Optical and hydrophobic properties of co-sputtered chromium and titanium oxynitride films

Applied Surface Science 257 (2011) 8755–8761

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Optical and hydrophobic properties of co-sputtered chromium and titanium oxynitride films Sushant K. Rawal a,b , Amit Kumar Chawla b , R. Jayaganthan a,c , Ramesh Chandra a,b,∗ a b c

Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India Nano Science Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

a r t i c l e

i n f o

Article history: Received 25 January 2011 Received in revised form 1 March 2011 Accepted 1 March 2011 Available online 12 April 2011 Keywords: Chromium oxynitride Titanium oxynitride Co-sputtering Stress Strain Contact angle Optical properties

a b s t r a c t The chromium and titanium oxynitride films on glass substrate were deposited by using reactive RF magnetron sputtering in the present work. The structural and optical properties of the chromium and titanium oxynitride films as a function of power variations are investigated. The chromium oxynitride films are crystalline even at low power of Cr target (≥60 W) but the titanium oxynitride films are amorphous at low target power of Ti target (≤90 W) as observed from glancing incidence X-ray diffraction (GIXRD) patterns. The residual stress and strain of the chromium oxynitride films are calculated by sin2 method, as the average crystallite size decreases with the increase in sputtering power of the Cr target, higher stress and strain values are observed. The chromium oxynitride films changes from hydrophilic to hydrophobic with the increase of contact angle value from 86.4◦ to 94.1◦ , but the deposited titanium oxynitride films are hydrophilic as observed from contact angle measurements. The changes in surface energy were calculated using contact angle measurements to substantiate the hydrophobic properties of the films. UV–vis and NIR spectrophotometer were used to obtain the transmission and absorption spectra, and the later was used for determining band gap values of the films, respectively. The refractive index of chromium and titanium oxynitride films increases with film packing density due to formation of crystalline chromium and titanium oxynitride films with the gradual rise in deposition rate as a result of increase in target powers. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Multi-target sputtering is used extensively to fabricate coatings, multi-layers, films with graded compositions to achieve excellent structural and functional properties of these materials since last decade [1–4]. Since their deposition requires certain precaution such as the careful control of nitrogen and oxygen sources for preparing oxynitride films due to difference in reactivity of both gases, the studies pertaining to oxynitrides have not been as systematic as oxides [5,6]. Titanium dioxide has attracted much attention due to its favorable biocompatible and hydrophilic properties [7–11]. It has been investigated extensively for various applications including photocatalysis, gas sensing, photoconductivity, optical coating, UV filtering and pigments [12,13]. Photocatalytic reactions are considered as one of the most promising routes for solar energy conversion [14]. TiO2 is one of the most

∗ Corresponding author at: Nano Science Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India. Tel.: +91 1332 285743; fax: +91 1332 286303. E-mail address: ramesfi[email protected] (R. Chandra). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.012

frequently used photocatalysts as it is chemically inert [11]. However, the effective photo excitation of TiO2 semiconductor particles requires the application of light with energy higher than the titanium dioxide band gap energy (Eg ). For anatase phase, Eg is 3.2 eV and for second rutile phase, Eg is 3.02 eV, therefore the absorption thresholds correspond to 380 and 410 nm for these two phases of TiO2 , respectively [15–18]. Therefore, TiO2 exhibit photocatalytic activity only under the ultraviolet light, which represents about 3% of the total amount of solar radiation [19]. Meanwhile, much attention has also been paid to fabricating supra-amphiphilic surfaces of which the contact angle for either water or oily liquids closes to 0◦ . Results showed that UV illuminating on a titania surface could lead to supraamphiphilicity of the surface [20–22], which has been attributed to the formation of both hydrophilic and oleophilic micro-domains in the same area. This kind of surfaces has promising potential applications in anti-fogging, anti-bacteria and self-cleaning, etc. [23]. It has been reported that ion implantation of transition metals such as V, Cr, Co, and Ni, shift the absorption edge to the visible region, resulting in enhanced photocatalytic activity of TiO2 in the visible region [24]. Asahi et al. [25] reported that nitrogen-doped titania possessed the photocatalytic activity on acetaldehyde pho-

