Mist CVD‑based growth of crystalline alpha chromium oxide ( α ‑Cr 2 O 3 ) on c ‑plane sapphire substrate with chromium acetylacetonate as a precursor

α -Cr 2 O 3 is used as a buffer layer for the growth of α -Ga 2 O 3 on sapphire for power devices. Presently, the growth of crystal-line corundum-structured metal oxides layers, except for α -Cr 2 O 3 , is performed with metal acetylacetonates. This article investigates the development of a crystalline α -Cr 2 O 3 thin films on c -plane sapphire with chromium acetylacetonate (CrAc) as precursor over a wide temperature range varying from 400 to 550 °C. The temperature range not only ensures the compatibility of the process with α -Ga 2 O 3 technology but also satisfies the requirement that the window is large enough to adequately optimize the quality of crystalline α -Cr 2 O 3 thin film. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were performed to analyse the quality of the crystalline α -Cr 2 O 3 layer. The XPS result showed that the ratio of different oxidation states of α -Cr 2 O 3 changes with deposition temperature. An optimal deposition temperature at 500 °C with the molarity of CrAc being 0.05 M is achieved for better quality α -Cr 2 O 3 thin film deposition. Thin film of α -Cr 2 O 3 of thickness 530.5 nm has been deposited at optimal condition with a deposition rate of 35.37 nm/min and has a crystallite size of 31.21 nm and root mean square value of surface roughness of 0.647 nm.


Introduction
Chromium oxide (Cr 2 O 3 ) is a p-type wide bandgap (3.2 eV) semiconductor material that finds application in gallium oxide power devices (Ghosh et al. 2019;Almaev et al. 2021). Gallium oxide (Ga 2 O 3 )-based power devices with the alpha phase of Ga 2 O 3 (α-Ga 2 O 3 ) are grown on sapphire substrates with mist CVD (Shinohara and Fujita 2008;Kawaharamura et al. 2012Kawaharamura et al. , 2016Mesfets et al. 2015). A large lattice mismatch exists between sapphire and α-Ga 2 O 3 (Kaneko et al. 2012;Schewski et al. 2015) and can be reduced by using α-Cr 2 O 3 as the buffer layer. The a-plane lattice mismatch between α-Ga 2 O 3 and α-Cr 2 O 3 is 0.4%, while the c-plane lattice mismatch is 1.2% (Marezuo and Remeika 1967), (February and Kristallstrukturen 1962). Additionally, as p-type doping is not possible for Ga 2 O 3 (Kyrtsos et al. 2018;Tadjer et al. 2019;Zhang et al. 2020), α-Cr 2 O 3 provides a means to achieve p-n heterojunction with α-Ga 2 O 3 as the n-doped layer and α-Cr 2 O 3 as the p-doped layer.
Chromium oxides can be deposited with a variety of techniques such as thermal CVD method (Tamura et al. 1996;Sousa et al. 2005), vacuum evaporation (Kadari et al. 2017), rf and dc reactive sputtering (Pedersen et al. 2010;Mohammadtaheri et al. 2018) and plasma spraying (Babu et al. 2018). The use of different precursor materials has also been reported, chromium hexacarbonyl precursor used in the APCVD process (Ivanova et al. 2001) while chromium nitrate nonahydrate with urea was used in spray pyrolysis (Kamble and Umarji 2012). The metal-organic CVD technique allows to produce thin films with uniform surface and thickness at relatively low temperatures (Carta et al. 2005). Recently published work on mist CVD deposited layer of α-Cr 2 O 3 on sapphire substrate uses ammonium dichromate as the precursor (Dang et al. 2018). A good quality epitaxial film with a reasonable growth rate has been reported for the same. An alternative set of precursors are metal acetylacetonates (chromium acetylacetonates, in this case) that is being used extensively. Kentaro used mist CVD to grow α-Cr 2 O 3 with CrAc as the precursor (Kaneko et al. 2010). However, the process required a significantly high growth temperature (700 °C) and provided a low growth rate. A temperature of 700 °C is deleterious for α-Ga 2 O 3 (Roy et al. 1952;Lee et al. 2013;Playford et al. 2013;Jinno et al. 2020;McCandless et al. 2021), and thus, the process is not compatible with α-Ga 2 O 3 -based technology .
In this paper, we have developed a mist CVD-based process for the deposition of α-Cr 2 O 3 on a sapphire substrate with CrAc as the precursor and analysed the properties of the thin film with multiple characterization techniques. Additionally, the effect of deposition temperature and precursor concentration on the quality of deposited film is also investigated. The upper bound of deposition temperature was restricted at 550 °C to ensure that α-Cr 2 O 3 film developed in this work is compatible with α-Ga 2 O 3 technology. The novelty of the work is the development of a mist CVD-based growth process of α-Cr 2 O 3 on sapphire that is compatible with α-Ga 2 O 3 and uses CrAc as the precursor. Figure 1 shows the in-house built experimental setup which has been used for the deposition of α-Cr 2 O 3 . As seen from the figure, the mist of precursor solution was generated using an ultrasonic atomizer which is submerged in water. The generated mist was then transported to the reaction chamber using an inert gas (nitrogen, in this case). The flow of nitrogen was controlled by a flow meter. The reaction chamber, in which the substrate is placed, is made up of stainless steel and is heated to the desired temperature with the help of the heater and appropriate electrical controls. The exhaust is kept at atmospheric pressure, and an exhaust unit transports the outgoing gas to a water trap unit, where the carrier gas along with the gaseous product of the reaction gets trapped.

