Hidden impurities in transparent conducting oxides: study of vacancies-related defects and impurities in (Cu–Ni) co-doped ZnO films

The effect of hydrogen and nitrogen impurities on the physical properties of transparent conductive oxides is investigated in this study. Therefore, 5 wt.% of copper and 5 wt.% of nickel co-doped zinc oxide ((Cu–Ni)/ZnO) films were prepared using the sol–gel method. The (Cu–Ni)/ZnO films were annealed in an oven at 500 °C for 2 h under air, vacuum, nitrogen, and argon atmospheres. The synthesized zinc hydroxide film was transformed to zinc oxide film during the annealing by evaporating H2O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{2}\mathrm{O}$$\end{document}. Films annealed under the mentioned atmosphere including as-prepared one were characterized by analyzing with UV–Vis and FTIR spectra in addition to the 2D mapping electrical conductivity of the surface measured by the 4-point probe. The annealed films under air, vacuum, and argon atmospheres led to generate H-related impurities bounded to the oxygen vacancy (HO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{H}}_{\mathrm{O}}$$\end{document}) which they act as shallow donor defects resulting in forming (Cu–Ni)/ZnO films into n-type materials. Whereas, the film annealed under a nitrogen atmosphere has N-related defects bounding to the zinc vacancy (NZn\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{N}}_{\mathrm{Zn}}$$\end{document}) which they act as shallow acceptor defects resulting in transforming the film from n-type to p-type. These defects affect the optical, electrical, and optoelectronic properties of the (Cu–Ni)/ZnO films.

Doping ZnO films with different elements, such as group III, transition metals, and rare-earth metals, have variated the physical and chemical properties for various potential applications [11]. Copper (Cu) and nickel (Ni) are considered ideal dopants for ZnO films, since their atomic radius as well as electronic shells are comparable to those of Zn atoms. Therefore, the replacement of Zn by Cu and/or Ni does not lead to vary the lattice constants substantially [12]. Cu-doped ZnO films were used as a NO 2 gas sensor [13], photocatalyst [14], and UV photodetector [15]. Moreover, Ni-doped ZnO films were used in spintronics applications [16] and UV photodetector applications [17]. Moreover, Cu has a deep impurity level, and Ni reveals ferromagnetic nature at room temperature, thus the resultant doping combination (Cu-Ni)/ZnO is a motivating candidate for attaining innovative properties [18].
A. Ahmad et al. [19] stated that the films prepared by sol-gel synthesis technique using zinc acetate dehydrated, ethanol, and ethanolamine are amorphous zinc hydroxide ( Zn(OH) 2 ), while the same film annealing at temperatures above 300 °C was transformed into crystalline zinc oxide (ZnO). However, G. Alvin Shi et al. [20] show that annealing ZnO films near 400 °C have H-containing defects that have the role as shallow donors. Hydrogen, oxygen, and nitrogen impurities are the main causes of defects in TCO films [21]. Hydrogen (H) is a widespread impurity in TCO films which affect optical, optoelectronic, and electrical properties of these films via shallow donors formation, since it forms a covalent bond with neighboring vacant oxygen atoms [22]. Hydrogen impurities in TCO films can be metastable and diffuse through the lattice [23]. The n-type conductivity of metal oxide films such as ZnO can be explained using H impurities that act as shallow donors [20]. The hydrogen impurity forms shallow donor defects in the ZnO structure, affecting the optical, optoelectronic, and electrical properties of ZnO [24][25][26][27].
Generally, there are various processes for defect creation in the TCO films during film deposition and post-annealing. Defects creation results in the incorporation of impurities from the ambient during the deposition and/or post-annealing. The defects vary the charge state of the film due to trapping the electrons and holes. The defects interact mutually with each other causing the formation of various forms of defects that diffuse throughout the material.
