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Article Effect of the Incorporation of on the Optical Properties of ZnO Thin Films: From Doping to Mixed Formation

Miriam Yuste 1, Ramon Escobar-Galindo 1,2 , Noelia Benito 3,4, Carlos Palacio 4, Oscar Martínez 5, Jose Maria Albella 1 and Olga Sánchez 1,*

1 Instituto de Ciencia de Materiales de Madrid (CSIC), 28049 Madrid, Spain; [email protected] (M.Y.); [email protected] (R.E.-G.); [email protected] (J.M.A.) 2 Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, IMEYMAT, Universidad de Cádiz, 11510 Puerto Real, Spain 3 Dept. de Física, Universidad de Concepción, Casilla 160-C, Concepción 4030000, Chile; [email protected] 4 Dept. de Física Aplicada, Universidad Autónoma de Madrid, 28049 Madrid, Spain; [email protected] 5 UGdS-Optronlab Group, Dpto. Física Materia Condensada, Universidad de Valladolid, Paseo de Belén 11, 47011 Valladolid, Spain; [email protected] * Correspondence: [email protected]

 Received: 17 January 2019; Accepted: 5 March 2019; Published: 8 March 2019 

Abstract: ZnO films with Ti atoms incorporated (TZO) in a wide range (0–18 at.%) have been grown by reactive co-sputtering on silicon and glass substrates. The influence of the titanium incorporation in the ZnO matrix on the structural and optical characteristics of the samples has been determined by Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicate that the samples with low Ti content (<4 at.%) exhibit a wurtzite-like structure, with the Ti4+ ions substitutionally incorporated into the ZnO structure, forming Ti-doped ZnO films. In particular, a very low concentration of Ti (<0.9 at.%) leads to a significant increase of the crystallinity of the TZO samples. Higher Ti contents give rise to a progressive amorphization of the wurtzite-like structure, so samples with high Ti content (≥18 at.%) display an amorphous structure, indicating in the XPS analysis, a predominance of Ti–O–Zn mixed . The energy gap obtained from absorption spectrophotometry increases from 3.2 eV for pure ZnO films to 3.6 eV for those with the highest Ti content. Ti incorporation in the ZnO samples <0.9 at.% raises both the blue (380 nm) and green (approx. 550 nm) bands of the photoluminescence (PL) emission, thereby indicating a significant improvement of the PL efficiency of the samples.

Keywords: Ti doped ZnO; thin film; co-sputtering; UV-visible

1. Introduction oxide (ZnO) is a semiconductor material characterized by a broadband gap (approx. 3.2–3.3 eV), a high transmission in the visible spectral range, a native n-type conductivity as well as by its excellent photoluminescence (PL) properties, which can be modified by the incorporation of impurities in its structure [1,2]. The PL emission of ZnO films presents essentially two peaks, in the UV and in the visible spectrum, at approximately 380 nm and in the 450–730 nm range, respectively. It is well-known that the degree of crystallinity of the films has a strong influence on the UV emission efficiency [3,4]. On the other hand, inherent defects in the material, such as zinc (Zn) or oxygen (O) interstitials or vacancies controlled by the method and growth conditions, determine the presence of PL emissions in the visible range [2].

Coatings 2019, 9, 180; doi:10.3390/coatings9030180 www.mdpi.com/journal/coatings Coatings 2019, 9, 180 2 of 12

A subject that currently interests numerous researchers is the improvement of the optical and electrical behaviour of ZnO films with the incorporation of metals in its structure (Al, Ga, In, Ti, etc.), demanded from the point of view of new potential applications: transparent conductive contacts (TCOs), laser diodes including UV light emitting diodes, thin film transistors, mobile phones, microwave dielectrics, etc. [5–8]. Actually, n-type doping controlled by the incorporation of certain elements in the ZnO network is a recurrent method for tailoring both the bandwidth energy and electrical conductivity by increasing the concentration of carriers while keeping a high transparency in the visible range [9]. Most of previous work on doped ZnO films is related to doping with group III elements (Al, Ga and In), while most recently, new studies have been conducted on quadrivalent dopants like titanium, which can provide two free electrons per atom to improve the conductivity of the ZnO host and to modify the photoresponse. Furthermore, ZnO ceramic materials with a high Ti content have also been searched because of their applications in paint , gas sensors, catalytic sorbents for removal of contaminants from hot coal gases (H2S, As, etc.), photocatalytic splitting of water and degradation of organic compounds, as well as in anodes of Li-ion batteries, microwave devices and low-temperature co-fired ceramics [10,11]. For these reasons, the study of ZnO films with Ti atoms incorporated and the understanding of the Ti incorporation into the ZnO structure, either as dopant or in the form of a mixed oxide, are still under debate and constitute a very interesting challenge. To obtain Ti-incorporated ZnO (TZO) thin films, sputtering techniques have been reported in the literature [12–14]. However, in most of the reports concerning sputtering techniques, RF magnetron sputtering and ZnO targets [13] or ZnO:Ti targets [14] were used. In the present project, we propose the deposition of ZnO:Ti thin films in a wide range of Ti atomic concentrations by the means of the reactive DC magnetron co-sputtering technique and by using two separate targets of pure Ti and Zn and no intentional heating. We already used this approach in a previous work [12] for ultrathin films, and due to the excellent results, in terms of the range of %Ti that was incorporated in the films, we have applied the same methodology here. In our work [12], our group explored the structural phase and composition of TZO ultrathin films (<40 nm) using X-ray absorption spectroscopy (XANES) and Rutherford Backscattering Spectroscopy (RBS). TZO films with an increasing Ti content show a gradual transition from the typical tetrahedral coordination for titanium atoms at lower contents (<3 at.%) to the octahedral coordination for higher contents (up to 22 at.%) with a hexagonal ZnTiO3-like structure. In order to further investigate the effect of Ti addition on the properties of sputtered ZnO films, we present, in this work, the structural and optical characterization of thicker TZO films (approx. 200 nm) and their evolution with the titanium content. The crystalline structure determined by low angle X-ray diffraction, observing how the highly oriented deposits grown with a preferential (002) direction of the original w-ZnO structure, is affected by the incorporation of titanium. The composition of the films has been obtained from RBS, while the oxidation state of incorporated Ti atoms to the ZnO structure has been examined by X-ray photoelectron spectroscopy (XPS). The surface morphology of the samples has been observed by scanning electron microscopy (SEM). In addition, the variation in the optical properties (transmittance and optical band gap) with the Ti content has been measured by ultraviolet-visible (UV-Vis) spectrophotometry, observing an improvement in the transparency at low wavelengths when titanium is substitutionally incorporated in the ZnO lattice. The presence of defects (excitons) has been also analysed by photoluminescence (PL) measurements.

