Tailoring Crystalline Structure of Titanium Oxide Films for Optical Applications Using Non-Biased Filtered Cathodic Vacuum Arc Deposition at Room Temperature

coatings Article Tailoring Crystalline Structure of Titanium Oxide Films for Optical Applications Using Non-Biased Filtered Cathodic Vacuum Arc Deposition at Room Temperature Elena Guillén1, Matthias Krause 2, Irene Heras 3, Gonzalo Rincón-Llorente 4and Ramón Escobar-Galindo 5,*   Citation: Guillén, E.; Krause, M.; Heras, I.; Rincón-Llorente, G.; Escobar-Galindo, R. Tailoring Crystalline Structure of Titanium Oxide Films for Optical Applications Using Non-Biased Filtered Cathodic Vacuum Arc Deposition at Room Temperature. Coatings 2021,11, 233. https://doi.org/10.3390/coatings 11020233 Academic Editor: Christian Mitterer Received: 31 December 2020 Accepted: 10 February 2021 Published: 15 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Profactor GmbH, Im Stadtgut A2, 4407 Steyr-Gleink, Austria; [email protected] 2Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany; [email protected] 3Advanced Center for Aerospace Technologies (CATEC), C/Wilbur y Orville Wright 19, 41309 La Rinconada, Sevilla, Spain; [email protected] 4C/Goles 26, 41002 Sevilla, Spain; [email protected] 5Departamento de Física Aplicada I, Escuela Politécnica Superior, Universidad de Sevilla, Virgen de África 7, 41011 Sevilla, Spain *Correspondence: r[email protected] Abstract: Titanium oxide films were deposited at room temperature and with no applied bias using a filtered cathodic vacuum arc (FCVA) system in a reactive oxygen environment. The dependence of film growth on two process parameters, the working pressure (Pw) and the O 2 partial pressure (p O2 ), is described in detail. The composition, morphological features, crystalline structure, and optical properties of the deposited films were systematically studied by Rutherford Back Scattering (RBS), Scanning Electron Microscopy (SEM), X-Ray diffraction (XRD), Raman Spectroscopy, UV-vis spectroscopy, and spectroscopic ellipsometry. This systematic investigation allowed the identification of three different groups or growth regimes according to the stoichiometry and the phase structure of the titanium oxide films. RBS analysis revealed that a wide range of TiO x stoichiometries ( 0.6 < ×< 2.2 ) were obtained, including oxygen-deficient, stoichiometric TiO 2 and oxygen-rich films. TiO, Ti 2 O 3 , rutile-type TiO 2 , and amorphous TiO 2 phase structures could be achieved, as confirmed both by Raman and XRD. Therefore, the results showed a highly versatile approach, in which different titanium oxide stoichiometries and crystalline phases especially suited for diverse optical applications can be obtained by changing only two process parameters, in a process at room temperature and without applied bias. Of particular interest are crystalline rutile films with high density to be used in ultra-high reflectance metal-dielectric multilayered mirrors, and reduced-TiO 2 rutile samples with absorption in the visible range as a very promising photocatalyst material. Keywords: titanium oxide films; filtered cathodic vacuum arc; rutile; optical coatings 1. Introduction Great attention has been focused on the different polymorphs and stoichiometries of titanium oxide thin films due to their application in a wide range of fields [ 1 ]. In the case of TiO 2 films, different crystalline phases are described, namely anatase, rutile, and brookite, which determine its properties and applications. The first two phases are the most used for technological applications. TiO 2 films with anatase phase are used as self-cleaning windows, antifogging glass, self-sterilizing, and anti-bacterial tiles, as well as for water purification devices [ 2 ]. Anatase TiO 2 is also known for its use as a photoanode in Dye Sensitized Solar Cells [ 3 ]. Among the TiO 2 phases, the rutile phase is chemically and thermodynamically more stable and has the highest refractive index and hardness [ 4 ]. Some technological applications of rutile films include its use as optical coatings, as a dielectric layer in microelectronic applications, and as a protective layer on biomedical implants [ 5 ]. Rutile-type TiO 2 films can be especially suited as the high refractive index Coatings 2021,11, 233. https://doi.org/10.3390/coatings11020233 https://www.mdpi.com/journal/coatings Coatings 2021,11, 233 2 of 16 