The main objective of this study is to investigate the hidden impurities and the vacancies-related defects in TCOs by exploring the chemical and structural properties of asprepared and annealed (Cu-Ni)/ZnO film at 500 °C under 1 atmospheric pressure of air, vacuum, nitrogen, and argon. The chemical and structural properties were investigated relying on using FTIR spectra and XRD pattern. In addition, the effect of these impurities on the optical, optoelectronic, and electrical properties of (Cu-Ni)/ZnO films were investigated by extracting the refractive index, bandgap energy, band structure from the UV-Vis spectra with addition to the electrical conductivity properties from 4-points probe. The final solution was centrifuged at 500 rpm for 10 min and then filtered using 0.45 μm filter paper. The (Cu-Ni)/ZnO films were deposited on fused silica substrates (UV grade, 220 ~ 2500 nm wave band, Double side optical-polished, and refractive index about 1.47) using dip coating technique. Each glass substrate was immersed in (Cu-Ni)/ZnO solution with the selected concentration for 2 h. Later on, coated films were dried using an oven at 110 °C for 15 min and thickness were estimated from crosssectional SEM image and found to be around 500 nm. As a final step, annealing of the films was conducted at 500 °C for 2 h and cooling down to room temperature at ambient conditions under air, vacuum, nitrogen, and argon atmospheres in the vacuum oven (Electric furnace, SA2-2-12TP) in different individual experiments. The n/p-type was measured using Avometer after heating the films compared with an n-type silicon substrate. Remarkably, the film annealed under nitrogen atmosphere is a p-type, while other films are n-type.

Experimental set-up and sample preparation
Dissolving ZnAc in EtOH by adding ethanolamine as a stabilizer at ambient conditions produces a clear and homogenous solution of Zn(OH) 2 due to reaction between the Zn 2+ ions revealed from the ZnAc and OH − ions revealed from the interaction of EtOH with the ethanolamine. Adding CuAc and NiCl 2 with calculated concentrations into the starting solution results in (Cu − Ni)∕Zn(OH) 2 solution [28]. Moreover, the (Cu − Ni)∕Zn(OH) 2 solution is deposited as a film and transformed into (Cu − Ni)∕ZnO film by annealing it at temperatures above 300 °C (Fig. 1). The main process is evaporating the H 2 O from the film producing metal oxide film from the metal hydroxide state. However, high temperatures create defects such as hydrogen, oxygen, and nitrogen found in the air. These defects diffuse throughout the film structure via the post-annealing process.
X-ray diffraction (XRD) patterns were obtained at room temperature with XRD instrument (Malvern Panalytical Ltd, Malvern, UK) diffractometer using CuKα radiation (0.1540598 nm) with the incident angels between 30° and 60° with a step of 0.02° and energy resolution of 20%. Fourier transform infrared spectroscopy (FTIR) spectra were obtained with (Bruker Tensor 27) spectrometer in the wavenumber range of 4000-400 cm −1 . The UV-Vis spectrophotometer (U-3900H) was used for conducting the UV-Vis spectra in the wavelength range of 250-700 nm. The 2D electrical conductivities at room temperature were obtained with a 4-point probe (Microworld Inc.) equipped with a high-resolution multimeter (Keithley 2450 Sourcemeter).

