Low-temperature growth and physical investigations of undoped and (In, Co) doped ZnO thin films sprayed on PEI flexible substrate

Accepted Manuscript Low-temperature growth and physical investigations of undoped and (In, Co) doped ZnO thin films sprayed on PEI flexible substrate S. Ben Ameur, A. Barhoumi, R. Mimouni, M. Amlouk, H. Guermazi PII: DOI: Reference:

S0749-6036(15)00248-7 http://dx.doi.org/10.1016/j.spmi.2015.04.028 YSPMI 3745

To appear in:

Superlattices and Microstructures

Received Date: Revised Date: Accepted Date:

14 February 2015 16 April 2015 28 April 2015

Please cite this article as: S. Ben Ameur, A. Barhoumi, R. Mimouni, M. Amlouk, H. Guermazi, Low-temperature growth and physical investigations of undoped and (In, Co) doped ZnO thin films sprayed on PEI flexible substrate, Superlattices and Microstructures (2015), doi: http://dx.doi.org/10.1016/j.spmi.2015.04.028

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Low-temperature growth and physical investigations of undoped and (In, Co) doped ZnO thin films sprayed on PEI flexible substrate S. Ben Ameur1, A. Barhoumi1, R. Mimouni2, M. Amlouk2, H. Guermazi1 1. Research Unit: Physics of insulators and semi insulator materials (PISIM), Faculty of Science of Sfax, Road of Soukra Km 3.5, B.P: 1171 3000 Sfax, University of Sfax,Tunisia. 2. Research Unit: Physics of semi-conductor devices, Faculty of Science of Tunis, Tunis El Manar University, 2092 Tunis, Tunisia.

Abstract ZnO thin films were deposited on polymer substrate Polyethyerimide (PEI) at 250°C by spray pyrolysis technique. The effects of different doping elements (Co and In) on physical properties of ZnO thin films were investigated. Thin film characterizations were carried out using X-ray diffraction technique, UV-Vis-NIR spectroscopy, Photoluminescence (PL) spectroscopy and the contact angle measurement method. XRD measurement showed a successful growth of crystalline films on polymer substrate at low temperature by the spray pyrolysis process. XRD patterns revealed that all films consist of single ZnO phase and were well crystallized with preferential orientation towards (101) direction. Doping by cobalt has effective role in the enhancement of the crystalline quality, increases in the band gap according to Burstein Moss effect. Doping with indium leads rather to the decrease of both crystallinity and optical band gap energy value. Photoluminescence of the films showed UV emission (NBE) and visible emission related to defects. The contact angles were measured to study the effect of various doping elements on the hydrophobicity of the film depending on surface roughness. Results showed strong dependence on the doping element. In fact, doping with cobalt element increases the roughness of ZnO films and reinforces the surface from hydrophilic to hydrophobic (θ>90°). Keywords: doped ZnO, PEI, Thin films, Spray, Optical constants, photoluminescence. I. Introduction Transparent conductive oxides (TCO) are wide band gap semiconductors combining high optical transmittance and low electrical resistivity. ZnO constitutes one of the most important classes of these materials due to their opto-electrical properties, high electrochemical stability, abundance in nature and the non toxicity [1]. ZnO exhibits native n-type conductivity with a large band gap (>3 eV) like other metallic oxide such as SnO 2 [2], and which was influenced by the experimental conditions [3, 4], the method of deposition [5-7] 1

