Investigation on the Mist Intensity to Deposit Gallium Oxide Thin Films by Mist Chemical Vapor Deposition

In this study, a novel, simple, and robust mist chemical vapor deposition is demonstrated, compatible with existing industrial practices to deposit gallium oxide thin films and influence of mist intensity on the properties of gallium oxide. The intensity of the mist generation is optimized to obtain smooth and uniform thin films. The thin film deposited in this work is mixed phase polycrystalline gallium oxide. Ultraviolet–visible–near‐infrared spectroscopy and photo response of thin film unveil that gallium oxide thin film is responsive to ultraviolet wavelengths including deep ultraviolet‐C and ultraviolet‐B bands and the mist‐generation intensity has negligible influence on the bandgap of the thin film. Thickness of thin film can be altered by varying the mist intensity. It is observed that there is no appreciable impact on refractive index of varying mist intensity. Morphological studies prove the formation of ultrasmooth thin film with root mean square value of 0.628 nm; which is closer and/or better than conventional semiconductor thin‐film deposition processes used for depositing Ga2O3.


Introduction
The environmental concerns related to the electronics today, both in terms of the materials utilized and synthesis methods, are very worrisome.Most of the current methods of synthesis of functional materials and manufacture of electronic devices require a high temperature that results in an increased carbon footprint.Therefore, it is utmost important to develop new functional materials and manufacturing methods to meet the demand for futuristic technologies and eco-friendly material manufacturing methods.Existing materials such as silicon has been widely used in most walks of life and has been pushed to theoretical limits of its underlying material properties.Wide bandgap materials semiconductor enables to achieve smaller device size, lesser weight, and higher efficient usage of energy as compared to silicon-based devices and reduces the lifecycle cost of device.
In comparison with Si, wide bandgap semiconductors allow fabrication of devices that can demonstrate operation at high temperatures, high breakdown voltage, increased switching frequency, and enhanced heat resistance ability. [1]Several wide bandgap semiconductors are being explored, for example, gallium nitride (GaN), silicon carbide (SiC), aluminum nitride (AlN), and gallium oxide (Ga 2 O 3 ).Metal-oxide-based semiconductor materials are arising as predominant material for next-generation electronic and optoelectronic applications.
Gallium oxide (Ga 2 O 3 ) is an ultrawide bandgap semiconductor reported in the range from 4.9 to 5.6 eV at room temperature. [2]It exhibits higher bandgap than the commonly used wideband semiconductor such as SiC and GaN, high dielectric constant, high breakdown field, and high Baliga's figure of merit.Thus, it has attracted interest in power electronics device fabrication (high power field effect transistors, Schottky junction diodes) as it could lead to high operating voltage, low on-resistance, in optoelectronics applications (deep UV photodetectors, transparent electrodes, novel alternative of phosphor used in vertical light emitting diode (LED) fabrication, and UV-LEDs) without any bandgap engineering, and in electronic memory applications (resistive random access memory and spintronic devices).Single crystals of gallium sesquioxide exhibit several distinct polymorphs.[7] These polymorphs belong to different space group, demonstrate variance in band alignment, [8] and contain differing Ga ion coordination number.Among these polymorphs of Ga 2 O 3 , monoclinic β-Ga 2 O 3 is chemically and thermodynamically most stable variant up to its melting point (1780 °C [9] ) and exhibits bandgap of 4.6-4.9eV.Rhombohedral α-Ga 2 O 3 is the metastable with the bandgap of this polymorph is 5.3-5.6 eV.