Results and discussion
The FTIR spectra of (Cu-Ni)/ZnO films exhibit three main bands of stretching Zn−O bonds (420 cm −1 ) [29], stretching Ni−O bonds (610 cm −1 ) [30], and stretching Cu−O bonds (670 cm −1 ) [31] (Fig. 2a). The weak stretching Zn−O bond in the as-deposited film increases of annealing under air and argon atmospheres. While, it disappears in the case of vacuum and nitrogen environments. Remarkably, Ni − O band does not appear in the as-deposited films; and showed up obviously after annealing regardless of the environment. The Cu − O band was shown up more strongly once annealing under nitrogen atmosphere while it is moderated when the annealing is under air, vacuum and argon. The vibrational bands (800 and 1200 cm −1 ) are associated to the acetate group (carbon, oxygen, and hydrogen), the residuals from the starting materials [32]. H impurities in the metal oxides was determined via the bending O−H bonds (1250-1700 cm −1 ) and the stretching ZnO ∶ H bonds (~ 3400 cm −1 ) (Fig. 2a). Annealing (Cu-Ni)/ZnO film at 500 °C in different environments reduces the H impurities [33]. However, the existence of bending O−H bonds and stretching ZnO ∶ H bonds in the annealed films suggests the existence of hydrogen shallow donors in the (Cu-Ni)/ZnO films, that bounds to the oxygen vacancy ( H O ) [34]. The fact that(Cu-Ni)/ZnO film annealed under nitrogen environment switched from n-type to p-type explained the appearance of the band at 450 cm −1 , this band represents Zn-N bond, this appearance confirms the existence of nitrogen shallow acceptors bounded to zinc vacancy ( N Zn ) [35]. Generally, H and N impurities related to the oxygen atoms ( H O ) and zinc atoms ( N Zn ) influence the physicochemical properties of metal oxide films [36]. The effect of H-containing defects bounded to the oxygen vacancy ( H O ) and N-containing defects bounded to the zinc vacancy ( N Zn ) on the microstructural properties of (Cu-Ni)/ZnO film, such as crystalline domain size ( D) and microstrain , were also investigated using Williamson-Hall (WH) method and its modifications.
The XRD diffraction patterns of (Cu-Ni)/ZnO films (Fig. 2b) were indexed by qualitative phase analysis software  [12]. Annealed films under vacuum, nitrogen, and argon atmospheres were led to enhance the crystallinity degree due to reducing the H-containing defects in the films [19].
The crystallinity degree ( X cryst % ) is defined as the ratio between the integrated area under the diffraction peaks ( A crys ) to the total area ( A crys + A amorph ) ( Table 1) [37,38]. The film annealed under air atmosphere exhibit a lower crystallinity degree (35%), while those annealed under vacuum or argon exhibited higher degree of crystallinity (66%). This is attributed to the lower H impurities in the (Cu-Ni)/ZnO film, as shown in Fig. 2a. However, the (Cu-Ni)/ZnO film annealed in nitrogen exhibits a lower crystallinity degree than the annealed samples in vacuum or argon due to N impurities, which appears at FTIR band of 450 cm −1 (Zn-N bond). The lattice constants of hexagonal structure were calculated using a = √ 4∕3d 100 and c = 2d 002 (Table 1), where d hkl is the interplanar distance calculated from Bragg's law ( n = 2d hkl sin hkl ) [39,40].
The crystallite size ( D ) and microstrain ⟨ ⟩ were calculated using Williamsons-Hall (WH) method [40]. This method states that the total linewidth of the XRD peaks ( total ) is composed of a superposition of the particle size linewidth ( size ) and microstrain linewidth ( strain ), as [41]: The values of D and ⟨ ⟩ were investigated using the modified WH equation via the uniform deformation model (UDM) [42], Plotting hkl cos versus 4sin illustrates the microstrain from the slope, while the crystallite size is calculated using the intercept (Fig. 2c). The average D and ⟨ ⟩ deduced from WH-UDM as explained above are listed in Table 1. The obtained microstrain of the films was the lowest in the case (1) total = size + strain .
