Properties of TiO2 thin films deposited by rf reactive magnetron sputtering on biased substrates

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Properties of TiO2 thin films deposited by rf reactive magnetron sputtering on biased substrates Sawsen Nezar a,b,∗ , Nadia Saoula a , Samira Sali c , Mohammed Faiz d , Mogtaba Mekki d , Nadia Aïcha Laoufi b , Nouar Tabet e a Equipe Plasma & Applications, Division des Milieux Ionisés et Lasers, Centre de Développement des Technologies Avancées, Cité du 20 Aout 1956, Baba Hassen, Alger, Algeria b Laboratoire des phénomènes de transfert, génie chimique, Faculté de Génie des procèdes, USTHB, BP 32 El-alia, Bab Ezzouar, Alger, Algeria c Centre de Recherche en Technologie des Semi-conducteurs pour l’Energétique (CRTSE Algiers), Algeria d Physics Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia e Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University (HBKU), Doha, Qatar

a r t i c l e

i n f o

Article history: Received 15 January 2016 Received in revised form 22 August 2016 Accepted 23 August 2016 Available online xxx Keywords: TiO2 thin films Magnetron sputtering Anatase Rutile Bias voltage

a b s t r a c t TiO2 thin films are of paramount importance due to their pervasive applications. In contrast to previous published works where the substrate was heated at high temperatures to obtain TiO2 crystalline phase, we show in this study that it is possible to deposit crystalline TiO2 thin films on biased and unbiased substrate at room temperature using reactive rf magnetron sputtering. The bias voltage was varied from 0 V to −100 V. The deposited films were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy, Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and atomic force microscopy (AFM). The average crystallite size was estimated using x-ray diffraction. The results showed that the application of negative bias affects the surface roughness of the films and favors the formation of the rutile phase. The root mean square roughness (Rrms ), the average grain size and the optical band gap of the films decreased as the substrate bias voltage was varied from 0 to −100 V. The UV–visible transmittance spectra showed that the films were transparent in the visible range and absorb strongly in the UV range. This study shows that biasing the substrate could be a promising and effective alternative to deposit TiO2 crystallized thin films of engineered properties at room temperature. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Titanium dioxide (TiO2 ) also known as titanium oxide or titanium IV oxide or titania, is the natural oxide of titanium. It is a versatile transition-metal oxide and a useful material in various applications related to catalysis, electronics, photonics, sensing, medicine, and controlled drug release. Titanium oxide is used also in various fields such as solar cells and optical coatings due to its high refractive index. It has also been widely used in memory devices due to its high dielectric constant [1]. TiO2 is one of the most widely used photocatalysts in many fields such as anti-bacterial applications, water purification, decomposition of various organic pollutants and solar cell applications because of

∗ Corresponding author at: Equipe Plasma & Applications, Division des Milieux Ionisés et Lasers, Centre de Développement des Technologies Avancées, Cité du 20 Aout 1956, Baba Hassen, Alger, Algeria. E-mail address: [email protected] (S. Nezar).

its non-toxic nature, high chemical stability, outstanding optical and electrical properties and ease of mass production [2]. TiO2 plays a significant role as photo-catalyst, helping to solve many serious environmental and pollution challenges and is also used in photovoltaic and water splitting devices [3]. Titanium dioxide is the most investigated crystalline system in the literature of surface of metal oxides [4]. Three distinct phases exist in pure titanium dioxide polymorphs: rutile (tetragonal structure, P42 /mnm), anatase (tetragonal structure, I41 /amd) and brookite (orthorhombic structure, Pbca) phases [5]. However, only anatase and rutile structures are commonly observed in thin film form [6]. In addition to the three crystalline phases, amorphous TiO2 is often observed when the substrate temperature during deposition is very low [7]. TiO2 thin films can be prepared by a variety of methods such as sol–gel method [8], pulsed laser deposition [9], chemical vapor deposition [10], spray pyrolysis [11] and sputtering techniques [12]. RF magnetron sputtering method can produce highly uniform films having good adherence to the substrate. This method also

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A(200) A(105) A(211)

A(004)