and the dopant element [8-10]. It has high exciton binding energy of the order of 60 meV [11]. Because of these properties, ZnO is very suitable for many applications such as gas sensors [12], photocatalysts [13], solar cells [14], and light emitting diodes [15]. ZnO thin films can be produced by several techniques such dip-coating [5], sputtering [6], spray pyrolysis [7]. In this work, ZnO thin films have been deposited by the spray pyrolysis. In fact, this technique has been widely used because it is a cost effective method, more reproducible, without toxicity and leads to large area thin film surface. The doping of semiconductors with appropriate metals is generally one of the most effective ways in research for developing sensitivity applications. Indeed, dopants such as Al, B, Ni, Mo, and Ga have been suitably added to increase the optoelectronic performance of ZnO films [8-10]. Cobalt doped ZnO (CZO) and Indium doped ZnO (IZO) thin films have been extensively studied because they exhibit a good optical transparency and a good electrical conductivity. Glass substrate is currently widely used for display applications, but it is too heavy and brittle to be easily deformed. To overcome these problems, many researchers have grown ZnO on flexible substrates such as polypropylene adipate (PPA), polyethylene terphthalate (PET) [10], TPT [16], polyethylene naphthalate PEN [17] and polyethersulphone (PES) [15]. Polyethyerimide PEI can be used as a substrate because it has a high glass transition temperature (T=220°C) excellent optical transparency and mechanical properties [18]. These outstanding advantages make Polyethyerimide (PEI) a good candidate as a substrate for growth ZnO thin films. In this paper, undoped and (Co, In) doped ZnO thin films were deposited by the spray pyrolysis on PEI substrate at 250°C. Structural, morphological and optical properties of thin films were analyzed in order to optimize the effect of doping element. This may be of interest in many sensitive applications since a simple method has been used to deposit doped ZnO thin films on PEI flexible substrates. II- Experimental details II-1- films preparation Undoped ZnO thin films have been prepared at 250°C on Polyethyerimide (PEI) substrates by spray pyrolysis method [7]. These films must be sprayed onto PEI substrate at low temperature because most flexible substrate is easily degraded by high substrate temperature. The aqueous solution of zinc acetate dehydrates (Zn (CH 3COO) H2.2H2O) 10-2 M contained a mixture of distilled water and propanol with fraction of ¼ and ¾ respectively [8]. Consecutively, under similar experimental conditions, doped ZnO thin films have been elaborated by adding CoCl2 and InCl3 as sources of Co and In respectively .The molar ratios 2

(Co/Zn) and (In/Zn) were 0% and 1% wt. Nitrogen was used as the gas carrier (pressure at 0.35 bar) through a 0.5 mm-diameter nozzle. As reported previously, the nozzle-to-substrate plane distance was fixed at the optimal value of 27 cm [19]. During the deposition process, the precursor mixture flow rate was taken constant at 4ml/min throughout the thin films deposition. Film thicknesses were found to be approximately to 200 nm. II-2- Characterization techniques The crystal structure and orientation of the elaborated ZnO, IZO (In (1%) doped ZnO) and CZO (Co (1%) doped ZnO) thin films were investigated by x-ray diffractometer (Analytical X Pert PROMPD) with Cukα radiation (λ=1.5406Å). The optical properties of these films were measured by UV-Vis-NIR spectrophotometer (Shimadzu UV 3100) in the wavelength range from 300 to 1800 nm. PL measurements were performed at room temperature using Perkin Elmer LS-55 Luminescence/Fluorescence spectrophotometer with 325 nm excitations. Finally, the wattability of thin films was examined by means of the water Contact Angle using a contact angle meter (Micro-Drop analysis DSA 100M) at ambient temperature. III- Results and discussion III-1- Structural properties The crystallinity of undoped ZnO, IZO and CZO thin films was analyzed by XRD method. Figure 1 shows the XRD patterns of undoped and (Co, In) doped ZnO thin films. The observed peaks matched well with the hexagonal wurtzite ZnO structure. These films were polycrystalline with (101) preferential orientation. This orientation was obtained for ZnO films grown at low temperature by spray pyrolysis. The absence of (002) orientation usually obtained at higher temperature using this technique , can be explained by the fact that there is no enough energy at low temperatures for atoms to move to low-energy sites which induces strain in the films [20]. XRD spectra revealed that no Co xOy peak neither InxOy or impurity peak were observed. This is due to the low percentage of doping element and to the successfully incorporation of the dopants in the lattice [21]. Also, we can note sharpness of ZnO peaks intensities with doping by Cobalt, which means an improvement in the crystalline quality of films. In addition, this indicates that the Co 2+ ions systematically substituted the Zn2+ ions in the lattice [22-26]. Because the Co is abundant electron states, the ionic radius of Co2+ (0.072 nm) is close to that of Zn2+ (0.074 nm), and the large solubility in the ZnO matrix [27].