Scientific research exhibited that Ga 2 O 3 thin films could be epitaxially grown using various deposition techniques such as molecular beam epitaxy, [10] chemical vapor deposition (CVD), [11] halide vapor phase epitaxy, [12] metal-organic CVD (MOCVD) or metal-organic vapor phase epitaxy (MOVPE), [13] and atomic layer deposition (ALD). [14]Although it is possible to attain uniform, pure, and stoichiometric thin film of Ga 2 O 3 using these techniques, it is important to consider valid concerns regarding complexity, footprint on environment, and cost associated with aforementioned deposition methods. [15]Scientific study has already exhibited simpler and robust approach of growing Ga 2 O 3 using mist CVD (M-CVD). [16]High-crystalline Ga 2 O 3 thin films are epitaxially grown on various substrates using atmospheric pressure M-CVD.The M-CVD or mist deposition is the variant of conventional CVD process that involves the generation of mist through the atomization of the innocuous solution of source materials and transferring it by a carrier gas to the deposition area to perform thin-film deposition of desired materials.The deposited material thickness can be controlled up to few atomic layers.It is a low temperature process and can be operated at atmospheric conditions [17] in contrast to the usage of extremely volatile and toxic gaseous precursors used in conventional CVD or MOCVD or MOVPE processes for some instances. [18]-CVD method of material synthesis utilizes a simple and non-vacuum setup and a water/alcohol-based precursor solution enabling to be safe, and economic in terms of cost and energy and scalable growth.It is imperative to exhibit device fabrication through M-CVD for future mass production aligned with sustainable development goals. [19]M-CVD method has some advantages for the growth of Ga 2 O 3 thin films including precise control over the delivery of precursor through controlling parameters, higher precursor utilization efficiency leading to a lesser precursor material wastage.Additionally, deposition and growth can be achieved at lower temperatures that diminishes thermal stress on the substrate and the thin film enabling to achieve quality of the film with less defects and dislocation, and doping is easily incorporated with the addition of dopant precursor to the same solution.
In this article, interest is focused on effect of mist intensity on the growth, morphology, and quality of deposited gallium oxide thin films.The quality of deposited films is determined by surface roughness, which is an important parameter.Feng et al. regulated the surface roughness of the grown films through altering chamber pressure in MOCVD process, which exhibited the increase in surface roughness from 3.41 to 5.67 nm) as the pressure is increased.Adatoms desorption and diffusion on the surface may cause this. [20]The innovative development of pulsed MOCVD technique led to a decrease in surface roughness from 30.1 to 5.4 nm.This is due to suppression of desorption of molecules and providing sufficient time for the migration and reaction to occur with the influx of precursor molecules over the thin film/islands present over the substrate, thereby facilitating the micro-islands to merge and form better film leading to a 2D and -3D growth. [21]Zhang et al. reported a decrease in root mean square (RMS) roughness (25.4-13 nm) of thin films grown using low-pressure MOCVD as the growth temperature is increased from 600 to 800 °C.The increase in temperature enables adatom migration on the substrate surface, which improves 2D growth, resulting in smoother films with lower RMS values.However, further increase in temperature could result in an increased decomposition rate of Ga 2 O 3 , which makes it difficult to control the surface roughness. [22]In this study, the growth of Ga 2 O 3 thin films on quartz substrates involving M-CVD process using gallium acetylacetonate, dissolved in ethanol as precursor, was performed.To the best of our knowledge, this is the first study that provides first proof of understanding on the effect of mist intensity on the growth of gallium oxide and relevant properties.In this work, mist intensity is controlled by the nebulization/atomization rate (rate at which a precursor solution is converted into a mist or aerosol), and it is controlled by varying the power applied to the device.It is worth to mention that excessive mist intensity can lead to poor film quality, uneven deposition due to uneven distribution of the mist within the reactor chamber.

Experiment Section
The novel instrumentation setup was a combination of two stages (Figure 1a).The first stage was atomization stage.The physical state transformation of the precursor solution into mist droplets was achieved by using an ultrasonic transducer operating at a resonant frequency of 1.70 MHz.The spherical shape and size of the droplets were dependent on atomization process parameters such as flow rate, process temperature, pressure, resonant frequency, gas-to-liquid mass ratio, viscosity, density, surface tension of the fluid. [23]Smaller droplets offered better coverage and reduced reagent volumes and it exhibited higher surface area to volume ratio that accelerated the evaporation rate of fluid in the reaction zone of M-CVD reactor tube.Microscale droplets allowed a significant increase in reaction kinetics because of faster mixing.In the next stage, as the generated mist droplets didn't have initial velocity, they were transported using carrier gas into quartz tube that forms reaction unit of the M-CVD setup.Substrates were placed on an aluminum holder with a 22.5°ramp facing the gas inlet, followed by plateau.Distance from the mist generator to the substrate was 15 cm.The sample holder was kept inside in the midzone of the quartz tube area.The tilted angle position of substrate was preferred to get a uniform thickness of Ga 2 O 3 over the substrate.It might also result in a higher deposition rate.In case of lower tilted angle of the substrate, smooth and uniform epilayer could be formed. [24] heating source was placed, externally, underneath the substrate holder to provide an adequate thermal energy for decomposition of the precursor in mist to form a nonvolatile residue and facilitate growth of thin film over the substrates.The generated mist propagated along the mist outlet from the tube that was connected to the container filled with solvent to dissolve the solute contained in the mist.