Experimental details and methods
The experiment can be divided into three steps, (1) conditioning of the wafer before growth (sapphire substrate conditioning), (2) preparing the appropriate precursor (precursor preparation), and (3) the eventual deposition of the α-Cr 2 O 3 (α-Cr 2 O 3 deposition) thin film. Below are the details of each of the process steps:

Sapphire substrate conditioning
A single side polished c-plane sapphire substrate (0001) of 400-micron thickness and 1 cm × 1 cm dimension has been used to grow thin films of α-Cr 2 O 3 . Multiple steps have been taken to clean the sapphire substrate before placing it in the reaction chamber. First, it was placed in soapy water for 5 min and then wiped with a gem cleaning cloth to remove any form of oil from the surface and then rinsed with deionized (DI) water. Subsequently, the sapphire substrate was soaked in isopropyl alcohol for a span of three to five minutes, followed by a soak in acetone for five minutes. Finally, the substrate was dried and placed in the reaction chamber where α-Cr 2 O 3 is deposited on top of polished side.

Precursor preparation
The precursor is prepared by dissolving CrAc in an additive mixed non-polar solvent (toluene, in this case) CrAc easily dissolve in toluene and allow us to reach higher deposition rate. The molarity of the CrAc was varied, and the values of 0.1 M, 0.05 M, and 0.025 M were used for the experiments. The prepared precursor solution was stirred for 10 min and thereafter filtered using PTFE (0.45 micron, nexflo) syringe filter before the solution was used.

α-CrO 3 deposition
The α-Cr 2 O 3 deposition was performed in a specially designed stainless steel chamber. The deposition temperature and concentration of the solution were varied. An optimized Fig. 1 Schematic of in-house made experimental setup where the wall of the chamber was heated from the bottom side only flow rate of nitrogen that varies between 1.0 and 1.5 L/min was used for the experiment. The films were deposited for the combination of four different temperatures of 400 °C, 450 °C, 500 °C, and 550 °C and three different precursor concentrations of 0.1 M, 0.05 M, and 0.025 M. The deposition was carried out for 15 min for all the samples, except for the one using a 0.1 M concentration of precursor solution. The deposition time for that set of samples was set for 30 min to observe the discrepancy on the growth rate.

Characterization methods
The deposited thin films of α-Cr 2 O 3 were investigated by various characterization techniques including powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) experiment was performed with Bruker D8 advance™ diffractometer. In the experiment, Cu K α (λ = 1.5418 Å) radiation was used, and the scan rate was fixed at 1°/min with a step size of 0.02°. The surface morphology and thickness of the deposited films were observed using FESEM (model-GeminiSEM 500) at several resolution. The surface roughness was investigated using AFM (model-Dimension icon, Probe-Tespa, Material-Antimony n-doped silicon) of force 42 N/m at the resonance frequency of 320 kHz. X-ray photoelectron spectroscopy (XPS) was used in the work to analyse the chemical composition and chemical state of the α-Cr 2 O 3 thin film. The XPS instrument is manufactured by Thermo Scientific (model-NEXSA surface analysis), with a micro-focused X-ray (400 μm, 72 W, 12,000 V), measurement angle 45°, monochromatic Al-Kα source (hν = 1486.6 eV), spectrometry work function 4.2 eV, a hemispherical analyser, and a 128-channel plate detector.