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For IZO thin films we remark the decrease of the intensity of (101) peak. This indicates the degradation of the crystalline quality of IZO film [28, 29] which can be explained by the difference in size and valence between In3+ and Zn2+. In fact, the indium is trivalent while Zn is bivalent element and the ion radius of In3+ (0.8Å) is larger than the Zn2+ (0.74Å). Thus, Indium incorporation in ZnO leads to a distortion of the lattice which results in a strained structure. The microstrain (ζ) parameter is indeed calculated using the following relation [30]:

x=

b 4 tan (q )

(1)

Where β is the full width at half maximum of the peak (FWHM) and θ is the Bragg’s diffraction angle of the peak. The calculated values were gathered in Table I for all films. Results showed that IZO thin film is highly strained compared to ZnO and CZO films. In the same line, from XRD patterns, the crystallite size of thin films was calculated by using the well-known Debye –Scherrer’s formula [31]: D=

0,9l b cosq

(2)

Where λ is the wavelength (1.54Å) .The values of D are gathered in Table I. The crystallite size was not affected by doping with Co, which confirms that the Co 2+ ions were incorporated in substitution to the Zn2+ ions in the ZnO lattice. Moreover, for In-doped ZnO films the crystallite size decreases which leads to the increase of the grain boundaries in films, then the increase of strains. This confirmed the degradation of the crystalline quality of films. This can also prove that In3+ ions was incorporated either in the interstitial sites of ZnO lattice or in the substitution to Zn2+. Finally, to reach more information on the crystal quality of films, the dislocation density (δ) is calculated from the grain size by the formula [32]:

d=

1 D2

(3)

The dislocation density is defined as the length of dislocation lines per unit volume of the crystal. Larger D and smaller δ values proves a better crystallinity of CZO films, Table 1. III.2. Optical study III.2.1.1. Transmittance spectra

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The transmittance spectra of ZnO, IZO and CZO thin films are shown in figure 2. All films exhibit a high transmission ≥ 85% in the 400-1800 nm wavelength region. Doping with Co leads to a slight variation in the optical transmittance of ZnO thin film. This suggests that those films showed a good optical quality due to low scattering or absorption losses which confirmed that films grown on PEI substrate are very suitable for solar cell applications. It can be seen that the interference fringe patterns are absent in all transmittance spectra due to weak multiple reflections at the interface. All films exhibit a sharp absorption edges in the ultraviolet region (360-400) nm. As shown in figure 2, we can note that CZO is more UV blocker than ZnO and IZO. III. 2. 1.2. Optical band gap energy and Urbach energy From the transmittance spectra, the absorption coefficient α of ZnO films with various doping was calculated using the following formula [33]:

a=

1 æ1ö Lnç ÷ ……………….. (4) d èT ø

Where T is the optical transmittance and d is the thickness of films (d=200 nm). The optical band gap energy Eg can be calculated using the Tauc model related to allowed direct transitions [34]:

(ahn )2 = A(hn - E g )

(5)

With hυ is the photon energy Eg is the optical band gap energy and A is a constant characteristic of the material. The optical band gap energy was obtained by the extrapolating of the linear portion of the curve (ahn ) 2 versus (hυ) presented in figure 3. Two linear regions were detected. The lower energy value (around 3.07 eV) matched well with the gap energy of PEI substrate as seen in the related curve (figure 3). The second was attributed to the ZnO films. The obtained gap energy values were 3.220, 3.335 and 3.185 eV for undoped ZnO, CZO and IZO, respectively. It can be seen that the optical band gap energy increases with Co doping. The bleu shift of Eg was attributed to the Burstein Moss effect [5, 21, 23, 35] which was caused by the increase in the carrier concentration. Brustein Moss theory points out that the lifting of Fermi level into the conduction band leads to the energy breading (bleu shift) effect. This lifting of the Fermi level is due to the increasing of the carrier concentration. Moreover, doping with In leads rather to the decrease of the optical band gap Eg in agreement with literature [36]. The evolution of the band gap energy is consistent with the crystalline

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quality of ZnO thin films. In fact, IZO showed the lower crystallite size and the higher lattice defects which leads to the decrease of Eg. Disorder in the elaborated films can be investigated by Urbach model in which the Urbach energy Eu has been interpreted as the width of the localized states in the band gap. It has been determined from the following equation [37]: Ln(a ) = Ln(a 0 ) +

hn Eu

(6)

