO films grown on sapphire and SiO2 glass substrates by pulsed laser deposition, Thin Solid Films 519 (2011) 5875–5881, https://doi.org/10.1016/j.tsf.2011.02.087. M. Zhao, F. Shang, J. Lv, Y. Song, F. Wang, Z. Zhou, G. He, M. Zhang, X. Song, Z. Sun, Y. Wei, X. Chen, Influence of water content in mixed solvent on surface morphology, wettability, and photoconductivity of ZnO thin films, Nanoscale Res. Lett. 9 (2014) 485, https://doi.org/10.1186/1556-276X-9-485. N.F. Ab Rasid, S.N. Mohd Tawil, N. Che Ani, M.Z. Sahdan, Effect of crystallite size on the optical and structural properties of gadolinium-doped zinc oxide, Adv. Mater. Res. 1133 (2016) 414–418 https://doi.org/10.4028/www.scientific.net/ AMR.1133.414. P. Gu, X. Zhu, D. Yang, Structural, optical and photoelectric properties of Mn-doped ZnO films used for ultraviolet detectors, RSC Adv. 9 (2019) 8039–8047, https://doi. org/10.1039/C9RA01099H. S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, Photoelectrochemical properties of highly mobilized Li-doped ZnO thin films, J. Photochem. Photobiol. B Biol. 120 (2013) 1–9, https://doi.org/10.1016/j.jphotobiol.2013.01.003. M. Musaab Khudhr, K.H. Abass, Effect of Al-doping on the optical properties of ZnO thin film prepared by thermal evaporation technique, Int. J. Eng. Technol. 7 (2016) 25–31 https://doi.org/10.18052/www.scipress.com/IJET.7.25. C.M. Vladut, S. Mihaiu, E. Tenea, S. Preda, J.M. Calderon-Moreno, M. Anastasescu, H. Stroescu, I. Atkinson, M. Gartner, C. Moldovan, M. Zaharescu, Optical and piezoelectric properties of Mn-doped ZnO films deposited by sol-gel and hydrothermal methods, J. Nanomater. 2019 (2019) 1–12, https://doi.org/10.1155/2019/ 6269145.

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