material of choice in combination with a low refractive index material as SiO 2 to achieve highly reflecting multilayer stacks in dielectric mirrors [ 6 ]. Regarding amorphous TiO 2 , it has been synthesized as a tinted or enhanced photocatalyst, used to purify dye-polluted water, and applied to resistive random access memory applications [7]. In addition to the possibility to fabricate different crystalline phases, the desired properties of the TiO 2 films can be tuned by slight changes in their stoichiometry. TiO 2 is a wide bandgap semiconductor (3.0 and 3.2 eV for rutile and anatase, respectively), which is photoactive when irradiated with UV light. To widen the use of TiO 2 as a photocatalyst, its activity needs to be extended beyond the ultraviolet regime into the visible region [ 8 ]. In oxygen-deficient TiO x films (x < 2), oxygen vacancy energy states are formed below the conduction band minimum [ 8 ]. This results in a reduction in the energy required for photoexcitation of electrons, making it possible for the films to absorb visible light. Oxygen-deficient TiO 2 films also have potential application as electrode material for alkaline-electrolyte batteries since they can be electrically conductive and yet maintain their corrosion-resistant properties [ 9 ]. However, there is no industrially scalable process available for high throughput synthesis of oxygen-deficient TiO2. The final properties of titanium oxide films strongly depend on the deposition technique employed for their fabrication (i.e., sol-gel, Chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD)). In particular, TiO 2 films deposited at low temperatures by most deposition methods exhibit an amorphous phase. Film crystallization is usually performed either by substrate heating during deposition or by a post-deposition annealing process. In addition, in PVD processes, very often the application of a bias to the substrate is required to obtain TiO 2 crystalline films. However, a myriad of recent applications require the TiO 2 films to be coated on heat-sensitive and/or non-conductive substrates (e.g., flexible electronics, antireflective coatings on plastic lenses, etc.). Therefore, it is important to optimize growth conditions in order to obtain crystalline films and/or the desired stoichiometry at low temperature using any type of substrate. To achieve that, it would be interesting to rely on other parameters of the deposition process different from the applied bias or the temperature. In this regard, there are some previous studies on the effect of the working pressure and the O 2 partial pressure on titanium oxide film formation [4,10]. Among the different PVD techniques, filtered cathodic vacuum arc (FCVA) has the highest plasma ionization ratio. This results in denser thin films and better adhesion than those obtained by technologies like magnetron sputtering or thermal evaporation [ 11 ]. Hence, this technique is especially suited for optical applications. The substrate temperature and the ion energy mainly determine phase formation during deposition by FCVA. In the case of TiO2, amorphous films are obtained at low temperature and low ion energy. Therefore, synthesizing crystalline TiO 2 by FCVA usually requires either heating or biasing the substrate during deposition [ 5 , 12 ]. The phase diagram proposed by Löbl et al. [ 13 ] shows that with particle energies between 4–30 eV, mixed phases of rutile and anatase can be obtained at room temperature, meanwhile only at particle energies over 30 eV pure rutile phases can be obtained. These high ion energies are in the range of a FCVA deposition process. In this work, a systematic study of the effect on titanium oxide films of the working pressure and oxygen partial pressure in a FCVA deposition process is carried out. We demonstrate that a fine tune of these parameters leads to the formation of a wide variety of stoichiometries and crystalline phases, from amorphous to rutile, at room temperature without the application of any bias during the deposition. 