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todegradation and the hydrophilicity under visible-light irradiation [26]. The incorporation of nitrogen into TiO2 , named as nitrogendoped titanium oxide or titanium oxynitride changes the refraction index, band gap as well as the mechanical properties such as hardness, elastic modulus and internal stresses of the base oxide films since metal–nitrogen bonds are less polar than the substituted metal–oxygen bonds [27]. Titanium oxynitride films have recently attracted much attention owing to their remarkable optical and electronic properties, which depend significantly on the N/O ratio [28]. Nitrogen-rich titanium oxynitride has been widely used in many applications, such as anti-reflective coating [29] and biomaterials [30], while oxygen-rich titanium oxynitride has been used in thin film resistors [28,31]. Titanium substituted chromium oxides (CTOs), have been developed recently as gas sensitive resistors at elevated temperatures for the detection of combustible and toxic gases [32,33]. Recently, these nonstoichiometric ternary oxides (CTO) have been extensively used as solid state sensors for relatively low concentrations of gases (e.g. NH3 , CO, H2 S, alcohols) in the temperature range of 300–500 ◦ C [32,34–36]. Titanium-substituted chromium oxide (CTO) exhibits easy fabricability, chemical stability at operating temperature, measurable resistance change, and a sensitive gas response as reported in the literature [37]. Chromium oxynitride possesses the combined properties of chromium oxide and chromium nitride and shows excellent structural, physical, optical and mechanical properties depending upon the deposition conditions. Chromium oxynitride and nitride double-layered coatings on tool steel were shown to be a promising coating for wear and corrosion resistance applications [38–40]. Chromium oxynitride thin films present various colors that vary with thickness and composition. The chromium oxynitride has a higher corrosion resistance and the layer has a better adhesion and a uniform structure as compared to Cr2 O3 [41,42]. Pure and metal-doped metal oxide films are deposited using various techniques including chemical vapor deposition (CVD), sol–gel deposition, sputtering deposition, vacuum arc deposition and plasma oxidation [11,19,43–45]. Using magnetron sputtering technology, it is possible to realize coatings with optimal optical properties. This technology is suitable to deposition of coatings on large areas with reproducible values and with relatively high deposition rate [46]. The deposition of co-sputtered chromium and titanium oxynitride films as function of power variation of both targets are so far not found in literature. This may lead to formation of Ti-doped chromium oxynitride and Cr-doped titanium oxynitride films, and the studies relating its structural, hydrophobic and optical properties are also not found in literature. Therefore, the present work has been focused to study the effect of power variation of two metallic targets such as chromium and titanium on the structural, optical, and hydrophobic properties of Cr and Ti oxynitride films deposited by reactive magnetron cosputtering. A constant flow of nitrogen besides oxygen was also used as reactive gases during sputtering that causes formation of metal oxynitride films. Helium has been used as inert gas in sputtering; its effect on hydrophobic properties of the films has not been reported so far. Moreover the titanium and zirconium oxynitride films deposited in helium exhibit good hydrophobic properties as compared to argon as reported in our earlier work [47,48]. The effect of Ti doping in chromium oxynitride and Cr doping in titanium oxynitride films on structural, hydrophobic and optical properties of films has been investigated in the present work.

2. Experimental details Two metallic targets of chromium and titanium (99.99% pure, 2 in. diameter and 5-mm thick) were used for depositing metal

Fig. 1. XRD micrographs of chromium and titanium oxynitride films at different variation of Cr and Ti target powers.

oxynitride films, by reactive RF magnetron sputtering, onto glass substrates. The targets were arranged confocally for co-sputtering and the substrate to target distance was kept constant at 50 mm during deposition. The substrates were cleaned by rinsing in ultrasonic baths of acetone and methanol. The base pressure was better than 4 × 10−6 Torr and the sputtering was carried out in a helium atmosphere along with oxygen and nitrogen as reactive gases. The deposition time, temperature, and gas pressure were kept constant at 60 min, 500 ◦ C and 15 mTorr, respectively, for all depositions. Before starting the actual experiment, the target was pre-sputtered for 15 min with a shutter located in between the target and the substrate. This shutter is also used to control the deposition time. First, the power of chromium target was varied at 60 W, 90 W and 120 W with a fixed power of 30 W for titanium target represented by samples CT-1, CT-2 and CT-3, respectively. Subsequently, a power of 75 W was used for both targets for sample CT. The power variation of titanium target at 60 W, 90 W and 120 W with fixed power of 30 W for chromium target is indicated by samples TC-1, TC-2 and TC-3. The structural characterization of the films was carried out by Bruker D8 Grazing-incidence X-ray diffractometer (GIXRD), which was also used for the stress measurement by sin2 method. The surface morphology of the films was characterized using Atomic Force Microscope (NT-MDT Ntegra). The elemental analysis of these films was carried out using an energy dispersive X-ray analysis (FEI, Quanta 200F). The wettability studies of films were made by contact angle measurement system (Kruss DSA 100 Easy Drop) to find the contact angle of water with the films. The thicknesses of the samples were examined by Surface Profilometer (Ambios Technology XP-200). Optical transmission and absorption of the films were measured by UV–vis–NIR spectrophotometer (Varian Cary 5000). 3. Results and discussions The identification of phases having various orientations with respect to power variation of chromium and titanium targets was made by X-ray diffraction analysis and is shown in Fig. 1. In sample CT-1 [Cr (60 W)–Ti (30 W)], Cr2 O3 having (1 0 4) and (1 1 6) orientations is observed along with weakly crystalline (1 1 0) Cr2 N