-100V

-75V

Intensity (arb.unt)

offers the advantage of depositing films on a large scale area which makes it suitable for industrial applications [6]. Kondaiah et al. [1] studied the influence of substrate bias voltage on the physical, electrical and dielectric properties of TiO2 films deposited by rf magnetron sputtering. The Raman analysis done in Kondaiah work showed that the unbiased films were amorphous and the crystallinity of the films was improved when a bias voltage was applied to the substrate during deposition. However, only anatase phase was obtained. Chandra et al. [13] studied the influence of substrate bias on the structure, the electric and the dielectric properties of TiO2 films deposited by dc magnetron sputtering. The films deposited on unbiased substrates were amorphous while those deposited on negative biased substrates were polycrystalline as confirmed by spectroscopic Raman measurements. In the work of Wang et al. [14], amorphous TiO2 films were obtained on glass substrate using rf magnetron sputtering. However, anatase phase was obtained only after annealing at 400 ◦ C. Singh and Kaur in Ref. [15] stated that the crystallization of TiO2 films at room temperature is mainly due to energetic particle bombardment (electrons, atoms, ions, molecules and even charged clusters) during sputtering. Hence, the deposition parameters are found to have great impact on the structural properties of the films. In this work, the structural and optical properties of TiO2 thin films deposited on glass substrate by rf magnetron sputtering at room temperature have been studied. We show that it is possible to obtain nanocrystalline anatase TiO2 thin films on both biased and unbiased substrates by controlling the sputtering conditions. The aim of this work is to investigate the effect of the substrate bias on the quality of the TiO2 films deposited on glass substrate by reactive rf magnetron sputtering at room temperature.

A(101) R(110)

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2

-50V

0V

15 20 25 30 35 40 45 50 55 60 65 70 75 80

2 theta (degree) Fig. 1. X-ray diffraction patterns of TiO2 thin films for different applied bias voltages (0, −25, −50, −75 and −100 V), 25% oxygen content in the plasma, and 2.66 Pa pressure.

2. Samples and methods 2.1. Thin film preparation TiO2 thin films were deposited on glass substrates by using a homemade radio-frequency (13.56 MHz) magnetron sputtering system. Pure titanium target (99.99%), of 76 mm diameter and 5 mm thick, was used as a sputter target. The gases used are pure argon (99.99990%) as the sputtering gas and pure oxygen (99.99%) as the reactive gas. Titanium target was cleaned to remove the surface oxide and also the commercial glass substrate were cleaned ultrasonically with anhydrous ethanol, acetone, and deionized water for 10 min and then dried before loading in the deposition chamber. All the glass slides were fixed on the substrate holder (diameter 100 mm) and the distance between the target and the substrate holder was set to 30 mm. The target was continuously cooled by water. Before each deposition, the sputtering chamber was evacuated down to a pressure of 6.6 × 10−3 Pa, then argon and oxygen gases were introduced at constant flow rates. The negative voltage applied to the substrate during deposition is expected to repel electrons and accelerate positive ions towards the substrate [16,17]. The voltage applied to the substrate was varied (0, −25, −50, −75 and −100 V) whereas all other deposition parameters were kept constant as follows: sputtering power = 250 W, deposition time = 60 min, working gas pressure = 2.66 Pa, 25% of oxygen flow rate, all experiments were done at room temperature.

a copper anode CuK␣ radiation ( = 1.5406 Å). The accelerating voltage and current used were 40 kV and 15 mA respectively, the 2␪ ranged from 15◦ to 80◦ . Diffraction patterns were collected under ambient conditions. The Raman spectra were collected using Raman spectrometer (Senterra, Bruker). An Argon-ion laser was used as monochromatic exciting source of 532 nm wavelength at a magnification of 50 times. The optical transmittance and reflectance of the films were measured by mean of UV–vis–NIR double beam spectrophotometer (PerkinElmer model Lambda 950) in the wavelength range between 300 and 2500 nm. Fourier transform infrared (FTIR) analysis has been done in reflection mode in the setting (70, polarized light) with wave numbers in the range 4000–400 cm−1 using a (Thermo Nicolet Nexus) spectrometer equipped with a deuterated triglycinesulphate detector. The composition of the films were analyzed by XPS technique using Thermo Scientific ESCALAB 250Xi Spectrometer equipped with a fully automated flood gun for charge compensation and an ion gun for depth profiling. An aluminum anode was used as X-ray source emitting Al K␣ photons of 1486.7 eV energy. In order to investigate the bulk composition of the films XPS analyses were carried out after short etching cycles (30 s–90 s) using Ar+ of 0.5 keV energy. Before these measurements, the samples were softly etched by Ar+ sputtering at 0.5 keV. The morphology of the surface of the films was analyzed using atomic force microscopy (Innova-Veeco instrument). 3. Results

