Investigation of Structural and Optical Properties of ZnO Thin Films Grown onDifferent Substrates byMist-CVDEnhanced with Ozone Gas Produced by Corona Discharge Plasma

+is study focuses on the growth and physical properties of ZnO thin films on different substrates grown by mist-CVD enhanced with ozone (O3) gas produced by corona discharge plasma using O2. Here, O3 is used to eliminate the defects related to oxygen in ZnO thin films. ZnO thin films are grown on amorphous soda-lime glass (SLG) and single crystals SiO2/Si (100) and c-plane Al2O3 substrates at 350°C of low growth temperature. All ZnO thin films show dominant (0002) diffraction peaks from X-ray diffraction (XRD). As expected, full width at half maximum (FWHM) of (0002) is decreasing in ZnO thin films on single-crystal substrates, especially c-Al2O3 due to similar crystal structure. It is found that the strain in the films is lowest in ZnO/c-Al2O3. +e surface morphologies of the thin films are studied with atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. Grown ZnO films have a hexagonal and triangular nanostructure with different nanostructure sizes depending on the used substrate types.+e calculated surface roughness is dramatically decreased in ZnO/c-Al2O3 compared to the other grown structures. +e confocal Raman measurements show the E2(H) peak of ZnO thin films at 437 cm. It is suggested that O3 gas produced by corona discharge plasma using O2 can be useful to obtain better crystal quality and physical properties in ZnO thin films.


Introduction
Zinc oxide (ZnO), which is one of the most studied transparent conductive oxide (TCO), is a wide bandgap semiconductor material with a 3.37 eV bandgap value and a large exciton binding energy of 60 meV [1,2]. ZnO material has a huge potential for both electronic and optoelectronic applications such as light-emitting diodes (LED), solar cells, transistors, gas sensing, and UV-photodetectors (PDs) [3][4][5][6][7][8][9][10][11]. In the last decade, bulk ZnO and its related heterostructures and many probable nanostructures have been investigated by different experimental techniques because of the possibility that a wide range of applications can be done with them [12][13][14].
e ZnO thin films can be prepared by various techniques such as the sol-gel method, spray pyrolysis, molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD) growth method [15][16][17][18]. Concerning many different types of chemical vapor deposition, ultrasonic spray or mist chemical vapor deposition (USCVD or mist-CVD) can be accepted as a new technology for growing oxide semiconductors [19][20][21]. Several studies used mist-CVD as a growth method and have successfully shown that the procedure allows low-cost, easy maintenance, simple system configuration, and highquality semiconductor oxide films [22,23]. Because the types of mist-CVD growth systems and used parameters are still a subject of research, there are many important questions about the growth mechanism and the related chemical processes that need to be answered.
In general, the mist-CVD growth method can be controlled by reactor temperature, frequency, and voltage of the ultrasonic transducer, growth time, precursor molarity, carrier gas flow rate, and carrier gas type. Previously, the usage of oxygen (O 2 ), argon (Ar), and nitrogen (N 2 ) as a carrier gas was reported in some studies [24][25][26]. In addition to these carrier gases, hydrogen peroxide (H 2 O 2 ) can be used as an oxidant in ZnO growths [27]. Also, the usage of ozone gas as an oxidant in MBE, ALD, and MOCVDgrown ZnO crystals is a known process [27,28]. In ZnO thin film growths, some known structural defects or disorders such as Zn vacancy (V Zn ), O vacancy (V 0 ), or interstitial of O and Zn atoms can occur. ese defects can give rise to the poor physical properties at ZnO, especially in both electronic and optical properties. Among these defects, the V o defect is unintentionally formed in the crystal without doping and has a luminescence of about 500 nm that is called green luminescence. To minimize these defects in the lattice system, especially V o , O 3 is used as an oxygen source in the MBE system, and more oriented ZnO films can be obtained [29]. O 3 -rich growth could be better than conventional using O 2 to oxidize the Zn metal in the precursor. Also, in this study, ZnO thin films are on different substrates SLG, SiO 2 /p-Si(100), and c-Al 2 O 3 by mist-CVD with O 3 -rich O 2 gas where O 3 gas is generated by a corona discharge generator. e structural and optical properties of the grown thin films are studied by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM)\, and confocal Raman spectroscopy measurements.

Experimental Details
ZnO thin films were grown by the mist-CVD method on soda-lime glass (SLG), SiO 2 /p-Si, and c-Al 2 O 3 substrates. As a source material of ZnO, zinc acetate dihydrate salt (ZnAc), which has the chemical formula Zn(CH 3 COO) 2 .2H 2 O, was used. e deposition parameters of ZnO thin films are given in Table 1. e substrates were first cleaned with acetone, ethanol, and deionized water (DIW), respectively. All substrates were dried in the N 2 atmosphere. Samples were named A, B, and C according to ZnO grown on SLG, SiO 2 /p-Si (100), and c-Al 2 O 3 substrates, respectively. e 90 nm thick SiO 2 is formed by thermal oxidation on the p-Si substrate. XRD measurements were done using a Rigaku SmartLab with CuKα 1.54Å of the X-ray wavelength. To determine the morphological structure and growth mode, AFM (AFM workshop TT-2) and SEM (Hitachi SU 5000) measurements were taken. Confocal Raman measurement was carried out using the Jasco NRS-4500 system with a 532 nm green laser. Figure 1 shows the mist-CVD system used in ZnO thin film growth. e mist-CVD system was enhanced with a corona discharge plasma generator. e O 2 came from the gas flowmeter going into the plasma generator and O 3riched gas from the generator goes into the reactor as an oxidation source in this system. O 3 -riched gas is generally formed according to the following equations.
(1) e liquid precursor using an ultrasonic transducer has become a cold mist vapor, and the formed mist vapor with O 3 -riched gas goes to a hot-wall CVD reactor. A polypropylene (PP) film was used between the ultrasonic transducer and liquid precursor to avoid damaging the transducer from acetic acid. After the decomposition process of mist particles, ZnO nucleation layers are formed and then turned into thin films on the substrate surface.