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Fig. 2. Texture coefficients of deposited chromium oxynitride films at varying power of Cr target.

peak. In sample CT-2 [Cr (90 W)–Ti (30 W)], the intensity of both orientations increases, also showing weakly (0 1 2) and (1 0 10) oriented peaks of Cr2 O3 and Cr2 N, respectively. In sample CT-3 [Cr (120 W)–Ti (30 W)], preferable (1 0 4) and (1 1 6) orientations of Cr2 O3 along with its less intense (0 1 2), (0 2 4) and (1 0 10) oriented peaks are observed. The weakly crystalline (1 1 0) and (0 0 2) peaks of Cr2 N are also observed. In CT [Cr (75 W)–Ti (75 W)], (0 1 2), (1 0 4) and (1 1 6) oriented Cr2 O3 peaks are observed. The weakly crystalline (0 2 4) and (1 0 10) orientations of Cr2 O3 are observed only at 120 W Cr target power. The peaks of titanium dioxide or oxynitride are absent in chromium oxynitride films, which has high Cr content. When the Cr content is high (CT-1, CT-2, and CT-3) for chromium oxynitride films, the Ti4+ ions may have dispersed in Cr2 O3 . The formation of titanium dioxide may be feasible but the presence of high chromium content may slow down this process leading to well crystalline, Cr2 O3 phase. Also, when the source proportion of Cr and Ti is 50:50 (sample CT), it preferably leads to formation of Cr2 O3 as reported in the literature [13]. This may be the major reason for the absence of titanium dioxide or titanium oxynitride peaks in samples of chromium oxynitride films with high Cr content. In order to calculate the average crystallite size, ‘d’ of the films, Scherrer’s formula is used [49]. The calculated average crystallite sizes are shown in Table 1. The average crystallite size decreases from 73 to 47 nm with increase in power of Cr target. For sample TC-3 [Cr (30 W)–Ti (120 W)], XRD peaks reveals the presence of titanium oxynitride phase with (0 1 1) and (3 0 2) orientations. The average crystallite size of titanium oxynitride films is around 61 nm; but for TC-1 [Cr (30 W)–Ti (60 W)] and TC-2 [Cr (30 W)–Ti (90 W)], the films are amorphous in nature. The Cr3+ and Ti4+ ions have almost identical ionic radius of 0.063 and 0.068 nm, respectively. The doping of Cr above 3 at.% leads to decline in crystallinity of titanium dioxide films as reported in literature [50]. In the case of high Ti content (TC-1, TC-2 and TC-3) for titanium oxynitride films, when the proportion Cr ions are high (>3 at.%), due to same ionic radius of both ions, it leads to oxide of each metal doped with other metal, but the inclusion of Cr3+ ions into titania is more leading to an amorphous product as reported in literature [13]. In our case for TC-1 and TC-2, the Cr content in the films is 9 at.% and 5 at.%, respectively, as obtained from EDS analysis. So, the higher Cr content may be the possible reason for the amorphous nature of films in both cases. The texture coefficients of the chromium oxynitride films at varying power of Cr target are calculated from their respective XRD peaks using the following formula [51,52] as per Eq. (1) which are shown in Fig. 2. texture coefficient (T ) =

I(h k l) I(0 1 2) + I(1 0 4) + I(1 1 6) + I(1010)

(1)

The texture coefficient of (1 0 4) is high as compared to other orientations and therefore, it was used for determination of the