2.2. Thin film characterization 3.1. XRD patterns Raman spectroscopy and X- ray diffraction (XRD) have been used to investigate the structure of the deposited films. XRD measurements were carried out using a Bruker Axe D8 advance diffractometer in Bragg–Brentano (␪–2␪) geometry equipped with

The phase composition of the deposited TiO2 films was characterized by XRD. Fig. 1 shows the diffraction patterns of films deposited under a reactive plasma containing 25% oxygen and 75%

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D=

k (ˇ cos)

(1)

B1g(1)E

Eg(1) Eg(2) SOE

g(R)

B1g(2) A

1g(R)

Eg(3)

A1g

Raman Intensity (arb.unt)

argon for different values of the voltage applied to the substrate. Notice the presence of a strong peak at 2␪ = 25.4◦ which corresponds to (101) plane of anatase in all diffraction patterns. The observed anatase diffraction peaks correspond to (004), (200), (105) and (211) are visible in all samples prepared under different bias voltages. As the substrate bias voltage was increased (≥−75 V), a mixture of anatase and rutile phases appeared. The most intense diffraction peak of rutile (110) was observed at 27.7◦ but the anatase phase remains dominant. The average crystallite size was estimated by Debye–Scherrer formula [18,19] as follows:

3

R: Rutile

0V -25 V -50 V -75 V -100 V

where: 100

• D is the average crystallite size; • ␭ is the X-ray wavelength of the incident radiation ( = 0.15406 nm) for CuK␣; • k is the shape factor with a typical value of 0.89; • ␤ is the line broadening full width at half maximum (FWHM) in radians; • ␪ is the Bragg angle.

fA =

1 I

1 + 1.26 IR

(2)

300

400

500

600

700

800

-1

Raman Shift (cm ) Fig. 2. Raman spectra of TiO2 thin films.

1280

1450

Log (1/R)

The average crystallite size was estimated based on the value of the line width (FWHM) of the most intense peak which is associated to (101) of the anatase phase. The line width (FWHM) was obtained by fitting the diffraction peak to Lorentzian distribution. As the substrate bias voltage was varied from 0 to −100 V, the grain size decreases from 31 nm to 21 nm, indicating a grain refinement. The weight fraction of the anatase in the samples was estimated by using XRD (Eq. (2)) as suggested by Spurr and Myers [20]:

200

565

428

-75 V -100 V -50 V -25 V 0V

A

4000

3500

where: • fA is the weight fraction of anatase in the film; • IA is the intensity of the peak attributed to anatase phase (25.4◦ ); • IR is the intensity of the peak attributed to rutile phase (27.7◦ ). 3.2. Raman spectroscopy The Raman spectroscopy is a sensitive tool to investigate the modification of the vibrational structure of TiO2 (anatase, rutile and brookite) even in very small quantities. The intensity, positions and widths of Raman bands of materials are related to their vibrational and structural properties [21]. Fig. 2 shows Raman spectra for unbiased and biased substrates where crystalline titanium dioxide was identified. Raman spectra were recorded in the range of 100–900 cm−1 . Six Raman active vibrational modes of tetragonal anatase phase appear at frequencies around 143, 195, 396, 513, 517 and 639 cm−1 . However, for the rutile phase, Raman active modes are located at around 246 (SOE), 431 (Eg(R) ) and 603(A1g(R) ) cm−1 . 3.3. FTIR studies of TiO2 thin films FTIR analysis was used to identify the absorption bands and confirm the formation of crystalline TiO2 thin films. Fig. 3 shows the FTIR spectra of TiO2 films formed under different substrate biases varying from 0 to −100 V in range of 4000–400 cm−1 . The peaks at 428, 565 1280 and 1450 cm−1 are present in the FTIR spectra and the formation of Ti O and Ti O Ti bonds were observed. These results are discussed below.

3000

2500

2000

1500

1000

500

-1

Wavenumbers (cm ) Fig. 3. FTIR spectra of TiO2 thin films.