Precursor solution was prepared by dissolving anhydrous gallium acetylacetonate in ethanol with a molar concentration of 0.0038 mol L À1 .To understand the effect of mist intensity on optical properties, thickness, uniformity, deposition rate, and surface roughness, different mist intensities, i.e., 10%, 20%, 50% and 100% were selected.Intensity referred to the variation in the applied power to the device from 20 W (10%) to 24 W (100%).For this experimental study, the optimized growth temperature and flow rate of the oxygen (O 2 ) carrier gas were fixed at 400 °C and 0.2 L min À1 , respectively.

Results
From transmittance spectra of the thin films (Figure 2a), 90% transparency is shown in visible and near-infrared wavelengths of the electromagnetic (EM) spectra.The sharp absorption exhibited by the thin film indicates Ga 2 O 3 could be polycrystalline with no amorphous phase.The blueshift of the fundamental absorption edge was noticed with the increase of the mist-generation intensity.For mist intensity  higher than 50%, shift was considerable which implies that the mist-generation intensity influences the bandgap of the deposited thin film.This claim is further supported by the direct bandgap analysis of deposited thin films deduced using Tauc method where the bandgap of deposited thin film with minimum mist-generation intensity is 5.81 eV.Similar bandgap value for gallium oxide was reported in refs.[25,26].The bandgap obtained for the thin film with maximum mist intensity increased to 5.99 eV.
Bandgap of Ga 2 O 3 thin films increased slightly (about 0.2 eV) with the increase in the mist intensity from 10% to 100%.This is influenced by various factors during their growth process, at higher mist intensity growth process is rapid and mist particles are larger and nonuniform due to amalgamation of droplets because of coarse mist.This is observed in the morphological study as shown in the atomic force microscopy (AFM) images.As mist intensity increases, the growth pressure increases, this can reduce surface desorption of Ga adatoms and limit surface diffusion at a higher O 2 partial pressure. [27]Hence stoichiometry of the films is altered which may alter the bandgap as shown in Figure 2b.Apart from fundamental absorption all films exhibited exciton absorption centered at 300 nm represented by the nonlinear curve in bandgap optical curve due to presence of intermediate state in between conduction band minima and valence band maxima that could be attributed to the presence of impurities and/or defects.It is also possible that carbon from the precursor is not completely removed and trapped within Ga 2 O 3 thin film.Hence, this impurity could lead to introduction of intermediate state within the bandgap.
The morphological studies of thin films using noncontact mode AFM over scanning area of 1 Â 1 μm 2 exhibited (Figure 3a-d) that for the mist-generation intensity less than 50%, the nano-grains present in the thin film are fine, small, and uniform.As the mist intensity became greater than or equal to 50% range, deposited particles are larger and nonuniform due to amalgamation of droplets because of coarse mist (Section S2, Supporting Information).Films deposited at high-mist intensity exhibited pin holes which were not present in the thin films deposited with the low-mist intensity.Topographical analysis showed the RMS roughness of the thin film increased from 0.628 to 4.356 nm as mist-generation intensity increased from minimum to maximum, likely due to simultaneous etching while deposition as the solvent present in droplet could etch the deposited film at the same time of deposition.Owing to amalgamation of droplets at higher mist intensity, the etching rate is higher which results in rougher surface.The deviation in surface roughness with change in mist-generation intensity denotes the significant variation of the thin-film crystallinity.Further details of morphological study of thin films are mentioned in Section S1, Supplementary Information.
For the same growth time (30 min) in deposition processes using different mist intensity, thickness of the thin films increased (Figure 3f ) as the mist-generation intensity was decreased.This could be attributed to generation of fine and coarse mist depending on the intensity.Generated mist droplets propagating to the proximity of the substrate get heated and evaporated before adhering to the heated substrate resulting in development of steam film encapsulating the mist droplets.It is evident that with the increase in mist-generation intensity, e) the surface roughness of thin film increases.f ) Thickness and g) refractive index measurement carried out for thin films obtained at different mist generation using ellipsometer (Rudolph AutoEL II with λ = 632.8nm).As it appears, thickness of the thin film decreased with higher mist intensity.This could be attributed to higher etching rate during deposition at high-mist intensity.Refractive index remained almost uniform irrespective of mist-generation intensity used in growth condition.