Fig. 3. The calculated residual stress and strain values of chromium oxynitride films.

residual stress in the films. For stress measurement by XRD technique, sin2 method is used. The lattice spacing d is a function of the angle between surface normal and the diffraction vector. From d , the strain ε( ) is given by Eq. (2) ε( )

d␺ − d0 d0

=



2S1h k l +

1 hkl sin2  S 2 2





(2)

where ‘d0 ’ is the strain free lattice spacing and S1h k l and S2h k l /2 are the X-ray elastic constants (XEC) of the film material. The XEC are a function of the Miller indices (h k l), the single crystal compliances, the connectivity of the grains in the films and texture. By plotting the measured lattice parameter d , or the strain ε( ) as a function of sin2 , the stress in the films can be determined from the slope of the straight line [53,54]. It is well known that the residual stress in thin films is composed of two parts, i.e. the thermal stress and the intrinsic stress [55]. Thermal stress is caused by differential thermal expansion coefficients between the films and the substrate [56]. The deposition temperature was kept constant, so the thermal stress of all the films are assumed to be of same magnitude. The extent of intrinsic stresses in sputtered films is found to be related to the microstructure of films, i.e., morphology, texture, grain size depicted by the structure-zone diagrams [57]. In a polycrystalline material, the stresses can be generated inside grains through crystalline lattice deformation or at the grain boundaries. In the latter case, the stresses can be attributed to impurities segregation at the grain boundaries or to due to the presence of a high grain boundary density. It is believed that the grain boundary density contributes to increasing the tensile component of the residual stress [58]. As per the analysis done by Doljack and Hoffman [59], tensile stress is generated at the grain boundaries by shrinking the distance between adjacent grains due to interatomic forces across the grain boundary together with the adherence of the film to the substrate. The measured residual stress which is tensile and corresponding strain for chromium oxynitride films are shown in Fig. 3. There may be two factors responsible for the development of tensile stress in the chromium oxynitride films namely; high grain boundary density and lattice deformation. With the increase of power of the Cr target, the deposited chromium oxynitride films are found to be polycrystalline by nature showing the evolution of various textures as shown in Figs. 1 and 2. Moreover, the average crystallite size of the films decreases as shown in Fig. 4 with the increase of Cr target power resulting in increase of the grain boundary density among the deposited films. Secondly, there is inclusion of titanium ions in the deposited chromium oxynitride films which is evident from the EDS data due to the co-sputtering process. There is also incor-

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Table 1 Calculated parameters of chromium and titanium oxynitride thin films. Sample name

Sputtering power of targets

Cr (at.%)

Ti (at.%)

O (at.%)

N (at.%)

Avg. d(XRD) (nm)

Band gap (eV)

Thickness (nm) by profilometer

CT-1

Cr (60 W)–Ti (30 W) Cr (90 W)–Ti (30 W) Cr (120 W)–Ti (30 W) Cr (75 W)–Ti (75 W) Cr (30 W)–Ti (60 W) Cr (30 W)–Ti (90 W) Cr (30 W)–Ti (120 W)

21

12

65

02

73

1.80

263

28

07

63

02

65

1.98

315

36

02

59

03

47

2.18

397

24

18

56

02

58

2.31

278

09

16

74

01

–

2.07

108

05

22

71

02

–

2.11

145

02

34

58

06

61

2.35

223

CT-2 CT-3 CT TC-1 TC-2 TC-3

poration of N ions, which has higher ionic radius of 0.171 nm as compared to 0.132 nm of O ions. So, these factors may have led to the lattice distortion which is shown in Fig. 3 as the measured strain is increased for the deposited films. Hence, for the samples CT-1, CT-2 and CT-3 with the increase of Cr target power; there is increase of tensile stress in the deposited chromium oxynitride films as shown in Fig. 3. For sample CT, when the power of both Cr and Ti targets are kept fixed at 75 W, the amount of Ti ions present is higher as compared to the Ti content in the samples CT-1, CT-2 and CT-3 causing greater amount of lattice distortion. So, this may be the possible reason for the greater magnitude of tensile stress and strain for sample CT as compared to samples CT-1, CT-2 and CT-3. The residual stress is not measured for titanium oxynitride films due to lower intensity of XRD peaks as observed from Fig. 1.

