Fabrication of Transparent Conductive Zinc Oxide Co-Doped with Fluorine and Zirconium Thin Solid Films by Ultrasonic Chemical Pyrolysis: Effects of Precursor Solution Aging and Substrate Temperature

Highly transparent, conducting zinc oxide [ZnO] thin films co-doped with fluorine and zirconium have been deposited on glass substrates by the ultrasonic chemical spraying technique. The effects of aging of the starting solution and substrate temperature on the structural, morphological, and electrical properties of the ZnO:F:Zr films have been studied. The resistivity of the films decreases with the aging time of the starting solution until the seventeenth day reaching a minimum of about 1.2 × 10−2Ω cm and then increases. Though all the samples are of polycrystalline hexagonal wurtzite type and grow preferentially with (002) plane parallel to the substrate, their morphology depends strongly on the aging time of the reaction solution. The optical transmittance of all the films remained around 80% in the visible spectral range. These highly transparent, low resistive thin films are expected to be highly useful as transparent electrodes in the fabrication of thin film solar cells.


Introduction
The demand of low-cost and high-performance optoelectronic devices leads to the development of more efficient transparent conductive oxide (TCO) thin films.Among the popular TCOs, the last decade has seen the emergence of zinc oxide [ZnO] as one of the most important materials for manufacturing transparent electrodes utilized in the fabrication of amorphous silicon-based solar cells.Apart from their high optical transparency, the ZnO thin films are highly stable in the hydrogen-plasma environment commonly used for the fabrication of silicon-based p-i-n structures [1].
ZnO thin films have been deposited by a wide variety of techniques, like evaporation [2], sputtering [3], chemical vapor deposition [4,5], sol-gel [6,7], and chemical spray [8,9], among others.Among those, the chemical spray technique has been successfully used for the deposition of conductive and transparent ZnO thin films.Keeping in mind the high optical transparency as one of the basic requirements, incorporation of several dopants has been tried to reduce the resistivity of ZnO films.On the other hand, like all the chemical deposition techniques, chemical spray technique involves several parameters or deposition conditions which control the physical properties of the fabricated thin films.However, a complete knowledge of the effect of deposition conditions on the physical characteristics of ZnO thin films is far from being reached.
The spray pyrolysis technique is based on the pyrolytic decomposition of small droplets of a zinc-containing solution onto a heated substrate, under atmospheric conditions [10].The low cost of the equipment, the simple atomization process, and both temperature and solution flow control are the main advantages of this technique.Although it is important to highlight that, a deposition process at ambient atmosphere is a strong limiting factor in some applications requiring a clean atmosphere [11].
For electrode manufacturing, oriented ZnO films doped with adequate impurities are required for their low resistivity in the order of 10 −3 Ω cm.In this respect, stable ZnO thin films of low resistivity [12] could be fabricated utilizing group III elements like aluminium [Al] and gallium [Ga] as dopants.However, frequently an additional postdeposition annealing either in vacuum [13,14] or under molecular hydrogen atmosphere [15] was required to achieve their low resistivity.On the other hand, incorporation of some other elements with higher valencies into the ZnO lattice is viewed as another chance in the quest of obtaining low-resistivity thin films [16,17].As a matter of fact, recently Paul et al. [18] have deposited zirconium-doped ZnO thin films [ZnO:Zr] by the sol-gel technique, finding that the doping concentration affects both their surface texture and resistivity.ZnO films with resistivity as low as 7.2 × 10 −2 Ω cm could be deposited with an optimum zirconium [Zr] concentration of 1.5 at.%.It is worth to mention that to achieve low resistivity those films were heat-treated several times either in air or in vacuum between 50 and 550 ∘ C. The reduction of resistivity of the ZnO:Zr films was considered to be due to the substitution of divalent Zn atoms from the lattice sites of ZnO by tetravalent Zr atoms.However, to the best of our knowledge, there is no report on a detailed study of the effect of Zr on the electrical properties of chemically sprayed ZnO thin films applicable as TCOs.
On the other hand, anionic impurities like fluorine [F] have also been tried to reduce the resistivity of semiconductor oxide thin solid films [19], through the substitution of oxygen atoms of their lattice.It has been observed that the incorporation of F in the oxide matrix depends strongly on the aging time of precursor solution containing F. Low resistive ZnO:F films could be obtained by increasing the aging time of the precursor solution.Taking into account that the incorporation of both anionic and cationic impurities helps to reduce the resistivity of several metal oxide systems [20], we tried to incorporate both F and Zr into the ZnO lattice to obtain its thin films of lower resistivity.
An ultrasonic spray pyrolysis technique was employed to prepare zinc oxide co-doped with fluorine and zirconium (ZnO:F:Zr) thin solid films on glass substrates at different aging times of the precursor solution and at different substrate temperatures to study the effects of these two parameters on their morphological, optical, and electrical properties.

