Structural, optical and electrical characterization of

Materials Science and Engineering B 130 (2006) 220–227

Structural, optical and electrical characterization of spray-deposited TiO2 thin films H.P. Deshmukh a , P.S. Shinde b , P.S. Patil b,∗ a

Department of Physics, Bharati Vidyapeeth Deemed University, Y. M. College, Pune-07, India b Department of Physics, Shivaji University, Kolhapur-416004, India

Received 24 December 2005; received in revised form 7 March 2006; accepted 18 March 2006

Abstract Titanium oxide (TiO2 ) thin films were deposited onto glass substrates by means of spray pyrolysis method using methanolic titanyl acetyl acetonate as precursor solution. The thin films were deposited at three different temperatures namely 350, 400 and 450 ◦ C. As-deposited thin films were amorphous having 100–300 nm thickness. The thin films were subsequently annealed at 500 ◦ C in air for 2 h. Structural, optical and electrical properties of TiO2 thin films have been studied. Polycrystalline thin films with rutile crystal structure, as evidenced from X-ray diffraction pattern, were obtained with major reflexion along (1 1 0). Surface morphology and growth stages based on atomic force microscopy measurements are discussed. Electrical properties have been studied by means of electrical resistivity and thermoelectric power measurements. Optical study shows that TiO2 possesses direct optical transition with band gap of 3.4 eV. © 2006 Elsevier B.V. All rights reserved. Keywords: Titanium oxide; Thin films; Spray pyrolysis; AFM; Optical property

1. Introduction TiO2 has attracted much attention in various fields of science and technology because of its remarkable optical and electronic properties [1,2]. It has high refractive index [3] and dielectric constant [4], and is transparent to visible light [5]. TiO2 thin films have successfully been used in photodecomposition of water [2–6], purification of environmental pollutants [7], and preparation of solar energy cells [8]. It is also suitable for potential applications in optical filters [9], gas sensors [10], ceramic membrane [11], waveguide [12], photocatalyst [13], and antireflection coatings [14]. TiO2 thin films have been synthesized by a plethora of methods like sol–gel [15–17], chemical vapor deposition [18,19], evaporation [20], sputtering [21–23], pulsed laser deposition [24], electrodeposition [25], and spray pyrolysis [26–33]. Of all the afore-mentioned thin film fabrication methods, spray pyrolysis is widely used because of its simplicity, commercial viability, and potential for cost-effective mass production.

∗

Corresponding author. Tel.: +91 231 2690571; fax: +91 231 2692333. E-mail address: psp [email protected] (P.S. Patil).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.03.016

Consequently, preparation of thin films by chemical spray pyrolysis has extensively been undertaken, since the pioneering work by Chamberlin and Skarman [34] in 1966 on cadmium sulphide films for solar cells. Several state-of-art review articles [35–39] covered the thin film formation mechanism, and various static and dynamic chemical steps involved in spray pyrolysis to tailor the thin film properties for transparent conducting oxides and solar cells, and wide variety of the films that could be deposited. The method involves spraying of a desired titanium precursor through an atomizer onto preheated substrates maintained at suitable temperature. The properties of spray-deposited TiO2 thin films were found to be dependent on the processing conditions and precursors. Formation of TiO2 phase, crystallinity, structure, morphology, growth and electrical and optical properties of the thin films depend on a kind of precursor used, mainly because of their thermal decomposition behaviour. The precursors like titanyl acetyl acetonate (TiAcAc) [TiC10 H14 O5 ], Ti(i-OC3 H7 )4 2-propanol, titanium tetrachloride (TiCl4 ), titanium(IV) isobutoxide [Ti((CH3 )2 CHCH2 O)], peroxo-titanium complex solution, etc. have so far been reported [26–33]. Castaneda et al. [27] prepared anatase TiO2 thin films by spray pyrolysis using TiAcAc in the temperature range of 300–500 ◦ C and achieved rutile phase upon annealing the films