Structural analysis by X-ray diffraction
Figures 2 and 3 show the two-theta/theta (2θ/θ) scanning profile of α-Cr 2 O 3 thin films deposited on the c-plane sapphire substrate. Figure 2 is the XRD profile of films deposited with the precursor concentration remaining fixed at 0.05 M and with a variation of temperature from 400 to 550 °C in steps of 50 °C. The minimum deposition temperature (400 °C) was chosen as it was found that the deposition of the thin film does not occur below 400 °C. The maximum deposition temperature (550 °C) ensures that α-Cr 2 O 3 developed in this work is compatible with α-Ga 2 O 3 technology. As it may be recalled, α-Ga 2 O 3 is a metastable phase, and at a higher temperature, phase change to β-Ga 2 O 3 starts to initiate (Kaneko et al. 2010). Figure 3 shows the XRD pattern of films deposited at a constant temperature of 500 °C but with varying precursor concentrations of 0.025 M, 0.05 M, and 0.1 M.
An analysis of Fig. 2 shows that alongside the substrate peaks at 2θ = 41.68° (Roberts et al. 2018;Gao et al. 2004), peaks at 2θ = 39.69°, 39.72°, 39.74°, and 39.77° can be observed. Since the peaks of the deposited films lie between 39.5° and 40°, and as other known polymorphs do not diffract in this range, it can be implied that the deposited film is α-Cr 2 O 3 (Kaneko et al. 2010;Dang et al. 2018), with the peaks corresponding to the (0006) reflection of α-Cr 2 O 3 . Additionally, among the peaks that are present between 39.5° and 40°, the higher intensity peak corresponds to the film deposited at 500 °C indicating higher film thickness (Vani et al. 2013), while the quality of the thin film, as observed from the XRD pattern, degrades with either increase or decrease in temperature from a value of 500 °C. From Fig. 3, we can observe that at a growth temperature of 500 °C and for a precursor concentration of 0.1 M, the peak corresponding to 0006 reflection of α-Cr 2 O 3 is absent, and only the sapphire substrate peak at 2θ = 41.68° is present, it might be due to the fact that the film quality is not good enough to diffract the X-rays. Furthermore, if the concentration is decreased from 0.05 to 0.025 M, a shift in the peak intensity to lower values and broadening in the peak at 2θ = 39.72° has been observed. The shift in the peak intensity to lower values can be attributed to a reduction in the thickness of the deposited film while peak broadening might be because of imperfect crystal lattice (Ungár 2004;Ying et al. 2009;Vani et al. 2013). Thus, with CrAc, 0.05 M and 500 °C are the optimum molarity and temperature for the deposition of α-Cr 2 O 3 thin film.
Further analysis of the dependence of crystallite size and peak position on temperature has been carried out and is illustrated in Fig. 4. The crystallite size of α-Cr 2 O 3 has been calculated using the Scherrer equation, as mentioned below where D is the mean size of the crystallites, K is the Scherrer constant of value about 0.9, λ is the wavelength of incoming X-rays, β is the full width at half maximum of the peak, and θ is the peak position. While calculating the crystallite size, we did not remove the broadening due to the instrument, and data were used as it was obtained from the XRD measurement. Figure 4 shows the effect of deposition temperature on the crystallite size, peak position and FWHM of α-Cr 2 O 3 thin film deposited with 0.05 M precursor solution. As shown from Fig. 4, the peak position shifted from 39.77° (1) D = K ∕ cos to 39.69° as the deposition temperature increased from 450 to 550 °C. Similarly, crystallite size decreased almost linearly as the deposition temperature increased this is due to the increase in full width at half maximum (FWHM) of the peak with the increase in deposition temperature. Evidently, the dependence of crystallite size and peak position on the deposition temperature is almost linear, which would allow us better control over the quality of deposited film for future development.  (Fig. 5b) and 500 °C (Fig. 5c) show the best morphology. We can see from the figures that the morphology appears close to being a single crystal with no grain boundary. The morphology of the surface degrades when the deposition temperature is either increased from 500 to 550 °C (Fig. 5d) or decreased from 450 to 400 °C (Fig. 5a). The degradation in surface morphology can be explained due to the existence of large number of grains. The best morphology for sapphire substrate was observed at a deposition temperature of 500 °C and had a thickness of about 530.5 nm. Figure 6 shows the SEM images of α-Cr 2 O 3 thin films deposited onto the c-plane sapphire substrates at 500 °C  (Fig. 6a) and 0.1 M (Fig. 6b) concentration of precursor solution. The films shown in Fig. 6 have poor morphology compared to the films deposited with 0.05 M concentration of precursor solution at 500 °C (Fig. 5c). From Fig. 6a, b, it is evident that the deposited films are composed of non-uniform grains leading to a poor surface morphology and affecting the crystallinity of the film implying that α-Cr 2 O 3 thin films degraded with either increase or decrease in the concentration of precursor solution from 0.05 M (Fig. 5c). The surface morphology of the α-Cr 2 O 3 thin film grown with 0.025 M precursor solution has better surface morphology than the film grown with 0.1 M precursor solution. Considering in-house built AACVD setup which is quite inexpensive, the quality of the deposited α-Cr 2 O 3 thin film is comparable to other deposition methods and precursor materials reported in the literature. Figure 7a, b shows the dependence of the deposition rate of α-Cr 2 O 3 thin films on the temperature and concentration of precursor solution, respectively. The deposition rate of the films increased almost linearly with respect to temperature up to 500 °C and then decreased, which means that at a higher temperature, the deposition rate is comparatively slower. We can explain this observation through a closer analysis of the growth mechanisms. In a mist CVD growth process, the pyrolytic activity taking place on the surface, which leads to growth, competes with the surface evaporation of the precursor. As the temperature starts to increase, the evaporation process starts to dominate, resulting in a lower growth rate. Similarly, as the concentration of the precursor solution increases, the deposition rate also increases almost linearly. This linear dependence of deposition rate on deposition temperature as well as on precursor concentration would provide a better control on the film thickness for future development. The maximum growth rate of 46.63 nm/min for sapphire substrate was observed at 500 °C with 0.1 M precursor solution, but there was no peak observed on the diffraction pattern. The surface roughness of the film has also increased, which eventually decreases the quality of the deposited film. The best result with a deposition rate of 35.37 nm/min was observed at 500 °C with 0.05 M solution. Figure 8a1, b1, and c1 shows the 2D morphology, while Fig. 8a2, b2, and c2 shows the 3D morphology of the deposited layer. The topographical 2D and 3D micrographs are recorded for the α-Cr 2 O 3 sample over an area of 5-micron × 5-micron. The scan rates are 0.5-0.7 Hz and 256 pixels/line, and 256 lines/scan are used to produce the scan of the entire sample. To quantify the surface roughness, we will use the root mean square (RMS) value of the surface roughness. From here on, we will refer to the RMS value of surface roughness, simply as the RMS value.