Where a 0 is a constant. The width of the located states (band tail) energy or Urbach energy Eu was estimated from the slopes of Ln (α) versus (hυ) plots of the films (figure 4). The values of Eu were found to be 230, 235 and 122 meV for ZnO, IZO and CZO, respectively. Doping ZnO with Co leads indeed to a decrease in the disorder in ZnO network. This is consistent with XRD results described above. The effect of impurity or disorder and any other effects in semi-conductors lead generally to local electric field that affect the band tails near the band edge are well known. Figure 5 shows the variations of the optical gap energy together with the Urbach tail energy as a function of doping element. As shown in figure 5, the low Eg value corresponds to the high Eu value which corresponds to IZO films. The band gap narrowing is due to the increase in the band tail width. The In introduction into the film was then followed by the lattice distortion and consequently disorder creation which caused the optical gap reduction as reported elsewhere [29, 36].Moreover the optical band gap energy increased with doping by Cobalt which can be related to the decrease of the structural defects and band tail width. III. 2.1. 3. Refractive index and extinction coefficient The transmittance and reflectance data can be analyzed to determine optical constants such as refractive index n and extinction coefficient k, in the spectral domain varying from 400 to 1800 nm .In this case, we have used an approach detailed previously [38]: k=

al 4p

æ1+ R ö n=ç ÷+ è1- R ø

(7)

4R - K 2 ……………………… 2 (1 - R )

6

(8)

Figure 6 shows that the refractive index n of undoped and doped ZnO films decreases with the wavelength showing the same trend. The n values range from 2 to 2.6 for thin films. As shown, CZO thin films have the greater refractive index in the IR region. The variation of refractive index with the nature of the doping element suggests that the optical properties can be controlled by changing the doping element which is important for the optoelectronic applications. The extinction coefficient decreases with the wavelength (figure 7). It varies from 0.01 to 0.05. Figure 7 shows that the extinction coefficient of CZO exhibits a significant increase in the IR region, whereas doping with In varied slightly the extinction coefficient in the IR region. The extinction coefficient k represents the losses energy and was related to the crystalline quality and surface morphology of the sample. In fact, the surface of CZO is rougher than that of both ZnO and IZO (as it will be proved by wettability characterization in section III.3) which leads to the increase of the extinction coefficient and a decrease of transmittance of CZO in IR region compared to other oxides. The light scattering is indeed due to surface roughness. Moreover, CZO film has the little extinction coefficient leading to a little optical loss [5] in the visible range (400-600 nm). III. 2.1. 3. Dielectric constants Using the obtained values of the refractive index n and the extinction coefficient k, the real and imaginary parts of the dielectric constant are calculated from the following expressions [39]:

e1 = n 2 (l ) - k 2 (l )

(9)

e 2 = 2n(l )k (l )

(10)

For all films, it is found that in infrared range the dispersion of ε1 is a linear function of the square of the wavelength λ2 (figure 8) while the absorption ε2 is linear with λ3 (figure 9). This behavior is in good agreement with the classical theory of dielectric constant which can be expressed by the following system in the near infrared region (wτ>>1) [40, 41]:

e1 » e ¥ -

e ¥w p2 2 l 4p 2c 2

(11)

7

e ¥w p2 3 e 2 = 2nk » 3 3 l 8p c t

(12)

Where e¥ is the dielectric constant at high frequencies, ωp the plasma frequency and τ is the relaxation time. The exploration of these curves ε1 (λ2) and ε2 (λ3) led to determine the optical constants such as e¥ , ωp and τ. The obtained values were gathered in table II. We have also calculated the values of the carriers concentration to effective mass ratio N

m*

from the

following well-known equation:

w p2 =

4pNe 2 e ¥ me*

(13)

Doping with Co and In increased the dielectric constant at high frequencies e¥ , plasma frequency, relaxation time and the free carrier’s concentration to effective mass ratio. It is found that CZO thin film exhibits the higher e¥ , ωp, τ and N

m*

constants values.