Experimental Details
2.1.Fabrication of ZnO:F:Zr Thin Solid Films.Ultrasonic spray pyrolysis (USP) is a versatile technique capable of producing nanostructured powder and thin film samples.The particle/grain size in these samples can be straightforwardly controlled by varying the concentration of the source solution and the atomization parameters.The deposition system used in this work includes a variable frequency piezoelectric transducer, which was set to 1.2 MHz and operated at 120 W ultrasonic powers.In the ultrasonic spray pyrolysis system, a geyser is formed in the liquid surface by the action of a beam of ultrasonic vibrations which is directed to a liquid-gas interface.This geyser is followed by the generation of a spray formed by microscopic droplets.A carrier gas transports and directs the spray toward the surface of a hot substrate where the film formation is produced.The experimental setup used for the fabrication of ZnO:F:Zr films in this work The thin films were deposited on glass substrates (alkalifree borosilicate) with an area of 2.5 × 2.5 cm 2 , and the substrates have a thickness of 0.5 cm.Prior to film deposition, all the glass substrates were cleaned using the procedure as follows: (i) sonication for five minutes in detergent free of sodium, followed by (ii) sonication for five minutes in trichloroethylene [C 2 HCl 3 ] for degreasing the substrates, followed by (iii) sonication in methyl alcohol [CH 3 OH] and (iiii) sonication in acetone [CH 3 COCH 3 ], and finally (v) the substrates are dried by a jet of pure and dry nitrogen [N 2 ].Then the substrates were placed on a fused tin bath of the deposition system.The substrate temperature (  ) was measured by putting a chromel-alumel thermocouple placed below.The deposition system or the reactor was covered with a stainless steel metal jacket during the pyrolysis process.The substrate temperature (  ) was varied from 450 to 500 ∘ C in 25 ∘ C steps, within an accuracy of ±1 ∘ C. Pure N 2 was used both as the solution carrier and precursor directing gas, maintaining its flow rates at 3.5 and 0.5 L/min, respectively.High resistive films were obtained when the depositions were performed at lower and higher temperatures of the substrate out of this range (450 ∘ C ≥   ≥ 500 ∘ C).The deposition time was controlled in order to get thickness values around 550 nm.
2.2.Characterization of ZnO:F:Zr Thin Solid Films.The crystallinity and microstructure of the deposited films were determined by X-ray diffraction (XRD) technique by recording the XRD spectra of the samples in -2 scan mode in the range of 30 to 80 ∘ , utilizing the Cu K 1 ( = 0.15405 nm) radiation.The thickness and the roughness of the films were obtained using a profilometer on the step formed right after deposition.The morphology of the films was studied by scanning electron microscopy (SEM).Before observations were made, the samples were coated with a very thin metallic deposit in order to improve the resolution of the micrographs, which in turn were directly recorded in a PC system connected to the microscope.Optical transmittance spectra of the films in the 200 to 1000 nm spectral range were obtained in a spectrophotometer, using a cleaned glass substrate as reference.The electrical sheet resistance of all the samples was measured by the conventional four-aligned probe method with the appropriate correction factors; the characterization was carried out in an open-air atmosphere.