H.P. Deshmukh et al. / Materials Science and Engineering B 130 (2006) 220–227

at 850 ◦ C. Thin films prepared at 450 ◦ C were amorphous, partially crystallizing to anatase phase near 500 ◦ C [28]. Natrajan et al. [29] deposited anatase TiO2 films from aqueous peroxotitanium complex solution at 300 ◦ C. Okuya et al. [32] have studied role of additives in the precursor solution on the mechanism of crystallization of TiO2 . In this paper, effects of substrate temperature and postannealing treatment on structural, morphological, optical and electrical properties of spray-deposited TiO2 thin films are discussed. 2. Experimental details TiO2 thin films were deposited using AR grade (99.99% pure, MERK made) TiAcAc as a precursor. The starting solution, 20 ml 0.05 M TiAcAc plus methanol, was sprayed through a pneumatic glass nozzle using ambient air as a carrier gas onto the ultrasonically cleaned preheated glass (soda lime silica) substrates. Basically three samples of area 0.135 cm × 2.5 cm × 7 cm were prepared for each substrate temperature. Film deposition was observed to be more on the middle substrate than on the others since there remained a temperature gradient of about 50 ◦ C as one go away from center to the periphery of the heating plate (circular). Therefore we have chosen only those films, which were deposited onto the middle substrates. The chosen middle sample was then cut into four equal pieces and subjected to the annealing treatment for 1 h at 500 ◦ C in air. The best sample was chosen for each substrate temperature depending upon the nature of the XRD patterns after annealing. The chosen annealed sample then used for all the characterizations. Preparative parameters viz. solution concentration, quantity of spraying solution, nozzle-to-substrate distance and solution flow rate were carefully optimized to obtain uniform, pin-hole free and adherent thin films. Films were redeposited with identical conditions and characterized in order to check their reproducibility. It confirms from our observation that films are quite reproducible. Optimized preparative parameters are given in Table 1. Substrate temperature was varied from 350 to 450 ◦ C, in steps of 50 ◦ C with an accuracy of ±5 ◦ C. Chromel–alumel thermocouple was used to measure temperature of the supporting hot plate. The films prepared at substrate temperatures 350, 400 and 450 ◦ C are denoted by T350 , T400 and T450 , respectively, and after annealing denoted as TA350 , TA400 and TA450 , respectively.

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Thermal decomposition behaviour of TiO2 precursor (TiAcAc powder) was studied using thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA). TGA and DTA measurements were performed in the temperature range between 30 and 800 ◦ C with Al2 O3 as a reference material at the scan rate of 10 ◦ C min−1 . The chamber was purged with oxygen at the flow rate of 100 cm2 min−1 . Thickness of the thin films was measured by using step profilometer. The X-ray diffractometer (Philips PW-1710), with Cu ˚ (operated at 25 kV, K␣ radiation of wavelength 1.5405 A 20 mA) was employed for structural studies. Morphology of the deposited TiO2 thin films was revealed using atomic force microscopy (AFM). The scans were performed with 1 ␮m × 1 ␮m scanner. The contact mode AFM images of the TiO2 thin films prepared at three different substrate temperatures were obtained using JEOL, JSPM-4200 scanning probe microscope. Electrical resistivity of the annealed films (2 cm × 1 cm) was determined in the temperature range 350–460 ◦ C, using four-probe method (Scientific equipments, Roorkee, India) with ±5 ◦ C accuracy. The distance between the probes was 0.2 cm. The Seebeck measurements were carried out with the help of home-made thermoelectric power (TEP) unit in the temperature range of 340–460 ◦ C with ±5 ◦ C accuracy. The optical absorption and transmission spectra were recorded in the wavelength range of 300–900 nm at room temperature using UV–Vis, V530 spectrophotometer (JASCO). 3. Results and discussion 3.1. Thermal decomposition characteristic of TiAcAc precursor Fig. 1 shows the thermogram recorded for TiAcAc (C10 H14 O5 Ti) powder. Thermal evolution in oxygen atmosphere takes place in five consecutive stages with weight losses for which the inflection point coincides with the temperature corresponding to the endotherms and exotherms in DTA trace. The weight loss begins at 165 ◦ C.