2. Materials and Methods Titanium oxide films were grown on Si (100) and on glass substrates by means of a DC Filtered Cathodic Vacuum Arc (FCVA) system PFCVA-450 from Plasma Technology Limited, Hong Kong. The system is provided by a 90 ◦ curved electromagnetic filter which guides the ions in the plasma to the deposition chamber. The substrates were placed Coatings 2021,11, 233 3 of 16 240 mm away from the filter duct exit. For each deposition process, base pressure in the chamber was ~2 × 10 −3 Pa. Prior to deposition, samples were sputter cleaned by Ar + bombardment for 15 min. In a FCVA system like the one used in this work, when no bias is applied during the process, the temperature at the surface of the sample reaches a maximum of ~100 ◦ C after 40 min of deposition process. A Ti cathode with 65 mm diameter was used for the deposition of the films in an oxygen/argon atmosphere. The total working gas flow was fixed at 60 sccm, and different percentages of reactive gas during depositions were obtained by changing the Ar/O 2 ratio. The cathode current used was 50 A. All the depositions processes were carried out at room temperature. Deposition time was 15 min and the substrate holder was rotating during the whole process at 5 rpm. Depth-resolved composition of the samples was determined by Rutherford backscattering spectroscopy (RBS) using 4+ He incident ions with an energy beam of 1.7 MeV. The data were acquired with a silicon barrier detector located at a backscattering angle of 170 ◦ , with an energy detector resolution of 13 keV. The experimental spectra were fitted using the SIMNRA software [ 14 ]. The thickness of the films was measured using a mechanical stylus profilometer (Veeco Dektak 150). Morphology was analyzed by scanning electron microscopy (SEM) with a Hitachi S4800 SEM-FEG microscope of high resolution (1–3 nm), equipped with a Bruker X flash 4010 EDX detector with a resolution of 133 eV, field emission gun, and STEM detector system. The phase structure of the thin films was determined by X-ray diffraction employing grazing incidence geometry (GIXRD) using a Rigaku Ultima IV diffractometer with Cu-K α radiation ( λ = 1.5406 Å). The incident angle was 0.4 ◦ , and the XRD patterns were measured in the diffraction angle range of 20–100 ◦ in steps of 0.02 ◦ . Raman spectra were measured in 180 ◦ backscattering geometry using a micro Raman LabramHR system (Horiba GmbH) equipped with a LN2 cooled CCD detector. For excitation, the frequency-doubled 532 nm emission of a Nd:YAG laser was focused to a 1 µ m spot with 1 mW power on the sample surface. Spectroscopic Ellipsometry (SE) was used to determine the refractive index nof the samples. Measurements were performed with a rotating compensator ellipsometer M-2000FI (J.A. Woollam, Inc.) in the wavelength range of 211–1688 nm with a fixed angle of polarized light incidence and reflection of 75◦. 3. Results 3.1. Thin Film Growth and Film Composition The deposition of the films was performed at three different working pressures (P w ), 0.1, 0.25, and 0.5 Pa. For each P w , the oxygen/argon ratio was varied by changing the O 2 gas flow from ~20 to ~80%. Consequently, the oxygen partial pressure (p O2 ) during deposition ranged from 0.02 to 0.41 Pa. The O/Ti ratio for the different films obtained by RBS are shown in Figure 1. RBS profiles (not shown) indicated that the composition is uniform through the thickness of the films. It was found that the samples could be divided into three main groups according to their stoichiometry. In Figure 1, it is shown that the group 1 (G1) samples have an O/Ti ratio lower than 2 (0.6 ≤ O/Ti ≤ 1.6), samples in group 2 (G2) exhibit an O/Ti close to 2 (1.9 ≤ O/Ti ≤ 2.0), and finally, samples belonging to group 3 (G3) have an O/Ti ratio higher than 2 (O/Ti ≥ 2.1). This classification of the deposited films in three different groups is further supported by the differences in crystallinity of the samples, as will be shown by XRD and Raman results in Section 3.2. G1 samples are obtained at the lowest working pressure (0.1 Pa) or at the intermediate pressure (0.25 Pa) and low oxygen partial pressure (<0.1 Pa). Stoichiometric TiO 2 samples in G2 can be obtained at 0.25 Pa working pressure provided that the p O2 is over a threshold value of~> 0.1 Pa . Finally, over stoichiometric (G3) films are obtained for the working pressure of 0.5 Pa in the entire oxygen partial pressure range analyzed. Coatings 2021,11, 233 4 of 16 Coatings 2021, 11, x FOR PEER REVIEW 4 of 18 0.25 Pa working pressure provided that the pO2 is over a threshold value of~ > 0.1 Pa. Finally, over stoichiometric (G3) films are obtained for the working pressure of 0.5 Pa in the entire oxygen partial pressure range analyzed. Figure 1. O/Ti