Vacuum xxx (xxxx) xxx Contents lists available at ScienceDirect Vacuum journal homepage: http://www.elsevier.com/locate/vacuum The effect of thickness on surface structure of rf sputtered TiO2 thin films by XPS, SEM/EDS, AFM and SAM Feyza Güzelçimen a, *, Bükem Tanören b, Çağlar Çetinkaya a, f, Meltem Dönmez Kaya c, H. İbrahim Efkere c, d, Yunus Özen c, e, Doğukan Bingöl f, Merve Sirkeci f, Barış Kınacı a, c, M. Burçin Ünlü b, g, Süleyman Özçelik c, e a Physics Department, Faculty of Science, Istanbul University, TR-34134, Istanbul, Turkey Department of Physics, Bogazici University, TR-34342, Istanbul, Turkey Photonics Research Center, Gazi University, TR-06500, Ankara, Turkey d Department of Metallurgical and Materials Engineering, Faculty of Technology, Gazi University, TR-06500, Ankara, Turkey e Department of Physics, Faculty of Science, Gazi University, TR-06500, Ankara, Turkey f Graduate School of Engineering and Sciences, Istanbul University, TR-34116, Istanbul, Turkey g Global Station for Quantum Medical Science and Engineering, Global Institution for Collaborative Research and Education (GI-CoRE), Hokkaido University, 060-8648, Sapporo, Japan b c A R T I C L E I N F O A B S T R A C T Keywords: TiO2 thin film rf sputtering system X-ray photoelectron spectroscopy Atomic force microscopy Scanning acoustic microscopy Hardness In the current study, silicon was utilized as the substrate material and, then, the TiO2 depositions with 100 nm, 300 nm, 500 nm and 700 nm were done onto substrates as thin films at room temperature by a radio frequency (rf) magnetron sputtering method. The binding energy, the surface roughness, elemental analysis and the specific acoustic impedance have been determined via X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) and scanning acoustic mi­ croscopy (SAM), respectively. AFM analysis represented that the root mean square roughness values changed in the range of 0.72 nm–1.22 nm, gradually by the increase in thickness. Two-dimensional acoustic images were recorded by SAM with 80 MHz transducer. The mean and standard deviation values of acoustic impedance were found as 3.151 ± 0.080 MRayl for 100 nm, 3.366 ± 0.080 MRayl for 300 nm, 3.379 ± 0.067 MRayl for 500 nm and, 3.394 ± 0.065 MRayl for 700 nm. SAM results pointed out that the hardness of films increased with increasing thickness. Moreover, the surface defects at the micrometer level were demonstrated. The success of imaging films indicated the potential of SAM in monitoring as well as the inspection of flat two-dimensional surfaces. 1. Introduction Since TiO2 (titania) has a wide band gap, high k-constant, high hy­ drophilicity and photocatalytic activities, there are many materials and systems using TiO2 thin films [1–16]. Due to it being a transparent semiconducting metal oxide, it is used as an electron transport layer (ETL) in perovskite-based structures [17]. Due to high photosensitivity, non-toxicity, strong oxidizingability, and chemical stability, it is used in the photocatalysis applications such as air purification, water purifica­ tion, photochemical cancer treatment, self-sterilizing, fog-proof and self-cleaning surfaces [18–21]. Due to its high dielectric constant, it is used in the metal-insulator-semiconductor (MIS) structures as insulator layer [22,23]. However, fast recombination of electron− hole pairs, wide bandgap, major absorption in the UV light, high recycling cost limit, and slow charge carrier transfer restrict the widespread usage of TiO2 [18, 19]. In order to solve these problems and obtain a higher quality ma­ terial, their surface areas should be modified by doping with different materials. In literature, some well-known systems such as dc- and rf-magnetron sputtering, chemical vapor deposition (CVD), spray, electron beam, liquidphase deposition, ion beam-assisted deposition, pulsed laser deposition, and sol-gel were used to obtain TiO2 thin film [22,24–27]. Among these systems, the commonly used system is the sol-gel method because of its low-cost. However, this method has been replaced by * Corresponding author. E-mail address: [email protected] (F. Güzelçimen). https://doi.org/10.1016/j.vacuum.2020.109766 Received 11 May 2020; Received in revised form 4 September 2020; Accepted 5 September 2020 Available online 14 September 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved. Please cite this article as: Feyza Güzelçimen, Vacuum, https://doi.org/10.1016/j.vacuum.2020.109766 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx layer thickness, substrate temperature and doping ratio with different materials are very important in determining the surface properties of a thin film material. It is well-known that the structural and morpholog­ ical properties of titania films depend on both deposition conditions and preparation method. It is worthwhile to study its optical and structural properties for promoting TiO2 applications more effectively. A great importance of TiO2 