The AFM images of chromium oxynitride films are shown in Fig. 4a. The average crystallite size decreases with the increase in power of Cr target for the deposited films. The thickness of the deposited films also increases with it due to greater deposition rate with a gradual rise in surface roughness. Fig. 4b represents the AFM images of titanium oxynitride films. At low power of Ti target, the films are almost amorphous with very low surface roughness value. When the power of Ti target is increased, the films exhibit weak crystalline structure with greater surface roughness values compared to previous one. Wenzel equation [60] gives the relation between the surface roughness and water contact angle, which is described elsewhere [48]. The contact angle was measured by sessile drop technique; ten readings were taken for each sample with average value shown in

Fig. 4. (a) AFM micrographs of chromium oxynitride films at varying power of Cr target (b) AFM micrographs of titanium oxynitride films at varying power of Ti target.

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Table 2 Calculation results of dispersion component, polar component, and surface energy of deposited chromium and titanium oxynitride films. Parameters of liquids used in evaluating surface energy

Water (H2 O) Diiodomethane (CH2 I2 ) Sample name

CT-1 CT-2 CT-3 CT TC-1 TC-2 TC-3

 L (mN/m)

(Ld ) (mN/m)

72.8 50.8

21.8 50.8

Owens–Wendl Method

p

(L ) 51 0

Wu method

 S (mN/m)

(Sd )

22.77 18.51 17.50 20.33 37.50 33.27 25.94

14.20 11.55 10.92 12.68 23.39 20.76 16.18

(mN/m)

p (S )

(mN/m)

8.56 6.96 6.58 7.65 14.10 12.51 9.76

p

 S (mN/m)

(Sd ) (mN/m)

(S ) (mN/m)

29.65 25.61 24.64 27.35 43.22 39.34 32.60

18.26 15.90 15.33 16.92 26.09 23.86 19.97

11.39 9.71 9.31 10.43 17.13 15.48 12.63

Fig. 5. The chromium oxynitride films are hydrophilic for CT-1 sample having contact angle values of 86.4◦ with surface roughness of 2.7 nm. As the power of Cr target is increased, the films deposition rate increases with rise in surface roughness of 3.6 nm leading to high contact angle values of 94.1◦ for sample CT-3. The chromium oxynitride films exhibit hydrophobic behavior when the power of Cr target is (≥75 W) which represents samples CT-2, CT-3 and CT. The titanium oxynitride films are hydrophilic for all the samples namely TC-1, TC-2 and TC-3. Initially, the surface roughness is lower giving smaller contact angle values, the major reason may be the amorphous nature of the deposited films for TC-1 and TC-2 samples. Sample TC-3 shows higher contact angle values of 82.3◦ with surface roughness of 2.3 nm but is still hydrophilic in nature.The relation of surface energy at 3-phase boundary is given by Young’s equation [61,62]. To calculate the surface free energy of solid, an additional relation to Young’s equation is required. Owens–Wendl [63] used geometric mean approach and Wu used harmonic mean approach to obtain the relation for surface energy of solids which are discussed in detail elsewhere [48,64]. The surface energy conp sists of two parts: polar (S ) and dispersive (Sd ). Two liquids such as water and diiodomethane with known surface tension values p as given in Table 2 are used to calculate (S ) and (Sd ) values of the films. The surface energy values calculated by Owens–Wendl and Wu method are also summarized (Table 2), which are in good conformity with each other. Surface energy and contact angle are connected conversely, whereas the latter is directly proportional to surface roughness. Initially, at lower power of Cr target for CT-1 sample, the surface roughness is less giving lower contact angle and greater values of surface energy. With the gradual increase of Cr target power for CT-2, CT-3 and CT samples, it facilitates the crystallization of Cr2 O3 phases having different textures along with the formation of weakly crystalline Cr2 N phases (Figs. 1 and 2). This may be due to more availability of Cr ions as evident from EDS analysis, thus resulting