ratio obtained from RBS for the different samples as a function of pO2. The different groups are indicated by color and legend code: group 1 (square, red), group 2 (circle, green) and group 3 (triangle, blue). Samples deposited at the same Pw are connected through lines, and the value of the working pressure is indicated in the graph. In Figure 2, a summary of the different deposited samples is depicted, indicating the three identified groups. All the samples in G1 are opaque. With respect to G2, visual inspection revealed that a slight variation in the stoichiometry of the samples (from O/Ti = 1.9 to O/Ti = 2) led to strong variation in transmittance. This can be observed by comparing G2 samples prepared at % O2 gas flow of 40% and 60%. For the former, a reduced TiO2 material is obtained (O/Ti = 1.9). TiO2 bulk reduction results in color centers, which produce a pronounced color change of TiO2 single crystals from initially transparent to visible light to eventually exhibiting a dark blue colour [2]. A similar result is observed in the current work for the reduced TiO2 sample, which exhibits a blueish color. The stoichiometric TiO2 samples prepared at 60 and 80% O2 gas flow are transparent, as expected for a wide band gap semiconductor. Finally, all G3 samples are transparent, regardless of the oxygen partial pressure used during the deposition process. Figure 1. O/Ti ratio obtained from RBS for the different samples as a function of p O2 . The different groups are indicated by color and legend code: group 1 (square, red), group 2 (circle, green) and group 3 (triangle, blue). Samples deposited at the same P w are connected through lines, and the value of the working pressure is indicated in the graph. In Figure 2, a summary of the different deposited samples is depicted, indicating the three identified groups. All the samples in G1 are opaque. With respect to G2, visual inspection revealed that a slight variation in the stoichiometry of the samples (from O/Ti = 1.9 to O/Ti = 2) led to strong variation in transmittance. This can be observed by comparing G2 samples prepared at % O 2 gas flow of 40% and 60%. For the former, a reduced TiO 2 material is obtained (O/Ti = 1.9). TiO 2 bulk reduction results in color centers, which produce a pronounced color change of TiO 2 single crystals from initially transparent to visible light to eventually exhibiting a dark blue colour [ 2 ]. A similar result is observed in the current work for the reduced TiO 2 sample, which exhibits a blueish color. The stoichiometric TiO 2 samples prepared at 60 and 80% O 2 gas flow are transparent, as expected for a wide band gap semiconductor. Finally, all G3 samples are transparent, regardless of the oxygen partial pressure used during the deposition process. In Figure 3, the thicknesses of the different samples obtained by profilometry are plotted as a function of the oxygen partial pressure, indicating also the samples that were deposited at the same working pressure. A gradient color can be observed in transparent samples due to interferences (Figure 2), which must be caused by non-homogeneity of the thickness along the substrate. For this reason, at least three measures of the thickness in different substrate positions were performed and the average and error values were obtained. It is observed that the sample thickness is affected by both, the p O2 and the P w . Higher thicknesses are obtained for lower working pressures and/or lower oxygen partial pressures. However, the working pressure is the factor that affects final thickness the most. For similar p O2 around 0.2 Pa, a sample deposited at Pw 0.25 Pa exhibits ~700 nm thickness, and one deposited at Pw 0.50 Pa is only 300 nm thick. At similar oxygen partial pressure, thickness decreases with working pressure, as the main free path of the species diminishes. Deposition rates were ~56 nm/min for G1, 45 nm/min for G2, and 24 nm/min for G3. Coatings 2021,11, 233 5 of 16 Coatings 2021, 11, x FOR PEER REVIEW 5 of 18 Figure 2. Summary table of deposited samples with varying % of Ar/O2 gas flow, and Pw including: picture of each sample deposited on glass, pO2 during deposition and elemental at.