2. Materials and Methods TZO films have been deposited on both (100)-oriented silicon samples (resistivity > 6000 Ω·cm) and Corning glass slides, by reactive magnetron co-sputtering in an argon atmosphere with 20% oxygen and a total constant gas flow of 30 sccm. Two independent cathodes (Ti and Zn) fed with different DC power were used to control the Ti/Zn relative content in the films. All the samples were prepared after the complete poisoning of both cathodes. No intentional heating was applied during Coatings 2019, 9, 180 3 of 12 deposition, though the measured substrate temperature was approx. 90 ◦C. Table1 shows the power applied in the Zn and Ti cathode sources (PZn and PTi, respectively). The deposition time was adjusted to obtain in all samples a thickness of 200 nm except in the sample deposited at the lowest PZn power (Zn/Ti-10/100) due to the low sputtering rate of Ti (approx. 4 times lower than Zn). The thickness of the layers was determined by profilometry (Veeco, model Dektak 150, Plainview, NY, USA).

Table 1. The deposition conditions, composition and density of the samples. The error in the composition was estimated at 1%.

Thickness Composition (at.%) Density Sample PZn (W) PTi (W) 3 (nm) Ti Zn O (g/cm ) ZnO 100 0 199 ± 5 – 48 52 5.6 Zn/Ti-100/10 100 10 197 ± 5 0.2 45.9 53.9 4.6 Zn/Ti-100/60 100 60 200 ± 9 0.9 44.9 54.3 4.5 Zn/Ti-100/100 100 100 200 ± 7 1.2 43.5 55.3 4.2 Zn/Ti-60/100 60 100 207 ± 3 4.1 40.8 55.1 4.8 Zn/Ti-10/100 10 100 57 ± 2 18.2 17.7 64.1 3.6 TiO2 0 100 18 ± 2 30.8 – 69.2 3.8

The chemical composition of the films was obtained by RBS with the 5 MV HVEE Tandetron accelerator at the Autonomous University of Madrid. These experiments were performed using He+ ions of 3.035 MeV, corresponding to the resonance of oxygen atoms and thus improving the oxygen atoms sensitivity. The chemical composition values have been extracted using the RBX software [15]. The crystalline structure of the TZO films with different Ti contents was identified by X-ray diffraction with the Bragg–Brentano configuration (Siemens D 5000, Berlin, Germany, with Cu anode, Kα radiation = 1.54178 Å). A low incidence angle (0.7◦) was used to obtain the preferred orientation of the crystals. An XPS analysis were carried out in an ultrahigh vacuum system (residual pressure approx. 8 × 10−8 Pa) equipped with a hemispheric analyser (SPECS Phoibos 100 MCD-5, Berlin, Germany). The pass energy was 9 eV, giving a constant resolution of 0.9 eV. A twin anode (Al–Mg) X-ray source was operated at a constant power of 300 W, using the Mg Kα (1253.6 eV) radiation. The Au 4f 7/2, Ag 3d5/2 and Cu 2p3/2 lines at 84.0, 368.3 and 932.7 eV, respectively, of the reference samples were chosen for the calibration of the binding energy (BE). The possible charging effects produced during XPS measurements were corrected using as reference the BE of the C 1s peak (285 eV) due to the adventitious C. The optical characteristics (transmittance) and energy gap, Eg, have been obtained from optical spectrophotometry in the wavelength range from 300 to 2000 nm using a SolidSpec-3700 spectrophotometer (Shimadzu Corp., Kyoto, Japan). In order to assess the effect of Ti addition in the ZnO matrix, PL measurements were also carried out using a Labram system, model HR800 UV, provided with a He–Cd laser (325 nm) and a diffraction grating of 150 lines/mm with 40× lenses. SEM images of the surface morphology of ZnO films with different Ti contents were obtained with a Philips XL30 SFEG system (Amsterdam, The Netherlands).

3. Results and Discussion

3.1. Film Composition and RBS Analysis Figure1 shows both the experimental and the fitted RBS spectra of the series of TZO samples, including pure ZnO and TiO2 reference films. A zoom (×20) of the energy zone between 2000 and 2500 keV is also attached to the spectra to better observe the metal signal. An excellent fitting of the experimental spectra was obtained by assuming a single layer of Zn–Ti–O on the silicon substrate. The results indicate a good quality of the substrate-film interface (within an estimated resolution Coatings 2019, 9, 180 4 of 12 about 5 nm), thus confirming that the composition of the layer is very homogeneous throughout the wholeCoatings thickness. 2019,, 9,, xx FORFOR PEERPEER REVIEWREVIEW 4 of 11

FigureFigure 1.1. TheThe RutherfordRutherford backscatteringbackscattering spectroscopyspectroscopy (RBS)(RBS) spectraspectra (experimental(experimental andand simulated)simulated) ofof ZnOZnO filmsfilms withwith anan increasingincreasing TiTi content content (from (from below below to toto above): above):above): A AA zoom zoomzoom ( ×(×20)(×20)20) isis includedincluded forforfor thethethe highhighhigh energyenergy regionregion toto magnifymagnify thethe signalsignal ofof ZnZn andand Ti.Ti.