in gradual increase in deposition rate and hence the thickness of the films. So the chromium oxynitride films show hydrophobic (≥90◦ ) behavior from the previously observed hydrophilic (≤90◦ ) one for sample CT-1. Hence, it leads to decline in surface energy values due to transformation to hydrophobic nature. For samples TC-1 and TC-2 of titanium oxynitride films, the XRD patterns show that they are amorphous. The possible reason may be the higher Cr incorporation in the films as discussed earlier. This may have led to lower contact angle and high surface energy values. For sample TC-3, weakly crystalline titanium oxynitride film is observed with increase in contact angle from 69.1◦ to 82.3◦ but the films are still hydrophilic in nature. Hence, the surface energy of titanium oxynitride films is higher than those observed for chromium oxynitride films. So as the deposited films are not that much hydrophilic, the contact angle properties are moderate, it can be too early to conclude that it can be used as the potential applications discussed in Section 1. The transmittance and absorbance spectra for all samples were measured by UV–vis–NIR spectrophotometer. The transmission spectra for all samples are shown in Fig. 6. It is observed that the transmittance of the chromium oxynitride films is less as compared to titanium oxynitride films. The model proposed by Manifacier et al. [65] is used to obtain refractive index of the films from transmission data, the details are given in detail else where [48]. The films packing density can be determined using the expression of

Fig. 5. Variation of contact angle and surface roughness at different power of Cr and Ti targets.

Fig. 6. Transmission spectra of chromium and titanium oxynitride films at varying powers of Cr and Ti targets.

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of weakly crystalline Cr2 O3 phase as shown in XRD graph. With the increase in the power of Cr target, the availability of the Cr ions as evident from the EDS analysis increases, facilitating the formation of highly crystalline Cr2 O3 phase with various textures, contributing to the higher band gap values. It also leads to smaller crystallite size, causing greater proportion of grain boundary density that acts as a source of scattering resulting in higher band gap values. For titanium oxynitride films, for samples TC-1 and TC-2, the films are amorphous; the reason may be due to higher atomic concentration of Cr ions as explained earlier giving lower band gap values. For sample TC-3 when the power of Ti target is high, it generates adequate Ti atoms to form crystalline titanium oxynitride phase, due to which a slight increase in the band gap values is observed. 4. Conclusions

Fig. 7. Refractive index versus packing density of chromium and titanium oxynitride films at varying powers of Cr and Ti targets.

Bragg and Pippard model [66,67]. p=

n2f − 1 n2b + 2 n2f + 2 n2b − 1

(3)

where nf is the refractive index of the films at the given wavelength and nb is the bulk value of refractive index. The variation of refractive index with films packing density is shown in Fig. 7. The refractive index of chromium oxide and titanium dioxide is 2.1 and 2.79, respectively, at 800 nm. Initially, at low values of target powers, the films packing density and refractive index is less but with the formation of polycrystalline films, due to increase of deposition rate, an increase in film packing density is observed. From the absorption spectra of the films, the optical band gap of the films was determined by the absorption coefficient (˛) using the Tauc relation [68], which is described in detail elsewhere [47]. Fig. 8 shows the band gap values of chromium and titanium oxynitride films. The band gap of chromium oxynitride films varies from 1.80 to 2.31 eV for samples CT-1, CT-2, CT-3 and CT and for titanium oxynitride from 2.07 to 2.35 eV for samples TC-1, TC-2 and TC-3. The band gap variation is sensitive to the structural and the atomic concentration of the ions. Initially, lower values of band gap is observed when the power of Cr target is low due to the formation

The effects of power variation on structural, hydrophobic, and optical properties of chromium and titanium oxynitride films, deposited by reactive magnetron sputtering, are investigated. The average crystallite size of chromium oxynitride films decreases with increase of Cr target power. The crystalline titanium oxynitride films are formed only at high Ti target power. The residual stress of chromium oxynitride films is tensile and increases with the decrease in average crystalline size that leads to more grain boundary density. The chromium oxynitride films changes from hydrophilic to hydrophobic at greater power of Cr target, whereas the titanium oxynitride films are hydrophilic for all power variations studied of Ti target. The hydrophobic chromium oxynitride films gives lower surface energy values of 17.5 mN/m as compared to 22.77 mN/m for hydrophilic ones. The film packing factor increases with the deposition rate and films thickness for both systems. The variation in band gap values was observed due to the structural variation and chemical composition of the deposited films for both the systems. Acknowledgements One of the authors, Sushant K. Rawal would like to express his gratitude to Principal, Charotar Institute of Technology-Changa Anand, Gujarat for granting study leave to pursue the research work and AICTE-NDF for financial assistance. This work has been supported by CSIR grant number 03(1148)/09/EMR-II and DRDO grant number ERIP/ER/0800354/M/011125. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

Fig. 8. Band gap of chromium and titanium oxynitride films at varying powers of Cr and Ti targets.

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