Surface roughness analysis by AFM
As shown in Fig. 8, when the film is deposited on the sapphire substrate at 500 °C with 0.1 M precursor solution (Fig. 8a1, a2), the RMS value is 1.96 nm. A decrease in the concentration of the precursor solution to 0.05 M (Fig. 8b1,  b2) reduces the RMS value to 0.647 nm. However, a further reduction in the concentration of the precursor solution to 0.025 M increases the RMS value to 1.12 nm. Figure 9 shows the AFM micrographs of α-Cr 2 O 3 thin film deposited at 550 °C. As shown from Fig. 9, for 550 °C deposition temperature, the RMS value decreases from 2.34 nm (Fig. 9a1, a2) to 0.636 nm (Fig. 9b1, b2) when the concentration of the precursor solution is reduced from 0.1 to 0.05 M. The RMS value at 550 °C shows the same trend as the one at 500 °C, with the RMS value increasing from 0.636 to 5.42 nm (Fig. 9c1, c2) when the concentration of the solution was decreased to 0.025 M.

Chemical composition and chemical state analysis by XPS
X-ray photoelectron spectroscopy (XPS) was used in the work to analyse the chemical composition and chemical state of the α-Cr 2 O 3 thin film. An analysis of the results of XRD, SEM, and AFM shows that the α-Cr 2 O 3 thin film deposition at 500 °C with 0.05 M concentration of precursor solution provides the optimum condition for the deposition of α-Cr 2 O 3. So, the XPS analysis was performed on the sample that was deposited at 500 °C with 0.05 M concentration of precursor solution. In addition, for purpose of comparison, we have also performed the XPS analysis on the sample that was deposited with the same precursor concentration of 0.05 M but at a lower temperature of 450 °C.   Figures 10 and 11 show the XPS survey spectrum of thin film α-Cr 2 O 3 deposited on the sapphire substrate at 450 °C and 500 °C, respectively. We will denote the sample deposited at 450 °C as sample 1 and the one deposited at 500 °C as sample 2. As shown in the XPS survey spectrum, the x-axis depicts the binding energy of the elements present in the investigated sample (in eV), and the x-axis denotes the intensity of the X-ray beam (in counts per second). From the XPS survey spectrum, we can see that oxygen 2s (O 2s), chromium 3p (Cr 3p), and chromium 3s (Cr 3s) peaks are present at the binding energy values of 22.9 eV, 43.9 eV, and 75.3 eV, respectively (Hassel and Freund 1996). The intensity of the signal from these peaks is relatively weak. The carbon 1s (C 1s) peak is at a binding energy value of 284.76 eV for both samples (Bumajdad et al. 2017). Chromium 2p (Cr 2p) peak is present at a binding energy value of 576.68 eV and 576.64 eV for samples 1 and 2, respectively (Hassel and Freund 1996), while oxygen 1s peak (O 1s) is observed at 529.66 eV for sample 1 and 529.62 eV for sample 2 (Bumajdad et al. 2017). Chromium 2s (Cr 2s) peak of relatively small intensity is present at a binding energy value of 698.9 eV (Hassel and Freund 1996). The peaks corresponding to auger emission for oxygen, chromium, and carbon are present at binding energy values of 976.8 eV, 1012.16 eV, and 1026.46 eV, respectively. The atomic percentage of carbon, oxygen, and chromium, as given by the analysis of XPS results through Avantage Software, are 22.15%, 53.99%, and 23.86%, respectively, for sample 1, while for sample 2, the corresponding values are 21.69%, 55.01%, and 23.31%, respectively.
To further analyse the XPS spectrum, we obtained a highresolution scan of carbon 1s (C 1s), chromium 2p (Cr 2p), and oxygen 1s (O 1s) peaks. Each peak was deconvoluted to adequately understand the chemical state of the constituent elements. CASA XPS was used for deconvoluting the peaks. First, the background was subtracted using the Shirley method, and thereafter, the background subtracted spectra were fitted with a Gaussian-Lorentzian line (GL10) shape. The fitted values are then transferred to XPS peak41 to obtain the final plots. The sub-sections below describe the analysis of C 1s, Cr 2p, and O 1s peaks in detail.

C 1s peak
As shown in Figs. 12 and 13, the carbon 1s (C 1s) peak can be deconvoluted into four different peaks for both samples. The most intense peak, or so-called main peak, is present at 284.76 eV and 284.70 eV for samples 1 and 2, respectively, and corresponds to the C-C bond. The value agrees with the literature (Bumajdad et al. 2017). The other three signals of Carbon 1s (C 1s) are detected at 285.39 eV, 288.71 eV, and 288.73 eV for sample 1 and 286.26 eV, 287.13 eV, and 288.58 eV for sample 2 and correspond to C-O bond, C-O bond, and O-C=O bond, respectively.