Table 1 Various preparative parameters and their optimized values Preparative parameters

Range studied

Optimized value

Precursor Solution concentration (M) Solution quantity (cm3 ) Nozzle tip diameter (cm) Spray rate (cm3 min−1 ) Nozzle-to-substrate distance (cm) Air flow rate (l min−1 ) Air pressure (kg cm−2 )

– 0.005–0.1 10–50 0.02–0.06 2–10 15–30 2–10 –

TiAcAc 0.05 20 0.03 4 25 5 2.5

Fig. 1. TGA-DTA thermograms of the TiAcAc precursor powder in air in the temperature range 30–500 ◦ C.

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Table 2 Effect of substrate temperature on properties of spray-deposited TiO2 thin films Sample

Thickness, t (nm)

T350 T400 T450 TA350 TA400 TA450

400 300 200 300 200 150

± ± ± ± ± ±

20 20 20 20 20 20

Band gap energy, Eg (eV)

τ at λ = 630 nm (%)

Crystallite size, D(1 1 0) (nm)

Room temperature resistivity, ρRT ( cm)

Activation energy, Ea (eV)

TEPS, S (␮V K−1 )

– – – 3.4 3.4 3.4

– – – 67 80 88

– – – 77 103 128

– – – 7.94 × 108 2.51 × 108 0.5 × 108

– – – 0.99 0.86 0.76

– – – 82 118 188

Step-I describes the weight loss due to evaporation of physisorbed water from the precursor. This is followed by threestep weight loss. A rapid weight loss was found in the temperature range between 200 and 250 ◦ C, corresponding to a rapid decay in the TGA curve (step-II), followed by a slow one between 250 and 370 ◦ C (step-III). Further, slow decay between 370 and 475 ◦ C is observed (step-IV). These consequent weight losses are attributed to the decomposition of acetate groups and expulsion of gases and, crystallization process, accompanied with an exothermic peak, indicating the formation of TiO2 phase. After 490 ◦ C, the DTA trace remains almost constant (step-V) with no further weight loss indicating formation of a stable and nearly stoichiometric TiO2 phase. Thus, TGA/DTA results indicated formation of TiO2 by complete decomposition of TiAcAc at 490 ◦ C. 3.2. Formation of TiO2 thin films The methanolic solution of TiAcAc was sprayed onto the preheated glass substrates through pneumatic glass nozzle. Every sprayed droplet reaching the hot substrate undergoes solvent evaporation, solute condensation and thermal decomposition, thereby resulting into formation of TiO2 thin films, according to the following chemical reaction (Ts -substrate or deposition temperature):

450 ◦ C; whereas upon annealing the films, it varied from 300 to 150 nm, respectively. There may remain difference in the coating thickness among different spatial locations on the samples. However thickness determined by us is the average thickness. The values are given in Table 2. Rise in substrate temperature causes increase in evaporation rate of the initial products leading to diminished mass transport towards the substrates, which results into decrement in film thickness. Hence, film thickness decreases almost linearly with substrate temperature. Upon annealing at 500 ◦ C for 2 h, thickness of the TiO2 films decreased considerably, owing to further decomposition and crystallization. 3.4. X-ray diffraction (XRD) studies Fig. 2 shows XRD patterns (obtained in the usual BraggBrentano θ/2θ configuration) of the as-deposited TiO2 thin films at three different substrate temperatures 350, 400 and 450 ◦ C. Presence of broad hump indicates the amorphous nature of the films. Upon annealing at 500 ◦ C for 2 h in an ambient air, these films became polycrystalline. Fig. 3 shows XRD patterns of the annealed films. The d (interplanar spacing) values of XRD

TiC10 H14 O5 + CH3 OH TS (350−450 ◦ C)