of the annealed film under a vacuum, revealing that film has the lowermost H impurities. In comparison, the higher microstrain was observed in the film annealed under nitrogen atmosphere, suggesting that defects associated with the N have higher impact on the film microstructure comparing to the defects associated with H impurities (Fig. 2d). The electrical conductivity of metal oxide films depends on many parameters, such as dopant type, dopant concentration, impurities, and crystallinity degree [43]. The asprepared film has an average conductivity of 2.93 ± 0.2 μS/ cm, while the annealed film under air vacuum, nitrogen, and argon atmospheres has average electrical conductivities of 3.77 ± 0.2, 6.71 ± 0.2, 6.48 ± 0.2, and 6.80 ± 0.2 μS/cm, respectively (Fig. 3a). The enhanced electrical conductivity in the annealed films is associated with the enhancement in the degree of crystallinity in addition to the reduction in the H impurities. In spite of that the shallow hydrogen donors bounded to O-vacancies ( H O ) are the origin of the n-type of metal oxides [36], the annealed film under nitrogen atmosphere has lower electrical conductivity compared to those annealed films under vacuum and argon due to the existence of shallow nitrogen accepters bounded to Zn-vacancies ( N Zn ) that are the origins of the p-type of the film. The conductivity mapping of (Cu-Ni)/ZnO film (Fig. 3b-f) shows variation in the surface conductivity due to the growth process and the quality of the transfer process.
The transmittance spectrum of the as-prepared film has a rapid jump in the transmittance from 0 to 90% associated with the increase in the wavelengths only from 300 to 400 nm, while no significant variation is observed in wavelengths between 400 and 700 nm (Fig. 4a). In the annealed films, the transmittance was decreased in the visible region, and the band edge was shifted to the red region (low energy), indicating a decrease in the bandgap energy. In addition, the reflectance was increased in all of the annealed films (Fig. 4b). The bandgap energies ( E g ) calculated according to Tauc equation ( hv) 2 = hv − E g , where is the absorption coefficient ( = (1∕d)ln((1 − R)∕T) ) [44], were decreased as the films were annealed (Fig. 4e). Moreover, the minimum occurrence in the obtained bandgap energy was associated with the film annealed under a nitrogen atmosphere, this can be connected to microstrain which has the highest value among all due to the defects induced by the nitrogen as it is presented above. The H shallow donor and N shallow acceptor defects influencing the valance (VB) and conduction (CB) bands occurred due the disorders were investigated by Urbach energy ( E U ) via the equation = α 0 exp(hv∕E u ) [45]. Based on the findings in the literature, the obtained band structure was investigated using ionization and electron affinity energies [46,47]. The band structure schematic diagram illustrated in Fig. 4f illustrates the shifts in the conduction bands (CB) toward less negative potential as well illustrates the shifts in the valance bands (VB) toward less positive potential in the annealed films compared to the as-prepared one. In addition, the as-prepared (Cu-Ni)/ ZnO film has higher Urbach energy, which means higher disorder, indicating a higher existence of H shallow donor defects [19]. However, Urbach energies of the annealed films under vacuum and argon atmospheres have similar and lowest values compare to the others, indicating lowest H shallow donor defects. While, the high Urbach energy value was observed in the annealed film under nitrogen atmosphere connected to the N shallow acceptor defects. Moreover, the average electrical conductivity and bandgap energy for the (Cu-Ni)/ZnO film annealed under vacuum is 6.71 μS/cm and 3.38 eV, respectively. However, the (Cu-Ni)/ZnO film annealed in argon has 6.80 μS/cm and 3.39 eV, average electrical conductivity, and bandgap energy due to the low microstrain from the low H-related defects. Moreover, the average electrical conductivity and bandgap energy for the (Cu − Ni)∕ZnO film annealed in nitrogen is 6.48 μS/cm and 3.22 eV, respectively.
The extinction coefficient ( k ) and refractive index ( n ) w e r e c a l c u l a t e d u s i n g k = ∕4 a n d [48]. The absolute values of k and n over the wavelength ranges from 250 to 700 nm were observed to be the highest in the case of the film annealed under nitrogen atmosphere (Fig. 4c, d).