Structural Evaluation of the ZnO:F:Zr Thin Solid Films.
Profilometry estimated thickness of the deposited films varied between 530 and 550 nm with an accuracy of 10%.The surface profile or the roughness of the films was measured by profilometry, and the values were 10 nm for one day, 13 nm for nine days, and finally 15 nm for twenty days.On the other hand, X-ray diffraction patterns of the as-grown ZnO:F:Zr thin films deposited using the precursor solutions aged 1 day, 9 days, and 20 days at 500 ∘ C of substrate temperature shown in Figure 2. From the XRD patterns it is clear that all the samples are of polycrystalline nature.The XRD peak positions of all the samples fit well to hexagonal wurtzite phase of ZnO (JCPDS no.36-1451).No diffraction peak related to zirconium or any compound of zirconium or fluorine was detected.All the samples revealed their preferential growth along the [002] direction, that is, (002) planes grown parallel to the substrate surface.The results indicate that in the studied range of 450-500 ∘ C the temperature of the substrate (  ) does not affect significantly the preferential growth of ZnO:F:Zr films on glass substrate.The signal intensities from each of the X-ray diffraction patterns contain information on the preferential or random growth of polycrystalline thin films following the expression [21] where   (hkl) is the corresponding texture coefficient, (hkl) are the X-ray diffraction intensities of the sample, and  is the number of diffraction peaks considered.  (hkl) are the intensities of the reference/standard XRD pattern (JCPDS no.36-1451).Considering the most intense (002) XRD peak, the texture coefficient for all the samples is estimated to be of the order of 0.3 (±10%).The estimated   values lower than 1 indicate the abundance of crystalline grains oriented along [002] direction in all the samples.The crystallite size was estimated using the (002) diffraction peak from the XRD data in accordance with the Debye-Scherer formula [22]: where  is the crystallite size in nanometers,  is the wavelength value of the Cu K 1 line,  is the Bragg diffraction angle, and  is the FWHM of the diffraction peak measured in radians.An average crystallite size value corresponding to films deposited at every substrate temperature with an accuracy of 10% is listed in Table 1.According to these results, the crystallite size of the ZnO:F:Zr thin solid films decreases from 20 to 100 nm as-grown ZnO:F:Zr thin solid films deposited using the precursor solutions aged 1-20 days at 500 ∘ C of substrate temperature.

Morphology Evaluation of the ZnO:F:Zr Thin Solid Films.
The surface morphology of ZnO:F:Zr, thin solid films deposited from starting solutions aged at one, nine, and twenty days, is shown in Figure 3. From the scanning electron microscopy images (SEM) slight changes are observed when the solution ageing time increases from one to twenty days.A porous surface consisting of definite rounded grains, with an average diameter around 120 nm, and separated by voids appears in the samples deposited from a one-day aged solution, as is shown in Figure 3(a).A uniform and more compact surface with uniform grain size (∼90 nm) is observed in the samples deposited from a nine-day aged solution, as can be seen in Figure 3(b).Finally, films deposited from a solution aged for twenty days (Figure 3(c)) with an irregular surface morphology, a higher roughness, and numerous voids, but similar grain size, can be evidenced at naked eye.It is worthy to mention that these characteristics have not been previously reported concerning ultrasonic chemical pyrolysis zirconium and fluorine doping ZnO thin films.

Optical Properties of the ZnO:F:Zr Thin Solid Films.
Figure 4 shows the optical transmission spectra of the ZnO:F:Zr films deposited at 475 ∘ C using the precursor solutions of different aging times.All the films were uniform and transparent to naked eyes.Optical transmittance of all the samples remained high (80-85%) in the visible spectral range, irrespective of their deposition temperature of the age of the used aliquot.Though the optical transparency of the films varied a bit with deposition conditions in the ultraviolet spectral range (which is irrelevant for the fabrication of TOC electrode applications), the position of their band edge did not vary significantly.Considering a direct allowed transition in the samples, the optical band gap energy of the samples estimated using the Swanepoel method [23] was close to 3.30 eV for   = 500 ∘ C and 3.33 eV for   = 475 ∘ C from starting solutions with different aging times.