−−−−−−−−−→TiOx + 11CO2 ↑ +9H2 O ↑

TiOx

(amorphous)

Annealing (2 h) at 500 ◦ C

−→

TiO2 (polycrystalline)

As-deposited films were amorphous. Subsequent annealing of these films at 500 ◦ C resulted into the formation of polycrystalline thin films. Films were uniform and strongly adherent to the substrates. Average transmittance of thin films in the visible region was >70%. 3.3. Thickness measurement The film thickness was measured using step profilometer with surface irregularities of the order of 0.2 nm. Thickness of the as-deposited films varied between 400 and 200 nm (±20 nm accuracy) as the substrate temperature increased from 350 to

Fig. 2. The superposed X-ray diffraction spectra of the samples deposited at three different substrate temperatures viz. 350, 400 and 450 ◦ C.


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3.5. Atomic force microscopy (AFM) Thin film growth generally comprises following stages [44]: i. Adsorption-diffusion-aggregation (ADA) to form welldefined grains to develop the 1st monolayer (ML) and onset of 2nd ML. ii. ADA and formation of 2nd matured ML and onset of 3rd ML and so on. iii. Formation of islands due to coalescence of clusters formed by diffusion and agglomeration of grains.

Fig. 3. The superposed X-ray diffraction spectra of the samples annealed at 500 ◦ C for 2 h in air exhibiting rutile (R) phase.

reflexions were compared with standard d values taken from Joint Commission on Powder Diffraction Standards (JCPDS) data. The films possess rutile as a prominent phase of TiO2 with tetragonal crystal symmetry [40]. The XRD patterns exhibit predominantly (1 1 0) reflexion, which is the most stable low index face of the rutile phase [41]. Additionally, some low intense (0 2 0) and (3 2 1) peaks corresponding to the substrate (SiO2 ) are observed [42]. The diffraction peaks ameliorate with increase in substrate temperature, indicating the grain growth. Liu et al. obtained rutile phase of TiO2 along (1 1 0) plane in the thin films prepared by pulsed vapor deposition [43]. To determine the crystallite size, slow scans of all the samples were carried out, with step of 0.02 degree min−1 having angle of diffraction between 28◦ and 29◦ for the (1 1 0) plane. The crystallite size is deduced from well-known Debye Scherrer’s formula, D(1 1 0) =

0.9λ β cos θ

(1)

where λ is the wavelength of incident X-ray radiation ˚ β the broadening of diffraction line measured at (1.5406 A); half of its maximum intensity in radians, i.e. full width at half maximum (FWHM) and θ the Bragg’s diffraction angle corresponding to the (1 1 0) peak. The factors viz. instrumental broadening, distortion of lattice, etc. were assumed to be common among all the samples. The crystallite size increases from 77 to 128 nm with substrate temperature (Table 2). This is due to the fact that smaller grains tend to have surfaces with sharper convexity and gradually disappear by feeding the larger grains, as temperature increases. The net effect is grain growth. Consequently, sample TA450 has larger crystallite size than other samples.