Cr 2p peak
The analysis of Cr 2p showed that Cr 3+ and Cr 6+ , where Cr 3+ comes from Cr 2 O 3 and Cr 6+ from CrO 3, are present as the oxidation states of chromium. It is clearly depicted that the XPS spectra of Cr 2p decompose into eight peaks, where both oxidation states of Cr 3+ and Cr 6+ have Cr 2p 3/2 and Cr 2p 1/2 and can further be deconvoluted into real, and satellite peaks of each and are shown in Figs. 14 and 15. The Fig. 10 The XPS survey spectrum of the α-Cr 2 O 3 layer deposited on the sapphire substrate at 450 °C has been properly depicted, which shows the presence of different elements (carbon, chromium & oxygen) with their intensity corresponding to the binding energy Fig. 11 The XPS survey spectrum of the α-Cr 2 O 3 layer deposited on the sapphire substrate at 500 °C has been properly depicted, which shows the presence of different elements (carbon, chromium & oxygen) with their intensity corresponding to the binding energy total area contribution considering all peaks of Cr (2p 3/2 ) and Cr (2p 1/2 ) is 66.66%, and 33.33%, respectively, which shows that the area ratio for the two spin-orbit peaks (2p 3/2 : 2p 1/2 ) will be 2:1 (corresponding to 4 electrons in the 2p 3/2 level and 2 electrons in the 2p 1/2 level). Out of these, the individual area contributions of Cr 3+ (2p 3/2 ), Cr 3+ (2p 1/2 ), Cr 6+ (2p 3/2 ), and Cr 6+ (2p 1/2 ) are 43.10%, 21.55%, 23.57%, and 11.78% for sample 1 and 44.96%, 22.46%, 21.74%, and 10.87% for sample 2, respectively. The corresponding binding energy of Cr 3+ (2p 3/2 ), Cr 6+ (2p 3/2 ), Cr 3+ (2p 1/2 ) & Cr 6+ (2p 1/2 ) peaks is 576.68 eV, 576.20 eV, 585.73 eV and 585.91 eV for sample 1 and 576.64 eV, 576.25 eV, 585.98 eV and 585.64 eV for sample 2, while their satellite peaks are at 575.14 eV, 579.45 eV, 588.36 eV and 587.38 eV for sample 1 (see Table 1) and at 575.15 eV, 579.75 eV, 587.92 eV and 585.17 eV for sample 2 (see Table 2), respectively. The most intense peak of chromium is found at 576.68 eV and 576.64 eV for samples 1 and 2, respectively. The peak belongs to Cr 3+ (2p 3/2 ), and this value agrees with the literature (Hassel and Freund 1996) Fig. 12 The XPS spectra of carbon 1s in the deposited α-Cr 2 O 3 at 450 °C, the peaks of carbon 1s orbital are properly delineated with their intensity corresponding to the binding energy Fig. 13 The XPS spectra of carbon 1s in the deposited α-Cr 2 O 3 at 500 °C, the peaks of carbon 1s orbital are properly delineated with their intensity corresponding to the binding energy increases with temperature. As Cr 3+ is the most stable phase of Cr 2 O 3 , it is expected that with the rise in temperature, the percentage fraction of Cr 3+ will be enhanced.

Conclusion
The paper reports on the development of α-Cr 2 O 3 thin film in an in-house built mist CVD setup using CrAc as the precursor. The analysis of the XRD, FESEM, and AFM results showed that the best α-Cr 2 O 3 thin film was obtained at 500 °C with the concentration of the precursor solution at 0.05 M. A change in the concentration of the precursor solution to either 0.025 M or 0.1 M results in a significant degradation of the thin film quality. A similar result is obtained with a change in temperature. A decrease in temperature degrades the film quality, with the films not being observed for a temperature below 400 °C. An increase in temperature to 550 °C decreases the growth rate with a corresponding decrease in the thin film quality. The XPS analysis of the α-Cr 2 O 3 film deposited at 500 °C with 0.05 M concentration of CrAc shows that the film is of mixed-phase, with the ratio of Cr 3+ corresponding to Cr 2 O 3 and Cr 6+ corresponding to CrO 3 being 2.07. The ratio decreases to 1.83 for the film deposited at 450 °C, with the concentration of the precursor remaining the same. Thus, we can conclude that although the paper reports for the first time a successful deposition of α-Cr 2 O 3 thin film on sapphire with CrAc as the precursor and at temperatures compatible with α-Ga 2 O 3 , further  . 16 The XPS spectra of oxygen 1s in the deposited α-Cr 2 O 3 at 450 °C, the peaks of oxygen 1s orbital are properly delineated with their intensity corresponding to the binding energy Fig. 17 The XPS spectra of oxygen 1s in the deposited α-Cr 2 O 3 at 500 °C, the peaks of oxygen 1 s orbital are properly delineated with their intensity corresponding to the binding energy improvement in the structural quality and phase purity will be necessary for the film to be incorporated in the device.