Electrical
Properties of the ZnO:F:Zr Films.The variation of electrical resistivity of the ZnO:F:Zr thin films with the aging time of precursor solutions is presented in Figure 5 for the samples deposited at different substrate temperatures (  ).As can be seen from the figure, the samples deposited using the precursor solution aged 1 day manifest highest resistivity 2-0.1 Ω cm.On increasing the age of the precursor solution, the resistivity of the deposited films decreases gradually, reaching a minimum value ∼1.2 × 10 −2 Ω cm for the 17-20 days of aliquot age, when they were deposited at   = 500 ∘ C.However, for the lower value of substrate temperature (e.g.,   = 450 ∘ C), the precursor solution had to be aged longer (25-28 days) to obtain the thin films of minimum resistivity (Figure 5).An increase in the resistivity values is observed for the films deposited using longer aging time.The observed variation of film resistivity with the aging time of the precursor solution can be tentatively explained considering the mechanism of incorporation of dopants in them.The Zr 4+ ions from the dissolved zirconium (IV) acetylacetonate can replace Zn 2+ ions from their lattice sites easily (fast   solution aging to obtain the films of minimum resistivity reduces gradually with the increase of substrate temperature.For example, the optimum aging time for the films grown at 450, 475, and 500 ∘ C substrate temperatures was 27, 20, and 13 days, respectively.Higher substrate temperature helps to reduce the Zn-O bond strength facilitating the substitution of O atoms by F. As has been mentioned earlier, a further increase of substrate temperature (  > 500 ∘ C) increases the resistivity of the films, which might be due to the escape of volatile F from the ZnO lattice.
Absence of XRD peaks associated with any other compound rather than ZnO in the diffraction patterns of the samples indicates both dopants incorporated into the ZnO lattice through substitution.It is worth mentioning that the minimum resistivity measured in our ZnO:F:Zr thin films is lower than the reported minimum resistivity of doped or undoped ZnO thin films fabricated by sol-gel process.Moreover, our USP fabricated ZnO:F:Zr films required no postdeposition heat treatment as in the case of several reported works.

Conclusions
In summary, we could fabricate smooth, pinhole-free, considerably low resistive ZnO:F:Zr thin films with high transmittance by ultrasonic spray pyrolysis technique.While the lowest resistivity of our films was ∼1.2 × 10 −2 Ω cm, their optical transparency was as high as 80%.Both these values are adequate for utilizing these thin films as TCO electrodes in solar cell.The resistivity of the films depends strongly on the aging time of precursor solution, as well as the temperature of substrates during their deposition.In the 450-500 ∘ C range of substrate temperature, ZnO:F:Zr thin films of lowest resistivity could be obtained for the aging time of precursor solution as short as 13 days.  = 500 ∘ C is seen to be optimum for obtaining lowest resistive ZnO:F:Zr thin solid films with shortest aging time of precursor solution.A higher substrate temperature inhibits the incorporation of F in ZnO films.The minimum resistivity value of our ZnO:F:Zr thin solid films is lower than the reported minimum resistivity values of ZnO thin films (either doped or undoped) fabricated by sol-gel process.As the films fabricated in this work need no postdeposition thermal treatment, they are expected to be highly attractive for transparent electrodes in solar cell fabrication.

Figure 1 :
Figure 1: Schematic diagram of the experimental setup used for the preparation of ZnO:F:Zr thin films.

Figure 2 :
Figure 2: X-ray diffraction patterns of ZnO:F:Zr thin films deposited at   = 500 ∘ C, utilizing the precursor solutions of (a) 1 day, (b) 9 days, and (c) 20 days of aging time.

Figure 3 :
Figure 3: SEM micrographs of ZnO:F:Zr thin films deposited at   = 500 ∘ C, utilizing the precursor solutions of (a) 1 day, (b) 9 days, and (c) 20 days of aging time.

Figure 4 :
Figure 4: Optical transmittance spectra of ZnO:F:Zr thin films deposited at   = 500 ∘ C utilizing the precursor solution aged one, nine, and twenty days.

Figure 5 :
Figure 5: Resistivity variation of the ZnO:F:Zr thin films as a function of aging time of the precursor solution.

Table 1 :
Crystallite size and thickness for the different substrate temperatures.