Formation dynamics of the steps depend both on deposition and post-annealing temperatures. The 1st and 2nd steps depend on deposition temperature, while the 3rd step occurs during annealing. Fig. 4a shows 2D AFM images of the TA350 sample. It exhibits beehives type structure with nanopores of different sizes ranging from 120 to 20 nm. It is apparent that three or more small pores break open and form a large pore after annealing. Grains become almost spherical and uniformly distributed, causing denser film at TA400 (Fig. 4b). For TA450 sample, the coalescence of grains to form ripple morphology is clearly seen (Fig. 4c). The average ripple wavelength was found to be 20 nm. The wrinkled ripple pattern (shown in Fig. 4c) is due to minimization of surface energy to form a stable selfassembled pattern. 3.6. Electrical resistivity Fig. 5 shows Arrhenius plots exhibiting temperature dependence of resistivity (ρ) for all the annealed films, supporting the semiconducting behaviour. The resistivity was measured using relation [45], π V ρ= t (2) ln 2 I where V is the applied voltage, I the current, and t the film thickness of the film. Here the correction factor is assumed to be unity. We do not expect different modes of electronic transitions with temperature in the studied range since the curvature of the plot is not that distinguishable though it looks like sigmoid shape. The room temperature electrical resistivity (ρRT ) was of the order of 108 cm. It varied from 7.94 × 106 cm for TA350 to 0.5 × 108 cm for TA450 annealed samples. The values are listed in Table 2. TiO2 stoichiometric oxide belongs to 3d0 insulators group with d0 electron configuration. The band gap is between filled band of bonding orbitals, with predominantly oxygen 2p atomic character, and an empty ‘metal d’ band of antibonding orbitals. Nevertheless, the d0 insulators are susceptible to loss of oxygen, which gives rise to non-stoichiometric or reduced compounds such as TiO2 − x , causing defects. Hence, extrinsic semiconducting properties, although intrinsic semiconduction is ruled out, take place. The (1 1 0) surface contains two types of oxygen vacancies (point defects), one in a row of bridging O-ions and one in the surface plane [46]. These defects create donor levels close to bottom of the conduction band, which ionize upon

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Fig. 4. Two-dimensional contact mode AFM images of the samples (a) TA350 , (b) TA400 and (c) TA450 .

heating the films. Recently, Bak et al. [47] have described details of defect chemistry and semiconducting properties of TiO2 with defect diagrams. Thermal activation energies were calculated by using the relation [48], Ea ρ = ρ0 exp (3) kT where ρ0 is the pre-exponential factor; k the Boltzmann constant; and T the absolute temperature. The calculated values of thermal activation energies for the samples are given in Table 2. Thermal activation energy represents the average energy of the carriers with respect to the Fermi energy, if the carriers can only move at the bottom or top of the well-defined band.

Negative temperature coefficient of resistance (TCR) is calculated by, αT =

R1 − R2 ρ1 − ρ 2 = RT (T1 − T2 ) ρT (T1 − T2 )

(4)

where RT is room temperature resistance (at 300 K). The negative TCR is of the order of 10−4 . Sample TA450 showed larger TCR of 3.8 × 10−4 K−1 while TA400 showed lowest TCR value of 1.804 × 10−4 K−1 . 3.7. Thermoelectric power (TEP) measurement TEP is the ratio of thermally generated voltage to the temperature difference across the semiconductor. Diffusion of thermally generated majority charge carriers occurs from high temperature


Fig. 5. Arrhenius plots showing variation of log ρ with 1/T for TA350 , TA400 and TA450 samples.

to the low temperature end, as a result of temperature difference ( T). This creates a positive space charge near high temperature end, which sets up an electric field or potential difference thereby giving rise to a thermal emf ( E). For TiO2 material, conduction electrons originate from ionized defects such as oxygen vacancies, rendering n-type conductivity. TEP was measured in the temperature range of 343–453 K. Fig. 6 shows variation of E with T for all the annealed films. TEP increases slowly for sample TA350 . The rate of increment is higher in the range of T from 355 to 375 K and thereafter slowed down. Initial increment is attributed to the increase in mobility of charge carriers and carrier concentration with rise in T. In case of TA400 and TA450 , there is almost no change in thermal emf for the T from 343 to 383 K and 343 to 443 K, respectively, indicating no change in carrier concentration. However, E increases linearly from 383 and 443 K onwards for the samples TA400 and TA450 , respectively. The magnitude of TEP increases with increase in substrate temperature. This may be attributed to the amelioration of crystalinity, due to

225

Fig. 7. Transmission spectra for the TA350 , TA400 and TA450 samples.