3.4. XPS analyses XPS is a powerful surface technique to elucidate the surface chemistry responsible for the electrochemical characteristics of the films [12]. In this study the technique was used to characterize the stoichiometry of the films and to identify the chemical state of Ti atoms. The C1s of adventitious carbon of the contamination layer (binding energy = 284.6 eV) was taken as reference for the charge shift correction of the binding energies [22]. Many authors have reported XPS analyses of TiO2 [8,23–26]. According to these references the Ti2p3/2 corresponding the Ti4+ is located at 458 eV binding energy and the O1s peak in TiO2 is observed at 529.4 eV. Furthermore, the presence of hydroxyl groups (OH− ) adsorbed on the surface is revealed by a signal O1s at 532 eV. We have analyzed the surface of the as grown films. Subsequently, we have also analyzed the samples after argon etching cycles of 30 and 90 s in order to probe the composition of the films after removing the contamination layer. 3.5. Atomic force microscopy (AFM) analysis The surface morphology of the films was investigated by recording AFM images over 2 × 2 ␮m2 area. Fig. 7 shows typical morphologies of samples prepared under −25 V and −50 V biases. Further, analysis of the roughness of the films surfaces revealed a

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clear effect of the substrate bias. The RMS roughness of the surface was computed using the standard definition [27]:

Rrms =

  n   z − zavg 2 i  i=1

N−1

(3)

where Zi is the current value of Z and Zavg is the mean value of Z on the scanned area and N is the number of points in the image region. 3.6. UV–vis measurements and optical band gap Both reflectance and transmittance were measured as functions of the wavelength of the incident light in order to study the influence of the substrate bias on the optical properties of TiO2 thin films. The obtained spectra are shown in Figs. 8 and 9. The optical band gap Eg was calculated using the Tauc plot method. The variation of the square root of the product of the absorption coefficient versus of the incident photon energy is plotted in Fig. 10. The optical gap was extracted from the x-intercept of the extension of the linear part of the graph [28]. The photon energy dependence of the absorption coefficient ‘␣’ can be described by the following equation [29]:



˛ (hv) = A hv − Eg

n

(4)

where A is a constant, Eg the optical band, ␣ is the optical absorption coefficient, h is the incident photon energy, h is the Plank’s constant and the exponent n characterizes the nature of the electron transition. The exponent n depends on band gap type of the semi2 conductor. For a direct gap n = 1/2. In this case, the graph (˛h) versus the photon energy (h) is linear over a certain energy range. The intercept of the straight line with x-axis is precisely the value of the optical gap. For indirect band gap n = 2, the value of the gap is obtained similarly from the x-intercept of the extrapolated linear 1/2 part of the graph (˛h) versus the photon energy (h) (Eq. (5)) [30,31]. We have adopted the second method which is widely used in the literature for TiO2 [32]. (˛hv)

1/2



= A hv − Eg



(5)

4. Discussion Fig. 1 shows the XRD patterns of the films obtained by reactive rf magnetron sputtering technique. All films deposited under biased or unbiased conditions exhibit a crystalline phase as shown on Fig. 1. A strong peak was observed at 2␪ = 25.4◦ in all diffraction patterns which corresponds to (101) plane of anatase. As the substrate bias voltage was increased (−75 V and −100 V), a mixture of anatase and rutile phases appeared. The most intense diffraction peak of rutile (110) was observed at 27.7◦ but the anatase phase remains the dominant one. These observations are in good agreement with the results reported by Zheng et al. [33]. The authors found that the deposited TiO2 films on unbiased substrates exhibit tetragonal anatase single phase with a strong (101) preferred orientation, which changes into (211) when the negative bias reaches −50 V. They observed that a further increase of the negative bias induced the formation of a mixture of anatase and rutile phases, and a pure rutile phase at high negative bias voltage (≥−100 V). However, Chandra et al. [13] reported the formation of only anatase phase by the application of a negative bias voltage to the substrate. Our results indicate the presence of anatase phase even for unbiased conditions and the formation of the rutile phase at −75 V and −100 V bias voltages. The slight differences in the results obtained by various authors can be related to the differences in sputtering conditions. For instance, Chandra et al. [13] deposited their films under a lower working pressure (P = 0.5 Pa instead of 2.6 Pa in our