TheThe densitydensity ofof thethe filmsfilms (Table(Table1 1)) was was calculated calculated from from the the RBS RBS curves curves using using the the film film thicknesses thicknesses obtainedobtained byby profilometry.profilometry. ItIt decreasesdecreases asas thethe TiTi incorporationincorporation increasesincreases fromfrom thethe referencereference valuevalue forfor 33 3 3 ZnOZnO (5.6(5.6 g/cm 3) )to to the the density density of of the the pure pure TiO TiO2 samplesample2 sample (3.8(3.8 (3.8 g/cmg/cm g/cm3). This). is This a consequence is a consequence of the oflower the loweratomic atomic mass massof Ti of(22 Ti amu) (22 amu) as compared as compared to Zn to Zn(30 (30 amu), amu), as as far far as as the the titanium titanium is incorporatedincorporated substitutionallysubstitutionally inin thethe ZnOZnO structure,structure, asas discusseddiscussed below.below. FigureFigure2 2gives gives the the variation variation of theof compositionthe composition of the of TZO the filmsTZO asfilms a function as a function of the power of the applied power toapplied one of to the one cathodes of the cathodes (either Zn (either or Ti), Zn keeping or Ti), constantkeeping constant the other the at 100other W. at According 100 W. According to the data to inthe Table data1 in, in Table the samples 1, in the deposited samples deposit with aed fixed with power a fixed in power the Ti cathode,in the Ti cathode,PTi = 100 P W,Ti == the 100100 Ti W,W, content thethe TiTi decreasescontent decreases abruptly abruptly from 30.8 from at.% 30.8 (pure at.% TiO (pure2) to 1.2TiO at.%2) to as 1.2 the at.% power as the in thepower Zn sourcein the Zn is increased. source is Conversely,increased.increased. Conversely,Conversely, in the samples inin thethe deposited samplessamples withdepositedeposite a constantd with powera constant of the power Zn cathode, of the ZnP Zncathode,= 100 W,PZn the == 100100 Ti contentW, the Ti takes content lower takes values lower from values 0 (pure from ZnO 0 sample)(pure ZnO to 1.2sample) at.%. Apartto 1.2 at.%. from theApart other from factors, the other this effectfactors, is this a consequence effect is a consequence of the larger of sputtering the larger yieldsputtering of Zn yield as compared of Zn as withcompared that of with Ti. Thus,that of the Ti. useThus, of the two use independent of two independent sputtering sputtering sources allows source as wideallows range a wide of compositionsrange of compositions in the TZO in the films TZO to befilms obtained. to be obtained.

FigureFigure 2.2. AA variationvariation ofof thethe TiTi concentrationconcentration inin thethe filmsfilms asas aa functionfunction ofof thethe appliedapplied powerpower toto thethe cathodescathodes (Zn(Zn andand Ti)Ti) forfor aa fixedfixed power power of of Ti Ti (upper (upper curve) curve) and and Zn Zn (lower (lower curve), curve),curve), respectively. respectively.respectively.

3.2. Crystalline Structure Figure 3 shows the X-ray spectra obtained for the samples disclosed in Table 1. As can be observed, a pure ZnO sample grown as a reference presents the typical polycrystalline hexagonal wurtzite structure with a (002) preferential orientation. The samples deposited with the lowest power applied to the Ti cathode (10 W), resulting in a Ti concentration of 0.2 at.%, show a peak centred in

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3.2. Crystalline Structure Figure3 shows the X-ray spectra obtained for the samples disclosed in Table1. As can be Coatingsobserved, 2019, a 9, pure x FOR ZnO PEER sampleREVIEW grown as a reference presents the typical polycrystalline hexagonal5 of 11 wurtzite structure with a (002) preferential orientation. The samples deposited with the lowest power theapplied position to the corresponding Ti cathode (10 to W), the resultingplane (002) in aof Ti the concentration ZnO but with of 0.2much at.%, higher show intensity a peak centred than that in obtainedthe position for correspondingthe reference ZnO to thesample plane (note (002) that of thethe ZnOintensity but withof the much spectrum higher is divided intensity by than a factor that ofobtained 3). In foraddition, the reference no TiO ZnO2 peaks sample were (note detected. that the intensityThese results of the spectrumindicate isthat divided the Ti by atoms a factor are of incorporated3). In addition, as no a dopant TiO2 peaks (electron were donor) detected. in the These wurt resultszite structure, indicate thatsubstituting the Ti atoms the zinc are incorporatedatoms in the ZnOas a dopantlattice (electronand significantly donor) in theimproving wurtzite structure,the crystallinity substituting of the zincdoped atoms ZnO in thesample. ZnO latticeThis interpretationand significantly is consistent improving with the crystallinity the smaller of radius the doped of Ti4+ ZnO (0.068 sample. nm) as This compared interpretation to the isZn consistent2+ radius with(0.074 the nm) smaller [7,16]. radius of Ti4+ (0.068 nm) as compared to the Zn2+ radius (0.074 nm) [7,16].

) 1) ) 2 0 3) 03 00 (1 10 (1 ( O ( x4 nO n O nO Z Z Zn Z 18.2 at.%Ti

4.1 at.%Ti

1.2 at.% Ti

0.9 at.% Ti Intensity(a.u.) 0.2 at.% Ti /3

ZnO

20 30 40 50 60 70 80 (ο) 2θ Figure 3. The X-ray X-ray spectra spectra of of Ti-incorporated Ti-incorporated ZnO (TZO) fi filmslms with different Ti content. (Notice that the intensity of the sample with a Ti content at 0.2 at.% is divided by 3, whereas the sample with 18.2 at.% Ti is multiplied by 4.).