% (O/Ti ratio) obtained by RBS. The code color indicates the samples belonging to group 1 (red), 2 (green), and 3 (blue). * For the lower working pressure (0.1 Pa), it was not possible to perform the deposition process with more than 70% of oxygen gas flow. In Figure 3, the thicknesses of the different samples obtained by profilometry are plotted as a function of the oxygen partial pressure, indicating also the samples that were deposited at the same working pressure. A gradient color can be observed in transparent samples due to interferences (Figure 2), which must be caused by non-homogeneity of the thickness along the substrate. For this reason, at least three measures of the thickness in different substrate positions were performed and the average and error values were obtained. Figure 2. Summary table of deposited samples with varying % of Ar/O 2 gas flow, and P w including: picture of each sample deposited on glass, p O2 during deposition and elemental at.% (O/Ti ratio) obtained by RBS. The code color indicates the samples belonging to group 1 (red), 2 (green), and 3 (blue). * For the lower working pressure (0.1 Pa), it was not possible to perform the deposition process with more than 70% of oxygen gas flow. Coatings 2021, 11, x FOR PEER REVIEW 6 of 18 Figure 3. Coating thickness on silicon as a function of pO2 for samples belonging to group 1 (square, red), 2 (circle, green), and 3 (triangle, blue). A linear fit has been obtained for the samples deposited with the same working pressure. The total deposition time was 15 min in all cases. It is observed that the sample thickness is affected by both, the pO2 and the Pw. Higher thicknesses are obtained for lower working pressures and/or lower oxygen partial pressures. However, the working pressure is the factor that affects final thickness the most. For similar pO2 around 0.2 Pa, a sample deposited at Pw 0.25 Pa exhibits ~700 nm thickness, and one deposited at Pw 0.50 Pa is only 300 nm thick. At similar oxygen partial pressure, thickness decreases with working pressure, as the main free path of the species diminishes. Deposition rates were ~ 56 nm/min for G1, 45 nm/min for G2, and 24 nm/min for G3. Provided the film thickness is known from profilometry, the density of TiO2 films can be obtained from RBS data [15]. Samples in G2 exhibit a density ~ 4.5 g cm−3, meanwhile the density of samples in G3 is ~3.6 g cm−3. As will be shown later, these results are in line with the crystallinity of the samples. Bulk theoretical values are 4.23 g cm−3 for rutile and 3.6 g cm−3 for amorphous TiO2 [16]. Bendavid et al. [17] reported a density of 3.62 g cm−3 for TiO2 amorphous films deposited by FCVA. 3.2. Structure and Morphology Cross-sectional SEM analysis of the morphology revealed remarkable differences among the three groups. A columnar structure is observed for G1. High energy particle bombardment at low pressure can be the cause for the elongated faceted texture of the film (Figure 4 a,b). For G2, two growth regimes are observed in the SEM images. During the first stages of the deposition process, the samples grow without any type of texturized structure (up to approximately 300 nm). Subsequently, the films develop a clear columnar structure (Figure 4 c,d). This is also in line with the crystallinity observed in G2 samples, as confirmed by XRD experiments. Less energetic particle bombardment at higher pressure resulted in featureless and non-textured films, which is an indication of amorphous nature in G3 samples (Figure 4 e,f). Figure 3. Coating thickness on silicon as a function of p O2 for samples belonging to group 1 (square, red), 2 (circle, green), and 3 (triangle, blue). A linear fit has been obtained for the samples deposited with the same working pressure. The total deposition time was 15 min in all cases. Coatings 2021,11, 233 6 of 16 Provided the film thickness is known from profilometry, the density of TiO 2 films can be obtained from RBS data [ 15 ]. Samples in G2 exhibit a density ~4.5 g cm −3 , meanwhile the density of samples in G3 is ~3.6 g cm −3 . As will be shown later, these results are in line with the crystallinity of the samples. Bulk theoretical values are 4.23 g cm −3 for rutile and 3.6 g cm −3 for amorphous TiO 2 [ 16 ]. Bendavid et al. [ 17 ] reported a density of 3.62 g cm −3 for TiO2amorphous films deposited by FCVA. 3.2. Structure and Morphology Cross-sectional SEM analysis of the morphology revealed remarkable differences among the three groups. A columnar structure is observed for G1. High energy particle bombardment at low pressure can be the cause for the elongated faceted