coating utilization, commonly on Si substrate, due to its good optical properties and high refractive index has been clearly seen in various studies for anti-reflection coating (ARC) purposes [28–32]. Anti-reflection coatings (ARC) act to reduce reflec­ tion as well as optical loss dependent of phase changes in light, thereby to improve the efficiency of solar cells [33]. Surface topological prop­ erties as roughness and hardness of materials are very important in ARC layer applications. In particular, it is well known that the roughness of an ARC layer used in solar cell applications prevents the reflection of the light propagating into the structure, thus more light penetrates into the material and contributes to photocurrent. In this case, there is an in­ crease in the short circuit current of the solar cell as well as the cell efficiency increases. Different suitable ARCs that can be used to improve the efficiency of solar cells are still searched in several researches [34,35]. Besides, the optimal thickness of an ARC such as TiOx layer is a focus parameter for characterizing an efficient solar cell [30]. X-ray photoelectron spectroscopy (XPS) and atomic force micro­ scopy (AFM) have been performed for determining the surface morphological characteristics such as binding energy, rougness and grain size of the TiO2 with different thicknesses as well as various layers. These parameters have been determined in several experimental studies [36–40]. Scanning acoustic microscopy (SAM) has an extensive coverage in controlling the quality of semiconductors, micro products, antireflective as well as protective coatings [41]. In semiconductor in­ dustry, the main field of SAM application is up to now to quantitatively detect the various surface defects (pinholes, cracks, scratches, orange skin, pores etc.). SAM as a robust and non-destructive method can be applied for single- and multi-layer coatings without specific sample preparation [42]. SAM is commonly used as an evaluation method which enables detection of hardness differentiation through the acoustic impedance values and surface acoustic visualization of semiconductors, polymer and composite materials [42–47] as well as of biological and medical samples [48–54]. SAM frequencies in the range of 5–300 MHz are typically used for semiconductor applications [46]. In this study, TiO2 thin films with different thicknesses were depos­ ited on n-Si substrates at room temperature using a TiO2 target by rfmagnetron sputtering system. The determination of deposition param­ eters and structural properties of films were investigated systematically by AFM and XPS methods. An elemental microanalysis of each asprepared coating was also examined by performing scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The current study mainly focused on the effect of acoustic waves on surface structure of TiO2 thin film coating. The SAM system with acoustic impedance mode at 80 MHz was applied to obtain the acoustic impedance and surface hardness values of TiO2 thin films by mapping the acoustic impedance distributions with a micrometer level resolution. Table 1 Deposition parameters of rf-sputtered TiO2 thin films. Film Thickness (nm) Base pressure of chamber (Pa) Sputtering time (sec) Deposition rate (Å/s) 100 5.4 × 10− 2460 0.04 − 4 11,040 0.02 4 300 5.4 × 10 500 5.2 × 10− 4 12,900 0.03 700 1.3 × 10− 4 10,500 0.06 Fig. 1. Schematic of SAM setup in acoustic impedance mode. Fig. 2. Principle of SAM in acoustic impedance mode. The acoustic waves re­ flected from the surfaces of distilled water and the target are collected by the same transducer and compared for the calculation of the acoustic impedance of the target. 2. Material and methods 2.1. Preparation of TiO2 thin films magnetron effective methods to solve the problem relevant to the poor electrical conduction of TiO2 nanoparticles. Although a variety of methods have been used to prepare TiO2 films, the materials grown by a rf magnetron sputtering system have parameters with uniform, dense and precise stoichiometric structure. Nowadays, the studies on improving the surface properties of TiO2 are still state of the art and continuing rapidly. The parameters such as TiO2 film-coated samples were deposited using a TiO2 target with high-purity (99% Plasmaterials Company, USA) were deposited onto (100)-oriented boron-doped n-type Si substrates via rf-magnetron sputtering method under identical vacuum processing and deposition conditions. The deposition was carried out in an atmosphere of 4 Pa Ar gas, with a power of 150 W and at a rotational speed of 5 rpm. The distance between the target and the substrate holder is 100 mm. The 2 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 3. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 100 nm, employing a Shirley type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments based on XPS fitting results. substrates were maintained at room temperature and thin films were not annealed after sputtering. The parameters of base pressure of chamber, sputtering time and deposition rate are given in Table 1. In vacuum processing, the system was cleaned using vacuum pumps. Subsequently, substrate and TiO2 target were placed in vacuum chamber and the chamber was evacuated to less than 10− 4 Pa in order to suffi­ ciently reach the base vacuum value. The value obtained in the present study is a very suitable value for deposition when we glance at literature studies performed by sputtering system [10,25,55]. After reaching the sufficient vacuum value, Argon gas is only leak to the chamber for convenient plasma conditions to be formed and deposition begins in plasma environment by applying power to TiO2 target as mentioned in the manuscript. Since titanium and oxygen were not separately sput­ tered into the system, oxygen gas deposition and contamination do not occur in the system at the high-vacuum value obtained. The thicknesses of the deposited TiO2 thin films were varied from 100 to 700 nm by changing the deposition time. The time of TiO2 sputtering at room temperature was set up to 2460 s, 11,040 s, 12,900 s and 10,500 s in order to achieve 100 nm, 300 nm, 500 nm, and 700 nm thick layers, respectively. The TiO2 thin films with thicknesses of monitored using a thickness-meter. 2.2. XPS, AFM and SEM/EDS analyses XPS is a powerful surface technique to elucidate the surface morphology of the films. The composition of the films was characterized with Omicron XPS device using non-monochromatic Mg Kα excitation source (hν = 1253.6 eV, 10 mA, 10 kV). The basic parameters of XPS system as beam power, beam size, and emission angle were 25 W, 10 mm, 45◦ , respectively. The spectra were recorded by 0.1 eV step. Charge Compensation was demonstrated using dual neutralization system consisting of low energy electron beam and ion beam. The pressure in the analytical chamber during spectral acquisition was about 10− 7 Pa. All analyses were carried out at room temperature. The spectra were obtained in the constant analyzer energy mode and the pass energies for survey and high-resolution scans were 50 eV and 20 eV, respectively. The surface morphology of the films was analyzed with high per­ formance atomic force microscope (NanoMagnetics Instruments Ltd., Oxford, UK) using dynamic mode scanning at scan area 10 × 10 μm2. All measurements at room temperature were performed at the scan speed 5 μm/s. The root mean square (RMS) and grain size values of the films were obtained by AFM images. To observe the surface topology of the thin films with a surface area of 5 × 5 mm, SEM method was performed. Before running the system, all samples were vacuumed up to ~1 Pa and were sputtered with a thin 3 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 4. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 300 nm, employing a Shirley type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments based on XPS fitting results. gold–palladium layer in 90 s (Quorum-SC7620). The surface morphol­ ogies were monitored using a Zeiss EVO LS 10 SEM device. The composition of all deposited thin films were studied by scanning elec­ tron microscopy (SEM) equipped with EDS analyzer. The EDS analysis has been carried out in order to validate the presence and to reveal the distribution of elements in samples. reflections from both surfaces of the reference (water) and the target cross-section on the substrate (Fig. 2). The 2-dimensional distributions indicate different acoustic properties due to the variation of elasticity within the targets. SAM in acoustic impedance mode measures the acoustic impedance of the target by comparing the reflected signal from the target with the one from the reference. The reflected signal from the reference is, 2.3. SAM analyses Sref = Scanning acoustic microscope is mainly composed of a transducer with an ultrasonic lens, a pulser/receiver, an oscilloscope and a com­ puter with a display monitor. 80 MHz transducer, which has a spot size of 17 μm and a focal length of 1.5 mm, generates single pulses of width of 5 ns with a repetition rate of 10 kHz and also collects the reflected acoustic waves, therefore, acts as a pulser/receiver. Water is chosen to be the coupling medium between the ultrasonic lens and the specimen. X–Y stage controlled by a computer is responsible in the twodimensional scanning of the transducer. The reflected signals from both the reference and target material are analyzed by the oscilloscope. Finally, acoustic intensity and impedance maps of the region of interest with 300 × 300 sampling points are visualized with a lateral resolution of approximately 20 μm. We analyzed TiO2 samples using the acoustic impedance mode (Fig. 1) of scanning acoustic microscope (AMS-50SI) developed by Honda Electronics (Toyohashi, Japan). In acoustic impedance mode, image is constructed using the acoustic Zref − Zsub S0 Zref + Zsub (1) where S0 is the signal generated by the transducer of SAM, Zref is the reference’s acoustic impedance (1.50 MRayl, 1 Rayl = 1 kg m− 2. s− 1) and Zsub is the substrate’s acoustic impedance. The signal reflected by the target is, Starget = Ztarget − Zsub S0 Ztarget + Zsub (2) Consequently, the target’s acoustic impedance is calculated as, Ztarget = 1+ 1− Starget S0 Starget Zsub S0 (3) 3. Results and discussion In this study, XPS analysis was performed in order to confirm the presence of phase of TiO2 grown on Si substrate. The films consist of 4 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 5. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 500 nm, employing a Shirley type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments based on XPS fitting results. three elements oxygen (O), titanium (Ti), carbon (C) in the range from 200 eV to 800 eV binding energy. It’s well known that XPS spectra are very sensitive to the environ­ ment around atomic species. The occurrence of adjacent to the C 1s peak is attributed to the surface contamination since the samples were exposed to air before XPS measurements [56–59]. Besides, a contribu­ tion by the adhesive tape, where the sample is held, is expected. Since TiO2 thin films investigated in the present study have been stored in air for longer than one month (relatively long periods of time), most of the adventitious carbon (AdC) found on the surface in XPS analysis originates from air exposure, i.e., the C 1s peak corresponds to the C–H bond and C–C bond in the hydrocarbon, which was from carbon contamination as the sample was exposed to air. For materials exposed to the air environment, the nature of the AdC peak with Carbon contamination on the surface depends on the sub­ strate, the environment, and the exposure time. In particular, the binding energy of the C-C/C-H peak of AdC depends on the substrate [58]. For materials on which AdC is accumulated, it may vary by as much as 2.66 eV. Therefore, in our study, we performed the binding energy referencing process by analytically examining the C 1s peak of AdC comprehensively. The binding energy (EB ) of the C-C/C-H peak of AdC varies depending on the work function (φSA ) of deposited sample [59]. EB + φSA , a constant value, indicates that C 1s does not change according to the vacuum level. This value is determined 289.58±0.14 eV as for various material systems exposed to air between seven minutes and 10 months [57,59]. In the light of these information, in the referencing process of XPS data, it is calculated that the binding energies of C-C/C-H peak of AdC C− C/C− H ) according to the following expression based on the work (EB function of the of TiO2 (φTiO2 ). EBC− C/C− H = 289.58 − φTiO2 (4) The work function for TiO2 deposited by magnetron sputtering method is 4.6 eV [60]. Thus, the binding energy of C-C/C-H peak of AdC for all thicknesses of TiO2 was obtained as 284.98 eV. The peak is setted at this value and all other core-levels, O 1s and Ti 2p, are shifted accordingly. C and H contamination elements and Ti/O stoichiometry were determined by analyzing of C 1s, Ti 2p and O 1s peak profiles. It is possible to make quantitative analysis separately by fitting the XPS experimental data for each peak. For a homogenous sample containing n elements the molar concentration xi of element i is then given by; Ai /si xi = ∑n i (Ai /si ) (5) where Ai is the area under the corresponding core-level peak and si is the relative sensitivity factor (RSF). 5 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 6. Detailed XPS peak modelling of core-level spectra acquired from air-exposed (more than one month) TiO2 film with thickness of 700 nm, employing a Shirley type background and Voigt functions: XPS spectrum of a) C 1s, b) Ti 2p, c) O 1s as well as d) binding energy levels, surface atomic concentrations and assignments based on XPS fitting results. A detailed XPS peak modelling of core-level spectra acquired from air-exposed TiO2 films with thickness of 100 nm, 300 nm, 500 nm and 700 nm by employing a Shirley type background [61] and Voigt function as shown in Figs. 3–6. The binding energy values of Ti 2p and O 1s referencing to the C 1s peak of AdC and the surface atomic concentra­ tions were reported. The bonding assignments were also unambiguously denoted. From XPS analyses, the details about chemical composition and the presence of titanium, oxygen and carbon were investigated. The carbon peak is observed and used as reference. The core level spectra reveal the presence of two prominent peaks of Ti 2p3/2 and Ti 2p1/2 positioned at between 458.97-459.10 eV and 464.72–464.90 eV, respectively. The binding energy separation between two peaks is dependent on the chemical state of the Ti atoms and the value between these spin-split components is between 5.72 and 5.75 eV as well as in good aggrement with recent literature [57,62,63]. The O 1s core level spectra exhibit that a broad peak at around 530 eV assigned to oxygen in TiO2 lattice which is presented in (Ti-O-Ti) manner. Because as-deposited thin films being exposed to air for different amount of time prior to analysis, various changes were observed in the formation of O-C=O and C-C/C-H peaks [57–59]. SEM/EDS method has also helped to measure quantitative elemental microanalyses of the as-prepared coatings. The element map scanning during the EDS analysis was conducted and the results were given in Figs. 7–10. Typical EDS patterns show two prominent peaks at around 0.5 eV and 4.5 eV which confirm presence of titanium and oxygen, respectively. From these spectra, it is observed that the TiO2 has a stoichiometric ratio of Ti (~33%) and O (~63%). The results of the EDS analyses obtained on the surface areas report that the atomic ratio of Ti/O was close to 1/2 and the two elements of Ti and O were homogenous dispersed in the for each sample. Surface morphology and roughness of films were assessed based on AFM measurements. Fig. 11 illustrates three-dimensional (3D) surface morphologies of TiO2 films deposited at different thicknesses for a scan area of 10 × 10 μm2. The surface morphology of the deposited films was influenced by the increase in film thickness. The RMS and grain size values of the TiO2 films were increased with increasing film thicknesses. The surface roughness is an important factor that influence physical behaviors of TiO2 thin films [4]. The RMS roughness values of the films deposited at different thicknesses are in the range of 0.72 nm–1.22 nm (Table 2). The surface morphology of the TiO2 films was found to be very smooth, i.e., the well-defined crystallinity and size of TiO2 films were 6 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 7. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 100 nm. a) SEM image of the surface topography in surface view. The EDS mapping of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 7a. (The scale bar is in 10 μm). confirmed by the AFM analysis. In this study, RMS values are closely observed to each other for rougness values by considering surface to­ pological results obtained from AFM since the values are in scale of nm (Table 2). All semiconductors have a mechanism that contains interaction be­ tween conduction electrons and acoustic wave. Moreover, electron ab­ sorption, relevant to this interaction, have a distribution in a wide frequency range up to GHz in many semiconductors [41]. Using SAM, we observed the two-dimensional images of TiO2 coat­ ings resulting from the reflection of material surfaces. Fig. 12a–d shows the acoustic impedance maps of TiO2 films. These images were con­ structed using the acoustic reflections from both surfaces of the refer­ ence (water) and the cross-sections of the films and operating SAM in acoustic impedance mode. For determining mean value of acoustic impedance, thin films of TiO2 with each film thickness of 100 nm, 300 nm, 500 nm and 700 nm were scanned and analyzed on five different surface areas of 0.3 mm × 0.3 mm. One representative surface image was chosen for each film thickness and given in Fig. 12. The acoustic impedance maps (as seen in Fig. 12) show few defects 7 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 8. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 300 nm. a) SEM image of the surface topography in surface view. The EDS mapping of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 8a. (The scale bar is in 10 μm). 8 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 9. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 500 nm. a) SEM image of the surface topography in surface view. The EDS mapping of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 9a. (The scale bar is in 10 μm). 9 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 10. Summary of SEM/EDS analysis results of TiO2 thin film, with thickness of 700 nm. a) SEM image of the surface topography in surface view. The EDS mapping of b) C, c) O, d) Ti elements in TiO2 coating deposited on Si substrate. e) The EDS spectrum of area shown in Fig. 10a. (The scale bar is in 10 μm). 10 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 11. 3D AFM images of TiO2 films with thicknesses of; a) 100 nm, b) 300 nm, c) 500 nm. and d) 700 nm. SAM measurement results are of great importance for the present study. According to the results obtained from SAM; while the hardness values of 300, 500 and 700 nm thicknesses of TiO2 thin films are very close to each other (~3.3 Mega-Rayl), the hardness value of TiO2 film with 100 nm thickness is ~3.1 Mega-Rayl and interestingly lower than others (as seen in Table 3 and Fig. 13). In summary from AFM and SAM results; there is close values for roughness of TiO2 thin films with increasing thickness, while the surface of the material distinctly hardens. Table 2 AFM parameters of TiO2 films. Film Thickness (nm) Grain size (nm) RMS roughness (nm) 100 300 500 700 11.5 13.2 13.6 15.9 0.72 0.90 1.09 1.22 4. Conclusion with different acoustic impedance values due to different