However, when the titanium concentration increa increasesses above 1.2 at.%, the intensity of of the (002) peak decreases and widens, suggesting a progressive amorphization of the sample [[16].16]. This is more apparent in the TZO samples with 4.1 at.% Ti, where the ZnO peaks suffer a shift towards lower diffraction angles. This fact has been explained by the compressive stress generated in the direction parallel to the surface, which induces an increase in the interplanar spacing, eventually leading to the peak displacementdisplacement atat lower lower angles, angles, as as detected detected in in Figure Figure3[ 163 [16,17].,17]. However, However, the the presence presence of some of some zinc zinctitanate titanate crystallites crystallites cannot cannot be discarded be discarded [18]. [18]. The lossloss ofof crystallinitycrystallinity when when the the amount amount of of Ti atomsTi atoms increases increases above above a certain a certain level level has alsohas beenalso beeninterpreted interpreted as a result as a result of the of intense the intense flux offlux sputtered of sputtered Ti atoms Ti atoms on the on sample the sample surface surface during during the film the filmgrowth growth [17]. Thus,[17]. Thus, the film the with film the with highest the Tihighest concentration Ti concentration (18.2 at.%) (18.2 is completely at.%) is amorphous,completely amorphous,not showing not any showing crystalline any planes crystalline associated planes with associated ZnO. However, with ZnO. these However, amorphous these samples amorphous could sampleskeep a certain could atomickeep a bondingcertain atomic structure bonding with anstructure octahedral with coordination an octahedral typical coordination of titanate typical phases of (ilmenite),titanate phases as we (ilmenite), previously as reported we previously in Caretti reported et al. [in12 ]Caretti for ultrathin et al. [12] TZO for samples ultrathin (<40 TZO nm) samples with a (<40high nm) Ti content with a (>18high at.%). Ti content (>18 at.%). 3.3. XPS Analysis 3.3. XPS Analysis XPS measurements were performed on selected TZO films to obtain a qualitative analysis of the XPS measurements were performed on selected TZO films to obtain a qualitative analysis of the chemical oxidation state of the elements in the compound. Figure4 depicts detailed windows for the chemical oxidation state of the elements in the compound. Figure 4 depicts detailed windows for the Zn 2p3/2, O 1s and Ti 2p levels of the analysed samples: pure ZnO and TiO2 reference films, along Zn 2p3/2, O 1s and Ti 2p levels of the analysed samples: pure ZnO and TiO2 reference films, along with with samples with low (0.9 at.%) and high Ti contents (≥18 at.%). As can be observed in Figure4a, the samples with low (0.9 at.%) and high Ti contents (≥18 at.%). As can be observed in Figure 4a, the Zn Zn 2p3/2 spectrum of the ZnO films displays a peak at 1022.5 eV (vertical dotted line) characteristic 2p3/2 spectrum of the ZnO films displays a peak at 1022.5 eV (vertical dotted line) characteristic for ZnO bonds, which slightly shifts (approx. 0.2 eV) to the high BE side once Ti is detected in the network, implying that the nature of the link between Zn and O atoms has been slightly modified. Similarly, Figure 4b shows the Ti 2p doublet for the TiO2 reference film (upper part) with a main peak at 459.1 eV and a spin-orbit splitting of approx. 5.7 eV that can be attributed to Ti–O bonds [19]. Those peaks shift approx. 0.2 eV to the high BE side with decreasing Ti contents in the films. In

Coatings 2019, 9, 180 6 of 12 for ZnO bonds, which slightly shifts (approx. 0.2 eV) to the high BE side once Ti is detected in the network, implying that the nature of the link between Zn and O atoms has been slightly modified. Similarly, Figure4b shows the Ti 2 p doublet for the TiO2 reference film (upper part) with a main peakCoatings at 2019 459.1, 9, eVx FOR and PEER a spin-orbit REVIEW splitting of approx. 5.7 eV that can be attributed to Ti–O bonds6 of [19 11]. Those peaks shift approx. 0.2 eV to the high BE side with decreasing Ti contents in the films. In addition, theaddition, signals the of signals Ti for the of TZOTi for samples the TZO are samples detected are with detected a very with low a intensity very low for intensity both levels, for both 2p3/2 levels,and 22pp1/23/2 and, characteristic 2p1/2, characteristic of the doublet of the TiOdoublet2. TiO2. AsAs forfor thethe OO 11ss peakpeak ofof thethe ZnOZnO referencereference film,film, FigureFigure4 4cc displays displays a a main main peak peak at at 531.2 531.2 eV eV that that cancan bebe attributedattributed toto Zn–OZn–O bondsbonds andand aa shouldershoulder atat approx.approx. 1.61.6 eVeV aboveabove thethe mainmain peakpeak thatthat hashas beenbeen attributedattributed to oxygen-defect oxygen-defect sites sites re relatedlated to to oxygen oxygen vacancies vacancies in inthe the matr matrixix [18]. [18 It]. is It worth is worth noting noting that thatthe shoulder the shoulder disappears disappears and that and thatthe main the main peak peak shifts shifts to 530.6 to 530.6 eV with eV withincreasing increasing Ti contents. Ti contents. The Thegradual gradual shift shift of the of theTi 2 Tip 2doubletp doublet to tothe the high high BE BE side side and and the the fact fact that that the the O O 1 1ss BE is somewheresomewhere betweenbetween thosethose ofof TiOTiO22 and ZnO indicates that Ti–O bondsbonds areare moremore ionicionic thanthan inin purepure TiOTiO22,, pointingpointing toto thethe formationformation ofof Ti–O–ZnTi–O–Zn mixedmixed oxidesoxides insteadinstead ofof thethe formationformation ofof singlesingle oxideoxide phasesphases asas reportedreported beforebefore forfor thethe Ti–O–SiTi–O–Si systemsystem [[20,21].20,21].