Which means that the photon penetrates the films with higher decaying or damping (Fig. 4c). The n−spectra of the films (Fig. 4d) exhibits two behaviors, the anomalous type which occurs in the wavelength region (250 ≤ < 400 nm), where the incident photon frequency balances the plasma frequency [49]. Besides, the normal behavior occurs in the wavelength region ( 400 ≤ < 700 nm) where  [50]. Generally, there are many processes for defect creation in TCO films during film deposition and post-annealing (Fig. 5a). Defects creation results in the incorporation of impurities from the environment during the deposition and/ or post-annealing. These defects change the film charge state due to electron and hole trapping. In addition, defects can react with each other to form different defects, and the final process is defect diffusion [21]. Based on the above results and their interpretation, annealing (Cu − Ni)∕Zn(OH) 2 film at 500 °C in an air environment produced (Cu − Ni)∕ZnO film through many processes (Fig. 5b). The main process that occurs is the H 2 O evaporation from the film to transform the film from metal hydroxide to metal oxide. However, high temperatures created defects, hydrogen, and oxygen, in the air components. These defects diffused into the film structure through the post-annealing process. Consequently, these defects could react with each other and/or changes the charge across the film. The annealing of the (Cu − Ni)∕Zn(OH) 2 film at 500 °C in a vacuum or argon environment produces (Cu − Ni)∕ZnO film (Figs. 5c, 4e) was led mainly to evaporate H 2 O from the film to produce zinc oxide film from zinc hydroxide, beside reaction between H impurities with oxygen to produce OH − . Finally, annealing the (Cu − Ni)∕Zn(OH) 2 film at 500 °C in a nitrogen environment produces (Cu − Ni)∕ZnO film with nitrogen defects (Fig. 5d). The main evidence of the existence of N-related defects bound to the zinc vacancy ( N Zn ) is the transformation from n-type to p-type [35], in addition to evaporating the H 2 O from the film. Nitrogen impurities may diffuse into the film, creating N-containing shallow acceptor defects.

Conclusions
The (Cu-Ni)/ZnO films were prepared using the sol-gel method and then annealed using an oven at 500 °C for 2 h in different environments (air, vacuum, nitrogen, and argon). The main process occurs through annealing the films is evaporating the H 2 O from the films to produce metal oxides from metal hydroxide material. The FTIR spectra confirm the existence of H-related defects bounded to the vacant oxygen ( H O ) in the annealed film under air, vacuum, or argon atmospheres that acts as shallow donor defects. In contrast to that, the (Cu-Ni)/ZnO film annealed under a nitrogen Fig. 4 The a transmittance, b reflectance, c extinction coefficient, and d refractive index spectra °C under air, vacuum, nitrogen and argon, e The calculated bandgap energies according to Tauc plot and the f schematic diagram of the band structure of (Cu-Ni)/ZnO film which are as-prepared and annealed at 500 °C under air, vacuum, nitrogen and argon atmospheres atmosphere has developed N-related defects bound to the vacant zinc ( N Zn ) that acts as shallow acceptor defects. The H-related defects shifts (Cu-Ni)/ZnO film to be an n-type material, while N-related defects shifts the film to be a p-type material. Consequently, the H-related defects were led to a high degree of crystallinity (66%) when films were annealed under vacuum or argon atmospheres. While the film annealed under nitrogen atmosphere exhibits a lower degree of crystallinity due to N-related impurities. Films were annealed under vacuum, nitrogen, and argon atmospheres show higher electrical conductivity compared to the as-prepared one. The bandgap energies of the (Cu-Ni)/ ZnO film were decreased as a result of annealing regardless the atmosphere where the annealing was performed; however, the bandgap energy showed the minimum value in the film annealed under a nitrogen atmosphere due to the high microstrain due to the N-related defects. In the film annealed under air atmosphere, the induced defects were led to reduce the degree of crystallinity by 35%. Moreover, the average electrical conductivity and bandgap energy are 3.77 μS/cm and 3.41 eV, respectively.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, Fig. 5 a Processes for defect creation, charge transfer, reactions and diffusion of the defects and charges within and between the TCOs film and the applied atmosphere [21]. Defect creation, possible reac-tions and diffusions within and between (Cu − Ni)∕ZnO film during the annealing under atmospheres of b air, c vacuum, d nitrogen, and e argon provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.