which inter-granular barrier height decreases, causing reduction in grain boundary scattering. The values of TEP lie in the range of 82–118 ␮V K−1 and are listed in Table 2. 3.8. Optical studies 3.8.1. Transmission measurements Fig. 7 shows transmission spectra of annealed samples. In case of TA450 , transmittance (τ) is as low as 75% at longer wavelengths (900 nm) and gradually rises towards shorter wavelengths until it reaches its maximum value of 90% at 560 nm, a value very close to the reference (bare substrate with τ = 93%). At shorter wavelengths, transmittance decreases rather quickly, shows a shoulder near 370 nm and approaches near zero at around 300 nm. The wavy nature of the curve between 350 and 850 nm is connected with the film thickness and consequently with the interference between TiO2 film and the substrate. On the other hand, a sharp decrease toward UV region (below 350 nm) is due to the fundamental absorption of light caused by the excitation of electrons from valence band to conduction band of TiO2 . Similar type of behaviour is observed for other two samples. From these spectra it is seen that average transmittance of the films increases with substrate temperature. The values of transmittance at 630 nm are 67, 80 and 88% for TA350 , TA400 and TA450 samples, respectively. Increase in transmittance with substrate temperature may be attributed to the thickness of the film, and nature of microstructure and surface morphology. 3.8.2. Optical absorption Optical absorption coefficient as a function of wavelength (αλ ) is calculated using the formula [21], 1 1 αλ = ln (5) t τλ

Fig. 6. Variation of thermal emf ( E) with temperature difference ( T) for TA350 , TA400 and TA450 samples.

where t is thickness of the film; τ λ transmittance of the film at a particular wavelength. The value of α is of the order of 104 cm−1 .

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phology having self-assembled wrinkles at highest deposition temperature. Acknowledgements Authors wish to acknowledge the UGC, New Delhi for the financial support through the UGC-DRS II phase programme. One of the authors (H.P. Deshmukh), is grateful to Hon’ble Dr. Patangrao Kadam, Chancellor Bharati Vidyapeeth Deemed University and Minister for Co-operation, Rehabilitation and Relief work, Maharashtra state and Dr. Shivajirao Kadam, Secretary, Bharati Vidyapeeth and Pro-Vice-chancellor, Bharati Vidyapeeth Deemed University, for the financial assistance from ‘Dr. Patangrao Kadam Pratishthan (University Fund)’. References Fig. 8. Plot of the (αhν)2 vs. photon energy (hν) for the TA350 , TA400 and TA450 samples.

Absorption coefficient decreases with increase in substrate temperature. The nature of optical transition is governed by a classical relation [49]: α=

α0 (hν − Eg )n hν

(6)

where α0 is a constant, a probability parameter for transition; Eg the optical gap energy; hν the photon energy and n constant. The value of n is 1/2 or 2 depending on presence of the allowed direct and indirect transitions. Fig. 8 shows the plots of (αhν)2 versus hν for annealed films. The nature of the plots suggests direct interband transition. The extrapolated linear portion of the curve and the horizontal baseline intersect at point X. The perpendicular drawn from X to zero absorption coefficient (α = 0), leads to estimation of band gap energy. Eg value remains same at 3.4 eV irrespective of the change in substrate temperature. It is due to the fact that lattice defects introduced by the mode of processing will cause only localized energy levels within the band gap without affecting its width. The obtained Eg value matches with spray pyrolysed TiO2 thin films reported by Xu et al. [26], Castaneda et al. [27] and others for sputtered thin films [27].

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

4. Conclusion To summarize, we have successfully prepared transparent nTiO2 thin films using methanolic TiAcAc precursor by employing a simple and inexpensive spray pyrolysis technique. Influence of substrate temperature on growth and morphology of TiO2 thin films is discussed. XRD shows that as-deposited films are amorphous and became polycrystalline with rutile crystal structure, oriented along (1 1 0) plane upon annealing at 500 ◦ C. Band gap energy (3.4 eV) was independent of both the substrate and annealing temperature. Annealed samples contained inter cross-linked structure leaving nano-pores in the films deposited at low substrate temperature. The uniform growth of almost spherical grains was seen at slightly elevated temperatures, which coalesce to form islands with ripple mor-

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