texture of the film (Figure 4a,b). For G2, two growth regimes are observed in the SEM images. During the first stages of the deposition process, the samples grow without any type of texturized structure (up to approximately 300 nm). Subsequently, the films develop a clear columnar structure (Figure 4c,d). This is also in line with the crystallinity observed in G2 samples, as confirmed by XRD experiments. Less energetic particle bombardment at higher pressure resulted in featureless and non-textured films, which is an indication of amorphous nature in G3 samples (Figure 4e,f). Coatings 2021, 11, x FOR PEER REVIEW 7 of 18 Figure 4. Representative SEM cross-section images for samples for G1 (a) and (b), sample deposited at Pw 0.1 Pa, pO2 = 0.06 Pa; G2 (c) and (d), sample deposited at Pw 0.25 Pa, pO2 = 0.20 Pa; and G3 (e) and (f), at Pw 0.5 Pa, pO2 = 0.31 Pa. The Glancing Incidence XRD (GIXRD) patterns of selected films for the different groups are represented in Figures 5 and 6. For the crystalline samples, interplanar spacing (d) and lattice constants (a and c) were calculated as an average of the main observed peaks. The crystallite sizes obtained from Scherrer’s formula [18] are also calculated for the TiO2 stoichiometric samples. The results are summarized in Table 1. In Figure 5, the diffractograms for the samples belonging to G1 are plotted. No peaks related to any TiO2 crystalline phases are observed. For samples with pO2 of 0.04, 0.05, and 0.06 Pa, despite the different stoichiometries, the peaks can be attributed to TiO (ICC card 01-077-2170) with a cubic structure, (rock-salt space group 225, Fm-3m) in all the samples. Reference crystal peaks at 2Ɵ = 36.60°, 42.87°, 62.38°, and 75.01° are associated with the crystal planes (111), (200), (220), and (311), respectively. For the sample with the strongest diffraction peaks in G1 (pO2 = 0.04), the diffraction peaks appear at slightly smaller diffraction angles (between 0.2 and 0.5°), corresponding to a larger lattice constant of the rocksalt-type sample than in the reference data set of TiO. The sample deposited at pO2 = 0.07 (O/Ti ratio of 1.5) shows diffraction peaks at 2θ = 35.00°, 40.66° and 54.21°. These peaks can be attributed to Ti2O3 (ICC card 00-010-0063), space group 167, R-3c, corresponding to the crystal planes (110), (113), and (116), respectively. Around 2θ = 62° a broad peak is observed which can be related to the contribution of crystal planes (214) and (300), related Figure 4. Representative SEM cross-section images for samples for G1 ( a , b ), sample deposited at Pw 0.1 Pa, p O2 = 0.06 Pa; G2 (c,d), sample deposited at Pw 0.25 Pa, pO2 = 0.20 Pa; and G3 (e,f), at Pw 0.5 Pa, pO2 = 0.31 Pa. Coatings 2021,11, 233 7 of 16 The Glancing Incidence XRD (GIXRD) patterns of selected films for the different groups are represented in Figures 5and 6. For the crystalline samples, interplanar spacing (d) and lattice constants (aand c) were calculated as an average of the main observed peaks. The crystallite sizes obtained from Scherrer’s formula [18] are also calculated for the TiO2 stoichiometric samples. The results are summarized in Table 1. Figure 5. GIXRD patterns for samples of G1 deposited on silicon substrates. TiO and Ti 2 O 3 reference peaks are included in the graph. Figure 6. Glancing incidence XRD patterns for samples of G2, and a representative sample of G3. Rutile-type TiO 2 reference peaks are included in the graph. Coatings 2021,11, 233 8 of 16 Table 1. Peak position, crystal planes, interplanar spacing, and lattice constants for TiO and Ti 2 O 3 reference cards and samples in group 1. pO2(Pa) Peak Position (2θ) (h k l) d (Å) a (Å) c (Å) Reference (TiO) (ICC card 01-077-2170) 37.18 43.20 62.74 75.24 (111) (200) (220) (311) 2.416 2.092 1.479 1.261 4.18 4.18 0.04 36.60 42.87 62.38 75.01 (111) (200) (220) (311) 2.45 2.10 1.48 1.26 4.25 4.21 4.21 4.19 4.25 4.21 4.21 4.19 0.05 37.71 43.72 - 76.54 (111) (200) (220) (311) 2.38 2.07 - 1.24 4.13 4.14 - 4.12 4.13 4.14 - 4.12 0.06 37.01 - 62.92 - (111) (200) (220) (311) 2.42 - 1.47 - 4.20 - 4.17 - Reference (Ti2O3) (ICC card 00-010-0063) 34.85 40.26 53.75 61.34 62.58 86.98 (110) (113) (116) (214) (300) (226) 2.57 2.23 1.70 1.51 1.48 1.11 5.13 13.65 0.07 35.00 40.66 54.21 61.23 63.40 87.69 (110) (113) (116) (214) (300) (226) 2.53 2.21 1.69 1.51 1.46 1.11 -- In Figure 5, the diffractograms for the samples belonging to G1 