elasticity on all TiO2 thin films. The reflection of acoustic waves causes a strong signal received from any porous region, while at the same time in SAM images occurs as visibly certain regions. These distributed nonhomogeneous regions identified defects with various size and shape on the coatings [44]. Table 3 summarizes acoustic impedance microscopy results of all thin films in this investigation; the mean acoustic impedance values with standard deviations of coatings are given. The acoustic impedance values are the mean values calculated by five different surface areas of 0.3 mm × 0.3 mm on the same sample. The standard deviations were also computed to be of less than 3%. The variations in acoustic imped­ ance correspond to the variations of elasticity and hardness. Therefore, we have concluded that the surface of the films has substantially gained higher hardness as the film thickness was increased. The SAM results noticed a prominent influence of film thickness on the hardness of nanostructures investigated and thus the sample of TiO2 thin film with thickness of 700 nm had the highest hardness displayed by the highest acoustic impedance value. The standard deviation values show us that the surface defect rate in the coatings prepared is at a significantly low-level. Both AFM and SAM outcomes revealed that an increase in layer thickness significantly affects both surface roughness and elasticity. Film surface was roughening with increasing thickness due to the change of surface morphology. Moreover, the grain size, acoustic impedance and surface roughness variations depending on the layer thickness of TiO2 films were investigated (Fig. 13). In this study, we dwell on the surface acoustic characterization of TiO2 layers with different thicknesses using acoustic waves generated by an acoustic microscope, while assisting these outcomes with surface roughness and grain size values observed by an atomic force microscope. The acoustic impedance values are evaluated for hardness characteris­ tics of semiconductor based materials without mechanical harm to the samples. When the acoustic impedance values are carefully examined (as seen in Fig. 13), a rapid increase from 100 nm to 300 nm is observed, but after 300 nm this increase appears to be saturated. It is generally known that single and multiple ARC coating materials use an average layer thickness of around 100 nm [28–35]. Based on this information, the thickness of 100 nm was taken as reference value and thicknesses were gradually increased in this study. In the light of AFM and SAM results; although it is seen that there is close values for roughness of TiO2 thin films with increasing thickness, the surface of the material distinctly hardens. The increase in thickness value for each material generates an extra resistance to the material, which is undesirable for electro-optic device applications. As a result, it is obviously seen that TiO2 thin film with 100 nm thickness has optimal properties for ARC applications as weel as it is very likely to be used in electro-optic applications. Credit author contributions statements Feyza Güzelçimen (F.G.) : Conceptualization, Investigation, 11 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx Fig. 12. Two-dimensional acoustic impedance maps of TiO2 films of thicknesses of a) 100 nm, b) 300 nm, c) 500 nm and d) 700 nm, recorded by 80 MHz scanning acoustic microscopy (SAM). The color bar represents the variation in acoustic impedance values. The scanning area is 0.3 mm × 0.3 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) H. İbrahim Efkere (H.I.E.) : Investigation, Formal analysis Yunus Özen (Y.O.) : Visualization, Original draft preparation Doğukan Bingöl (D.B.) : Investigation Merve Sirkeci (M.S.) : Investigation Barış Kınacı (B.K.) : Conceptualization, Resources, Writing Reviewing and Editing M. Burçin Ünlü (M.B.U.) : Writing - Reviewing and Editing, Supervision Süleyman Özçelik (S.O.) : Supervision Table 3 Mean acoustic impedance values and standard deviations for TiO2 thin films. Film Thickness (nm) Acoustic impedance (on Si) (MRayl) 100 300 500 700 3.151 ± 3.366 ± 3.379 ± 3.394 ± 0.080 0.080 0.067 0.065 Declaration of competing interest Visualization, Original draft preparation, Writing - Reviewing and Editing Bükem Tanören (B.T.) : Formal analysis, Writing - Reviewing and Editing Çağlar Çetinkaya (Ç.Ç.): Formal analysis, Investigation Meltem Dönmez Kaya (M.D.K.) : Investigation, Formal analysis The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 12 F. Güzelçimen et al. Vacuum xxx (xxxx) xxx [17] [18] [19] [20] [21] Fig. 13. The grain size, acoustic impedance and surface roughness variations depending on the layer thickness of TiO2 films. [22] [23] Acknowledgement X-ray photoelectron spectroscopy and atomic force microscopy studies were supported by the Directorate of Presidential Strategy and Budget of Turkey (Project No: 2019K12-92587). 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