These encapsulated mist droplets glide over the substrate surface with Leidenfrost motion and acquire adequate energy through heat influx.Therefore, precursors present within the mist droplets are supplied to the substrate via mass flux through developed vapor film.The reaction process persists till the droplets disappear.For the higher intensity of mist, the etching rate (redissolving of gallium acetylacetonate into incoming droplets) is higher as explained earlier which could be attributed to the decrease in film thickness.In this work, the film thickness decreased with the increase in mist-generation intensity and film roughness also increased (Section S3, Supporting Information).This indicates the roughness difference between the films deposited with different mist intensity is not due to thickness.Thus, mist-generation intensity is a vital growth parameter for deposition in M-CVD.
The color variation in the thin film was prominent on the silicon substrate due to thickness variation of the thin film as shown in Figure 4.The average thickness of the optimized thin film in this work was 130.42 nm.Hence, this implies that considerable thickness uniformity of the thin films was achieved in this work.Further optimization is required to reduce the variance present in the thin-film thickness across the substrate.The gradient in thickness achieved in this work could be attributed to the temperature gradient across the substrate present inside the reaction area and flow dynamics of the vapor.The temperature profile around the substrate is additionally subjected to substrate position and geometry of substrate holder owing to ramp structure of the substrate holder.Hence, heat penetration throughout the ramp could be different resulting in temperature gradient.It could be attributed to the higher temperature that could enable more dissociation of precursor, therefore offering higher growth rate whereas for lower-temperature areas would allow thinner growth, consequently, the variance in the thickness was observed.The small variation in the concentration of vapor arriving at the edges, due to flow dynamics (there is a possibility that the flow of vapors may not be streamline), will also contribute to this variation in the thickness.At this stage of development, it is not of concern, but it can be improved by selecting the right flow rate and a thermal blanket around the reactor tube to reduce any temperature variation.
Thin film deposited with mist-generation intensity of 10% showed the comparatively smoother surface than thin films deposited with higher mist intensity.Reported RMS roughness range of gallium oxide thin film are 0.16-1.830nm for ALD, [28][29][30] 1-6.3 nm for pulse laser deposition, [31,32] 0.38-3.91nm for MOCVD, [33][34][35] and 0.4-3 nm for sputtering, [36,37] which are similar to RMS surface roughness obtained in this work.Hence, this proves that it is possible to achieve superior and ultrasmooth thin films using this low-cost and simple deposition method.Therefore, surface and electrical characterization of thin film grown at 10% mist intensity is studied further to understand different properties.
X-Ray diffraction (XRD) spectra in Figure 5 show multiple diffraction peaks which implies Ga 2 O 3 thin film is polycrystalline.XRD spectra of thin film deposited at higher intensities (>10%) did not have any peaks indicating that the films are amorphous, and data is not shown here.However, thin film deposited at mist intensity of 10% revealed that the films are polycrystalline that is attributed to existence of arbitrarily oriented nanocrystals.The narrowness of the peak depicts small variation of the lattice constant for the scanned area in the film.Careful observation of the peaks showed presence of two split peaks at around diffraction angle of 32.9°and 69.1°.The ( 200) and (400) diffraction produces a distinct and well-defined doublet peak at an angle of 69.1°(Figure S3, Supporting Information).However, this peak is composed of two separate peaks that are independent of each other.These individual peaks can be attributed to the Cu-Kβ 1 line at 2θ of 69.14°and the Cu-Kβ 2 line at 2θ of 69.34°.Similarly, (200) diffraction results in a doublet peak at an angle of 32.9°, which is both weak and sharp.This doublet peak comprises two distinct peaks that are independent of each other.These individual peaks can be attributed to the Cu-Kα 1 line at 2θ of 32.97°and the Cu-Kα 2 line at 2θ of 33.05°( Table 1).
From the peak analysis, we are less certain; however, peaks for both α-Ga 2 O 3 and β-Ga 2 O 3 were found which indicate that the thin-film material is of mixed phase.Formations of mixed phase Figure 4. Gallium oxide thin film deposited on p-Si substrates with optimized growth conditions.Presence of color gradient in the films deposited on p-Si is attributed to thickness variation of the obtained thin film, although the film is quite uniform across much of the substrate.The variation at the edges may be due to the flow dynamics of the precursor vapors and the temperature gradient at the substrate surface.