FigureFigure 4. SpectraSpectra of of Zn Zn 2 2pp (a(a),), Ti Ti 2p 2 p(b()b and) and O O1s 1(sc)( cof) ofthe the TZO TZO films films with with low low (approx. (approx. 0.9 at.%) 0.9 at.%) and ≥ andhigh high ( 18 (at.%)≥18 at.%) Ti content, Ti content, including including the reference the reference ZnO ZnOand TiO and2 TiOsamples:2 samples: The vertical The vertical dotted dotted lines linesrepresent represent the position the position of the ofreference the reference energy energy of the ZnO of the (a), ZnO TiO (2 a(),b), TiO and2 (O–Znb), and and O–Zn O–Ti and (c) chemical O–Ti (c) chemicalbonds. bonds.

3.4.3.4. SEM Observations FigureFigure5 a5a shows shows the the SEM SEM image image of of the the surface surface of of as-deposited as-deposited pure pure ZnO ZnO films, films, made made of of inhomogeneousinhomogeneous polycrystallinepolycrystalline grains, grains, according according to to the the XRD XRD results results (Figure (Figure3). 3). The The incorporation incorporation of of a smalla small amount amount of Tiof (0.2 Ti at.%)(0.2 at.%) in the in ZnO the structure ZnO structure (Figure5 b)(Figure does not5b) producedoes not significant produce differencessignificant indifferences the morphology in the ofmorphology the sample withof the respect sample to thewith initial respect ZnO films,to the though initial theZnO distribution films, though of grains the isdistribution more homogeneous of grains is all more over homogeneous the surface. Thisall over confirms the surface. that the This low confirms doping ofthat ZnO the induceslow doping the filmof ZnO growth induces with the more film aligned growth and with ordered more crystalline aligned and structures ordered than crystalline that of purestructures ZnO films.than that As the of concentrationpure ZnO films. of TiAs in the the concentration sample increases of Ti (4.1 in the at.%), sample seen increases in Figure 5(4.1c, the at.%), grain seen size in decreases Figure 5c, and, the mostgrain notably,size decreases in the case and, of most films notabl with 18.2y, in at.%the case of Ti of (Figure films 5withd), where 18.2 at.% the grainof Ti (Figure boundaries 5d), arewhere barely the appreciated.grain boundaries The are surface barely morphology appreciated. in The these surface high Timorphology coatings is in close these to high that Ti of coatings those observed is close into as-deposited TiO films (Figure5e) that are fully amorphous [ 22,23], which is in agreement to the that of those observed2 in as-deposited TiO2 films (Figure 5e) that are fully amorphous [22,23], which findingsis in agreement of Mullerova to the etfindings al. [18]. of Mullerova et al. [18].

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addition, the signals of Ti for the TZO samples are detected with a very low intensity for both levels, 2p3/2 and 2p1/2, characteristic of the doublet TiO2. As for the O 1s peak of the ZnO reference film, Figure 4c displays a main peak at 531.2 eV that can be attributed to Zn–O bonds and a shoulder at approx. 1.6 eV above the main peak that has been attributed to oxygen-defect sites related to oxygen vacancies in the matrix [18]. It is worth noting that the shoulder disappears and that the main peak shifts to 530.6 eV with increasing Ti contents. The gradual shift of the Ti 2p doublet to the high BE side and the fact that the O 1s BE is somewhere between those of TiO2 and ZnO indicates that Ti–O bonds are more ionic than in pure TiO2, pointing to the formation of Ti–O–Zn mixed oxides instead of the formation of single oxide phases as reported before for the Ti–O–Si system [20,21].

Figure 4. Spectra of Zn 2p (a), Ti 2p (b) and O 1s (c) of the TZO films with low (approx. 0.9 at.%) and

high (≥18 at.%) Ti content, including the reference ZnO and TiO2 samples: The vertical dotted lines

represent the position of the reference energy of the ZnO (a), TiO2 (b), and O–Zn and O–Ti (c) chemical bonds.

3.4. SEM Observations Figure 5a shows the SEM image of the surface of as-deposited pure ZnO films, made of inhomogeneous polycrystalline grains, according to the XRD results (Figure 3). The incorporation of a small amount of Ti (0.2 at.%) in the ZnO structure (Figure 5b) does not produce significant differences in the morphology of the sample with respect to the initial ZnO films, though the distribution of grains is more homogeneous all over the surface. This confirms that the low doping of ZnO induces the film growth with more aligned and ordered crystalline structures than that of pure ZnO films. As the concentration of Ti in the sample increases (4.1 at.%), seen in Figure 5c, the grain size decreases and, most notably, in the case of films with 18.2 at.% of Ti (Figure 5d), where the grain boundaries are barely appreciated. The surface morphology in these high Ti coatings is close to Coatingsthat2019 of, 9those, 180 observed in as-deposited TiO2 films (Figure 5e) that are fully amorphous [22,23], which7 of 12 is in agreement to the findings of Mullerova et al. [18].

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FigureFigure 5. SEM 5. SEM images images of theof the surface surface of of the the ZnOZnO samples with with different different contents contents of Ti of (b Ti–d (b), –includingd), including the reference samples of ZnO (a) and TiO2 (e). the reference samples of ZnO (a) and TiO2 (e).