case) and a larger distance between the substrate and the target (80 mm instead of 30 mm in our case). Singh and Kaur [15] indicated that the crystallographic orientation of TiO2 films deposited on glass at room temperature is controlled by the sputtering parameters and especially the sputtering power which has a great influence on the structural properties of the films. Other parameters such as the type of substrate, its temperature and the energy of ions impinging the film during deposition can also affect drastically film properties [6]. In the present experiment, the crystalline formation of TiO2 was not due to the thermal process because the substrate was not heated but could be the result of the increasing energy of the positive ions hitting the film surface as suggested in Ref. [34]. Notice that the crystallite size calculated by Eq. (1), decreased from 31 nm to 21 nm as the bias voltage was changed from 0 to −100 V, this indicates the grain refinement induced by applying a negative substrate bias. These results are in agreement with the observations reported by Zheng et al. in Ref. [33]. The fractions of anatase (fA ) and rutile (fR =1 − fA ) corresponding to (−75 V and −100 V) were estimated using XRD (Eq. (2)). The fraction of rutile was found equal to 30% and 55% for the films deposited at −75 and −100 V respectively. Table 1 shows both the crystallite size of samples calculated from Scherrer’s formula and the relative fractions of anatase and the rutile in the films. Fig. 2 presents the Raman spectra of the TiO2 films in the 100–900 cm−1 spectral region. The rutile phase is the most common and stable crystal structure of TiO2 and Raman spectroscopy has been widely used to study it [35]. In Fig. 2, the Raman active modes observed at 143 (Eg(1) ), 195 (Eg(2) ), 396 (B1g(1) ), 513 (A1g ), 517 (B1g(2) ) and 639 (Eg(3) ) cm−1 can be attributed to the presence of tetragonal anatase phase (I41 /amd (D4h 19 )) space group [21,36–38]. According to factor group analysis, Ohsaka [38] and [21] concluded that six anatase Raman active modes are expected (A1g , 2B1g and 3Eg ) and appear at 144 (Eg ), 197 (Eg ), 399 (B1g ), 513 (A1g ), 519 (B1g ), and 639 (Eg ) cm−1 . According to Silva et al. in Ref. [36], the Raman active modes are located at around 443 (Eg(R) ) and 592 (A1g(R) ) cm−1 for the rutile phase. In addition, Xijun et al. in Ref. [39] observed two Raman features modes at 612 (A1g ) and 422 (Eg ) cm−1 for rutile structure in the nanophase. They found that when the annealing temperature increases, the 612 cm−1 Raman mode does not show any frequency shift but the frequency at 422 cm−1 increases to 446 cm−1 . They also report that the Raman normal mode A1g at 612 cm−1 is from Ti O stretching vibration. However, Zhang et al. in Ref. [35] observed four typical vibrational modes for rutile TiO2 around 145 cm−1 (B1g ), 445 cm−1 (Eg ), 610 cm−1 (A1g ), and 240 cm−1 for second-order effect (SOE). In Fig. 2, the Raman active modes located at 246 (SOE), 431 (Eg(R) ) and 603(A1g(R) ) cm−1 are characteristics of the tetragonal rutile phase (P42 /mnm (D4h 14 )), (R in Fig. 2 denotes rutile mode) [35,36,39]. These Raman modes are slightly shifted as compared to those reported in the literature. This could be related to the deviations from stoichiometry of the TiO2 films composition which are known to affect both Raman line positions and widths [36,40]. Moreover, non-stoichiometry affects strongly the lattice vibrational characteristics and induces the line shape changing of the Raman lines in oxides, for example in the nanocrystalline rutile [41]. We can observe on the spectra displayed in Fig. 2a significant shift of the high frequency Raman modes of the rutile phase, but no influence on the frequency of the main Eg mode in the anatase phase. Raman spectrum confirmed the presence of only anatase phase for the unbiased substrate and for the films formed at −25 and −50 V substrate bias voltages. Eg mode was observed at 143 cm−1 and corresponds to external vibration of the anatase structure [37]. These results are in agreement with the phase composition obtained from XRD analyses. Moreover, the Raman spectroscopy showed clearly that the phase transition from anatase to rutile happens at substrate bias voltages of −75 and

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Table 1 The Crystallite size and an estimation of anatase and rutile content. Sample

Biasing voltage (V)

FWHM (Degree)

Crystallite size (nm)

fA (%)

fR (%)

1 2 3 4 5

0 −25 −50 −75 −100

0.368 0.372 0.412 0.418 0.494

31 30 26 25 21

100 100 100 70 45

– – – 30 55

a Photoelectron Intensity (arb. units)

Ti 2p3/2 Ti 2p1/2

-100 V -75 V -25 V 0V 448

450

452

454

456

458

460

462

464

466

468

470

472

Binding Energy (eV)