are plotted. No peaks related to any TiO 2 crystalline phases are observed. For samples with p O2 of 0.04, 0.05, and 0.06 Pa, despite the different stoichiometries, the peaks can be attributed to TiO (ICC card 01-077-2170) with a cubic structure, (rock-salt space group 225, Fm-3m) in all the samples. Reference crystal peaks at 2 θ = 36.60 ◦ , 42.87 ◦ , 62.38 ◦ , and 75.01 ◦ are associated with the crystal planes (111), (200), (220), and (311), respectively. For the sample with the strongest diffraction peaks in G1 (p O2 = 0.04), the diffraction peaks appear at slightly smaller diffraction angles (between 0.2 and 0.5 ◦ ), corresponding to a larger lattice constant of the rocksalt-type sample than in the reference data set of TiO. The sample deposited at p O2 = 0.07 (O/Ti ratio of 1.5) shows diffraction peaks at 2 θ = 35.00 ◦ , 40.66 ◦ and 54.21 ◦ . These peaks can be attributed to Ti 2 O 3 (ICC card 00-010-0063), space group 167, R-3c, corresponding to the crystal planes (110), (113), and (116), respectively. Around 2 θ = 62 ◦ a broad peak is observed which can be related to the contribution of crystal planes (214) and (300), related to peaks at 61.34 ◦ and 62.58 ◦ in the reference card (see Table 1). In general, the peaks are very broad, except for the one at 2 θ = 35.00 ◦ . Parker et al [ 19 ] reported on the formation of the Ti 2 O 3 layer by bombardment with Kr ions on TiO 2 . A high dose ion impact led to preferential oxygen sputtering and/or internal precipitation of oxygen. Ion energy is higher in cathodic arc vacuum deposition, so, in the current work, the Ti 2 O 3 phase could be formed at low working pressures (where high ion energy of the ions is expected) and with a high oxygen partial pressure. A completely different scenario is observed for the three samples belonging to group 2 (Figure 6and Table 2). The diffractograms of samples belonging to this group showed a pattern corresponding to rutile TiO 2 with tetragonal structure (space group 136, P42-mnm). The main rutile peak (110) is observed at 2 θ = 27.39 ◦ , 27.38 ◦ and 27.3 ◦ for samples with p O2 of 0.10, 0.16, and 0.20 Pa, respectively. Hence these peaks are slightly shifted to higher Coatings 2021,11, 233 9 of 16 angles with respect to the reference pattern (2 θ = 27.29 ◦ ) for this crystal plane (110). The other main peaks are observed at 2 θ ~54 ◦ and 56 ◦ , related to crystal planes (211) and (220). These three main peaks have been selected to calculate the lattice parameter (aand c) and the crystallite size. Other minor peaks confirming the rutile phase observed at 2 θ ~38.97, 40.97, 43.80, and 68.56 ◦ are associated to the crystal planes (200), (111), (210), and (301), respectively, according to the reference card. Table 2. Peak position, crystal planes, interplanar spacing, and lattice constants for TiO 2 rutile reference cards and for samples in group 2. pO2 (Pa) Peak Position (2θ)(hkl)d(Å) a(Å) c(Å) Crystallite Size (nm) Average Reference Rutile TiO2 (ICC card 01-077-0444) 27.29 53.98 56.30 (110) (211) (220) 3.265 1.697 1.632 4.61 2.97 - 0.10 27.39 54.06 56.07 (110) (211) (220) 3.252 1.694 1.639 4.60 2.99 6.60 ±0.80 0.16 27.38 54.21 56.57 (110) (211) (220) 3.254 1.690 1.625 4.60 2.96 5.55 ±0.99 0.20 27.33 54.04 56.33 (110) (211) (220) 3.254 1.690 1.625 4.61 2.98 6.00 ±0.12 Finally, all group 3 samples showed only few broad and weak diffraction peaks, pointing to a predominantly X-ray amorphous phase structure with few nanocrystalline regions. See for example the diffractogram in Figure 6corresponding to samples deposited at 0.31 Pa of p O2 . This phase structure can be related to the higher working pressure, and therefore to the lower energy of species impinging on the substrate. Only a small and very wide peak can be inferred at angles corresponding to the crystal plane (110) of the rutile phase. The rutile (110) surface is the most stable crystal face [ 2 ]. In the case of vapor deposition, the different crystallographic planes grow at different rates depending on their surface formation energies. The slight crystallization observed for films in G3 in the rutile phase for the plane (110) can be attributed to the lower surface formation energy of this plane [ 4 ]. In relation to the SEM results, this can be an indication of a change in the growth regime that could lead to crystalline structures. Raman spectroscopy confirmed and complemented the data obtained