III.2.2.Photoluminescence spectroscopy: PL measurements were largely used to study the intrinsic and extrinsic defects in semiconductor materials. It gives abundant information on the energy states of impurities and defects, even at very low densities, which is helpful for understanding structural defects in semiconductors. The PL spectra of ZnO, IZO and CZO thin films, measured at room temperature with an excitation wavelength of 325 nm, were shown in figure 10. As can be seen in figure 10 (a), all films showed a broad overlapped luminescence peaks in the UV-vis range. It is easily noticeable that CZO is the least luminescent film while IZO is the most luminescent one. This is related to structural disorder and the presence of defects, already highlighted by XRD investigations. A Gaussian fit was used to detect the origin of particular emissions. Figures 10 (b-d) give the best fit to the measured PL peaks in ZnO, IZO and CZO films respectively. Hence, the undoped ZnO thin films exhibit a peak in the UV region at 383 nm and four weak peaks in the visible rang corresponding to blue emission at 412, 432 and 460 nm and a bleu green emission at 495 nm (figure 10(b)). The UV emission of IZO was detected at 385 nm with additional bleu emissions at 408,430, 462 nm, a weak bleu green emission at 504 nm and a weak green emission at 564 nm (figure 10(c)). After Cobalt doping, two UV emissions were detected at 366 and 390 nm (figure 10(d)). Furthermore, three weak blue emissions were identified at 411, 430 and 463 nm and a weak green emission 8

was detected at 526 nm. Here UV emission energy is slightly less or more than the optical band gap of all ZnO films and hence it corresponds to the near band edge (NBE) transition. The bleu shift of UV emission on CZO films indicates that the Co 2+ ions systematically substituted the Zn2+ ions. It is reported that the luminescence of ZnO films exhibits one emission peak in the UV region due to a recombination of free exciton (NBE) and one or more emission peaks in the visible spectral range, which are attributed to the defect emissions [42]. Liang et al [43] suggested that the origin of the bleu violet emission at λ = 410 nm (3.02 eV) could be due to the exciton recombination between the electrons in the conduction band and the holes localized at defect level associated with zinc vacancy (V Zn), and the origin of blue emission at 440 nm (2.8 eV) could come from the contribution of the exciton recombination between the electron localized at the interstitial zinc (Z ni) and the holes in the valence band. Also, Zeng et al. [44] reported that the blue emissions in the range of 455–488 nm could be due to the electron transition from extended Z ni states to the valance band, when Eg < Eex. The visible green emission peaks located for λ > 520 nm, observed in the CZO and IZO films, have been widely reported to be related to the oxygen vacancies. The decrease of visible emission intensity with Co doping confirmed the results of structural and UV spectroscopy. III. 3. Hydrophobicity: surface wettability In recent years, control over surface hydrophobicity is highly desirable due to the wide range of possible application of those surfaces such as lenses transparent window glass and self cleaning materials [45, 46]. Controlling the surface wettability is important which involves the interaction between a liquid and solid in contact. Measurement of surface water contact angle θ (figure 11) is inversely proportional to the wattability and can be determined by Young’s relation [47]: cos q =

Where g SV , g SL and

(g SV -g SL )

(14)

g LV

g LV refer to the interfacial tensions with solid-vapor, solid-liquid and

liquid-vapor, respectively. Young’s angle θ is a result of the thermodynamic equilibrium of the free energy at the solid-liquid-vapor interphase. In general, surface roughness is assumed to be among principal factors affecting surface wettability [48, 49]. Therefore, water contact angle measurement can allow us to study the surface morphology. The water contact angle measurement was done onto deposited thin films as shown in figure 12. The static contact angle for the PEI substrate was found to be 76.05°. The contact angle decreases with doping 9