Photoelectron Intensity (arb. units)

−100 V and the Eg peak of anatase remains at 143 cm−1 . These results are consistent with those reported in Ref. [42]. Fig. 3 shows the FTIR spectra obtained on the films prepared under various substrate voltages. According to Chandra et al. [13], the presence of absorption bands indicates that samples are crystalline. They report also that no absorption bands indicates the presence of amorphous films. Notice in Fig. 3 the presence of absorption bands for all substrate voltages which indicates that the samples are crystalline including the unbiased film. Moreover, the intensity of the absorption bands increases as the substrate voltage increases. The absorption band related to the Ti O Ti stretching vibration mode of TiO2 in anatase phase is at 430 cm−1 according to [43] but Chandra et al. in Ref. [13] shows that it is at 436 cm−1 . In Fig. 3, the strong absorption bands at 1280 cm−1 are attributed to the vibration of stretching and deformation of Si O Si bond stemming from the glass substrate because of the small thickness of the films. The absorption band around 430 cm−1 is attributed to the vibration of the Ti O Ti anatase bonds in TiO2 [13,43]. Other absorption bands are observed at 565 cm−1 and 1450 cm−1 which are assigned to the stretching vibration of Ti O [44,45] and Ti O Ti bonds, respectively. Ti2p (a) and O1s (b) XPS spectra of the as grown films are shown in Fig. 4. Two peaks corresponding to the core level binding energies of Ti2p3/2 and Ti2p1/2 are observed. The separation between the doublet is 5.74 eV which is characteristic to Ti4+ oxidation state in TiO2 according to [9,13]. The most conspicuous feature of the O1s signals is the presence of a doublet. The signal at about 529.4 eV corresponds to the oxygen atoms in TiO2 phase while the second peak observed at higher binding energy (531.5–532.2 eV) corresponds to hydroxyl groups as reported in the literature [8,23–26]. It can be clearly observed that the binding energy of titanium peaks and the O1s related to titania were quasi unchanged for all samples. However, a shift of the second peak towards lower binding energies was observed at low voltage (0 and −25 V). The ratio of oxygen of hydroxyl group (OH− ) to the oxygen bound to TiO2 can be obtained by fitting the O1s signal as shown in Fig. 5. The value of OH− to the total oxygen varied between 20% and 40%. The difference of amount of humidity adsorbed on the samples could be related to a difference in the surface porosity and roughness. However, a systematic study would be needed to validate such correlation. We have reported in Fig. 6 the Ti2p (a) and O1s (b) XPS spectra recorded after etching cycles with argon ions. We observe the presence of a second peak of Ti 2p at lower binding energy (457 eV). This peak could be assigned to the reduction of Ti4+ ions to Ti3+ as a result of argon etching under vacuum. The reduction of the oxidation state of titanium atoms under argon bombardment has been reported in the literature [22,46]. It can be noted that, as expected, the O1s signal associated with the hydroxyl groups is quasi absent in all spectra after etching except in the case of the sample prepared under −25 V bias where a small signal subsists. Fig. 7 shows the AFM images of TiO2 films formed at (a) −25 V and (b) −50 V. The surface morphology of the deposited films was significantly influenced by the substrate bias voltage. The AFM micrograph of TiO2 films formed at (a) −25 V (Fig. 7(a)) showed a smooth surface, which was homogenous and uniform with an average grain size of 30 nm. As the substrate voltage increased to

b

O1s (TiO2)

O1s (Hydroxyl)

-100 V -75 V -25 V 0V 520 522 524 526 528 530 532 534 536 538 540 542 Binding Energy (eV)

Fig. 4. Ti2p (a) and O1s (b) XPS spectra of as grown TiO2 thin films before etching.

−50 V, the grain size was decreased to about 26 nm as shown in Fig. 7(b). This can be explained by the increase in the crystallinity of the films as supported by Raman and XRD data. In addition, as the substrate bias was changed from −25 V to −100 V, the root mean square roughness (Rrms ) increased from 0.747 to 2.191 nm. This result may be related to the increased kinetic energy of Ti+ ions as the bias voltage on the substrate increases [1]. Figs. 8 and 9 show the reflectance and transmittance spectra of the TiO2 films recorded in the UV–vis region. All studied thin films were uniform and highly transparent to the naked eyes. The films were transparent in the visible region, and a sharp fall in the transmittance was observed below 400 nm. This decrease is due to the fundamental absorption of light due to the excitation of electrons from the valence band to the conduction band of TiO2 [47]. In Fig. 9, the transmittance spectra in the range of [300 nm,400 nm]