by X-ray diffraction. For G1 samples grown at the oxygen partial pressures of 0.02 Pa ≤ p O2 < 0.06 Pa, the Raman spectra exhibit two broad band-like features centered at around 250 cm −1 and at around 550 cm −1 (Figure 7). The corresponding line widths of 200 to 300 cm −1 are typical for phonon density of states-like Raman spectra of amorphous materials. It is tempting to attribute the low-energy band (50 to 350 cm −1 ) to titanium-based phonon states, and the high-energy band (400 to 700 cm −1 ) to oxygen phonon-based states. Thus, the Raman spectra indicate a significant amount of amorphous structures, in addition to the fraction of rock salt-type TiO detected by XRD. Since crystalline TiO is Raman-silent, the combination of XRD and Raman data point to the coexistence of nanocrystalline and amorphous TiO in these four samples of G1. The Raman spectrum of the sample grown at an oxygen pressure of 0.07 Pa shows, in addition to broad lines at the same positions as those of the other four G1 samples, several narrower features (Figure 7). The most prominent ones among them, located at 263 cm −1 and 334 cm −1 , can be assigned to the most prominent lines reported for Ti 2 O 3 at 269 cm −1 and 347 cm −1 (vertical bars in Figure 7) [ 20 ]. The observation of these lines supports the finding of Ti 2 O 3 by XRD. Moreover, the Raman spectra reveal a significant amorphous phase fraction within the film grown at an oxygen pressure of 0.07 Pa at a working pressure of 0.10 Pa. Coatings 2021,11, 233 16 of 16 29. Zhang, F.; Wang, X.; Li, C.; Wang, H.; Chen, L.; Liu, X. Rutile-Type Titanium Oxide Films Synthesized by Filtered Arc Deposition. Surf. Coat. Technol. 1998,110, 136–139. [CrossRef] 30. Bendavid, A.; Martin, P.; Jamting, Å.; Takikawa, H. Structural and Optical Properties of Titanium Oxide Thin Films Deposited by Filtered Arc Deposition. Thin Solid Films 1999,355–356, 6–11. [CrossRef] 31. Martin, P.; Bendavid, A.; Netterfield, R.; Kinder, T.; Jahan, F.; Smith, G. Plasma Deposition of Tribological and Optical Thin Film Materials With a Filtered Cathodic Arc Source. Surf. Coat. Technol. 1999,112, 257–260. [CrossRef] 32. Takikawa, H.; Matsui, T.; Sakakibara, T.; Bendavid, A.; Martin, P.J. Properties of Titanium Oxide Film Prepared by Reactive Cathodic Vacuum Arc Deposition. Thin Solid Films 1999,348, 145–151. [CrossRef] 33. Zhao, Z.; Tay, B.K.; Lau, S.P.; Yu, G. Optical Properties of Titania Films Prepared by off-Plane Filtered Cathodic Vacuum Arc. J. Cryst. Growth 2004,268, 543–546. [CrossRef] 34. Huang, A.; Chu, P.K.; Wang, L.; Cheung, W.; Xu, J.; Wong, S. Fabrication of Rutile TiO 2 Thin Films by Low-Temperature, Bias-Assisted Cathodic Arc Deposition and Their Dielectric Properties. J. Mater. Res. 2006,21, 844–850. [CrossRef] 35. Kleiman, A.; Márquez, A.; Lamas, D.G. Optimization of a DC Vacuum Arc to Obtain Anatase Phase TiO 2 Coatings. Fourth Huntsville Gamma-Ray Burst Symp. 2006,875, 223–226. [CrossRef] 36. Kleiman, A.; Marquez, A.; Lamas, D. Anatase TiO 2 Films Obtained by Cathodic Arc Deposition. Surf. Coat. Technol. 2007 , 201, 6358–6362. [CrossRef] 37. Zhang, M.; Lin, G.; Dong, C.; Wen, L. Amorphous TiO 2 Films With High Refractive Index Deposited by Pulsed Bias Arc Ion Plating. Surf. Coat. Technol. 2007,201, 7252–7258. [CrossRef] 38. Zhao, S. Spectrally Selective Solar Absorbing Coatings Prepared by Dc Magnetron Sputtering. Ph.D. Dissertation, 2007. Available online: http://uu.diva-portal.org/smash/get/diva2:169700/FULLTEXT01.pdf (accessed on 31 December 2020). 39. Bendavid, A.; Martin, P.; Preston, E. The Effect of Pulsed Direct Current Substrate Bias on the Properties of Titanium Dioxide Thin Films Deposited by Filtered Cathodic Vacuum Arc Deposition. Thin Solid Films 2008,517, 494–499. [CrossRef] 40. Paternoster, C.; Zhirkov, I.; Delplancke-Ogletree, M.-P. Structural and Mechanical Characterization of Nanostructured Titanium Oxide Thin Films Deposited by Filtered Cathodic Vacuum Arc. Surf. Coat. Technol. 2013,227, 42–47. [CrossRef] 41. Gjevori, A.; Gerlach, J.; Manova, D.; Assmann, W.; Valcheva, E.; Mändl, S. Influence of Auxiliary Plasma Source and Ion Bombardment on Growth of TiO2Thin Films. Surf. Coat. Technol. 2011,205, S232–S234. [CrossRef] 42. Šícha, J.; Musil, J.; Meissner, M.; ˇ Cerstvý, R. Nanostructure of Photocatalytic TiO 2 Films Sputtered at Temperatures below 200 ◦ C. Appl. Surf. Sci. 2008,254, 3793–3800. [CrossRef]