SummarizingSummarizing these these observations, observations, alongalong withwith the structure structure and and composition composition data data obtained obtained by by RBS, XRD, XPS and SEM, including previous XANES results [12], one can classify the sputtered TZO RBS, XRD, XPS and SEM, including previous XANES results [12], one can classify the sputtered TZO coatings in three compositional groups with different properties: coatings in three compositional groups with different properties: • Films with a low Ti content (<0.9 at.%), exhibit a highly crystalline wurtzite-like structure, where • Filmsthe with Ti4+ aions low are Ti incorporated content (<0.9 into at.%), the ZnO exhibit structure a highly by substituting crystalline the wurtzite-like Zn atoms, thus structure, forming where the Ticrystalline4+ ions are Ti-doped incorporated ZnO films. into the ZnO structure by substituting the Zn atoms, thus forming crystalline• TZO films Ti-doped with a ZnOmedium films. Ti content (up to 4.1 at.%) show a wurtzite structure which becomes • TZOamorphous films with with a medium increasing Ti contentTi content. (up to 4.1 at.%) show a wurtzite structure which becomes • amorphousTZO films with with increasing a high Ti Ti content content. (18.2 at.%) display a fully amorphous structure with a predominance of the Ti–O–Zn oxides (probably ilmenite type) and a minor content of wurtzite. • TZO films with a high Ti content (18.2 at.%) display a fully amorphous structure with a predominance3.5. Optical Properties of the Ti–O–Zn oxides (probably ilmenite type) and a minor content of wurtzite.

3.5. OpticalFigure Properties 6 shows the transmittance spectra obtained for the TZO samples with different Ti contents. We also included the spectrum of a stoichiometric ZnO film. As can be observed, the Figuresamples6 showspresent thea high transmittance transparency spectra (approx. obtained 80%) in forthe thewavelength TZO samples range 400–800 with different nm, except Ti contents. for We alsothe includedlayers with the a high spectrum Ti concentration of a stoichiometric (18.2 at.%), ZnO which film. the As transmittance can be observed, of is significantly the samples lower. present a high transparencyAs stated above, (approx. this sample 80%) shows in the an wavelength amorphous range structure, 400–800 made nm, of a except mixture for of the Ti–O–Zn layers withoxides a high Ti concentrationof a very small (18.2 column at.%), size. which According the transmittance to some authors, of is the significantly presence of lower.Ti–O aggregates As stated in above, these this samplefilms shows might an reduce amorphous the transmission structure, madeof light of in a mixturethe visible of Ti–O–Znrange as a oxides consequence of a very of smallthe light column scattering losses associated to a high density of grain boundaries [16]. Diffuse transmittance size. According to some authors, the presence of Ti–O aggregates in these films might reduce the experiments would be needed to confirm this hypothesis. However, such experiments are beyond transmission of light in the visible range as a consequence of the light scattering losses associated to a the scope of this work. In any case, the absorption edge of samples with a Ti content of ≤4.1% is shifted high densityto lower ofwavelengths grain boundaries for increasing [16]. DiffuseTi concentrations, transmittance as seen experiments in the Figure would 6. be needed to confirm this hypothesis. However, such experiments are beyond the scope of this work. In any case, the absorption edge of samples with a Ti content of ≤4.1% is shifted to lower wavelengths for increasing Ti concentrations, as seen in the Figure6.

Coatings 2019, 9, 180 8 of 12 CoatingsCoatings 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1111

FigureFigure 6.6. TheThe opticaloptical transmittancetransmittance spectraspectra ofof TZOTZO samplesamplessampless withwith differentdifferent TiTi contents,contents, includingincluding thethe ZnOZnO reference reference film:film:film: TheThe arrow arrow indica indicatesindicatestes thethe direction direction inin which which the the opti opticalopticalcal absorption absorption edge edge shifts shifts when when thethe TiTi contentcontentcontent increases.increases.increases.

FromFrom thethe aboveabove results,results, thethe opticaloptiopticalcal behaviourbehaviour has has been been analysed analysed in in more more detail detail in in termsterms ofof thethe α α n n calculatedcalculated absorption absorption coefficient,coefficient, coefficient, αα,, , ofof of thethe the samples.samples. samples. AnAn An adequateadequate adequate fittingfitting fitting ofof ofthethe the ((ααhh (νν))hn ν valuesvalues) values withwith with nn == 1 n½½ = oror⁄ 2 22or allowsallows 2 allows oneone one toto identifyidentify to identify thethe the typetype type ofof electronicelectronic of electronic transitionstransitions transitions acrossacross across thethe the bandgapbandgap bandgap (either(either (either indirectindirect indirect oror 2 ordirect,direct, direct, respectively)respectively) respectively) inin inthethe the lightlight light absorptionabsorption absorption process.process. process. FigureFigure Figure 777 depictsdepicts the the ((ααhhhννν)))22 valuesvaluesvalues (Tauc(Tauc plot) plot) correspondingcorresponding to to the the bestbest fitfitfit ofof thethe aboveabove alternatives,alternativalternatives,es, i.e.,i.e., directdirect transitionstransitions ofof thethe TZOTZO films.films.films. FromFrom thesethese curves,curves,curves, it itit is isis possible possiblepossible to toto obtain obtainobtain the thethe energy energyenergy gap gapgap by extrapolation byby extrapolationextrapolation of the ofof asymptote thethe asymptoteasymptote in the in high-energyin thethe high-high- range to the energy axis. The evolution of the E with theg titanium concentration in the TZO films is energyenergy rangerange toto thethe energyenergy axis.axis. TheThe evolutionevolution gofof thethe EEg withwith thethe titaniumtitanium concentrationconcentration inin thethe TZOTZO displayedfilmsfilms isis displayeddisplayed in Figure inin8 . FigureFigure 8.8. AsAs itit cancancan be bebe appreciated, appreciated,appreciated, the thethe gap gapgap energy energyenergy increases increasesincreases monotonously monotonouslymonotonously from fromfrom 3.2 3.23.2 eV eVeV for forfor pure purepure ZnO ZnOZnO to 3.3 toto eV3.33.3 foreVeV lowforfor lowlow Ti-doped Ti-dopedTi-doped films filmsfilms with withwith the wurtzite thethe wurtzitewurtzite structure. structure.structure. This increaseThisThis increaseincrease can be cancan interpreted bebe interpretedinterpreted in terms inin terms ofterms the Moss–Bursteinofof thethe Moss–BursteinMoss–Burstein model, model, whichmodel, states whichwhich that statesstates a virtual thatthat aa shift virtualvirtual of the shiftshift band ofof gapthethe forbandband highly gapgap dopedforfor highlyhighly degenerate dopeddoped semiconductorsdegeneratedegenerate semiconductorssemiconductors [24,25]. In this [24,25].[24,25]. case, theInIn this largethis case,case, electronic ththee largelarge population electronicelectronic in thepopulapopula conductiontiontion inin the bandthe conductionconduction leads to a displacementbandband leadsleads toto ofaa displacementdisplacement the Fermi level ofof above thethe FermiFermi the bottom levellevel abab ofoveove the thethe conduction bottombottom ofof band thethe conductionconduction so that the excitedbandband soso electrons thatthat thethe needexcitedexcited a larger electronselectrons energy needneed to aa jump largerlarger to energyenergy the Fermi toto jumpjump level toto (i.e., thethe the FermiFermi highest levellevel occupied (i.e.,(i.e., thethe levels).highesthighest occupiedoccupied levels).levels). AsAs statedstated above, above, for for a a larger larger TiTi contentcontent inin thethe films,films,films, thethe wurtzite-likewurtzite-like structurestructure progressivelyprogressively becomesbecomes moremore amorphousamorphous andand thethe gapgap risesrises upup toto 3.63.6 eVeV when when the the Ti Ti contentcontent isis 4.1 4.1 at.%. at.%. Finally, Finally, inin aa highlyhighly doped doped film filmfilm (Ti (Ti concentrationconcentration is is 18.218.2 at.%)at.%) the the bandwidth bandwidth increases increases to to 3.9 3.9 eV. eV. InIn thisthis case,case, thisthis valuevalue cannotcannot bebe justifiedjustifiedjustified byby thethe dopingdoping effect,effect, butbut instead,ininstead,stead, itit couldcould havehave arisenarisen by by thethe presencepresence ofof aa differentdifferent amorphousamorphous TZOTZO compound.compound.