Results and Discussion
XRD measurements of investigated structures have been carried out as shown in Figure 2. According to XRD results, ZnO thin films are preferentially grown in a wurtzite hexagonal crystal system without any other crystal phases. ree well-defined ZnO peaks have been seen, displayed as the (10-10), (0002), and (10-11) peaks showing the ZnO polycrystalline wurtzite structure. As can be seen, all the samples are oriented in the dominant (0002) direction. e (0002) peak position of ZnO thin films has been gradually shifted to the higher diffraction angles when the used substrate can become different from the crystal system of ZnO and thus lead to strained ZnO structures.
All investigated ZnO thin films show the compressive strain. e strain calculation was carried out via ε zz � (c 0 -c)/ c 0 , where c 0 is the lattice parameter of ZnO without strain [30]. From the relevant XRD peak results, to observe the crystallite size of the samples, Scherrer's formula has been used as given in equation (2).
where D is the crystallite size, β is the full width at half maximum (FWHM), λ is the wavelength of X-ray equal to 0.154 nm, and θ is the peak location of (0002) diffraction. e AFM images of the investigated ZnO thin films are shown in Figure 3. e surface morphology of ZnO thin films is different from each other due to using different types of substrates. As can be seen, the lower surface roughness  [31], and an RMS value for 98 nm thick ZnO thin films grown by the sol-gel process is found to be 5.8 nm [32]. Our results are found to agree with these literature values. Figure 4 shows the top-view SEM images of ZnO thin films with x20k and x90k magnification. Very homogeneous surface structures are seen in SEM images for each ZnO film. ZnO thin films have preferred to grow in triangular and hexagonal formed nanopyramids as can be seen from SEM images.
ese hexagonal and triangular pyramids are randomly grown on the substrate surface especially for sample A, and this results in polycrystalline hexagonal structure as shown in XRD results. ese pyramid forms of ZnO can be found in the literature [33][34][35]. e thicknesses of ZnO thin films are determined with cross-sectional SEM images as shown in Figure 5. e thickness of films ranges from 90 nm to 180 nm according to cross-sectional SEM images. ZnO films grown on single crystals have shown more uniform growing behaviors rather than ZnO grown on SLG. In our previous ZnO growths, we have a problem related to island-type growth like noncontinued film; however, after using O 3 -riched gas as the carrier gas, it is shown that these growths begin to grow the continued thin film also as shown in Figure 5 [36,37]. Moreover, ZnO/c-Al 2 O 3 films are highly oriented with columnar-like dense growth. Such columnar ZnO growths are known to have light-trapping properties in solar cells applications [38]. e vibration properties of ZnO thin films are investigated with confocal Raman spectroscopy measurement. e     Raman peaks, which are related to Zn vibration, are presented at 97 and 99 cm −1 for ZnO/SiO 2 /Si and ZnO/c-Al 2 O 3 , respectively. Also, for the grown ZnO thin films, the substrate-related Raman peaks are shown for SiO 2 /Si and c-Al 2 O 3 substrates as shown in Figure 6. In some ZnO thin films, multiple-phonon vibration modes are also observed. For sample A, the second-order peak of the longitudinal acoustic (LA) mode, which is related to LA overtones in the M-K zone, is observed at 483 cm −1 [40]. Some structural and vibrational parameters of the investigated ZnO thin films from AFM, XRD, and Raman measurements are given in Table 2. e peak shifts of the E 2 (H) Raman and (0002) direction of the XRD spectra are shown in Figure 7. Δω and Δθ are the peak shift values in the Raman and XRD spectra, respectively. Δω values in the E 2 (H) Raman spectrum are found to   it.

Advances in Condensed Matter Physics
Exp.    be 9.8, 2.8, and 0.8 cm −1 for samples A, B, and C, respectively, and also, Δθ values are determined as 0.22, 0.12, and 0.08 for samples A, B, and C, respectively, as given in Table 3.
One of the causes of the strain is the lattice-mismatch between ZnO and substrates. With a decreasing lattice mismatch, the redshift of E 2 (H) is gradually decreased with a reduction of local strain in the ZnO lattice which shows a decrease in Δθ values.

Conclusion
In the present study, ZnO thin films have been grown via mist-CVD enhanced with O 3 gas produced by corona discharge plasma using O 2 . e continuous ZnO films are obtained on the substrate surfaces. Grown ZnO films are mostly preferred in (0002) direction and have a polycrystalline pyramid-like structure. e island-like growing behavior in mist-CVD grown polycrystals has changed to smooth columnar-type growth after using O 3 -riched gas. e surface roughness of ZnO thin films is gradually decreased from using amorphous to hexagonal substrates. e wurtzite-type ZnO thin film formation is also supported with Raman spectroscopy. e characteristic ZnO vibration modes and the peak shifts of these modes due to usage of different substrates are shown. e redshifts of the E 2 (H) mode in the Raman spectrum are observed to decrease as the strain declines in the ZnO thin film.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.