Figure 5. X-Ray diffraction (XRD) pattern exhibited multiple diffraction peaks which implied that the obtained thin film was polycrystalline and mixed phase in nature as peaks for both α-Ga 2 O 3 and β-Ga 2 O 3 peaks were detected.Ga 2 O 3 thin film was deposited over silicon wafer and hence peak corresponding to Si is from the bottom Si substrate.Please note that we are unable to assign some of the peaks at this stage of the study.gallium oxide were reported in refs.[38-40].It has been reported that for fabrication of high-power devices, an ultrathin mixed oxide phase may hold greater potential than a pure phase.The XRD analysis further proved that the deposited thin film is of gallium oxide.Currently, additional peaks are being identified.
To perform electrical characterization, 100 nm thick aluminum electrodes were deposited through thermal evaporation in a well-defined rectangular gap cell pattern using shadow mask to form Ohmic contact with deposited thin film.These measurements were carried out consisted of six contact pads with different interspacing varying from 100 to 600 μm at 100 μm steps.A symmetrical linear behavior for both negative and positive bias) was observed in current-voltage characteristics (Figure 6a), which implies that homogeneous Ohmic contacts were formed at the metal/film interface rather than Schottky behavior.
The time-dependent response of semiconductor upon exposing to light is a complex process that involves electron-hole pair generation upon excitation, trapping, and recombination of carriers.The photocurrent response could be influenced by the surface conduction of the thin film, penetration depth, the intensity of the incident light, and the ionization status of the level. [41]igure 7c-e demonstrates the time-dependent response of the gallium oxide thin upon exposed to light.The response was measured by turning on and off a 275 nm light and a 360 nm light with light intensity of 3 μW cm À2 periodically controlled by computer (20 s for illumination and dark phase) at fixed bias of 10 V.For illumination source with 360 nm wavelength, current versus time response of the thin film didn't show any change when exposed to light and after turning off the light periodically over 160 s.This is expected photo response of the thin film as the transmittance spectra and bandgap analysis showed in Figure 2, and wavelength of these light sources lies beyond the fundamental absorption edge of the thin film under test.Upon illumination of light source with wavelength 275 nm at applied bias of 10 V, the measured current value increased.It is noticeable that the photo response of the thin film at this wavelength was highly stable over long time, robust, and reproducible.The resistance of the thin film decreased noticeably over long time, and was robust and reproducible.The resistance of the thin film decreased noticeably under 275 nm illumination.Under light exposure, larger number of photogenerated carriers are generated and that leads to reduction of resistance, resulting in the reduction of the effective barrier. [42]Under exposure of light source, time versus current photo response showed that the current increased swiftly from 1.6 to 2.6 pA through a single surge step (1 s elapsed time) and when the light source was turned off, the current slowly decayed and reached to value equivalent to dark current value (elapsed time for transition:19 s).Therefore, such thin film could be used for the fabrication of solar blind photodetector.Required time for the current to rise Figure 6.a) Current-voltage (I-V ) characteristics exhibited linear relationship between aluminum contact electrodes and the thin film for the applied voltage ranging between À100 and 100 V.The linear and symmetrical (for both positive and negative voltage scan) I-V implied the formation of Ohmic aluminum contact with the deposited thin film.b) The linear fit to the resistance as a function electrode gap, this further validates measured electrical behavior is of the bulk material and rather than the contribution coming from the interface.The resistance is quite high; this could be due to unintentional impurities (e.g., source gallium is not pure of semiconductor standards).
Table 1.List of diffraction peaks obtained from XRD spectra and the corresponding value.