by Indium, so the surface roughness of the ZnO film decreases with In content. This confirms that low concentration of In doping favors smoothing the films surface. IZO films exhibit a hydrophilic surface. Moreover, CZO thin film has a hydrophobic surface (θ > 90°), thus, it can used on self cleaning application [47] due to its relatively high static contact angle. It’s known that the wattability of the surface can be enhanced by increasing the surface roughness, within a special size range the air trapped between the solid surface and the water droplet can minimize the contact [49]. So, doping with Co increases the surface roughness of films. Moreover, In-doped ZnO has a smoother surface than that for undoped ZnO, as it can be expected from XRD data by the decrease of crystallite’s size of IZO films. Conclusion Undoped ZnO and (Co, In) doped ZnO thin films were prepared by spray pyrolysis on PEI substrate at 250 °C. A successful deposition of ZnO thin films at low-temperature on PEI substrates was obtained by using the spray pyrolysis. The effect of Co and In doping in physical properties of ZnO thin films was investigated in this work. Strong dependence of the microstructure, crystal quality, optical and morphological properties on the nature of the doping element was found. XRD results showed that ZnO thin films deposited on PEI substrates have a hexagonal structure. The crystallite size was found to be around 83 nm for undoped and Co-doped ZnO and 41 nm for In-doped ZnO. This shows that CZO have a high crystalline quality than IZO films. All films with thicknesses of 200 nm were found to be highly transparent in the Vis-NIR region with an average transmittance over 85%. Calculated values of the band gap energy were around 3.22, 3.33 and 3.18eV for ZnO, CZO and IZO respectively. This evolution of Eg was attributed to the Burstein-Moss effect for CZO and to the lattice distortion and structural defects due to the incorporation of In into ZnO. Room temperature photoluminescence reveals that the UV peak position for IZO samples slightly red shift in comparison with the pure ZnO which can be attributed to the change in the acceptor level induced by the incorporation of In3+, and the band-gap narrowing of ZnO with the In doping. The bleu shift of UV emission on CZO films indicates that the Co 2+ ions systematically substituted the Zn2+ ions. Wattability properties demonstrated that doping by cobalt enhanced the surface roughness and change the surface nature from hydrophilic to hydrophobic. Moreover, for In-doped ZnO thin films, we showed the same surface nature than the undoped ZnO. Good agreement was found between structural, morphological and optical investigation results.

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Figure

Figure 1: XRD patterns of ZnO, IZO, CZO thin films and PEI substrate

Figure

Figure 2: Transmittance spectra of ZnO, CZO IZO thin films and PEI substrate

Figure

Figure 3: Optical band gap energy estimation of ZnO, IZO and CZO thin films and PEI substrate

Figure

17

ZnO pur ZnO:Co ZnO:In

Ln()

16

15

14

2,8

2,9

3,0

3,1

3,2

3,3

3,4

3,5

3,6

h (eV)

Figure 4: Ln (α) versus energy hυ plots of the films

Figure

Figure 5: The evolution of Eg and Eu of ZnO thin films with doping element

Figure

Figure 6: Refraction index n(λ) of undoped and (Co, In)-doped ZnO thin films deposited on PEI substrate at 250°C

Figure

Figure.7: Variation of the extinction coefficient k (λ) for all films

Figure

Figure.8: ε1 versus λ2

Figure

0.30

ZnO pur ZnO:In 1% ZnO:Co 1%

0.25

2

0.20

0.15

0.10

0.05

0.00 0

9

1x10

9

9

2x10

3x10 3

3

 (nm )

Figure.9: ε2 versus λ3

9

4x10

9

5x10

Figure

(a)

Figure 10: (a) experimental PL spectra of ZnO, IZO and CZO thin films, and the best Gaussian fit to the measured PL peaks in (b) ZnO, (c) IZO and (d) CZO films

Fig.11 Schematic diagram of the contact angle and interfacial tensions of the three surfaces at the three-phase

15

Figure

Figure 12.contact angle measurement of PEI substrate, ZnO, I ZO and CZO thin films

Table I: XRD parameters of undoped and doped ZnO thin films deposited on PEI substrate

ZnO CZO IZO

2 θ (°) (101)

FWHM (°)

36.503

0.100

36.509

ζ (10-4)

D (nm)

13

83.285

13

83.287

27

41.66

0.100

36.503

0.200

δ (nm-2 10-4) 1.441 1.441 5.761

Table II: Values of e¥ , ωp, τ and N/m*

ZnO IZO CZO

e¥

w p (1014 ) rad s-1

t (10 -15 ) s

4.45 4.86 5.59

3.5 3.7 3.8

5 8 7

16

N

m*

(1046)g-1 cm-3 1.5 1.9 2.2

Highlights -

A successful deposition of ZnO thin films at low-temperature on PEI substrates was obtained by using the spray pyrolysis.

-

The crystallite size was found to be around 83 nm for ZnO and CZO and 41 nm for IZO.

-

Room temperature photoluminescence reveals that all ZnO films have UV emission and visible defects.

-

Wattability properties show that doping by cobalt enhanced the surface roughness and change the surface nature from hydrophilic to hydrophobic.

17