22 FigureFigure 7.7. TheThe variation variation in in the the ( (ααhhνν)))2 valuesvalues as as a a function function of of ener energyenergygy for for ZnO ZnO films filmsfilms withwith differentdifferent concentrationsconcentrations ofof TiTi incorporationincorporincorporationation (from(from(from 000 tototo 18.218.218.2 at.%).at.%).at.%).

Coatings 2019, 9, x FOR PEER REVIEW 9 of 11 Coatings 2019, 9, 180 9 of 12 Coatings 2019, 9, x FOR PEER REVIEW4.0 9 of 11 mixed oxides 3.9 4.0 3.8 mixed oxides 3.9 3.7 3.8

(eV) 3.6

g 3.7

E 3.5 w-ZnO + mixed oxides

(eV) 3.6

g w-ZnO

E 3.4 3.5 w-ZnO + mixed oxides 3.3 w-ZnO 3.4 3.2 ZnO 3.3 0 2 4 6 8 101214161820 3.2 ZnO [Ti] incorporated (at.%) 0 2 4 6 8 101214161820 [Ti] incorporated (at.%) Figure 8. The energy gap, Eg, for TZO films as a function of the incorporated Ti content (the solid line is drawn as a guide for the eye). Figure 8. The energy gap, E , for TZO films as a function of the incorporated Ti content (the solid line Figure 8. The energy gap, Eg, for TZO films as a function of the incorporated Ti content (the solid line is drawn as a guide for the eye). 3.6. Photoluminescenceis drawn as a guide (PL)for the eye). 3.6. Photoluminescence (PL) 3.6. PhotoluminescenceFigure 9 illustrates (PL) the PL spectra of selected TZO samples. A reference spectrum of stoichiometricFigure9 illustrates ZnO is included the PL spectra as well. of selected The pure TZO ZnO samples. films exhibit A reference a PL peak spectrum in the of UV stoichiometric region (380 Figure 9 illustrates the PL spectra of selected TZO samples. A reference spectrum of ZnOnm) attributed is included to as the well. near The band pure ed ZnOge excitation films exhibit of defects, a PL peak as well in theas a UV wide region green (380 band nm) (approx. attributed 550 stoichiometric ZnO is included as well. The pure ZnO films exhibit a PL peak in the UV region (380 tonm) the of near lower band intensity edge excitation due to shallow of defects, defects as well [7,26] as. aThe wide incorporation green band of (approx. Ti (0.2 550at.%) nm) in ofthe lower ZnO nm) attributed to the near band edge excitation of defects, as well as a wide green band (approx. 550 intensityfilms raises due both to shallow PL signals, defects increasing [7,26]. The the incorporation intensity of the of Ti blue (0.2 band at.%) in thea factor ZnO of films 2 and raises thereby both nm) of lower intensity due to shallow defects [7,26]. The incorporation of Ti (0.2 at.%) in the ZnO PLindicating signals, increasinga significant the intensityincrease of in the PL blue efficiency band in ain factor the of UV 2 and range, thereby which indicating corroborates a significant the films raises both PL signals, increasing the intensity of the blue band in a factor of 2 and thereby increaseimprovement in PL efficiencyof the crystallinity in the UV of range, low-doped which ZnO corroborates films as thedetected improvement by XRD. of This the behaviour crystallinity is indicating a significant increase in PL efficiency in the UV range, which corroborates the ofexplained low-doped by the ZnO increase films as of detected exciton defects by XRD. caused This behaviourby the substitution is explained of the by Zn the2+ increaseby Ti4+ ions of excitonin ZnO improvement of the crystallinity of low-doped ZnO films as detected by XRD. This behaviour is defectslattice [7,27]. caused by the substitution of the Zn2+ by Ti4+ ions in ZnO lattice [7,27]. explained by the increase of exciton defects caused by the substitution of the Zn2+ by Ti4+ ions in ZnO lattice [7,27].