XRD peaks Type 23.25°α-Ga 2 O 3 (012) [48] 32.97°3 3.05°o Si (200) Kα 1 [49,50]   Si (200) Kα 2 [50,51]   36.147°α-Ga 2 O 3 (110) [49] 39.577°β-Ga 2 O 3 (110) [49] 43.336°β-Ga 2 O 3 (311) [48] 48.65°β-Ga 2 O 3 (À510) [52] 61.972°Si (400) Kβ [48] 69.14°6 9.34°o Si (400) Kβ 1 [50]   Si (400) Kβ 2 [50]  from its 10% to 90% of its peak value and vice versa is represented by the rise (t r ) and decay time (t d ), respectively.For Ga 2 O 3 thin films deposited in this work, the t r is about 1 s and t d is 19 s. small value of rise time and longer decay time of current could be attributed to presence of lower density traps, which also implies that better crystalline quality of the thin film. [43,44]As a result of that, a smaller number of generated carriers could get trapped leading to increment of the lifetime of the photogenerated carriers.It is known that gallium oxide is intrinsically n-type semiconductor and majority carriers are electrons. [45]The incident light can perform the ionization of the natively neural oxygen vacancy state to shallow donor state.An energy barrier of %0.97 eV is created that surrounds the shallow-donor states due to outward relaxation process of chemical bonds. [46]These energy bands tend to prevent the recombination of electrons and shallow donor states and that results in longer lifetime of shallow donor states than that of holes, resulting in a longer recovery time.These ionized vacancies would contribute to internal gain. [47]The slow decaying process of the photocurrent of the thin film could hinder its prospective application as photodetector.This persistent photoconductivity (PCC) effect needs to be prevented to enable high gain and faster recovery time for the photodetectors.Thermal relaxation could be applied to achieve these characteristics. [47]It was observed that the maximum value photocurrent increased from 2.6 to 4.5 pA with the increase in applied bias voltage from 10 to 20 V keeping the same light source of 275 nm wavelength.The increase in photocurrent with increase in applied bias voltage could be attributed to the collection of large number of charge carriers at a higher electric field.The higher electric field could influence the carrier numbers resulting in enhancement of photocurrent. [42]

Conclusion
This work reported the mist intensity, and its flow could substantially influence the growth of thin film using M-CVD process.
The study showed that mist intensity affects the bandgap of the thin film as the blueshift of the fundamental absorption edge with the increase of the mist-generation intensity was observed.However, it is a negligible change.Morphological studies of thin films showed surface roughness increased with the increase in mist intensity and further analysis (mentioned in Supporting Information) proved the uniformity of surface roughness in the thin film.XRD analysis of the optimized thin film showed diffraction peaks for both α-Ga 2 O 3 and β-Ga 2 O 3 and implies that thin film is polycrystalline-mixed phase of Ga 2 O 3 .Aluminum was a selected metal for electrodes formation on optimized thin film.Contact resistance, sheet resistance, transfer resistance, and specific contact resistance are determined.The photoconductivity measurement using 275 nm light with incident light intensity of 3 μW cm À2 source showed change in current from 1.6 to 2.6 pA upon exposure.This implies that thin film could be used in photodetector applications.This study proves that it is imperative to optimize the mist-generation intensity to deposit ultrasmooth, dense film with repeatable characteristic.To the best of author's knowledge, this is the first report unfurling the effect of mist intensity on the properties of gallium oxide thin film.

Figure 2 .
Figure2.a) Ultraviolet-visible-near-infrared (UV-vis-NIR) spectra exhibited blueshift of absorption edge with increase in the mist intensity; most prominent for 100% mist intensity.The direct bandgap analysis using Tauc method (inset graph) showed the increase in bandgap with the increase in mist intensity.b) Increase in the bandgap of Ga 2 O 3 thin film with increase in the mist intensity, which may be attributed to the change in stoichiometry of the thin film at higher intensities.

Figure 1 .
Figure 1.a) Mist chemical vapor deposition (M-CVD) instrumentation setup consisted of mist-generation unit, reaction area, mist controller unit, heating source, and exhaust for carrier gas.The mist-generation unit and quartz tube-based reaction unit are coupled together using Union and O rings.b,c) Mist generated from precursor liquid carried by carrier gas was coming out of mist outlet and it formed mist streamline before dispersing into the air.

Figure 3 .
Figure 3. Topographical analysis of thin films deposited with a) 10%, b) 20%, c) 50%, and d) 100% mist intensity over 1 Â 1 μm 2 area.It is evident that with the increase in mist-generation intensity, e) the surface roughness of thin film increases.f ) Thickness and g) refractive index measurement carried out for thin films obtained at different mist generation using ellipsometer (Rudolph AutoEL II with λ = 632.8nm).As it appears, thickness of the thin film decreased with higher mist intensity.This could be attributed to higher etching rate during deposition at high-mist intensity.Refractive index remained almost uniform irrespective of mist-generation intensity used in growth condition.