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Photoelectron Intensity (arb. units)

O1s (TiO2)

O1s (Hydroxyl)

520 522 524 526 528 530 532 534 536 538 540 Binding Energy (eV) Fig. 5. XPS spectra of O1s fitted spectrum.

showed a strong absorption while the transmittance decreases in the UV region due to band to band electronic transitions. A transmittance value of 78–87% was recorded in the range from 400 nm to 900 nm for the films deposited under 0 V, −25 V and −75 V substrate bias. However, the films deposited under −50 V and −100 V showed a transmittance exceeding 95% in the region between 400 and 500 nm.

4+

a Photoelectron Intensity (arb.units)

Ti2p3/2 Ti

Ti2p1/2 4+

Ti 3+

Ti

3+

Ti

-100 V -75 V -25 V 0V 452 454 456 458 460 462 464 466 468 470 472 Binding Energy (eV)

Photoelectron Intensity (arb.units)

b

Fig. 7. AFM Topography images of the films: (a) −25 V, (b) −50 V.

We used the transmittance data and the graph [(Log 1/T) h]1/2 versus photon energy (Fig. 10) to extract the value of optical gap of the films [48,49]. The obtained value of the optical gap decreased from 3.45 eV to 3.22 eV as the bias of the substrate was varied from 0 to −100 V. Table 2 shows a summary of several parameters of the films such as: the bias voltage, energy band gap, crystallite size and roughness.

O1s

100 0V -25 V -50 V -75 V -100 V

90

-100 V -75 V

-25 V 0V 524

526

528

530

532

534

536

538

540

Binding Energy (eV)

Reflectance (%)

80 70 60 50 40 30 20 10 0 300

400

500

600

700

800

900 1000 1100 1200

Wavelength (nm) Fig. 6. Ti2p (a) and O1s (b) XPS spectra of TiO2 thin films after etching cycles with argon ions.

Fig. 8. UV–vis reflectance spectra.

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Table 2 Energy band gap, crystallite size and roughness for TiO2 thin films deposited at various substrate bias voltages. Sample

Biasing voltage (V)

Energy band gap (eV)

Crystallite size (nm)

(RMS) roughness

1 2 3 4 5

0 −25 −50 −75 −100

3.45 3.42 3.39 3.35 3.22

31 30 26 25 21

/ 0.74 1.53 1.74 2.19

TiO2 . The grain size and the optical gap decreased as the substrate bias was varied from 0 V to −100 V. Our results suggest the possibility to engineer the structural and optoelectronic properties of the films by controlling the synthesis conditions during growth. Biasing substrate voltage is a promising alternative that enable the deposition of crystalline TiO2 films without heating the substrate.

100 90

Transmittance (%)

80 70 60 50

0V -25 V -50 V -75 V -100 V

40 30 20 10 400

500

600

700

800

900 1000 1100 1200

Wavelength (nm) Fig. 9. UV–vis transmittance spectra.

5

4

(αhυ)

This work was funded by the Ministry of High Education and Scientific Research of Algeria (MESRS/FNR 2014). References

0 300

1/2

Acknowledgment

0V -25 V -50 V -75 V -100 V

3

2

1

0 1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

Photon Energy (eV) Fig. 10. The variation of the square of the absorption coefficient (␣h)1/2 versus the photon energy h of TiO2 thin films (Eg (0 V) = 3.45 eV, Eg (–25 V) = 3.42 eV, Eg (−50 eV) = 3.39 eV, Eg (−75 eV) = 3.35 eV and Eg (−100 eV) = 3.22 eV).

5. Conclusion In this work, TiO2 thin films have been successfully deposited on glass substrates by rf reactive magnetron sputtering without external heating. A negative voltage was applied on the substrate and was varied from 0 to −100 V. Our results showed that all samples were crystalline and that the substrate bias voltage can be used to control the nature of crystalline phases present in the films as well as their grain size. In addition, the experiments showed that the substrate bias affects the surface roughness of the films. Only anatase phase was obtained at 0, −25 and −50 V voltages whereas a mixture of anatase and rutile phases appeared at −75 and −100 V. XPS analyses revealed that titanium atoms were present on the form of Ti4+ state corresponding to the chemical composition of

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