FigureFigure 9.9. Photoluminescence (PL)(PL) spectraspectra ofof thethe TZOTZO samplessamples withwith differentdifferent TiTi contents.contents. ForFor higherhigherFigure Ti Ti9. concentrationsPhotoluminescenceconcentrations inin the(PL)the film filmspectra structurestructur of the eTZ (4.1(4.1O samples at.%at.% andand with 18.218.2 different at.%),at.%), Ti the contents. PL spectra show, inin bothboth cases,cases, aa tailtail atat wavelengthswavelengths aboveabove 360360 nm,nm, suggestingsuggesting thethe presencepresence ofof thethe correspondingcorresponding For higher Ti concentrations in the film structure (4.1 at.% and 18.2 at.%), the PL spectra show, emissionemission peaks,peaks, likelylikely ofof lowlow intensity,intensity, deeperdeeper inin thethe blueblue rangerange (out(out ofof thethe detectiondetection limitlimit ofof thethe in both cases, a tail at wavelengths above 360 nm, suggesting the presence of the corresponding spectrophotometer).spectrophotometer). AsAs aforementioned,aforementioned, thesethese samplessamples correspondcorrespond eithereither toto aa mixturemixture ofof crystallinecrystalline emission peaks, likely of low intensity, deeper in the blue range (out of the detection limit of the ZnOZnO and other other amorphous amorphous compounds compounds (for (for Ti Ti= 4.1 = 4.1at.%) at.%) or to or the to amorphous the amorphous titanate titanate phase phase(for Ti spectrophotometer). As aforementioned, these samples correspond either to a mixture of crystalline (for= 18.2 Ti at.%). = 18.2 Indeed, at.%). the Indeed, optical the energy optical band energy gaps band for these gaps materials for these are materials 3.6 and are3.9 eV, 3.6 respectively and 3.9 eV, ZnO and other amorphous compounds (for Ti = 4.1 at.%) or to the amorphous titanate phase (for Ti respectively(Figure 8), which (Figure may8), whichgive rise may to givenear riseedge to excitonic near edge transitions excitonic in transitions that region. in that region. = 18.2 at.%). Indeed, the optical energy band gaps for these materials are 3.6 and 3.9 eV, respectively Generally,Generally, thethe greengreen bandband ofof thethe PLPL spectraspectra ofof TZOTZO filmsfilms isis attributedattributed toto singlesingle ionizedionized oxygenoxygen (Figure 8), which may give rise to near edge excitonic transitions in that region. vacanciesvacancies [[2,24,27],2,24,27], inin agreementagreement toto ourour XPSXPS resultsresults (see(see above).above). TheThe large emission intensityintensity observedobserved Generally, the green band of the PL spectra of TZO films is attributed to single ionized oxygen vacancies [2,24,27], in agreement to our XPS results (see above). The large emission intensity observed

Coatings 2019, 9, 180 10 of 12 for the sample with 4.1 at.% Ti corroborates the existence of w-ZnO aggregates within the titanate phase, which is supposed to bear a large defect concentration. On the other hand, the decrease of the emission in this optical range for the sample with a much higher Ti content (18.2 at.%) can be associated to the amorphous nature of pure titanate films, which present a broad low emission band in the 500–600 nm range, as observed in Figure9. This phenomenon has been related to the presence of delocalized electronic levels (Ti–O5 clusters) in the optical bandgap [28]. As it is well-known, amorphous semiconductors have a large population of electronic states close to the band edges (mobility gap) which obviously may smooth the PL spectra of the green band. Nevertheless, the green luminescence processes in ZnO materials are still a matter of intense debate [1].

4. Conclusions The above results demonstrate a close relationship between the crystalline structure of sputtered TZO coatings and their optical response, determined by the atomic Ti concentration in the films. Specifically, films with a low Ti content (≤0.9 at.%) exhibit a highly crystalline wurtzite-like structure, with the Ti4+ ions substitutionally incorporated into the ZnO structure, forming Ti-doped ZnO films. Their energy gap (3.3 eV) is slightly higher than in reference ZnO films (3.2 eV). Higher Ti contents still keep the wurtzite structure, which becomes progressively amorphous with increasing Ti content, widening the energy gap. At Ti concentration of 18.2 at.%, the material displays a fully amorphous structure of Ti–O–Zn mixed oxides and the gap further increases to 3.9 eV. On the other hand, the PL characteristics of pure ZnO films show the typical blue band (which the position of correlates with optical gap), as well as the green band, associated to the near edge excitation of defects, likely ionized oxygen vacancies. Ti addition below 0.9 at.% in the films raises the UV band emission, thereby indicating a significant improvement of PL efficiency. For larger Ti contents (4.1 at.%), there is a shift of the blue band to the deep UV range, whereas the green band increases likely due to a larger defect concentration in the band gap. Finally, the amorphous nature of the films for Ti at 18.2 at.% preserves a similar PL emission (blue and green band), both with much lower intensity. Hence, we prove that there is a very narrow window of titanium content to be incorporated into the structure of ZnO films to improve its crystalline structure as well as its luminescence and other optical properties.

Author Contributions: Conceptualization, R.E.-G. and O.S.; Methodology, R.E.-G and O.S.; Formal Analysis, R.E.-G. and O.S. and M.Y.; Investigation, M.Y., N.B. and O.M.; Writing—Original Draft Preparation, J.M.A. and O.S.; Writing—Review and Editing, O.S., C.P., R.E.-G. and J.M.A. Funding: We acknowledge financial support from the Comunidad Autónoma de Madrid under project DIMMAT ref: CAM S2013/MIT-2775 and from the Spanish government under the project MAT2015-68892-R. Acknowledgments: We acknowledge Javier Ortiz for his technical support. Conflicts of Interest: The authors declare no conflict of interest.

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